Definition
A secondary nut tightened against a primary nut or threaded fitting to lock it in place and prevent it from loosening due to vibration or rotation.
Plain English
A second nut screwed up tight against the first one so the first one cannot vibrate loose.
Context Anchor
Seen during aircraft maintenance and preflight discussions around adjustable rods, linkages, and other threaded parts.
Derivation
From 'jam' meaning to wedge or press tightly. The two nuts are jammed against each other so neither can turn.
Why Pilots Care
Loose fasteners in flight can lead to component failure; jam nuts provide a simple, reliable way to maintain torque in vibrating environments.
Analogy
It is like using a second cap on a threaded bottle top to keep the first cap from turning. The pressure between the two helps keep things from backing off.
Intuition Check
Do not read “jam” as “stuck by accident.” Here it means intentionally tightened against another part to keep an adjustment from moving.
Example Sentence 1
After adjusting the control rod to the correct length, the mechanic tightened the jam nut to lock the setting.
Example Sentence 2
After torquing the main nut, the technician ran a jam nut up against it and tightened it to specification.