landing gear project report

8/10/2019 landing gear project report

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Project Landing Gear 2A2G

Table of ContentsIntroduction ............................................................................................................................................. 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 ......................................................................................................... 222.1.1 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




2.2.4 Internal forces ............................................................................................................... 28

2.3 Forces on Materials ............................................................................................................... 28

2.4 Conclusion ............................................................................................................................. 30

3 Troubleshooting ............................................................................................................................ 313.1 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




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IntroductionIn 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. ALAalso wants to know the maintenance procedure and costs of the maintenance when these failuresshow up.

The research will be in the form of a report and a presentation to the ALA. The report will be handedout in week 7 of the academic year 2010 and the presentation will take place in the first week ofNovember 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 Boeing737-300 is a retractable landing gear system. Main functions of the landing gear are supporting theaircraft’s weight and absorbing the landing shock, allowing the aircraft to manoeuvre on ground andbraking. 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 includingthe 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 thechoice 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 themaximum 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 anoverstressed torsion link can occur. To fix these failures there is a maintenance program made. Theserepairs 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 appendicesthe project assignment (appendix XI) and the process evaluation report (appendix XII) are added.




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SummaryThe team had chosen a Boeing 737-300 for the analysis of the landing gear and a discussion of twofailures and the adaption of the maintenance program and the costs involving the adaption of themaintenance 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 thehistory 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 thelanding 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 pressuresystem. 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. Theseshock absorbers are oleo-pneumatic absorbers, they work with oil and nitrogen. To manoeuvre theplane on the runway and taxiway two types of steering are placed, one is the rudder steering used bytake-off and landing. For taxiing there is a control wheel to make smaller rates of turn. To slow theaircraft 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 isused with four amounts of deceleration, 1, 2, 3 and MAX. For rejected take-off (RTO) the auto brakewill make to stop the aircraft as fast as possible. To prevent the wheels from blocking during brakingan anti-skid system is installed. When the landing gear cannot be extended with hydraulic pressure itmust be extended manually by the gravity. In the CS-25 (Certification Specifications) of the EuropeanAviation 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 maingears. The force on the main gear is the greatest because of the small distance between the centre ofgravity (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 nosewheel will rotate to the ground. When adding crosswind in the landing the touchdown will occurdifferent with headwind or no wind. First the upwind main gear touches down, then the downwindmain gear and at last the nose gear. The materials which can be used can be found in the CS-25. Thespecifications of the materials have to be high strength and stiffness, low cost and weight, and havegood machinability, weldability, and forgeability. They also must be resistant to corrosion, stresscorrosion, hydrogen embrittlement, and crack initiation and propagation. The alloys which can beused are steel, aluminium and titanium.

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




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1 Systems Landing Gear B737-300The 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 AspectsDuring the history of the aircraft, the landing gear had undergone a great development (1.1.1) . Whenthe 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 gearIn the beginning there was one type of landing gear, the feet ofthe airman. In 1891, Otto Lilienthal was one of the first men whoflew above the earth. Lilienthal’s airplane wa s not more than twowings and one stabilizer. To fly, he jumped of a hill and landed onhis feet (figure 1.1) .After Lilienthal, the Wright Brothers made the first poweredflight. The landing gear of the Wright Flyer I was made of skis.After the Wright Brothers had flown, the airplane was furtherdeveloped, so did the landing gear.

1.1.2 Modern types of landing gearThe first type of landing gear that was used on a large scale was the conventional landing gear. Theconventional gear consist of three wheels. The main gear, which is under or in front of the wings andone 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-3has a retractable landing gear. The retractedlanding gear was invented by two Fren-chmen in 1876, but only used on large scaleafter 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 onsmaller 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 theroll-out after the landing the plane could make a ground loop. When steering to much or in case ofwind 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 onthe wings, they could be damaged too. At last, the visibility in front of the plane is less then when thefuselage 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, butthe 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 threewheels, 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 gearThe B737-300 is using a variant of the tricycle landing gear. It has two nose wheels and two pair ofwheels for the main gear. One part of the main gear has two wheels. The figures below show themeasures 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 gear2. Nose gear

The measurements of the longitudinal axis can be viewed in the side view (figure 1.4) . The position ofthe main gear is slightly aft of the center of gravity (CG) of the airplane, 3,1 m (1) , it is positioned 16,5meters from the nose (2), while the CG is positioned 13,4 meters from the nose (3). The purpose ofplacing the main gear aft the CG is to create a negative turning moment by touchdown of the maingear. 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 9,4m 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 gear2. Nose - main gear3. Nose gear - main gear4. Nose - CG5. Nose gear - CG

1.2 Landing Gear SystemsThe Boeing 737-300 consist of many landing gear systems. Hydraulic systems are used to retract orextend the landing gear (1.2.1) . The shock absorbers absorb the landing shock when touching theground (1.2.2). In order to obtain directional control during ground maneuvers and taxiing, steeringat 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) .

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1.2.1 Retract / extendTo 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 gearis, just like the main landing gear, able to retract (1.2.1d) and extend (1.2.1e) .To retract and extendthe crew has to operate the landing gear lever (1.2.1f) .

1.2.1a Four-bar linkageThe most used landing gear is a four-bar system. The system consists of four arms and is ideal tostore 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 pointthere 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).

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Figure 1.5 Four-bar system

1.2.1b Retraction main landing gearThe Boeing 737 has two main retract and extended landing struts. The main gear (figure 1.6) can becontrolled by using the landing gear control handle moving into UP or DOWN position. The main gearactuator applies the force to raise and lower the gear. A lock actuator is used to lock the gear in up ordown position. An actuator (1) is a hydraulic mechanism type for using controlled movementsThe main landing gear actuator and walking beam (2) work together to raise and lower the maingear. The walking beam reduces the reaction force going into the aircraft structure from the maingear actuator. When retracting the main landing gear an inboard force from the actuator i s 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 andpressure 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 pushesthe hook into final position where it is locked.

