ASY500 - Landing Gear - Grade: 100 PDF

Preview

Summary

Landing gear systems discussion for the A320, B777 and B787...

Description

Landing Gear & Brake Systems ASY500 October 16, 2018 AIRCRAFTS: A320, B777 & B787

Table of Contents:  Introduction

2

 Canadian Aviation Regulations Section 525 for “Landing Gear”

3

 Landing Gear Statistics

““5

 Airbus A320 - Landing Gear

6

 A320 Landing Gear Incident - JetBlue Airways

15

 Boeing 777 - Landing Gear

“ ““

16

 B777 Landing Gear Incident - Air France - Korean Air - Air Canada  Pilot’s Perspective  Bibliography

25

“ “ “ 26 42

1

Introduction In this report, the landing gear system will be examined for the Boeing 777, Boeing 747 and the Airbus A320. The contrast between the two manufacturers and how they designed their landing gear will also be studied. The landing gear system has many aspects that come to play in its design, the shock absorption, retracting mechanism, the tires and the braking system is all included and integrated. Each will be looked at carefully, with how the design accommodates for the Canadian Aviation Regulations that govern those components. This report will also include case studies on real life accidents where the landing gear system have failed on these aircrafts. The purpose of the landing gear is to allow the plane to maneuver on the ground, whilst also able to support the weight of the plane and allow it to take the forces applied to it in the event of a landing. Certain aircrafts also have the ability to retract their gear, this is to fold the landing gear into the fuselage or into the wing of the aircraft by the pilot’s input. There are many ways a landing gear can be retracted, this includes being folded sideways (both outward or inward direction of the fuselage) and even folding backwards and forwards. The method it is retracted depends on the design of the aircraft, and the size of the fuselage and wing. There are even some retractable landing gears made to rotate on their way up as they fold into the fuselage. The primary purpose of the shock absorber is to prevent any landing forces from damaging the fuselage or body of the aircraft. This is a necessary component to include in the landing gear since pilots cannot always guarantee soft landings. If the landing forces are not properly distributed or absorbed in this case, these forces will inevitably cause structure failure or damage to the aircraft. There are 4 general types of shock absorbers: low pressure tires, oleo, rubber and spring steel. The oleo is the one that will be most mentioned in this report. The way this system works is that the force is dissipated by forcing oil (an incompressible fluid) from one piston to another, usually one of a different size through a small opening. The displacement of the oil helps cushion the shock of the forces from a landing. This oil is then displaced and will not return to its original holding sink until it leaves the ground again. Further forces that may be experienced during taxiing or on take off can be then further dampened by another valve that contains compressed air. This is called an oleopneumatic system. In this report, the braking systems for the Boeing 777, 787 and the Airbus A320 will also be discussed. Brakes are advantageous on a plane as they provide quick deceleration after landing and better maneuverability on the ground. The deceleration is important in shorter runways. Braking is also useful as it can reduce the radius of turn on the ground while taxiing the aircraft. One crucial element in the braking system is the hydraulic fluid. This is what aids in the application of the brakes, lubricates the moving parts and keeps the system cool. More details on the type of braking system in these aircrafts will be further discussed below.

Canadian Aviation Regulations Section 525 for “Landing Gear” The Canadian Aviation Regulations help specify the requirements for the landing gears in operation of every aircraft. This is to ensure the safest possible outcome in any abnormal or emergency situation. As a report, a summary on some of these requirements is as seen.

