Landing Gear Systems

Main Functions and Types

The landing gear system enables the aircraft to taxi, take off, and land safely. It absorbs the vertical impact of touchdown, supports the aircraft's weight, and resists lateral and torsional loads during ground operations. The most common aircraft landing gear types are the tricycle (nose wheel) and tailwheel configurations, with large transport aircraft often using complex multi-bogie arrangements to distribute loads.

Key Components Explained

• Oleo Leg/Shock Strut: Absorbs and dampens vertical landing loads using a combination of compressed gas and hydraulic fluid.
• Axles: Support the wheels and transfer loads to the strut.
• Bogies and Bogie Beam: Allow multiple wheels per gear leg, spreading loads and reducing pavement stress.
• Drag Struts: Prevent rearward collapse during braking.
• Side Stays/Struts: Resist lateral forces, especially during crosswind landings.
• Torsion Links (Scissor Links): Prevent rotation of the shock strut piston, ensuring wheel alignment.
• Locks (Over-centre): Secure the gear in the extended or retracted position.
• Gear Doors: Streamline the undercarriage bay and reduce drag when the gear is retracted.

Retraction and Extension

Retractable landing gear reduces drag for better fuel efficiency and performance. Retraction is usually hydraulic, sometimes electric on smaller types. Emergency extension methods include gravity/free-fall (most common on modern jets), compressed air/nitrogen, or manual means.

Indication and Warning Systems

Position indicators show gear status (up, down, or in transit). Warning systems alert the crew if the gear is not down and locked when landing is imminent, often using throttle and flap position logic or GPWS inputs. Ground locks are used on the ground to prevent accidental retraction.

Typical Sequence of Operation

1. Gear doors open (if fitted).
2. Gear unlocks and extends/retracts.
3. Gear locks in position.
4. Doors close (if applicable).

Landing Gear Failures

Failures can include inability to extend/retract, indication faults, or mechanical jams. Emergency procedures depend on the aircraft type and system design.