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Engine Failure Speed
The speed at which the Critical Engine is assumed to fail during takeoff.
Engine Failure Speed (VEF) is an aircraft design speed. It is the maximum aircraft speed at which the most critical engine can fail without compromising the safe completion of the takeoff after failure recognition. Considering that it is generally assumed humans have a reaction time to an unexpected event (such as a failure) of 1 second, V1 must be greater than VEF. In addition, if an engine failure occurs at VEF, then it must be possible to continue and safely achieve takeoff with takeoff power set on the remaining engine(s). This means that VEF must also be greater than Vmcg.
Aircraft Performance & Definitions