landing gear project report

Project Landing Gear 2A2G

Table of Contents
Introduction ............................................................................................................................................. 1
Summary ................................................................................................................................................. 2
1 Systems Landing Gear B737-300 ..................................................................................................... 3
1.1 General Aspects ....................................................................................................................... 3
1.1.1 History of the landing gear .............................................................................................. 3
1.1.2 Modern types of landing gear ......................................................................................... 3
1.1.3 B737-300 landing gear ..................................................................................................... 4
1.2 Landing Gear Systems ............................................................................................................. 4
1.2.1 Retract / extend ............................................................................................................... 5
1.2.2 Shock absorption ............................................................................................................. 7
1.2.3 Steering ............................................................................................................................ 8
1.2.4 Brakes ............................................................................................................................ 10
1.2.5 Wheels/Tires .................................................................................................................. 11
1.2.6 Nose landing gear shimmy ............................................................................................ 12
1.3 Related Systems .................................................................................................................... 13
1.3.1 Auto braking .................................................................................................................. 13
1.3.2 Antiskid System ............................................................................................................. 14
1.3.3 Air/Gound Logic ............................................................................................................. 15
1.3.4 Manual Gear Extension ................................................................................................. 15
1.4 Legal Requirements ............................................................................................................... 16
1.4.1 Requirements landing gear system ............................................................................... 17
1.4.2 Maintenance requirements ........................................................................................... 20
1.4.3 Minimum Equipment List (MEL) .................................................................................... 21
2 Forces B737-300 ........................................................................................................................ 22
2.1 No-wind landing forces ......................................................................................................... 22
2.1.1 Centre of gravity ............................................................................................................ 22
2.1.2 Theories and formulas ................................................................................................... 23
2.1.3 Aircraft standing on ground .......................................................................................... 24
2.1.4 Aircraft touchdown ....................................................................................................... 24
2.2 Crosswind Landing Forces ..................................................................................................... 25
2.2.1 Touchdown upwind wheel ............................................................................................ 26
2.2.2 touchdown downwind wheel ........................................................................................ 27
2.2.3 Nose Gear ...................................................................................................................... 28

Project Landing Gear 2A2G

2.2.4 Internal forces ............................................................................................................... 28
2.3 Forces on Materials ............................................................................................................... 28
2.4 Conclusion ............................................................................................................................. 30
3 Troubleshooting ............................................................................................................................ 31
3.1 Failures .................................................................................................................................. 31
3.1.1 Hydraulic shimmy damper failure ................................................................................. 31
3.1.2 Main gear torsion link failure ........................................................................................ 32
3.2 Controlling maintenance ....................................................................................................... 33
3.3 Cost ........................................................................................................................................ 33
3.3.1 Aircraft on ground ......................................................................................................... 34
3.3.2 Employers ...................................................................................................................... 34
3.3.3 Leasing landing gear ...................................................................................................... 34
3.4 Conclusion ............................................................................................................................. 34
Bibliography ........................................................................................................................................... 36
List of appendices .................................................................................................................................. 39

Project Landing Gear 2A2G

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Introduction
In the first period, academic year 2010, Amstel Leeuwenburg Airlines (ALA) gave the technical engi-
neering department the task to analyse one or more failures of a Boeing 737-300 landing gear. ALA
also wants to know the maintenance procedure and costs of the maintenance when these failures
show up.

The research will be in the form of a report and a presentation to the ALA. The report will be handed
out in week 7 of the academic year 2010 and the presentation will take place in the first week of
November 2010. The report and the presentation will lead to an overview of the maintenance pro-
gram and these failures.

This report consists of three chapters conform Wentzel.

Main aspect for this research is the theory behind the landing gear. The landing gear of the Boeing
737-300 is a retractable landing gear system. Main functions of the landing gear are supporting the
aircraft’s weight and absorbing the landing shock, allowing the aircraft to manoeuvre on ground and
braking. To carry these main functions, the landing gear system consist of components and subsys-
tems. In order to guarantee safety and to reduce the risk of failures the landing gear system including
the subsystems are bound to regulations. These regulations consist of certifications and limitations.
(1)

During the landing procedure, the landing gear is exposed to severe loads. These loads result in the
choice of different materials which can be used in the landing gear. Material aspects such as fatigue,
durability, stiffness and hardness are taken into consideration when choosing materials. Also the
maximum loads on the landing gear are measured during a landing or rejected take off. (2)

During the landing procedure, landing gear failures such as a failure of a shimmy damper and an
overstressed torsion link can occur. To fix these failures there is a maintenance program made. These
repairs and maintenance checks are related to the use of an aircraft and its landing gear. This influ-
ences the costs which the airline will have to make. (3)

The main references used for this report are: Flight systems from C.J.A. Langedijk and the mainte-
nance manuals of Boeing. An overview of all references can be found on page 35. In the appendices
the project assignment (appendix XI) and the process evaluation report (appendix XII) are added.


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Summary
The team had chosen a Boeing 737-300 for the analysis of the landing gear and a discussion of two
failures and the adaption of the maintenance program and the costs involving the adaption of the
maintenance program.

The main purposes of the landing gear are to absorb the kinetic energy of the landing and to ma-
noeuvre the aircraft on the ground, in the landing and on the parking space. The landing gear is in the
history of aviation improved to the modern tricycle landing gear, one nose gear and two main gears.
The landing gear is a disadvantage for the aerodynamics, therefore it is able to extend and retract the
landing gear. Every gear of the B737-300 is a four-bar-linkage, the ideal gear construction for a com-
pact storage in the fuselage and the wing. The extension and retraction occur by a hydraulic pressure
system. When the landing gear is extended it will holds in its place by a kink in one of the struts. In-
side these struts shock absorbers are placed to absorb the kinetic energy during the landing. These
shock absorbers are oleo-pneumatic absorbers, they work with oil and nitrogen. To manoeuvre the
plane on the runway and taxiway two types of steering are placed, one is the rudder steering used by
take-off and landing. For taxiing there is a control wheel to make smaller rates of turn. To slow the
aircraft down on the ground, the B737 has multi disk brakes, which operates with hydraulic pressure.
The brakes are mounted in the wheels. The six tires are designed to withstand a lot of forces. To pre-
vent the tires from exploding by overpressure a thermal plug is used. In the landing the auto brake is
used with four amounts of deceleration, 1, 2, 3 and MAX. For rejected take-off (RTO) the auto brake
will make to stop the aircraft as fast as possible. To prevent the wheels from blocking during braking
an anti-skid system is installed. When the landing gear cannot be extended with hydraulic pressure it
must be extended manually by the gravity. In the CS-25 (Certification Specifications) of the European
Aviation Safety Agency (EASA) the requirements for the landing gear are given.

The landing gear has to carry the weight of the aircraft, divided over the nose gear and the main
gears. The force on the main gear is the greatest because of the small distance between the centre of
gravity (CG). When landing, the forces on the landing gear are greater than the weight of the aircraft.
The aircraft is first touching down on the main gear, with the CG in front of the main gear, the nose
wheel will rotate to the ground. When adding crosswind in the landing the touchdown will occur
different with headwind or no wind. First the upwind main gear touches down, then the downwind
main gear and at last the nose gear. The materials which can be used can be found in the CS-25. The
specifications of the materials have to be high strength and stiffness, low cost and weight, and have
good machinability, weldability, and forgeability. They also must be resistant to corrosion, stress
corrosion, hydrogen embrittlement, and crack initiation and propagation. The alloys which can be
used are steel, aluminium and titanium.

The first failure the team has chosen is the hydraulic shimmy damper failure. This failure occurs when
the hydraulic system has a leak. To prevent this failure, the hydraulic lines have to be checked for
leaks. The second failure is a fracture in a torsion link of the right main gear. Because of this fractured
torsion link, one tire has been deflated and the gear has been rotated 45° to the right. When one of
the two failures will occur, it can be prevented in the A, B, C or D-check. With the A and B-checks the
most critical parts are checked and with the C and D-checks the aircraft is taken apart. These checks
will costs 2,165,000 Euros per five years, included employer costs and aircraft on ground costs.


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1 Systems Landing Gear B737-300
The landing gear is a complex part of the airplane. The landing gear can be designed in many forms
(1.1). The B737 has essential components to operate the landing gear (1.2). Beside those systems,
other parts are installed at the landing gear (1.3). There are many rules that has to be taken into ac-
count (1.4).
1.1 General Aspects
During the history of the aircraft, the landing gear had undergone a great development (1.1.1). When
the first production aircraft were made, standard types of gears were developed (1.1.2). The B737-
300 is using one of these standard types of gear, the tricycle landing gear (1.1.3).
1.1.1 History of the landing gear
In the beginning there was one type of landing gear, the feet of
the airman. In 1891, Otto Lilienthal was one of the first men who
flew above the earth. Lilienthal’s airplane was not more than two
wings and one stabilizer. To fly, he jumped of a hill and landed on
his feet (figure 1.1).
After Lilienthal, the Wright Brothers made the first powered
flight. The landing gear of the Wright Flyer I was made of skis.
After the Wright Brothers had flown, the airplane was further
developed, so did the landing gear.
1.1.2 Modern types of landing gear
The first type of landing gear that was used on a large scale was the conventional landing gear. The
conventional gear consist of three wheels. The main gear, which is under or in front of the wings and
one small wheel under the tail. These air-
crafts are called tail draggers. A well-known
tail dragger is the DC-3 (figure 1.2). The DC-3
has a retractable landing gear. The retracted
landing gear was invented by two Fren-
chmen in 1876, but only used on large scale
after 1930. The main gear of the DC-3 is par-
tially retractable in case of a gear down fail-
ure, the tail wheel isn’t.

The tail draggers were made for decades on
smaller aircraft, but it had a few disadvan-
tages. At first, when the pilot braked too
much, the airplane could make a nose-over. In this case the propeller would break. Secondly, in the
roll-out after the landing the plane could make a ground loop. When steering to much or in case of
wind shear, the tail of the plane would turn. The consequences of a ground loop can be different.
One will have no damage, while the other has damaged wingtips. When the engines are located on
the wings, they could be damaged too. At last, the visibility in front of the plane is less then when the
fuselage is horizontal.
Therefore a new type of gear was invented, with the fuselage of the airplane horizontal. In most cas-
es it means that the plane has a tricycle landing gear. The main gear will stay under the wings, but
the tail wheel has become a nose wheel. By doing that, the maneuverability will get better. The tri-
cycle landing gear has the most variants. For smaller aircraft, like the Cessna C-172, with three
wheels, but also variants for airliners, like the B737-300.

Figure 1.1 Lilienthal in flight

Figure 1.2 DC-3

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1.1.3 B737-300 landing gear
The B737-300 is using a variant of the tricycle landing gear. It has two nose wheels and two pair of
wheels for the main gear. One part of the main gear has two wheels. The figures below show the
measures of the gear. The width of the main gear is 5,2 meters (figure 1.3) (1). The nose gear is cen-
tered in the latitudinal axis of the fuselage (2). The position of the main gear is slightly aft of the cen-
ter of gravity of the airplane and the nose wheel is positioned four meters from the nose (figure 1.4).


Figure 1.3 Front view B737-300



1. Width mean gear
2. Nose gear
The measurements of the longitudinal axis can be viewed in the side view (figure 1.4). The position of
the main gear is slightly aft of the center of gravity (CG) of the airplane, 3,1 m (1), it is positioned 16,5
meters from the nose (2), while the CG is positioned 13,4 meters from the nose (3). The purpose of
placing the main gear aft the CG is to create a negative turning moment by touchdown of the main
gear. The negative turning moment will result in moving the nose gear on the ground. The position-
ing of the main gear will prevent the airplane from standing on its tail. The nose gear is positioned 9,4
m in front of the CG (4) and 12,5 m from the main gear (5).


