Definition
A second nut tightened against a primary nut or against a threaded fitting to prevent it from loosening or rotating out of its set position. The check nut does not carry the main load; it locks the adjustment in place.
Plain English
A locking nut added behind another nut to stop it from working loose. Once the first nut is set where you want it, the check nut is tightened against it to hold everything in position.
Context Anchor
Seen in aircraft maintenance on adjustable rods, fittings, and other parts that must stay fixed after they are set.
Derivation
Check' here comes from the older sense of 'to stop or restrain,' as in 'check the motion of something.' A check nut checks (stops) the primary nut from turning.
Why Pilots Care
Keeps critical fasteners secure in high-vibration environments, directly affecting flight safety and component reliability.
Intuition Check
“Check” does not mean “inspect” here. It means “hold in place” or “stop movement.”
Example Sentence 1
After adjusting the pushrod length, the mechanic tightened the check nut to lock the setting.
Example Sentence 2
Vibration loosened the mixture control cable until a check nut was added to hold everything secure.