Project Landing Gear - Report

2008
Hogeschool van Amsterdam Amsterdamse Hogeschool van Techniek Aviation studies

Project Landing gear- Boeing 737-800

Imad Aarrass

Gear down, 3 green

Omer Badal Jordi van de Bogaart Sara van Grieken Ahmet Pehlivan Benjamin Wennink Reha Yali

Preface
This project group report is produced by group 2A2E. The process of making the report was a educating and challenging one. The group would like to take this opportunity to thank Mr. P. van Langen for his guidance and assistance during the process of making this report. We would also like to thank Mrs. Wentzel for providing the guidance of proper reporting techniques and Benjamin Vlijm for his part of the project and we regard the fact that he had to stop early on.

The group 2A2E consists out these seven persons:

Ahmet Pehlivan – Sara van Grieken – Benjamin Wennink – Benjamin Vlijm (ex member) – Omer Badal – Reha Yali Jordi van den Bogaart – Imad Aarrass

Table of contents
Summary .................................................................................................................................... 1 Introduction ............................................................................................................................... 2 1. Landing gear Boeing 737-800 NG .................................................................................. 3
1.1 Purpose ....................................................................................................................................3 1.2 Structure ..................................................................................................................................3 1.2.1 General overview................................................................................................................4 1.2.2 Nose landing gear struts .....................................................................................................5 1.2.3 Main landing gear struts......................................................................................................7 1.2.4 Wheels and tyres................................................................................................................7 1.3 Functioning ...............................................................................................................................9 1.3.1 Extension and retraction mechanisms...................................................................................9 1.3.2 Brakes .............................................................................................................................12 1.3.3 Steering...........................................................................................................................14 1.4 Regulations .............................................................................................................................15 1.4.1 General ...........................................................................................................................16 1.4.2 Shock absorption..............................................................................................................16 1.4.3 Retracting/extending mechanism .......................................................................................16 1.4.4 Brakes .............................................................................................................................17 1.4.5 Wheels and tyres..............................................................................................................17 1.4.6 Steering systems ..............................................................................................................18

2.

Analysis landing gear Boeing 737-800 NG .................................................................. 19
2.1 Design aspects ........................................................................................................................19 2.1.1 Durability .........................................................................................................................19 2.1.2 Safety .............................................................................................................................20 2.2 Forces on the landing gear during landing..................................................................................20 2.2.1 Pre-calculations ................................................................................................................20 2.2.2 Forces during landing........................................................................................................22 2.2.2 Forces and kinetic energy during landing ............................................................................23 2.3 Forces during a rejected take-off...............................................................................................23 2.3.1 Forces during take-off.......................................................................................................23 2.3.2 Forces during rejected take-off ..........................................................................................25 2.4 Vibrations ...............................................................................................................................28 2.4.1 Occurring forces ...............................................................................................................28 2.4.2 Stress..............................................................................................................................28

3.

Troubleshooting............................................................................................................. 30
3.1 Malfunctions............................................................................................................................30 3.1.1 Main wheel tyre thread separation .....................................................................................30 3.1.2 Nose gear collapse ...........................................................................................................31 3.1.3 Insufficient lubrication of the nose landing gear ..................................................................33 3.2 Maintenance............................................................................................................................34 3.2.1 Checks ............................................................................................................................34 3.2.2 Maintenance manual.........................................................................................................35 3.3 Costs ......................................................................................................................................35 3.3.1 General costs ...................................................................................................................35 3.3.2 Costs for tyres and brakes .................................................................................................36 3.4 Conclusion ..............................................................................................................................36

Bibliography ............................................................................................................................. 38 List of abbreviations ................................................................................................................ 39 List of appendices .................................................................................................................... 40

Hogeschool van Amsterdam

Aviation Studies

Summary
Airline Amstel Leeuwenburg Airlines has assigned the engineering department to perform an investigation of possible malfunctions which can occur in the landing gear and related systems. Therefore an analysis of the landing gear functions and lay-out need to be performed. In addition to this the influence on the aeroplanes dispatch, to be precise the airworthiness, is investigated. This investigation is specified to one type of aeroplane that Amstel Leeuwenburg Airlines possesses; the Boeing 737-800 NG. The Boeing 737-800 NG landing gear is based on a conventional tricycle gear, involving two main gear assemblies and one nose gear assembly. The landing gear fulfils two main tasks; supporting and manoeuvring the aeroplane. The nose and main gear structure consist of three strut assemblies. This structure is based on a four bar undercarriage system involving; shockstruts, drag struts and side struts. This system is designed to support the weight of the aeroplane, to absorb the landing shock, provide damping and keeping the landing gear extended during different phases of operation. The landing gear is equipped with wheels to provide a smooth manoeuvring ability and some damping. The tyres need to provide grip and are mostly inflated with nitrogen gas which is kept between 117 and 205 psi of pressure. The landing gear is equipped with a retraction and extending mechanism to keep the aeroplane more operational efficient. The retraction and extension can be regulated with the landing gear lever. The locking mechanism keeps the landing gear in the desired position. Uplocks, downlocks and doors are linked to a light indicating system to provide information to pilots and a safe operation. The landing gear is provided with hydraulic actuators for the operation. Because of this an alternate extension mechanism is present in case of a hydraulic failure. Other systems related to the landing gear are the brakes and the steering system. The brakes provide the ability to stop the moving aeroplane early so that there can be landed on shorter runways. On the Boeing 737-800 NG multiple disc brakes are used. The braking system is provided with subsystems like; alternate brakes, auto brakes and the anti-skid system. These subsystems contribute to a more efficient way of braking. The steering system is based on push and pull actuators which provide a movement to the left or right. The steering can be established by moving the rudder pedals or the nose landing gear steering wheel. The main landing gear wheels are equipped with a shimmy damper to control the vibrations of the wheels during manoeuvring. An airworthy landing gear has to comply with the regulations prescribed in Certification Specifications 25 of the European Aviation Safety Agency. Regulations concerning shock absorption, locking mechanisms, brakes, wheels, and tyres and steering are analyzed for the specifications of which the landing gear has to comply with. Part of the analysis of the landing gear is the so called design aspects. These aspects describe the durability, safety and the costs of the aeroplanes materials and operation. The landing gear has to cope with the taxiing, landing and take-off forces. The dispatch of the aeroplane can be influenced by the extreme use of the landing gear. Therefore an analysis of two extreme situations is made; forces on the landing gear during landing and forces on the landing gear during take-off/Rejected Take-off. The forces on the landing gear during require a sophisticated calculation. Therefore assumptions are made to simplify the calculations. Assumed is that the sum of the horizontal and vertical forces are equal to zero while the aeroplane maintains a constant velocity. When the aeroplane attempts to land a landing flare is performed just before touchdown. In this extreme situation this will not be done. The vertical velocity will be higher and resulting in a force on the main landing gear shockstruts of almost twice the weight of the Maximum Landing Mass. When the aeroplane has touched down the brakes have to absorb kinetic energy from the aeroplane. The other extreme situation is when an aeroplane with a Maximum Take-Off Mass must perform a Rejected Take-off. The brakes must be fully applied and a max deceleration rate of 4.27 m/s² can be achieved. Here is also the kinetic energy calculated which the brakes must absorb from the aeroplane. This kinetic energy of the aeroplane is transformed to heat. The temperature change of the brakes during a Rejected Take-Off is 462.2 Kelvin. An aeroplane during operation has to cope with different kinds of forces. These forces are; pressure forces, pull forces, tearing forces and drag forces. These forces cause different kinds of stress on the materials and structure of the aeroplane. This has different consequences for the operation and durability of the aeroplanes materials. Based on the definition of the landing gear and the analysis, three occurring malfunctions in terms of the landing gear are investigated; main wheel tyre thread separation, nose gear collapse and lubricating the steering system. Tyres are components which are exposed to high speeds and friction drag. This causes the tyre to wear during a certain time. When a tyre is worn out it can be recovered. The thread of the tyre is replaced by a new one. When this is done more than four times the chance of the thread coming loose is large. When this happens it mostly causes damage to the structure of the aeroplane. This sometimes results in an inoperative aeroplane. The nose landing gear is vulnerable when the aeroplane is towed or push backed. This can result in a nose gear collapse. This incident causes structural damage to the fuselage of the aeroplane and the nose landing gear. When this malfunction occurs the aeroplane is always inoperative and needs to be repaired. When the nose gear is not lubricated properly it produces awkward noises. As a result of this is that some passengers were worried about the flight safety. This malfunction is harmless but can cause some wear and tear due to friction on the part. When a malfunction is not urgent enough to immediately keep the aeroplane on the ground, the problem can be dealt with during a maintenance check. There are different maintenance checks that variate to a walk around check to a heavy maintenance check. The maintenance crew makes use of different prescribed documents like the Dispatch Deficiency Guide and the Minimum Equipment List. The reports of the malfunctions are reported and processed by the Operations Control Centre. The costs of maintaining a landing gear are not so high because the systems are tough and kept basic. Therefore not many reports of malfunctions concerning the landing gear are experienced. The brakes and tyres are the main parts that need regular maintenance. These parts are mainly responsible for the yearly costs of the landing gear, excluding unforeseen costs. Concluding, the landing gear of the Boeing 737-800 NG is not very vulnerable to malfunctions which involve the dispatch of the aeroplane. When it does, the aeroplane is sufficiently damaged which influences the airworthiness.

Landing gear
Project group: 2A2E

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Hogeschool van Amsterdam

Aviation Studies

Introduction
The first project in the second study year is about landing gear. The assignment is to choose an existing aeroplane and provide an analysis of the layout and the operation of its landing gear. The failures that could occur on the landing gear system and the influence these failures could have on the airworthiness of the aeroplane as a whole are required to be part of the analysis as well. The end product of the group project is a report which meets the requirements concerning reporting techniques. The report will be clarified by a group presentation. The main part of the report is divided in to three chapters, in order to provide a proper analysis of the chosen landing gear. The aeroplane that is chosen for analysis is the Boeing 737-800 NG, that has a tricycle landing gear. The layout of the nose landing gear and the twofold main landing gear is designed regarding the construction of the rest of the aeroplane and the position of the centre of gravity. The analysis of the operation is divided in to the analysis of separate components of the landing gear. The components are the struts, the retracting and extension mechanism, the wheels, the brakes and the steering system of the nose landing gear. (1) The forces that exist on the landing gear are included in the analysis of the landing gear. When performing a landing or a rejected take-off, these forces reach their highest values. The vibrations due to movement of the aeroplane and applying brakes are relevant for analysing the landing gear as well. The design aspects of the landing gear consisting of the design durability and safety are described to clarify the analysis. (2) Three malfunctions concerning landing gear are described. This is done by defining the malfunction and its causes. Than the influence the malfunctions have on the airworthiness and the solutions that are designed for the certain failures are described as well. The maintenance of the landing gear is done at the hand of several checks. Maintaining and replacing the landing gear brings certain costs. (3) The main used sources for the project report are: Wentzel (2007) for reporting techniques, IJspeert (2008) for the definition of the assignment (Appendix I) and the Aircraft Maintenance Manuals of the Boeing 737 for the knowledge of the landing gear. more sources are found in the bibliography (p. 38). All abbreviations are marked with a * and in italic. The abbreviations are recited in a list (p. 39). A list of appendices is found on p. 40. It contains the pyramid model (Appendix II), the project planning (Appendix III) and the process evaluation (Appendix XXIX).

Landing gear
Project group: 2A2E

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Hogeschool van Amsterdam

Aviation Studies

1. Landing gear Boeing 737-800 G
The landing gear of a Boeing 737-800 NG* (B737) is used to carry the aeroplane around on the ground during take-off, landing, taxiing, towing and parking. The landing gear is designed to support the weight of the aeroplane and absorb a part of the shock during landing. (1.1) The landing gear of a B737 consists of a three strut assembly and wheels and tyres. (1.2) The landing gear parts have different functions. The extending and retracting mechanism makes it possible to extent en retract the landing gear, the brakes makes sure the aeroplanes is able to stop while on the ground and the steering mechanism to control the aeroplane while on the ground. (1.3) The European Aviation Safety Agency* (EASA) established regulations which have to be followed when designing and using a landing gear. (1.4) The main sources of this chapter are: Boeing 737 Aircraft Maintenance Manual and EASA document CS-25.

1.1

Purpose

The landing gear of the B737 is a conventional tricycle gear; this means that there is one gear assembly in the front of the aeroplane and two equally divided gear assemblies at the back. Looking form a top down view, the shape of the landing gears connected to each other would have the shape of a triangle. The nose gear assembly contains two wheels and the Main Landing Gear* (MLG) contains two assemblies each with two wheels. The landing gear of the B737 supports the aeroplane on the ground and besides that it has several functions like, taxiing, landings and take-offs. The landing gear is retractable for efficient aerodynamic operation. To take-off and land, the aeroplane must taxi to and from the runway. Landing gear has to withstand take-off velocities and landing impacts. An aeroplane must be capable of manoeuvring on the runway, platform and taxiways. The function of the landing gear can be categorized in two functions: 1. Support 2. Manoeuvring Ad 1. Support An aeroplane needs support when it is in rest on the ground or airport platform. An element between the fuselage and the ground prevents the aeroplane of damage and keeps the aeroplane in equilibrium (fig. 1). Landing gear also creates a space for further ground handling (1). The weight of the aeroplane is supported by the landing gear (2). 1. 2. Upward forces (landing gear) Downwards force (centre of gravity)

Figure 1: B737 in a equal static rest Ad 2. Manoeuvring The landing gear of the B737 is capable of manoeuvring on the ground. This process is also called taxiing. This can be arranged by its own propulsion or by towing. This can be achieved efficient by the wheels. In this situation the gear is extended and carries the weight of the aeroplane. The landing gear contains brakes and these stop the wheels turning by creating friction. The B737 has to cope with take-off forces and velocities. The wheels enable the aeroplane to move over the ground. This manoeuvring is achieved with the nose gear steering system.

1.2

Structure

The landing gear of the B737 consist of three strut assemblies, these assemblies are equally divided in the shape of a triangle. Every assembly has its own structure, and that would mean that each landing gear is built up of a four bar undercarriage system (1.2.1). The assembly in front of the ‘’triangle’’ is constructed in a certain way in order to fulfil the purposes of supporting and manoeuvring (1.2.2). The two equal assemblies at the back of the ‘’triangle’’ is constructed parallel to each other and fulfil the similar function (1.2.3). Wheels and tyres are made to interact the strut efficiently over the ground (1.2.4).

Landing gear
Project group: 2A2E

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Hogeschool van Amsterdam

Aviation Studies

1.2.1

General overview

The construction of the landing gear of the B737 is a sophisticated system. This system fulfils functions like steering and keeping the aeroplane level when it is on ground. The explanation of the overview is divided in two sections: 1. Nose landing gear 2. Main landing gear Ad 1. Nose landing gear The NLG is responsible for steering and also bears shock impacts (fig. 2). It is located at the front of the aeroplane so the aeroplane can make the turns more lucrative. The gear has a wheel well in the fuselage. This wheel well stores the gear when it is in flight. The NLG consists of a shockstrut, with an inner- and outer cylinder for the shock absorption (1). Holding the NLG in an extended or retracted position is being done by the drag strut (2). The locking mechanism locks the drag strut in the upper or lower position of the landing gear (3). The lock actuator activates the locking mechanism when the gear is up or down (4). Retracting and extending the gear is done by the NLG actuator (5). The shockstrut contains oil and nitrogen for shock absorption, the gas and oil can be charged by the valves (6). The torsion link (7) prevents the inner- and outer strut of internal rotation. The heel of the NLG is attached to the integral Axle (8). If wheel replacement is necessary, the inner cylinder can be lifted by the jack pad (9). Tow fitting is available if towing is necessary (10). The doors (11) on each side of the struts close the wheel well in flight and are connected to the shockstrut. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Shockstrut Drag strut Locking mechanism Lock actuator NLG actuator Oil/nitrogen valve Torsion link Integral axle Jack pad Tow fitting NLG doors

Figure 2: Nose landing gear and door B737 Ad 2. Main landing gear The MLG is located at the back of the aeroplane. It contains two assembly’s, on each side of the wing. When the aeroplane retracts it gear the wheel retract sideways in the fuselage of the aeroplane (fig. 3, p. 5) Each MLG contains a shockstrut (1), and it can be considered as the primary supporting of the landing gear. The shockstrut is stabilized by the drag strut (2). The side strut holds the gear in extended position (3). The strut contains oil and nitrogen for shock absorption, the gas and oil can be charged by the valves (4). During an active actuator, the walking beam (5) decrease forces that goes to the aeroplane construction. The shockstrut is connected to the fuselage by the reaction link (6). The torsion link (7) prevents the inner- and outer strut of internal rotation. Heavy vibrations during braking and taxi are decreased by the shimmy dampers (8). The wheels and brakes can be attached to the axle assembly (9). If wheel replacement is necessary, the inner cylinder can be lifted by the jack pad (10). The inner, outer and centre door (11) closes the fuselage in flight.

Landing gear
Project group: 2A2E

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Hogeschool van Amsterdam 1.Schock strut 2.Drag strut 3.Side strut 4.Oil/nitrogen valve 5.Walking beam 6.Reaction link 7.Torsion link 8.Shimmy dampers 9.Axle assembly 10.Jack pad 11.MLG doors

Aviation Studies

Figure 3: Right main landing gear B737, looking aft.

1.2.2

ose landing gear struts

The NLG is a four bar system. Meaning that the NLG uses three struts and the fuselage to extend, retract and support the nose of the aeroplane. The four bar system is the most common setup for a retractable landing gear. This is due to the fact that a four bar system allows a landing gear to be relatively light and compact compared to other systems. While all the struts in the NLG combined serve the purpose of supporting the nose of the aeroplane. Each separate strut of the NLG on the contrary, fulfils a task to accomplish the main purpose of the NLG. In the struts of the NLG a distinction can be made between the following three struts: 1. Shockstrut 2. Drag strut 3. Torsion links Ad 1. Shockstrut The NLG shockstrut fulfils two functions, the main function is to support the weight of the nose of the aeroplane. The second function is to absorb the landing shock which acts like a damper. The damper of the NLG is based on an oleo-pneumatic shockstrut (fig. 4, p. 6). It consists out an outer cylinder (1), which is attached to the aeroplane and an inner cylinder (2), which is attached to the axle and tyres. The outer and inner cylinder can move in and out of each other. The outer cylinder features two outer (3) and one centre (4) chamber that are filled with nitrogen or dry air. In the centre chamber this nitrogen or dry air is kept at a lower pressure than the two outer chambers. The inner cylinder has one chamber (5) filled with hydraulic fluid. The inner cylinder chamber is divided into two sections by and orifice (6) and a tapered rod (7) that can move in and out of the orifice.