Figure 1.4 Side view B737-300

1. CG - main gear
2. Nose - main gear
3. Nose gear - main gear
4. Nose - CG
5. Nose gear - CG
1.2 Landing Gear Systems
The Boeing 737-300 consist of many landing gear systems. Hydraulic systems are used to retract or
extend the landing gear (1.2.1). The shock absorbers absorb the landing shock when touching the
ground (1.2.2). In order to obtain directional control during ground maneuvers and taxiing, steering
at the nose wheel is provided (1.2.3). For slowing down the aircraft during ground rolling there are
different brakes systems (1.2.4). The Boeing 737 nose and main landing gear has totally six wheels.
(1.2.5). A problem able to appear in the landing gear is shimmy (1.2.6).
1
2
1
2
3
4
5

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1.2.1 Retract / extend
To improve the aerodynamic aspects and to prevent extra drag on the Boeing 737 the landing gear
can be retracted and extended. Most airplanes use a four-bar linkage construction to retract or ex-
tend the landing gear (1.2.1a). The Boeing 737-300’s main landing gear system is able to retract
(1.2.1b) and extend it (1.2.1c). The landing gear is also equipped with a nose landing gear. This gear
is, just like the main landing gear, able to retract (1.2.1d) and extend (1.2.1e).To retract and extend
the crew has to operate the landing gear lever (1.2.1f).
1.2.1a Four-bar linkage
The most used landing gear is a four-bar system. The system consists of four arms and is ideal to
store the landing gear. A four-bar linkage consist of the fuselage and struts.
There are different four-bar linkage (figure 1.5) constructions. The simplest way is the sideway (1)
construction. The pivot point (2) is a construction that is placed in the main strut. Beside pivot point
there is also a pivot point in supporting strut construction (3). This construction is able to form a kink.
Finally there is a backwards construction (4).


1
2

3

4
Figure 1.5 Four-bar system

1.2.1b Retraction main landing gear
The Boeing 737 has two main retract and extended landing struts. The main gear (figure 1.6) can be
controlled by using the landing gear control handle moving into UP or DOWN position. The main gear
actuator applies the force to raise and lower the gear. A lock actuator is used to lock the gear in up or
down position. An actuator (1) is a hydraulic mechanism type for using controlled movements
The main landing gear actuator and walking beam (2) work together to raise and lower the main
gear. The walking beam reduces the reaction force going into the aircraft structure from the main
gear actuator. When retracting the main landing gear an inboard force from the actuator is applied di-
rectly to the gear. Hydraulic pressure transported directly through the modular package and the ac-
tuator. The flow of hydraulic pressure to the main actuator is controlled by flow limiting valves and
pressure relief valves. When retracting the gear, the up-lock actuator cuts off the pressure and pre-
vents the up-lock hook from moving back into locked position. The pressurized lock actuator pushes
the hook into final position where it is locked.
1. Side way
2. Pivot point in main strut
3. Pivot point in supporting strut
4. Backwards

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1.2.1c Extension main landing gear
When extending the
landing gear, hydraulic
pressure is moved di-
rectly to the main gear
actuator and the modu-
lar package. The transfer
valve controls the
amount of pressure to
extent the main gear.
The landing gear ex-
tends by actuator force,
landing gear weight, and
air loads. After extend-
ing, the landing gear is
moved to the lock posi-
tion (figure 1.6). When
the landing gear is in locked position there is a upper side strut (3), a lower side strut (4) and shock
strut (5) to hold the landing gear in position.
1.2.1d Retraction nose landing gear
The Boeing 737-300 has a double nose gear (figure 1.7). The double nose gear has more advantages
than a single nose gear, for example more stability and the ability to operate with one flat tire. When
the landing gear control handle is moved into UP or DOWN position the nose landing gear retracts or
extends at the same time as the main landing gear. The nose gear actuator (1) uses hydraulic pres-
sure to raise or lower the gear. The lock strut (2) is used for locking the nose gear by spring bungees
(3). The spring bungees are placed on each side of the lock brace.
To retract the nose gear, the control lever has to be placed into UP position. The hydraulic pressure is
directly transported through the selector valve and through the nose gear modular package. Then
the hydraulic pressure transported to the gear and the lock actuator. Using a downward force the
lock is pulled over centre. The lock link causes a 90 degrees swing that changed the landing gear from
horizontal to vertical position. The lock actuator (4) retracting force is opposed to the lock link
movement until the gear is almost retracted. In time of opposition the larger main gear actuator
overpowers the lock actuator.
1.2.1e Extension nose landing gear
To extent the nose gear, the control lever has to be placed into DOWN position. The hydraulic pres-
sure will be transported to the gear actuator and lock actuator in the opposite direction when re-
tracted the landing gear. When the gear is locked, there is a heavy weight on the lock strut. This
heavy weight causes a force that release the lock. To support the extension there is a transfer cylin-
der that equalizes hydraulic pressure on either sides of the nose gear.

1
2

3

4
5
Figure 1.6 Gear down and locked

1. Main gear actuator
2. Walking beam
3. Upper side strut
4. Lower side strut
5. Shock strut

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1
2

3

4
Figure 1.7 Nose landing gear

1.2.1f Operation
For operation of the landing there are two different types to use it. The landing gear lever controls
the different movements of the landing gear.
1. Retraction
2. Extension
ad. 1 Retraction
The landing gear is normally controlled by the landing gear lever. On ground, the lever lock prevents
the landing gear lever from moving to the up position. When the landing gear lever is moved up, the
landing gear start to retract. During retraction the brakes automatically stop rotation of the main
wheels. The nose wheels retract forward into the wheel well and the rotation is stopped by snub-
bers. Hydraulic system B pressure is used for raising the landing gear.
ad. 2 Extension
When the landing gear lever is moved to DOWN, the hydraulic system A pressure released the up-
locks. The landing gear extends by hydraulic pressure, gravity and air loads. Mechanical and hydraulic
locks stops the gear from retracting.
1.2.2 Shock absorption
The Boeing 737-300 has an oleo-pneumatic shock absorber (1.2.2a). The shock absorber works with
oil and nitrogen (1.2.2b).
1.2.2a Function shock Absorber
The impact of landing must be absorbed. This is done by the oleo-pneumatic shock absorber. This
absorber is mostly used by large aircraft. The main advantage of this absorber is that it provides
shock absorption as well as effective damping. There are three types of oleo-pneumatic shock absor-
bers; the telescopic strut, the articulating strut and the semi-articulating strut. The telescopic strut is
the only one used in a Boeing 737-300. This strut is housed within the main vertical strut of the land-
ing gear. This is very compact, but it is difficult to maintain.
1. Nose gear actuator
2. Lock strut
3. Spring bungees
4. Lock actuator

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1.2.2b Design Shock Absorber
The oleo-pneumatic shock absorber (figure
1.8) consists of two separated chambers. One
chamber is filled with nitrogen (1) and the oth-
er chamber is filled with oil (2). If the aircraft
lands, the oil chamber will be pushed against
the nitrogen chamber, the gas and oil will be
compressed. The kinetic energy is damped by
the oil which is being forced through orifices
(3). The rebound of the landing is controlled
by the gas pressure forcing the oil back into
its chamber through recoil orifices (4). The
orifices must be calibrated, because if the oil
flows back too fast, the aircraft will bounce
upwards. If the oil flows back too slow, the
shock absorber will not damp adequately. This
could happen during taxiing, the bumps on the
taxiway would not be absorbed because the
absorber has not restored itself quickly enough
to the static position.
1.2.3 Steering
In order to obtain directional control during ground maneuvers and taxiing, steering at the nose
wheel is provided.
When the pilot wants to turn (figure 1.9) during taxiing, he normally uses the steering wheel (1) lo-
cated at the left and right side of the cockpit. When turned at the control wheel, the cable get tensed
and rotate the pulleys (2), after the pulleys comes the rudder pedal steering mechanism (3).







Eventually (figure 1.10) the cables (1,2) pull the steering valve (3) which provides the actuator (4) of
hydraulic pressure, this actuator pulls the wheels from side to side with hydraulic power.

Figure 1.8 Shock absorber



Figure 1.9 Steering system

3
1

2
1. Chamber filled with Nitrogen
2. Chamber filled with Oil
3. Orifices
4. Recoil Orifices
1






2
3


4
1. Steering wheel
2. Pulleys
3. Rudder steering mechanism


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To turn the aircraft from standard to the maximum turn rate of 78 degrees, hydraulic pressure from
system A is send to the nose wheel. The pressure needed to turn the wheel is 3000 psi. When the
nose wheel is unturned the steering valve keeps a pressure from 70 to 130 psi against the actuator
pistons to act as an shimmy damper. Hydraulic pressure from system B is to engage the alternate
nose wheel steering system to turn the nose wheels if the pressure from system A is lost. This alter-
nate system can be set trough a switch on the captains forward panel.
Rudder panel steering is available during take-off or landing. When moving on high speed small di-
rectional changes are required. Full input of the rudder pedals can produce about seven degrees of
nose wheel steering. In order to use this type of steering the squat switch must be activated which
regulates the steering actuator to minimize the rotation. This switch is activated by the weight of the
airplane compressing the shock strut.
The nose gear steering mechanism at the shock strut (figure 1.11) consist of a steering collar (1), two
actuators (2) and an inner cylinder (4). When a turn is made, the two cylinders separately retract and
extract and the pistons (3) will set the steering collar in motion. When a force is applied to the steer-
ing collar, this will move the inner cylinder to turn the wheel.













Figure 1.10 Steering wheel system

1. Cable system A
2. Cable system B
3. Steering valve
4. Steering actuator


1. Steering collar
2. Actuator
3. Piston
4. Inner cylinder
5. Pressure of 70-130psi


Figure 1.11 Nose wheel straight

Nose wheel in left turn at 0°-33°

Nose wheel in left turn 33°-78°

1
2
3
4
1 1
4
2
3
5
3000
psi
3000
psi

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1.2.4 Brakes
The brake system is powered by hydraulic system A and controls the braking of the aircraft during
ground operation. The Boeing 737-300 uses a multi disk brakes (figure 1.12). A multi disk brake uses
hydraulic pressure to control a rotor (1)-stator (2) unit. The brakes consists of multiple steel discs. An
adjuster giving hydraulic pressure to the pressure plate (3). For safety reasons companies use a brake
wear pin to indicate when the brakes has to be replaced.

Pushing the brake pedal opens the alternate brake metering valve, that allows pressure to pass
through the alternate antiskid valves to the brakes. The brakes can be divided into:
1. Normal brake system
2. Alternate brake system
3. Accumulator brakes
ad 1 Normal brake system
The normal brake system can be used during standard situations. The brakes are controlled by the
two rudder pedals in the cockpit. The brake pedal mechanism sends inputs to the brake metering
valves. The system is pressurized by hydraulic system. The hydraulic system results in a 3000 PSI
pressure.

At the main gear the alternate brake metering valves are fitted together. The metering slide of each
brake metering valve systems is connected up to a rotation of crank assembly to the meter hydraulic
pressure. When the pedals are pushed, cables are used to open the metering valve slide. Directly the
pressure port opens and provides the pressure to the brakes. Because of a feedback force, the valve
is closing. The force provides a feeling to the pedals. When the pedals are released the pressure valve
slide opens.

