What is defined as the engine whose failure would most negatively impact an aircraft's performance or handling?

The engine that is classified as having the most significant potential impact on an aircraft's performance or handling in the event of its failure is referred to as the critical engine. In the context of multi-engine aircraft, the failure of the critical engine can lead to a marked reduction in performance due to factors such as yaw, which can adversely affect controllability.

The critical engine is typically identified based on the aircraft's design and performance characteristics, including its center of gravity and overall aerodynamic profile. This classification is essential for pilots and flight operations to understand because it informs procedural responses in the event of an engine failure, ensuring that corrective measures can be taken to maintain control of the aircraft.

In this scenario, understanding the term "critical engine" is vital, as it plays a crucial role in determining how aircraft handle engine failures and informs a pilot's training and operational protocols during flight. The other terms provided, such as backup engine or emergency engine, do not convey the same importance regarding performance impact when one engine fails.