How high did the Concorde fly?

Flying at twice the speed of sound at the altitude of a conventional airliner would have been impractical. Aerodynamic drag grows with air density, and in supersonic flight the penalty is much greater. That is why the Concorde climbed up to 18,300 metres (60,000 ft), its certified ceiling, where density is a fraction of that at sea level and the four afterburning Olympus 593 engines could sustain Mach 2 without ruinous fuel consumption. The typical profile involved a gradual climb during cruise: as it burned fuel and lost weight, the aircraft climbed, a procedure known as cruise-climb.

At that altitude, above almost all the air mass, the sky appeared a very dark blue and the horizon showed a noticeable curvature. The window, notably smaller than on a conventional airliner to limit the effect of a decompression at that altitude, grew warm to the touch: air friction raised the nose temperature above a hundred degrees and stretched the structure by several centimetres in flight.

Flying that high came with trade-offs. The crew followed specific emergency descent procedures, because the time of useful consciousness without oxygen at 18,000 metres is measured in seconds. Cosmic radiation dose, which increases with altitude, was also monitored with an onboard radiometer. No passenger aircraft has operated regularly at that altitude since the type was retired in 2003.