Chapter 16: Landing Gear

Arrangements and construction

Nearly all transport aircraft use a tricycle layout — a steerable nose gear and two (or more) main gears behind the CG. Light aircraft may be tailwheel (conventional) types. Main gears carry most of the weight, so heavy aircraft spread it over many wheels.

• Shock strut — the main structural leg containing the shock absorber, hinged to the wing or fuselage at its trunnion.
• Drag brace and side brace — diagonal members that take fore-aft and side loads; on retractable gear they fold, and an overcentre lock (with lock springs and a lock actuator) holds them straight when the gear is down.
• Torque links (scissors) — connect the inner and outer cylinders so the wheel cannot rotate on the strut, and limit the strut's extension.
• Bogie beam and pitch trimmer — on multi-wheel gears, the beam carries the axles and an actuator sets its angle for stowage and touchdown.
• Shimmy damper — a hydraulic damper on the nose gear (or the steering actuators acting as dampers) that stops the rapid side-to-side oscillation called shimmy.
• Other shock absorbers: spring-steel legs and rubber bungees (light aircraft), liquid springs.

Oleo-pneumatic shock struts

• Energy absorption: on landing, fluid is forced through a small orifice (damping, energy turned into heat) while the gas (dry nitrogen) compresses, acting as the spring. A metering pin varies the orifice area through the stroke to give an even deceleration; a recoil (snubber) valve damps the rebound.
• Servicing: the correct extension depends on aircraft weight, so the AMM gives a chart of extension ("X" dimension) against pressure. Fluid level is checked with the strut fully deflated (compressed) — on jacks or as the AMM directs — then the nitrogen is charged to the pressure the chart gives for the measured extension.
• Defects: fluid leaks at the seals, pitting or scoring of the chromed inner cylinder (which destroys seals), a strut sitting too low (low gas charge or overloaded) or too high, and "bottoming" on landing.

Tyres and wheels

• Construction: bias-ply (cross-ply) or radial carcass, steel-wire beads, tread and sidewall rubber, inner liner on tubeless tyres. Some nose tyres have a chine moulded into the sidewall that deflects runway water away from rear-mounted engines.
• Inflation: transport aircraft tyres are inflated with dry nitrogen — it does not support combustion, keeps out moisture and slows rubber oxidation. Pressure is checked cold (typically at least 2–3 hours after flight) with a calibrated gauge, normally under load: loaded and unloaded readings differ by up to about 4 %. The AMM gives the actions for low pressure (re-inflate and re-check, or remove the tyre and often its axle partner).
• Inspection: tread wear (to the limit given — often the bottom of a groove or before the reinforcing ply shows), cuts, bulges (remove the tyre), flat spots from skidding, heat damage, and blocked vent holes.
• Wheels are split (two halves bolted together) forgings of aluminium or magnesium alloy, running on tapered roller bearings. Thermal (fusible) plugs in the inner half have a low-melting-point core that deflates the tyre before heat from heavy braking can make it burst; a tyre whose plug has melted is scrapped and the wheel inspected. An over-inflation safety plug stops the tyre being over-pressurised during inflation.

Extension and retraction

• Normal operation: the gear selector lever (electrically or mechanically) positions a selector valve; hydraulic pressure unlocks and opens the gear doors, releases the up- or downlocks and drives the retraction actuators. Sequence valves (or the computers on modern aircraft) make things happen in order: doors open → gear moves → gear locks → doors close.
• Uplocks hold the retracted gear; downlocks (the overcentre lock of the brace, held by springs and a lock actuator) hold it extended. On the ground, ground lock pins with red streamers are fitted in the locks for maintenance and towing and removed before flight.
• A gear lever lock (a solenoid controlled by air-ground sensing) stops the lever being moved to UP while the aircraft is on the ground; an override is provided.
• During retraction the main wheels are braked automatically (and nose wheels stopped by snubbers) so they do not spin in the wheel well.
• Emergency (alternate) extension: pulling the free-fall handle mechanically releases the door and gear uplocks and disconnects the hydraulics, so the gear falls by gravity, helped by air loads and springs, into its downlocks. Other types use an alternate hydraulic or electric pump, a nitrogen blow-down bottle, or (light aircraft) a hand crank.

Indications and warnings

Positions are sensed by proximity sensors on the locks, gear and doors and processed by landing-gear control and interface units, which also drive the ECAM/EICAS gear page and report faults.

Wheels, brakes, antiskid and autobrake

• Brake systems: normal brakes from one hydraulic system through brake metering valves operated by the toe pedals (or electrically by a brake control unit); alternate brakes from another system through shuttle valves; a brake accumulator for emergency and parking braking. The B787 uses electric brakes.
• Wear is shown by wear pins read with the parking brake set; a brake is changed when the pin is flush (or at the AMM limit).
• Brake temperature is monitored on many aircraft (sensors in the heat stack, shown on the gear page), and some have brake cooling fans. Hot brakes after a rejected take-off can melt the thermal plugs minutes later — keep people clear.
• The parking brake traps pressure in the brakes, from the accumulator if needed; check accumulator pressure before relying on it.

• Autobrake — selected before landing (e.g. LO / MED / MAX or 1–4), it applies the brakes automatically at touchdown (after wheel spin-up or ground-spoiler deployment) and regulates pressure for the selected deceleration. The RTO mode applies maximum braking if the thrust levers are retarded during a take-off run above a set speed. It disarms when the pilot brakes manually or advances the thrust levers.

Steering

• Nose-wheel steering is hydraulic: actuators (push-pull or rack-and-pinion) turn the steering collar, controlled by a tiller for large angles while taxiing (about 60–80°) and by the rudder pedals for small corrections (a few degrees) during take-off and landing.
• Centring cams inside the strut straighten the nose wheel when the strut extends after lift-off, so it fits the wheel well.
• Large aircraft add body or main-gear steering (for example the aft axle of B777 bogies or the A380 body gear) to reduce tyre scrubbing in tight turns.
• Towing: steering is depressurised or disconnected with a towing (bypass) pin before towing; exceeding the towing angle limits (often marked on the gear doors) can damage the steering — many aircraft record an over-steer event for inspection.

Air-ground sensing

Proximity sensors or switches on the shock struts (strut compression) or torque links tell the aircraft whether it is in the air or on the ground. Two or more independent sensors feed interface units that supply the air/ground logic to many systems: the gear-lever lock, ground spoilers and autobrake, thrust-reverser deployment (ground only), pressurisation ground mode, stall-warning and GPWS modes, minimum idle, APU, flight/ground modes of the maintenance computers, and more. A faulty air-ground sensor can therefore cause many apparently unrelated defects.

Tail protection

Aircraft with long rear fuselages (stretched narrow-bodies and some wide-bodies) can strike the tail on over-rotation at take-off or a high-attitude landing. A tail skid — fixed, or retracting with the gear — has a replaceable wear shoe and an energy-absorbing crushable cartridge or small shock strut; a tail-strike indicator or the cartridge's compression marking shows whether it has been hit. Any tail strike requires the AMM inspection of the skid, the aft fuselage skin, frames and the aft pressure bulkhead.