1. Side way2. Pivot point in main strut3. Pivot point in supporting strut4. Backwards




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1.2.1c Extension main landing gearWhen extending thelanding gear, hydraulicpressure is moved di-rectly to the main gearactuator and the modu-lar package. The transfervalve controls theamount of pressure toextent the main gear.The landing gear ex-tends by actuator force,landing gear weight, andair loads. After extend-ing, the landing gear ismoved to the lock posi-

tion (figure 1.6) . Whenthe landing gear is in locked position there is a upper side strut (3), a lower side strut (4) and shockstrut (5) to hold the landing gear in position.

1.2.1d Retraction nose landing gearThe Boeing 737-300 has a double nose gear (figure 1.7) . The double nose gear has more advantagesthan a single nose gear, for example more stability and the ability to operate with one flat tire. Whenthe landing gear control handle is moved into UP or DOWN position the nose landing gear retracts orextends 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 isdirectly transported through the selector valve and through the nose gear modular package. Thenthe hydraulic pressure transported to the gear and the lock actuator. Using a downward force thelock is pulled over centre. The lock link causes a 90 degrees swing that changed the landing gear fromhorizontal to vertical position. The lock actuator (4) retracting force is opposed to the lock linkmovement until the gear is almost retracted. In time of opposition the larger main gear actuatoroverpowers the lock actuator.

1.2.1e Extension nose landing gearTo 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. Thisheavy 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.

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Figure 1.6 Gear down and locked

1. Main gear actuator2. Walking beam3. Upper side strut4. Lower side strut5. Shock strut




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Figure 1.7 Nose landing gear

1.2.1f OperationFor operation of the landing there are two different types to use it. The landing gear lever controlsthe different movements of the landing gear.

1. Retraction

2. Extension

ad. 1 RetractionThe landing gear is normally controlled by the landing gear lever. On ground, the lever lock preventsthe landing gear lever from moving to the up position. When the landing gear lever is moved up, thelanding gear start to retract. During retraction the brakes automatically stop rotation of the mainwheels. 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 ExtensionWhen 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 hydrauliclocks stops the gear from retracting.

1.2.2 Shock absorptionThe Boeing 737-300 has an oleo-pneumatic shock absorber (1.2.2a) . The shock absorber works withoil and nitrogen (1.2.2b) .

1.2.2a Function shock AbsorberThe impact of landing must be absorbed. This is done by the oleo-pneumatic shock absorber. Thisabsorber is mostly used by large aircraft. The main advantage of this absorber is that it providesshock 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 isthe 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 actuator2. Lock strut3. Spring bungees4. Lock actuator




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1.2.2b Design Shock AbsorberThe oleo-pneumatic shock absorber (figure

1.8) consists of two separated chambers. Onechamber is filled with nitrogen (1) and the oth-er chamber is filled with oil (2) . If the aircraftlands, the oil chamber will be pushed againstthe nitrogen chamber, the gas and oil will becompressed. The kinetic energy is damped bythe oil which is being forced through orifices(3) . The rebound of the landing is controlledby the gas pressure forcing the oil back intoits chamber through recoil orifices (4). Theorifices must be calibrated, because if the oilflows back too fast, the aircraft will bounceupwards. If the oil flows back too slow, theshock absorber will not damp adequately. This

could happen during taxiing, the bumps on thetaxiway would not be absorbed because theabsorber has not restored itself quickly enoughto the static position.

1.2.3 SteeringIn order to obtain directional control during ground maneuvers and taxiing, steering at the nosewheel 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 tensedand 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) ofhydraulic pressure, this actuator pulls the wheels from side to side with hydraulic power.

Figure 1.8 Shock absorber

Figure 1.9 Steering system

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1. Chamber filled with Nitrogen2. Chamber filled with Oil3. Orifices4. Recoil Orifices

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1. Steering wheel2. Pulleys3. Rudder steering mechanism




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To turn the aircraft from standard to the maximum turn rate of 78 degrees, hydraulic pressure fromsystem A is send to the nose wheel. The pressure needed to turn the wheel is 3000 psi. When thenose wheel is unturned the steering valve keeps a pressure from 70 to 130 psi against the actuatorpistons to act as an shimmy damper. Hydraulic pressure from system B is to engage the alternatenose 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 ofnose 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 theairplane compressing the shock strut.The nose gear steering mechanism at the shock strut (figure 1.11) consist of a steering collar (1), twoactuators (2) and an inner cylinder (4) . When a turn is made, the two cylinders separately retract andextract 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 A2. Cable system B3. Steering valve4. Steering actuator

1. Steering collar2. Actuator3. Piston4. Inner cylinder5. 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°

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3000psi

3000psi




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

Pushing the brake pedal opens the alternate brake metering valve, that allows pressure to passthrough the alternate antiskid valves to the brakes. The brakes can be divided into:

1. Normal brake system

2. Alternate brake system3. Accumulator brakes

ad 1 Normal brake systemThe normal brake system can be used during standard situations. The brakes are controlled by thetwo rudder pedals in the cockpit. The brake pedal mechanism sends inputs to the brake meteringvalves. The system is pressurized by hydraulic system. The hydraulic system results in a 3000 PSIpressure.

