Definition
The calibrated airspeed at which the critical engine is assumed to fail during takeoff in a multi-engine airplane, used by the manufacturer to determine V1 and other takeoff performance speeds.
Plain English
The speed the manufacturer assumes an engine quits during the takeoff roll when calculating the airplane's takeoff performance numbers.
Context Anchor
Seen in multiengine airplane takeoff performance planning and discussions of what happens if an engine fails during takeoff.
Derivation
V is the standard aviation prefix for a velocity (speed), borrowed from the physics symbol. EF stands for engine failure. So VEF reads literally as 'velocity, engine failure.'
Why Pilots Care
It sets the basis for accelerate-stop and takeoff distances that guarantee safe performance if an engine fails.
Intuition Check
VEF is not a speed the pilot tries to sense by feel during takeoff. It is an assumed speed used in the performance planning for an engine failure.
Example Sentence 1
The performance chart lists VEF for this weight and configuration, and V1 is derived from it with a one-second recognition delay added.
Example Sentence 2
Certification data lists VEF as the reference speed for all engine-out takeoff calculations.