Landing gear
Project group: 2A2E

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Hogeschool van Amsterdam 1. 2. 3. 4. 5. 6. 7. Outer cylinder Inner cylinder Outer chambers Centre chamber Inner cylinder chamber Orifice Tapered rod

Aviation Studies

Figure 4: Shockstrut lay-out. When the shockstrut is compressed during landing, the inner cylinder is forced into the outer cylinder. This will result into the compression on the nitrogen in the upper chambers. This compression will absorb most of the energy that is created during the landing shock, thus absorbing the shock. The general gas law (Formula 1) explains why the landing shock is absorbed due to the compression of the nitrogen. When the inner cylinder is forced into the outer cylinder the volume will decrease, this means that the pressure has to increase in accordance to the gas law. It can now be concluded that a certain amount of volume has been “traded” against a higher pressure, this change absorbs most energy of a landing impact. At the same time, when the inner cylinder is forced into the outer cylinder, the hydraulic fluid which is uncompressible will be forced through the orifice. During this process the energy from the compression will be converted into heat due to the friction between the fluid and the orifice. This conversion from energy is responsible for the damping of the aeroplane during landing. Note, that the tapered rod decreases the size of the orifice depending on the compression. This means that when the shock is compressed further, more force is needed to force the hydraulic fluid through the orifice. This will automatically result in more friction, thus more energy is converted which means more dampening. The two nitrogen chambers under different pressures make sure that when the shockstrut is decompressed, the decompression happens gradually. Formula 1 p = Pressure [Pa] V = Volume [M3] T = Temperature [K] C = Constant The shockstrut also features gas and oil changing valves. These valves allow personnel to pressurize the shockstrut and also allow hydraulic servicing of the shockstrut. The axle of the NLG is directly attached to the inner cylinder of the shockstrut. Thus meaning that when the axle has to be replaced, the entire inner cylinder has to be replaced as well. The axle of the NLG also supports the possibility to be towed by a tow bar. A jacking pad is also provided to enable to jack the NLG and replace the tyres. A jack pad is basically a bulge on the bottom of the shockstrut. This bulge fits into a bulge shaped hole on the top of a jack. When the shockstrut is jacked this hole will be placed over the bulge, which makes sure that the jack can not move when the weight of the shockstrut is carried by the jack. Ad 2. Drag strut The drag strut is used for holding the shockstrut in the extended position. This is accomplished by preventing the shockstrut from folding back into the retracted position. The drag strut consists out off an upper and a lower link and is hinged in the centre. The upper link is connected to the nose wheel side wall structure, while the lower link is connected to the outer cylinder of the shockstrut. During retraction and extension the drag strut will fold at the hinge, thus allowing the shockstrut to retract.

Landing gear
Project group: 2A2E

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Hogeschool van Amsterdam

Aviation Studies

Ad 3. Torsion links The torsion links prevent the inner cylinder of the shockstrut from turning inside the outside cylinder of the shockstrut. Only when a steering force is applied, is it possible to let the two cylinders turn inside each other. The torsion links do allow vertical movements of the shockstrut. Torsion links consists out of an upper link and are connected at the aft ends. The upper link is connected to the outer cylinder, while the lower link is connected to the inner cylinder. When a steering force is applied to the outer cylinder, the force will be transmitted through the upper link to the lower link. The lower link on its turn, turns the inner cylinder, allowing the wheel to turn.

1.2.3

Main landing gear struts

The MLG consists out two identical gears placed on either side of the aeroplane. The MLG consists out a four bar type landing gear. This results into the fact that each strut of the landing gear fulfils a part in supporting the aeroplane. The MLG consists out the following struts: 1. Shockstrut 2. Drag strut 3. Torsion links 4. Side strut 5. Axle assembly 6. Reaction link 7. Walking beam Ad 1. Shockstrut The shockstrut of the MLG supports most of the aeroplane’s weight and is basically a bigger version of the shockstrut of the NLG. The MLG needs a bigger shockstrut because it supports more weight than the NLG and the B737 touches down with the MLG first. Meaning that it needs to absorb a bigger shock. Ad 2. Drag strut The drag strut stabilizes the shockstrut of the MLG in a fore and aft position. The drag strut is an integral part of the MLG shockstrut. Ad 3. Torsion links The torsion links on the MLG serve the same functions as the torsion links on the NLG. The torsion links on the MLG are also installed exactly the same as on the NLG. Ad 4. Side strut The side struts fulfils the same functions as the drag strut does in the NLG, it holds the shockstrut of the MLG in the extended position. This is accomplished by preventing the shockstrut from folding back into the retracted position. The side strut connects to the shockstrut, the fuselage and reaction link and is hinged in the centre. Ad 5. Axle assembly The axle assembly of the MLG attaches to the bottom of the inner cylinder of the MLG shockstrut. The axle assembly supports the wheels and brakes and is removable in case of damage. Ad 6. Reaction link The reaction links are responsible for transferring most of the side loads from the landing gear to the upper end of the shockstrut. The transfer of forces is possible due to the connection with the side strut and shockstrut. The reaction link connects to the upper side of the shockstrut, the fuselage and the side strut. Ad 7. Walking beam The walking beam is used to decrease the forces that act on the aeroplane’s fuselage when MLG actuator is used. This is accomplished by directing a part of the forces that are acting on the fuselage back to the landing gear. The walking beam is attached to the fuselage and the top of the shockstrut.

1.2.4

Wheels and tyres

Each MLG strut of the B737 has two tyre and wheel assemblies. The NLG strut has also two tyre and wheel assemblies. The wheels of an aeroplane makes it possible to operate on the ground and are attached to the axle of the strut (1.2.4.A). To ensure damping and grip on the ground, tyres are placed on the wheels (1.2.4.B).

Landing gear
Project group: 2A2E

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Hogeschool van Amsterdam

Aviation Studies

1.2.4.A Wheels The wheels of a B737 make use of the split wheel system (fig. 5). The advantage of this system is that a tyre can be replaced very easily without damaging the tyre. Another system which can be used, are wheels with a detachable rim edge. It uses the same principle as the split wheel, the only difference is that the rim edge must be removed to place a tyre. The wheels are made of a forged aluminium alloy, which makes the wheel strong but relatively light of weight. The split wheel system consists of two halves, an inner (1) and outer (2) half. Those two halves are jointed together by bolts (3). The MLG wheels have four thermal fuse plugs (4), which are located in the inner wheel half. The purpose of the valve is to prevent tyre explosion, which can be caused by hot brakes. At a temperature of approximately 465 Kelvin, the plugs will melt and as a result of that tyre pressure releases. 1. Outer half 2. Inner half 3. Bolts 4. Thermal fuse plug

Figure 5: Main gear Wheel There are two different types of valves which are assembled in the wheels: 1. Over pressure relief valve 2. Tyre inflation valve Ad 1. Over pressure relief valve The NLG wheels (Appendix IV) and MLG wheels both have an over pressure relief valve. The valve ensures that all of the pressure in the tyre releases when the pressure increases more than 375-450 psi. The valve prevents high pressure in the tyres, and works the same for the NLG wheels and MLG wheels. The only difference is that, the over pressure relief valve of the NLG wheels is assembled in the outer wheel half. While the valve of the MLG wheels is assembled in the inner wheel half. Ad 2. Tyre inflation valve The MLG and NLG wheels also have a tyre inflation valve to release pressure. To ensures a maximize tyre life and minimize runway stress, the pressure in the tyres needs to be correct. A high pressure in the tyres causes a relative small surface of contact between, the ground and the tyre. As a result of this the tyre shall mainly wear in the middle (Appendix V). A low tyres pressure results in a wear to the outside edge of the tyre. 1.2.4.B Tyres The main functions of the tyres on an aeroplane are to provide grip on the runway and absorb part of the shocks during landing and take-off. The tyres of an aeroplane have to be made strongly, because the tyres need to support a full loaded aeroplane. The tyres need to withstand high temperatures. The tyres are made out of different materials and structures, to make the tyres strong enough to bear great forces (fig. 6, p 9). The tread is made of rubber and provides grip, it is resisted against high temperatures (1). The tread is reinforced on the inside with nylon layers (2). The profile ensures drainage of water (3). Furthermore the tyre exists out of a chord body, which follows the shape of the tyre (4). And out of a bead section, which consists of several wires made from steel (5). The purpose of the tyre beads is to strengthen the tyre and retain it on the wheel rim.

Landing gear
Project group: 2A2E

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Hogeschool van Amsterdam 1 3 2 1. Tread 2. Nylon layers 3. Profile 4. Chord body 5. Tyre beads

Aviation Studies

4

5 Figure 6: Tyre There are several circumstances which influence the performances of the tyre, points which have influence on the performances of the tyres are : 1. Size of the tyre 2. The contents of the tyre 3. Hydroplaning Ad 1. Size of the tyre On basis of the static loading case can be decided which size of main wheel tyre is going to be used. By means of the formula for the MLG load (Appendix VI), the size of the tyre can be determined. The total load on the strut is divided equally over the tyres. Ad 2. The contents of the tyre The tyre can be filled with compressed air or nitrogen. Nitrogen is used on modern aeroplane tyres. The advantage of tyres filled with nitrogen in comparison with compressed air, is that oxygen molecules are smaller than nitrogen molecules. This results in a longer preserve of pressure, for tyres which are filled with nitrogen, than of those filled with oxygen. Another advantage of tyres filled with nitrogen is to prevent fire in the event of a blow out. The tyre pressure is supposed to amount between 117-205 psi, which is equal to eight to fourteen bar. Ad 3. Hydroplaning Hydroplaning is a phenomenon that can appear on a wet runway. When the aeroplane lands on a dry runway, the tyres make directly contact with the runway. This ensures a high speed rotation of the tyres. But when an aeroplane lands on a wet runway, the tyres first make contact with the water. As a result the tyres shall water-ski upon the water, which ensures a lower speed rotation of the tyres. When the pilot slows down the temperature of the tyres will increase, as a result the water evaporates and steam will develop. Generally this results in wear of the tyres (Appendix VII).

1.3

Functioning

The several components of the landing gear all have their specific function. The goal for all the components together is to fulfil the main function of the landing gear as a whole. The retraction and extending mechanism (1.3.1) is responsible for letting the landing gear down or up. This system makes the aeroplane more aerodynamic when airborne. The wheels and the brakes (1.3.2) make the aeroplane able to make touchdown and move on the ground. The brakes make sure that the aeroplane can decelerate in order to stop when landing and to control the speed when moving on ground. When on ground the steering system (1.3.3) can direct the aeroplane in order to taxi to the destined apron. The NLG wheel is manly responsible for the steering on the ground. The NLG wheel steering is controlled from the cockpit.

1.3.1

Extension and retraction mechanisms

The landing gear of a B737 uses a number of components to extend and retract the landing gear (1.3.1.A). When all of the actions of these components are combined, the extension and or retraction of the landing gear can be explained (1.3.1.B)

Landing gear
Project group: 2A2E

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1.3.1.A Components Both the NLG and MLG use a number of components to ensure the gear can be extended and retracted safely. These seven components are used to extend or retract the NLG and MLG: 1. Actuators 2. Landing gear lever 3. MLG Uplocks 4. MLG Downlocks 5. NLG Locking mechanism 6. Doors 7. Manual extension Ad 1. Actuators Both of the NLG and MLG feature one actuator to extend or retract the landing gear. These actuators are powered by hydraulic pressure (Appendix VIII) from system A but can use system B to retract the landing gear in case of the loss of pressure from system A. In the NLG the actuator is connected to the drag brace links. When the NLG has to be extended the NLG actuator will retract, thus pulling the drag brace links straight. This will force the shockstrut into its down position. When the NLG has to be retracted The NLG actuator extends, thus folding the drag brace links. This will force the shockstrut into its up position. The MLG on the contrary has an actuator that is attached to the shockstrut. During extension the actuator will retract and thus pulls the MLG out of its wheel well. During retraction, in the contrary the actuator will extend and will push the MLG in its wheel well. Ad 2. Landing gear lever The landing gear lever (Appendix IX, fig.1) is responsible for the control over the landing gear. The landing gear lever consists out of the control lever (1), four positions switches (2), lock mechanism (3) and the lever lock solenoid (4). The control lever has three positions, up, off and down. While the up and down position are self-explaining, the off position is not. When the lever is in the off position, all hydraulic pressure will be removed from the landing gear system. Thus meaning that the landing gear can not accidently deploy during flight. The four position switches determine in to which position the lever is moved and relay this info to the Proximity Sensor Electronic Unit* (PSEU). The PSEU is an computer that has control over several systems, it task is to determine if the aeroplane is airborne or on the ground by a set of sensors. When it the PSEU knows the aeroplane’s status it will activate or deactivate systems according to software. The lock mechanism ensures that the lever can not be moved into the up position while the aeroplane is on the ground and is operated by the lever lock solenoid. When the aeroplane takes-off, the solenoid will be powered and will move the locking mechanism to the unlocked position. This system can be overridden by the override trigger (5). When the pilot wants to move the lever the pilot firstly has pull the lever, before he can select the desired position. When the lever is moved, it moves a push pull rod (6). Which on its turn the forward quadrant (Appendix IX, fig.2) through a push-pull gearbox (2). The forward quadrant moves a cable (3) which is connected to the selector valve, which controls the hydraulic fluid to the landing gear. Ad 3. Main landing gear uplocks The MLG uplock mechanism is responsible for holding the landing gear in a secured up and locked position when the landing gear lever is not in the down position. Each MLG has one uplock mechanism that is located on the ceiling of the MLG wheel well. A single uplock (Appendix X, fig 1) consists out of an uplock hook (1), two springs (2), a uplock actuator (3) and an uplock roller (4). During extension (Appendix X, fig 2) the uplock actuator (1) will retract thus releasing the roller out of the hook (2) and enabling the MLG to extend due to the MLG actuator, air loads and gravity. The two springs (3) hold the hook in an up and locked position when no hydraulic pressure is presented to the lock actuator. During retraction the roller will move into the hook, after which the actuator will extend thus closing the hook and locking the gear in place. Ad 4. Main landing gear downlocks The MLG downlock mechanism is responsible for holding the landing gear in a secured down and locked position when the landing gear lever is in the down position. Each MLG has one downlock mechanism (Appendix XI) that is located between the reaction links (1) and the side strut (2). A single downlock consists out of a hinged downlock link (3), two springs (4) and a downlock actuator (5). During extension the downlock actuator will retract, thus pulling the hinged downlink to the position that can be seen in the appendix. In this position the downlock link prevents the sidestrut from folding around its hinge. Thus meaning that the entire MLG can not retract and is safely secured in the down position. During extension the downlock actuator will extend, thus forcing the downlock link the fold around its hinge. After the downlock is folded the sidestrut will regain the ability to fold and is thus able to retract. The springs hold/force the downlock in its down and locked position when no hydraulic pressure is available.

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Ad 5. Locking mechanism The locking mechanism of the NLG is responsible for holding the NLG secured and locked in both the down and up position. The NLG locking mechanism (Appendix XII, fig. 1 and 3) is located in the NLG wheel well and is connected to the upper and lower drag brace hinge (1) and is hinged at the aft bulkhead (2) in the NLG wheel well. The locking mechanism consists out of a hinged lock link (3), two springs (4) and a NLG lock actuator (5). During extension (Appendix XII, fig. 2) the lock actuator will extend, thus forcing the hinged lock link to turn around its hinged (1a, 2a, 3a) to a horizontal position (2,3). During this transition from the vertical position to the horizontal position, the rest of the NLG is allowed to extend. This is now possible because the hinged lock link is no longer preventing the drag links from deploying. After the rest of the NLG is extended, the lock actuator will extend a little further thus locking the hinged lock link in a horizontal position. In this horizontal position the hinged lock link again prevents the drag links from folding and thus from retracting. During retraction the lock actuator will retract, thus unlocking the hinged lock link from its horizontal position. Then same sequence as with the extension will take place, only in reverse. This sequence will end with the lock actuator retracting a little bit further, thus locking the hinged lock link in de vertical position. The two springs attached to the hinged lock link are used to hold the hinged lock link in either the extended or retracted position. Ad 6. Doors The landing gear features a number of doors to seal the wheel wells to make the wheel wells more aerodynamically efficient. The NLG features two doors, on at each side of the wheel well. The NLG doors (Appendix XIII, fig. 1) are operated by series of control rods (2) that are connected to the top (3) of the shockstrut. This means that when the shockstrut is moving to the down position, the doors will automatically open. During the retraction, the shockstrut will move to the up position and will thus close the doors. A MLG (Appendix XIII, fig.2) on the other hand features three doors, the outer (1), centre (2) and inner door (3). The outer door is directly connected with a hinge (4) to the wing structure (5). The outer door is controlled by a push rod (6) that is connected the shockstrut and the outer door. The centre door is connected to the shockstrut by two adjustable tie rods (7) and follows the movement of the shockstrut. The inner door on the other hand is connected with a hinge to the centre door and is operated with a pushrod that is connected to the sidestrut. Thus as with the NLG the movement of the landing gear controls the deployment of the doors. Note: the wheels of the MLG are not covered by any form of door. Instead the wheels are sealed inside the wheel well with a rubber seal. This rubber seal makes this configuration as aerodynamically as possible. Boeing opted not to go with doors because its investigation stated that the cost and weight of mounting doors, would offer little advantage over the seals. Ad 7. Manual extension The manual extension system is a separated system that allows the extension of the landing gear, when pressure from system A is lost. The manual extension system is operated by handles that are hidden under a hatch in the cockpit. When a manual extension is needed, firstly the hatch has to be opened. By opening the hatch the selector valve will connect all hydraulic components in the landing gear to the return system. This will allow the deployment of the landing gear without the hydraulic system resisting it. Now three handles can be seen. Both outer handles deploy the MLGs, left and right. And the centre deploys the NLG. When one of the outer handles is pulled, it will move a quadrant below the floor. The quadrant on its turn will move a cable to the MLG extension linkage (Appendix XIV, fig. 1). This linkage consists out of a quadrant (1), a rod (2) and a lever (3). When the cable movement (4) arrives at the extension linkage, it will move the quadrant. Which on its turn will move the rod. The rod will pull the lever, which releases the uplock (5). The MLG will now deploy due to gravity and air loads. When the centre handle is pulled, it moves a quadrant (Appendix XIV, fig. 2). The quadrant moves a cam (2), which releases the roller (3). This roller pushes the upper drag link of the NLG (4), which on its turn unlocks the NLG locking mechanism. The NLG will now extend due to air loads and gravity. 1.3.1.B Operations and indications If all the actions taken by each component are placed behind one another the following takes places. During retraction (Appendix XV, fig.1): the lever is moved to the up position. Red lights on the instrument panel, one for each gear will illuminate. The cables that are moved due to the lever will operate the selector valve. This valve will supply the correct actuator with the correct pressure. First the downlock actuator will receive pressure (1a) thus unlocking it, then the MLG actuator will extend (1b). This will force the shock to fold sideways into the wheel well (1c). Due movement of the shockstrut, the sidestrut has to fold (1d). The uplock hook (1e) is still open to allow the uplock roller to move into the hook. Now the movement (2) of all components during transition (2a-2e) can be seen. After the uplock roller has moved into the hook the uplock actuator (3e) extend and thus closing the hook and locking the landing gear. Now the landing gear can be seen in the up and locked position (3), also the location of all components in up and locked position can be seen (3a-3e). The NLG retracts simultaneous (Appendix XV, fig 2) to the MLG with the following actions. The locking mechanism will unlock (1a), the NLG actuator (1b) will retract, thus forcing the drag brace links (1c) to fold. This movement will pull the shockstrut up (1d). Now all NLG components can be seen while the components are transitioning (2a-2d) into the up position.

