INTRODUCTION

An exactly analysis of the landing gear, as designed on modern aircraft’s, is necessary to understand the construction and systems of the landing gear of the Airbus A320. Operation of the landing gear is made possible through extension/retraction, steering, braking and damping systems. In order to maintain the safety of the aircraft, the design of the landing is satisfied to the legislation of the European Aviation Safety Agency . With the knowledge of the landing gear construction of the Airbus A320, the forces on the construction calculated during different flight phases. In these flight phases the aircraft endures several forces. The materials that are used depends on the forces at the aircraft . Then, with a good insight of the A320’s landing gear I’m able to provide an in detail overview of the common faults and problems of the A320’s landing gear. These faults and problems have consequences for the aircrafts airworthiness. Change in the aircraft’s airworthiness requires maintenance with inevitable costs for the airline .
The used main sources, serve as information to learn how the landing gear of the Airbus A320 operates.

Literature review

The Airbus A320 family consists of short- to medium-range, narrow-body, commercial passenger jet airliners manufactured by Airbus. The family includes the A318, A319, A320 and A321, and the ACJ business jet. Final assembly of the family in Europe takes place in Toulouse, France, and Hamburg, Germany. Since 2009, a plant in Tianjin in the People's Republic of China has also started producing aircraft for Chinese airlines. In June 2012, Airbus announced plans to begin building the 319, 320, and 321 variants in Mobile, Alabama. The aircraft family can accommodate up to 220 passengers and has a range of 3,100 to 12,000 km (1,700 to 6,500 nmi), depending on model.
The first member of the A320 family—the A320—was launched in March 1984, first flew on 22 February 1987, and was first delivered in 1988. The family was soon extended to include the A321 (first delivered 1994), the A319 (1996), and the A318 (2003). The A320 family pioneered the use of digital fly-by-wire flight control systems, as well as side-stick controls, in commercial aircraft. There has been a continuous improvement process since introduction.
On 1 December 2010, Airbus officially launched the new generation of the A320 family with the A320neo "New Engine Option". The new generation offers a choice of the CFM International LEAP-X or Pratt & Whitney PW1000G, combined with airframe improvements and the addition of winglets, named Sharklets by Airbus. The aircraft will deliver fuel savings of up to 15%. Virgin America will be the launch customer for the aircraft in spring of 2016. As of 31 December 2011, a total of 1,196 A320neo family aircraft have been ordered by 21 airlines making it the fastest ever selling commercial aircraft.
As of August 2012, a total of 5,232 Airbus A320 family aircraft have been delivered, of which 4,858 are in service. In addition, another 3,621 airliners are on firm order. It ranked as the world's fastest-selling jet airliner family according to records from 2005 to 2007, and as the best-selling single-generation aircraft programme. The family's direct competitors are the Boeing 737, 717, 757 and the McDonnell Douglas MD-80.

The following table provides details on the A320 EASA ETOPS approvals.

Landing gear A320

1.Component and system

The landing gear of the A320 is controlled by several landing gear systems, but the basis lay-out is the same as any other aircraft of the same class. There are several systems that control the landing gear during different flight phases, these systems must somehow be activated at the right time. This is done by the basis lay-out slowly evolved to a retractable lay-out which is standard for large civil aircraft these days. The landing gear is also the only way to control the aircraft direction on the ground. Due to the excessive use of the landing gear on the ground, the tires and wheels must be able to endure this stress for longer periods of time. But if they need to be replaced, it must be a easy routine job which does not require the disassembly of the landing gear. One of the main causes of wear to the tires is braking, because the tires are kept on the skid limit. Before the brakes are used during landing, the aircraft has made a touchdown. This touchdown requires another landing gear system, namely the shock absorbers which reduce the impact on the rest of the aircraft. Some of these systems are required by law, especially for civilian aircraft. The source that is most used for this chapter is: Technical training manual (1999-2000).

1.1 General description

The landing gear or undercarriage of the A320 has four main functions. It separates the aircraft from the ground, allows the aircraft to manoeuvre on the ground with a steerable nose wheel, it softens the shock during landing and slows the aircraft down by using the brakes. The landing gear used on the A320 it has a tricycle gear layout. It has one nose landing gear leg and two main landing gear legs that are placed under the wing and retract into the fuselage.

