Definition
Small metal fasteners, usually a folded pin with two prongs, inserted through a hole in a bolt or shaft and then bent open to lock a castle nut, clevis pin, or similar fitting in place so it cannot rotate loose or slide out. On aircraft they are single-use safety devices: once removed and straightened, they are discarded and replaced with a new one.
Plain English
A small split pin you push through a hole in a bolt and then bend open so the nut can't unscrew itself in flight. If you take one out, you throw it away and fit a new one.
Context Anchor
Seen during aircraft inspections and preventive maintenance when checking or replacing small hardware that must not loosen in service.
Derivation
‘Cotter’ comes from an old English word meaning a wedge or pin used to hold parts together. The ‘key’ part reflects its locking function — like a key, it stops something from moving when it is in place.
Why Pilots Care
Vibration can loosen fasteners; an unsecured bolt on a control surface or propeller can lead to loss of control or engine failure.
Analogy
A cotter key works a little like putting a small pin through a hole and bending the ends so it cannot slide back out.
Intuition Check
Do not think of cotter keys as keys for opening locks. In aircraft maintenance, they are small locking pins used to keep hardware from moving out of place.
Example Sentence 1
After torquing the castle nut on the wheel axle, the mechanic installed a new cotter key and bent the prongs to secure it.
Example Sentence 2
Before flight, the pilot verified that every cotter key on the main gear was properly installed and bent.