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After the NLG is raised (3), the locking mechanism (3a) will again receive pressure to lock the drag brace (3c) links and shockstrut (3d) in their position. The NLG is now in the up and locked position. When the gear lock sensors indicate that all gears are locked, the red lights will turn into green lights. During extension the same process will occur, only in reverse. The sensors that indicate if the landing gear is up or down and locked, are located in the up and downlocks. Each MLG uplock contains two sensors, as does each downlock. The NLG locking mechanism also features two sensors for the up and locked position and two sensors for the down and locked position. All sensors use a target and a sensor to determine if the lock is in the correct position for a certain action. All the sensors report to the PSEU, which on its turn can illuminate the landing gear lights or auxiliary lights. Also an audio warning can be given.

1.3.2

Brakes

When on ground, an aeroplane must be able to decelerate. This is done by brakes attached to the landing gear (1.3.2A). On the B737 only the MLG has brakes. The anti-skid system is an aiding system which prevents the wheels of skidding during braking applications (1.3.2.B). Besides the normal braking system, an alternate braking system (1.3.2.C) and an auto braking system (1.3.2.D) are available. 1.3.2.A Normal brakes The braking system of the landing on a B737 consists out multiple steel discs (fig. 7). Steel is chosen for constructing the brake discs, because of its strength and its ability to withstand high temperatures. This is also the reason that multiple discs are better than a single disc. More steel can absorb more heat. These discs are driven by hydraulic actuators (1) powered by hydraulic system B. There are two kinds of discs: Rotor blades (2) that spin with the same velocity as the wheels and the stator blades (3) that do not spin. The brakes are controlled by the braking pedals in the cockpit. The top of the rudder pedals can be pushed back which activates the left brake, the right brake or both brakes. When the brakes are activated, the hydraulic actuators press the discs together, which cause the wheel connected to the brake to decelerate. When an aeroplane is to be parked, the parking brake is activated by pushing the pedals to their maximum and pulling the parking brake lever. This locks the brakes into a braking position. To deactivate the parking brake, the pedals are pushed to their maximum again. 1. 2. 3. Hydraulic actuators Rotor blade Stator blade

Figure 7: Multiple steel disc brakes 1.3.2.B Anti-skid system To reduce the distance needed to stop after braking and to avoid wheel lockdown when activating the brakes, the B737 uses the anti-skid system (fig. 8), which works automatically. Skidding is when the velocity of the wheel rotating is lower than the velocity of the aeroplane. This means that the product of the angle speed and the radius of the wheel is smaller than the velocity in the direction of the movement. Skidding is monitored by an anti-skid transducer (1), which is installed in the axis of each wheel. The anti-skid transducer is an electromagnetic device with an internal rotor. It measures the speed of the wheel it’s attached to. This signal is sent to the Brake System Control Unit* (BSCU) (2). This unit is also called the anti-skid/auto brake control unit. The BSCU is a computer which compares the input signals of the anti-skid transducers with the speed of the NLG wheel, which has no brakes, to monitor whether a MLG wheel is kidding or not.

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A certain amount of skidding is always desired, because of the friction heat it reduces. If there is no skidding at all, the heat caused by friction would be concentrated in one point of the wheel. This desired amount of skidding however is also present at the nose wheel. The BSCU controls the anti-skid valves (3) which can regulate the amount of pressure to a brake. The anti-skid valves are directly connected to the brake hydraulic cylinders (4). Anti-skid protection is provided for both MLG wheels individually. 1. 2. 3. 4. Anti-skid transducer BSCU Anti-skid valves Hydraulic cylinders

Figure 8: Anti-skid system schematic 1.3.2.C Alternate brakes When hydraulic pressure is low or lost in the normal brakes, the alternate brakes are automatically activated by the BSCU. The alternate brakes are driven by hydraulic system A. Antiskid protection is available when the alternate brakes are activated. However, in contrast to the normal brakes, the antiskid protection is not provided for each wheel individually. When the alternate brakes lose pressure as well, the aeroplane can still apply brakes. In this situation the brake accumulator is activated automatically. The accumulator is a cylinder with stored hydraulic pressure and it can provide pressure for six braking applications. When the hydraulic pressure in system B is lost, pressure in system A closes an isolation valve, thus storing pressure in the accumulator. The parking brake can be activated by pressure provided by the accumulator as well. 1.3.2.D Auto brakes On B737 aeroplane automatic brakes are provided. These automatic brakes are activated by the air/ground logic system. This system detects whether the aeroplane is airborne or on ground by two sensors in each strut. These sensors are triggered by weight changes. The sensors in struts of the landing gear detect the compression in the struts when the weight of the aeroplane presses on the ground. These input signals are send to the PSEU. The PSEU sends the signals to the BSCU, which activates the brakes. Other systems activated by the air/ground logic system are the landing gear position indication and warning system and the landing gear compression system. When performing a landing, the auto brakes are immediately activated when the aeroplane touches the ground and when the thrust levers are returned to idle. There are several modes that can be selected concerning auto brakes. Before landing modes 1, 2, 3 and max can be selected depending on the deceleration rate that is desired. This can be done by the auto brake selector. The Rejected Take-Off* (RTO) mode is selected prior to take off. The auto brakes will be triggered when the thrust levers are pulled to idle during take off. The relation between selected modes, maximum brake pressure and deceleration rate is shown in (table 1). Table 1: Auto brakes Auto brake Selector 1 2 3 Max Max RTO

Max Pressure at Brakes (PSI) 1250 1500 2000 3000 3000 Full

Deceleration Rate (ft/sec²) 4 5 7.2 12 (below 80 knots* (kts) 14 (above 80kts) Not Controlled

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1.3.3

Steering

There is a NLG steering system placed to manoeuvre during taxiing. The NLG wheel steering system supplies the ground directional control of the aeroplane (1.3.3.A). The NLG wheel can be controlled by two different mechanics; the first one is the captain or first officer steering wheel and the second one are the rudder pedals. The steering system makes use of two hydraulic systems on different conditions (1.3.3.B). When the aeroplane is in the air it is dangerous that the steering system is active. To prevent nose wheel steering in the air a rotary actuator and centring cams are placed (1.3.3.C). The steering metering valve controls the flow of hydraulic pressure to the steering actuators (1.3.3.D). During a high speed taxiing will the vibrations on the MLG will influence the steering. To prevent the vibrations on the MLG a shimmy damper placed (1.3.3.E). 1.3.3.A Controls The NLG wheel can be controlled by two different mechanisms (fig. 9), these are; the rudder pedals (1) and the NLG steering wheel (2). The maximum reach of the rudder pedals is 7 degrees on left and right direction. The NLG wheel steering wheel of the captain and the first officers and the rudder pedals are connected with a cable loop (3). The range of the steering wheel is 78 degrees on the left and the right direction. The reason of the limitation of the turning range is because of the steering action. At large angles it will eliminate the use of steering actuators. The steering wheel of the captain is coupled on the steering wheel of the first officer. Moving the NLG wheel steering wheel will pull the control cables of the steering system. The movement of the cables goes to the summing mechanism (8) at the NLG wheel. The summing mechanism mixes steering wheel input and NLG gear position feedback to control the NLG wheel steering metering valve (9). The NLG wheel steering metering valve controls the hydraulic flow to the steering actuators (7). When the steering metering valve gets a signal from the summing mechanism it will move the NLG wheel to the desired direction. 1. 2. 3. 4. 5. 6. 7. 8. 9. Rudder pedals Steering Wheel Cable loop Steering quadrant Rotary actuator Shockstrut cylinder Steering Actuator Summing lever Steering metering valve

Figure 9: Nose wheel steering system 1.3.3.B Hydraulic There are two hydraulic system on board, hydraulic system A and B. To switch between the hydraulic systems an alternate NLG wheel steering switch is placed. The NLG wheel steering switch has a normal position and a alternate. It will activate the transfer valve which changes the pressure supply on the NLG wheel. When the NLG wheel steering switch is on the alternate position will the transfer valve change the pressure supply on the NLG wheel. If there is a technical problem with the hydraulic system A the pilot can switch to the alternate position, the alternate position makes use of the hydraulic system B. 1.3.3.C Rotary actuator The function of the rotary actuator (5) is to disconnect the steering arm from the steering quadrant (4). The rotary actuator engages the rudder pedal steering when the NLG is on the ground and disengages it in the air. When the aeroplane goes in to the air, a signal from the air/ground logic system sends power to the rotary actuator. This causes a disconnection between the NLG steering systems. The ground procedure is almost the same but otherwise.

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1.3.3.D Steering metering valve The steering metering valve controls the hydraulic flow to the steering actuators. The steering metering valve gets an input from the rudder pedals, this will activate the steering metering valve to send the pressure to the NLG wheel steering actuator and that will turn the NLG wheel to the desired direction. The metering valve is connected to the shockstrut (6) of the NLG. 1.3.3.E Shimmy damper The shimmy damper is placed at the MLG wheel and it decreases vibrations between the outer and the inner cylinders during high speed taxi and heavy brake use. The shimmy damper is attached between the torsion links. The function of the shimmy damper is to prevent the inner and outer cylinder from turning inside each other. This causes the shimmy damper piston (2) to move from side to side inside the housing assembly. When the piston moves, hydraulic fluid moves through the damping orifices (7). This decreases the piston movement. The damper connects to the return line of the MLG. The compensator (1) maintains system pressure between 18 and 23 PSI. The inlet check valve (3) controls the hydraulic fluid flow rate into the damper 50 PSI. The relief valve (5) protects the compensator if the pressure increases to more than 240PSI. The two bleed plugs (6) are used to remove trapped air in the piston house. The check valve is used to block the reverse flow of the oil that comes from the inlet check valve (Fig 10). 1. 2. 3. 4. 5. 6. 7. Compensator (18-23 PSI) Damper piston Inlet check valve Check valve Relief valve Bleed plug Damping orifice

Figure 9: B737 shimmy damper

1.4

Regulations

A landing gear has to comply with regulations that are set up by the European authority. The EASA is responsible for the certification of large aeroplanes. The regulations for the certification are prescribed in the EASA book Certification Specifications* (CS)-25. The different regulations concerning the landing gear are divided into different subjects. Before a landing gear is airworthy, it has to comply with the general regulations (1.4.1). To withstand the forces occurring on the landing gear shock absorbers need to absorb the shocks on the landing gear. The landing gear has to support the aeroplane in different phases of the flight like landing (1.4.2). Large aeroplanes must have an retractable landing gear. Therefore regulations are set up (1.4.3). The brakes have to be approved and must have back-up procedures in case of failure (1.4.4). The forces on the wheels and tyres of the aeroplane have to be tested with the maximum design weight (1.4.5).

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The steering systems must be easy to handle by the pilot and must not interfere with the extending and retracting mechanism when accidently used while extending or retracting (1.4.6).

1.4.1

General

The landing gear system must be designed so that if it fails due to overloads during take-off and landing the failure mode is not likely to cause a fire hazard in the event of: • Aeroplanes with a passenger seating configuration (excluding pilots seats) of ten or more. • When the aeroplane is under control it can be landed on a paved runway with one or more landing gear legs not extended without causing a structural component failure. The landing gear has to comply with the regulations above to be certified as airworthy. Analysis, tests or both may be shown if the aeroplane complies with the provisions.

1.4.2

Shock absorption

The analytical representation of the landing gear dynamic characteristics that is used in determining the landing loads must be validated by energy absorption tests and must also comply with: • • The energy produced on the shocks during landing must be absorbed. Therefore the aeroplanes designed landing or take-off weight must be used, whichever produces the greater value of landing impact energy The reserve energy absorption capacity descent velocity of 3.7 m/s (12 fps) at the design landing weight may not result in a landing gear failure. This assuming that the aeroplanes lift is not greater than the aeroplanes weight during landing impact.

1.4.3

Retracting/extending mechanism

The retracting and extending mechanism consists of different parts that have to comply with the regulations of CS-25. these parts are: general, landing gear lock, emergency operation and position indicators. For aeroplanes with retractable landing gear the following regulations are applicable: Ad 1. General • The landing gear retracting mechanism, wheel well doors and supporting structure must be designed so that it can support the loads occurring in the flight conditions when the landing gear is in the retracted position. Also including the combination of friction loads, inertia loads, brake torque loads, air loads and gyroscopic loads resulting from the wheels rotating at peripheral speed. • The landing gear doors and supporting structures must withstand the yawing manoeuvres in addition to the conditions of airspeed and load factor. Ad 2. Landing gear lock • The landing gear locks must keep the landing gear extended in flight and on the ground. Also in the air the gear and doors must be held in the correct retracted position. Ad 3. Emergency operation • In the event of an hydraulic, electrical or equivalent energy supply failure the landing gear must able to be extended. Ad 4. Position indicators • For retractable landing gear, position indicating lights for ‘down and locked’ and ‘up and locked’ need to be visible to the pilots. • Warnings must be given when a landing is attempted and the gear is not locked down so that a go-around can be made in time. • A clear indication or warning must be provided whenever the landing gear position is not consistent with the landing gear selector lever position. Ad 5. Protection of equipment Equipment that is essential for the safe operation of the aeroplane and which is located on the landing gear and in the wheel wells must be protected from the damaging effects of:

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1.4.4

Brakes

The brakes are explained in CS 25.735. The regulations about the brakes can be divided in the following: 1. Brakes 2. Brake controls 3. Parking brake 4. Anti-skid system 5. Severe landing stop 6. Brake wear indicators 7. Over-temperature burst prevention Ad 1. Brakes • Each assembled brake need to be approved. • When the braking system and/or the hydraulic system fails the aeroplane has to be able to stop and this stop must not be more than twice the distance of a normal stop. • If the hydraulic system fails and leaks, because of a failure in the brakes, this leak must be insufficient to cause a hazardous fire during the flight or on the ground. Ad 2. Brake controls • The brake controls must be designed and constructed so that it can be handled without using excessive control force. • The pilot must be able to arm and disarm the automatic braking system and the pilot must be able to override the system when braking manually. Ad 3. Parking brake • The aeroplane must be equipped with a parking brake. • The parking brake of the aeroplane, when selected on, must be able to prevent the aeroplane from rolling on level and dry paved runway while using maximum thrust on one engine or maximum ground idle thrust on any or all the engines. • A system in the cockpit must indicate the pilot when the parking brake is not fully released. Ad 4. Anti-skid system The anti-skid system on the aeroplane must be working in all expected kinds of runway conditions, without the system needs external adjustments. Ad 5. Severe landing stop It must be demonstrated that during the first five minutes after an emergency stop, no conditions may occur that could prevent the safe and complete evacuation of the aeroplane. Ad 6. Brake wear indicators An indicating system must be installed that provides for each brake assembly an indication when the heat sink is worn to the permissible limit. This indicator must be reliable and easy to check. Ad 7. Over-temperature burst prevention There must be means in each braked wheel to prevent the tyre to burst, the wheel to fail or both, because of elevated brake temperatures.

1.4.5

Wheels and tyres

The regulations about the wheels can be divided in two parts: 1. Wheels 2. Tyres

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Ad 1. Wheels • The regulations about the wheels are in CS 25.731. • Each wheel used on the NG and MLG must be approved by EASA. • The wheels have to be tested with the maximum design weight and the critical centre of gravity. In this test the maximum static load rating on each LG wheel must be more or the same than the corresponding static ground reaction and the maximum limit load rating must be more or the same then the maximum radial limit load. • The wheels have to be protected against overpressure burst. Excessive pressurisation and tyre assembly may cause the tyre to burst or fail, every tyre needs means to prevent this from happening. Ad 2. Tyres • In CS 25.733 the regulations about the tyres on the LG are described. • On each wheel used on the NG and MLG the suitable tyre have to be assembled. • The forces on the NG and MLG must be tested while the aeroplane carries the maximum design weight. When retracted every tyre assembled on the LG must be clear from the surrounding structure and systems. • The braked tyres on the B737 have to be filled with dry nitrogen or any other gas that does not consist of more than 5% of oxygen, unless it can be shown that when heated the material will not produce a volatile gas or the temperatures do not reach unsafe levels

1.4.6
• •

Steering systems
The steering systems are described in CS 25.745. The steering mechanism of the NG must be easy to handle during landing and take-off without the use of exceptional skill even in the case of cross winds and in the event of sudden power-unit failure. This must be shown by tests. If the pilot accidently uses the steering controls while the LG is retracting or extending, this must not interfere with the retracting or extending of the LG. When the steering mechanism fails to work, the wheel have to have a position that will not cause a dangerous situation. The NG must be designed to prevent damage to the steering mechanism when towed.

• • •

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2. Analysis landing gear Boeing 737-800 G
The landing gear of the B737 has had several changes since its design in the year 1964. Over almost four decades the B737 was manufactured in almost nine different types. The wishes and comfort of the airliners always had the priority by designing the aeroplane. (2.1) By making the right manoeuvrings the B737 can make a landing without disadvantages consequences. (2.2) When the B737 has to break roughly, the struts of the landing gear also has to cope with horizontal forces. (2.3) By applying regularly forces on the construction of the landing gear, it must have the characteristic to deal with stress. (2.4) The wishes of customers never stand still, neither does the technical development of the B737. The main sources of this chapter are: Airplane Characteristics Boeing 737 (2005) and Binas (2004).

2.1

Design aspects

When designing a landing gear it is important to following some guidelines. These guideline are called design aspects. When parts of the landing gear have a really short durability the parts have to be replaced often, this will cost a lot of money and man hours (2.1.1). When the landing gear is not working properly it is important to have back-up possibilities (2.1.2).

