Landing Gear Systems

29.1 Purpose and Components

• The landing gear supports the aircraft on the ground, absorbs landing shocks, and allows for
• taxiing and steering.
•  Oleo-Pneumatic Strut: The most common type of shock absorber. It consists of a cylinder
• containing both hydraulic fluid and compressed nitrogen gas.
• During compression (landing), the fluid is forced through a small orifice, which damps the
• impact.
• The compressed nitrogen acts as a spring, supporting the aircraft's weight and re-extending
• the strut.
•  Torque Links (Scissors): A hinged link that connects the inner and outer cylinders of the
• strut, allowing it to compress while preventing it from rotating.
•  Nose Wheel Steering: Allows the pilot to steer the aircraft on the ground using tiller wheels
• r rudder pedals.
•  Shimmy Damper: A small hydraulic damper on the nose gear that prevents rapid, unstable
• scillations (shimmy) of the nose wheel during takeoff and landing.

29.2 Retraction Systems

•  Operation: Retractable landing gear systems are typically operated hydraulically. When the
• gear is selected UP, hydraulic pressure is directed to actuators that retract the gear into wheel
• wells, which are then covered by doors to maintain aerodynamics.
•  Locks: Up-locks and down-locks are mechanical locks that ensure the gear is held securely
• in the extended or retracted position.
•  Safety Devices:

• A selector lock prevents the gear lever from being moved to the UP position while the
• aircraft is on the ground. This is controlled by weight-on-wheels switches (also known as
• squat switches or an air/ground logic system).
• Ground locks (safety pins with red flags) are inserted manually on the ground to prevent
• accidental gear retraction.
•  Emergency Extension: In case of a hydraulic failure, an emergency system allows the gear
• to be lowered. This often involves mechanically releasing the up-locks and allowing the gear
• to free-fall into the down-and-locked position due to gravity and air loads.