1.1.1 Tricycle gear layout

The tricycle gear (figure 1.1) consists of one nose landing gear leg and two main landing gear legs. When using a tricycle gear, around 13% of the total weight of the airplane acts down on the nose landing gear and 87% acts down on the main landing gear. The weight distribution depends on the position of the center of gravity. The center of gravity on the A320 must be between 17% and 38, 5% on the Mean Aerodynamic Chord (MAC). The location of the MAC can be calculated using the aircraft’s dimensions .

Figure 1.1 Airbus 320 Landing gear layout

The advantage of the tricycle gear is the stabilizing momentum during touchdown when the aircraft’s longitudinal axis is not parallel to the runway. The drag created by the two main gear wheels is parallel to the motion direction (figure 1.2). This means there is no momentum created however the drag that works on the two main gear wheels to the side creates a negative momentum. This momentum turns the aircraft parallel to the runway again.

Fig 1.2 Tricycle gear

Another advantage of the tricycle gear is that the aircraft is in a horizontal position when parked on the ground. Unlike the conventional landing gear configuration which has a tail wheel instead of a nose wheel. With the tail wheel causes the airplane to stand in a nose up position therefore making loading the airplane more difficult The disadvantage of the tricycle gear is that it is heavier than a conventional configuration. Also when loading an aircraft equipped with a tricycle gear you cannot load the aircraft in such a way that the center of gravity is shifted to behind the two main gear wheels. This causes the aircraft to tip over and hit the ground with its tail.

1.1.2 Nose and main landing gear

The A320 uses a four bar linkage on both nose and mean landing gears (figure 1.3). The four bar linkage is used. This four bar linkage has one degree of freedom it can extend and retract. When using the four bar linkage it does not matter if it is retracted straight or retracted sideways.

1.2 Steering

During taxiing, landing and takeoff, the pilots need directional control of the aircraft. The nose landing gear steering system controls the direction of the aircraft on ground. The steering system has limitations to prevent dangerous situations or damage to the aircraft. The ECAM display shows information about the steering system.

1.2.1 Steering system

The steering system is hydraulically powered and controlled by an electrical servomechanism from the cockpit via the Braking and Steering Control Unit (BSCU). The steering system is controlled by two hand wheels during taxi, or with the rudder pedals during landing and takeoff. The pilots give steering commands from the cockpit. The signals are processed by the BSCU (A) which controls the hydraulic block (B) . The hydraulic block powers the steering actuator (C) that turns the nose landing gear.

A. Braking and Steering Control Unit
The BSCU gets commands from the hand wheels or the rudder pedals. The orders are converted to an electrical signal and sent to the BSCU. The orders are processed before they reach the hydraulic block.
1. Aircraft speed
2. Anti skid and nose wheel steering switch
3. LGCIU
4. Towing control lever

1. Aircraft speed
The aircraft reference speed for the nose wheel steering is received by the ADIRUs. The information is used for a correct steering angle at the relative aircraft speed.

2. Anti skid and nose wheel steering switch
The anti skid and nose wheel steering switch, which is placed in the cockpit, must be in the ‘’ON’’ position to send the signals to the hydraulic block.
3. LGCIU
When the nose landing gear is extended and the nose gear doors are closed, the hydraulic block is pressurized. Also the main landing gear has to be compressed. When both demands are met, both chambers of the steering actuator are supplied and the nose wheels return to an angle of 0° for landing.
4. Towing control lever
The towing control lever, which is located on the strut of the nose landing gear, must be in the ‘’NORMAL’’ position. Only then it is possible for the pilots to manoeuvre with the aircraft. The towing control lever has to be deactivated for towing purposes. When it is deactivated, the BSCU will not send orders to the hydraulic block. It is one of the safety measures in the steering system. Because of this it is impossible to damage the steering system of the nose wheel landing gear.

