What is considered a critical engine?

A critical engine is defined as an engine whose failure would have the most significant negative impact on an aircraft's ability to operate safely and effectively. This is particularly important in multi-engine aircraft where the loss of a single engine can dramatically alter the aircraft's performance characteristics, including its ability to maintain controlled flight, climb, or sustain altitude.

For example, if an engine on a multi-engine aircraft fails, it can lead to a loss of thrust that makes the aircraft difficult to control, requiring additional pilot skill and attention. Because of this potential for severe consequences, identifying and focusing on the critical engine in training and operational protocols is essential for maintaining safety.

The other provided options do not capture this essence. While fuel efficiency and thrust generation are important aspects of an engine's performance, they do not directly relate to the critical nature of engine failure. Redundancy is a safety feature in aviation design but does not directly define which engine is critical in terms of failure consequences.