Plain English Meaning
A small metal pin shaped like a folded hairpin. You push it through a hole, then bend its two ends apart so it cannot slip back out. It stops nuts and pins from working loose.
Full Definition
A split metal fastener inserted through a hole in a bolt, shaft, or castellated nut, with its two prongs bent apart to lock the assembly and prevent it from backing off or coming loose under vibration.
Context Anchor
Seen during aircraft maintenance, preflight inspections, and control-system checks where nuts, bolts, or linkage pins must stay secured.
Derivation
From the old English word 'cotter,' meaning a wedge or pin that holds something in place. The name describes the job: a pin that 'cots' or fixes parts together so they cannot move apart.
Why Pilots Care
Loose fasteners can lead to control surface failure or landing gear collapse; cotter pins provide a simple, reliable secondary locking method.
Intuition Check
Do not think of a cotter pin as the main bolt or pin carrying the load. Its job is to secure another fastener so that fastener stays in place.
Example Sentence 1
After torquing the castellated nut, the mechanic installed a new cotter pin and bent the prongs around the bolt to lock it in place.
Example Sentence 2
During the annual inspection, all cotter pins on the flight control linkages were inspected for proper installation and corrosion.