How high do planes fly?

An airplane drawing a white contrail line across a blue sky is clearly thousands of feet above the ground, but exactly how high is it flying? It turns out that the exact altitude an airliner is at at any given point in the flight has to do with a variety of factors, such as the weight of the aircraft, the temperature and weather, the pilot’s desires, a protocol, which indicates the direction of the aircraft, and of course what air traffic control requires.

When it comes to aircraft altitudes, here’s what’s going on.

On another plane

“Most of the time planes fly in the mid 30,000s [in terms of feet]says John Cox, a retired airline pilot who now runs a consulting firm called Safety Operating Systems. “They can be as high as 40 to 41,000, but that’s relatively rare.”

Tom Adcock, a retired air traffic controller and now director of safety and technology for the National Air Traffic Controllers Association (NATCA), gives a similar estimate, suggesting that most traffic occurs in the “upper 30s” and some reaches Altitudes of 41,000 or 43,000. A Boeing 757 can fly up to 42,000 and a 767 up to 43,000; 747-400 can go higher. Different aircraft types have different maximum service caps.

[Related: The illuminating tech inside night vision goggles, explained]

Controllers take into account the compass direction an aircraft is flying when giving a pilot an altitude. Although heights like 38,000 and 39,000 are both even numbers, “38” and “39” are even and odd. Westbound flights are given even numbers like 38,000 feet, while eastbound flights are given odd numbers like 39,000. This way planes flying in opposite directions have a built in vertical distance between them. Airplanes flying northeast or southeast would still fly at an odd altitude, while northwest or southwest would have an even altitude. “There are exceptions to the rule,” Cox says, noting that if he were going east at night and wanted to reach 32,000 feet, he would hypothetically still ask for it. “Worse, they can say no.”

In a way, this odd-even system reflects a pattern on the ground far below where interstate freeways were historically given numbers that reflect their direction: freeways running east-west were given even numbers (Route 80, for example) along with it lowest numbers heading south, and the north-south interstates are odd-numbered, with lowest numbers starting in the west (Route 5). Here is more about this street numbering system.

Reach a better height

A number of variables go into determining the exact altitude an aircraft is at at any given time, and Cox says that higher altitudes are generally better. “You want to be as high as possible,” he says. “The jet engines are more efficient at higher altitudes, and there is less drag.” A pilot has an incentive to use less fuel because he would rather complete his flight with more fuel than with less in reserve to assist him in the event of delays in flight giving the air more options, says Cox.

He says the “higher is better” rule of thumb also applies to short jumps. “You would be amazed – especially on short flights [fuel] The most efficient way to do that is to board the planes to high altitudes, cut off the power, and then go back down,” he says. “I can walk up to 31,000 feet and I won’t be there for five minutes.”

The flight management computer gives a pilot information about the aircraft’s optimum altitude, as well as its maximum altitude, taking into account the aircraft’s weight and air temperature. An airplane can climb higher after it burns fuel and becomes lighter.

“Pilots stay as close as possible to the optimal altitude or the maximum altitude,” says Cox. The goal is calm air and light headwinds when flying west – and climbing high can achieve that. Meanwhile, an air traffic controller may ask an aircraft to climb to a higher altitude than the aircraft can handle at the time, forcing the pilot to decline the request.

Commercial airliners aren’t the only planes in the sky. A pilot in a Cessna 172 heading out for a Sunday excursion will be under 10,000 feet (the planes are unpressurized) and maybe fumbling around at 5,000 feet or so. A commercial turboprop would be above those planes, but below the jets — a Bombardier Q400, like the one Alaska Airlines flies, for example, isn’t designed to fly above 25,000 feet. Turboprops like this could be in the “low 20s,” says NATCA’s Adcock.

Leading the way are the jewelry-covered people in private jets, where Learjets and Gulfstreams take the thin air at about 45,000 or higher, even as high as 51,000 feet.

Something special happens even higher. Cox recalls the supersonic Concorde flying at 60,000 feet. “When you get that high, the sky gets real, real, real dark blue and you can see the curvature of the earth,” he says. (Space itself doesn’t technically begin until you’re much, much higher.) “After you’ve been on Concorde and been at 60,000 feet, you can see it pretty clearly there.”

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