How fast does a Boeing 747 take off?

A commercial jet takeoff is planned around three reference speeds. V1 is the decision speed: above it the takeoff continues even if an engine fails. VR is the rotation speed, the moment the pilot pulls back on the column and the nose lifts. V2 is the safety speed the aircraft must maintain after leaving the runway with one engine inoperative. On a 747-400 loaded for an intercontinental flight, those values are around 150, 165 and 180 kn (280, 305 and 335 km/h) respectively.

The number changes on every flight. The greater the mass, the more lift is needed and the faster the aircraft must go before rotating. Flaps change the wing's shape: with more deflection, the aircraft can take off at a lower speed, at the cost of a worse climb rate. Airport temperature and altitude reduce air density, which forces higher actual speeds and longer runways. That's why a 747 leaving Mexico City on a summer afternoon needs much more runway than one taking off from Amsterdam in winter. A headwind shortens the takeoff run, because part of the airspeed is already provided by the moving air.

At maximum mass, a 747-400 needs about 3,000 m of runway to take off, and the heavier 747-8 needs somewhat more. The manufacturer's performance tables, now built into the flight management system, calculate the combination of thrust, flaps and speeds that guarantees that, even with an engine failure at V1, the aircraft can either stop on the remaining runway or continue and clear the obstacles. That double margin is what actually sets the figure pilots see on their screen, and it's also why many takeoffs are made with reduced thrust: if there's runway to spare, the engines are saved.