At the main gear the alternate brake metering valves are fitted together. The metering slide of eachbrake 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 thepressure port opens and provides the pressure to the brakes. Because of a feedback force, the valveis closing. The force provides a feeling to the pedals. When the pedals are released the pressure valveslide 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 systemThe alternate brake system is powered by hydraulic system B and results in a 1500 PSI pressure. Thealternate brake selector valve and system A supply pressure to the alternate brakes. When system Bis 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

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Figure 1.12 Brake system

1. Rotor2. Stator3. 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 ofsystem 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 Bis lost. When all the hydraulic power is lost, the accumulator brake system can controlled six brakesystems. A accumulator brake system use mechanical force to store pressurized energy and providedback-up power for the brakes. When system B and system A lost pressure, the accumulator brakesystem valve opens and provide several applications of brake power through the normal brake lines.

1.2.5 Wheels/TiresThe Boeing 737-300 has totally six wheels, to withstand high rolling speeds. To keep the strength ofthe gear, wheels are provided (1.2.5a) . To damp the little vibrations rubber tires are mounted aroundthe wheel (1.2.5b) .

1.2.5a WheelsFor 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 theinner (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 thewheel is an axle (5) for the wheel to make it spin, this is provided with a seal that keeps the lubricantinside and keeps the dirt and moisture outside.

Figure 1.13 Wheel

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1.2.5b TyresThe main landing gear is provided with tubeless tyres and designed to withstand the forces to 195knots. When an aircraft is making a rejected take-off (RTO), the breaking will generate a lot of heatwhich that must be cooled (figure 1.14) . This is the reason why four thermal relief plugs (1) , equallylocated and mounted on the inner wheel half (2), are protecting the wheel from excessive brakeheat, 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 weights2. Inner wheel3. Outer wheel4. Secured bolt5. 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 inflatethe tubeless tire. There is also an over inflation plug installed in the inner wheel half (3) and goes halfout trough the outer wheel (4) . It prevents over inflation by means of a seal that breaks when overinflation occurs, it will deflate the tyre to ‘zero pressure’ . 3412

Figure 1.15 Valve extension

1.2.6 Nose landing gear shimmyA 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 shimmySteerable 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 dynamicselasticity of the tire. The wheel gets unbalanced, when a critical low friction force is applied to thewheel, because of a minimum contact area. This force is getting critical when the weight on the fronttire is too low. During the landing, the aircraft has a high velocity, which cause the nose gear to act asa gyroscope. With the property of precession the gear wants to balance the wheel, but unfortunatelyit 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 onthe landing gear system. Shimmy can be reduced in several ways:

1. Provision of a hydraulic lock across the steering jack piston2. Fitting a hydraulic damper3. Fitting heavy self-centering springs4. Double nose wheels5. Twin contact wheels

ad 1 Provision of a hydraulic lock across the steering jack pistonIn 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 wheelstrut (2). Shimmy mainly occurs at an high velocity when the forces are noticeable. To reduceshimmy the hydraulic pressure (3) must be locked. When closing the restrictors (4) the piston (5)

Figure 1.14 Tire

1.Thermal plug2. Inner wheel3. Outer wheel4. Heat shield

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1. Valve2. Extension3. Inner wheel half4. Outer wheel half

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

Figure 1.16 Steering mechanics of the nose wheel.

ad 2 Fitting a hydraulic damperIn the same situation like above, the valve will keep a pressure of 120 psi. Unlike the hydraulic lockthe piston will not be locked but there will be a small movement with a lot of resistance, with theresult 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. Theforces in direction of the centerline provide a self centering system, when shimmy shows up.

ad 4 Double nose-wheelsThe double nose-wheel on the Boeing will lead to an increase of contact area and therefore groundfriction. 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 acontrolled deceleration rate, the auto-brake system can be used (1.3.1) . Because the pilot gets nofeedback 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 themain extraction of the landing gear fails, a manual extension system is used (1.3.4)

1.3.1 Auto brakingAn auto brake system provides direct braking after touchdown of the aircraft (1.3.1a) . The systemmeasures and regulates a selected deceleration, by controlling the brake pressure (1.3.1b) .

1.3.1a Function Auto brakeThe 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 differentmodes. These modes are:

1. Landing mode2. Rejected Take-Off mode (RTO)

ad 1 Landing modeFor 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 levelbefore 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 areIDLE and the rotating speed of the wheel is measured. The system stops when the roll out is ended orwhen the pilots take over control.

1. Steering jack2. Nose weheel strut

3. Hydraulic pressure4. Restrictor5. Piston

152

3

4




14

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 lastmode is used to stop the aircraft after a rejected take-off. The RTO auto brake system is the primaryway to stop the aircraft during a RTO. When pilots arm the RTO auto brake system, the auto brakesystem will directly initiate full brake pressure after the thrust levers are back to the fully retardedposition or thrust reverse is used. The system then checks the wheel speed to determine auto brakedeceleration rate, th is will limit the crew’s handling activities during the RTO. Stopping distance andthe risk to overrun the runway will be minimized by the system.

1.3.1b Auto brake OperationOperating the auto brake starts with the selectionmade 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 autobrake disarm light (1) . This light is illuminated in thefollowing conditions:

- During RTO or landing the speed brake leveris 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 differentoptions, separated into three categories. The first is OFF, this means the auto brake system is turnedoff. 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 thrustreversers will however not lower the brake pressure when option MAX is selected. When using theoption MAX, the switch must be pulled to select it. The last possible selection is RTO. This selection isused prior to start. When applied the system will automatically use maximum brake pressure whenthrust 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 andcontrols the brake pressure that goes with the desired deceleration rate. The control module gets itsinformation about wheel speed and deceleration rate from the antiskid/auto brake control unit.

1.3.2 Antiskid SystemThe 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 systemThe antiskid system has different functions. This system is mainly installed to prevent the wheelsfrom blocking while the plane is de-accelerating. The stopping way is greatly increased when the tiresare skidding over the runway, instead of rolling. Another function of this system is to prevent thebreaks from blocking the wheels before touchdown. This insures that the wheels are rotating beforethey are being slowed down by the breaks.

