With left engine critical engine, the most adverse is?

• A. Right engine, thrust arm
• B. Left engine, thrust arm
• C. Center engine, thrust arm
• D. Both engines equally adverse

Answer: (A)

The main concept is yaw control from engine failure in a twin. The critical engine is the one whose failure would produce the greatest yawing moment about the aircraft’s center of gravity. In this configuration, the left engine is designated as the critical engine, so losing that engine creates the largest adverse yaw moment. The remaining right-hand engine’s thrust is applied offset from the CG, and aerodynamic effects like P-factor, accelerated slipstream, and gyroscopic precession on the operating engine further amplify the turning tendency. Because the left-side thrust line and these effects combine to produce a larger yaw than losing the right engine, the left-engine failure is the more adverse event. The center engine option isn’t applicable to a typical twin, and both engines being equally adverse would only be true if their lever arms and aerodynamic effects were identical, which they generally aren’t.