When the captain says you’ve reached cruising altitude, you’re usually around 10 to 12 kilometres above the ground. Here’s why planes fly that high, and what decides exactly how high.
Most passenger jets cruise at between about 30,000 and 40,000 feet, or around 9 to 12 kilometres above sea level. That’s higher than Mount Everest, which is about 29,000 feet tall.
Thinner air, less fuel
The main reason is efficiency. Air gets thinner the higher you go. Thinner air creates less drag, the resistance that slows a plane down, so planes can fly faster while burning less fuel.
Jet engines are designed to work most efficiently at those heights. But there’s a limit. If the air is too thin, the wings can’t generate enough lift and the engines can’t get enough oxygen. The best altitude is a balance between those factors.
As a plane burns fuel and gets lighter on a long flight, its best altitude rises, so pilots often climb in steps during the flight.
Above the weather
Most clouds, storms and turbulence occur in the lowest part of the atmosphere. Cruising high lets planes fly over much of that weather, making flights smoother and safer, although some thunderstorms can reach above 40,000 feet and are flown around.
Planes can also take advantage of jet streams, fast-moving bands of wind high in the atmosphere. Flying with a jet stream can cut flight times and fuel, while flying against one can add to them. That’s one reason flights in one direction can be much faster than the return trip.
Safety
Altitude gives pilots more time and options if something goes wrong. A plane that loses engine power at cruising altitude can glide a long way, giving the crew time to deal with the problem and find somewhere to land.
Who decides
Air traffic control assigns altitudes to keep planes safely apart. Planes flying in different directions are usually given different altitudes, and there must be at least 1,000 feet of vertical separation between planes in most busy airspace.
Pilots and airline flight planners choose the best altitude based on the aircraft’s weight, the weather, winds and the length of the flight. Short flights often don’t climb as high, because they’d spend most of the flight climbing and descending.
Pressurised cabins
At cruising altitude, the air outside is far too thin to breathe, and temperatures can fall below minus 50 degrees Celsius.
Planes are pressurised so the air inside is similar to being at about 6,000 to 8,000 feet, the altitude of a mountain town. That’s why ears can pop during climb and descent. If a cabin loses pressure, oxygen masks drop from the ceiling and pilots descend quickly to a lower altitude where the air is breathable.
