Not technically illegal, but it's a violation for civil air crafts to break the sound barrier and cause any damage or disturbance.
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Fifty years ago, the federal government banned all civilian supersonic flights over land. The rule prohibits non-military aircraft from flying faster than sound so their resulting sonic booms won't startle the public below or concern them about potential property damage.
Currently, U.S. law prohibits flight in excess of Mach 1 over land unless specifically authorized by the FAA for purposes stated in the regulations. The two supersonic rulemaking activities would not rescind the prohibition of flight in excess of Mach 1 over land.
Supersonic speed is the speed of an object that is faster than the speed of sound, measured at about 768 miles per hour at sea level. Supersonic speed is one of the four “regimes of flight” (subsonic, transonic, supersonic, hypersonic).