For example, the Airbus A320 passenger jet has a maximum ceiling of over 39,000 ft (12,000 m), but its certified engine relight envelope only extends to 30,000 ft (9,100 m). Up to that altitude, a windmill restart can be attempted at airspeeds greater than 260 knots (480 km/h; 300 mph); below that speed, a starter-assisted relight is required.[5]