B. Hydraulic block
The BSCU sends signals to the hydraulic block .The hydraulic block is placed at the rear of the nose landing gear, and is powered by the hydraulic green system. With a pressure of 4000 Pressure per square inch (Psi) from the hydraulic green system, it is not possible to accurately control the steering actuator, because the bypass valve is opened and tries to decrease the pressure in the hydraulic block. The hydraulic block calculates the correct pressure by using the signal from the BSCU. The adjustable diaphragm, located on each output line of the servo valve, adjusts the flow to the actuating cylinder chamber and thus the wheel steering speed.

C. Steering actuator
With a hydraulic flow from the hydraulic block, the steering actuator pushes the rotating tube, which is part of the nose landing gear structure, via a rack-and-pinion assembly to the correct position. There is one anti-shimmy valve per steering actuator camber, which prevents that the steering wheel is influenced.

1.3 Wheel systems
The six tires on the A320 are mounted on divided wheels. The tires have different zones and have a complicated design.

1.3.1 Wheels
The A320 has divided wheels because the tires are mounted tight on the wheel for an air tight seal. Replacing the tires would be impossible without the divided wheel because pulling the tire over the wheel is impossible. The divided wheel consists of two forged light alloy halves and is connected with high tensile steel bolts and nuts (figure 1.5,). The tires are mounted directly on the wheel, so an airtight seal is needed between the two halves. This seal is made by using an O ring (2). The tire is filled with nitrogen, because by using this, the air pressure is also maintained for a longer period of time. The inflating of the tire is done using an inflation valve (3). When braking the aircraft, heat is building up in the brake packs. This heats up the wheel and tire causing high pressure and result in the possibility of tire blowout. To prevent tire blowout due to heat two sets of fuse plugs have been installed. The first set placed inside the wheel keys melts at 300 °C (4). The second set placed on the wheel web blows out at 183 °C (5).

1.4 A320 Brakes

The A320 has brakes on the main landing gear only. During manual operation of the brakes, the normal brake system is used. To get the most efficient use out of these brakes and to stop the aircraft from skidding an anti skid system is installed. However some phases of the flight or on the ground require a different operation of the brakes. If a failure occurs in one of these brake systems, the A320 has several alternate brake systems, so it is still able to decelerate or stop safely to prevent other damage to the aircraft and its load. The pilots must know when the normal systems or one of the alternate systems is active and where the failure is located, this is projected on several displays in the cockpit.

1.4.1 Brake components

The brakes of an A320 are hydraulic multiple disc brakes. Each main landing wheel is equipped with one brake, so this means that there are a total of four brakes on the aircraft. The braking principle is based on friction between two different parts of the brake. The plates that are used for the brakes are made of carbon because carbon wears less than normal steel. The multi disc brake (figure 1.7) consists of a piston house, which is connected to the axe of the tire. Each brake consists of fourteen pistons, seven of these are used for the normal braking and the others are used for the alternate braking. In the hydraulic system there is also a bleed valve. The purpose of this valve is to get rid of air in the hydraulic fluid. The carbon heat packs (rotor) is attached to the wheel and rotate together. The carbon plates (stator) are attached to the piston house. The measurement of the wear pin indicator is based on the amount of extension of the pin. The further the extension is the less wear there is. The pistons driven by hydraulic fluid push the carbon heat pack on the carbon plates, the green system is used as normal and yellow as alternate. When the heat pack and the plates are compressed by the pistons, friction is created between these two parts and the aircraft slows down. By creating friction between the two parts, the temperature increases. Therefore a temperature sensor is added to the brakes. The brakes are not allowed to get a higher temperature then 300°C when the brake fans are off. In order to lower the temperature of the brake fans can be enabled. The maximum temperature of the brakes is 150°C in this case. The brake temperature indicator measures the potential difference between the two different metals.

1.4.2 Normal brake system

To control the normal brakes, the system has an electronic system (A) and a hydraulic system (B).

