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The strength of the yawing moment caused by an engine failure on a twin-engine aeroplane...
  • A
    will be greater when the CG is located at the aft CG limit.
  • B
    will be greater when the CG is located at the forward CG limit.
  • C
    will be greater when the CG is located halfway between the forward and aft CG limits.
  • D
    is independent of the location (fore or aft) of the CG.

Refer to figure.
When an engine fails on a multi-engine aircraft there will be a decrease in thrust and an increase in drag on the side with the failed engine:

  • airspeed will decay.
  • the nose will drop and.
  • most significantly, there will be an immediate yawing moment towards the failed engine.
The forces and moments acting on an aircraft following failure of the left engine are shown in figure 1. The aircraft has a yawing moment towards the dead engine. The pilot has applied rudder to stop the yaw. The vital action when an engine fails is to stop the yaw.
The rolling and yawing moments and the power of the flight controls to balance them will determine the controllability of an aircraft with asymmetric thrust. Rolling and yawing moments with asymmetric thrust are affected by:
  • Thrust on the live engine
  • Altitude
  • Drag from the dead engine and propeller
  • Asymmetric blade effect (also known as ‘P’ Factor)
  • CG position
The fore and aft CG location has no effect on the yawing moment from a failed engine, but will influence the rudder arm, hence the rudder moment. CG on the aft limit will give the smallest rudder arm and the least ability to oppose the yawing moment from a failed engine.
  • Torque reaction
  • Difference in lift due to slipstream
  • Rolling moment due to sideslip
  • Weight
  • Airspeed
Note: This question is out of the scope of performance (032). However, a student has reported that s/he came across it in his/her performance exam. Please, let us know if you come across it.

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