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Vr is defined as the speed at which the rotation of the aircraft should be initiated to takeoff attitude.
This speed cannot be less than V1 and if it is greater than V1 and it is found at Vr that rotation cannot be in imitated, then a rejected take off may not be achievable within the remaining runway length.
Vr is a function of aircraft weight and flap setting but may also vary with pressure altitude and temperature.
In the engine failure case, Vr must allow for acceleration to V2 at screen height - 35 feet10.668 m above the level of the runway surface for aircraft certificated as meeting Performance 'A'.