2.1.1

Durability

The durability of the landing gear depends mostly on the different types of take-offs and landings the landing gear experiences. Some parts of the landing gear have a longer durability than others. The landing gear is designed in the sidelife design. This means that the system is designed to work properly for years without any major repairs. Described are the three following parts: 1. Gear 2. Brakes 3. Wheels and tyres Ad 1. Gear The gear is designed to cope with hard landings and vibrations. The shockstrut is made out of titanium. Titanium is strong, light (table 2) and corrosion-resistant material. This material is capable of handling big temperature differences. These characteristics make titanium durable and ideal for the landing gear. Table 2: Comparison steal and titanium Characteristic Density [kg/m3] Pull strength [psi]

Steel 7800 50000

Titanium 4540 80000

Ad 2. Brakes The B737 has steel brakes. In 1996 new steel brakes where designed with improved durability because of increased energy absorption and a shorter cool down time. The new steel brakes have a new friction material, this increases the durability with 30%. The newest B737 have carbon brakes. The durability of the carbon brakes is twice as long as the steel brakes and the carbon brakes are 300kgs lighter than the steel brakes (table 3). The carbon brakes absorb the heat better than the steel brakes, because of this the carbon brakes last longer. The carbon brakes are more expensive but this compensates with the longer durability and the extra weight the aeroplane can take. Table 3: Comparison steal and carbon Characteristic Density [kg/m3]

Steel 7800

Carbon 2620

Ad 3. Wheels and tyres When one of the two NLG tyres has to be replaced because of worn down tyres, both have to be replaced, otherwise the replaced tyre will wear down faster because of the difference in size. This is called mismatch tyres. In the MLG this makes no difference because there are more tyres to divide the forces and weight. The durability of the tyres depends on the landing conditions and the brake conditions. When the tyres are worn down the manufacture off the tyres will retreat them. The tyres are retreaded after approximately 300 landings. The wheels and tyres are completely replaced in the d-check, this is once in the five years.

Landing gear
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Hogeschool van Amsterdam

Aviation Studies

2.1.2

Safety

When designing a landing gear the designers keep fail safe in mind. Fail safe means that if the landing gear fails to work it will cause no harm or the harm is minimized. The safety features of the following two parts are described: 1. Gear 2. Brakes Ad 1. Gear When the aeroplane is on the ground the landing gear cannot be retracted because of the landing gear lever lock. In case of an emergency this lock can be override by a override trigger. When trying to extent the landing gear and the landing gear lever has no effect there is a manual gear extension access hatch (fig. 11) in the cockpit. The pilots have to open the hatch (1) and pull a handle (2) in the cockpit 61 centimetres, this will extend and lock the landing gear manually. 1. 2. Hatch Handle

Figure 11: Manual gear extension access hatch If the three green landing indicator lights in the cockpit are not working properly the pilots can check if the gear is extended and locked with an extra set of green gear down lights. The second set of lights are located in the overhead panel in the cockpit and are on a different circuit than the normal indicator lights. Ad 2. Brakes Hydraulic system B is used for the brakes. When this system fails system A takes over. When both fails there is a brake accumulator which has enough hydraulic pressure stored to use the brakes. The accumulator stores air pressure, it is pressurised by hydraulic system B . The stored air pressure can be used for six brake applications when both the normal braking system and the alternate braking system fails. The accumulator is also used to make sure the hydraulic pressure does not fluctuate and an instantaneous flow of fluid to the brakes. The B737 only has one accumulator on board.

2.2

Forces on the landing gear during landing

To ensure a safe landing the landing gear has to cope with several forces. Before any force can be determined some precalculations and assumptions need to be made for the points of implement, forces and values (2.3.1). With use of the assumptions and calculated forces and values the forces on the aeroplane during the approach and landing can be calculated. Therefore different equations are used, like the second law of Newton; the force equals the product of mass and acceleration. In this way there can be determined which forces are dealt with in terms of the landing gear (2.3.2). The energy that a moving aeroplane or a part of a aeroplane has, can be calculated by the kinetic energy. Energy can transformed in a other kind of energy (2.3.3). These processes proves that the landing gear is a solid, but also a very dynamic component.

2.2.1

Pre-calculations

The calculations of the forces on the landing gear during different stages of the flight is a sophisticated procedure. To simplify this some assumptions need to be made (2.2.1.A). The forces like lift and thrust apply in different points. Therefore the points in which they apply must be determined (2.2.1.B). Values like the landing mass and the landing velocity can be pre determined. This will be used later on with the actual force calculations (2.2.1.C). 2.2.1.A Assumptions There are dozens of aspects involved in terms of forces on the aeroplane and in particular the forces on the landing gear. These aspects can not all be taken into account. Therefore some assumptions need to be made to be able to calculate the points of implement, forces during the approach, forces before landing impact and the forces in the shockstruts during landing impact. These assumptions are:

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Hogeschool van Amsterdam

Aviation Studies

• The first assumption is that the most extreme situation is used. This means that the aeroplane has its Maximum Landing Mass* (MLM) and that the aeroplane has a vertical velocity which is in range of the maximum vertical velocity which the landing gear must survive. In this case the aeroplane that lands does not flare. This means that the vertical velocity will not be reduced by slowing down the vertical velocity to reduce the landing impact shock. • The second assumption is that the Centre of Gravity* (CG) lies in the middle of the aeroplane. In the horizontal plane the CG has an angle of 15˚ with the MLG. When the aeroplane is flying an approach it is assumed that the CG shifts another 15˚ with the MLG. This will eventually be the assumed position of the CG in the horizontal plane. By assuming this, the position of the payload and the fuel is at an fixed position. This will simplify the calculations because the CG is not at a variable distance from the MLG. • The third assumption is that the Centre of Pressure* (CP), which is located between 9,4 and 27% of the Mean Aerodynamic Cord* (MAC), now lies at 27% of the MAC. The MAC is assumed to be in the middle of the aeroplane so that the CP is at the same height as the CG. When the aeroplane is flying an approach it is assumed that the CP does not shift forwards or backwards. • The fourth assumption is that the aeroplane flies the approach with a pitch attitude of 3˚ nose up. The aeroplane follows the Glide Path* (GP), which has an angle of 3 degrees between the CG and the runway. In this situation the aeroplane descents with a constant velocity and a constant acceleration. The aeroplane does not flare when the landing impact is made. The fifth assumption is that the thrust settings at 15˚ flaps is 48% N1. This is because the aeroplane does not land with high drag forces when the flaps are 40˚ out. When the B737 has the maximum flap position the percent of N1 will be 80%. 2.2.1.B Points of implement In the simplified aeroplane, which is used for the calculations, there are three points of implement: 1. Centre of gravity 2. Centre of pressure 3. Centre of thrust Ad 1. Centre of gravity The CG depends on different factors. Factors like payload septum, angle of attack and fuel quantity and placement. Therefore the assumptions are made. According to this, the position of the CG during an approach and landing can be determined (Appendix XVI). First the distance from the MLG in the horizontal plane is calculated with the 15˚ angle. Second the distance in the horizontal plane when the aeroplane is making an landing and approach is calculated. This results in a horizontal distance of 0.771 m. Ad 2. Centre of pressure The CP is the point where the lift and drag component applies. This point has a certain dimension in the horizontal plane. The distance from the beginning of the wing to the MLG is known. The CP is located several percents of the MAC. It is assumed that the CP does not shift when the aeroplane is flying at an angle towards the undisturbed incoming airflow. According to this, the position of the CP during an approach and landing can be determined (Appendix XVII). The MAC length is 3.96 m, this value is multiplied times a factor 0,27 to calculate the position of the CP on the MAC. The height of the CP is assumed to be 2.878 m and the angle between the CP and the height is 3˚, now the horizontal distance form the MLG can be calculated. This results in a horizontal distance of 0.151 m. Ad 3. Centre of thrust The centre of thrust is the point where the thrust force applies. It is assumed that this point is at the front of the engine. The distance from the MLG to the front of the engine is 6.33 m. It is assumed that the centre of thrust applies in the middle of the engine. This distance is 1.34 m. The distance becomes then 1.672 m. Because the approach is flown at an angle of 3˚ nose up the height can be determined (Formula 2). Formula 2 Tan (α) = o / a

α = angle [degrees] o = opposite side [m] s = diagonal side [m]

Tan (3) = o / 6.33 O = Tan (3) · 6,33 = 0.332 m Total height (h) = 0.332 +1.34 = 1.672 m h = 1.672 m

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Hogeschool van Amsterdam

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2.2.1.C Forces and values The forces like the landing mass and the thrust settings are available. Only the correct values must be calculated for the determination of the forces. The landing speed value is a standard value set up in a table. 1. Maximum landing mass 2. Thrust 3. Approach speed Ad 1. Maximum Landing Mass An aeroplane is capable of having a certain MLM. The aeroplane’s weight has to be equal or under the MLM to operate safely. The reason why the MLM is a safe operating mass is because of the extra buffer calculated. The actual maximum allowance is called the design landing weight. The actual certified MLM (Appendix XVIII) of the B737 is 66,361 kg. Ad 2. Thrust The maximum thrust of one engine of the B737 is 12,383.07 kg. The landing is never done with maximum thrust. A percentage of N1 is used for the landing. During the approach and the landing flare this percentage is 48% of N1. The B737 has 2 · 12.383,07 = 24.766,14 kg’s of maximum thrust available. 48% of N1 power is 11,887.75 kg’s of thrust available. The force will eventually be 116.618,80 N of thrust. Ad 3. Approach speed When flying an approach the B737 has a certain configuration. This configuration can variate in flaps, speed, landing mass and Angle Of Attack* (AOA). The configuration used for the calculation is a B737 with a MLM is 48% of N1 thrust and a flap setting of 15˚. According to the MLM of which the B737 is certified, Boeing has set up a table (Appendix XIX) which includes approach speeds. The approach speed of a B737 with a MLM is 142 Knots Indicated AirSpeed* (KIAS).

2.2.2

Forces during landing

To determine the forces on the landing gear during landing is sophisticated and complex. Therefore some assumptions are made. During the approach the aeroplane is static, this means that the forces in the vertical and horizontal plane are equal to zero (2.2.2.A). While the aeroplane has a constant velocity the horizontal and vertical component can be dissolved. With this the acceleration and time of the compression are determined (2.2.2.B). Eventually the actual force which the shockstruts must absorb is calculated (2.2.2.C). 2.2.2.A Forces during the approach When the aeroplane approaches the runway the acceleration and the velocity are assumed to be constant. The aeroplane has a assumed speed of 142 kts. The lift and drag force are unknown. These forces can be determined because, according to the assumptions, the forces in the vertical and horizontal planes need to be zero. To simplify the calculations the axes are turned 3 degrees so that the drag force is parallel to the x-axis and the lift force parallel to the y-axis. This means that when the aeroplane is approaching the runway the angle between the fixed x-axis and the thrust force is 6 degrees. According to this the angle between the gravitation force and the y-axis is 3 degrees (Appendix XX). 2.2.2.B Forces before landing impact With the sum of the horizontal and the vertical forces equal to zero, and still having a constant velocity parallel to the GP, we can consider the aeroplane as a point mass. The vector of the velocity which is 142 kts; 73,05 m/s, is at an angle with 3 degrees of the horizontal plane (Appendix XXI). The velocity can be dissolved in a vertical and a horizontal value. This results in a horizontal velocity of 72,95 m/s and a vertical speed of 3,82 m/s. To determine the acceleration the B737 has in the vertical plane, the time in which the shockstrut absorbs the force must be determined (Formula 3). Formula 3 v = ds dt

v = Velocity [m/s] ds = Distance of compression [m] dt = Time of compression [s]

3,82 = 0,751 / dt dt = 0,75/3,82 = 0,196597 s t = 0,197 s The acceleration in the vertical plane can be determined with the vertical velocity and the time in which the shockstrut compresses (Formula 4).

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Hogeschool van Amsterdam Formula 4 a = dv dt a = Acceleration [m/s²] dv = Vertical velocity [m/s] dt = Time of compression [s]

Aviation Studies

a = 3,82/0,196597 = 19,431 m/s² a = 19,431 m/s² 2.2.2.C Forces in the main landing gear shockstruts The shockstruts are compressed during landing impact. It is assumed that the shockstruts are compressed with 0,75 m. This value is estimated because the placement of the B737 is not very likely to be large. Because the shockstrut is at an angle of 3˚ with the ground, the length of the shockstruts compression can be determined (Appendix XXII). This results in a shockstrut compression of 0.751 m. With the force on the MLG shockstruts and the compression length of the shockstruts known the force that have to be absorbed by the shockstruts can be determined with Newton’s first law (Appendix XXIII); F = m · a. The mass (66.361 kg) and the acceleration (19,431 m/s²) are known, the landing impact shock which has to be absorbed by the MLG shockstruts is 1.289.460,591 N. This is 644,730.296 N per strut.

2.2.2 Forces and kinetic energy during landing
At the moment the B737 lands on the runway the fuselage has to bear forces and with this process also energy will be generated. The MLG makes first contact and is responsible for the shock absorption (2.2.2.A). Putting the aeroplane on the ground, creates high stress on components (2.2.2.B). The B737 has to decelerate of a speed of 73.05 m/s to a standstill position, brake action is inevitable (2.2.2.C). All of the process during landing is done at maximum allowed values. 2.2.2.A Forces on the MLG For the calculation of the forces on the MLG during landing the lift force, drag force, weight, horizontal and vertical speed force are needed. All the forces are determined to calculate the force on the MLG during landing. The forces can be used for the sum of the horizontal and vertical forces. The equations (Appendix XXIV) can now be solved. 2.2.2.B Generated vertical kinetic energy When the landing gear of the B737 touches down on the runway during landing, the MLG will absorb this shock. The inner and outer strut merge and this process generates a kinetic energy. To calculate the energy that is generated in each strut (Appendix XXIV), the MLM and the vertical speed is relevant. The two struts also has to be taken into account. 2.2.2.C Generated kinetic energy by brakes. From the moment the B737 is on the ground, the aeroplane has to decelerate so it can stop. In this situation the brakes will be responsible for the whole braking process. The aeroplane has a MLM and all four wheels that contains brakes do have to be taken into account. Also the speed on the runway is relevant, to calculate the kinetic energy that is generated by the brakes. After calculating the kinetic energy of each brake, the temperature difference also can be determined (Appendix XXV).

2.3

Forces during a rejected take-off

A RTO is a situation where a pilot has to abort the take-off during the ground roll. The pilot will abort a take-off when a failure occurs during the ground roll that could endanger a safe flight. There are basically two RTO levels, low and high. Low means that a take-off is aborted before 80 kts, meaning that the energy of the aeroplane is relatively low. A high level RTO can be made between 80 kts and the V1 speed or decision speed. V1 is the last moment that the pilot can decide to abort the landing by initiating the brakes, after V1 the remaining runway length is insufficient to safely stop the aeroplane before the end of the runway. During the following analysis two situations will be investigated, the first will analyse the forces on the aeroplane when it hits V1 speed (2.3.1). While the other is analysis of the forces on the aeroplane when a RTO is initiated (2.3.2).

2.3.1

Forces during take-off

When a RTO is going to be made the aeroplane has to cope with a number of forces to safely bring the aeroplane to a halt. To give a good impression of the forces during the RTO, there has to be made a calculation of the forces during the take-off first. To calculate the values of these forces a couple of assumptions have to be made (2.3.1.A). When the assumptions are made, the forces during the take-off can be calculated (2.3.1.B).

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Hogeschool van Amsterdam

Aviation Studies

2.3.1.A Assumptions Because not all values are known, a number of assumptions have to be made to calculate the forces on the aeroplane. Therefore the following four values are assumed: 1. Frontal surface 2. Drag 3. CL coefficient 4. Drag coefficient of the wheels Ad 1. Frontal surface In order to calculate the drag which is caused by air flow around the aeroplane, the total front surface of the B737 is needed. It is assumed that the B737 has a frontal surface of 52 m² during the take-off. Ad 2. Drag It is assumed that there are two different kinds of drag. Drag which is caused by air flow around the aeroplane, and drag caused by the contact of the wheels with the ground. The total drag is the sum of the drag caused by air flow and the drag caused by the wheels. The situation where the aeroplane has a speed of 149 kts (V1) during the take-off is used as starting point, to stipulate the forces on the aeroplane. When the aeroplane has a constant speed, the thrust will be equal to the total drag. The maximum thrust per engine of the B737 amounts 121,478 N. So assumed is that the aeroplane provides a thrust of 242,956 N, which is needed for a constant speed of 149 kts. This means that the total drag is also 242,956 N. Because the lift and the friction factor of the wheels are also known, the drag caused by airflow can be determined (Appendix XXVI). By means of the total drag formula, the Cw coefficient can be calculated, which ends up on a value of 0.76 (Formula 5). Formula 5 Fw = A · Cw · ρ · v² 2

Fw = friction force [N] A = frontal surface [m²] Cw = friction coefficient [dimensionless] ρ = density [kg/m³] v = speed [m/s]

Ad 3. CL coefficient For the value of CL, is assumed that it amounts 0.3. Because in the CL – α graphic a value of 0.3 is given, for an alpha of zero. Now the value of CL is determined the lift can be calculated, by means of the lift formula (Formula 6). Formula 6 CL = Lift coefficient ρ = Density [kg/m²] v = Speed [m/s²] S = Wing surface [m2] L = Lift [N]

L = CL — ½ — ρ — v2 — S

Ad 4. Drag coefficient of the wheels When the wheel move over a surface, drag shall be caused as a result of the contact of the wheels with the ground. The rolling resistance is calculated for the NLG and MLG. This can be done by means of the drag formula for the wheels (Formula 7). Assumed is that the value of the drag coefficient of the wheels amounts 0.2. Formula 7 D=N—µ D = drag [N] N = normal force [N] µ = drag coefficient

2.2.1.B Forces during take-off During the take-off there are five forces acting on the aeroplane: 1. Gravitation force 2. Thrust 3. Drag of the B737 4. Normal forces on the landing gear

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Hogeschool van Amsterdam

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Ad 1. Gravitation force One of the vertical forces which is working on the aeroplane during the take-off is the gravitation force. The gravitation force can be determined by means of the gravitation formula (Formula 8). The Maximum Take-Off Mass* (MTOM) of the B737 during the take-off amounts 79,010 kg. When this value is multiplies with the fall acceleration of 9.81, the gravitation force can be calculated. The gravitation forces amounts 775,090 N (Appendix XXVI). Formula 8 Fg = m — g

Fg = Gravitation [N]
m = Mass [Kg] g = Acceleration of gravitation [m/s²]

Ad 2. Thrust Each engine of the B737 provides a Maximum Thrust of 27,300 lb or 12,383.07 kg. When this is calculated to Newton, each engine provides a maximum thrust of 121478 N. Because the B737 has two engines, the total thrust shall amount 242,956 N. Ad 3. Drag of the B737 For the aeroplane moves with a constant speed, the total drag has to be equal to the thrust. The total drag exists out of the drag caused by air flow around the aeroplane, and drag caused by the wheel in contact with the ground. By means of the drag formula for the wheels and the drag formula for the total drag caused by airflow, the drag forces can be determined. The drag force on the NLG wheel during take-off amounts 4,597.51N, the drag of the main wheels 48,345.64N and the drag caused by airflow amounts 141,667.23 N. The sum of these forces is 242,956 N, which is equal to the thrust. Ad 4. Normal forces on the landing gear By means of the sum of the moments around the MLG, the normal force on the NLG can be determined (Appendix XXVI). The normal force on the NLG amounts 22,987.53 N (5% of the total weight). Using the sum of the vertical forces, which has to be equal to zero, the normal force on the MLG can be calculated. Because the normal force on the MLG is must greater than the normal force on the NLG. The normal force on the MLG amounts 241,728.2N (95% of the total weight).

