FAA Aviation Mechanic Airframe (AMA)Airframe StructuresEasy
A technician installs a clevis bolt securing a flight control push-pull rod end to a bellcrank. The bolt has a drilled shank and is secured with a castellated nut. Which safetying method must be used to complete the installation?
- AUse a self-locking nut in place of the castellated nut with no cotter pin required
- BInsert a cotter pin through the drilled bolt shank and the slots of the castellated nut
- CApply a bead of thread-locking compound to the nut threads instead of mechanical safetying
- DTwist a length of safety wire around the nut only, without engaging the bolt
Show answer & explanationAnswer & explanation
Correct answer: B. Insert a cotter pin through the drilled bolt shank and the slots of the castellated nut
Castellated nuts are designed to be used with drilled-shank bolts and secured with a cotter pin passed through the bolt hole and the nut's slots, providing positive mechanical safetying against rotation.
Why the other options are wrong
- A. Substituting hardware types changes the approved installation and is not a valid safetying substitute without proper authorization.
- C. Thread-locking compound is not an approved substitute for mechanical safetying on flight control hardware.
- D. Safety wire alone around the nut without passing through the drilled bolt does not provide the required mechanical lock.
Castellated Nut & Cotter Pin Safetying
A castellated nut has slots cut into its top so a cotter pin can be inserted through the drilled bolt shank and nut, mechanically preventing the nut from backing off.
- Requires a bolt with a drilled shank
- Cotter pin prongs are bent to secure it in place
- Common on clevis bolts and landing gear bolts
- Provides positive mechanical lock, unlike friction-based locknuts
Memory trick: Castle nut needs a pin guard at its gate — the cotter pin.