Takeoff is the phase of flight in which an aircraft goes through a transition from moving along the ground (taxiing) to flying in the air, usually starting on a runway. For balloons, helicopters and some specialized fixed-wing aircraft (VTOL aircraft such as the Harrier), no runway is needed. Takeoff is the opposite of landing.
For light aircraft, full power is used during takeoff. Large transport category (airliner) aircraft will usually use a derated power takeoff, where less than full power is applied, with unneeded power held in reserve in case of emergency. Before takeoff, the engines, particularly piston engines, are routinely run up at high power to check for engine-related problems. The aircraft is permitted to accelerate to rotation speed (often referred to as Vr). The term rotation is used because the aircraft pivots around the axis of its main landing gear while still on the ground, usually due to manipulation of the flight controls to make this change in aircraft attitude.
The nose is raised to a nominal 5°–20° nose up pitch attitude to increase lift from the wings and effect liftoff. For most aircraft, attempting a takeoff without a pitch-up would require cruise speeds while still on the runway.
Fixed-wing aircraft designed for high-speed operation (such as commercial jet aircraft) have difficulty generating enough lift at the low speeds encountered during takeoff. These are therefore fitted with high-lift devices, often including slats and usually flaps, which increase the camber of the wing, making it more effective at low speed, thus creating more lift. These are deployed from the wing prior to takeoff, and retracted during the climb. They can also be deployed at other times, such as prior to landing.
The speeds needed for takeoff are relative to the motion of the air (indicated airspeed). A headwind will reduce the ground speed needed for takeoff, as there is a greater flow of air over the wings. Typical takeoff air speeds for jetliners are in the 130–155 knot range (150–180 mph, 250–290 km/h). Light aircraft, such as a Cessna 150, take off at around 55 knots (63 mph, 100 km/h). Ultralights have even lower takeoff speeds. The takeoff speed is usually directly proportional to the aircraft weight; the heavier the weight, the greater the speed needed. Some aircraft specifically designed for short takeoff and landing can take off at speeds below 40 knots (74 km/h), and can even become airborne from a standing start when pointed into a sufficiently strong wind.
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.