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.
Plain English
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.
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.