For taxiing there is a brake feel augmenter connected on each normal brake metering valve to im-
prove brake pressure controlling. The brake feel augmenter is only fitted on the normal brake sys-
tem.
ad 2 Alternate brake system
The alternate brake system is powered by hydraulic system B and results in a 1500 PSI pressure. The
alternate brake selector valve and system A supply pressure to the alternate brakes. When system B
is lost, the valve will open to use system A pressure. After takeoff when the landing gear is retracted,
system B hold the selector valve closed. The alternate brake metering valves stop the rotation of the

1
2

3
Figure 1.12 Brake system

1. Rotor
2. Stator
3. Pressure plate

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wheels. Normally the system B holds the select valve closed preventing system A pressure from en-
tering the alternate braking system. When system B pressure is lost the valve opens with pressure of
system A and closes the alternate braking system to landing gear retraction pressure.
ad 3 Accumulator brake system
The accumulator brake system is used for emergency brake pressure when hydraulic system A and B
is lost. When all the hydraulic power is lost, the accumulator brake system can controlled six brake
systems. A accumulator brake system use mechanical force to store pressurized energy and provided
back-up power for the brakes. When system B and system A lost pressure, the accumulator brake
system valve opens and provide several applications of brake power through the normal brake lines.
1.2.5 Wheels/Tires
The Boeing 737-300 has totally six wheels, to withstand high rolling speeds. To keep the strength of
the gear, wheels are provided (1.2.5a). To damp the little vibrations rubber tires are mounted around
the wheel (1.2.5b).
1.2.5a Wheels
For the dynamic balances of the split-typed wheel (Figure 1.13), balance weights (1) are provided.
The wheel is split-typed to make the mounting of the tire possible. To keep the wheel together the
inner (2) en outer (3) wheel are fastened together by 16 secured bolts (4). To prevent the in-
ner/outer wheel connection from leaking packing is mounted on each side. In the middle of the
wheel is an axle (5) for the wheel to make it spin, this is provided with a seal that keeps the lubricant
inside and keeps the dirt and moisture outside.

Figure 1.13 Wheel



3
2

5




1

4
1.2.5b Tyres
The main landing gear is provided with tubeless tyres and designed to withstand the forces to 195
knots. When an aircraft is making a rejected take-off (RTO), the breaking will generate a lot of heat
which that must be cooled (figure 1.14). This is the reason why four thermal relief plugs (1), equally
located and mounted on the inner wheel half (2), are protecting the wheel from excessive brake
heat, which otherwise will result in a blowout trough the increase of air pressure. This is made possi-
ble due to the inner core of the thermal relief plug, which is made off fusible metal like magnesium.
It has the characteristic that it melts at a predetermined temperature, releasing the air in the tire.
1. Balance weights
2. Inner wheel
3. Outer wheel
4. Secured bolt
5. Axle


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In the inner wheel (figure 1.15) there is a valve (1) extended (2) to the outer wheel used to inflate
the tubeless tire. There is also an over inflation plug installed in the inner wheel half (3) and goes half
out trough the outer wheel (4). It prevents over inflation by means of a seal that breaks when over
inflation occurs, it will deflate the tyre to ‘zero pressure’. 3412

Figure 1.15 Valve extension

1.2.6 Nose landing gear shimmy
A wheel is said to shimmy when it oscillates about its vertical axis. It can be caused by a lack of tor-
sion stiffness in the gear, improper wheel balancing, worn parts and difference in tire pressure
(1.2.6a). Because of the dangerous consequents of shimmy, the B737-300 is extended with a stan-
dard hydraulic shimmy damper (1.2.6b).
1.2.6a Explanation of shimmy
Steerable nose wheels are mostly vulnerable to shimmy, because of the single castored wheel, vari-
ous methods are used to damp it. The exact cause of shimmy is very complex due to the dynamics
elasticity of the tire. The wheel gets unbalanced, when a critical low friction force is applied to the
wheel, because of a minimum contact area. This force is getting critical when the weight on the front
tire is too low. During the landing, the aircraft has a high velocity, which cause the nose gear to act as
a gyroscope. With the property of precession the gear wants to balance the wheel, but unfortunately
it makes the wheel more unstable to knock the wheel to the other side.
1.2.6b Solution of shimmy
Reducing the shimmy effect on the nose gear can be done by improvement of certain elements on
the landing gear system. Shimmy can be reduced in several ways:

1. Provision of a hydraulic lock across the steering jack piston
2. Fitting a hydraulic damper
3. Fitting heavy self-centering springs
4. Double nose wheels
5. Twin contact wheels
ad 1 Provision of a hydraulic lock across the steering jack piston
In the steering mechanism of the aircraft (figure 1.16) situated at the shock strut are two actuators
also called steering jacks (1). These steering jacks directs the directional control of the nose wheel
strut (2). Shimmy mainly occurs at an high velocity when the forces are noticeable. To reduce
shimmy the hydraulic pressure (3) must be locked. When closing the restrictors (4) the piston (5)

Figure 1.14 Tire

1.Thermal plug
2. Inner wheel
3. Outer wheel
4. Heat shield
3
2
1
4
1. Valve
2. Extension
3. Inner wheel half
4. Outer wheel half

3

4
1

2

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cannot move in the steering cylinder. Doing this the nose wheel strut can no longer be turned and
shimmy no longer occurs.





Figure 1.16 Steering mechanics of the nose wheel.
ad 2 Fitting a hydraulic damper
In the same situation like above, the valve will keep a pressure of 120 psi. Unlike the hydraulic lock
the piston will not be locked but there will be a small movement with a lot of resistance, with the
result that the force will be damped. This is an advantage because the forces are not applied on the
construction materials that causes the metals to fatigue.
ad 3 Fitting heavy self-centering springs.
These springs are meant for centering the nose-wheel, using the characteristics of a spring. The
forces in direction of the centerline provide a self centering system, when shimmy shows up.
ad 4 Double nose-wheels
The double nose-wheel on the Boeing will lead to an increase of contact area and therefore ground
friction. This increase will lead to a decrease of the chance of shimmy.
1.3 Related Systems
Besides normal landing gear systems, the landing has also features for optimum operation. For a
controlled deceleration rate, the auto-brake system can be used (1.3.1). Because the pilot gets no
feedback from the speed of the wheels, he does not know if the tires are rolling or skidding. To pre-
vent the wheels from skidding, the anti-skid system releases brake pressure of the brakes (1.3.2).
The air/ground Logic system is a system that indicates the touchdown of the 737 (1.3.3). When the
main extraction of the landing gear fails, a manual extension system is used (1.3.4)
1.3.1 Auto braking
An auto brake system provides direct braking after touchdown of the aircraft (1.3.1a). The system
measures and regulates a selected deceleration, by controlling the brake pressure (1.3.1b).
1.3.1a Function Auto brake
The main function of the auto brakes is to stop the aircraft after touchdown or Rejected Take-Off
(RTO) with a controlled deceleration rate. The auto brake system in a B737-300 got two different
modes. These modes are:
1. Landing mode
2. Rejected Take-Off mode (RTO)
ad 1 Landing mode
For the landing mode of the auto brake, the pilot has four different auto brake levels to choose from.
These levels are 1, 2, 3 and MAX and can be chosen by an selector switch. The pilot will arm this level
before the touchdown. During the rollout the auto brake system will control the braking at the de-
sired rate and compensates for effects like drag. This auto braking starts when the thrust levers are
IDLE and the rotating speed of the wheel is measured. The system stops when the roll out is ended or
when the pilots take over control.

1. Steering jack
2. Nose weheel strut
3. Hydraulic pressure
4. Restrictor
5. Piston
1
5
2
3
4

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ad 2 Rejected Take-Off mode (RTO)
The auto brake in the B737-300 consists of the landing mode, but also of the RTO mode. This last
mode is used to stop the aircraft after a rejected take-off. The RTO auto brake system is the primary
way to stop the aircraft during a RTO. When pilots arm the RTO auto brake system, the auto brake
system will directly initiate full brake pressure after the thrust levers are back to the fully retarded
position or thrust reverse is used. The system then checks the wheel speed to determine auto brake
deceleration rate, this will limit the crew’s handling activities during the RTO. Stopping distance and
the risk to overrun the runway will be minimized by the system.
1.3.1b Auto brake Operation
Operating the auto brake starts with the selection
made by the pilot. A Boeing 737-300 auto brake sys-
tem has a selector switch in the center of the instru-
ment panel (figure 1.17). This panel contains an auto
brake disarm light (1). This light is illuminated in the
following conditions:
- During RTO or landing the speed brake lever
is move to down detent.
- Manual brakes are used.
- Thrust lever(s) advanced.
- Landing with RTO mode selected. (light will
illuminate after two minutes)
- RTO mode selected, while on ground. (extin-
guishes after one to two seconds)
- Malfunction in auto brake system.

The light is out when the auto brake switch is set to off or if the auto brakes are armed, and nothing
is wrong.
The second function on the panel is the auto brake selector switch (2). The switch has five different
options, separated into three categories. The first is OFF, this means the auto brake system is turned
off. The second is the auto brake landing category. This category contains four selections, named: 1,
2, 3 and MAX. The pilot uses them to select the desired rate of deceleration. Using the thrust revers-
ers will still hold the deceleration rate. This means the brake pressure can be lowered. Using thrust
reversers will however not lower the brake pressure when option MAX is selected. When using the
option MAX, the switch must be pulled to select it. The last possible selection is RTO. This selection is
used prior to start. When applied the system will automatically use maximum brake pressure when
thrust is IDLE and speed is at or above 90kts.

The selection made by the pilot will go to the Auto brake control module. This module measures and
controls the brake pressure that goes with the desired deceleration rate. The control module gets its
information about wheel speed and deceleration rate from the antiskid/auto brake control unit.
1.3.2 Antiskid System
The antiskid system prevents the tires from skidding over the runway (1.3.2a). It measures the dece-
leration of each wheel and releases brake pressure on the wheels (1.3.2b).
1.3.2a Function antiskid system
The antiskid system has different functions. This system is mainly installed to prevent the wheels
from blocking while the plane is de-accelerating. The stopping way is greatly increased when the tires
are skidding over the runway, instead of rolling. Another function of this system is to prevent the
breaks from blocking the wheels before touchdown. This insures that the wheels are rotating before
they are being slowed down by the breaks.


1



2







Figure 1.17 Auto brake selector switch
1. Auto brake disarm Light
2. Auto brake selector
switch


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1.3.2b Antiskid operation
The antiskid system has influence on the
amount of pressure given to each of the landing
gear brakes (Figure 1.18). The system measures
the velocity of each wheel with the transducers
(1). These transducers send the signal of the
rotating speed to the antiskid auto brake con-
trol unit (AACU) (2). This control unit has the
actual groundspeed for reference. It compares
the rotating speeds of each wheel with the other
wheels of the landing gear. When one wheel is
de-accelerating faster than the other wheels,
the AACU sends a signal to the antiskid control
valve (3). This valve is located after the brake
valves of the hydraulic system. The antiskid con-
trol valve will narrow, so that less hydraulic
brake pressure (4) is going to the brake. As a
result, the rotating speed of the wheel increas-
es, until this is the same speed as the other
wheels.
1.3.3 Air/Gound Logic
The air/ground Logic system is a system that indicates the touchdown of the Boeing 737-300 (1.1.2a).
The system has sensors on the actuators (1.1.2b).
1.3.3a Function Air/Ground Logic System
The function of the air/ground logic system is to determine if the airplane is flying or is located on the
ground. This information is distributed to many systems in the airplane. Those systems are for in-
stance ground spoilers, auto breaking or the anti-collision system. The air/ground logic system will
arm the warning systems which are only applied on ground. For instance the deployment of the anti-
ice. This could damage the wings on the ground. The system is also used for preventing pilot errors,
such as retracting the landing gear while standing on the ground.
1.3.3b Air/Ground Logic system operation
The air/ground Logic system contains six sensors. Each strut has two sensors, one for system one and
the other for system two. Both systems send a signal of the condition of the strut to the proximity
switch electronics unit. This unit is connected to all of the systems that are in need of the position of
the B737. An indication of failure is a warning light in the cockpit.
1.3.4 Manual Gear Extension
The Manual Gear Extension will only be used if the gear cannot deployed. If that happens, the pilots
take some preparations prior to the manual extension (1.3.4a). After the preparations the gear will
be extended by gravity (1.3.4b). Subsequently the pilot needs to check if the gear is lowered proper-
ly and everything is in place (1.3.4c).
Figure 2.18 Antiskid operation
1. Transducer
2. AACU
3. Antiskid control
valve
4. Hydraulic brake
pressure

