Definition
The amount a rotating shaft deviates from true rotation about its centerline, measured as the total side-to-side or end-to-end movement of the shaft surface as it turns. Runout is checked with a dial indicator and is expressed as Total Indicated Runout (TIR), which is the difference between the highest and lowest readings during one full rotation. Excessive runout indicates the shaft is bent, misaligned, or has worn bearings.
Plain English
How much a spinning shaft wobbles instead of turning perfectly straight. Mechanics measure the wobble with a gauge while slowly rotating the shaft; if it moves too far off-center, the shaft is bent or the bearings are worn.
Context Anchor
Seen in aircraft maintenance when checking rotating parts such as crankshafts, drive shafts, pumps, starters, generators, and propeller-related components.
Derivation
From 'run' (to rotate or operate) and 'out' (away from true position). Literally, how far the shaft 'runs out' from its intended axis as it turns.
Why Pilots Care
Excessive runout creates vibration that can loosen fasteners, accelerate bearing wear, and lead to in-flight component failure.
Intuition Check
Shaft runout does not mean the shaft has run out of length, time, or service life. It means the shaft is not rotating perfectly straight and centered.
Example Sentence 1
After a prop strike, the mechanic checked the crankshaft for shaft runout before returning the engine to service.
Example Sentence 2
High propeller shaft runout caused noticeable vibration during the preflight engine run-up.