1

2

Figure 1.17 Auto brake selector switch

1. Auto brake disarm Light2. Auto brake selectorswitch




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1.3.2b Antiskid operationThe antiskid system has influence on theamount of pressure given to each of the landinggear brakes (Figure 1.18) . The system measures

the velocity of each wheel with the transducers(1) . These transducers send the signal of therotating speed to the antiskid auto brake con-trol unit (AACU) (2). This control unit has theactual groundspeed for reference. It comparesthe rotating speeds of each wheel with the otherwheels of the landing gear. When one wheel isde-accelerating faster than the other wheels,the AACU sends a signal to the antiskid controlvalve (3). This valve is located after the brakevalves of the hydraulic system. The antiskid con-

trol valve will narrow, so that less hydraulicbrake pressure (4) is going to the brake. As aresult, the rotating speed of the wheel increas-es, until this is the same speed as the otherwheels.

1.3.3 Air/Gound LogicThe 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 SystemThe function of the air/ground logic system is to determine if the airplane is flying or is located on theground. 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 willarm 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 operationThe air/ground Logic system contains six sensors. Each strut has two sensors, one for system one andthe other for system two. Both systems send a signal of the condition of the strut to the proximityswitch electronics unit. This unit is connected to all of the systems that are in need of the position ofthe B737. An indication of failure is a warning light in the cockpit.

1.3.4 Manual Gear ExtensionThe Manual Gear Extension will only be used if the gear cannot deployed. If that happens, the pilotstake some preparations prior to the manual extension (1.3.4a) . After the preparations the gear willbe 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. Transducer2. AACU3. Antiskid controlvalve

4. Hydraulic brakepressure

4

3

12




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1.3.4a FunctionThe manual gear extension will be used in case of a failure of hydraulic system A. System A is thehydraulic system that controls the landing gear extension and retraction. During normal operationthe pilot will lower the gear by pullingthe gear lever down. If one or more ofthe gears fails to extend, the emergencymanual gear extension has to be used.The manual gear extension handles canbe found on the cockpit floor at the F/Oside (Figure 3 .19) . There are threeswitches that can be pulled out (1) . Themiddle one controls the nose wheel, theleft one controls the left main gear andthe right one controls the right maingear. The switches are connected by acable which will be connected to the

landing gear (2).

1.3.4b System OperationBoth main gears are working identical (figure1.20) . The cable (1) which comes from themanual gear extension handles goes to a hook(2) . If the cable is pulled to its limit, about 45cm, the up-lock hook which hold the gear upand the gear falls down by gravity. The gearwill become in down-locked position (3), thismeans the gear is locked and cannot move.

The nose gear works slightly different. Thecable should be pulled approximately 20 cm,then the up-lock will be released and the gearfalls down. The nose gear doors will be openedby the weight of the nose gear. The gear willcontinue to fall by the gravity until it is lockedin down-lock position.

1.3.4c VerifyingAfter an alternate extension the pilots must verify if the gear is down and locked. Therefore there aresmall 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 cansee two parts of the down-lock strut. On these parts are two red arrow heads shown. Indication thatthe 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 nearbythe 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 RequirementsThe European Aviation Safety Agency (EASA) is the authority in Europe that defines the rules of anairplane. 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 levers2. Cables to landing gear

1. Cable2. Hook3. 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 afire 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 rationalanalysis, the side loads must be assumed to be up to 20% of the vertical load or 20% of thedrag load, whichever is greater.

- The aeroplane must be designed to avoid any rupture leading to the spillage of enough fuelto constitute a fire hazard as a result of a wheels up landing on a paved runway, under thefollowing minor crash landing conditions:

1. Impact at 1,52 m/s (5 fps) vertical velocity, with the aeroplane under control, atMaximum 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° yawangle.

- 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 notlikely 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. Theairworthiness 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 systemThe 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 thisshock 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 brakingsystems 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 systemA retractable landing gear on an aircraft, including; the retracting mechanism, the wheel well doorsand 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 SteeringCertifying 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 forthe handling of it, during take-off and landing, including the case of cross-wind and in theevent of a sudden power-unit failure at any stage during the take-off run. This must beshown by tests (AMC 25.745)

- It must be shown that, in any practical circumstances, the steering columns in the cockpitcan’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 failurewill 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 damageto the steering system.

1.4.1d BrakesAfter the touchdown of an aircraft the brakes and the braking systems are the most important partsof 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 ispossible to bring the aeroplane to rest with a braked roll stopping distance of not more thantwo 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 brakesis 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 themost adverse combination of maximum thrust on one engine and up to maximum groundidle 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 thecockpit 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/TyresTo make a movement on the ground there are two parts needed to ensure that. These parts arewheels and tyres.

1. Wheels2. Tyres

ad 1 WheelsThe wheel of a landing gear is an important part. To make sure that nothing will happen with thewheel 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 TyresTo make a movement on the ground only a wheel is not enough. There is also a tire. To know whatkind of tire there must be used, are there an amount of regulations for tires. These regulations canbe found in the CS-25.733.

- The applicable ground reactions for nose wheel tyres are as the static ground reaction for thetyre corresponding to the most critical combination of aeroplane weight (up to maximumramp weight) and centre of gravity position with a force of 1·0 g acting downward at the ce n-tre of gravity.

- When a landing gear axle is fitted with more than one wheel and tyre assembly, such as dualor 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 loadrating approved by the Agency that is not exceeded by the loads on each main wheel tyrecorresponding 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 thetyre type expected in service, have a clearance to surrounding structure and systems that isadequate to prevent unintended contact between the tyre and any part of the structure orsystems.