4




3


1
2

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1.3.4a Function
The manual gear extension will be used in case of a failure of hydraulic system A. System A is the
hydraulic system that controls the landing gear extension and retraction. During normal operation
the pilot will lower the gear by pulling
the gear lever down. If one or more of
the gears fails to extend, the emergency
manual gear extension has to be used.
The manual gear extension handles can
be found on the cockpit floor at the F/O
side (Figure 3.19). There are three
switches that can be pulled out (1). The
middle one controls the nose wheel, the
left one controls the left main gear and
the right one controls the right main
gear. The switches are connected by a
cable which will be connected to the
landing gear (2).
1.3.4b System Operation
Both main gears are working identical (figure
1.20). The cable (1) which comes from the
manual gear extension handles goes to a hook
(2). If the cable is pulled to its limit, about 45
cm, the up-lock hook which hold the gear up
and the gear falls down by gravity. The gear
will become in down-locked position (3), this
means the gear is locked and cannot move.
The nose gear works slightly different. The
cable should be pulled approximately 20 cm,
then the up-lock will be released and the gear
falls down. The nose gear doors will be opened
by the weight of the nose gear. The gear will
continue to fall by the gravity until it is locked
in down-lock position.
1.3.4c Verifying
After an alternate extension the pilots must verify if the gear is down and locked. Therefore there are
small windows installed. The window for the nose gear is placed at the cockpit floor nearby the cock-
pit door. The window is called nose gear viewer. If the pilots take a look through the viewer they can
see two parts of the down-lock strut. On these parts are two red arrow heads shown. Indication that
the nose gear is down and locked is provided by observing that the two red arrow heads are in con-
tact.
There are two windows for the main gears. These are installed at the passenger cabin floor nearby
the wing emergency exit. On the main gear side struts and down-lock are red paint stripes painted.
The gear is down and locked if the stripes are aligned.
1.4 Legal Requirements
The European Aviation Safety Agency (EASA) is the authority in Europe that defines the rules of an
airplane. The Certification Specifications-25 (CS-25) shows the airworthiness regulations of EASA for
turbine powered large aeroplanes with a Maximum Take Off Weight (MTOW) of 5700 kilograms (kg)
or more. The group 2A2G, technical engineers of the university of applied science in Amsterdam,

Figure 3.19 Manual Gear Extension Handles

1


2



Figure 1.20 Main Gear Extension
1




2






3
1. Manual Gear levers
2. Cables to landing gear
1. Cable
2. Hook
3. Down Locked Position

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need this airworthiness regulations to analyse the landing gear. There are an amount of general regu-
lations for a landing gear.
- The landing gear system must be designed so that when it fails due to overloads during take-
off and landing, the failure mode is not likely to cause spillage of enough fuel to constitute a
fire hazard. The overloads must be assumed to act in the upward and aft directions in com-
bination with side loads acting inboard and outboard. In the absence of a more rational
analysis, the side loads must be assumed to be up to 20% of the vertical load or 20% of the
drag load, whichever is greater.
- The aeroplane must be designed to avoid any rupture leading to the spillage of enough fuel
to constitute a fire hazard as a result of a wheels up landing on a paved runway, under the
following minor crash landing conditions:
1. Impact at 1,52 m/s (5 fps) vertical velocity, with the aeroplane under control, at
Maximum Design Landing Weight,
i. With the landing gear fully retracted and, as separate conditions,
ii. With any other combination of landing gear legs not extended.
2. Sliding on the ground, with-
i. The landing gear fully retracted and with up to a 20° yaw angle and, as sepa-
rate conditions,
ii. Any other combination of landing gear legs not extended and with 0° yaw
angle.
- For configurations where the engine nacelle is likely to come into contact with the ground,
the engine pylon or engine mounting must be designed so that when it fails due to overloads
(assuming the overloads to act predominantly in the aft direction), the failure mode is not
likely to cause the spillage of enough fuel to constitute a fire hazard.

These regulations are the general regulations of a landing gear which stands in the CS-25.721. The
airworthiness regulations of a landing gear can also be divided in the requirements of landing gear
system (1.4.1), maintenance requirements (1.4.2) and minimum equipment list (MEL) (1.4.3).
1.4.1 Requirements landing gear system
The landing gear is built on several subsystems. For these subsystems there are separate regulations.
The CS-25.729 gives an overview for the regulations of the retract mechanism (1.4.1a). The CS-
25.723. shows the regulations for shock absorption and how the landing gear can be test with this
shock absorption (1.4.1b). The CS-25.745 gives an overview of the regulations of steering (1.4.1c).
These regulations are limited to nose wheel steering. Also there different kind of brakes and braking
systems on the landing gear. The CS-25.735 shows the regulations of the brakes and braking system
(1.4.1d). The CS-25.731 and the CS-25.733 shows the regulations of the wheels and tyres (1.4.1e).
1.4.1a Retract / Extend system
A retractable landing gear on an aircraft, including; the retracting mechanism, the wheel well doors
and supporting structure, has to apply the requirements that are made in EASA Certification Specifi-
cations number 25 (CS-25).
A landing gear is designed for:
- The loads occurring in the flight conditions, when the gear is in retracted position.
- The combination of friction loads, inertia loads, brake torque loads, air loads and gyroscopic
loads which will be the effect of the wheels rotating.
- Yawing maneuvers of the aircraft.

A landing gear is required to have:
- Positive means of staying extended when the aircraft is at the ground or in the air.
- Emergency means in case any element in the retraction system or the energy supplying sys-
tem fails.

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- A retracting position indicator, if the landing gear is retractable.
- Protection to elements of the aircraft which can be affected by damaging effects of, for ex-
ample; a bursting tire, a loose tire or possible wheel brake temperatures.
1.4.1b Shock absorption
CS-25 includes only a shock absorption test for the requirements of the shock absorbers. When test-
ing the shock absorbers, energy absorption tests will be executed:
- by using the maximum landing weight of the aircraft, because of the maximum landing im-
pact energy.
- by paying attention to the attitude of the landing gear and appropriate drag loads on the air-
plane when landing related to the limit loads
The landing gear may not fail when demonstrating its reserve energy absorption capacity. The analyt-
ical representation has to be valid for the design conditions specified in CS 25.473.
1.4.1c Steering
Certifying the nose wheel steering system is done by five requirements included in CS-25:
- The nose wheel steering system must be so designed that exceptional skill is not required for
the handling of it, during take-off and landing, including the case of cross-wind and in the
event of a sudden power-unit failure at any stage during the take-off run. This must be
shown by tests (AMC 25.745)
- It must be shown that, in any practical circumstances, the steering columns in the cockpit
can’t interfere with the landing gear when retracting or extending it.
- When a failure shows up, the arrangement of the system must be such that no single failure
will result in a nose wheel position which can lead to a hazardous effect.
- The designs of the attachment for towing the airplane on the ground may not cause damage
to the steering system.
1.4.1d Brakes
After the touchdown of an aircraft the brakes and the braking systems are the most important parts
of the landing gear. In the EASA there are many regulations about the brakes and braking systems.
The following regulations for brakes and braking systems are:
- Each assembly consisting of a wheel(s) and brake(s) must be approved.

The brake system, associated systems and components must be designed and constructed so that:
- If any electrical, pneumatic, hydraulic, or mechanical connecting or transmitting element
fails, or if any single source of hydraulic or other brake operating energy supply is lost, it is
possible to bring the aeroplane to rest with a braked roll stopping distance of not more than
two times that obtained in determining the landing distance as prescribed in CS25.125.
- Fluid lost from a brake hydraulic system following a failure in, or in the vicinity of, the brakes
is insufficient to cause or support a hazardous fire on the ground or in flight.

The brake controls must be designed and constructed so that:
- Excessive control force is not required for their operation.
- If an automatic braking system is installed, means are provided to arm and disarm the sys-
tem, and allow the pilot(s) to override the system by use of manual braking.

There are also separate regulations for the parking brake of the aeroplane.
- The aeroplane must have a parking brake control that, when selected on, will, without fur-
ther attention, prevent the aeroplane from rolling on a dry and level paved runway when the
most adverse combination of maximum thrust on one engine and up to maximum ground
idle thrust on any, or all, other engine(s) is applied. The control must be suitably located or

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be adequately protected to prevent inadvertent operation. There must be indication in the
cockpit when the parking brake is not fully released.

If an anti-skid system is installed the following regulations are applied:
- It must operate satisfactorily over the range of expected runway conditions, without external
adjustment.
- It must, at all times, have priority over the automatic braking system, if installed.

The other regulations of the brake and braking system can be found in CS-25.735.
1.4.1e Wheels/Tyres
To make a movement on the ground there are two parts needed to ensure that. These parts are
wheels and tyres.
1. Wheels
2. Tyres
ad 1 Wheels
The wheel of a landing gear is an important part. To make sure that nothing will happen with the
wheel there are also regulations for this part. These regulations are from the CS-25.731.
- Each main and nose wheel must be approved.
- The maximum static load rating of each wheel may not be less than the corresponding static
ground reaction with design maximum weight and critical centre of gravity.
- The maximum limit load rating of each wheel must equal or exceed the maximum radial
limit load determined under the applicable ground load requirements of this CS-25.
- Overpressure burst prevention. Means must be provided in each wheel to prevent wheel
failure and tyre burst that may result from excessive pressurisation of the wheel and tyre as-
sembly.
- Braked wheels. Each braked wheel must meet the applicable requirements of CS-25.735.
ad 2 Tyres
To make a movement on the ground only a wheel is not enough. There is also a tire. To know what
kind of tire there must be used, are there an amount of regulations for tires. These regulations can
be found in the CS-25.733.
- The applicable ground reactions for nose wheel tyres are as the static ground reaction for the
tyre corresponding to the most critical combination of aeroplane weight (up to maximum
ramp weight) and centre of gravity position with a force of 1∙0 g acting downward at the cen-
tre of gravity.
- When a landing gear axle is fitted with more than one wheel and tyre assembly, such as dual
or dual-tandem, each wheel must be fitted with a suitable tyre of proper fit with a speed rat-
ing approved by the Agency that is not exceeded under critical conditions, and with a load
rating approved by the Agency that is not exceeded by the loads on each main wheel tyre
corresponding to the most critical combination of aeroplane weight (up to maximum weight)
and centre of gravity position, when multiplied by a factor of 1∙07.
- Each tyre installed on a retractable landing gear system must, at the maximum size of the
tyre type expected in service, have a clearance to surrounding structure and systems that is
adequate to prevent unintended contact between the tyre and any part of the structure or
systems.
- For an aeroplane with a maximum certificated take-off weight of more than 34019 kg, tyres
mounted on braked wheels must be inflated with dry nitrogen or other gases shown to be
inert so that the gas mixture in the tyre does not contain oxygen in excess of 5% by volume,
unless it can be shown that the tyre liner material will not procedure a volatile gas when

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heated, or that means are provided to prevent tyre temperatures from reaching unsafe lev-
els.
These regulations are in application for the Boeing 737-300. The details en the rest of the regulations
about tyre can be found in CS-25.733.
1.4.2 Maintenance requirements
When continuing airworthiness of a certified aircraft, maintenance has to take place by periods of
time. Therefore, different rules and requirements have been set up by the EASA to maintain aircraft
certified. EASA part M is the primary legislation when looking at the maintenance requirements of
aircraft. Part M contains the measures which will have to continue airworthiness and the conditions
required to the staff in the process of maintaining an aircraft. Part M subpart G contains the Continu-
ing Airworthiness Management Organisation Approval (CAMOA). This organisation is certified to
approve Airworthiness Review Certificates (ARC) of aircraft, which will determine either the aircraft is
airworthy for a maximum of one year or not. The staff which directly run the maintenance of the
aircraft must be certified by the requirements set up in part 66 – Certifying staff.

