Landing Gear

What Is Landing Gear?

Landing gear (also called the undercarriage) is the structural system that supports an aircraft on the ground and absorbs the kinetic energy of landing. It allows the aircraft to taxi, take off, and land safely, carrying the full weight of the aircraft plus dynamic landing loads that can momentarily reach two or three times the aircraft's maximum landing weight at the point of touchdown.

Function and Purpose

Landing gear must perform under demanding and contradictory requirements: it must be strong enough to absorb hard landings, light enough to minimize weight penalty, and retractable on most commercial aircraft to reduce aerodynamic drag in flight. The primary shock absorption is performed by an oleo-pneumatic strut — a combination of compressed nitrogen gas and hydraulic fluid — that progressively resists compression, smoothing the transition from flight to ground roll.

Braking is provided by carbon or steel disc brakes on the main wheels, with anti-skid systems (analogous to automotive ABS) preventing wheel lockup on wet or contaminated runways. On large jets, the nose gear steers the aircraft on the ground, supplemented by asymmetric thrust or differential braking for tight turns.

Types and Variations

• Tricycle gear: The dominant configuration for commercial jets — one nose gear unit forward and two (or more) main gear units beneath the wings or fuselage. Provides excellent ground stability and forward visibility during taxi.
• Bogie (truck) configuration: Wide-body jets use multi-wheel bogies (e.g., four- or six-wheel trucks) to distribute weight over a larger area, meeting pavement load limits at major airports.
• Body gear: Very large aircraft like the Boeing 747 and 777 carry additional centerline body gear beneath the fuselage to help distribute the massive landing weight.
• Tail-dragger (conventional gear): A configuration with main gear forward and a small tail wheel aft, now limited to older or light aircraft.

Notable Examples

The Boeing 747-8 lands on 18 wheels across five gear struts — two nose wheels, four six-wheel main bogies on the wings, and two four-wheel body bogies under the fuselage. The Airbus A380 uses 22 wheels across four main bogies and a twin nose gear, enabling it to operate at airports that would otherwise be structurally unable to support its 575-tonne maximum takeoff weight. The Boeing 787 employs a lightweight composite main gear door to save weight.

Related Components

Landing gear wells are cut into the fuselage and/or wing structure, creating significant openings that must be carefully reinforced. Gear retraction and extension relies on hydraulic actuators powered by the main hydraulic systems, and the APU can provide hydraulic power on the ground if main engines are off. The flaps are typically extended in conjunction with gear deployment during approach, both increasing drag and reducing speed for a controlled landing.