Why Do Planes Fly at 35,000 Feet?
Open any flight on a live map and look at the altitude. Most airliners in cruise sit between 30,000 and 41,000 feet, which is roughly 9 to 12.5 km above the ground. Why so high?
Thinner air means less drag
The higher you go, the thinner the air. Thin air pushes back less against the plane, so it can fly fast while using less fuel. At cruise height an airliner covers far more distance per kilogram of fuel than it would close to the ground. Jet engines are designed for this: they are most efficient in that thin, cold air.
But not too high
Air that is too thin is a problem too. Wings need air to create lift and engines need oxygen to burn fuel. Every aircraft has a maximum certified altitude, often around 41,000 to 43,000 feet for airliners. Pilots choose a height that gives the best balance for the plane's weight that day.
That's why a heavy plane at the start of a long flight often cruises lower, then steps up to higher levels as it burns fuel and gets lighter. You can see these step climbs on the flown path of long-haul flights on fly24.
Above most of the weather
Most clouds, rain and bumpy air sit lower down. Cruising above them gives a smoother ride. Big thunderstorm clouds can reach even higher, and pilots steer around those using weather radar.
Out of the way of other traffic
Small planes, helicopters and most birds stay far below. Up high, airliners share the sky with fewer aircraft, and controllers separate them by at least 1,000 feet vertically on most routes. That's why altitudes are usually round numbers: 33,000, 35,000, 37,000 feet.
Why short flights stay lower
On a short flight, like Lahore to Islamabad, there isn't time to climb all the way up before it's time to come down again. These flights often cruise lower, sometimes around 20,000 to 30,000 feet. Turboprops like the ATR 72 typically fly lower still, around 17,000 to 25,000 feet.
See it for yourself
Pick any plane on the live map and watch its altitude: climbing after take-off, flat in cruise, then descending on approach. The colour of the flown path changes with height, so you can see the whole flight profile at a glance.