Each aircraft has its own mechanical and electric systems and so its own maintenance manual. For
this reason, each aircraft has its own Aircraft Maintenance Plan (AMP). This plan describes how to
maintain each particular system on the aircraft, step by step. This AMP is been set up by using the
maintenance recommendations of the producer of the concerning aircraft.

Maintaining the landing gear system of the Boeing 737-300 is done by following the AMP, as written
above. A maintenance schedule of an Boeing 737-300 (appendix I) is given to show in which way the
particular system has to be checked. Each complicated part of the landing gear system has its own
checklist, for example; the main- and nose landing gear shock strut and the nose wheel steering me-
chanism. Each system has its own time interval in which this check is being executed. These intervals
can be expressed in several parameters of time:

- Flight hours – FLH
- Flight cycles – CYC
- Days – DAY
- Months – MTH
- Years – YRS
- Trip check – T

These maintenance requirements of the landing gear system will be reviewed in chapter 3, when
looking at the effect of several errors on the maintenance program.

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1.4.3 Minimum Equipment List (MEL)
The Minimum Equipment List (MEL) is a document developed for a specific aircraft set up by the
manufacturer and is approved by the EASA in Europe and by the FAA in the United States. It lists the
instruments and equipment (1) that may be inoperative without harming the safety of the aircraft.
The MEL also includes procedures for flight crews to follow when the instruments and equipment are
deactivated inoperative. In appendix (II) a complete overview of a MEL for the landing gear of a Boe-
ing 737-300 is given. In figure 1.21 is a part of a MEL for a landing gear for a Boeing 737. The number
in the MEL shows the number of specific component (2) and shows in category (A-D) (3) when it
needs to be repaired. The minimum number required for dispatch show the minimum number of
components that is needed to be operative (4). Also, there are some remarks (5) by the numbers
required for dispatch. These remarks gives a specific overview which components had to be opera-
tive.


Figure 1.21 MEL
1. Component
2. Number of specific com-
ponent
3. Category
4. Number of component
required for dispatch
5. Remarks


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2 Forces B737-300
To study the failures of the landing gear of the B737-300, the external and internal forces must be
calculated. Because both failures have once been taken place during landing, the external forces will
be calculated in a no-wind landing (2.1) For the maximal crosswind landing, external and internal
forces will be determined (2.2). As a result, the materials and treatments for those materials can be
chosen (2.3). A conclusion of this forces study is given (2.4).
2.1 No-wind landing forces
To calculate the no-wind landing forces the first thing that has to be determined is the centre of grav-
ity (CG) (2.1.1). The CG is the average location where all the forces come together and changes dur-
ing flight or loading the aircraft. The calculation of the CG and the forces during a no-wind landing
can be done by using different formulas (2.1.2). For calculating the no-wind landing forces the meas-
urements of the aircraft are used. The no wind landing forces can be divided into two landing parts.
After landing the aircrafts main and nose gear are both on ground (2.1.3). Before rolling on ground
the main gear touches the ground first (2.1.4).
2.1.1 Centre of gravity
The CG is the average location where all the forces come together. During the flight the CG will
change, due to fuel consumption and passenger movements. To calculate the landing forces, the CG
will be determined in most forward position to get the highest critical landing force on the landing
gear. The limits of the CG have been expressed in a rate of the mean aerodynamic chord (MAC). The
MAC is located on the reference axis and is the chord of a rectangular wind. The MAC referenced to
the location of the aircraft’s centre of gravity (appendix III).

The following formula gives the measurement (chord) of the MAC (formula 2.1)(figure 2.1).It does
not give the span wise location of the MAC (appendix IV). The CG can be measured from any point
along the span from the leading edge of the wing if the wing has a constant chord with no sweep.
The MAC value and the position of MAC can be calculated.






(2.1)
C
t
= Wingtip chord
C
r
= wind root chord
S = halve wingspan
H = distance to MAC from symmetry line

For the calculations of different situations the most forward position of the CG has been chosen. If
the CG is in the most forward position the aircraft is most stable. Because the nose heaviness mo-
ment should be collected by the tail heaviness moment. The negative lift of the stabilizer increases.
To move the aircraft around its CG a bigger negative force around the elevator must be neutralize
this.

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Figure 2.1 MAC

2.1.2 Theories and formulas
For a clear understanding of the formulas and forces the basic knowledge of mechanics is essential.
Most of the force calculations are based on the laws of Newton:
- First law of Newton: An object or particle that is in rest or moves with a uniform motion will
continue to do so, unless a force is subjected to it (2.2).
- Second law of Newton: An object or particle that is subjected to a net force will experience
an acceleration or deceleration proportional to the force and inversely proportional to its
mass.
- Third law of Newton: When two objects interact, they both produce a similar, opposed and
collinear force (so called action is reaction). For calculating the aircraft landing forces the
third lay is not used.

The three laws have different formulas to calculate forces. A force is any influence that causes a free
body to undergo an acceleration.






(2.2)
F = force (N)
m = mass (kg)
a = acceleration (m/s
2
)


To calculate moments on the aircraft the Newton’s law using a zero formula. With this formula forces
can be calculated in different directions.






(2.3)
F
x
= horizontal force (N)
F
y
= vertical force (N)
M = moment (N/m)

To calculate the acceleration speed the following formula is used. The distance between the shock
strut extended and extracted when touching the ground is 0,356 m.


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(2.4)
V
t
= vertical speed reaching the ground (m/s)
V
0
= vertical speed descending (m/s)
a = acceleration / deceleration (m/s
2
)
ds = distance difference between the shock
strut extended and extracted when touching
the ground.


The roll forces working horizontally as external forces can be calculated with the following formula.
The μ is the cause of friction and is the force resisting the relative motion of solid surfaces.





(2.5)
Frol = Vertical force during rolling (N)
F
n
= Vertical landing force (N)
μ = cause of friction (0,4)

2.1.3 Aircraft standing on ground
When the aircraft is on the ground, the weight of the aircraft is the only force that is working on the
aircraft. The main landing gear carries more weight than the nose landing gear because the centre of
gravity is closer to it. The arm between the CG and the main landing gear is smaller than the arm
between the CG and the nose landing gear. With the theory of all the moments must be zero, the
forces can be calculated. The force of the main landing gear needs to be bigger to compensate the
bigger arm between the nose landing gear and the CG. That is the reason that the main landing gear
consist of more struts and more wheels than the nose landing gear. The arm between the CG and the
nose gear is 9,03 meters. The arm between the CG and the main gear is 3,47 meters.

When the aircraft is standing on ground with the CG in most forward position, the distance to the
nose wheel is 9.03 meters (figure 2.2). The distance from the CG to the main landing gear is 3.47
meters. According to the maintenance manual (appendix V) the maximum landing weight of a Boe-
ing 737-300 is 51,709 kg (507,260.29 N (F
MLW
)). Using the zero formula the forces on the landing gear
can be calculated. This results in a 140,845.44 N force (F
n,nose
)

for the nose landing gear and a
366,450.61 N force (F
n,main
) for the main landing gear.

Figure 2.2 Aircraft on ground during landing
2.1.4 Aircraft touchdown
During touchdown the main landing gear touches the ground first. When the main landing gear
touches the ground, there are several forces that work on the main landing gear (figure 2.3). There is

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a vertical force that needs to be absorbed by the shock strut. The shock strut should be able to ab-
sorb a maximum vertical landing force of 3.70 m/s vertical speed (CS-25).
The normal landing speed of a B737 is 135 kts IAS (69.449 m/s). When the aircraft touches the
ground the pilot flayers the planes up to make a comfortable landing. This landing is done with an
vertical speed of 3 fps (0.9144 m/s)

The acceleration speed can be calculated with formula 2.4.





 


   





The negative value means there is a deceleration. With the deceleration the force can be calculated.

 

External vertical forces on the main landing gear are the forces of the landing with the vertical speed
plus the amount weight of the aircraft. The total landing force on the main landing gear is
60721.8787 N. The maximum landing weight of the aircraft working from the centre of gravity:

The total amount of vertical forces is  . The landing
forces of a B737 are divided to both main landing gear:


= 283,993.5844 N

The roll forces working horizontal can be calculated with the landing forces on the main landing gear
(formula 2.5).


  is the external roll friction force working horizontally
on the main landing gear.

Figure 2.3 Aircraft touchdown
2.2 Crosswind Landing Forces
When landing with crosswind, wind from the sides, the landing process is different in comparison
with a no wind situation. The usual way to land an aircraft during crosswind is in a slip manoeuvre.
Landing during this manoeuvre will happen in three stages. Just before touchdown the aircraft will be
put straight. This will lower the upwind wing and will make the aircraft land on its upwind main land-
ing gear wheel (2.2.1). Directly after touchdown of the upwind wheel, the weight of the aircraft will
force the downwind main landing gear wheel to the ground (2.2.2). After the main landing gear has
complete contact with the ground, the nose gear can be lowered to the ground (2.2.3). At last, the
internal forces are calculated (2.2.4).

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2.2.1 Touchdown upwind wheel
The first stage of a crosswind landing is the landing on the upwind wheel. Assuming the lift the air-
craft still generates will compensate for the weight of the aircraft, the only vertical force on the land-
ing gear during the landing with the upwind wheel is the vertical speed impact. This vertical force can
be calculated with formula 2.2.

To maximize the force on the upwind wheel during the crosswind landing, the maximum landing
weight (MLW) of the B737-300 is used. This weight is limited by the aircraft strength and airworthi-
ness requirements.

In order to calculate the acceleration at the point the aircraft touches down with 1 wheel, the mini-
mum required sink rate the aircraft's landing gear has to handle will be used. At maximum design
landing weight, this minimum required sink rate is 10fps (3.048 m/s). Based on CS criteria this sink
rate will probably deliver an Hard landing as a normal landing will only produce sink rates between 2
and 3 fps. An hard landing will discomfort the passengers.

Now the vertical speed is known, the vertical acceleration or deceleration can be calculated. This will
happen with the formula 2.4.
By filling in the formula, the acceleration or deceleration speed will be found:




    
 


   





The outcome is negative, which means it's deceleration instead of an acceleration. This is obvious,
because the aircraft is landing and not climbing. This situation is also based on a landing with consis-
tent speed, so the force on the aircraft is consistent as well.

Now the landing weight and the deceleration rate is calculated, the vertical force on the upwind
wheel can be calculated. Formula 2.1 is used for this calculation.
 
 

The external force on the main landing gear during the first stage of the crosswind landing (figure
2.4), is the force created by the vertical speed during the landing. Because the lift during this stage is
sufficient to counteract the gravitation, the vertical speed force is the only force acting on the main
landing gear during this situation. Because the first stage of the crosswind landing holds a landing on
the upwind wheel, the complete vertical force doesn't have to be divided into two. This means the
complete vertical force of 674,699.032 N acts on the upwind wheel.