A. Electronic brake system
The electronic system is used to control the hydraulic system and to collect and process the feedback it gains from the hydraulic system. When the brake pedals are used, the mechanical signal of the pedals is transformed into an electronic signal by the brake pedal transmitter unit. The electronic signal is send to the Brake and Steering Control Unit (BSCU). The BSCU is connected to the LGCIU, the ADIRU and the Spoiler Elevator Computers (SECs) that supply data to the BSCU like wheel rotation speed and deceleration/acceleration data. The BSCU consists of two exactly the same systems which work simultaneously (figure 1.8). If one system fails, the BSCU can still operate with the other one.

B. Hydraulic brake system
The BSCU processes the electronic signal and activates the selector valve which allows green hydraulic pressure into the system. The pressure goes through the throttle valve to the automatic selector valve which puts the yellow system on standby and allows green pressure into the brakes manifold. The throttle valve prevents quick returning of the automatic selector valve when it is de-energized. This is to avoid instantaneous release of the brakes. The normal brake manifold consists of two normal servo valves and a hydraulic fuse, there is one normal manifold for each wheel. Depending on the brake force needed by the pilots, the BSCU sends a certain current to the normal servo valves. These valves convert this current to a certain amount of braking pressure by which braking is realized. The hydraulic fuse plugs the line in case of a leakage. Two master cylinders are also present to give an artificial feel at the braking pedals.

1.4.3 Anti skid

The purpose of the anti skid is to keep the maximum braking efficiency on each wheel in order to avoid a skid situation. The BSCU provides anti-skid control during normal and alternate braking, the yellow or green system is used for this. The BSCU provides this information to two systems, system one or system two, which work simultaneously.
The speed indication of the wheels is giving by the tachometer. The tachometer measures the rotation speed of the wheels. The tachometer consists of two rings. One ring is attached to the axle and the other is attached to the wheel itself. The rotation speed is measured by variations in the induction frequency, this variations appear because a piece of metal moves along the ring that is attached on the axle.
This speed is compared to the speed that is supplied by the air date inertial reference unit. As soon as the indication of the tachometer goes below 0.87 of the speed that is provided by the ADIRU, the brakes are released. When the speed of a single wheel comes below the speed of 0.87 this single wheel, orders are given to release the brakes on this wheel to avoid skidding. A situation where a form of skidding happens pretty often is hydroplaning, what happens in this case is that a layer of water comes between the tires and the ground. The aircraft will lose traction and because of this it will not slow down when the brakes are used. Anti skid can be disabled by putting the A/SKID & N/W STR switch to off.

1.4.4 Alternate brake systems

When there is a problem with the normal brake system, the alternate system is automatically switched on. In order to still brake as efficient as possible anti skid is still active (A). When the anti skid function is lost, the aircraft needs a longer distance to decelerate because the pilots need to brake more careful to prevent skidding (B). If this alternate system fails, the parking brake could be used as an emergency brake (C).

A. Alternate braking with anti skid
When green pressure is lost the automatic selector pressurizes the alternate system with yellow pressure. The mechanical signal from the pedals is no longer converted into an electronic signal but directly into low hydraulic pressures by the auxiliary low pressure control system. The pressure goes to the dual valve which converts this in the proper amount of braking pressure for the left and right-hand brakes using yellow pressure. This pressure passes the dual shuttle valve which is now in alternate selection instead of parking brake selection. Braking is realized by the alternate braking manifold. The anti skid is regulated by the BSCU using the alternate brakes.

B. Alternate braking without anti skid
Anti skid is no longer available when the anti skid is turned off, when there is a power supply failure or when only accumulator pressure is available. If the alternate system is supplied by the accumulator only, then only seven full brake actions can be performed. The anti skid is not available anymore, pilots need to reduce the amount of braking to avoid skidding. This means that the aircraft decelerates slower and needs a longer distance to reduce speed.

C. Emergency parking brake
As a last resource, the parking brake could be used in an emergency. The dual shuttle valve selects the parking brake system which then supplies the alternate brakes and brake pressure is released. This is only for emergency situations because it can cause skidding, overheating and damage to the wheels and brake systems.

1.5 Damping system

During the landing, the landing gear endures huge forces. The shock absorbers soften the forces that are involved. Torque links are placed at the rear of the nose landing gear. Torque link dampers decrease the vibrations through the torque links during a landing.

