Critical engine

The critical engine of a multi-engine, fixed-wing aircraft is the engine a failure of which would most adversely affect the performance or handling abilities of an aircraft(§1.1). On propeller aircraft, there is a difference in the remaining yawing moments after failure of the left or the right (outboard) engine when all propellers rotate in the same direction due to the P-factor. On turbojet/turbofan aircraft, there usually is no difference between the yawing moments after failure of a left or right (outboard) engine.

When one of the engines on a typical multi-engine aircraft becomes inoperative, a thrust imbalance exists between the operative and inoperative sides of the aircraft. This thrust imbalance causes several negative effects in addition to the loss of one engine's thrust.

During the design phase of the aircraft, the tail design engineer sizes the vertical stabilizer/ tail to comply with the controllability and performance requirements after engine failure in Aviation Regulations.

During the experimental flight test phase of a multi-engine aircraft, the Experimental Test Pilot and Flight Test Engineer determine which of the engines is the critical engine. Definitions of the critical engine can also be found in Flight Test Guides for Part 23 and Part 25 airplanes, issued by the FAA and by EASA.

When one engine becomes inoperative, a yawing moment develops, the magnitude of which is equal to the lateral distance of the thrust vector of the operative engine to the center of gravity (C.G.), also called moment arm, multiplied by the thrust of the engine. In addition, a rolling moment might develop due to asymmetrical propulsive lift generated by the wing section behind the operative propeller. These moments yaw and/ or roll the aircraft towards the inoperative engine, a tendency which must be counteracted by the pilot's use of the flight controls: rudder and ailerons. Due to P-factor, a clockwise rotating right-hand propeller on the right wing typically develops its resultant thrust vector at a greater lateral distance from the aircraft's C.G. than the clockwise rotating left-hand propeller (Figure 1). The failure of the left-hand engine will result in a larger remaining yawing moment by the operating right-hand engine, rather than vice versa. Since the operating right-hand engine produces a larger yawing moment, the pilot will need to use larger control deflections in order to maintain aircraft control, or a higher speed. Thus, the failure of the left-hand engine is less desirable than failure of the right-hand engine, and the left-hand engine is called critical.

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