Any speed over the speed of sound, which is 1,225 km/h at sea level, is said to be supersonic. Many modern fighter aircraft are supersonic. The Concorde was a supersonic passenger aircraft, but, since its final retirement flight on November 26th 2003, there is no supersonic passenger aircraft in service. Speeds greater than 5 times the speed of sound are sometimes referred to as hypersonic.

Chuck Yeager is the first person to travel faster than Mach 1, the speed of sound, in level flight. On October 14, 1947, Yeager broke the sound barrier flying the Bell X-1 at an altitude of 45,000 feet.

It is believed that the Messerschmitt Me 262, the first operational jet powered aircraft, was able to break the sound barrier during dives. There are numerous reports of pilots of sudden, strange aerodynamic behavior during dives that are consistent with the effects while breaking the sound barrier, plus a number of unexplained crashes during high speed dives. Pilots described it as the plane being hit with a large hammer, often resulting in damage to the plane.

See also:

sound barrier
Mach number