At what altitude do airliners fly?

A commercial jet normally cruises between flight levels 300 and 410, that is, between 9,100 and 12,500 m (30,000-41,000 ft). Short flights, under an hour, barely reach 8,000-9,000 m because climbing further is not worthwhile; long-haul flights start cruise around 10,000 m, with the aircraft heavy with fuel, and step climb up to 12,000 m as they get lighter. The certified ceiling sets the limit: around 12,000-12,500 m (39,000-41,000 ft) on the Airbus A320 or Boeing 737, and 13,100 m (43,000 ft) on the A350 and 787. Business jets reach 15,500 m (51,000 ft).

At 11,000 m air density is less than a third of sea level's. With less drag the aircraft needs less thrust to maintain speed, and turbofans, besides, perform better in cold air. The result is fuel burn per kilometre far lower than at 3,000 m. Flying high also places the aircraft above most weather: storm clouds can exceed that altitude, but they are flown around, and the convective turbulence of the lower layers is left below. Climbing even higher is not worthwhile, because thin air reduces lift and narrows the margin between stall speed and maximum speed.

The level is negotiated with air traffic control: by convention, eastbound flights use odd flight levels and westbound flights even ones, vertically separated by 300 m (1,000 ft). The pilot requests the optimum level calculated by the onboard computer from weight, wind and temperature, and the controller grants it if the airspace is free. That is why two aircraft of the same type on the same route can cross at different altitudes.