- For an aeroplane with a maximum certificated take-off weight of more than 34019 kg, tyresmounted on braked wheels must be inflated with dry nitrogen or other gases shown to beinert 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




20

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 regulationsabout tyre can be found in CS-25.733.

1.4.2 Maintenance requirementsWhen continuing airworthiness of a certified aircraft, maintenance has to take place by periods oftime. Therefore, different rules and requirements have been set up by the EASA to maintain aircraftcertified. EASA part M is the primary legislation when looking at the maintenance requirements ofaircraft. Part M contains the measures which will have to continue airworthiness and the conditionsrequired 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 toapprove Airworthiness Review Certificates (ARC) of aircraft, which will determine either the aircraft isairworthy for a maximum of one year or not. The staff which directly run the maintenance of theaircraft 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. Forthis reason, each aircraft has its own Aircraft Maintenance Plan (AMP). This plan describes how tomaintain each particular system on the aircraft, step by step. This AMP is been set up by using themaintenance 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 writtenabove. A maintenance schedule of an Boeing 737-300 (appendix I) is given to show in which way theparticular system has to be checked. Each complicated part of the landing gear system has its ownchecklist, 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 intervalscan 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, whenlooking 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 themanufacturer and is approved by the EASA in Europe and by the FAA in the United States. It lists theinstruments 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 aredeactivated 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 numberin the MEL shows the number of specific component (2) and shows in category (A-D) (3) when itneeds to be repaired. The minimum number required for dispatch show the minimum number ofcomponents that is needed to be operative (4). Also, there are some remarks (5) by the numbersrequired for dispatch. These remarks gives a specific overview which components had to be opera-tive.

Figure 1.21 MEL

1. Component2. Number of specific com-ponent

3. Category4. Number of componentrequired for dispatch5. Remarks




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2 Forces B737-300To study the failures of the landing gear of the B737-300, the external and internal forces must becalculated. Because both failures have once been taken place during landing, the external forces willbe calculated in a no-wind landing (2.1) For the maximal crosswind landing, external and internalforces will be determined (2.2) . As a result, the materials and treatments for those materials can bechosen (2.3) . A conclusion of this forces study is given (2.4) .

2.1 No-wind landing forcesTo 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 landingcan 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 groundthe main gear touches the ground first (2.1.4).

2.1.1 Centre of gravityThe CG is the average location where all the forces come together. During the flight the CG willchange, due to fuel consumption and passenger movements. To calculate the landing forces, the CGwill be determined in most forward position to get the highest critical landing force on the landinggear. The limits of the CG have been expressed in a rate of the mean aerodynamic chord (MAC). TheMAC is located on the reference axis and is the chord of a rectangular wind. The MAC referenced tothe 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 doesnot give the span wise location of the MAC (appendix IV) . The CG can be measured from any pointalong 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)Ct = Wingtip chordCr = wind root chordS = halve wingspanH = 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 neutralizethis.




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Figure 2.1 MAC

2.1.2 Theories and formulasFor 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 willcontinue 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 experiencean acceleration or deceleration proportional to the force and inversely proportional to itsmass.

- Third law of Newton: When two objects interact, they both produce a similar, opposed andcollinear force (so called action is reaction). For calculating the aircraft landing forces thethird lay is not used.

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

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

To calcula te moments on the aircraft the Newton’s law using a zero formula. With this formula forces

can be calculated in different directions.

(2.3)Fx = horizontal force (N)Fy = vertical force (N)M = moment (N/m)

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




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(2.4)Vt = vertical speed reaching the ground (m/s)V0 = vertical speed descending (m/s)a = acceleration / deceleration (m/s 2)ds = distance difference between the shock

strut extended and extracted when touchingthe 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)Fn= Vertical landing force (N)μ = cause of friction (0,4)

2.1.3 Aircraft standing on groundWhen the aircraft is on the ground, the weight of the aircraft is the only force that is working on theaircraft. The main landing gear carries more weight than the nose landing gear because the centre ofgravity is closer to it. The arm between the CG and the main landing gear is smaller than the armbetween the CG and the nose landing gear. With the theory of all the moments must be zero, theforces can be calculated. The force of the main landing gear needs to be bigger to compensate thebigger arm between the nose landing gear and the CG. That is the reason that the main landing gearconsist of more struts and more wheels than the nose landing gear. The arm between the CG and thenose 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 thenose wheel is 9.03 meters (figure 2.2) . The distance from the CG to the main landing gear is 3.47meters. 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 gearcan be calculated. This results in a 140,845.44 N force (F n,nose ) for the nose landing gear and a366,450.61 N force (F n,main ) for the main landing gear.

Figure 2.2 Aircraft on ground during landing

2.1.4 Aircraft touchdownDuring touchdown the main landing gear touches the ground first. When the main landing geartouches 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 theground the pilot flayers the planes up to make a comfortable landing. This landing is done with anvertical 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 is60721.8787 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 horizontallyon the main landing gear.

Figure 2.3 Aircraft touchdown

2.2 Crosswind Landing ForcesWhen landing with crosswind, wind from the sides, the landing process is different in comparisonwith 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 beput 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 willforce the downwind main landing gear wheel to the ground (2.2.2). After the main landing gear hascomplete contact with the ground, the nose gear can be lowered to the ground (2.2.3) . At last, theinternal forces are calculated (2.2.4) .




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2.2.1 Touchdown upwind wheelThe 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 canbe calculated with formula 2.2 .

To maximize the force on the upwind wheel during the crosswind landing, the maximum landingweight (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 designlanding weight, this minimum required sink rate is 10fps (3.048 m/s). Based on CS criteria this sinkrate will probably deliver an Hard landing as a normal landing will only produce sink rates between 2and 3 fps. An hard landing will discomfort the passengers.

