Definition

The critical engine of a multi-engine, fixed-wing propeller-driven aircraft is the one whose failure would result in the most adverse effects on the aircraft's handling and performance.

Description

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. For reasons listed below, the left engine of a conventional twin-engine propeller-driven aircraft is usually considered critical.

Asymmetrical Yaw

When one engine becomes inoperative, a torque develops which depends on the lateral distance from the center of gravity (C.G.) to the thrust vector of the operating engine, multiplied by the thrust of the operating engine. The torque effect attempts to yaw the aircraft's nose towards the inoperative engine, a yaw tendency which must be counteracted by the pilot's use of the flight controls.