Definition
A mechanical locking arrangement used in retractable landing gear systems in which the linkage that holds the gear in the down-and-locked position is pushed slightly past its straight (centered) alignment, so that any load trying to fold the gear actually drives the linkage harder against a stop rather than collapsing it. This geometric lock holds the gear rigid without relying on hydraulic pressure or a separate latch.
Plain English
A way of locking the landing gear down by pushing the linkage just past straight, so that weight on the gear pushes the lock tighter instead of folding it up.
Context Anchor
Seen in landing gear discussions, especially when learning how retractable landing gear stays down and locked for landing.
Derivation
The phrase comes from mechanical engineering. A linkage is 'on center' when its joints are perfectly aligned in a straight line. Pushing it slightly past that alignment is 'over center.' Once over center, the load no longer tries to fold the linkage — it tries to push it further into its stop, which is what creates the lock.
Why Pilots Care
The locks prevent the gear from collapsing on touchdown or during ground operations if hydraulic pressure fails, directly affecting landing safety.
Analogy
Think of a folding card-table leg. When the brace is pushed just past straight against its stop, the leg won't fold no matter how hard you press down on the table. Pull the brace back the other way and the leg collapses.
Intuition Check
Do not picture a lock like a padlock or key lock. Here, “lock” means a mechanical linkage position that holds the landing gear securely in place.
Example Sentence 1
After the gear handle was lowered, the over-center locks engaged and the three green lights confirmed the gear was down and locked.
Example Sentence 2
During the after-landing checklist the pilot confirmed the over-center locks remained engaged before taxiing back to the ramp.