Aircraft Landing Gear Systems: Operation, Locking, and Emergency Extension

021-04-01Pablo Asensio Martínez2026-03-273 min

The landing gear system supports the aircraft during taxi, takeoff, and landing. It is a complex system involving retraction mechanisms, locking devices, and emergency extension capabilities. Understanding its operation and limitations is crucial for safe flight.

Landing Gear Operation and Control

Most modern aircraft landing gear systems are active systems, powered by hydraulics.

  • Power Source: The hydraulic pressure is primarily provided by engine-driven pumps, which are driven by the accessory gearbox. While electric pumps exist, the heavy load during retraction is mainly handled by the engine-driven pumps.
  • Indication:
    • Down and Locked: Indicated by 3 green lights in the cockpit.
    • Transit/Unsafe: A red "gear unsafe" light illuminates if the gear position does not match the selector lever or is not locked.
    • Aural Warning: An aural warning must sound if a landing is attempted when the landing gear is not locked down (typically triggered by low throttle settings or radio altitude).

Retraction and Locking Mechanisms

To prevent inadvertent operation and ensure the gear remains in the correct position, several safety mechanisms are used:

  • Anti-Retract Latch: Physically prevents the gear selector lever from being moved to the "UP" position when the aircraft is on the ground.
  • Uplocks: Mechanical locks (often hook locks) that hold the gear in the retracted position.
  • Downlocks (Overcentre Locks):
    • Geometric/Overcentre Lock: Mainly used as a downlock. It uses the geometry of the drag/side stays to lock the gear in the extended position.
    • Mechanism: It requires a small force to lock (often locking automatically upon extension) but needs hydraulic or mechanical force to unlock (break the overcentre condition).

Emergency Extension

If the main hydraulic system fails, an emergency extension system is available to lower the gear.

  • Methods:
    • Gravity Free-Fall: The most common method. Mechanical uplocks are released, and the gear falls under its own weight.
    • Mechanical Hand Crank: Manually turning gears to operate actuators.
    • Hand Pump: Manually pumping hydraulic fluid to actuators.
    • Compressed Nitrogen/Air (Blowdown): Using stored gas pressure to release locks or power actuators.
  • Sequence:
    1. Release Hydraulic Pressure: Often by bypassing the cut-off valve.
    2. Unlock Doors: Release door uplocks.
    3. Unlock Gear: Release mechanical uplocks.
  • Result: The gear extends and locks down automatically (via overcentre springs/geometry). Gear doors usually remain open as there is no system to retract them. This is typically a one-time, non-reversible operation.

Component Functions

  • Torsion Link (Torque Link):
    • Function: Connects the inner (sliding) and outer (fixed) cylinder of the oleo strut to prevent the inner cylinder from twisting or rotating within the outer cylinder.
    • Stress: It transmits steering torque and prevents shimmy. It experiences maximum stress during tight turns on the ground.
  • Shuttle Valve: Allows an actuator to be powered by an alternate pressure source (emergency) if the primary source fails.

Speed Limitations

Operating the landing gear incurs drag and structural loads, necessitating specific speed limits:

  • $V_{LO}$ (Landing Gear Operating Speed): The maximum speed at which it is safe to extend or retract the landing gear. If these differ, they are designated as $V_{LO(EXT)}$ and $V_{LO(RET)}$.
  • $V_{LE}$ (Landing Gear Extended Speed): The maximum speed at which the aircraft can safely fly with the gear extended and locked. This is typically higher than $V_{LO}$.