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15. Landing gear

The main gear are dual wheel and retract inboard. The nose gear is also dual wheel, and retracts forward.

The main gear are equipped with:

The nose gear is equipped with a nose wheel steering system.

The gear and gear doors are controlled by two Landing Gear Control and Interface Units (LGCIUs). Gear sequencing is electrical and actuation is hydraulic (green system). A complete gear cycle is controlled by a single LGCIU, with an automatic toggle to the other LGCIU at the end of each retraction. There is also an automatic switchover if a failure is detected. A seperate safety valve is also incorporated. This shuts off hydraulic pressure to the actuators when the aircraft exceeds 260kt, and restores pressure only when the gear lever is selected down with the aircraft below 260kt.

Emergency extension of the gear is possible. This is acheived through a cut off valve that isolates the landing gear from the hydraulics and by manually removing gear and door uplocks. The gear extends via a combination of gravity, aerodynamic forces and locking springs. The gear doors will remain open after extension. Emergency extension is controlled by a crank located at the rear of the center pedestal. Reset is possible in flight.

The LGCIUs also gather and processes data from proximity detectors on:

Gear lock position and gear door position are presented on the ECAM WHEEL page and on the LDG GEAR indicator. On the ECAM page, each gear is represented by two triangles, one per LGCIU. When down and locked, the triangles are green. When unlocked, the triangles are red. When fully retracted, the triangles are not shown. A green indication on either triangle is sufficient to determine that the gear is down and locked. The lights on the LDG GEAR indicator are controlled by LGCIU 1 only, and have their standard meanings.

The LGCIUs uses the shock absorber position to supply air/ground data to various client systems.

The gear, gear door and shock absorber proximity detectors are monitored for electrical failure. If a proximity detector suffers an electrical failure, flight condition is assumed for the detector (i.e. shock absorber extended, landing gear uplocked), and the unaffected LGCIU takes over landing gear operation. Mechanical failure of proximity detectors is not monitored. In the event of electrical failure of an LGCIU some client systems may receive incorrect air/ground data.

A subset of the cargo door proximity detectors are monitored for electrical failure. When these fail, a non locked condition is assumed.

Data from the flap disconnect proximity switches is forwarded to the SFCCs. SFCCs are not monitored by the LGCIUs.

The nosewheel steering is controlled electrically by a dual channel Brake and Steering Control Unit (BSCU) and actuated hydraulically (yellow system). The BSCU receives steering requests from the steering handwheels, the rudder pedals and the autopilot. The steering handwheels have authority for 75° of nose wheel deflection up to 20kt, reducing linearly to 0 at 80kt. If inputs are made on both handwheels, the request generated is the sum of the deflections requested. The rudder pedals have authority for 6° of deflection up to 40kt, reducing linearly to 0 at 130kt. The steering servo valve is electrically inhibited when the aircraft is on the ground and when neither engine is running. It may also be inhibited with the ASKID NWSTRG switch or with the towing contol lever. When the steering is deactivated with the towing control lever "NW STRG DISC" is displayed on the memo display and the nosewheel can be moved through 95°.

A switch is provided on the steering handwheel to electrically disconnect the rudder pedals from the BSCU in order that rudder control checks may be carried out.

The nosewheel is centered after takeoff by an internal cam mechanism.

Two braking systems are provided. The normal system utilises green system hydraulics, and the alternate system uses the yellow system. The alternate system also has a backup hydraulic accumulator designed to supply at least seven full brake applications or provide parking brake pressure for a minimum of 12 hours.

Braking requests may be initiated by rudder pedal deflection, by the autobrake system or automatically during gear retraction. When the normal system is active, these requests are processed by the BSCU. When the alternate system is active, they are processed by the Alternate Braking Control Unit (ABCU). The alternate system is activated automatically when green system pressure is insufficient or manually using the ASKID NW STRG switch.

The BSCU also provides:

The master BSCU channel automatically toggles at each DOWN selection of the landing gear lever, or when a fault is detected.

The anti-skid system determines wheel slip ratio by comparing aircraft speed from the ADIRUs with tyre speed from tachometers on the wheel. When tyre speed reduces to 0.87 of aircraft speed, a servo valve is actuated to release the brakes. If all ADIRUs have failed, the maximum main gear speed may be used to approximate aircraft speed. The anti-skid will be unavailable with complete BSCU failure or when yellow and green hydraulic sytems are both lost. It may also be turned off with the ASKID & NW STRG switch. If the anti-skid system is not available, brake pressure is automatically limited to 1000psi.

The autobrake system allows braking to be applied on a deceleration schedule. It is only available with the normal braking system. It is armed by selecting the LO, MED or MAX autobrake pushbutton switches (MAX is for RTO and can only be selected on the ground). Activation is linked to ground spoiler extension. The autobrake system will therefore not activate on an RTO where speed remains less than 72kt since the ground spoilers will not automatically deploy below this speed. LO mode applies the brakes 4 seconds after ground spoilers deploy and decelerates the aircraft at 1.7m/s². MED mode applies the brakes 2 seconds after ground spoilers deploy and delerates the aircraft at 3m/s². MAX mode applies maximum brake pressure as soon as the ground spoilers deploy. The green DECEL light on the AUTOBRK selector indicates that at least 80% of the required deceleration rate has been acheived. The system is deactivated when the ground spoilers retract, or when it is disarmed. Disarming occurs automatically when the landing gear is fully retracted. The system may also be disarmed by manually braking or by deselecting the armed autobrake pushbutton.

The parking brake applies full pressure from either the yellow hydraulic system or accumulator via the parking brake control valve. If no pressure is available from these sources, the normal braking system is enabled and the brake pedals may be used.

THE BSCU also sends brake temperature data to the ECAM WHEEL page. If any brake is >100°C, a green arc appears on the ECAM page. If the temperature exceeds 300°C, an amber arc appears, indicating that takeoff must be delayed to let the brakes cool. If the brake fans are on, the brake temperature sensors are not accurate. Takeoff must be delayed if brake temperatures are over 150°C with the brake fans on. After landing, if the brake temperatures on one gear differ by more than 150°C or the difference in the mean temperature of each gear exceeds 200°C, maintenance action is required.

Brake wear indicators are provided on the brakes. They must be checked with the parking brake on. If any of the brake wear indicator is showing, the brake unit is serviceable.

During gear retraction, the main gear is automatically braked. The nose gear is braked by a brake band.

Various messages may be displayed on the ECAM WHEEL page. The UPLOCK message on the ECAM WHEEL page indicates that the landing gear is down and locked with an uplock engaged. The L/G CTL message on the ECAM wheels page indicates a discrepancy between demanded landing gear position and actual position.