A jet aircraft (or simply jet) is an aircraft (nearly always a fixed-wing aircraft) propelled by jet engines. Jet aircraft generally fly much faster than propeller-powered aircraft and at higher altitudes – as high as 10,000–15,000 metres ( ). At these altitudes, jet engines achieve maximum efficiency over long distances. The engines in propeller-powered aircraft achieve their maximum efficiency at much lower altitudes. Some jet aircraft can move faster than sound.
