Definition
A pivot or pin assembly that supports a structural component and allows it to rotate or swing about a fixed axis. On aircraft, the trunnion is the fitting on a landing gear strut that attaches it to the airframe and provides the pivot point about which the gear retracts and extends.
Plain English
The hinge-like fitting at the top of a landing gear leg that lets the gear swing up into the wheel well and back down for landing.
Context Anchor
Seen in aircraft maintenance discussions about landing gear attachment, gear inspection, removal, installation, and repair.
Derivation
From the French 'trognon', meaning stump or core. Originally used to describe the short pivot pins on either side of a cannon barrel that let it tilt up and down. The same idea carries directly into aviation: a fixed pin or pair of pins that something heavy pivots on.
Why Pilots Care
Proper trunnion function prevents binding or collapse of landing gear during takeoff, landing, and retraction cycles.
Analogy
A trunnion works somewhat like the pin area of a door hinge: it gives the part a strong fixed point to turn around. In an aircraft, that point must also carry heavy landing and ground loads.
Intuition Check
A trunnion is not the whole landing gear assembly. It is the mounting-and-pivot point that helps attach a part and let it move correctly.
Example Sentence 1
The mechanic inspected the trunnion for cracks before signing off the landing gear retraction check.
Example Sentence 2
Hydraulic pressure moves the actuator attached to the nose gear trunnion, swinging the assembly into the wheel well.