In a twin-engine airplane, which engine is considered the "critical" engine?

In the context of twin-engine airplane operations, the "critical" engine is defined as the engine that, when lost, provides the greatest adverse yaw and control difficulty. This typically refers to the engine that is furthest from the aircraft's centerline.

When a twin-engine airplane loses an engine, the asymmetrical thrust created can lead to a yawing motion toward the inoperative engine due to the position of the remaining operational engine. The engine located further away from the centerline generates a greater moment that affects the aircraft's performance and handling characteristics. This makes managing the flight more challenging and necessitates immediate corrective actions by the pilot.

Consequently, the engine that presents the greatest challenge in the event of failure is identified as the critical engine because of the additional rudder input required to counter the yaw moment and maintain controlled flight. Understanding which engine is critical aids pilots in effective training and operational decision-making, particularly during takeoff and climb phases where engine failures are most critical.