How fast does a supersonic plane fly?

An aircraft becomes supersonic when it exceeds the speed of sound in the air it's moving through — Mach 1, which in the stratosphere equals roughly 1,062 km/h and near the ground about 1,225 km/h. Most aircraft capable of reaching that speed stay within a relatively narrow band, between Mach 1.6 and Mach 2.5, for a reason rooted in materials: up to about Mach 2.2, conventional aluminum can withstand aerodynamic heating, which at that speed reaches roughly 100-130°C on the aircraft's skin; beyond that, titanium, special steels, or high-temperature composites are needed, all far more expensive. That's why Concorde, built from aluminum alloy, topped out at Mach 2.04 — about 2,180 km/h at 18,000 m, with the nose reaching 127°C; the Tupolev Tu-144 reached Mach 2.15; and fighters like the F-15, MiG-29, or Mirage 2000 sit around Mach 2.2-2.5. Only the MiG-25 and MiG-31, built with steel structures, approach Mach 2.8-3.

On a Paris-New York crossing, Concorde accelerated subsonically out to the coast, lit its afterburners over the Atlantic, and crossed Mach 1 around 8,500 m; at 13,000 m it reached Mach 1.7, shut off the afterburners, and kept accelerating to Mach 2 on dry thrust alone from its four Olympus 593 engines. Cruise took place between 15,000 and 18,300 m, at about 2,180 km/h relative to the air, and the whole flight lasted around 3 hours 30 minutes, compared with 7-8 hours on a conventional jet. At that speed the fuselage stretched about 20 cm from thermal expansion, the aircraft covered a kilometer in just over 1.6 seconds, and, flying westward, it landed in New York at a local time earlier than its departure time from London. The sonic boom, however, closed off overland routes and limited its service to little more than a dozen airliners.

The Lockheed SR-71 Blackbird cruised at Mach 3.2 (about 3,400 km/h) above 24,000 m and set the 1976 speed record for a manned jet aircraft, 3,529.6 km/h, which still stands. Above that lies the hypersonic regime, starting at Mach 5, where only rocket planes have flown with a crew on board: the X-15 reached Mach 6.7 (7,274 km/h) in 1967. Uncrewed vehicles have gone further still, with NASA's X-43A hitting Mach 9.6 in 2004 thanks to a scramjet engine. For the ordinary passenger, supersonic speed will return, if it does, around Mach 1.7 — the figure current airliner projects are working with to balance fuel burn, noise, and materials.