2

In section 525.721 (for aeroplanes that have any amount of passenger seats), the landing gear must be designed such that overloading of the landing gear during takeoff or landing would not cause any fuel spillage to the amount it becomes a fire hazard. However for aeroplanes that have 10 or more passenger seats, they must be designed such that when any one or more landing gear legs is unable to extend, upon landing on a paved surface - it would not sustain structural damage that is likely to cause a fire from fuel spillage. These requirements may be proven through data or through tests, or both. In 525.723, it details about the requirements done in the shock absorption tests. Landing gear dynamic characteristics can be shown and validated by energy absorption tests. These tests must be done to validate the design conditions specified in section 525.473 which will be mentioned later. In the energy absorption tests, the minimum condition must include the design landing or takeoff weight, the one with a greater load must be chosen for the test to simulate the worst possible outcome. The limit loads must be proportionate to the test altitude and drag loads, conducted in realistic and consistent conditions. The landing gear must have reserve energy absorption capacity, simulating the design landing weight with a descent velocity of 12 feet per second where lift is not greater than weight - the landing gear must not fail the tests. Any changes in previously approved design weights may be approved through analysis by previous tests done in similar conditions. The CARs also specifies the retracting mechanisms of aeroplanes (525.729). They must be designed for loads experienced during flight when the gear is in the up position, this is in different configurations (flaps extended) and must consider the gyroscopic force when processing the gear in the up position. The aircraft must also be able to extend or keep landing gear (and doors) in the retracted position while in flight and extended (and on the ground). The flight crew must also have an emergency procedure which they can follow in order to extend the landing gear when hydraulics/electric/energy source of the landing gear is insufficient or any reason of failure in the normal retraction system happens. The reaction mechanism must be demonstrate positive results in operation tests. The flight crew must also have a form of indication which can inform them whether the landing gear is in the UP or DOWN position, and if their associated doors are OPENED or CLOSED. Aural warnings must function as well, indicating to the flight crew if the landing gear is not locked down when intended to land. In addition, any equipment located on the landing gear and its wheel wells must safe from any effects of a possible bursting tire, wheel brake temperatures or loose tire tread. For the wheels, which is specified in 525.731 must carry a minimum of the designed maximum weight and critical centre of gravity. It should also equal or exceed the maximum radial limit load. They should also be designed to prevent any failures or tire bursting from the high pressurization of the wheel and tire assembly. The Minister must approve a suitable tire of designed fit with proper speed rating and load rating that must withstand most critical combination of maximum weight and centre of gravity. They must also be sized such that they have enough space to clear the surrounding structure in a retractable landing gear system. For aeroplanes with a maximum certified takeoff weight of 75,000 lbs or more, the tires on braked wheels must be inflated with a gas mixture with less than 5% oxygen in volume (for example: dry nitrogen) this would ensure no dangerous gases are produced when heated. In the brakes and braking system written in section 525.735 of the CARs, they must be designed such that if any single brake operating energy supply fails it is still possible to bring the aeroplane to rest by 2 times the braking distance specified in 525.125. If there is any fluid loss in the brake hydraulic system, the brakes would not have sufficient cause to start a hazardous fire

3

in any part of the phases of flight. The brake controls must not need excessive control force to operate them, and if there is an auto-brake system installed - there must be means to arm and disarm the system and allow the pilot to provide manual braking. The aeroplane must also have a parking brake and an indication if the parking brake is not fully released. The antiskid system (if installed) must perform satisfactorily in expected runway conditions without readjustments from the outside and if at times, have priority over the auto braking system. If there is a stored energy system installed, an indication to the flight crew must be provided. A minimum of 6 full applications of the brakes when the anti-skid is not operating must be available in the stored energy. The brakes must have a form of brake wear indicator which shall be easily available and reliable to indicate any wear. They must also have means for the braked wheels to prevent wheel failure, tire burst caused by extreme high temperatures produced by the brakes. Beginning of section 525.471, the detailing of the ground loads, landing load conditions and assumptions, landing gear arrangement, level landing conditions and side load conditions are all mentioned. These however will be referred to later in the report if they relate to the aircraft being studied. In conclusion, references to many more specifications can be found in the CARs, detailing the minimum design qualities and characteristics the landing gear/braking systems should have. These all help design a safe landing gear system, fit for the public and for commercial use.

