Critical engine
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.
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.
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 a critical engine in an aircraft with propellers in a push-pull configuration typically will not generate large yawing or rolling moments.
The standards and certifications that specify airworthiness require that the manufacturer determine a minimum control speed (VMC) 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.
When any one of the other engines fails or is inoperative, the actual VMC that the pilot experiences in flight will be slightly lower, which is safer, but this difference is not documented in the manual.
The critical engine is defined in aviation regulations for the purpose of designing the tail, and for experimental test pilots to measure VMCs in flight.
Other factors like bank angle, and thrust have a much greater effect on VMCs than the difference of a critical and a non-critical engine.
Only rapid reduction of thrust of the opposite engine, or increased airspeed can restore the required control power to maintain straight flight following the failure of a feathering system.
