The critical engine of a multi-engine fixed-wing aircraft is the engine that, in the event of failure, would most adversely affect the performance or handling abilities of an aircraft. 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 and turbofan twin-engine aircraft, there usually is no difference between the yawing moments after failure of a left or right engine in no-wind condition.
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.
The tail-design engineer is responsible for determining the size of vertical stabilizer that will comply with the regulatory requirements for the control, and performance of an aircraft after engine failure, such as those set by the Federal Aviation Administration and European Aviation Safety Agency. The experimental test pilot and flight-test engineer use flight testing to determine which of the engines is the critical engine.
Factors affecting engine criticality
Asymmetrical yaw
When one engine fails, a yawing moment develops, which applies a rotational force to the aircraft that tends to turn it toward the wing that carries the engine that failed. A rolling moment might develop, due to the asymmetry of the lift in each wing, with a greater lift generated by the wing with the operating engine. The yawing and rolling moments apply rotational forces that tend to yaw and roll the aircraft towards the failed engine. This tendency is counteracted by the pilot's use of the flight controls, which include the 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 center of gravity than the clockwise rotating left-hand propeller (Figure 1). The failure of the left-hand engine will result in a larger 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 deflections of the flight controls or a higher speed in order to maintain control of the aircraft. Thus, the failure of the left-hand engine has a greater impact than failure of the right-hand engine, and the left-hand engine is called the critical engine. On aircraft with propellers that rotate counter-clockwise, such as the de Havilland Dove, the right engine would be the critical engine.
Most aircraft that have counter-rotating propellers do not have a critical engine defined by the above mechanism because the two propellers are made to rotate inward from the top of the arc; both engines are critical. Some aircraft, such as the Lockheed P-38 Lightning, purposefully have propellers that rotate outward from the top of the arc, to reduce downward air turbulence, known as downwash, on the central horizontal stabilizer, which makes it easier to fire guns from the aircraft. These engines are both critical, but more critical than inward-rotating propellers.
Aircraft with propellers in a push-pull configuration, such as the Cessna 337, may have a critical engine, if failure of one engine has a greater negative effect on aircraft control or climb performance than failure of the other engine. The failure of a critical engine in an aircraft with propellers in a push-pull configuration typically will not generate large yawing or rolling moments.
frame|none|Fig. 1. With clockwise-rotating propellers (as seen by the pilot), the operating right-hand engine will produce a more severe yawing moment towards the dead engine, thus making the failure of the left-hand engine critical.
Effect of the critical engine on minimum control speed
The standards and certifications that specify airworthiness require that the manufacturer determine a minimum control speed (V<sub>MC</sub>) at which a pilot can retain control of the aircraft after failure of the critical engine, and publish this speed in the section of the airplane flight manual on limitations.