1.5.1 Shock absorber

The shock absorber must soften the impact of the touchdown. The main landing gear is in normal conditions the first part of the aircraft which touch the ground (A). The nose landing gear is the last part of the landing gear which touches the ground (B). A. Main landing gear
The shock absorber makes the touchdown more smoothly and gradually. The shock absorber is a telescopic oleo-pneumatic unit. Oleo-pneumatic means that the shock absorber works with hydraulic fluid and pneumatics combined. The reason that hydraulics and pneumatics are combined is that if there are only pneumatics, the aircraft gets a recoil. With only hydraulics, there will be no compression at all. When the shock absorber compresses, the forces of the touchdown are transmitted to the hydraulic fluid and nitrogen gas. The recoil stroke is slow to make sure that the aircraft does not become airborne again. The shock absorber is a two stage unit and contains four chambers.
• First stage gas chamber contains a gas at low pressure and some hydraulic fluid.
• Recoil chamber that contains hydraulic fluid.
• Compression chamber that contains hydraulic fluid.
• Second stage gas chamber contains a gas at high pressure.
The damping tube, which contains the first stage orifice, attaches to the head of the second stage cylinder and has a fluid connection. The movement of the damping tube through the orifice block decreases the fluid flow in the first stage damping. This increases the damping effect. A floating piston in the second stage cylinder separates the hydraulic fluid of the compression chamber and the gas of the second stage chamber. During compression, the floating piston does not move down until the gas pressures of the first stage and the second stage chambers are equal.
As noted earlier, the shock absorbers of the main landing gear are running a few stages when the aircraft makes a touchdown.
1. Compression
2. Recoil
3. Compression and recoil – second stage gas chamber

1. Compression
During the compression, the sliding tube slides into the main fitting. The volume of the shock absorber reduces, which compress the gas. The fluid flows from the compression chamber to the first stage gas chamber, through the first stage orifices of the damping tube. While the shock absorber compresses, the damping tube and the first stage orifices go through the damping head. The flow through the first-stage orifices stops, and the flow limits to that through the compression-orifice plate. This produces a two stage damping effect. The gas compression in the second stage chamber, pushes on the floating piston to help the damping. It also helps to make the damping effect and the compression effect of the oleo smooth. At the same time, the fluid goes from the first stage gas chamber into the recoil chamber, through the openings in the upper bearing. This flow of fluid moves the recoil-orifice plate against the flange of a retaining ring, to let the fluid flow fully. The gas compression and the fluid transfer absorb the shock-loads from the main landing gear.

2. Recoil
There is still energy in the gas of the first and second stage gas chambers. This gas wants to expand. The fluid goes through the recoil-orifice and the compression-orifice plates. The flow of fluid moves these plates to their almost closed position, so that the fluid movement is slow. This decreases the speed of the recoil travel. A flow of fluid through the first-stage orifices in the damping tube only occurs if the recoil orifice and the compression-orifice plates go into the compression chamber again. The gas in the second stage chamber helps to make the extension effect of the shock absorber smooth.

3. Compression and recoil – second stage gas chamber
The compression and the expansion of the gas in the second stage gas chamber help to make the effect of the shock absorber smooth. This is transmitted through the floating piston to the oil and then to the remaining parts of the shock absorber assembly. This procedure helps to make a smooth landing.

B. Nose landing gear

The shock absorber in the nose landing gear is a single chamber type and handles the shock at the touchdown in two stages. The reason that the nose landing gear is not a four chamber shock absorber is because it does not have to catch the huge forces of the main landing gear. The shock absorber is filled with hydraulic fluid and nitrogen through a single standard servicing valve in the upper part of the leg.

2.Landing gear operations

During the operational time of the A320’s landing gear malfunctions will occur in the landing gear. To prevent that all malfunctions ground the aircraft and create high costs, the Master minimum equipment list and Minimum equipment lists have been created. Two of these malfunctions, the 90° turned nose wheel and leaking shock absorber, have been analyzed for cause and damage to the landing gear. The 90° turned nose wheel and the leaking shock absorber cause the airplane to go into maintenance and create large costs. However by doing proper maintenance by following procedures the safety of the airplane increases and costs will decrease because of less malfunctions. Following from this information a conclusion has been made on how the maintenance effects costs and its positive effect on costs in the long term. The most used sources for this chapter are Aircraft maintenance manual (2004) and the Master minimum equipment list (2009).

