Definition
A condition in a multi-engine aircraft in which one engine has failed or has been intentionally shut down while the remaining engine or engines continue to operate. Performance data, climb gradients, and obstacle clearance requirements published for this condition assume the failed engine's propeller is feathered (or otherwise configured for minimum drag) and the aircraft is being flown at the appropriate single-engine speed and configuration.
Plain English
One of the aircraft's engines has stopped working, but the other engine (or engines) is still running. The aircraft is being flown on the engines that remain.
Context Anchor
Seen in multiengine aircraft performance planning, especially for departures, climbs, missed approaches, and obstacle clearance.
Derivation
Inoperative comes from in-, meaning “not,” and operative, meaning “working.” In this phrase, the engine is still part of the aircraft, but it is not doing useful work for flight.
Why Pilots Care
It sets the minimum runway length and climb performance needed to continue flight safely after losing an engine.
Intuition Check
Do not read this as just “one engine is turned off.” In this context, it means the aircraft must be planned or flown with one engine providing no usable power.
Example Sentence 1
The departure procedure required a climb gradient of 250 feet per nautical mile, so we checked the one engine inoperative performance chart before takeoff to confirm we could meet it.
Example Sentence 2
With one engine inoperative the airplane still maintained a positive rate of climb on the remaining engine.