At what altitude do airplanes fly?

There isn't one flying altitude, but a band for each class of aircraft. A piston-engine propeller light aircraft, without a pressurised cabin, normally operates between 300 and 3,000 m: above roughly 3,800 m regulations require supplemental oxygen for the crew, and the engine loses power as the air thins out. Regional turboprops, such as the ATR 72 or the Dash 8, fly pressurised between 5,000 and 8,000 m, where the propeller still performs well. Commercial jets cruise between 9,000 and 12,000 m (30,000-40,000 ft), and the most capable business jets reach 15,500 m (51,000 ft), above airline traffic.

A turbofan and a swept wing are designed for thin air. At 11,000 m the air density is less than a third of that at sea level: aerodynamic drag falls in the same proportion, and although available thrust drops too, fuel burn per kilometre flown is much lower than down low. The aircraft is also above most storm clouds and can either ride or avoid the jet stream. The limit is set by pressurisation (the cabin cannot maintain an acceptable equivalent altitude if the aircraft climbs too high), the margin between stall speed and buffet speed, which narrows with altitude, and engine temperature.

A fighter such as the F-16 or F-35 has a service ceiling above 15,000 m, but spends most of its life between 5,000 and 12,000 m, dropping to a few hundred metres when the mission requires hiding from radar. Reconnaissance aircraft such as the U-2 were designed to sustain cruise above 21,000 m, with the pilot in a pressure suit. At the other extreme, crop-spraying and firefighting aircraft work just a few metres above the ground. And above everything, without being aircraft, stratospheric balloons rise to 30 or 40 km.

In most controlled airspace, aircraft fly at flight levels set in hundreds of feet above a standard reference pressure, not above the ground. Below the transition altitude, local pressure is used; above it, everyone sets their altimeters to 1013.25 hPa, so two aircraft at FL350 are actually at the same altitude even if the altimeter doesn't show the exact height above sea level. Eastbound headings use odd levels and westbound ones even levels, with 1,000 feet of vertical separation, which allows dozens of routes to be stacked in the same corridor.