2.1 MMEL & MEL

The Master minimum equipment list (MMEL) and the minimum equipment list (MEL) inform the operator or mechanic about the airworthiness of the aircraft when a failure occurred. The documents are used as handbook to decide if an aircraft can dispatch or need to stay on the ground and undergo maintenance. Each failure has a different impact on the system. For some failures it is easy to recognize if the aircraft is still able to dispatch, for example a leak in the green hydraulic system it is more difficult to determine. During the development process of the MMEL different technicians work together, in order to create the MMEL in such a way that the acceptable level of safety is guaranteed while there is no loss in profitability of the aircraft. Every failure that can occur has impact on several systems instead on only the failed system, multiple failures need to be avoided at all cost since these result in an unacceptable level of safety. The difference between a MMEL and the MEL, is that the MMEL is made by the manufacturer the moment the aircraft is designed. The MEL is made by the operator which includes his own demands, that are as strict or stricter then the MMEL. The MEL is also specified to the layout of operator’s aircraft.

The MMEL has a standard lay-out (figure 3.1). In the column item, the items are listed for which a failure can occur. The rectification interval can be classified as A,B,C or D. When an item is classified as A there is no maximum timeframe when the rectification should take place, for B there is a maximum of 3 days, C 10 days and 120 days for D. The column for number installed refers to the amount of installed items there are in total on the aircraft. Number required for dispatch refers to the amount of items that need to be operative in order to have a safe dispatch. The column remarks or exceptions gives an overview of all exceptions there can be for a certain item that must be met before a dispatch can take place.

Figure 3.1 example of a MMEL

2.2 A320 failures

2.2.1 Nose landing gear failure

On the 21 of September 2005 a JetBlue A320, on a flight from California to New York, had a problem with its nose wheel. After take-off, when the pilot tried to retract the landing gear, two warning lights light up. These lights indicated that there are problems with the shock absorber and steering on the nose gear. The problem was that the nose landing gear turned 90°. During the emergency landing when the nose gear touched the ground, the rubber tires shredded away until the metal wheel scraped the runway pavement. This friction caused a trail of white smoke and finally sparks and flames. The failure could be a result of a malfunction in one of the control systems. Malfunctions in other mechanical components can also result in this failure. In this case, the real cause was a wrongly installed hydraulic shock absorber. During the installation of the shock absorber the upper part of the inner cylinder, which contains anti-rotation lugs, was wrong installed. Anti-rotations lugs are installed On the top of the shock absorber inner cylinder to protect the inner cylinder against rotating and twisting. The anti-rotation lugs must correctly seat in the slots. Because of the wrong installation the anti-rotation lugs were rotated 20-30°, so they are not properly fitted in the back plate slots. The A320 cannot be airworthy with a wrong installed hydraulic shock absorber. The nose wheel cannot be retracted and because of the 90° nose wheel turn the landing cannot be performed smoothly. There will be severe damage to the nose landing gear and the aircraft.

2.2.2 Shock absorber failure

The shock absorber could have a hydraulic leakage which results in less effective or no absorption during landing. This failure could occur because of poor maintenance.
There could be a leakage of hydraulic oil in several points (figure 3.2). Leaks between the main fitting and the gland housing or between the gland housing and the sliding tube are possible. One of the leakage causes is poor maintenance of the chrome sliding tube. In this case the chrome is damaged by dust that is not removed during maintenance. When the chrome is damaged, the sliding tube is not sealed any more. It is also possible that the leak is caused by damaged grand seals which separates the hydraulic fluid from the sliding tube through the main fitting. The aircraft is not airworthy with a hydraulic leak in the shock absorber. The shock absorber cannot bear the forces during touchdown, so the safety is not guaranteed.

