Following a flameout, jet engines can normally be restarted in flight, provided the aircraft is flying within the portion of its flight envelope defined as the engine relight envelope. Depending on where in the relight envelope the restart is attempted (that is depending on the aircraft's airspeed and altitude), the procedure may simply rely on the airflow (windmill restart) or require the use of the starter (starter-assisted restart) in order for the compressor to achieve sufficient rotational speed for successful ignition.[4]