2.3.2

Forces during rejected take-off

When an RTO is initiated a B737 has to cope with a number of forces to safely bring the aeroplane to a halt. In order to calculate the values of the forces a number of assumptions have to be made (2.3.2.A). When the assumptions are made an analysis can be made to determine the values and effects of the forces during a RTO (2.3.2.B). 2.2.2.A Assumptions As mentioned earlier the dimensions of the B737 are published by Boeing and thus do not have to be calculated. But when considering the forces that act on the B737 during an RTO a number of assumptions have to be made. The following six values are assumed: 1. Lift 2. Cw coefficient 3. Frontal surface 5. Drag coefficient of the wheels 6. RTO speed and deceleration rate Ad 1. Lift During an RTO the ground spoilers will be fully deployed to slow the aeroplane down. The spoilers, especially the ground spoiler will increase the drag and thus slow the aeroplane down. However the main purpose of spoilers is not to create extra drag but to destroy most of the lift that is produced by the wings. Therefore this analysis assumes that the spoilers destroys all lift. Ad 2. Cw coefficient The drag of a B737 during an RTO consists out of a number of different kinds of aerodynamic drag. To simplify the calculation it is assumed that one total drag force acts on the B737 and that this drag force acts on the CG. The drag of the B737 is calculated. In the formula a drag coefficient is present. This coefficient strongly depends on several factors: are the (ground) spoilers deployed, angle of attack, thrust reversers and flap and slat setting. For this analysis it is presumed that the Cw coefficient is 0,76.

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Hogeschool van Amsterdam

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Ad 3. Frontal surface In order to calculate the total drag, the total front surface of the B737 is needed. With flaps at setting 5, ground spoiler deployed and level it is assumed that the frontal surface of the B737 is 52.00 m². This is estimated by using the official dimensions of Boeing. Thus meaning multiplying the length with width for all separate surfaces of the fuselage, meaning: wings, fuselage, tail section etc. Ad 4. Drag coefficient of the wheels When wheels move over a surface friction will occur between the wheel and that surface. This friction is called rolling resistance. Rolling resistance is calculated per wheel, meaning that there is a separate resistance for the NLG wheel and both of the MLG wheels. Rolling resistance can be calculated by (Formula 9). In order to calculate a value in this formula it is assumed that the Drag coefficient of the wheels if of the wheels is 0.2. For example the drag coefficient of winter tyres for a car is around the 1.1, an aeroplane has a far lower drag coefficient due the simplicity of the grooves. Formula 9 D=N—µ D = drag [N] N = normal force [N] µ = drag coefficient

Ad 5. RTO speed and deceleration rate An RTO can be initiated at any speed before the decision speed. After the decision speed the pilot has to rotate due to the fact that he does not have enough runway left to stop. For the worse case scenario it is presumed that the RTO is initiated by the pilots at the moment the aeroplane hits its V1 speed. Also It is presumed that the B737 that will abort its landing is at MTOM, which is 79,010 kg according to Boeing. The V1 speed at MTOM is around the 149 KIAS, sea level, no headwind and on a dry runway. It is also assumed that the deceleration rate is fourteen feet per second or 4.75 m/s2. 2.2.2.B Forces during RTO When a B737 enters an RTO the following four forces act on the aeroplane: 5. Mass of the B737 6. Thrust 7. Drag of the B737 8. Force of the brakes Ad 1. Mass of the B737 Before the RTO the 79,010 kg mass of the B737 will be accelerated to 149 kts. At this speed the mass of the B737 will have a considerable amount of energy. When the RTO is started this energy has to be converted into other kinds of energy like heat. Thus when all energy is converted, the aeroplane will be at a stand still. Firstly it is important to calculate the amount of energy the B737 contains at the moment the RTO starts, this can be calculated with (Formula 10). Formula 10 Ek = 0.5 · m · V2 Ek = kinetic energy [J] m = mass [kg] V2 = velocity [m/s]

If it is presumed that the B737 has a velocity of zero end the end of the RTO, an estimation can be made about the energy the B737 contains at 149 kts. According to the formula this results into 232,100,664.90 J (Appendix XXVII). Thus meaning that the aerodynamic drag, rolling resistance and brakes have to convert 232,100,664.90 J into other forms of energy. Ad 2. Thrust During the RTO the thrust reversers are used, however the engine is also running in idle meaning that there is still a certain amount of thrust that is produced. While idle, the engines are at 20% N1 (Fan rpm). If it is assumed that the relationship between the thrust produced and N1 is linear, than 48,591.17 N of thrust is produced. Ad 3. Drag of the B737 When are assumption are used the drag can be calculated according to the drag formula. This results into 142,215.63 N of aerodynamic drag. Next to the aerodynamic drag there is also the drag that is created by the rolling wheels. This drag also calculated an results into 147.614,53 N of rolling resistance for the MLG and 7,403.09 N for the NLG. According to (Formula 11) the B737 travelled a distance of 687.97 m. Thus by multiplying the force with the travelled distance, the energy that is absorbed by the drag can be calculated (Formula 12): the aerodynamic drag absorbs: 97,840,090.99 J and the MLG rolling resistance absorbs: 23,300,216.12 J While the energy for the NLG is: 5,009,852.58 J

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Hogeschool van Amsterdam Formula 11 V22 = v12 +2a(∆S) V2 = start velocity [m/s] v12 = end velocity [m/s] a = acceleration [m/s2] ∆S = travelled distance

Aviation Studies

Formula 12 W=F·s W = work [J] F = force [N] s = travelled distance [m]

Ad 4. Force of the brakes Now all other horizontal forces that occur during the RTO have been calculated and the total energy is calculated. Thus it can be concluded that the brakes have to be responsible for converting the rest of the energy. After calculations it can be said that the brakes have to convert 139,379,772.40 J. Thus meaning that the brakes are one of the primary reason that the aeroplane stops during an RTO. This is correct and can be compared with the following example. When a Cessna 152 lands and it would only use aerodynamic braking. Meaning that after the landing the nose is kept at a certain angle of attack, thus placing the fuselage and the wings at an angle and use them to create more drag. The aeroplane would not stop before the end of the runway. This due to the fact that the aerodynamic drag decreases as the speed drops and below a certain speed it will become almost ineffective. This would mean that without using brakes in this case, still some kinetic energy is still present. Thus meaning that the aeroplane still has a certain velocity and will overshoot the runway when no braking are applied. Therefore meaning that during a landing or RTO brakes always have to be used to stop the aeroplane. Therefore runway calculations are preformed while using the brakes only. While 5,672,455.93 indicates the amount of energy the brakes have to absorb, it is not the braking power. The braking power can be calculated by (Formula 13) and results into 337,372.70 N of braking force. Now the brake temperature can be calculated by dividing the specific heat with the mass of the brakes. This results into a 287.87 K temperature increase per brake from the original temperature of the brakes. Formula 13 F=m·a F = force [N] m = mass [kg] a = acceleration [m/s2]

When considering forces one of the most important factors is how the forces affect the structure and one of these effects is created by an momentum. Now all forces that act on the landing gear during RTO are calculated, a momentum can be calculated. This momentum has a size of: 391,264.95 Nm in an anti clockwise direction. Due to this momentum the NLG will be compressed. Also the MLG shockstruts will have the tendency to bend (fig. 12). This due to the fact that there are two opposing forces that act on the shockstrut. These opposing forces are created by the forward energy of the aeroplane and the braking forces and drag of the wheels. The fact that these opposing forces act on both ends of the shockstrut only increases their tendency to bend, due to the longer momentum arm. The NLG on the contrary will also have a tendency to bend, only its tendency will be a lot smaller. This can be explained due to the fact that the NLG lacks any form of brakes and the rolling resistance is smaller. Overall the aeroplane will decelerate as expected due to the energy conversion of the brakes and drag. 1. 2. Force of the moving aeroplane Braking force

Figure 12: Bending of the shockstrut

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2.4

Vibrations

During a landing will the landing gear persist different kind of forces. Every part in the landing gear has there own material characteristic. These forces are mostly occurring because of the vibrations during the landing or high speed taxiing (2.4.1). The forces that occurs on the landing gear create stress. The stress that mostly occurs on the landing gear is fatigue stress. Fatigue stress is something that occurs at parts that one way or the other changed dear material characteristic as resulting tiny tear on the material (2.4.2).

2.4.1

Occurring forces

The vibration that occurs during a landing or approach causes forces on the construction of the aeroplane. These vibration forces can be absorbed by different kind of systems. Most of the vibrations are absorbed by the shockstruts and the shimmy damper that is placed on the MLG. Some of the vibrations is being absorbed by the construction of the aeroplane because the materials that is been used at the construction has an elasticity range. The elasticity of the material will decrease the vibration on the construction. Most of the heavy vibration forces are occurring during high speed taxiing or during a landing. The vibration on the landing gear arises during high speed taxiing on the runway. The vibrations of the taxiing will arise different kind of forces on the landing gear and the construction. These five forces are: 1. Forces on the structure of the aeroplane 2. Pressure forces on the materials 3. Drag forces on the aerodynamic components 4. Temperature forces 5. Tearing forces Ad 1. Forces on the structure Most of the forces during a flight works on the structure of the aeroplane. The forces rise during landing and high speed taxiing. The consequences of the vibration forces on the structure of the aeroplane creates stress on the material. Ad 2. Pressure forces The pressure forces are forces that are occurring during a touchdown of the flight. Most of the pressure forces are being absorbed by the shockstruts and the special made materials. The forces can give the materials tiny strain, and metal fatigue. The disadvantage of this kind of forces are that the materials are wearing out faster, so it has to be check a lot of time and sometimes it has to be replace. Ad 3. Drag forces Drag force is the force that resists the movement of the aeroplane trough the air. The vibration of this kind of forces are occur during a high speed flight with an over speed, it gives the flight a lot of stress on the material because of the drag forces. Ad 4. Temperature forces This force is existing during a flight on a high flight level where the temperature is around - 50 ˚C. The wearing out of the material is because of the temperature variation from hot air to extreme cold air. This variation in temperature gives the material a different characteristic. Some places on the aeroplane are sensitive for this kind of variation so it is necessary to maintain all these sensitive parts. Ad 5. Tearing forces The vibrations on the aeroplane occurring tearing forces on some of the parts. These forces mostly exist at places like at the doors of the landing gear.

2.4.2

Stress

The different occurring forces cause different kinds of stress on the components of the landing gear. The landing gear is divided in to two groups of components in order to give a clear explanation. This deviation is based on the function of the components, because different functions cause different forces and stress. The materials used for constructing the components of the landing gear play a significant role when explaining the stress that occurs. The two different groups are struts (2.4.2.A) and wheels (2.4.2.B). 2.4.2.A Struts During landing the struts have to endure the vertical forces caused by the aeroplane hitting the ground. There are also vertical forces the entire time that the aeroplane is on the ground caused by the weight. This is why struts are made out of Titanium. (See chapter 1.2.2 (See chapter 1.2.2 and 1.2.3)). Titanium is a strong and relatively elastic material. The strength is needed for withstanding the forces. Especially the vertical forces when the aeroplane is on the ground.

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The elasticity is needed to withstand the vibrations. Vibrations are caused by the movement of the aeroplane. When airborne, the landing gear is retraced and locked by the up-locks. Although it is locked, movement of the aeroplane with high velocity trough the air makes vibration of the struts inevitable. When a strut vibrates a force makes part of it move a small distance one way. The other part is still in place or moves the opposite direction. This makes the strut extend or compress a little. These vibrations have a great influence on the durability of the struts. When on ground the struts have to damp every bump or every pot-hole that is in the runway. The vibrations caused by the movement of the aeroplane are still existent, only now the struts are locked down. The forces caused by vibrations are non-existent when the aeroplane is on ground and in rest. 2.4.2 B Wheels In contrary to the struts, wheels and tyres are made of numerous different materials. A deviation is made based on the materials used: 1. The base of the wheel and the braking discs 2. The tyres Ad 1. Rims and the brakes The rims and the brakes are mainly made out of steel. The steel used for the multiple disc brakes allows the thermal energy, which is converted from the kinetic energy when braking, to be absorbed. The larger the amount of mass in steel disc, the larger the amount of energy that can be absorbed without permanent damage. An other reason why steel is used for the base of the wheel and the brakes is, because the same vibration forces as in the struts can occur when the aeroplane is in movement. In addition to these forces there are vibration forces caused by the other wheels. One wheel may go faster which causes the other wheel to vibrate. This vibration is called shimmy. These vibration forces will be less damaging when the landing gear is retracted than when extended, because when retraced the upwards force and shimmy caused by the ground is non-existent. Ad 2. Tyres The tyres are made of rubber and filled up with nitrogen gas. The vibration forces that occur in the retraced mode are insignificant because rubber is a very elastic material. The forces that occur on the ground however, have great influence on the tyres and their durability. When a wheel spins on the ground, friction forces occur as a result of the tyre being in contact and moving over the ground. This friction force does not seize in one point, but in the whole surface of the wheel that makes contact with the ground at that point. This makes the part of the upper layers of the rubber come off and stay on the ground. During landing and RTO brakes are used. When applying the brakes the friction force becomes larger, because the wheel is slowed down. This means that the velocity of the aeroplane forwards is greater that the velocity of the spinning of the wheel. Because of the elastic properties of rubber, this friction force during braking causes the tyre to deform. The part that is in contact with the ground stays a little behind and is dragged over the ground by the upper part of the tyre. If the wheel skids (see chapter 1.3.2), the friction force becomes even larger, because the same part of the tyre is dragged over the ground. This means that the same part of the wheel will lose material as a result of the friction force and the tyre will be permanently damaged.

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3. Troubleshooting
The landing gear of an aeroplane consists out different systems and materials. When there is a failure in one of the landing gear components, it could cause a malfunction in the landing gear or the landing gear systems. (3.1) Because the materials of the landing gear experience high forces and different kinds of weather conditions. It can occur that the strength of the materials of the landing gear decreases, as a result of corrosion and high stress of the materials. Therefore there has to be done different maintenance checks, to prevent possible failures in the landing gear. (3.2) The maintenance of an aeroplane bring several costs with itself. These costs are useful for the airliner to know. (3.3) After an analysis of the landing gear of the B737 and an overview of the costs and possible failures that can occur, a conclusion can be made. (3.4) The main sources of this chapter are: Technical Information Publications (TIP) and the NTSB accident report.

3.1

Malfunctions

A good analysis of the landing gear has to include the mentioning of certain malfunctions in the landing gear. To give a proper image of what can fail concerning landing gear related systems, three malfunctions are described. These malfunctions are chosen regarding the diversity between the malfunctions in order to provide an overall analysis. The first malfunction that is described is main wheel tyre thread separation (3.1.1). This malfunction is related to the main landing gear and the tyres. The second malfunction is nose gear collapse (3.1.2). This malfunction is related to the struts in the nose landing gear. The third malfunction is lubricating the nose landing gear (3.1.3). This malfunction is related to the movement of the nose landing gear. For each of these malfunctions the causes and solutions will be described. The effects these malfunctions can have on the airworthiness of an aeroplane will be attended as well.

3.1.1

Main wheel tyre thread separation

This malfunction can only occur in the main wheel tyres. This is because on these tyres great forces exist when applying brakes and when performing a landing. Brakes are only installed in the main landing gear and when a touchdown is made, the MLG wheels hit the ground first. A definition of the malfunction (3.1.1.A) and a description of the causes (3.1.1.B) is given. The influence on the airworthiness of the aeroplane (3.1.1.C) and the solutions that resulted from investigating the malfunction are described as well (3.1.1.D). 3.1.1.A Problem definition The B737 had problems with the main wheel tyre thread. The problem was that the main wheel tyre thread separated during take-off. When this problem was investigated there was concluded that the main wheel tyre thread separations occurred on recovered tyres. The incidents involved tyres which were recovered more than five times. When a tyre is recovered more than five times it is indicated as an R5 or R-level 5 or higher. Measures where taken to prevent the tyres form bursting by having a tyre not more recovered than five times. But still there were incidents involving tyres recovered higher than R4. The tyre thread separation problems caused substantial damage to the aeroplane structure, in particular the flaps. 3.1.1.B Causes Three of the causes that are mainly responsible for this failure are: 1. Recovering 2. Leakage 3. Deterioration Ad 1. Recovering The thread on the tyres of the landing gear can be replaced. The tyre is not replaced as a whole, it is only recovered. The rubber that is used to construct the surface of the tyre undergoes serious reduction in quantity due to friction. When regarding main wheel thread separations, at four of the twelve incidents that had occurred since 1988, retreading had taken place more than four times. After this number of recovering procedures, the cohesion between the replaced layer and the inner layer gets to weak to withstand the great friction forces that it has to be able to endure during landing or RTO. Ad 2. Leakage Leaking of the tyre causes gasses to flow though the tyre layers, which causes the separation of the tread. After investigation, the main reason why leakage had occurred in the tyres is the thermal relief plug. This plug is meant to release pressure when the tyre reaches high temperatures, caused by braking. The thermal relief plug melts at these temperatures, which causes the tyre to depressurise. When the plug is still solid, it should close up the tyre in order that the gas remains inside. Vibrations, touchdown and severe braking can make the plug move a little. This movement is the cause of non-

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hermetic closing of the tyre. The gap that is created between the thermal relief plug and the rubber of the tyre can cause the damaging leakage. Ad 3. Deterioration When an aeroplane is taxiing for longer than normal distances, the temperature of the tyres reaches high levels, which causes the tyres to deteriorate faster than they should. When this is done at high speeds the tyres deteriorates even faster. Deterioration means that the quality of the tyres gets less. This is calculated in, but when abnormal manoeuvres on ground are carried out, the deterioration rate accelerates. Deterioration takes place in the rubber, the nylon cords and in the cohesion between the different layers of the tyre. Insufficient quality of the material makes tyre wheel separation only a matter of time. 3.1.1.C Influence on airworthiness A separated tyre has a certain effect on the airworthiness of the aeroplane. When substantial damage to the aeroplanes structure is caused, the aeroplane can not be declared as airworthy and needs to be fixed. When a tyre separation problem occurs during take-off before 80 kts, the take-off should be rejected. The debris of the tyre can form a potential danger, not only for the departing RTO aeroplane but also for the other aeroplanes which will land or take-off on the same runway. When a tyre separation problem occurs after 80 kts it is recommended that the take-off should proceed. This is because braking efficiency can be significantly reduced, which will affect the needed stopping distance for a RTO. When sustainable structural damage to different parts can be determined, the procedure needs to be started to land the aeroplane safely. The departing aeroplane does not always need to return to the airport. When the flight safety can be maintained and the support facilities and equipment are not available at the present airport, it can proceed the flight. In this case the passenger serving costs are lower and the efficiency of the reparations time can be optimized. Otherwise the aeroplanes schedule can be disordered and the costs will significantly increase. 3.1.1.D Solutions For the different causes of wheel tyre tread separation, different solutions are available: 1. Recovering 2. Leakage 3. Deterioration Ad 1. Recovering To prevent tyres that are recovered more than four times, tyres where classified. New tyres where classified R1 and after each replacement of the tread the number after the R adds one. After investigating the tread history of all the tyres on aeroplanes as in the shops, where replaced. Tyres with classification above R4 can not be used anymore. Ad 2. Leakage The thermal relief plug that caused the leakage, where replaced. A new type was developed and installed in every wheel on the aeroplanes and in the shops. The new plugs are more resistant to vibrations and are marked with an orange dot for clarity reasons. In addition to the new thermal relief plug, a pre-flight check concerning the tyre pressure was set mandatory in order to prevent tyre leakages. Ad 3. Deterioration To ensure that a tyre has enough quality overall, a monitoring system for tyre pressure is set up. This system uses a table with improved limitations to the tyre. The tyre pressure can indicate the quality of the materials as well. A minimisation of taxiing and speed during taxiing is a precaution to decelerate deterioration as well.

