Note: This is a Principles of Flight question and should not be in Performance, so should have been moved by EASA, please let us know if you see this in the Performance exam, thank you.

The thrust line of a clockwise rotating propeller, when viewed from the rear, is displaced to the right of the engine center line. If both engines rotate clockwise, the right engine will have a longer thrust arm than the left engine. If the left engine fails, the thrust of the right engine acts through a longer thrust arm and will give a bigger yawing moment.

The critical engine is the engine, the failure of which would give the biggest yawing moment.

To overcome the disadvantage of the critical engine, engines may be designed to counter-rotate. This means that the left engine rotates clockwise, and the right engine rotates anti-clockwise, giving both engines the smallest possible thrust arm.