Critical engine failure is defined as:

Critical engine failure in aviation refers specifically to the failure of a single engine in multi-engine aircraft. This definition is crucial for understanding the implications that engine failure has on flight safety and aircraft performance.

In multi-engine aircraft, the loss of one engine can significantly impact the aircraft's ability to maintain controlled flight, especially in terms of performance, controllability, and the ability to climb. The remaining engine must compensate for the thrust loss, which can lead to asymmetric thrust conditions. Pilots must be well-trained in handling such situations, as the aircraft's handling characteristics change dramatically when operating on one engine.

The concept of critical engine failure is particularly important during phases of flight such as takeoff and initial ascent, where the aircraft is most vulnerable. Additionally, understanding this definition is essential for developing the necessary emergency procedures and performance calculations, which inform pilots about the altitude and airspeed required to maintain control of the aircraft in the event of an engine failure.