At what altitude does a plane fly?

The altitude of a passenger aircraft is not a single figure but a profile. After takeoff it climbs at near-maximum thrust to about 1,500 feet above the ground, where flaps are retracted and power is reduced; at 10,000 ft, around 3,000 m, it is no longer limited to 250 knots and accelerates. The climb continues at a constant indicated airspeed until it intercepts a fixed Mach number, and the aircraft reaches its initial cruise level. On a medium-haul flight that level sits around 11,000 m (36,000 ft), within a range of 9,000 to 12,500 m depending on the model, takeoff weight and route length. Descent begins about 200 km from the destination and follows a gentle slope, with the engine near idle.

The altitude is proposed by the airline in the flight plan, calculated by a system that looks for the lowest-cost level given that weight and those winds, and it is authorised by air traffic control, which may assign a different one to separate traffic. En route, pilots request step climbs as the aircraft loses mass from burned fuel. The type's certified ceiling sets the final limit: around 12,000 m for an A320 and 13,100 m for a 787; the Concorde, a special case, cruised between 17,000 and 18,000 m.

The higher it flies, the less dense the air and the lower the drag at a given true airspeed, so the aircraft covers more kilometres per kilo of kerosene. Turbofans prefer cold air, which improves their thermal efficiency. In exchange, lift also drops with density, and to maintain it the aircraft must fly faster relative to the air, approaching the speed at which shock waves appear over the wing. The balance point between these two limits is precisely the 10,000-12,000 m band occupied by commercial jets since the 1960s.

A regional turboprop flies lower, around 6,000-7,500 m; a training light aircraft rarely exceeds 1,500 m; a transport helicopter works at a few hundred metres. A fighter jet can exceed 15,000 m and a stratospheric reconnaissance aircraft can exceed 20,000. So when someone asks at what altitude "the plane" flies, the useful answer is the airliner's: about eleven kilometres, more than two kilometres above the summit of Everest.