FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesEasy
During assembly, a technician installs a castellated nut on a drilled bolt and inserts a cotter pin through the aligned holes. What is the primary purpose of this cotter pin installation?
- ATo seal the joint against moisture intrusion
- BTo indicate proper torque was applied to the nut
- CTo provide electrical bonding across the joint
- DTo prevent the nut from loosening due to vibration by mechanically locking it to the bolt
Show answer & explanationAnswer & explanation
Correct answer: D. To prevent the nut from loosening due to vibration by mechanically locking it to the bolt
A cotter pin passed through the drilled bolt shank and the slots (castellations) of the nut mechanically prevents the nut from backing off under vibration. It provides no electrical, sealing, or torque-indicating function.
Why the other options are wrong
- A. Cotter pins do not seal against moisture.
- B. Torque is verified with a torque wrench, not by cotter pin fit.
- C. Bonding jumpers, not cotter pins, provide electrical continuity.
Cotter Pin Safetying
A cotter pin inserted through a drilled bolt and a castellated nut prevents the nut from rotating loose under vibration.
- Requires a castellated (slotted) nut and drilled bolt
- One prong bent over the bolt end, other along the nut flat
- Never reuse a bent/removed cotter pin
Memory trick: 'Pin it so it can't spin it'