Definition
The amount of resistance, measured in inch-pounds or foot-pounds, required to turn a self-locking fastener (such as a self-locking nut) on its bolt or stud before the nut contacts the part being fastened. This resistance comes from the locking feature of the fastener itself, not from clamping load.
Plain English
It is the friction you have to overcome just to spin a self-locking nut down the bolt, before it ever pulls anything tight. That built-in stiffness is what stops the nut from backing off in service.
Context Anchor
Seen in aircraft maintenance when installing self-locking nuts, bolts, or other threaded fasteners that have a locking feature.
Derivation
‘Prevailing’ comes from the Latin praevalere, meaning ‘to be stronger’ or ‘to persist.’ In this context it describes the resistance that persists throughout the run-down of the nut — it is always there, even before the nut is seated.
Why Pilots Care
Low prevailing torque can allow a fastener to loosen under vibration, affecting engine or airframe security.
Intuition Check
“Prevailing” does not mean “most common” here. It means the resistance that is already present from the fastener’s locking friction before the parts are tightened.
Example Sentence 1
Before seating the nut, the technician measured the prevailing torque and added it to the value specified in the maintenance manual.
Example Sentence 2
If prevailing torque falls below spec, the self-locking nut may not stay tight in flight.