uplock
A mechanism that secures an aircraft's landing gear in the retracted position, helping to prevent an uncommanded gear deployment during flight.
uplock: the lock that keeps landing gear folded up
An uplock is a mechanical or hydraulic locking device that holds an aircraft's landing gear firmly in its retracted position after takeoff. Once the gear retracts into the wheel wells, the uplock engages to prevent any inadvertent extension caused by vibration, pressure fluctuations, or hydraulic leakage. Without it, a gear extension mid-flight would create dangerous drag, structural stress, and potential loss of control.
The uplock typically consists of a hook or pawl mechanism attached to the gear strut that latches into a fixed catch on the airframe structure. Hydraulic pressure or a spring-loaded arm holds the latch closed. On retraction, as the gear fully enters the wheel well, a striker on the gear linkage engages the uplock mechanism, setting it automatically. The design ensures that even total hydraulic pressure loss cannot release the gear until the pilot manually unlocks it for descent.
Two main types exist: positive mechanical uplocks, which use rigid hooks and require pilot or automatic electrical solenoid actuation to release, and hydraulic uplocks, which rely on pressurized fluid to maintain the lock. Many modern aircraft use both: a mechanical uplock as the primary safety barrier, backed by a hydraulic uplock as redundancy. The uplock is distinct from the downlock, which secures the gear in the extended position on the ground.
Uplock failure or sticking is a serious maintenance concern. A failed uplock may allow the gear to drift down during flight, a condition pilots detect through a warning light triggered by a gear position sensor. Conversely, a stuck uplock can prevent gear extension for landing, making it a potential emergency. Maintenance checks include visual inspection for corrosion, travel tests to confirm engagement and release, and functional testing of any associated electrical or hydraulic circuits.
The term reflects the device's core function: it locks the gear in the up position. This straightforward nomenclature contrasts with older slang like "up-and-lock mechanism," but uplock has standardized across manufacturers and regulatory bodies including the FAA and EASA. Understanding uplocks is essential for troubleshooting retractable-gear aircraft: a gear that won't stay retracted or won't come down often points directly to uplock malfunction.