3.1.2

ose gear collapse

Between 2004 and 2006 six B737 suffered from NLG wheel collapses during towing and push back operations. These collapses were caused by fractures in certain NLG struts (3.1.2.A). These fractures were caused by excessive braking during towing or push back operations, which resulted into overstressing the NLG structure (3.1.2.B). Without a NLG a aeroplane is incapable of flying, however if the problem occurs throughout the aeroplane type, the entire fleet can be grounded (3.1.2.C). To prevent NLG collapses it is possible to modify the NLG or to change the operating procedures (3.1.2.D). 3.1.2.A Problem definition During the towing and push backs operations brakes were applied which resulted into the collapse of the NLG. The collapses resulted into minor damage to all six aeroplane’s. The damage included, damage to the frontal fuselage and damage to the NLG. The NLG was fractured on several places (fig. 13, p. 32), including: The locking mechanism was fractured near the hinge (1), the lower drag brace link was fractured near the centre of its length (2) and the tow bar shear pin was also fractured .

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The tow bar shear pin, is a pin in the tow bar that is meant to break when a excessive force is exerted on the tow bar. 1. 2. Locking mechanism fracture location Lower drag brace link fracture location

Figure 13: Fracture points Normally the locking mechanism prevents the drag brace links from folding around its hinge and thus prevents the NLG from retracting. However due to the fraction in the locking mechanism and the drag brace link, the shockstrut was no longer locked and was now able to retract. If the locking mechanism would have fractured while the aeroplane was parked, nothing would have directly happened, because no horizontal forces are acting on the NLG. During towing or push back operations horizontal forces are present (fig. 14). Between these horizontal forces a distinction can be made between the force of the tow truck (1) and the force of the aeroplane and the brakes resisting the movement (2). These opposing forces act at both end of the NLG shockstrut. The same forces apply for the drag brace link (3,4). When the locking mechanism is no longer holding the gear locked, the NLG shockstrut will rotate around the suspension due to the forces and thus folds into the wheel well. This results into the gear collapse of the NLG. 1. 2. 3. 4. Pulling force on the shockstrut Resisting force on the shockstrut Pulling force on the drag brace links Resisting force on the drag brace links

Figure 14: Opposing forces during towing operations 3.1.2.B Cause of the problem After the incidents all NLG parts were investigated to determine what caused the fractures. In this investigation the primary objective was to determine if the fractures were caused by pre-incident cracks or by metal fatigue. This due to the fact that if this B737 contained cracks or metal fatigue, it would be possible that other B737’s could encounter the same problems. According to the results no cracks of metal fatigue were present before the incident. However, it was possible to determine that the fractures in the locking mechanism and lower drag brace link were caused by overstressing the NLG. This overstress situation was created due to applying excessive braking during push back or towing situations. This excessive braking results into higher loads on the attachments points of the NLG. Normally during push back or towing four forces act on the NLG struts (the four forces in fig. 14). However, when brakes are applied the forces that resist the NLG from moving forward increase significantly. When the aeroplane is still being pushed back or towed, this can result into a situation with loads that exceed the limits of the NLG. The attachment points however where strong enough to resist the increased load.

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The locking mechanism and lower drag brace link, were not and therefore fractured when these bars were exposed to these loads. This led to the collapse of the NLG. Normally, the tow bar shear pin should have broken when excessive force is applied on the NLG thus preventing damage. In this case however, the tow bar shear pin broke to late, thus the damage was already inflicted. 3.1.2.C Influence on airworthiness When the NLG of an aeroplane collapse, it is not possible to take-off. However, inspection procedures can be done, to look for damage and overstressed materials. Therefore there can be decided if the other aeroplanes are worthy of flying safe. And to look if the aeroplane on which the NLG collapsed if it is able of flying. 3.1.2.D Solution There are several manners to reduce the chance of collapsing of the NLG. The manners which apply are: 1. Design modifications 2. Recommendations for operators Ad 1. Design modifications By means of a design modification the chances on collapse of the NLG can be made smaller. A modification should minimise the movements of the drag brace resulting from loads applied to the NLG. And to make sure enough force is applied to the drag brace to retain it in the locked condition. Ad 2. Recommendations for operators A recommendation can be given to operators, with the intension to minimize the likelihood of NLG damage or collapse during towing and pushback operations. An example for a recommendation can be, that the pilot has to apply the aeroplane brakes at a minimum while towing the aeroplane.

3.1.3

Insufficient lubrication of the nose landing gear

The Royal Dutch Airlines, also known as KLM. Is keeping up a technical review of malfunctions that occurs on their fleet. One of their complaints is a hammering noise that creates a unsafe feeling in the cabin (3.1.3.A). The maintenance manual always has to be consulted by a new type of aeroplane (3.1.3.B). Malfunctions in certain values, does not always has to affect the safe feeling in the cabin (3.1.3.C). Changes in the manual always has to be notified by the maintenance crew (3.1.3.D). Precautionary measures prevents further damage to components. 3.1.3.A Problem definition After multiple complaints of passengers and flight crew on a KLM aeroplane about vibrations and loud hammering noises near de nose of the aeroplane, the aeroplane was kept on the ground for an inspection. During the inspection radial and axial movement was found in the suspension of the NLG. Because this complaints are not described in the maintenance manual, further steps where taken, by informing Boeing. The movement on suspension was within the limiting value of Boeing, but flight operations crew was suspicious about the continuing hammering noises and vibrations. 3.1.3.B Causes First thought was that the cause was a defect trunnion bushing. A trunnion bushing is a movement pin used to mount the struts to the fuselage of an aeroplane. On a regular basis the same complaints were described, because the insufficient lubrication of the NLG bushing persisted vibrations and noises. 3.1.3.C Influence on airworthiness In this case the airworthiness of the aeroplanes is not affected because the freedom of movement is within the boundaries Boeing calculated for the B737. The situation created an aggravating atmosphere for the passengers and crew. For this reason the aeroplane was kept on the ground. 3.1.3.D Solutions The fist solution that was attempted was by putting new trunnion bushings at the struts in the aeroplane. But in this case non where available. Because of the unavailable bushings at Boeing, the first attempt that was made by lubricating the bushings regularly. For evaluation it was lubricated daily and after good results it was continued weekly. The maintenance crew was noticed that the lubrication nipples where mounted on a different position than the older B737-300/400 series (fig. 15, p. 34). The B737 has lubricating valves at the trunnion bushings (1) and the upper drag strut bushings (2).

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Hogeschool van Amsterdam 1. 2. 2x Trunnion bushings 2x Upper drag strut bushings

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Figure 15. Bushing lubricating valves It is of importance that on a regular base, also the described valves are filled with lubrication oil. By this the vibrations and the hammering noise can be prevented.

3.2

Maintenance

Every airliner needs to be maintained to make sure that the aeroplane is safe. To guarantee the safety of the landing gear all visible part has to be checked after every landing. The checks can be divided into different kind of checking periods these checks are dependent of the flying hours (3.2.1). The maintenance of the landing gear can be done by an order of rank this can be done by the Aircraft Maintenance Manual* (AMM). The safety of the landing gear is depending on the way of how the maintenance is done (3.2.2).

3.2.1 Checks
The checks can be divided into different kind of checks depending on the flight hours. Checks are divided into an A, B, C and D check. The duration of the A check is approximately two man hours and it has to happen every day. The purpose of this check is to do a visual check in and around the aeroplane. During the A check the maintenance crew checking the logbook of the aeroplane. The content of the logbook is to check when a part has to be maintained. The B check is a check that must be happen every month and the duration of every B check is approximately 250 man hours for a B737. The purpose of the B check is to check of every instrument in the aeroplane and make the aeroplane fully operational. The purpose of the C check is to check every part including the frame accurately. The duration of the check is around 10,000 man hours and it must be done every 15 months. The most complicated and the most specific check is the D check. In this check the aeroplane must be split up in parts and must be inspected precisely. This check must be happen each 5 years and the duration of this check is around 10 months or sometimes longer. During a check are some component of the landing gear firmly. The places and things that must be checked are:

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Hogeschool van Amsterdam • • • • • • • • • • • • • • • • • Landing gear Air/Ground system MLG and doors NLG and doors Extension and retraction Landing gear control system MLG extension and retraction NLG extension and retraction MLG manual extension system NLG manual extension system Wheels and Brakes Hydraulic brake system Antiskid system Parking brake system Tyres Nose wheel steering system Landing gear position indicating and warning

Aviation Studies

All these systems must be checked during C and D check because it is necessary to check all of the parts of the landing gear for the airworthiness of the aeroplane.

3.2.2

Maintenance manual

There are different kinds of standard that deal with the maintenance. These standard are needed to fly safe. The three standards that are used are: 1. Minimum Equipment List 2. Dispatch Deficiency Guide 3. Operations Control Centre Ad 1. Minimum equipment list The Minimum Equipment List* (MEL) is a list with content of aeroplane components that are needed during a flight. The MEL is needed for example during an emergency flight with a broken component. In this situation it is necessary to check in the MEL to read what to do during this kind of situations Ad 2. Dispatch deficiency guide The Dispatch Deficiency Guide* (DDG) is a guide that permits operations with unserviceable items of equipment for a period. Until the repair is done and permits the dispatch with inactive components. Ad 3. Operations control centre The Operations Control Centre* (OCC) is a staffed 24/7 crew and they are responsible for the collecting, tracking and reporting data from field organization for facilities and service interruption, special events and disasters. The OCC monitors critical situations.

3.3

Costs

When an analysis is made concerning a landing gear, costs always play an important factor. This due the fact that the airline wants to operate its fleet as efficiently as possible, meaning that the airline cuts costs where possible. A airline that operates a B737 has a number of options concerning the landing gear (3.3.1.A). Due to the fact the brakes a tyres are two components in the landing gear that have to be replaced regularly, a separate analyse is made (3.3.2.B).

3.3.1

General costs

Normally when an airline purchases an airframe it is bought as a complete aeroplane, meaning that the airline owns the entire aeroplane. However parts of an aeroplane can be leased, including the landing gear. By leasing the landing gear the airline can save some cost involved in the purchase and maintenance of the landing gear. It has to be noted the all cost that are involved with the use of tyres and brakes are not included in a lease. Due to the fact that some cost can be saved on maintenance of the landing gear, leasing is especially attractive for small airlines. If an airline chooses to lease a landing gear the cost involved would be around the 36,696 euro per year per airframe.

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However if an airline owns the entire airframe, it is naturally responsible for its own maintenance. Besides the maintenance of the tyres and brakes a number of other components have to be maintained. These components include, the hydraulic system of the landing gear, air ground logic, nose wheel steering, struts and extension and retraction systems. All these system are very reliable and thus meaning that almost no components have to be replaced on a regular base (excluding wheels and tyres). However, all components have to be checked to discover cracks or other irregularity’s before the flight safety of the airframe is compromised. Man-hours involved in checking the landing gear cost around 130,000 euro on a yearly basis. On a yearly basis some parts have to be replaced accounting for a cost of 10,000 euro. The airline also has a certain budget for unforeseen circumstances, like when an aeroplane that has to stay on the ground due longer due to landing gear failure.

3.3.2

Costs for tyres and brakes

The parts of the landing gear which are relatively expensive in maintenance are: 1. Tyres 2. Brakes Ad 1. Tyres The reason why the tyres are an expensive part of the landing gear, is because they wear during the operation of the aeroplane. The lifetime of a tyre can be different for each landing gear, but most of the time the lifetime of the tyre is approximately 200 flights. One Michelin NLG tyre of the B737 costs 1,708 euro. And a MLG tyre of the mark Michelin for the B737 costs 3,378 euro (Appendix XXVIII). The airline has also the opportunity to only replace the tyre tread, in case only the tread is worn. The advantage is, that it is cheaper to only recover the tyre tread. A disadvantage is that it can not be done endlessly, this is because of the safety. Ad 2. Brakes Just like at the tyres the brakes are a relatively expensive part of the landing gear. This is also because the brakes wear during the operation of the aeroplane. The stator and rotor plates have to be replaced when, the wear pin has become to short and so reached its limit. The pin has reached its limit when it no longer sticks out. The price of new wheels and brakes for the B737NG, is approximately € 280,000.00 Euro.

3.4

Conclusion

During the analysis made concerning the landing gear of a B737 three points were investigated. These points include, the purpose and definition of the landing gear, the design of the landing gear and three possible malfunctions that can occur. During the investigation the following results can be concluded: • The landing gear allows the movement of the aeroplane on the ground and it also absorbs the landing shock during a landing. The B737 has a landing gear which shape resembles a triangle, therefore it is called a tricycle gear. This tricycle gear consists out of a nose landing gear and a main landing gear. The nose landing gear consists out of a shockstrut an a number of associated struts. The shockstruts is an oleo based shock damper which damps and absorbs shocks. The associated struts are meant to hold the shockstrut in either an up and locked or a down and locked position. The main gear uses two gears based on the same principle. The B737 hydraulic system is used to extend or retract the landing gear. When these hydraulic systems fail, the landing gear can be extended manually. Each landing gear is equipped with tyres that are filled with nitrogen, due to its lighter mass and anti fire properties. The main gears also features multiple steel disc brakes. These steel disc brakes transfer the kinetic energy of the moving aeroplane into heat. The brakes also have an auto brake option which allows the aeroplane to brake upon touch down without any further manual input. The aeroplane can also be steered while operating on the ground, this is a accomplished by nose wheel steering actuators. The aeroplane can be steered either by using the rudder pedals or the steering wheel. However, the rudder pedals only give limit control over the nose wheel steering. • When a manufacturer designs an new airframe, it has to consider several options for the landing gear configurations. This due to the fact the landing gear of the new airframe has to comply with the rules and regulations to make it airworthy. Also The landing gear has to be as durable as possible, which is affected by the choice of materials. The single most important factor of a landing gear design is the distribution of the forces during different flight phases. Therefore an analysis is made by this project group regarding the distribution of forces on the 737-800 landing gear. There is concluded that during a static rest, around 95% of the weight is carried by the main landing gear, while around 5% of the weight is carried by the nose gear. During the landing the initial shock will be absorbed due to the properties of the oleo shockstrut, this shock not only contains vertical forces. After touch down the kinetic energy of the aeroplane has to be transformed into heat. Most of this energy is absorbed by the brakes, which causes a temperature increase in the brakes. During a RTO the same principle applies. This means that brakes are the primary reason an aeroplane comes to a stand still during a landing or RTO.

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Also during a RTO, the nose gear will be compressed due to a anti clockwise momentum. Besides this momentum, the struts of the landing gear have a tendency to bend, due to the large opposing forces that act on them. The designer of the landing gear also has to consider all the vibrations and the resulting forces that act on the landing gear. For example the landing gear has to withstand large temperature differences, tearing forces and pressure forces without compromising the flight safety of the aeroplane and its passengers. • However, malfunctions are always possible due to unforeseen circumstances or human mistakes. Therefore three malfunctions where investigated, these included: main wheel tyre thread separation, nose gear collapse and faulty lubrication of the nose landing gear. While each of these malfunctions had a separate cause and solution a number of observations could be made. When a malfunctions occurs in the landing gear, the airworthiness is compromised. However, to which degree depends on the malfunctions. If the aeroplane is allowed to continue its flight depends on the guidelines in the MEL. However, a number of rare occasions are not described in the MEL, like a nose gear collapse. It is however obviously that an aeroplane with a nose gear collapse is not airworthy. Next to airworthiness one of the most interesting aspects of the landing gear is costs. This due to that an airline always wants to operate as efficiently as possible and to cut costs where possible. Besides regular checks of the landing gear, the only components that have to replaced regularly due to wear are the brakes and tyres. Therefore the brakes and tyres are responsible for most of the yearly costs.

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Bibliography
Books:
Gerrit Verkerk Binas (2006) The Boeing Company Operations manual 737-600/-700/-800/-900 (2002) Anderson, John D., jr Introduction to Flight (2008) C.J.A.Langedijk Vliegtuigsystemen deel 3 (1978) The Boeing Company 737 Characteristics for airport handling (2005) Ted L. Lomax Structural loads analysis for commercial transport aircraft (1996) Royal Dutch Airlines Technical Information Publications (2008) Wentzel, Tilly Het projectgroepsverslag (2007) F.J. Siers Methodisch ontwerpen: volgens H.H. van den Kroonenberg (2004)

Websites:
http://www.boeing.com/commercial/737family/pf/pf_800tech.html http://www.cfm56.com/index.php?level2=products&level3=cfm56-7b http://www.b737.org.uk/ http://www.smartcockpit.com/pdf/plane/boeing/B737/systems/0021/ http://www.aaib.dft.gov.uk/cms_resources/dft_avsafety_pdf_024576.pdf http://www.boeing.com/commercial/737family/pf/pf_800tech.html

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List of abbreviations
Abbreviation Explanation A B C AMM AOA B737 BSCU CG CP CS D E G K L M DDG EASA GP KIAS kts LE MAC MEL MLG MLM MTOM N O P R NLG OCC PSEU RTO Aircraft Maintenance Manual Angle Of Attack Boeing 737-800 NG Brake System Control Unit Centre of Gravity Centre of Pressure Certification Specifications Dispatch Deficiency Guide European Aviation Safety Agency Glide Path Knots Indicated AirSpeed Knots Leading Edge Mean Aerodynamic Cord Minimum Equipment List Main Landing Gear Maximum Landing Mass Maximum Take-Off Mass Nose Landing Gear Operations Control Centre Proximity Sensor Electronic Unit Rejected Take-Off First Appearance § 3.3 § 2.2.1.C §1 § 1.3.2.B § 2.2.1.A § 2.2.1.A § 1.4 § 3.3.2 §1 § 2.2.1.A § 2.2.1.C § 1.3.2.D § XVII § 2.2.1.A § 3.3.2 § 1.1 § 2.2.1.A § 2.2.1.B § 1.1 § 3.3.2 § 1.3.1.A § 1.3.2.D

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List of appendices
Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix I II III IV V VI VII VIII IX X XI XII XIII XIV XV XVI XVII XVIII XIX XX XXI XXII XXIII XXIV XXV XXVI XXVII XXVIII XXIX Project assignment................................................................................ 1 Pyramid model ....................................................................................... 2 Planning ................................................................................................. 3 Nose gear wheel .................................................................................... 5 Tire pressure .......................................................................................... 6 Size of tire .............................................................................................. 7 Hydroplaning ......................................................................................... 8 Hydraulics .............................................................................................. 9 Landing gear lever ................................................................................11 Main landing gear uplocks ...................................................................12 Main landing gear downlock ................................................................13 Nose landing gear locking mechanism................................................14 Doors .....................................................................................................16 Manual extension system ....................................................................17 Retraction actions of the landing gear ................................................18 Calculating the position of the centre of gravity ................................19 Location of the centre of pressure ......................................................20 Maximum Landing Mass .......................................................................21 Approach speeds ..................................................................................22 Lift and drag force during landing .......................................................23 Before landing impact speed ...............................................................25 Movement of the main gear shock struts ...........................................26 Force in the main gear shock struts ....................................................27 Kinetic energy on the main landing gear ............................................28 Kinetic braking energy .........................................................................29 Forces during take-off ..........................................................................30 Calculations of paragraph 2.2.2 ..........................................................32 Tire costs ...............................................................................................36 Process report .......................................................................................37

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Appendix I

Project assignment

De Projectopdracht De projectopdracht vraagt om een analyse van het ontwerp van een landingsgestel van een modern verkeersvliegtuig, en het doen van onderzoek naar mogelijke storingen. Hiervoor is een aantal richtlijnen gegeven, waar het onderzoek zich op moet richten. Voor de uitvoering van het project zijn weer randvoorwaarden opgesteld. Het eindresultaat is een projectverslag, dat nu in vlekkeloos Engels is geschreven. • Uitgangssituatie Luchtvaartmaatschappij Amstel Leeuwenburg Airlines [ALA] geeft de afdeling Engineering opdracht een onderzoek in te stellen naar storingen die voor kunnen komen bij onderstellen en daaraan gerelateerde systemen. Hierbij kan gedacht worden aan het remsysteem inclusief “auto brakes” en “anti skid”, en “air/ground logic” voor onder andere het “auto spoiler” systeem of het beschikbaar komen van de “thrust reverser”. • Opdrachtformulering Als projectteam van de maatschappij gaan jullie het ontwerp van een landingsgestel analyseren, met een verantwoording van de opbouw en werking van het gekozen systeem. Daarnaast wordt een analyse gemaakt van voorkomende storingen en hoe deze de “dispatch” van vliegtuigen, met name de luchtwaardigheid van het toestel, kunnen beïnvloeden. Hiervoor zal men de MEL of DDG moeten raadplegen.

