What is Vef?

• A. Maximum engine thrust speed
• B. Cruise speed
• C. Speed at which an engine is assumed to fail
• D. Minimum engine idle speed

Answer: (C)

Vef is the speed used in takeoff performance analysis to model an engine becoming inoperative. It represents the airspeed at which the calculation assumes one engine has failed, so you can evaluate whether you can still achieve required performance (such as a safe climb with the remaining engine and obstacle clearance) after that event. This makes it the best answer because it directly describes the moment an engine failure is assumed for performance calculations. The other speeds described don’t fit this purpose: maximum thrust speed isn’t a standard performance marker, cruise speed is simply the normal operating speed, and minimum engine idle speed is just the lowest idling setting, not the point at which an engine failure is assumed.