13.16 Landing gear (ATA 32) — systems, anti-skid, autobrake and air–ground sensing

Scope

B2 and B2L Airframe Systems.

• 13.16(a) Description — construction, shock absorbing, tyres — level 1.
• 13.16(b) Systems — normal and emergency extension/retraction; indications and warnings; wheels, brakes, anti-skid and autobraking; steering — level 3.
• 13.16(c) Air–ground sensing — level 3.

Construction and shock absorbing (level 1)

• Tricycle gear (nose + main) on most aircraft; tail-wheel on older/light types.
• Oleo-pneumatic struts: nitrogen gives the spring; hydraulic fluid metered through an orifice (often with a metering pin) gives damping. Serviced by fluid level and nitrogen pressure/extension per chart.
• Other types: spring steel legs, rubber discs/bungees.
• Torque links (scissors) keep the piston aligned with the cylinder and transmit steering torque; drag and side braces with overcentre locks.
• Tyres: tubeless radial or bias-ply, inflated with nitrogen; pressure checked cold; wheels with fusible plugs (melt to release pressure after hot brakes) and over-pressure relief valves.

Extension and retraction

• Normally hydraulic on transports (electric on some light aircraft). The gear selector lever (with a solenoid lock released by the air/ground signal) commands a selector valve; sequence valves or a gear controller open doors, unlock, move the gear, lock it and close the doors.
• Uplocks hold the gear up; downlocks (overcentre braces with lock springs/actuators) hold it down.
• Electronic control: a landing gear control and interface unit (LGCIU) or proximity switch electronic unit (PSEU) processes proximity sensors and controls valves; two channels alternate between flights.
• Emergency (alternate) extension: mechanical or electrical release of the uplocks and door locks, then gravity free-fall assisted by aerodynamic loads and springs; or alternate hydraulic/electric pump, or nitrogen blow-down bottle on some types.
• Safety: ground lock pins with red flags, door safety pins, gear-down lock checks before towing or maintenance.