Critical Engine

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.