Now the vertical speed is known, the vertical acceleration or deceleration can be calculated. This willhappen 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 upwindwheel 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 (figure2.4) , is the force created by the vertical speed during the landing. Because the lift during this stage issufficient to counteract the gravitation, the vertical speed force is the only force acting on the mainlanding gear during this situation. Because the first stage of the crosswind landing holds a landing onthe upwind wheel, the complete vertical force doesn't have to be divided into two. This means thecomplete vertical force of 674,699.032 N acts on the upwind wheel.

Figure 2.4 Aircraft touchdown upwind wheel




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12

3

2.2.2 touchdown downwind wheelAfter the first contact on the ground by the upwind main landing gear, the downwind landing geartouches down. Assuming that the pilot will instantly open the ground spoilers, the lift of the wingsare reduced to zero. Also the lift of the horizontal stabilizer is reduced to zero.. The forces on thelanding gear can now be separated into two parts, namely:

1. Crosswind component2. Forces on main landing gear

Ad. 1 Crosswind componentThe crosswind component can be defined by the drag of the wind on the flank of the B737 (formula2.6)

F =

(2.6)F = Force (Newton)ρ = density of the air (kg/m 3)v= velocity (m/s)

CD = Drag constantS = 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 ofthe 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 certainheight: 1,957 meter above the centre of gravity.

Ad. 2 Forces on main landing gearWith the calculations of the wind and with the gravitythe forces on the main landing gear (figure 2.5) . Themomentum of the wind practicing on the CG is

. The gravity is themaximum landing weight, 51,709 Kg. This force can bedivided equally over the two struts. Each strut has

to bare. The force of

the wind is pushed on the downwind main landinggear, so the force is the momentum of the wind divedby the arm of the downwind gear: . This extra force can be added to the253,632.645 N. The result is 279,456.445 N. The sameforce 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 thethree 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 meter2. 1.957 meter3. 2.615 meter

Figure 2.5 Crosswind on B737




28

3,47 34,506 = Force of nose gear 9,03.The force on the nose gear is 13,250 NThe 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 willdescent 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 forcesAfter the calculation of the external forces, the internal forces can be determined. The most forcesare practised on the downwind main landing gear during a crosswind landing. Therefore, the internalforces on the nose gear (2.2.3a) and the main gear (2.2.3b) are calculated.

2.2.4a Nose landing gear internal forcesAs 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 isthen .

2.2.4b Downwind landing gear internal forcesThe 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 is10,623 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 MaterialsThere are also requirements for materials compiled by EASA. if the materialmeets 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 arealloys (2.3b) . The alloys will be stronger through a heat treatment and a surface treatment (2.3c) .

2.3a Material RequirementsThe 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 toCS-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




29

weldability, and forgeability. They also must be resistant to corrosion, stress corrosion, hydrogenembrittlement, and crack initiation and propagation. All these requirements needs to be establishedon the basis of experience or tests. Conform to approved specifications, that ensure their having thestrength and other properties assumed in the design data.

2.3b Alloy UsedThere are three types of alloys used for landing gears. These alloys meet all requirements of CS-25.These alloys are:

3. Steel Alloy4. Titanium Alloy5. Aluminum Alloy

Ad. 1 SteelThe most used steel for a landing gear of a Boeing 737 is the 300M alloy. 300M is a low alloy, thismeans 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 Co300M 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 ofstrength (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 becoated by an anti-corrosion layer.The 300M alloy is fabricated as a forging. Forging is a metalworking process, where the desired alloywill 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 AlloyThe most used titanium alloy is Ti-AL6-V4, this is a non-ferrous alloy. This alloy is used throughout anentire aircraft and landing gear. Because it has a light weight and a high strength (1150 MPa). It ismore 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 weightreduction due to the low density of the metal or by the increased life of the component due to highcorrosion 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 AluminumThere are two types of aluminum alloy which are used for a landing gear of a 737. The alloys are 7075and 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, buthas less resistance to corrosion. The total chemical composition of AL 7075 is listed in table 2.3 . Thetotal characteristics of AL 7075 are listed in appendix VII .

Table 2.3




30

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

Table 2.2

2.3c Treatment of the AlloyThe 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-typeatmosphere furnaces. The alloy will be heated to the temperature where the mechanical propertiesare its best. The temperature of steel is 870 ºC. The temperature for titanium is 730 ºC. Then it willcool 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 landinggear against the nature elements.

2.4 ConclusionThere 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 singleshock 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 materialcan resist all forces of the landing, acceleration and deceleration during start and landing. Only thestrongest material alloys will be used, such as steel, titanium and aluminum. The material propertieswill 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 alwaysexceptions. Like a Con tinental airlines 737. There was an “overstressed torsion link” and a shimmydamper 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 arethe questions that will be answered in chapter three. It is necessary to prevent or solve the problemswith the current maintenance program if ALA wants to expand, because every unknown error causesa safety risk and a non affordable delay of an aircraft.




31

3 TroubleshootingNow 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, agood check of this fix must be performed (3.2) . by knowing that, a good financial view of the problemand fix can be set up (3.3) . A final conclusion of these aspects will be given (3.4) .

3.1 FailuresGaining 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 bedescribed with use of cause and solution is the shimmy damper failure due to the hydraulic systemfailure (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 failureThe first failure which will be analyzed is the failure of the shimmy damper due to a leaking hydraulicsystem which caused pressure loss (3.1.1a) . A shimmy damper is meant to resolve the shimmy effecton the wheels of the landing gear. Shimmy is caused by critical forces appearing on the wheels, thisoscillation is being resolved using the hydraulic shimmy damper (3.1.1b) .