Landing Gear Statistics An overview of some of the statistics of accidents involving landing gears, and their failures will emphasize a greater importance on the research of this system. According to the Transportation Safety Board of Canada published report done in 2009, a summary of aviation occurrences can be found. These statistics by the TSB were updated till March 2, 2010. The table below contains data taken from this report. Canadian Registered Aircraft - Aeroplanes Involved in Accidents by First Event Fault

2000

2001 2002 2003 2004

2005

2006 2007 2008 2009

Landing Gear Collapsed/Retracted

8

7

10

9

10

3

3

9

8

8

Total Accidents

258

243

210

242

206

206

208

237

201

215

%

3.1

2.9

4.8

3.7

4.9

1.5

1.4

3.8

4.0

3.7

4

This would mean by an average between 2000 and 2009, the accidents that were related to a faulty landing gear failure was 3.8% of the accidents by first event. Another statistic reported by the TSB for Canadian registered aircrafts involved in incidents, between 2005 and 2009 in the comparison of first event type of failure and incident type. 22.7% of those declaring an emergency experienced a landing gear failure. In another recent report done by the TSB, they concluded between 2007 and 2017, out of the 1261 aeroplanes that experienced an accident during landing phase - 21% of them were related to having the landing gear system fail. In the year 2017 alone, this statistic was at 24%. In 2016, Boeing’s record showed for aviation accidents related for commercial aircraft had experienced 12 incidents involving collapsed or failed landing gears.

Airbus A320 - Landing Gear The landing gear of the Airbus A320 includes two main gears, a nose gear that both have the ability to retract into the aircraft during flight. There are doors that enclose the landing gear, where both the gear and doors are controlled electrically but move and operate hydraulically. These doors fit on the landing gear struts, and open when the gear is either retracting or extending in the phases of landing and take off. This ability is controlled by the two landing gear control and interface units, which is also known as the LGCIUs. The LGCIUs send information to the ECAM that indicate when the aircraft is in flight, on the ground or in another phase of flight to other aircraft systems. In addition, a manual hand crank is placed in the center pedestal which enables the flight crew to still extend the landing gear in the event where the hydraulic or electrical systems fail. For the Airbus A320, every main gears consists of twin wheels that are equipped with an oleopneumatic shock absorption system as well as an anti-skid brake. The nose gear, is twowheeled that has a steering system and a oleopneumatic shock strut. On a normal operation basis, the flight crew is able to control the landing gear through a lever on the center instrument panel. The landing gear control interface units electrically control both the gear and doors such that they synced during control. The first LGCIU controls an entire gear cycle, then the system will switch over automatically to the other LGCIU once the retraction cycle has been completed. This will also be done when of the LGCIUs fail. In the following

5

diagram, the green hydraulic system will actuate all the gears and doors. A safety valve cuts the hydraulic supply to the landing gear when the aircraft is flying speeds above 260 knots, and when it is flying under that speed; it will remain closed in the event that the landing gear lever is still in the up position. Like most things in aviation, there is a contingency plan always present. In the even where the normal system fails to allow the gear to extend hydraulically, a hand crank can be mechanically turned by the flight crew. Once it is turned, the crank will then separate the landing gear from the green hydraulic line and unlock the landing gear doors including both the nose and main gear. Next, it will use gravity to extend the gear to the full extended position. Two factors help assist the flight crew to manually crank the gears into the locked position. Locking springs help the main gear, while aerodynamic forces assist the nose gear. In this emergency extension event, the gear doors would remain open during the duration of the manual crank but can be reset by the flight crew so long as the green hydraulic pressure is still intact.

