Definition
A measurement of how much an engine crankshaft deviates from a perfectly straight rotational axis as it turns. Runout is checked by mounting a dial indicator against the crankshaft and rotating it through a full revolution; the total movement of the indicator needle shows how far the shaft is bent or out of true. Excessive runout means the crankshaft is bent beyond the manufacturer's allowable limit and the engine is unairworthy.
Plain English
It's a check to see if the engine's main spinning shaft is bent. A gauge is held against the shaft while it's turned by hand, and the amount the gauge moves shows how far off-straight the shaft is.
Context Anchor
Seen in aircraft engine maintenance after a propeller strike, sudden engine stoppage, overhaul, or vibration complaint.
Derivation
Runout' comes from machining and engineering, describing how far a rotating part 'runs out' from its true centerline as it turns. The word captures the idea of a shaft that doesn't run true — it wobbles or traces a slightly off-center path.
Why Pilots Care
Excessive runout produces damaging vibrations that accelerate bearing wear and can lead to engine failure.
Analogy
It is like spinning a slightly bent pencil between your fingers. Instead of the tip staying in one place, it traces a small circle as it turns.
Intuition Check
Crankshaft runout does not mean the engine has run out of oil, fuel, or time. It means the rotating crankshaft is not staying perfectly centered as it turns.
Example Sentence 1
After the prop strike on landing, the mechanic performed a crankshaft runout check and found the shaft was within limits.
Example Sentence 2
High crankshaft runout caused noticeable vibration during the test run.