3.1.1a Cause failureThe shimmy damper failure is caused by a failure of the hydraulic system. The hydraulic fluid can leakout of the lines, or the lines are filled with air bubbles . When the system is missing the pressure , itsmissing its power to operate and so the damper is not able to intercept these oscillations. In case ofa shimmy damper failure, when the force partition is critical, shimmy does take place.

3.1.1b Solution of the maintenance failureIn 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 theshimmy 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 withoutleaking 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 mustbe 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 canaffect the pump by a phenomenon called cavitations . When this occurs the bubbles will implode anddamage 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 ensurethat the oil is vacuum.




32

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 lotof 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 failureThe second failure is a failure of the torsion link apex bolt, which caused a malfunction in the torsionlink (3.1.2a) . When the failure is analyzed a solution for the maintenance program can be found(3.1.2b) .

3.1.2a Cause failureThe second failure is the main gear torsion link failure (figure 3.2) . The failure was that the right MLGwas twisted. The lower portion of the right main gear was rotated about 45 degrees to the right. Theaircraft could not continue taxiing. After research, the conclusion was that the inboard tire on theright 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 tothe torsion link was bent rearward 20 degrees, but still intact and activated. The damage of the rightMLG 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 severalreasons 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 completefailure report can be found in appendix (IX) .

Figure 3.2 Torsion link system

1. Lower torsion link

2. Upper torsion link3. Apex bolt

1

2

3




33

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

After the failure the aircraft could not taxi, so the aircraft was towed into the hangar, where thedamage 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 maintenanceTo make sure the aircraft's maintenance is done correctly the maintenance has to be checked. Thishappens during regular checks.

The nose gear shimmy damper will be checked during the nose wheel steering system test. This testis done on the steering system of the nose wheel and is used to find if the steering rate, steeringwheel centering and the angle of travel are in the necessary limits. The test also checks if the systemis without leaks.To do this test it is necessary that the hydraulic system is serviced and the steering system of thenose wheel is adjusted correctly.To make sure the shimmy damper works correctly, the hydraulic system of the nose gear must bebled 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 nosewheel torsion link. After the test the bolt will be placed into normal position. The problem of theloose 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 torsionlink maintenance check the maintenance crew must check if the bolt is tightened and secured. Bythis 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 boltshave 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 wiregetting tightened, so the bolt will not move. This wire alsomakes it more easy to check if the bolt is still in place.

3.3 CostThe maintenance of the landing gear results in some cost for the airline. There are different checks toreplace or just check all the components of the landing gear. During these checks the aircraft has tostay on the ground (3.3.1) . To execute the maintenance different ground staff and engineers arenecessary (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




34

3.3.1 Aircraft on groundWhen the landing gear is maintained the aircraft cannot fly, and therefore will generate costs. Whenthe aircraft is on ground the airline cannot use the aircraft for transporting passengers or cargo. TheA-check is done once a month and takes seven hours to complete. The A-check is mostly to check themost critical parts of the aircraft. The B-check is done four times in five year. Finally there is a C-checkand a D-check to strip the landing gear and maintain it. This check take place once in five years ofoperation. 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 differenthours to complete. The total costs of all the checks during aircraft on ground are €1.875.000,-.

Check Numbers ofchecks (per fiveyears)

Hours Hours on ground Total costs (inf ive years)

A 40 5 200 500.000B 15 10 150 375.000C 4 50 200 500.000D 1 200 200 500.000

€ 1.875.000

3.3.2 EmployersThe aircraft must be checked by well educated staff. The engineers salary is taken into account to thetotal 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 thelanding gear are more intensive checks and can be done by 6 engineers. The C-check takes 15 hoursto complete and the D-check take 2500 man hours to complete. The total cost of the landing gearmaintenance per 5 years is almost 300.000 Euros.

Check Numbers ofchecks (per fiveyears)

Man hours perinspection

Total man hours Total costs

A 40 5 200 10.000B 15 8 120 60.000C 4 70 280 140.000D 1 2500 2500 125.000

€ 290.000

3.3.3 Leasing landing gearMaintenance of the landing gear are major costs for the airliner, therefore smaller airline companiesare able to lease their landing gears. The total lease costs of a Boeing 737-300 landing gear is about24,000 Euros a year. When an airliner leases a landing gear the only cost of maintenance are thebrakes 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 ConclusionThis report is started with a main question, which meets the demand of the managing board of theALA and the main purpose of the project team 2A2G;




35

'' 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 arecertified 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 onthe landing gear system of the B737. Answering this main question is done by using the informationgained and written in all the previous chapters. The costs and airworthiness are distinguished to get abetter view of both aspects.

According to the team’s forces calculation of the landing gear, the materials can be selected, such assteel, 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.




36

Bibliography

Boeing 737 Manuals and Readers Aircraft illustrated parts B737-300 Aircraft maintenance manual Boeing 737-300Boeing 737 Flight Crew operation Manual – Landing GearContinental maintenance Manual Boeing 737CS-25 amendment 7Mel (minimum equipment list) Boeing 737 all typesSystem schematic Manual

BooksBenjamin Chartier, Brandon Tuohy, Jefferson Retallack, Stephen TennantLanding Gear Shock Absorber3016 Aeronautical Engineering I2006

Hibbeler, Russel CSterkteleer voor techniciPearson Education Benelux, Amsterdam

Hibbeler, Russel CMechanica voor technici, StaticaPearson Education Benelux, Amsterdam

Hibbeler, Russel CMechanica voor technicie, Dynamica

Pearson Education Benelux, Amsterdam

Jelle Hieminga, Simon Ijspeert, Pieter van LangenProjectboek periode 5: Landing gearAmsterdam, Augustes 2010Hoge school van Amsterdam, Aviation studies

Langedijk, C.J.A.VliegtuigsystemenHogeschool van AmsterdamAmsterdamse Hogeschool voor techniek

Mason'sLanding Gear DesignVirginia tech college of engineering1997

Norman S. CurreyLanding Gear Design: Principles and PracticesAmerican Institute of Aeronautics and Astronautics, Inc.1984

Wenztel, TillyHet projectgroepsverslagAmsterdam, 2008




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2009 Smart cockpit

The Boeing Companyhttp://www.boeing.com/commercial/airports/acaps/737sec2.pdf1995 - 2010 Boeing

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




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




40

I Maintenance Schedule




1




2




3




4




5




6




7




8




9




10

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 ofthe MEL item.

