착륙 장치 구조

Overview

The landing gear is the undercarriage structure that supports the aircraft during ground operations — taxiing, take-off, and landing. It must absorb the kinetic energy of landing impact, support the full maximum take-off weight during ground manoeuvring, withstand braking loads that decelerate an aircraft from over 160 knots to rest, and retract cleanly into the airframe in flight to minimize drag. Despite operating for only a small fraction of flight time, landing gear accounts for roughly 3–5% of aircraft structural weight and represents some of the most highly loaded components in the airframe.

Commercial aircraft use tricycle gear arrangements — main gear aft of the centre of gravity and a nose gear forward — because this configuration is stable during ground roll and provides good visibility for pilots during taxi. Larger aircraft may use multiple main gear bogies to distribute ground loads within airport pavement strength limits.

How It Works

Each main gear leg is built around an oleo-pneumatic shock absorber (oleo strut) — a telescoping cylinder containing hydraulic fluid and compressed nitrogen. On touchdown, the upper and lower cylinders compress, forcing hydraulic fluid through a calibrated orifice to dissipate landing energy while the nitrogen acts as a spring. This two-stage energy absorption system converts the impact energy into heat in the hydraulic fluid, limiting peak loads transmitted to the airframe structure. On a typical commercial aircraft, the oleo strut reduces landing loads by approximately 80% compared to a rigid strut.

The gear retracts via hydraulic actuators, folding the assembly into wheel-well bays within the fuselage or wing. Uplocks hold the gear retracted in flight; downlocks secure it extended for landing. Free-fall extension using gravity and aerodynamic drag provides a backup if the primary hydraulic retraction system fails.

Key Components

• Main gear legs: Primary load-bearing struts incorporating oleo shock absorbers; manufactured from high-strength steel or titanium forgings.
• Nose gear: Smaller steerable strut with oleo absorber; incorporates steering actuators and anti-shimmy damping.
• Bogies (trucks): Multi-wheel axle assemblies on large aircraft; the 777 uses six-wheel main bogies, and the A380 uses four main bogies with a total of 20 wheels.
• Wheels and tires: Aviation-specific designs rated for high-speed operation; main gear tires are inflated to 150–200 psi and must withstand the thermal and structural loads of hard landings and aborted take-offs.
• Brakes: Carbon–carbon multi-disc assemblies generating enormous heat loads; anti-skid systems maximize braking while preventing tire blowout.
• Retraction actuators: Hydraulic rams and torque tubes driving gear uplocks, door actuators, and bogie positioning.
• Gear bay structure: Reinforced airframe structure surrounding the wheel well, including drag struts, side struts, and gear beam that transmit loads into primary fuselage or wing structure.

Aircraft Applications

The Airbus A380, at a maximum take-off weight of 575 tonnes, uses the most complex landing gear system in commercial service: two four-wheel body gear units under the fuselage and two six-wheel wing gear units — 20 wheels in total. This arrangement ensures that pavement loading (the load per unit area on airport surfaces) remains within international standards even at maximum weight. The Boeing 777-300ER similarly uses six-wheel main bogies to manage its 352-tonne MTOW. The Boeing 787 uses four-wheel main bogies and nose gear with carbon brakes, while the 737 and A320 families use conventional two-wheel single-axle main gear adequate for their lower weights.

Advantages and Limitations

Advantages: Modern oleo-pneumatic struts are highly efficient energy absorbers that protect both passengers and airframe structure from hard landings; carbon–carbon brakes offer much higher heat absorption capacity and lower weight than older steel disc brakes; electric braking systems on some aircraft provide precise anti-skid control and eliminate hydraulic fluid from the brake circuit.

Limitations: Landing gear represents a large proportion of airframe weight and drag (when deployed). High-strength steel components require careful corrosion protection and regular inspection for fatigue cracking, especially in the bogie pivot area and gear attachment lugs. Carbon brakes require careful thermal management — overheating from rapid successive landings or rejected take-offs can result in brake fires. The retraction kinematics of large multi-bogie gear require complex mechanical linkages that must be maintained to close tolerances.