Inlet spike

A sharp-pointed spike in the inlet air duct of a supersonic aircraft. This spike forms an oblique, or angled, shock wave inside the inlet duct as it passes through the air. Air entering the first-stage compressor of a gas turbine engine must flow at a speed below the speed of sound, and when the airplane is flying faster than sound, the air must be slowed before it enters the compressor. When air passes through the oblique shock wave, it slows down, but is still moving faster than the speed of sound. Just inside the inlet duct, a second shock wave, this time a normal shock wave, is produced. This normal shock wave is at right angles to the airflow, and air passing through it is slowed to a speed below the speed of sound.