Figure 2.4 Aircraft touchdown upwind wheel

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1
2

3
2.2.2 touchdown downwind wheel
After the first contact on the ground by the upwind main landing gear, the downwind landing gear
touches down. Assuming that the pilot will instantly open the ground spoilers, the lift of the wings
are reduced to zero. Also the lift of the horizontal stabilizer is reduced to zero.. The forces on the
landing gear can now be separated into two parts, namely:
1. Crosswind component
2. Forces on main landing gear
Ad. 1 Crosswind component
The crosswind component can be defined by the drag of the wind on the flank of the B737 (formula
2.6)

F =
(2.6)
F = Force (Newton)
ρ = density of the air (kg/m
3
)
v= velocity (m/s)
C
D
= Drag constant
S = Surface (m
2
)



The two main surfaces that are influenced by the crosswind are the fuselage and the horizontal stabi-
lizer. The C
D
value of the flank of the fuselage is 1 and the surface area is 4,01 x 29,54. The force of
the wind on the fuselage:


 

The force on the vertical stabilizer:


 

The total horizontal force of the wind is 34,506.49 Newton. The wind pushes the airplane at a certain
height: 1,957 meter above the centre of gravity.
Ad. 2 Forces on main landing gear
With the calculations of the wind and with the gravity
the forces on the main landing gear (figure 2.5). The
momentum of the wind practicing on the CG is
 . The gravity is the
maximum landing weight, 51,709 Kg. This force can be
divided equally over the two struts. Each strut has


  to bare. The force of
the wind is pushed on the downwind main landing
gear, so the force is the momentum of the wind dived
by the arm of the downwind gear:
 


 . This extra force can be added to the
253,632.645 N. The result is 279,456.445 N. The same
force must be deducted of the upwind main LG, that results in 227,813.73 N.

Besides the vertical forces, the wind is pressing the aircraft horizontally. That force is divided over the
three landing gears. The distance between the CG and the main landing gear is 3,47 meter, the dis-
tance between the CG and the nose landing gear is 9,03 meter. The total wind force is 34,506 N.

1. 0.85 meter
2. 1.957 meter
3. 2.615 meter
Figure 2.5 Crosswind on B737

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3,47 34,506 = Force of nose gear 9,03.
The force on the nose gear is 13,250 N
The force on one main landing gear is (34,506 – 13,250) / 2 = 10,623 N
2.2.3 Nose Gear
After the pilot has managed to get the main landing gear on the ground, the nose landing gear will
descent to the ground. The force of the wind stays the same, but the force of the gravity is distrib-
uted differently. Therefore, the calculations made in the same situation without the crosswind ap-
plies. The forces of the crosswind are then added, so the Force on the upwind LG is 183,225.31 –
25,823.8= 157,401.51 N. The forces on the downwind strut is 183,225.31 + 25,823.8 = 209,049.11 N.
The force on the nose gear stays 140,845.44 N.
2.2.4 Internal forces
After the calculation of the external forces, the internal forces can be determined. The most forces
are practised on the downwind main landing gear during a crosswind landing. Therefore, the internal
forces on the nose gear (2.2.3a) and the main gear (2.2.3b) are calculated.
2.2.4a Nose landing gear internal forces
As stated earlier the force on the nose gear is 140,845.44 N
(figure 2.6). This force is the vertical force in A. The horizon-
tal (crosswind) force in A is 13,250 N. This force is trans-
ported to beam B-C and B-D. Therefore the horizontal force
on both of these beams is


 . The
vertical component of both beams is then
  . Combined this force is
= 18,555.5 N This force will be deducted from
the total vertical force. The vertical force on beam B-E is
then
 

 .

2.2.4b Downwind landing gear internal forces
The forces of the particles of the main landing gear (figure 2.7) can be calcu-
lated. The vertical force in A is 279,461.3 N. The horizontal force in A is
10,623 N. The horizontal force is transported to beam C-D, so the force in C-
D is


 . The Vertical component of C is
 . This force can be deducted from the vertical force on
A to obtain the vertical force on B:  .
2.3 Forces on Materials
There are also requirements for materials compiled by EASA. if the material
meets all requirements, the materials will also be tested (2.3a). There are
only a few materials which are suitable for the landing gear. The ma-
terials are mostly composed chemical elements. These materials are
alloys (2.3b). The alloys will be stronger through a heat treatment and a surface treatment (2.3c).
2.3a Material Requirements
The materials selection is an important step of the design. Most accidents are caused by human er-
rors or wrong materials. To prevent this, there are requirements that are listed in CS-25. According to
CS-25 the materials must be of high strength and stiffness, low weight, and have good machinability,

Figure 2.6 Nose landing gear scematic
Figure 2.7 Main landing gear scematic

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weldability, and forgeability. They also must be resistant to corrosion, stress corrosion, hydrogen
embrittlement, and crack initiation and propagation. All these requirements needs to be established
on the basis of experience or tests. Conform to approved specifications, that ensure their having the
strength and other properties assumed in the design data.
2.3b Alloy Used
There are three types of alloys used for landing gears. These alloys meet all requirements of CS-25.
These alloys are:
3. Steel Alloy
4. Titanium Alloy
5. Aluminum Alloy
Ad. 1 Steel
The most used steel for a landing gear of a Boeing 737 is the 300M alloy. 300M is a low alloy, this
means that there is one predominant chemical element. In this case, iron. The total chemical compo-
sition of 300M alloy is listed in table 2.1.
Fe C Mn Si Ni Cr Mo Co
300M 94.0 0.46 0.75 1.65 1.80 0.80 0.40 -
Table 2.1
300M is vacuum melted steel of very high strength. This alloy has a very good combination of
strength (1,930 to 2,100 MPa), toughness, fatigue strength, and good ductility. It is a through harden-
ing alloy to large thicknesses. But it is very sensitive for corrosion. So the component of steel must be
coated by an anti-corrosion layer.
The 300M alloy is fabricated as a forging. Forging is a metalworking process, where the desired alloy
will be shaped trough a press. The advantage of forgings is that it will improve the strength characte-
ristics. This will also be used with Titanium and aluminum alloys.
Ad. 2 Titanium Alloy
The most used titanium alloy is Ti-AL6-V4, this is a non-ferrous alloy. This alloy is used throughout an
entire aircraft and landing gear. Because it has a light weight and a high strength (1150 MPa). It is
more resistant against fatigue than steel. It is also more corrosion resistant than steel. The big disad-
vantage of titanium is the high cost. But this is compensated for either by the advantages of weight
reduction due to the low density of the metal or by the increased life of the component due to high
corrosion resistance of the metal. The total chemical composition of Ti-AL6-V4 is listed in table 2.2.
The total characteristics of Ti-AL6-V4 are listed in appendix VI.

Table 1.2
Ad. 3 Aluminum
There are two types of aluminum alloy which are used for a landing gear of a 737. The alloys are 7075
and 7175. Both aluminum alloys are low cost. Aluminum alloy 7075 is the highest strength of alumi-
num alloys (530 Mpa). It has strength comparable to many steels, and has good fatigue strength, but
has less resistance to corrosion. The total chemical composition of AL 7075 is listed in table 2.3. The
total characteristics of AL 7075 are listed in appendix VII.

Table 2.3


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Aluminum 7175 is a variation of 7075 with less chemical components. Therefore it is more resistance
to corrosion, but it is weaker than AL 7075 (480 MPa). The total chemical composition of AL 7075 is
listed in table 2.4.

Table 2.2
2.3c Treatment of the Alloy
The alloys will be treated by heat treatment. There is a heat treatment applied on the alloy to im-
prove the mechanical properties. The heat treatment of landing gear is done in large gantry-type
atmosphere furnaces. The alloy will be heated to the temperature where the mechanical properties
are its best. The temperature of steel is 870 ºC. The temperature for titanium is 730 ºC. Then it will
cool down, this is done by oil. Oil cools slowly down, that avoids cracking of the material.
There is also a surface treatment this is mostly an anti corrosion layer. This layer protects the landing
gear against the nature elements.
2.4 Conclusion
There are several forces that work on the landing gear during landing. The biggest forces will be col-
lected by the main gear and during a crosswind landing. Those forces will be collected by a single
shock strut, this is 279,461.3 Newton. By a normal landing is this force 236,568.675 Newton.
The materials must resist all these forces, so therefore there are requirements listed. All these re-
quirements needs to be established on the basis of experience or tests, to assure that the material
can resist all forces of the landing, acceleration and deceleration during start and landing. Only the
strongest material alloys will be used, such as steel, titanium and aluminum. The material properties
will improve even more due to a heat treatment.

All those measures, like heat treatment and tests could prevent a lot of errors, but there are always
exceptions. Like a Continental airlines 737. There was an “overstressed torsion link” and a shimmy
damper problem onboard. Are these errors caused by wrong material choices? Or was it a mainte-
nance problem, and could the maintenance program solve and prevent these problems? These are
the questions that will be answered in chapter three. It is necessary to prevent or solve the problems
with the current maintenance program if ALA wants to expand, because every unknown error causes
a safety risk and a non affordable delay of an aircraft.


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3 Troubleshooting
Now that the forces are known and the material properties are discussed, the failures can be studied
(3.1). Those failures must be fixed during maintenance. To assure this failure will not happen again, a
good check of this fix must be performed (3.2). by knowing that, a good financial view of the problem
and fix can be set up (3.3). A final conclusion of these aspects will be given (3.4).
3.1 Failures
Gaining knowledge for the maintenance program and the costs of the maintenance is done by ana-
lysing two big errors on the landing gear system of a Boeing 737-300. The first problem that will be
described with use of cause and solution is the shimmy damper failure due to the hydraulic system
failure (3.1.1). The second error is about the torsion link which fails due to a loose apex nut (3.1.2).
3.1.1 Hydraulic shimmy damper failure
The first failure which will be analyzed is the failure of the shimmy damper due to a leaking hydraulic
system which caused pressure loss (3.1.1a). A shimmy damper is meant to resolve the shimmy effect
on the wheels of the landing gear. Shimmy is caused by critical forces appearing on the wheels, this
oscillation is being resolved using the hydraulic shimmy damper (3.1.1b).
3.1.1a Cause failure
The shimmy damper failure is caused by a failure of the hydraulic system. The hydraulic fluid can leak
out of the lines, or the lines are filled with air bubbles . When the system is missing the pressure , its
missing its power to operate and so the damper is not able to intercept these oscillations. In case of
a shimmy damper failure, when the force partition is critical, shimmy does take place.
3.1.1b Solution of the maintenance failure
In order to minimize hydraulic system failure, the components of the system have to be maintained.
A failure can be noticed by the manometric valve, it has the ability to notice pressure drops. When
this occurs it means that the system has a leakage or one of the components is not functioning.
When pressure drop is caused by the lines, this often is at the connection point from the line to the
shimmy damper (appendix VIII). The connector mounted on the shimmy damper is the Quick Dis-
connect (QD) coupling. Hydraulic QD’s are used in a hydraulic system to connect lines quickly without
leaking hydraulic fluid or losing fluid pressure. At inspection before the maintenance, regularly lea-
kage is found at parts that are exposed to vibrations on this coupling (figure 3.1). This coupling must
be inspected often to ensure the shimmy damper is provided of hydraulic power. This happens dur-
ing hydraulic system checks. The shimmy damper is part of hydraulic system B, and when a compo-
nent of this system fails, the aircraft must stand on the ground.

Figure 3.1 Fractured QD
The system is powered by an hydraulic pump. Too much bubbles in the viscose fluid in the lines can
affect the pump by a phenomenon called cavitations. When this occurs the bubbles will implode and
damage the scoops of the pump. This will lead to power loss of the pump. To ensure the usage life of
the pump, the system must be bled often and when refilling the system, the mechanic must ensure
that the oil is vacuum.

