Plain English Meaning
The engine is running, but not smoothly. It feels shaky, sounds uneven, or seems to be working harder than it should — a sign that something is wrong and needs attention.
Full Definition
An engine that is operating unevenly, producing irregular power, vibration, or audible roughness rather than the smooth, steady output expected in normal operation. Causes can include fouled spark plugs, a faulty magneto, improper fuel mixture, induction system icing, contaminated fuel, or a mechanical problem inside the engine.
Context Anchor
A pilot may notice a rough-running engine during the pre-takeoff engine check, after changing power, during cruise, or while troubleshooting an engine problem in flight.
Why Pilots Care
A rough-running engine signals a problem that can reduce available power, increase pilot workload, or progress to complete engine failure if left unresolved.
Intuition Check
Do not confuse a rough-running engine with rough air. Rough air shakes the airplane from outside; a rough-running engine is the engine itself operating unevenly.
Example Sentence 1
During the run-up, the pilot noticed a rough-running engine on the left magneto and taxied back to have the spark plugs inspected.
Example Sentence 2
After leaning for cruise, the engine began running rough until the pilot enriched the mixture slightly to restore smooth operation.