In a twin-engine configuration where the left engine is the critical engine, which statement is correct?

• A. Left engine fails, greater arm
• B. Right engine fails, greater arm
• C. Both engines failing has the same effect
• D. There is no yaw moment from engine failure

Answer: (A)

In this setup, the concept being tested is which engine’s failure produces the larger yawing moment about the aircraft’s center of gravity—the critical engine. Since the left engine is the critical engine, its loss creates the greater adverse yaw moment than a loss of the right engine. When the left engine fails, the surviving right engine’s thrust pulls on the right side of the aircraft, and the combination of thrust asymmetry and the propeller rotation effects increases the yaw tendency toward the left. That means the moment arm from the CG to the thrust of the surviving engine generates the larger yawing moment.

If the right engine fails, the yaw moment is smaller because the left engine’s thrust still acts closer to the CG in a way that produces less adverse yaw. If both engines fail, you lose thrust on both sides, but the single-engine-out case with the greater yaw tendency is the one described by the statement about a larger moment arm. There is a yaw moment whenever an engine fails, so saying there is no yaw moment is incorrect.