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When shimmy occurs, vibrations comes with it. These vibrations are damped by the shimmy dam-
per. These vibrations will move the piston rapidly up and down in the cylinder, which generates a lot
of heat. This is automatically cooled by the hydraulic system. When too much bubbles are in the sys-
tem the actuator will overheat and the piston and cylinder will melt together.
3.1.2 Main gear torsion link failure
The second failure is a failure of the torsion link apex bolt, which caused a malfunction in the torsion
link (3.1.2a). When the failure is analyzed a solution for the maintenance program can be found
(3.1.2b).
3.1.2a Cause failure
The second failure is the main gear torsion link failure (figure 3.2). The failure was that the right MLG
was twisted. The lower portion of the right main gear was rotated about 45 degrees to the right. The
aircraft could not continue taxiing. After research, the conclusion was that the inboard tire on the
right MLG was punctured on the inboard sidewall, and had deflated. This failure is caused by a frac-
ture in the lower torsion link (1). There are two torsion links in the MLG of the Boeing 737. The upper
torsion link (2) is connected with an apex nut (3) to the lower torsion link. The apex nut was loose,
therefore the most of the forces went through the lower torsion link. The shimmy damper next to
the torsion link was bent rearward 20 degrees, but still intact and activated. The damage of the right
MLG was limited, but the aircraft could not taxi further.

Now the problem is analysed there can be concluded that the apex nut was loose. There are several
reasons that the apex nut can be loose. One of the reasons is that the apex nut get loose by vibra-
tions. Another reason is that the apex nut not is tighten by the maintenance engineer. The complete
failure report can be found in appendix (IX).


Figure 3.2 Torsion link system











1. Lower torsion link
2. Upper torsion link
3. Apex bolt

1
2
3

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3.1.2b Solution of the maintenance failure
The failure of the 737 with an “overstressed torsion link” was caused by a loose apex nut. This nut is
probably not checked during scheduled maintenance of this aircraft. Although it must be checked
according to the maintenance manual. The scheduled maintenance was done 21 hours before the
failure.

After the failure the aircraft could not taxi, so the aircraft was towed into the hangar, where the
damage was examined. The gear was completely removed and a new gear installed. This is a non-
scheduled maintenance because the aircraft is taken out of service. To replace a main gear, the air-
plane was grounded for two days. This has an impact on the flight schedule of the airline.
3.2 Controlling maintenance
To make sure the aircraft's maintenance is done correctly the maintenance has to be checked. This
happens during regular checks.

The nose gear shimmy damper will be checked during the nose wheel steering system test. This test
is done on the steering system of the nose wheel and is used to find if the steering rate, steering
wheel centering and the angle of travel are in the necessary limits. The test also checks if the system
is without leaks.
To do this test it is necessary that the hydraulic system is serviced and the steering system of the
nose wheel is adjusted correctly.
To make sure the shimmy damper works correctly, the hydraulic system of the nose gear must be
bled before it can be opened. When opened, malfunctions can be checked.

Another aspect of the nose wheel steering system test is the removal of the apex bolt of the nose
wheel torsion link. After the test the bolt will be placed into normal position. The problem of the
loose bolt, which caused the overstressed torsion link, may have been occurred by not properly re-
turning the bolt on the gear.

However the torsion link did not collapse on the nose gear, but on the main gear. During the torsion
link maintenance check the maintenance crew must check if the bolt is tightened and secured. By
this check the torsion links will be removed and installed back on the landing gear.
To remove the torsion links, the main tires have to be removed and the shock strut has to be ex-
tended. During the reinstallation of the torsion links all bolts
have to be tightened. This includes the torsion link apex bolt,
which was loose when the accident happened. After tighten-
ing, the bolt will be checked for being safe tied with lock wire
(appendix X).

This lock wire (figure 3.3) is used to make sure the bolt re-
mains tightened. Any losing of the bolt will result in the wire
getting tightened, so the bolt will not move. This wire also
makes it more easy to check if the bolt is still in place.
3.3 Cost
The maintenance of the landing gear results in some cost for the airline. There are different checks to
replace or just check all the components of the landing gear. During these checks the aircraft has to
stay on the ground (3.3.1). To execute the maintenance different ground staff and engineers are
necessary (3.3.2). To reduce the maintenance cost the airliner can lease the landing gear (3.3.3)

Figure 3.3 Example of the use of lock wire

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3.3.1 Aircraft on ground
When the landing gear is maintained the aircraft cannot fly, and therefore will generate costs. When
the aircraft is on ground the airline cannot use the aircraft for transporting passengers or cargo. The
A-check is done once a month and takes seven hours to complete. The A-check is mostly to check the
most critical parts of the aircraft. The B-check is done four times in five year. Finally there is a C-check
and a D-check to strip the landing gear and maintain it. This check take place once in five years of
operation. These landing gear checks take place when the whole aircraft is taken apart.

For an AOG a standard of 2500 Euros per hour will be charged. The different checks takes different
hours to complete. The total costs of all the checks during aircraft on ground are €1.875.000,-.

Check Numbers of
checks (per five
years)
Hours Hours on ground Total costs (in
five years)
A 40 5 200 500.000
B 15 10 150 375.000
C 4 50 200 500.000
D 1 200 200 500.000
€1.875.000
3.3.2 Employers
The aircraft must be checked by well educated staff. The engineers salary is taken into account to the
total costs of the maintenance. The different types of checks using different amount of engineers.
The hour costs per man are €50,-. The A-check can be done in five hours by four engineers. The B-
check take place 3 times a year and can also been done by four engineers. The C and D checks for the
landing gear are more intensive checks and can be done by 6 engineers. The C-check takes 15 hours
to complete and the D-check take 2500 man hours to complete. The total cost of the landing gear
maintenance per 5 years is almost 300.000 Euros.

Check Numbers of
checks (per five
years)
Man hours per
inspection
Total man hours Total costs
A 40 5 200 10.000
B 15 8 120 60.000
C 4 70 280 140.000
D 1 2500 2500 125.000
€290.000
3.3.3 Leasing landing gear
Maintenance of the landing gear are major costs for the airliner, therefore smaller airline companies
are able to lease their landing gears. The total lease costs of a Boeing 737-300 landing gear is about
24,000 Euros a year. When an airliner leases a landing gear the only cost of maintenance are the
brakes and tires. The costs of landing gear parts, that have to be replaced, are for the leasing compa-
ny and not for the airliner.
3.4 Conclusion
This report is started with a main question, which meets the demand of the managing board of the
ALA and the main purpose of the project team 2A2G;


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''In which way can the maintenance program be influenced by the impact of the B737's error, accord-
ing to the costs of the maintenance and the airworthiness of the aircraft and which materials are
certified for the forces on the landing gear? ''

The first two chapters of the report have been filled with technical and mechanical information of
the B737's landing gear system. In the third chapter, the attention went out to two specific errors on
the landing gear system of the B737. Answering this main question is done by using the information
gained and written in all the previous chapters. The costs and airworthiness are distinguished to get a
better view of both aspects.

According to the team’s forces calculation of the landing gear, the materials can be selected, such as
steel, aluminum and titanium alloys. Steel has a high strength, toughness, fatigue strength and ductil-
ity. Titanium and aluminum are both light weight and are having more corrosion resistant .

The maintenance program can be influenced by the errors in relation to continue the airworthiness,
by giving more priority to these errors which only can be prevented by scheduled checks. When fol-
lowing the maintenance programs which are being executed by certified personnel, the airworthi-
ness of the aircraft can be kept at a standard.

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Bibliography
Boeing 737 Manuals and Readers
Aircraft illustrated parts B737-300
Aircraft maintenance manual Boeing 737-300
Boeing 737 Flight Crew operation Manual – Landing Gear
Continental maintenance Manual Boeing 737
CS-25 amendment 7
Mel (minimum equipment list) Boeing 737 all types
System schematic Manual
Books
Benjamin Chartier, Brandon Tuohy, Jefferson Retallack, Stephen Tennant
Landing Gear Shock Absorber
3016 Aeronautical Engineering I
2006

Hibbeler, Russel C
Sterkteleer voor technici
Pearson Education Benelux, Amsterdam

Hibbeler, Russel C
Mechanica voor technici, Statica
Pearson Education Benelux, Amsterdam

Hibbeler, Russel C
Mechanica voor technicie, Dynamica
Pearson Education Benelux, Amsterdam

Jelle Hieminga, Simon Ijspeert, Pieter van Langen
Projectboek periode 5: Landing gear
Amsterdam, Augustes 2010
Hoge school van Amsterdam, Aviation studies

Langedijk, C.J.A.
Vliegtuigsystemen
Hogeschool van Amsterdam
Amsterdamse Hogeschool voor techniek

Mason's
Landing Gear Design
Virginia tech college of engineering
1997

Norman S. Currey
Landing Gear Design: Principles and Practices
American Institute of Aeronautics and Astronautics, Inc.
1984

Wenztel, Tilly
Het projectgroepsverslag
Amsterdam, 2008

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Hogeschool van Amsterdam, Domein en Techiek

Magazines
Aerospace Engineering
Article: Landing Gear Structural integrity
March 1996

ASM International
Article: Heat Treatment Landing Gears
May/June 2008

Websites
Aluminum and Aluminum Alloys :: KEY to METALS Article
http://www.keytometals.com/page.aspx?ID=CheckArticle&site=ktn&NM=2
2009

Aviation Accident Database Query
http://www.ntsb.gov/ntsb/query.asp
2010

Aviation Safety Network
http://aviation-safety.net/database/record.php?id=20041128-1
1996-2010 Aviation Safety Network

Boeing 727 Technical Site
http://www.b737.org.uk/flightcontrols.htm#Wing_Design
Chris Brady 1999

Crane Aerospace and Electronics
http://www.craneae.com/products/landing/downloads/AutobrakeTutorial.pdf
2010 Crane Aerospace & Electronics

European Aviation Safety Agency
http://easa.europa.eu
Notice 2003-2010 easa.europa.eu

NCBI Materials
http://www.ncbi.nlm.nih.gov/pubmed/7263759
2009

Nederlandse vereniging voor van luchtvaart technici
http://www.nvlt.org/archief/2006/thema-avond%20kl.pdf
Ronald Woudstra, 15 juni

Non-ferrous Metals Data Sheets
http://www.matbase.com/material/non-ferrous-metals/titanium/tial6v4/properties
2010

Smart Cockpit
www.smartcockpit.com

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2009 Smart cockpit

The Boeing Company
http://www.boeing.com/commercial/airports/acaps/737sec2.pdf
1995 - 2010 Boeing

Ungefallberichte November 2004
http://www.berlin-spotter.de/unfall/unfall2004/berichtenov.htm



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List of appendices
I Maintenance Schedule 1
II MEL Repair Time Categories 10
III Centre of Gravity 11
IV Wing Design 12
V General Characteristics 13
VI Titanium Aspects 14
VII Aluminum Aspects 15
VIII Hydraulic pressure hose QD coupling leakage 16
IX Main Gear Failure Continental 18
X Use of Lockwire 23
XI Project assignment 25
XII Process report 27
XIII Group Agreements 28
XIV Distribution Of Tasks 30
XV Group members 31


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I Maintenance Schedule







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II MEL repair time categories

The time within a component needs to be repaired is defined in the MEL by a categorized system
which runs from A to D.

Category A:
Items in this category shall be rectified within the time interval specified in the remarks column of
the MEL item.

Category B:
Items in this category shall be rectified within three consecutive calendar days following the day of
discovery.

Category C:
Items in this category shall be rectified within ten consecutive calendar days following the day of
discovery.

Category D:
Items in this category shall be rectified within one hundred and twenty consecutive calendar days
following the day of discovery.


