Definition
Mechanical devices that physically lock the landing gear in the extended (down) position so it cannot retract or collapse under the weight of the airplane during taxi, takeoff, and landing.
Plain English
Parts that hold the landing gear firmly in place once it is down, so the gear stays down and supports the airplane.
Context Anchor
Encountered during preflight inspection of airplanes with landing gear that can be raised and lowered.
Derivation
A straightforward compound: 'down' (the gear-down position) and 'lock' (to secure). The plural 'mechanisms' refers to the fact that each landing gear leg typically has its own locking device.
Why Pilots Care
A failed or unlocked downlock can allow the gear to collapse during taxi, takeoff, or landing, resulting in aircraft damage and potential injury.
Analogy
It is like a door latch: the door may be closed, but it is not truly secure until the latch catches and holds it there.
Intuition Check
Do not confuse a downlock mechanism with only a cockpit light or indication. The downlock mechanism is the physical locking hardware; the indication only tells the pilot what the system believes is happening.
Example Sentence 1
During preflight, the pilot inspected the downlock mechanisms on each gear leg to confirm they were secure and free of damage.
Example Sentence 2
Loss of hydraulic pressure will not retract the gear as long as the downlock mechanisms remain in place.