The takeoff speed required varies with air density, aircraft gross weight, and aircraft configuration (flap and/or slat position, as applicable). Air density is affected by factors such as field elevation and air temperature. This relationship between temperature, altitude, and air density can be expressed as a density altitude, or the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density.
Bulgarian-registered Wizz Air Airbus A320-200 takes off at London Luton Airport, England Pilots of large multi-engine aircraft calculate a decision speed (V1) for each takeoff that dictates action to be taken in case an engine fails. This speed is determined not only by the above factors affecting takeoff performance, but by the length of the runway and any peculiar conditions, such as obstacles off the end of the runway. Below V1, the takeoff is aborted; above V1 the pilot continues the takeoff and returns for landing. After the co-pilot calls V1, he/she will call Vr or "rotate," marking speed at which to rotate the aircraft. The Vr for transport category aircraft is computed such that three seconds after rotation is initiated the aircraft is in the liftoff attitude and at the liftoff speed. Then, V2 (the safe climb speed) is called. This speed must be maintained after an engine failure to meet performance targets for rate of climb and angle of climb.
In a single-engine or light twin-engine aircraft, the pilot calculates the length of runway required to take off and clear any obstacles, to ensure sufficient runway to use for takeoff. A safety margin can be added to provide the option to stop on the runway in case of a rejected takeoff. In most such aircraft, any engine failure results in a rejected takeoff as a matter of course, since even overrunning the end of the runway is preferable to lifting off with insufficient power to maintain flight.
If an obstacle needs to be cleared, the pilot climbs at the speed for maximum climb angle (Vx), which results in the greatest altitude gain per unit of horizontal distance travelled. If no obstacle needs to be cleared, or after an obstacle is cleared, the pilot can accelerate to the best rate of climb speed (Vy), where the aircraft will gain the most altitude in the least amount of time. Generally speaking, Vx is a lower speed than Vy, and requires a higher pitch attitude to achieve.
