How Aircraft Landing Gear Works

Types of Landing Gear

Commercial aircraft universally use the tricycle configuration: two main gear assemblies under the wings or fuselage, and a smaller nose gear under the forward fuselage. This arrangement provides stable ground handling — the aircraft naturally points forward because the center of gravity is forward of the main gear. Tricycle gear replaced the earlier tailwheel (taildragger) configuration on jet aircraft because it provides better forward visibility during takeoff roll and eliminates the tendency to ground loop.

Within the tricycle family, large aircraft vary in main gear complexity:

• Single-axle main gear (Airbus A320, Boeing 737): Each main gear has a single axle with two or four wheels. Sufficient for aircraft under approximately 100 tonnes MTOW.
• Multi-axle bogies (Boeing 777, Airbus A330): The main gear strut carries a bogie beam with two axles and four wheels each. The 777 has a six-wheel bogie on each main gear leg, distributing the 350-tonne maximum weight over twelve main gear wheels.
• Body gear (Boeing 747, Airbus A380): In addition to wing-mounted main gears, these aircraft have additional gear legs in the fuselage to distribute the enormous weight. The 747 has five gear assemblies with 18 main wheels; the A380 has four main gear assemblies with 20 wheels.

The Retraction Mechanism

Landing gear retraction is driven by the aircraft's hydraulic system — the same system that powers flight controls, brakes, and thrust reversers. When the gear lever is raised after takeoff, sequencing valves direct hydraulic fluid to actuators that unlock the downlocks (structural latches holding gear extended), retract the gear into its bay, and engage the uplocks (structural latches holding gear retracted). Gear doors open before retraction begins and close after gear is up.

The entire retraction sequence on a typical jetliner takes 6–12 seconds. Gear is usually retracted before reaching 1,000 feet to reduce drag during the critical initial climb phase. The gear retraction produces the recognizable "thump" heard shortly after takeoff as the gear doors close and the uplocks engage.

Gear must withstand enormous loads: the main strut is an oleo-pneumatic shock absorber — a telescoping cylinder containing hydraulic fluid and high-pressure nitrogen that compresses during touchdown, absorbing landing energy and slowly releasing it. A Boeing 777 landing at maximum landing weight (251 tonnes) at a sink rate of 10 feet per second generates over 1 million pounds of force during touchdown.

The Braking System

Commercial aircraft use hydraulically actuated multi-disc brakes on the main gear wheels. The brake assembly contains a stack of rotating discs (rotors) and stationary discs (stators) compressed together by hydraulic actuators to create friction. Heat generated during braking is immense — a maximum-energy rejected takeoff (RTO) from V1 on a heavy jet can heat the brakes to over 1,000°C.

Modern aircraft use carbon-carbon composite brakes (discussed in detail in the carbon brakes guide), replacing steel discs that were heavier and less heat-resistant. The brake system includes anti-skid computers (analogous to ABS in cars) that modulate braking pressure wheel-by-wheel to prevent skidding and maximize deceleration. The Boeing 787's brake-by-wire system adds electronic control on top of the hydraulic actuation.

Nose Gear Steering

The nose gear steers the aircraft on the ground using hydraulic actuators controlled by the tiller (a small wheel or handle on the cockpit side console) for large steering angles on the ground. The rudder pedals also provide limited nose wheel steering authority (typically ±5–7°) for alignment during the final moments of landing. Large aircraft have nose wheel steering capability of ±70–80° for tight turns at airport gates.

Some aircraft (notably the 747 and A380) also have steerable body gear or wing gear to reduce tire scrub during tight turns. The Airbus A380 features rear body gear that steers in the opposite direction to the nose gear, dramatically reducing the turning radius for an aircraft with a 79.75 m wingspan.

Emergency Extension

If hydraulic power is lost, landing gear must extend by an alternate means. Most aircraft use gravity extension as the primary backup: uplocks are mechanically released (via a cable connected to a cockpit handle or a dedicated hydraulic system), and the gear falls into the extended position under gravity and aerodynamic drag, then locks with downlocks. Crew must verify gear down by checking three green lights (one per gear assembly) and may need to perform a slow descent and gentle maneuver to ensure gear swings fully into the down-and-locked position.

Tire Technology

Aircraft tires are specialized for extreme conditions: high loads, high speeds, and resistance to the heat of hard landings and braking. A Boeing 777 main gear tire operates at approximately 220 psi inflation pressure (compared to a car tire's 35 psi) and carries up to 25 tonnes per tire. Tires are inspected before every flight for cuts, wear, and correct pressure. Nitrogen (not air) is used for inflation to avoid moisture condensation and to reduce fire risk if a tire overheats. Tire speed ratings for commercial jets typically reach 235–260 mph.