6

7

In brief, the LGCIUs (Landing Gear Control Interface Units) gather and process data from proximity detectors from the landing gear, cargo door and the landing flap systems. From the landing gear, the LGCIUs know through the proximity detectors when the gear is locked in the down or up position. They also receive when the shock absorbers are compressed or extended and when the landing gear door is open or closed. In the event of a failure of a proximity detector, an associated output will be signaled; for example “shock absorber not compressed” or “landing gear uplock”. In addition, the other LGCIU will then automatically gain control over the operation of the landing gear. On the other hand, if a mechanical failure were to happen, the LGCIU would not change the related output but would entirely depend on the condition of what exactly is being signalled incorrectly. While in an electrical failure of an LGCIU would first transfer control to the other LGCIU (assuming it is still operable). The failed LGCIU’s outputs would not be forced upon the safe flight condition but rather users would see either flight or ground condition. The visual of how the proximity detectors output their signals is in the following:

8

9

In the following table; it shows

The above lines mean that the service interphone receives the output n° 6 from both LGCIUs, while SFCC 1 receive the output 5 from LGCIU 1 and SFCC 2 the output 5 from LGCIU 2. The two additional columns give the system functioning when the aircraft is in flight and on the ground.

10

11

12

The landing gear selector is done through a lever, and can control the landing gear to be in either the up or down position. The ‘two-position’ (up/down) selector lever sends an electrical signal to both LGCIUs, which then controls the green hydraulic supply through selector valves sending it to their respective positions. The airspeed must be below 260 knots in both cases. In the up lever position, braking of the main gears is done automatically by the normal brake system. In the unlikely event where the up position is selected whilst the aircraft is on the ground, a interlock mechanism is placed such that when there is compression in the shock absorber in the main gear or when the nose wheel steering is not centered it would inhibit the retraction of the landing gear. In the extended position, the landing gear hydraulic system remains pressurized. In the event where the hydraulic systems fail the extension of the landing gear, the flight crew can extend it by gravity. This is done by a gear crank which is then pulled out and turned clockwise for 3 turns. A cutout valve depressurizes the hydraulic pressure to the landing gear system during this process. This complies with the CARs requirement 525.729. The landing gear braking system normally operates through the green hydraulic pressure, while the alternate choice is the yellow hydraulics complimented by the hydraulic accumulator. The main wheels use multi disk brakes which are actuated by those systems (independent of each other). The aircraft brakes when the pilot applies pressure on the brake pedals or when the auto brake system is applied by the pilots. Temperature of the brakes are monitored by 2 units on each main gear. Fusible plugs prevent the bursting of the wheels when the temperature rises when brakes are applied. Braking functions are controlled by a twochannel Brake and Steering Control Unit (BSCU).

13

Anti-skid system allows maximum braking by allowing the wheels to reach the point right before they begin to skid. This is advantageous for maximum braking efficiency. It can be controlled by a ON/OFF switch but also becomes deactivated when the ground speed is under 20 knots. The way it works is it compares the speed of each main gear wheel with the speed of the aircraft. This speed is given through a tachometer and uses the aircraft as a reference speed, where when the wheel’s speed becomes under 0.87 times the reference speed, the brakes will activate the anti-skid system. The deceleration is limited to 1.7 m/s or 5.6 ft/s. There are lights to indicate whether the auto There is also an auto brake system, made purposely to reduce the stopping distance when an aborted takeoff is executed or to establish a set deceleration speed set by the crew during landing to both alleviate crew workload and guarantee passenger comfort. The system is deactivated when it is disarmed, or when ground spoilers retract. In the landing gear, there is four modes of operating the braking system: normal, alternate with anti-skid, normal alternate without anti-skid and parking brake. The difference is highlighted in the table below: Normal Braking

Normal Alternate with Anti-skid

Normal Alternate without Anti-skid

Parking Brake

● Green hydraulic pressure sufficient ● A/SKID and N/W STRG switch is ON ● BSCU electrically controlled by pilot’s input

● Green hydraulic pressure insufficient (But Yellow hydraulic is sufficient) ● A/SKID and N/W STRG switch is ON ● BSCU acts through alternate servo valves

● Anti-skid system can be deactivated electrically or hydraulically ● Pilot must reference to triple indicator to avoid locking wheels while braking ● Acc...