•

Richtlijnen - Aan welke eisen moet het landingsgestel voldoen (regelgeving)? - Welke ontwerpaspecten liggen aan het systeem ten grondslag (onderhoud, duurzaamheid, veiligheid en kosten)? - Welke constructiemethoden zijn toegepast (let hierbij op: ligging zwaartepunt bij toepassing van een neus- of staartwiel; bevestiging aan vleugel of romp; besturingsmechanismen van neus- en hoofdonderstel; intrek-, up- en downlockmechanismen; verende stijlen; alternate gear extension; bediening en indicaties)? - Hoe heeft de fabrikant zijn materiaalkeuze bepaald (sterkte, duurzaamheid, gewicht)? - Welke krachten en momenten werken op het onderstel tijdens de verschillende vluchtfasen, met en zonder zijwind (taxiën, start en landing)? - In hoeverre kunnen trillingen ontstaan en hoe worden die voorkomen of gedempt? - Hoe is de werking van subsystemen, die afhankelijk zijn van of samenhangen met het gebruik van het landingsgestel? - Wat voor storingen kunnen zich zoal voordoen in het onderstel of de subsystemen

• Randvoorwaarden voor het project Randvoorwaarden zijn in feite eisen van de opdrachtgever, waar het project absoluut aan moet voldoen. Deze eisen zijn: - De tijdsduur van het project is zeven weken (week 35-41), het verslag moet ingeleverd zijn op 9 oktober 2008, vóór 16.30u. De toetsing vindt plaats in week 42, dit is kort na de inleverdatum dus plan goed wanneer de presentatie gemaakt gaat worden; - Uiterlijk aan het eind van de tweede week moet een Startdocument worden ingeleverd, waarin de projectplanning en de taakverdeling binnen de groep is opgenomen; - Het project wordt aangepakt volgens de algemene projectindeling en de methodiek van Van den Kroonenberg (Siers, 2004); - Het eindrapport wordt aan de directie van ALA gepresenteerd in de vorm van een verslag, dat voldoet aan het dictaat Wentzel (2007); - Het verslag wordt in het Engels geschreven, is ingevoerd in de computer en heeft, exclusief bijlagen, een omvang van 30-40 pagina’s.

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Appendix II

Pyramid model

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Appendix III

Planning

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(Continuance)

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Appendix IV

ose gear wheel

The nose wheel has also a inner and outer wheel half (fig. 1). The over pressure relief valve ensures that all of the pressure in the tire releases when the pressure increases more than 375-450 psi (1). The over pressure relief valve of the nose gear wheels is assembled in the outer wheel half (2). By means of the tire inflation valve, the pressure in the tires can be released (3). 1. 2. 3. Over pressure relief valve Outer half Tire inflation valve

Figure 1: Nose gear wheel

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Appendix V

Tire pressure

High pressure in the tires causes a relative small surface of contact between, the ground and the tire. Which causes that the tire shall mainly wear in the middle. A low tires pressure causes a relative great surface of contact which results in a wear to the outside edge of the tire (fig. 1).

Figure 1: Tire pressure

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Appendix VI

Size of tire

By means of the formula for the main gear load (Formula 1), the size of the tire can be determined. The total load on the strut is divided equally over the tires. Formula 1 = Distance from CG to nose gear. = Distance from CG to main gear = Weight of the aircraft. = Total main gear load [M] [M] [kg] [N]

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Appendix VII

Hydroplaning

When the pilot applies the brakes, the tires make contact with the runway, which causes that the temperature of the tires will increase. As a result the water evaporates and steam will develop (fig. 1).

Figure 1: Hydroplaning

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Appendix VIII

Hydraulics

The hydraulic system of a Boeing 737-800 NG consists out of system A, system B and an alternate system. While each of these systems power certain aircraft systems, only system A and B are used for operating the landing gear and brakes. Both system A and B are powered by engine driven pumps. Each system uses one pneumatic Engine Driven Pump (EDP) and one Electric Driven Motor Pump (EDMP). In the 737 engine number one powers system A’s EDMP and system B’s EDP, Engine number two in contrary powers system B’s EDP and system A’s EDMP. This means that the loss of one engine does not result in the complete loss of a hydraulic system. While operating normally both EDP’s act as the primary pressure source for each individual system, while the EDMP acts as a stand-by pump. While both systems EDMP’s act as a stand-by pump it has to be taken into account that an EDMP only delivers one fourth of the pressure of an EDMP. Both EDP’s and EDMP’s are controlled by the hydraulic panel (fig. 1), which is located on the overhead panel. The panel provides the option to turn the pumps on or off (1) and also provides a low pressure (2) or overheat warning light (3). The low pressure light will illuminate when the output f the associated pump is low. The overheat light will illuminate when the hydraulic fluid that is used to cool the pumps is overheated, or when the pump itself is overheated. 1. 2. 3. On/off Switches Low pressure indication light Overheat indication light

Figure 1: Hydraulic panel While the EDP’s and EDMP’s provided the hydraulic pressure for operating the different systems in het landing gear, two valves are installed that distribute the hydraulic pressure: 1. Landing gear transfer valve 2. Landing gear selector valve Ad 1. Landing gear transfer valve Normally system A provides pressure to raise and lower the landing gear. But when hydraulic pressure from system A is lost, system B pressure can be used to raise the landing gear. Note, systems A’s EDMP can’t provided enough pressure to raise the landing gear. In this case the landing gear transfer valve (fig. 2) switches a valve that enables the use of pressure from system B. The landing gear transfer valve is electronically controlled by the PSEU. The PSEU needs the following conditions before it switches to system B: • Airborne • left engine N1 speed drops below 50% • Landing gear lever is positioned up • Either main gear is not up and locked Ad 2. Landing gear selector valve The selector valve (2) receives pressure form the transfer valve and passes this pressure to certain components in a certain sequels. Which components receive pressure and in which sequels depends on if the landing gear is extending or retracting. For example when the MLG is extending, the uplock actuator will receive pressure first thus unlocking the gear, then after a certain time interval the MLG actuator will receive pressure. Meaning the MLG will start with extending. After a second time interval the downlock actuator will receive pressure, thus locking the gear in the down position. As with the landing gear lever the selector valve has three options, down, up and off. While down and up are self explaining, off is not. In the off position the hydraulic system is depressurized, meaning that the landing gear can not accidentally extend or retract. The selector valve can be controlled by two separate means. Normally the selector valve is operated by a cable that is connected to the landing gear lever. However when the landing gear manual extension door is opened a electrical signal is send to the selector valve. This electrical signal activates a bypass valve.

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Appendix VIII

(Continuance)

This bypass valve connects the hydraulic landing gear components to the return line of the hydraulic system. This depressurizes all hydraulic components in the landing gear system. Thus allowing the landing gear to safely deploy due to air loads and gravity without the hydraulic components resisting the deployment. 1. 2. Transfer valve Selector valve

Figure 2: Hydraulic system of the landing gear

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Appendix IX

Landing gear lever

The landing gear system uses one landing gear lever (fig. 1) that is located on the centre panel of the cockpit. This lever can be used to extend or retract the landing gear by hydraulic pressure. It also has the option off, which removes all hydraulic pressure form the landing gear system, thus denying the possibility of an accidental extension. Also a mechanism (fig. 2) is used to transform the lever input into a movement of a cable. 1. 2. 3. 4. 5. 6. Control lever Positions switches in two pairs of two Lock mechanism Lever lock solenoid Override trigger Push pull-rod

Figure 1: Landing gear lever. 1. 2. 3. 4. Forward quadrant Push-pull gearbox Cables Push-pull rod

Figure 2: Forward quatrant

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Appendix X

Main landing gear uplocks

A MLG uplock is responsible for holding the landing gear in the up and locked position when the landing gear is not in the down position. (fig. 1) desribes the uplock in the up and locked position. When the landing gear is extending or retraction the uplock will be in the position as is shown in (fig. 2). In this second position the uplock hook is open, thus allowing the uplock roller to enter or exit the hook due to actuator loads, air loads or gravity. Each MLG features one MLG uplock and is located ceiling of the MLG wheel well as can be seen in Appendix xx. Each uplock also contains two sensors. These sensors work with a target and use this target to determain if the uplock is in the correct position for a certain situation. 1. 2. 3. 4. Uplock hook Spring, one on each side Uplock actuator Uplock roller

Figure 1: Uplock mechanism, in the up and locked position 1. 2. 3. Uplock actuator Uplock hook Spring; one on each side

Figure 2: Uplock mechanism, in the down position

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Appendix XI

Main landing gear downlock

A MLG downlock is responsible for holding the MLG in the down position when the landing gear is not in the up and locked position or when it is in transition. (fig. 1) describes the downlock in the down and locked position. Each MLG features one downlock. A downlock is located between the sidestrut and the reactionlink. The way the downlock folds during retraction and extension can be seen in appendix xx. 1. 2. 3. 4. 5. Reactionlink Side strut Hinged downlock link Springs Downlock actuator

Figure 1: Downlock mechanism, in the down and locked position

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Appendix XII

ose landing gear locking mechanism

A NLG locking mechanism is responsible for holding the NLG locked in both the up and down position. (fig. 1) describes the uplock in the down and locked position. When the NLG is extending or retraction the mechanism will fold as is shown in (fig. 2). When the NLG locking mechanism is in the up and locked position it will be stored as can be seen in (fig. 3). The MLG locking mechanism is located between the drag brace links and the aft bulkhead of the NLG wheel well. The NLG locking mechanism also contains two sensors. These sensors work with a target and use this target to determine if the uplock is in the correct position for a certain situation. 1. 2. 3. 4. 5. Upper and lower drage brace link hinge Hinge at the Aft bulkhead NLG well Hinged lock link Springs NLG lock actuator

Figure 1: NLG locking mechanism, in the down and locked position. (horizontal position) 1. Retracted position 1a Hinge while retracted 2. In transition 2a. Hinge during transition 3. Down and locked 3a. Hinge while down and locked

Figure 2: NLG locking mechanism, in transition

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Appendix XII

(Continuance)
1. 2. 3. 4. 5. Upper and lower drage brace link hinge Hinge at the Aft bulkhead NLG well Hinged lock link Springs NLG lock actuator

Figure 3: NLG locking mechanism, in the up and locked position (vertical position)

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Appendix XIII

Doors

The landing gear uses a number of doors to seal the wheel wells in flight. This improves the overall aerodynamics of the aeroplane compared to a system without doors, thus meaning a lower fuel burn. The nose gear uses a two door system (fig. 1), while the main gear uses a three door system (fig. 2). 1. 2. 3. Door Control rod Control rod leading to the shock strut

Figure 1: Nose landing gear doors 1. 2. 3. 4. 5. 6. 7. Outer door Inner door Centre door Hinge Wing structure Push rod Adjustable tie rods

Figure 2: Main landing gear doors.

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Appendix XIV

Manual extension system

The manual extension system is used when no hydraulic pressure is available to extend the landing gear. The MLG manual extension system (fig. 1) is attached to the uplocks. While the NLG manual extension system is attached to the upper drag brace link (fig. 2). 1. 2. 3. 4. 5. Quadrant Rod Lever Cable Uplock

Figure 1: Main landing gear extension linkage 1. 2. 3. 4. Quadrant Cam Roller Upper drag link

Figure 2: Nose landing gear manual extension

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Appendix XV

Retraction actions of the landing gear

When the landing gear is retracted both the MLG (fig. 1) and NLG (fig. 2) follow certain actions to retract safely.

1. MLG extended 1a. Downlock while extended 1b. MLG actuator 1c. Side strut while extended 1d. Shock strut while extended 1e. Uplock while extended Figure 1: Main landing gear retraction

2. 2a. 2b. 2c. 2d. 2e.

MLG during transition Downlock during transition MLG actuator Side strut druing transition Shock strut during transition Uplock during transition

3. 3a. 3b. 3c. 3d. 3e.

MLG retracted Downlock while retracted MLG actuator Side strut while retracted Shock strut while retracted Uplock while retracted

1. NLG extended 1a. Lock while extended 1b. NLG actuator 1c. Brace links while extended 1d. Shock strut while extended Figure 1: Nose landing gear retraction

2. 2a. 2b. 2c. 2d.

NLG during transition Lock during transition NLG actuator Brace links druing transition Shock strut during transition

3. 3a. 3b. 3c. 3d.

NLG retracted Lock while retracted NLG actuator Brace links while retracted Shock strut while retracted

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Appendix XVI

Calculating the position of the centre of gravity

When using the angle of attack for the determination of the CG, the following steps need to be made to calculate the distance from the MLG. First the CG is calculated when the aeroplane is in a level position. According to the design guidelines the CG is positioned at an angle of 15 degrees from the MLG. The height of the aeroplane from the ground to top is 5,56 m. This is a value measured by Boeing when the aeroplane has it’s MLM. It is assumed that the CG is always in the middle of the aeroplane. This means that the CG height is 2,78 m. The CG in a horizontal situation can be calculated (Formula 1) by using the geometrical equation Cos. Formula 1 Cos (α) = a / s

α = angle [degrees] a = connected side [m] s = diagonal side [m]

Cos (15) = 2.780 / s O = 2.780 / Cos (15) = 2.878 m O = 2.878 m

The CG of the aeroplane moves when the landing is made because the pitch attitude changes. Assumed is that the CG shifts also 15 degrees when a landing with 3 degrees nose up is made. With the use of geometrical equation Tan (Formula 2) the length in the horizontal plane to the MLG can be calculated. Formula 2 Tan (α) = o / a

α = angle [degrees] o = opposite side [m] s = diagonal side [m]

Tan (15) = o / 2.878 O = Tan (15) · 2.878 = 0.771 m O = 0.771 m

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Appendix XVII

Location of the centre of pressure

The MAC can be used for determining the distance between the MLG and the CP. To calculate this, the following steps need to be made. The MAC (fig. 1) has a certain value which can be calculated. In this case Boeing has published the MAC in different sources so the MAC will not be calculated by the use of a formula. The red line in the figure is the MAC. The grey box is the margin of 9.4% – 27%. In this margin the CP is located. 1. 2. 3. Wing Mean Aerodynamic Cord 9.4% – 27% CP margin

Figure 1: Mean Aerodynamic Cord When an aeroplane makes an approach, landing or an RTO there is assumed the CP will more forwards or backwards. A fixed location is assumed for the CP. This location is at 27% of the MAC. Calculating the distance from the Leading Edge* (LE) of the wing to the CP goes as follows: Percent MAC · MAC = Distance from LE to CP 0.27 · 3.96 = 1.069 m

To determine the horizontal distance from the CP to the MLG there is assumed that the CG and the CP are at the same height. The x-axis distance can be determined by the geometrical function (Formula 1). Formula 1 Tan (α) = o / a

α = angle [degrees] o = opposite side [m] s = connected side [m]

Tan (3) = o / 2.878 O = Tan (3) · 2.878 = 0.151 m O = 0.151 m

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Appendix XVIII

Maximum Landing Mass

An aeroplane has to be certified by the EASA before it is airworthy. Involved in the certificating process is the MLM. This value is determined to have a maximum weight for which a landing still can be made safely. The value used in the calculations is the maximum design landing weight (table 1). Table 1: Boeing 737-800 aeroplane characteristics

The MLM of the B737 is 66,361 kg. This value will be used in the calculations.

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Appendix XIX

Approach speeds

The B737 is certified to operate with a MLM. In this configuration the landing speeds are determined. These speeds (table 1) are published by Boeing. Table 1: Approach speeds of Boeing aeroplanes

As shown in the red box of the table the B737 has a approach speed of 142 KIAS. This speed will be used in the further calculations.

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Appendix XX

Lift and drag force during landing

A B737 with flap setting 15˚ has an approach speed of 142 kts. This is 73.05 m/s. Because the acceleration and the velocity are constant factors there can be assumed that the forces in the horizontal and the vertical plane must be zero. The thrust (116,618.8 N) and the gravitation force (651,001.41 N) of the aeroplane are known (fig. 1).

1. Cenre of thrust 5. Lift force 2. Centre of gravity 6. Drag force 3. Centre of pressure 7. Thrust force 4. Mean aerodynamic cord 8. Gravity force Figure 1: B737 with occurring forces during the approach

FLift = ? FDrag = ? FThrust = 116,618.8 N FZ = 651,001.41 N

the lift and drag forces can be determined when the sum of the horizontal forces are equal to zero and when the sum of the vertical forces are equal to zero (Formula 1). To simplify the calculations the axis is rotated 3˚ to the left (fig. 2). The result is that the angle between the gravitation force of the aeroplane and the y-axis is 3˚ and the angle between the x-axis and the thrust force is 6˚. The lift force is perpendicular on the x-axis and the drag force is perpendicular to the y-axis.

