Definition
A self-locking nut that uses a fiber insert pressed into the top of the nut to grip the threads of the bolt. As the bolt is screwed in, it cuts its own threads through the unthreaded fiber collar, creating friction that resists loosening from vibration. Fiber locknuts are limited to use in low-temperature applications (typically below 250°F) and are generally not reused once removed.
Plain English
A nut with a small ring of fiber inside it. When you tighten a bolt into the nut, the fiber grips the bolt tightly so vibration can't shake it loose.
Context Anchor
Seen in aircraft maintenance, hardware identification, and instructions for securing parts that may be exposed to vibration.
Derivation
Fiber' refers to the nylon or similar fibrous insert in the nut. 'Locknut' simply means a nut designed to lock itself in place. The name describes exactly what it is: a nut that locks using a fiber insert.
Why Pilots Care
Loose fasteners from vibration can cause component failure or loss of control in flight; these nuts provide reliable locking without extra hardware.
Analogy
It is like a metal nut with a built-in grip. The metal part carries the load, and the fiber insert adds friction so the nut does not back off easily.
Intuition Check
Do not read fiber locknut as a nut made entirely of fiber. It is normally a metal nut with a fiber insert that provides the locking action.
Example Sentence 1
The mechanic replaced the fiber locknut on the seat track because once it had been removed, it could no longer be trusted to hold its grip.
Example Sentence 2
During preflight, several fiber locknuts on the exhaust clamps showed wear and were replaced.