How high planes fly?

Airliners spend most of a flight between flight level 300 and 410, that is 9,100 to 12,500 m. Two things pull them up there. Air at 11,000 m has about a quarter of sea-level density, so drag for a given true airspeed drops and the aircraft covers more ground per kilogram of fuel. The air is also colder, which suits the engines, and the weather is calmer: most convective cloud tops out below the tropopause. What stops them going higher is the same thin air. The wing needs a minimum indicated airspeed to keep flying, while the Mach limit sets a maximum true airspeed; as altitude increases those two limits converge into what pilots call the coffin corner. Engine thrust also falls with density. The certified ceiling of a Boeing 737 is 12,500 m (41,000 ft); the Airbus A350 reaches 13,100 m (43,100 ft). Heavy aircraft start lower and step-climb as fuel burns off.

Business jets are lighter for their wing and engine size and are certified higher: the Gulfstream G650 and the Bombardier Global 7500 are approved to 15,545 m (51,000 ft), above almost all airline traffic and weather. Regional turboprops such as the ATR 72 cruise at 5,000–7,600 m (17,000–25,000 ft), where the propeller is still efficient and the cabin pressurisation demands are modest. Light piston aircraft, usually unpressurised, stay below 3,000 m (10,000 ft) for routine flying; above 3,800 m (12,500 ft) regulations require the crew to use oxygen after thirty minutes and above 4,300 m (14,000 ft) at all times. Helicopters typically operate below 3,000 m, though the record for a landing stands at the summit of Everest, 8,848 m, achieved by a Eurocopter AS350 B3 in 2005.

Military and research aircraft go much higher. The Lockheed U-2 flies reconnaissance above 21,000 m (70,000 ft), where the pilot wears a full pressure suit because body fluids would boil without it. The SR-71 cruised at 26,000 m. The absolute altitude record for a jet-powered aircraft taking off under its own power is 37,650 m, set by a MiG-25 derivative in 1977, and the rocket-powered X-15 reached 107,960 m in 1963, past the boundary of space. At the other end, a passenger cabin is pressurised to the equivalent of about 1,800–2,400 m (6,000–8,000 ft) regardless of how high the aircraft is flying, which is why ears pop on climb and descent but not much more.