Critical Engine

The critical engine is the engine whose failure would most adversely affect the performance or handling qualities of a multi-engine aircraft. In a conventional twin with both propellers rotating clockwise (as viewed from the cockpit), the left engine is typically the critical engine due to P-factor and accelerated slipstream effects.

Why the Left Engine is Critical

In a twin with clockwise-rotating propellers, the descending blade on the right side of each propeller disc generates more thrust than the ascending blade on the left (P-factor). Both engines contribute to this asymmetric thrust, but the left engine’s thrust is centred further from the aircraft centreline — meaning if the left engine fails, the yawing moment is larger than if the right engine fails. The left engine’s failure is therefore more critical.

Counter-Rotating Propellers

Some twins use counter-rotating propellers (one clockwise, one counter-clockwise) specifically to eliminate the critical engine. With symmetrical thrust lines, neither engine is more critical than the other. The Piper Seminole is a well-known example.

On the TC Written Exam

The critical engine is a multi-engine rating topic. Know: which engine is typically critical and why; how counter-rotating propellers eliminate the critical engine issue; and the effect on V_MC (minimum control speed).

Related Terms

See also: Balanced Field Length, V_YSE.