Aircraft hijacking (also known as skyjacking and aircraft piracy) is the takeover of an aircraft, by a person or group, usually armed. In most cases the pilot is forced to fly according to the orders of the hijackers. Sometimes, the hijackers fly the planes themselves, as occurred in the September 11, 2001 attacks. In one case the official pilot hijacked the plane, when he diverted his internal Air China flight to Taiwan.