Critical engine
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The critical engine of a multi-engine airplane is that engine which, if any engine were to become inoperative, produces the least desirable effects.
Description
When a twin-engine airplane engine becomes inoperative, a thrust imbalance exists between the operative and inoperative sides of the airplane. 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 propellor airplane is typically considered critical.
Factors affecting engine criticality
Asymmetrical yaw
When one engine becomes inoperative, a torque will be developed which depends on the 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. Due to the P-factor effect, the right-hand engine typically develops its resultant thrust vector at a longer distance from the aircraft C.G. than the left-hand engine. The failure of the left-hand engine will result in a larger yaw effect via the operating right-hand engine, rather than vice-versa. Since the operating right hand engine produces a stronger yaw moment, the pilot will need to use larger control deflections in order to maintain aircraft control. Thus, the failure of the left-hand engine is less desirable than failure of the right-hand engine, and the left-hand engine is critical. It is important to note however that this example depends upon both engine propellers turning in the same direction (clockwise when view from the rear of the aircraft). Airplanes which have counter-rotating propellers do not have a critical engine.
Accelerated Slipstream
Lift can be roughly defined as an upwards force resulting from an airstream going over and under a wing. On aircraft with propellers mounted on the wing, the propwash from the engine will accelerate the airstream over the portion of the wing directly behind the propeller. This results in greater lift behind the propeller than at other spots on the wing. From the P-factor effect, the right wing's center of lift will be further from the C.G. than the left-hand wing. While failure of either engine will cause a rolling motion towards the inoperative side, the rolling motion will be more severe with the right engine operating. Thus, the failure of the left-hand engine is critical. Again, this example depends on both engines turning clockwise when viewed from the rear.
Non-aerodynamic criticality
On certain aircraft, hydraulic, pneumatic or electrical systems may be powered by one engine. This engine would therefore be critical in this respect.
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