What happens when a plane goes supersonic?

Sound is a pressure wave, and at subsonic speeds the air ahead of an aircraft receives advance warning through those waves: the flow begins to divert around the nose and wings before they arrive. At Mach 1 the aircraft is travelling as fast as the warning, and the disturbances can no longer propagate forward. They pile up into extremely thin regions where pressure, temperature and density jump almost discontinuously. These are shock waves, and every supersonic aircraft carries a set of them attached to its nose, canopy, wing leading edges, inlets and tail.

The uncomfortable part is not being supersonic but getting there. Air accelerates as it curves over a wing, so parts of the flow reach Mach 1 while the aircraft is still at Mach 0.8 or so. Local shock waves form on the upper surface, the boundary layer behind them separates, and drag rises steeply. On the early aircraft that probed this region the effects were dramatic: buffeting, a nose-down pitching tendency called Mach tuck, ailerons that reversed their effect, and elevators frozen by the shock wave sitting on the tailplane. The Bell X-1 solved the last of these with an all-moving horizontal tail, and the design tools that followed, swept wings, thin sections and area ruling, allowed later aircraft to pass through the region smoothly.

Once fully supersonic the flow becomes more orderly. The shocks stabilise, drag settles at a higher but predictable level, and the aircraft flies with the centre of lift moved rearward, which on Concorde was compensated by pumping fuel aft into a trim tank. The air compressed through the shocks also heats up: at Mach 2 the nose of Concorde reached about 127 °C, and the airframe stretched by around 25 cm in cruise. Engine inlets need movable ramps or spikes to slow the incoming air to subsonic speed before it reaches the compressor. The pilots feel little; the Mach meter is the only clear sign that anything has changed.

The shock waves do not stay with the aircraft. They spread outward in a cone and reach the ground as a sudden pressure rise followed by a drop, heard as a double bang. The boom is continuous along the entire supersonic track, which is why overland supersonic flight has been banned in most countries since the 1970s and why NASA's X-59 is testing a shape that softens the signature.