Describe the concept of "Critical Engine."

The concept of a "Critical Engine" in multi-engine aircraft refers to the engine whose failure most adversely impacts the aircraft's controllability and performance. Specifically, this engine is typically the one that, when lost, creates the most significant yawing moment towards the failed engine.

For example, in a conventional multi-engine aircraft configuration, the critical engine is often the one rotating in the opposite direction to the aircraft's directional control during flight. When this engine fails, the resultant asymmetrical thrust can lead to an inability to maintain controlled flight without significant corrective input from the pilot. This can make the aircraft challenging to control and can lead to decreased performance in terms of altitude and airspeed, making understanding the critical engine vital for safe operation.

Thus, knowing which engine is critical helps pilots prepare for engine-out scenarios and manage the aircraft more effectively in case of an emergency.