Critical Engine — The engine whose failure would most adversely affect aircraft performance or handling.

The critical engine is the engine whose failure would result in the most adverse effect on aircraft performance or handling characteristics. On multi-engine propeller aircraft, this is typically the left engine (on aircraft with clockwise-rotating propellers as seen from behind), because its failure produces the greatest yawing moment due to the combination of P-factor, accelerated slipstream, spiraling slipstream, and torque effects.

The concept is important because V-speeds and performance data in the Aircraft Flight Manual are calculated assuming failure of the critical engine. If the non-critical engine fails instead, actual performance will be slightly better than the published figures.

On jet aircraft with wing-mounted engines, both engines are generally considered equally critical because the thrust lines are symmetrical and there are no propeller effects. The concept still applies to three-engine configurations like the MD-11 or older trijet designs.