In a twin-engine airplane, the critical engine is the engine whose failure would result in greater loss of performance.

• A. Left engine
• B. Downwind engine
• C. Upwind engine
• D. Right engine

Answer: (B)

The concept being tested is the critical engine—the engine whose failure would cause the greatest loss of performance and controllability in a twin-engine airplane. When an engine quits, you get asymmetric thrust, which creates a yaw tendency toward the failed side. The remaining engine’s thrust, plus the airplane’s aerodynamics and propeller effects, determine how much control is lost.

In many common twin configurations, the downwind engine is the one whose failure leads to the largest adverse yaw and a bigger drop in overall performance. That’s because the interaction of the live engine’s thrust, the propeller rotation, and the wing/tail aerodynamics on the downwind side tends to produce a more pronounced yaw and loss of climb or acceleration than if the upwind engine failed. So the downwind engine is considered the critical engine, which is why that option is chosen.