Definition
The amount a rotating part deviates from true rotation about its axis, measured as the total variation observed at a reference surface as the part is turned through one full revolution. Runout is typically expressed in thousandths of an inch and is checked with a dial indicator against the part's outer surface or face.
Plain English
How much a spinning part wobbles or moves off-center as it turns. If a shaft, propeller flange, or disc is not perfectly true, one side will sit a little farther out than the other, and that difference is the runout.
Context Anchor
Seen in aircraft engine and propeller maintenance when checking rotating parts for bending, wear, or poor alignment.
Derivation
From 'run' (to rotate or operate) and 'out' (away from true position). The word literally describes how far a turning part 'runs out' from where it should be.
Why Pilots Care
Excessive runout produces vibration that can damage bearings, loosen fasteners, or lead to in-flight component failure.
Analogy
A bent bicycle wheel shows the idea: as it spins, the rim moves side to side instead of staying in one steady path. That side-to-side wobble is like runout.
Intuition Check
Runout does not mean using up fuel, time, or supplies. In maintenance, it means measured wobble or off-center movement in a rotating part.
Example Sentence 1
After a sudden propeller stoppage, the mechanic checked crankshaft runout with a dial indicator before approving the engine for further service.
Example Sentence 2
Propeller runout beyond limits requires hub or blade inspection before the aircraft returns to service.