Figure 3.2 Leakage points of a shock absorber

3.Maintenance and Servicing

To ensure and maintain the safety of the passengers and personnel, maintenance of the aircraft is required. Maintenance is divided in three different categories. When a failure occurs, the last of the three categories is needed which is non-scheduled maintenance.
Maintenance categories
There are three maintenance categories:
1. Inspections
2. Scheduled maintenance
3. Non-scheduled maintenance

1. Inspections
Inspections are visual and carried out at daily or weekly basis. The purpose of these inspections is to discover visual damage to the aircraft. These inspections are carried out by a qualified mechanic, which has to sign for approval. The walk around of the pilot before each flight is also a form of inspection.

2. Scheduled maintenance
Scheduled maintenance is carried out at a fixed time or on-condition basis. A component is replaced or overhauled after a number of flight hours or a cycle at a fixed time. There are three types of scheduled maintenance, the A/B-check, the C-check and the D-check. The A check is done after 550 flight hours (approximately three months) or 330 flight cycles. During the A/B check the oil filters are replaced which takes at least 16 hours. The C-check is more complex and is done after 4000 flight hours (approximately eighteen months) or after 3000 cycles. During this check all crucial elements of the landing gear and the aircraft are inspected and if necessary replaced.
The D-check is very time consuming, it takes about four weeks. During this period the entire aircraft is separated and each part is inspected. This check is done after 24.000 flight hours (approximately eight years) or after 18.000 cycles.

3. Non-scheduled maintenance
When a failure occurred during flight, or when a failure is found during inspection non-scheduled maintenance is required. Sometimes replacement of components can be deferred, if allowed by the minimum equipment list (MEL).

4.Safety Features
The most well-known safety feature of the A320 family is the fly-by-wire system. Prior to the launch of this aircraft, pilots had to fly planes through a cord and pulley system and did not control the plane electronically. The new system provides pilots with a completely overhauled system that controls the plane electronically. The sophisticated flight system contains automatic safety fallbacks, including protection against pilot manoevers that exceed the plane's thermodynamic capabilities or that pull the aircraft below stall speed.
Airbus fly-by-wire is now a wide family that may be considered as a legitimate reference for transport aircraft flight control system. Its main characteristics are recalled hereafter: * Lateral side-sticks, with constant feel force * Enhanced control laws and flight envelope protections: aircraft response is "slaved" to pilot orders. * Redundant and dissimilar fail-safe computers performing the entire fly - by- wire functions including servo-control command and monitoring. * Ultimate mechanical back up by rudder and THSA. On A330/A340, yaw damper is available in ultimate back-up mode.

Aircraft safety level is improved: * an electrical circuit may be installed in areas not suitable for hydraulic circuits and may be more segregated from the remaining circuits that the one it replaces. Survivability to peculiar hazard such as engine burst is consequently improved. * aircraft remains controllable if a common failure mode affect all the hydraulic circuits * with an electrical circuit, only "downstream" equipment are lost in the case of circuit rupture

CONCLUSION

After the three analyses that have been done regarding the construction, the different forces on the construction and the failures that can occur on the landing gear. The team can conclude the following points: * Landing gear is one of the most essential parts of the aircraft, it has influence on different systems as well as the design of the aircraft. * The materials are able to sustain the stress that is on the different components in the different phases of the flight. * Maintenance is required to prevent malfunctions, when this maintenance is not done in a sophisticated way as for example in the case of the JetBlue where the engineers did not rotate the shock absorber around its vertical axis before tightening the shock absorbers upper bolt, this malfunction would have been avoided. * the aircraft must follow their maintenance schedule to make sure the aircraft always in good condition. * A correct maintenance program does not only have a positive influence, the downside is that the aircraft can be less in the air. Beside this, a good maintenance program does not guarantee that no failure will occur anymore. * The fly-by-wire technology is the most development of aircraft for safety features.

Based on these points the advice of the team is to increase the frequency of the maintenance by 300% compared to the old maintenance program.

Reference * Airbus 320 maintenance manual * EASA Type Certificate Data Sheet for A320 * http://en.wikipedia.org/wiki/Airbus_A320_family * Analysis of landing gear system by Hogeschool van Amsterdam