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III Centre of Gravity


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IV Wing Design



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V General Characteristics



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VI Titanium



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VII Aluminum



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VIII Hydraulic pressure hose QD coupling leakage



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IX Main Gear Failure Continental


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X Use of Lockwire
Lock wiring* is the securing together of two or more parts with a wire which shall be installed in such
a manner that any tendency for a part to loosen will be counteracted by an additional tightening of
the wire.
For general purpose lock wiring, use the preferred sizes in Table 1-1. Use smaller diameter wire
where parts are too small to permit a hole diameter to accommodate the preferred sizes, or where
space limitations preclude the use of the preferred sizes. The larger sizes are used where stronger
wire is required. Wire diameter of .032 is the most commonly used.
The common method
of installing lock wire
shall consist of two
strands of wire twisted
together (so called
"Double Twist" me-
thod). (One twist is
defined as being pro-
duced by twisting the
wires through an arc of
180 degrees and is
equivalent to half of a
complete turn.) The
single strand method of
lock wiring may be
used for some applica-
tions, such as in a close-
ly spaced, closed geo-
metrical pattern (trian-
gle, square, rectangle,
circle, etc.) parts in
electrical system.
Where multiple groups
are locked by either the
double twist or the
single strand method,
the maximum number
in a series shall be de-
termined by the num-
ber of units that can be
lock wired by a twenty-four inch length of wire.
Wire shall be pulled taut while being twisted. The number of twists per inch as recorded in Table 1-1,
represents general practice and is given as guidance information only.
Caution must be exercised during the twisting operation to keep the wire tight without overstressing.
Abrasions caused by commercially available wire twisting pliers shall be acceptable but nicks, kinks,
and other damage to the wire are not.
Lock wire shall not be installed in such a manner as to cause the wire to be subjected to chafing, fati-
gue through vibration, or additional tension other than the tension imposed on the wire to prevent
loosening. In the event that no wire hole is provided, wiring should be to a convenient neighboring
part in a manner so as not to interfere with the function of the parts. Hose and electrical coupling
nuts shall be wired in the same manner as tube coupling nuts.
Various examples of lock wiring are shown in Figures 1-1 through 1-12. Figure 1-12 shows the single-
stranded method, while the other figures show the two-stranded or double twist method.

Figure 3 The use of lock wire

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Check the units to be lock wired to make sure that they have been correctly torqued. Under-torquing
or over-torquing to obtain proper alignment of the holes is not advisable. If it is impossible to obtain
a proper alignment within the specified torque limits, back off the unit and try it again or select
another unit.
In adjacent units, it is desirable that the holes be in approximately the same relationship to each oth-
er as shown in Figures 1-1 through 1-4 (for right-hand threads), thus the lock wire will have a tenden-
cy to pull the unit clockwise. This should be reversed for left-handed threads.
Where lock wire is used to secure a castellated nut on a threaded item, selection of locking hole di-
ameter for the item shall be based on cotter pin requirements.

Table 1-1
Lockwire and Lockwire Hold Data
Wire Diameter Twists per Inch Recommended Hole Diameter
0.020 9-12 0.037-0.057
0.025 9-12 0.060-0.080
0.032 7-10 0.060-0.080




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XI Project assignment


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XII Process Report
The start of the project was a bit cautious but good. In the first meeting we exchanged numbers and
mail addresses and we planned a brainstorm session, where the project has been inventoried.

The meetings where useful. Through these meetings could the project group quickly see which parts
is completed and need to be done. A few things can be done better. The project group needs to
create discussions, this is to create more ideas to enrich the information provision of the report. In
the start document a chairman and secretary list has been set up, where all the members of the
group first get a turn of secretary and then as chairman. The chairman functioned in general good,
this is due that every member of the group complied to the rules during an meeting. The agree-
ments and task distributions where good noted by the secretary. The secretary placed it within 24
hours on BSCW and sent it to all the members per e-mail with a hyperlink of it. Another thing what
can be done better is that all the minutes had been checked with feedback by the members , this is
to keep the quality of the minutes high and it is a very handy tool to keep the member posted of the
current situation. When a member was absent this always been reported beforehand the meeting to
the chairman, eventually the absent was noted on the minute.

The planning of the start document was very global. Every chapter was in general planned for a week
with a margin of a week at the end. This couldn’t get reached because the problems we encountered
during writing the report. This was not a big problem because the deviation was maximal a few days.
When a member couldn’t get his job done on time, then he must reported this on time to the group
with the question to finish this later on.

To ensure that there are no spelling or content errors the group used a buddy system and at the end
of every chapter a beamer session. The buddy checks the whole document of spelling and content ,
there are no documents rejected but there are many spelling mistakes made. We as group should
work on that. During an beamer session with the whole group it is hard to keep the members fo-
cused, that’s why it is better to divide the whole group in smaller groups to keep the attention level
high. At the end of the control rounds the group sends the chapters to the project manager for last
comments.

During this project, we have learned how the landing gear works and which systems are related to it.
The project group can apply mechanical calculations at various conditions or situations to the land-
ing gear. Also the group has analyzed the failures of the landing gear and adjusted the maintenance
schedule with the corresponding costs to it.

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XIII Group agreements
The project should be finished within a deadline. Therefore milestones are created which has its own
deadline. Each group member should complete their tasks correctly and also needs to be up to date
with changes and agreements within the group. For these reasons rules are made to even out every-
thing regarding group work and to avoid misunderstandings;

 Too late
 Too late is later than the agreeing time of the meeting, or later than the time the college
COM/Project-lesson is beginning with a margin of five minutes.
 Important: when a group member is too late he call or sent the captain a text message.
In case this is not possible call or sent the minutes secretary a text message. When you
can’t reach each of these people, call one other person of the group.
 When a group member is more than one time too late in the whole project time, with-
out a valid reason: note the protocol.

 Absent
 When a group member is absent it’s just as important that he call or sent the captain a
text message. The group member makes sure that he catch up with the subject materi-
al. The group member will do this as soon as possible, namely in consultation with the
group. The group will decide with the person who is absent a date when the subject ma-
terial should be finished. Eventually will the captain be the person who will tell the ab-
sent person when the subject material should be finished. The group member also
makes sure that he, before the next meeting, read the minutes of that meet-
ing/COM/project lesson. The group member makes sure that he often check his mail
that day.
 When a person is more than one time, without a valid reason, absent in the whole
project time: note the protocol

 Not completing assignments or minutes.
 When a group member thinks that he can’t make it to finish the assignment (under as-
signment we mean: assignments given by the project/COM-mentor and assignment giv-
en to each other at a groups meeting. So also presentations.) or minutes: tell it to the
captain at latest two days before the deadline. The group member also let the other
group members know that he could not make it so they can help him. Of course the
group member should have a valid reason for not finishing assignments. Eventually the
group will decide when this person should hand in the assignment.
 It is always possible that a group member don’t understand an assignment. The group is
a team so it is very important to let the group know that a person doesn’t understand,
so we can help each other. There is always a person who can help a group member to
explain something.
 When someone is not following the agreement more than one time in the whole project
time, without a valid reason: note the protocol.

 Protocol
 First time: the group member get a warning from the captain on behalf of the whole
group.
 Second time: During the group meeting the group will have a talk about the person who
is not following the agreement, without a valid reason. It could me possible that this
person is holding up the group. So the group will also have a talk about this with the
project mentor and with the person who is not following the agreements to find a solu-

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tion. It is possible that the person who isn’t following the agreements, will be put out of
the group.

 Valid reasons
 The next valid reasons will only be valid when these will be told on time to the captain
or other group mates. With on time the group means: before the meeting or
COM/project lesson is starting. When something happens on short time the group
member can tell the captain that something happened by text messages him or call him
(not too late). Valid example reasons are:

1. Problems at home.
2. Illness.
3. Delay with the train.
4. It could be possible that there are more reasons acceptable. This is what the
group could decide on that moment.

 It could also be possible that there is a person who is too many times too late or absent
and telling a valid reason. The group cannot always check this, so when this is starting to
be suspicious the group can decide whether de reason is always that valid or not.

 General
 Make sure that every group member checks his mail daily and when he has questions
about the minutes, ask then to the minutes secretary.
 It is very important to communicate with each other. When someone doesn’t under-
stand anything, can’t make it to finish something or can’t be an a meeting: communi-
cate.
 Deadlines are precisely decided in the planning.
 Before every COM/project lesson the captain makes a agenda. This agenda will be sent
to the teachers 24 hours before the lesson begins. After every group/COM/project
meeting the minutes secretary will make a minute and sent it 24 hours after the meet-
ing to the other persons in our group.
 Every Friday before 1 p.m. will the minutes secretary sent the most important minute to
the project mentor.
 The group will work with a buddy system.
 Every time someone has finished an assignment, he will put it an BSCW en the biblio-
graphy will be placed under need the text.
 On Monday the group will have a groups meeting the 5
th
hour and also on Thursday the
group will have a groups meeting this will be the 3
e
until the group needs or the 5
e
till
when the group needs. It is possible that these meeting can be changed from time and
date.
 The method of approach and report will be written in English.
 Each member is accessible on the given email and phone number (appendix XX).
 Each group member need to play the chairman or minutes secretary role (appendix XX)
 When typing/ writing a paragraph each group member will use the same lay-out there-
fore using the template document and the LG_start document uploaded on BSCW.



Hereby each group member declares that they have read and agree with the above mentioned rules
and statements;

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XIV Distribution Of Tasks
Introduction Reza Azizahamad 1.4.3 Minimum
Equipment List (MEL)
Reza Azizahamad
Summary Jurre Klein 2.1 No Wind Landing
Forces
Matthijs Niemeijer
Jurre Klein
1.1 General Aspects Jurre Klein 2.2 Crosswind Landing
Forces
Joost Broekhuizen
Rob van Loon
1.2.1 Retract/Extend Matthijs Niemeijer 2.3 Forces on Mate-
rials
Thijs de Wilde
1.2.2 Shock Absorption Thijs de Wilde

2.4 Conclusion Thijs de Wilde
1.2.3 Steering Sissai Gerezgiher 3.1.1 Hydraulic Shim-
my Damper Failure
Sissai Gerezgiher
Robin van Gemert
1.2.4 Brakes Matthijs Niemeijer 3.1.2 Overstressed
Torsion Link
Reza Azizahamad
1.2.5 Wheels/Tires Sissai Gerezgiher 3.2.1 Shimmy Damper
Replacement
Joost Broekhuizen
1.3.1 Auto Braking Rob van Loon 3.2.2 Torsion Link Re-
placement
Thijs de Wilde
1.3.2 Antiskid System Joost Broekhuizen 3.2.3 Controlling
Maintenance
Rob van Loon
1.3.3 Air/Ground Logic Joost Broekhuizen 3.3 Cost

Matthijs Niemeijer
1.3.4 Manual Gear
Extension
Thijs de Wilde 3.4 Conclusion Robin van Gemert
1.4.1 Requirements
Landing Gear System
Reza Azizahamad
Robin van Gemert
Process Report Sissai Gerezgiher
Jurre Klein
1.4.2 Maintenance
Requirements
Robin van Gemert




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XV Group members
Groupmembers from the left to the right
1. Thijs de Wilde
De Reit 4
3451KM Utrecht
[email protected]
0634095545

2. Sissai Gerezgiher
Oranjerivierdreef 8c
[email protected]
0614741391

3. Joost Broekhuizen
Veldhuyzen van Zantenpark 83
2163GC Lisse
[email protected]
0619028370

4. Reza Azizahamad
Vlieland 3
2631NT Nootdorp
[email protected]
0614239618



























5. Robin van Gemert
Steenweg 37bis
3511JL Utrecht
[email protected]
0646745262

6. Rob van Loon
Kapelakker 40
5763AG Milheeze
[email protected]
0623633182

7. Matthijs Niemeijer
Ruysdaellaan 23
3712AP Huis ter Heide
[email protected]
0646322770

8. Jurre Klein
Hemsterhuisstraat 50
1065KB Amsterdam
[email protected]
0648247003