Category B:

Items in this category shall be rectified within three consecutive calendar days following the day ofdiscovery.

Category C:

Items in this category shall be rectified within ten consecutive calendar days following the day ofdiscovery.

Category D:

Items in this category shall be rectified within one hundred and twenty consecutive calendar daysfollowing the day of discovery.




11

III Centre of Gravity




12

IV Wing Design




13

V General Characteristics




14

VI Titanium




15

VII Aluminum




16

VIII Hydraulic pressure hose QD coupling leakage




17




18

IX Main Gear Failure Continental




19




20




21




22




23

X Use of LockwireLock wiring* is the securing together of two or more parts with a wire which shall be installed in sucha manner that any tendency for a part to loosen will be counteracted by an additional tightening ofthe wire.

For general purpose lock wiring, use the preferred sizes in Table 1-1. Use smaller diameter wirewhere parts are too small to permit a hole diameter to accommodate the preferred sizes, or wherespace limitations preclude the use of the preferred sizes. The larger sizes are used where strongerwire is required. Wire diameter of .032 is the most commonly used. The common methodof installing lock wireshall consist of twostrands of wire twistedtogether (so called"Double Twist" me-thod). (One twist is

defined as being pro-duced by twisting thewires through an arc of180 degrees and isequivalent to half of acomplete turn.) Thesingle strand method oflock wiring may beused for some applica-tions, such as in a close-ly spaced, closed geo-metrical pattern (trian-gle, square, rectangle,circle, etc.) parts inelectrical system. Where multiple groupsare locked by either thedouble twist or thesingle strand method,the maximum numberin a series shall be de-termined by the num-ber of units that can belock 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 preventloosening. In the event that no wire hole is provided, wiring should be to a convenient neighboringpart in a manner so as not to interfere with the function of the parts. Hose and electrical couplingnuts 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




24

Check the units to be lock wired to make sure that they have been correctly torqued. Under-torquingor over-torquing to obtain proper alignment of the holes is not advisable. If it is impossible to obtaina proper alignment within the specified torque limits, back off the unit and try it again or selectanother 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




25

XI Project assignment




26




27

XII Process ReportThe start of the project was a bit cautious but good. In the first meeting we exchanged numbers andmail 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 partsis completed and need to be done. A few things can be done better. The project group needs tocreate discussions, this is to create more ideas to enrich the information provision of the report. Inthe start document a chairman and secretary list has been set up, where all the members of thegroup 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 24hours on BSCW and sent it to all the members per e-mail with a hyperlink of it. Another thing whatcan be done better is that all the minutes had been checked with feedback by the members , this isto keep the quality of the minutes high and it is a very handy tool to keep the member posted of thecurrent 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 weekwith a margin of a week at the end. This couldn’t g et reached because the problems we encounteredduring 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 groupwith 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 endof 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 shouldwork 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 attent ion levelhigh. At the end of the control rounds the group sends the chapters to the project manager for lastcomments.

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 maintenanceschedule with the corresponding costs to it.




28

XIII Group agreementsThe project should be finished within a deadline. Therefore milestones are created which has its owndeadline. Each group member should complete their tasks correctly and also needs to be up to datewith 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 youcan’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 thegroup. 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 alsomakes 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 mailthat 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 a s-

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 thecaptain at latest two days before the deadline. The group member also let the othergroup members know that he could not make it so they can help him. Of course thegroup member should have a valid reason for not finishing assignments. Eventually thegroup 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 toexplain something.

When someone is not following the agreement more than one time in the whole projecttime, 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 theproject mentor and with the person who is not following the agreements to find a solu-




29

tion. It is possible that the person who isn’t following the agreements, will be put out ofthe 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 orCOM/project lesson is starting. When something happens on short time the groupmember 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 absentand telling a valid reason. The group cannot always check this, so when this is starting tobe 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 unde r-

stan d anything, can’t make it to finish something or can’t be an a meeting: commun i-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/projectmeeting 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 tothe 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 tillwhen the group needs. It is possible that these meeting can be changed from time anddate.

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 rulesand statements;




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XIV Distribution Of Tasks

Introduction Reza Azizahamad 1.4.3 MinimumEquipment List (MEL)

Reza Azizahamad

Summary Jurre Klein 2.1 No Wind Landing

Forces

Matthijs Niemeijer

Jurre Klein1.1 General Aspects Jurre Klein 2.2 Crosswind LandingForces

Joost BroekhuizenRob 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 GerezgiherRobin van Gemert

1.2.4 Brakes Matthijs Niemeijer 3.1.2 OverstressedTorsion Link

Reza Azizahamad

1.2.5 Wheels/Tires Sissai Gerezgiher 3.2.1 Shimmy DamperReplacement

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 ControllingMaintenance

Rob van Loon

1.3.3 Air/Ground Logic Joost Broekhuizen 3.3 Cost Matthijs Niemeijer

1.3.4 Manual GearExtension

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 Klein1.4.2 MaintenanceRequirements

Robin van Gemert

landing gear project report

Documents

Transcript of landing gear project report