FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
A technician needs to install a self-locking nut on a bracket located near an exhaust duct where operating temperatures regularly exceed 250°F. Which type of self-locking nut is appropriate for this high-temperature location?
- AStandard plain hex nut without a locking feature
- BFiber (nylon) insert self-locking nut
- CWing nut for quick removal
- DAll-metal (spring or deformed-thread) self-locking nut
Show answer & explanationAnswer & explanation
Correct answer: D. All-metal (spring or deformed-thread) self-locking nut
Fiber-insert self-locking nuts are limited to about 250°F because the nylon insert degrades at higher temperatures. All-metal self-locking nuts use a spring or deformed thread design that maintains locking friction at much higher temperatures, making them suitable for hot areas such as near exhaust systems.
Why the other options are wrong
- A. A plain nut has no locking feature and would back off due to vibration.
- B. Nylon insert nuts lose their locking ability and can melt or degrade above about 250°F.
- C. Wing nuts are for quick manual removal, not vibration-resistant structural applications.
Self-Locking Nut Temperature Limits
Self-locking nuts use either a fiber insert or an all-metal locking feature to resist loosening from vibration; the type selected depends on the operating temperature of the installation location.
- Fiber-insert (nylon) locknuts limited to about 250°F
- All-metal locknuts suitable for higher temperature areas like engine/exhaust zones
- Both resist nut rotation from vibration but by different mechanisms
- Not used where the nut is expected to be removed and reinstalled excessively without inspection
Memory trick: Fiber melts near fire — use all-metal near heat.