Definition
Mechanical devices, typically over-center linkages or locking pins, that physically hold the landing gear in the extended (down) position so it cannot collapse under the weight of the airplane during landing, taxi, and takeoff roll.
Plain English
Built-in locks that keep the landing gear firmly in the down position so it can't fold up while the airplane is on the ground.
Context Anchor
Encountered when studying retractable landing gear, landing gear safety devices, gear indication systems, and abnormal gear-extension procedures.
Derivation
From 'down' (the gear position) and 'lock' (a device that holds something fixed). The word 'mechanical' is included to distinguish these physical locks from hydraulic pressure or electrical signals that might also be holding the gear in place.
Why Pilots Care
They provide a fail-safe mechanical backup so the gear cannot collapse even if hydraulic pressure is lost, directly protecting against gear-up landings or runway excursions.
Analogy
A mechanical downlock is like a deadbolt on a door. The door may be closed, but the deadbolt is what keeps it from swinging open under force.
Intuition Check
Do not assume “gear down” automatically means “gear safe.” In this context, the important idea is “down and locked.”
Example Sentence 1
After lowering the gear handle, the pilot waited for the green lights confirming the mechanical downlocks were engaged before continuing the approach.
Example Sentence 2
During the preflight walk-around the pilot checked that the mechanical downlocks were visible and properly engaged on both main struts.