The short answer? They don’t -- at least, not all the time. Even though it does always seem like the captain says, "We’ve reached our cruising altitude of 36,000ft" before switching off the seat belt sign.
So what determines the altitude at which your flight cruises? We wanted to know, and asked three commercial pilots for the lowdown.
There is an ideal altitude
Basically, the higher a plane flies, the thinner the air is. This is both good and bad. Good: there’s less drag on the plane, so less fuel is needed to hit the same speed. Bad: there are also fewer oxygen molecules to combust with fuel, so less power is generated. Higher altitudes also require a longer climb, which in turn means the airplane burns more fuel to reach its cruising altitude. So what the pilot wants is to find the sweet spot where he or she's flying as fast as possible, but burning the least amount of fuel.
Ideal altitudes differ between planes...
Ideal altitudes are based on the aircraft, its weight, and the current atmospheric conditions. For most airliners, that's between 30,000ft and 40,000ft.
...And are determined by a number of factors
Including the flight's direction, turbulence, and flight duration. For example, there's “the cardinal direction altitude rule.” This stipulates that commercial airliners traveling Northeast, East, Southeast, and one degree short of South fly at odd altitudes (31,000ft, 33,000ft, etc.), while those traveling all other directions fly at even thousands (32,000, 34,000ft, etc.).
Clear-air turbulence (CAT) affects altitude, as well. Pilots report any turbulence they encounter, and Air Traffic Control (ATC) uses that information to steer other planes above or below it.
Finally, longer flights benefit from flying at higher altitudes; the thinner air reduces drag, increases engine efficiency, and saves fuel.
Airline dispatchers calculate the ideal altitude for every flight
Airline dispatchers are employed by carriers, and generally work in the company’s HQ. They control the planning and execution of their airline’s flights -- keeping track of where they are, where they are going, their fuel/weight, as well as assigning them specific routes. Most importantly, they calculate a flight’s ideal altitude and request it from Air Traffic Control, who tries to accommodate the request when giving the pilot clearance to take off.
There are highways in the sky
They’re called jet routes, and ATC uses them to separate planes by a certain distance or altitude (generally around 1,000ft vertically) and adjusts them to control traffic flow.
Despite a ton of sky, however, "traffic jams" still happen. For example, when thunderstorms build up there can be hundreds of miles with only one or two suitable crossing points at specific altitudes. It's up to ATC to steer all the traffic through those limited windows.