1. Cenre of thrust 5. 2. Centre of gravity 6. 3. Centre of pressure 7. 4. Mean aerodynamic cord 8. Figure 2: B737 with axis twisted 3 degrees

Lift force Drag force Thrust force Gravity force

FLift = ? FDrag = ? FThrust = 116,618.8 N FZ = 651,001.41 N

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Appendix XX

(Continuance)

The lift force is determined as follows: +↑ ∑ Fy = 0 +↑ ∑ Fy = +↑ ∑ Fy = +↑ ∑ Fy = +↑ ∑ Fy = +↑ ∑ Fy = FThrust y + FLift – Fz y = 0 Sin (6) · 116,618.8 + FLift – Cos (3) · 651,001.41 = 0 12,189.98 + FLift – 650,109.23 = 0 FLift – 637,919.25 = 0 FLift = 637,919.25 N ↑

The drag force can be determined as follows: → + ∑ Fx = 0 → + ∑ Fx = → + ∑ Fx = → + ∑ Fx = → + ∑ Fx = → + ∑ Fx = – FThrust x – Fz x + FDrag = 0 – Cos (6) · 116,618.8 – Sin (3) · 651,001.41 + FDrag = 0 – 115,979.95 – 34,070.78 + FDrag = 0 FDrag – 150,050.73 = 0 FDrag = 150,050.73 N →

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Appendix XXI

Before landing impact speed

Now that the aeroplane can be considered as a PUNTMASSA the horizontal and vertical components of the velocity (fig. 1) can be determined. The angle between the horizontal velocity is 3˚ (Formula 1). The vertical velocity is higher than the prescribed value in EASA CS-25, this is because it is an extreme situation and the aeroplane does not flare during touchdown. Formula 1 Cos (α) = a / s Sin (α) = o / s

α = angle [degrees] a = connected side [m/s] o = opposite side [m/s] s = diagonal side [m/s]

vx = Cos (3) · 73.05 = 72.95 m/s vy = Sin (3) · 73.05 = 3.82 m/s vx = 72.95 m/s vy = 3.82 m/s

1. Resulting velocity (73.05 m/s) 2. Horizontal velocity (72.95 m/s) 3. Vertical velocity (3.82 m/s) 4. Aeroplane point mass Figure 1: Velocity point mass B737

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Appendix XXII

Movement of the main gear shock struts

The shock struts are compressed during landing impact, this value is estimated at 0,75 m (fig. 1). The length of the compression of the shock strut must be dissolved in the diagonal plane (Formula 1). Formula 1 Cos (α) = a / s

α = angle [degrees] a = connected side [m] s = diagonal side [m]

Cos (3) = 0.75 / s s = 0,75 / Cos (3) = 0.751 m s = 0.751 m

Figure 1: Main gear shock strut force damping movement

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Appendix XXIII

Force in the main gear shock struts

The force applied on the MLG shock struts and the acceleration are known. Now the landing impact shock which has to be absorbed by the shock struts can be determined (Formula 1) by Newton’s first law. Formula 1 F=m·a

F = Force [N] m = Mass [kg] a = Acceleration [m/s²]

F = 66,361 · 19.431 = 1,289,460.591 N F = 1,289,460.591 N

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Appendix XXIV

Kinetic energy on the main landing gear

With every movement action that is done by the landing, energy will be produced. During a landing of the B737 there are two main actions where high kinetic energy is being generated. During the landing touchdown shock and during braking. To calculate the kinetic energy that occurs on each strut (Formula 1), the MLM, that is a total weight of 66.361 kg is taken into account and the maximum vertical landing speed of 3.8 m/s. This speed is a described regulation by the EASA. With this calculation a situation with maximum allowed limits is described Formula 1 KEMLG = ½ · m · v²

KE = Kinetical energy (vertical) [J] m = mass [kg] v² = speed [m/s]

KEMLG = ½ · 66,361 · 3.8² = 479,126.42 J To calculate the kinetical energy on each gear, the answer has to be divided in two struts. KEstrut = (½ · 66,361 · 3,8²) / 2 = 239,563.21J KEMLG = 479,126.42 J KEstrut = 239,563.21 J

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Appendix XXV

Kinetic braking energy

To stop the B737 with its MLM and its maximum allowed speed of 73.05 m/s, the brakes of the wheels have to make friction. This process also generates kinetic energy. To calculate the kinetic energy of the brakes (Formula 1), the maximum landing speed and the MLM a situation with a maximum allowed limits will be described. Formula 1 KEbrake = (½ · m · v²) / N

KE = Kinetic energy brakes [J] m = mass [kg] v² = speed [m/s] N = amount wheels with brakes [dimensionless]

KEall-brakes = ½ · 66,361 · 73.05² = 177,061,185.1 J To calculate the kinetic energy on each brake, the answer has to be divided in four wheels that contains brakes. KEbrake = (½ · 66,361 · 73.05²) / 4 = 44,265,296.28 J KEall-brakes =177,061,185.1 J KEbrake = 44,265,296.28 J To calculate the temperature change (Formula 2) that occurs on each brake by landing. The kinetic energy, mass of the brake and specific heat of the material has to be known values. considered brake temperature before braking is 288 K. Formula 2 KEbrake = m · c · ∆T

KE = Kinetic energy [J] m = mass [kg] c = specific heat [J/(kg.K)] ∆T = temperature change [K]

44,265,296.28 = 2176.75 · 440 · ∆T Now the temperature change of the brake can be calculated. ∆T = 462.2 K ∆T = 462.2 K After braking the temperature of the brake will be 334.22 K

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Appendix XXVI

Forces during take-off

During the take-off (fig. 1) the aeroplane will have a constant speed of 149 knots. Which means that the thrust will be equal to the total drag. Further are the forces on the main gear equal to each other.

1. 4.09 m 2. 15.60 m 3. 6.33 m 4. 0.745 m 5. 0.600 m 6. Thrust force (Ft) 7. Drag force (Fw) Figure 1: Forces during take-off

8. 9. 10. 11. 12. 13.

Gravity force (FZ) Lift force (FL) Thrust application point Centre of gravity application point Centre of pressure application point Wing with MAC

The aeroplane provides a thrust of 242,000 N. For the mass of the aircraft the MTOM of 79,010 kg will be used. With the aid of the gravitation formula (Formula 4), the gravitation force in Newton’s can be stipulated. FZ = 775,090 N 79,010 * 9.81 = 775,090 N Besides the thrust and the gravitation force, also the drag force and lift force are needed. The lift can be calculated with the lift formula (Formula 2). The values which are needed to calculate the lift are: CL = 0.25 p = 1.225 kg/m3 v = 149 knots (76.6 m/s) S = 249.17 m2

Lift = 0.25 — ½ — 1.225 — 76.6 2 — 249.17

FL = 268,646.2 N

Because the total drag and the drag of the wheels are known, the drag caused by airflow around the aeroplane can be calculated. Dtotal = Dwheels + Fw Dtotal = N * µ + Fw Dtotal = (FL – FG) * µ + Fw Dtotal = (268646,2 – 775090) * 0,2 + Fw = 242,956 N This means that the drag caused by airflow around the aeroplane amounts 141,667.23 N. By means of the drag formula (Formula 1) the Cw value can be calculated.

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Appendix XXVI

(Continuance)

The values which are needed to calculate the Cw are: p = 1.225 kg/m3 v = 149 knots (76.6 m/s) A = 52 m2 141,667.23 = 52 · Cw · 1.225 · 76.6² 2 Cw = 0.76

Using the sum of the moments around point A which has to be equal to zero. And the sum of the vertical forces equal to zero. The forces FN and FM can be calculated. ΣMA = 0 - FN — 15.60 + Ft — 1,34 – FL — 0.60 + FZ — 0.745 – Fw — 2.705 = 0 FN — 15.60 = 242,956 — 1.34 –268,646.2 — 0,60 + 775090 — 0.745 – 141667.23— 2.705 15.60 FN = 358605.5 FN = 22,987.53 N FN = 22,987.53 N + ↑ΣFY = 0 FN – FZ + L + (2 — FM) = 0 22,987.53 – 775,090 + 268,646.2 + (2 — FM) = 0 2FM = 483,456.27 FM = 241,728.14 N

FM = 241728,14 N

Finally the drag forces of the nose and main gear, caused by the contact of the wheels with the ground can be calculated. This can be done by means of the drag formula (Formula 3). Drag nose wheel FDN = 22,987.53 * 0,2 = 45,97.51N Drag main wheel FDM = 241,728.14 * 0,2 = 48,345.63 N

DN = 5,252.48 N

DM = 48,345.63N

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Appendix XXVII Calculations of paragraph 2.2.2
This appendix contains all the calculations that are made for paragraph 2.2.2. The following five calculations are made: 1. Energy of the moving B737 2. Thrust 3. Drag 4. Braking forces 5. Momentum Ad 1. Energy of the moving B737 Formula 1 Ek = 0.5 · m · V2 Ek = kinetic energy [J] m = mass [kg] V2 = velocity [m/s]

Fv = 0.5 · 79,010 · 76.652 = 232,100,664.90 J Ek = 232,100,664.90 J • MTOM = 79,010 kg • ∆ v = 149 knots or 275.95 km/h or 76.65 m/s • t = 1 second Ad 2. Thrust • Maximum thrust of one CFMI CFM56-7 is 27.300 lbs • 27,300 lbs = 12.383,07 kg of thrust • Two engines provide 12.383,07 x 2 = 24766.14 kg of thrust • For 20% N1 = 0.2 · 24766.14 = 4953.23 kg x 9.81= 48.591,17N Formula 2 W=F ·s W = work [J] F = force [N] S = travelled distance [m]

W = 48,591.17 · 687.97 = 33,429,267.22 J, Engine thrust energy W = 33,429,267.22 J • • s = 687.97 m F = 48,591.17N

Ad 3. Drag Formula 3 (aerodynamic drag) Fw = A · Cw · ρ · v² 2

Fw = friction force [N] A = frontal surface [m²] Cw = friction coefficient [dimensionless] ρ = density [kg/m³] v = speed [m/s]

Fw = 52.0 · 0.76 · 1.225 · 76.65² 2 = 142,215.63 N Fw = 142,215.63 N • • • • A = 52.0m² Cw =0.76 ρ = 1.225 kg/m³ V = 76.65 m/s 32

Landing gear Appendices
Project group: 2A2E

Appendix XXVII (Continuance)
Formula 4 D=N—µ D = 168,340.34 — 0.2 = 33,868.07 N, Drag MLG Fw MLG = 33,868.07 D = 36,410.41 — 0.2 = 7,282.08 Drag NLG Fw NLG = 7,282.08 • • • N MLG = 168,340.34 N N NLG = 36,410.41 N Friction factor = 0.2 D = drag [N] N = normal force [N] µ = drag coefficient

Formula 5 V22 = v12 +2a(∆S) 76.652 = 02 + 2 — 4.27(∆S) = 687.97m ∆S = 687.97m • • • v1 = 0 m/s V2 = 76.65 m/s a = 4.27 m/s2 V2 = start velocity [m/s] v12 = end velocity [m/s] a = acceleration [m/s2] ∆S = travelled distance

Formula 6 W=F ·s W = work [J] F = force [N] S = travelled distance [m]

W = 142.215,63 — 687.97 = 97.840.090,99 J, Energy aerodynamic drag W = 97.840.090,99 J W = 33.868,07 — 687.97 = 23.300.216.12 J, Energy MLG drag W = 23.300.216.12 J W = 7.282,08 — 687.97 = 5.009.852,58 J, Energy NLG drag W = 5.009.852,58 J • s = 687.97 m • Fw = 142.215,63 • Fw MLG = 33,868.07 • Fw NLG = 7,282.08 Ad 4. Braking force, braking energy and temperature of the brakes Formula 7 F=m·a F = 79.010 · 4.27 = 337,372.70 N 337,372.70 N • Deceleration rate during RTO is 14 feet per second, or 4.27 m/s • M = 79.010 kg F = force [N] m = mass [kg] a = acceleration [m/s2]

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Appendix XXVII (Continuance)
Braking energy: Energy of: Forward motion + thrust – (Aerodynamic drag + Rolling resistance MLG + Rolling resistance NLG) = 232.100.664,90 + 33.429.267,22 – (97.840.090,99 + 23.300.216.12 + 5.009.852,58) = 139.379.772,40 J • Energy of the forward motion = 232.100.664,90 J • Energy of the thrust = 33.429.267,22 J • Energy of the aerodynamic drag = 97.840.090,99 J • Energy of the rolling resistance MLG = 23.300.216.12 J • Energy of the rolling resistance NLG =5.009.852,58 J Brake temperature: Formula 8 π · r2 = surface of a circle π · ((0.276/2) ²) = 0.06 m2 π · ((0.276/2) ²) = 0.06 m2 0.06 ·1.1303 = 0.07 m3 7860 · 0.07 = 550.2 kg 550.2 · 2 = 1100.4 139,379,772.40 / (440 · 1100.4) = 287.87 K • Diameter of the brakes = 0.276 m • Height of the brakes = 1.1303 m • “Soortelijke warmte” = 440 J-kg-K • “Soortelijke massa” = 7860 kg m3 Ad 5. Momentum ΣMMLG = Fm — 2.705 + Ft — 6.33 + Fn NLG— 15.60 - Fw — 2.705 - Fb — 0.57 - Fg — 0.745 = 337.370,33— 2.705 + 48,591.17— 1.34 + 36,410.41— 15.60 - 142.215,63 — 2.705 - 337,372.70 — 0.57 - 775,088.10 — 0.745 = 391.264,95 Nm • Fm = 232.100.664,90 / 687.97 = 337.370,33 • Ft = 48,591.17 • Fn NLG = 36,410.41 • Fw = 142,215.63 • Fb = 337,372.70 • Fg = 775,088.10 • Height CG = 5.41* / 2 = 2.705 *height of the fuselage above the ground when fully loaded (MTOM) • 44.5 inch = 113.03 cm / 2 = 56.515 =0.56515 = MLG wheel radius. π=π r2 [m] surface of a circle [m]

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Appendix XXVII (Continuance)

1. 4.09 m 2. 15.60 m 3. 6.33 m 4. 0.745 m 5. 0.600 m 6. Thrust force 7. Drag force 8. Gravity force 9. Lift force Figure 1: RTO forces overview

10. 11. 12. 13. 14. 15. 16. 17. 18.

Thrust application point Centre of gravity application point Centre of pressure application point Wing with MAC Fm Fw Fb Fn NLG Fn mLG

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Appendix XXVIII Tire costs
A new main landing gear tire, of the mark Michelin costs 3,378 euro (fig. 1).

Figure 1: Tire main landing gear A new nose landing gear tire, of the mark Michelin costs 1,708 euro (fig. 2).

Figure 2: Tire nose landing gear

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Appendix XXIX
Orientation

Process report

Reha- In the beginning it went alright but as we proceeded it was harder to find the right sources, especially in chapter two. Chapter two and three went a little chaotic and found it hard to find the right sources. Omer- Has no trouble with mechanics, because of this had no problems with chapter two. It was easy to find the information needed for chapter one. Had no difficulties overall and thought it went well. Imad- Sources where easy to find in the beginning. But the problem was that we planned to much time to orientate in the beginning at the easy parts of the project and had less time planned to for the more difficult to find sources. Jordi- Had no problems orientating except the time was sometimes too short to find the right sources. Some of the other group members did not do enough orientation. Benjamin- There were not enough checks if everybody did the orientation right. In the beginning Benjamin did the forces during landing his own way without orientation. Then he got stuck and had to start over with the use of sources. The part about brakes was first really bad because of bad orientation but was corrected. Ahmed- Did not orientate the right way at all times. Should have. Sara- Also thinks we started off alright with the orientation but after the first chapter the orientation was not done as much anymore. After the first chapter there was not enough time planned for the orientation. Meetings Reha- The meeting went well. The agenda point for the meeting where not always followed. Sometimes some of the members where distracted. Some of the discussions went on without end. In week six there were some unplanned meetings. This caused some irritations. It would be better if the meetings are announcement up front, if there are time limits and if everyone is familiar with the agenda before the meeting. Omer- The meetings were not optimized, often the discussion went too far and we ended on a different subject then we started with. Imad- Has the same opinion as Omer. Thinks the meetings started to slow. Sometimes there were discussions only involving three persons, doing the part about forces, while the rest had to be there for nothing because the discussions were not about group subjects. Jordi- The meetings where not going well. Only a few persons where really involved in the meeting, often people where with there thoughts at the meeting. Because of this some of the agreement went not well. Thinks that the meeting about the forces was necessary, he wanted the opinion about this part from the rest of the group. The set meeting dates went alright and there were not too much extra meetings. Benjamin- Thinks all of the above have a point, but thinks that how further we went into a important part the discussion was stopped. Because of this a lot of import parts of the project were lost. He did not really noticed who was the chairman each week. The minutes list were not checked the right way. Ahmed- Did not think the meetings went well. The group has to stay on one line. The chairman has to leas the group. The group has to support and help the chairman. Sara- Sometimes we got distracted and lost the subject we where discussing about. Sometimes the meetings were unnecessarily and this was unused time that could be spend well on the project.

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Appendix XXIX
Transfer of knowledge

(Continuance)

Reha- Has read all the parts of the other members and asked the writer in case of vagueness . Did not fall back. Omer- Thinks he has enough knowledge. Mechanics got more clear because the part of forces he has written. Imad- Gets the third chapter, but has difficulties with forces part in chapter two, because it was not clear what the definitive version was. But he thinks this is his own fault. Jordi- The group did not ask him any questions about his part except Benjamin. There was no or minimal knowledge exchange during meetings. Thinks he collected enough knowledge himself. Benjamin- Prefers using sources than the knowledge of other group members. He uses sources to be able to correct the other group members. Because of postponing some of the corrections where not done properly. Ahmed- Thinks the knowledge transfer went well. Sara- Has enough knowledge of chapter one and three, but had problems following the forces part in chapter two.

During the final project meeting some irritations were spoken out. This is an evalutation report of the group process that was discussed at that moment. Benjamin mentioned that appointments were not finsihed on time. The appointments in the minutes list were not fullfilled grouply. In his opinion not all agreements that were made in the groups meetings are discribed properly and that this creates an unpleasent working environment. Pieter replyes by asking if other group members have similair experencises and wht the reasons where why deadlines where not met. Benjamin also complains that he is the only one in the group who adresses these problems. Reha’s opinion is that Benjamin has a negative attitude which also creates an unpleasent working environment. The opinion of Sara, Imad and Omer is that regulations are not to be taken to strictly and a working in a group should be productive and pleasant as well. Pieter concludes that the problems in the group are caused by different ways of motivation of the several group members. One member is determined to do the very best he can and the other wants to get his credits and work in a pleasant efficient atmosphere.

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