What speed is defined as Liftoff Speed (Vlof)?

Liftoff Speed, abbreviated as Vlof, is specifically defined as the speed at which the main gear of the aircraft leaves the ground during takeoff. This is a crucial moment in the flight profile, as it marks the transition from ground operations to flight. At Vlof, the aircraft's wings generate sufficient lift to allow the airplane to become airborne, ensuring that aerodynamic forces can effectively support the aircraft's weight.

Understanding Vlof is essential for pilots during takeoff performance calculations, as it helps determine the optimal moment for rotation and ensures that the aircraft is operating within its performance parameters. It is vital for achieving safe and effective takeoffs, and pilots must be aware of this speed to successfully execute a takeoff maneuver.

The other options do not accurately define Liftoff Speed. While stability in flight, maximum operational speeds during ascent, and thresholds for engine failure are important aspects of flight operations, they do not pertain specifically to the moment of lift-off and the associated speed that signifies when the aircraft is no longer in contact with the runway.