What is the definition of critical engine failure for a multi-engine aircraft?

The definition of critical engine failure for a multi-engine aircraft centers around the failure of an engine that significantly hinders the aircraft's capability to maintain controlled flight. This concept is vital for ensuring safety and operational awareness in multi-engine aircraft, where the loss of one engine can lead to a substantial decrease in performance and control.

When an engine fails during flight, depending on the phase of operation and the configuration of the aircraft, the aircraft may still be able to maintain controlled flight. However, if the failure of an engine adversely affects the aircraft's ability to sustain altitude or control, it is categorized as a critical engine failure. This term acknowledges the importance of each engine's role in maintaining stability and control, particularly in situations such as takeoff, climb, and certain flight attitudes.

The other options describe common scenarios of engine failure but do not encapsulate the primary concept of critical engine failure. For instance, engine failure during takeoff or cruise may occur without severely impacting control, and the failure of the least powerful engine specifically during climb doesn't address the broader implications for control following an engine loss. Thus, the selected definition accurately captures the risks and implications associated with engine failures in multi-engine aircraft operations.