By Celine Hacobian
June 27, 2018

Over 42,000 planes take flight in the U.S. every day, with 5,000 in the sky at any given time, according to the Federal Aviation Administration. Coordinating that many arrivals, departures and flights without collisions requires masterful planning, particularly when it comes to how high planes fly.

It turns out there’s a specific altitude range airplanes must stay in while flying because of a few factors like the type of aircraft, the distance to your destination, the type of engine they have, the strength of the winds and the weight of the airplane.

Here’s what to know about how high planes really fly:

How high do planes actually fly?

One reason that planes cruise above the clouds is so they can fly fast. The higher airplanes climb, the thinner the air gets, and the more efficiently they can fly because of less resistance in the atmosphere, according to Ryan Jorgenson, an aviation data analyst.

“With these larger jets, when they take off from airports, their first job is essentially to get out and get as high as possible as quickly as possible,” says Adam Beckman, a lecturer for aviation studies at Ohio State University.

Commercial aircraft typically fly between 31,000 and 38,000 feet — about 5.9 to 7.2 miles — high and usually reach their cruising altitudes in the first 10 minutes of a flight, according to Beckman.

Planes can fly much higher than this altitude, but that can present safety issues. Flying higher means it would take a longer time to return to a safe altitude in case of an emergency, like rapid decompression, Beckman says. It also isn’t the most efficient use of fuel to fly that high in the first place, he says, since planes can fly at a lower altitude with the assistance of wind.

Another reason why planes don’t fly higher is due to the weight of the aircraft. “The more you weigh, the harder it is to get to a certain altitude,” says Jorgenson.

And the weight of the plane changes as the aircraft climbs higher into the sky. “Jet fuel weighs about 6.7 pounds per gallon, so the more that you burn as you’re flying, you would actually end up losing a lot of fuel weight,” Jorgenson says. This, combined with the thinner atmosphere at this height, creates less resistance.

The direction of the wind is also an important factor. “If I’m flying to Europe from Philadelphia, it’s going to take me less time to go over there than go back,” says Bob Cowgill, Managing Director at ACI Aviation Consulting. “Oftentimes you will find that you will fly at higher altitudes when the winds are at your back.”

An airplane’s potential speed also increases the higher it goes. “Ten thousand feet and above, you can go legally to a much higher speed,” says Dr. Thomas Carney, Professor of Aviation and Transportation Technology at Purdue University. This also explains why you feel the plane slowing down during landing.

Why don’t small private planes or helicopters fly at the same height?

So why don’t small private planes fly as high? In most cases, these planes use a piston-powered engine, which operates similarly to the engine in your car and with power that only allows for shorter flights, according to the National Business Aviation Association. This type of engine prevents these smaller planes from reaching the same altitudes as commercial aircraft.

“The plane that the average guy can rent and fly, those tend to stay usually below 15,000 feet and that’s just a limit on what the plane can do,” Beckman says.

Pilots also refrain from flying these types of planes at greater heights because of potential health risks like hypoxia, which is when tissues do not receive enough oxygen, according to the National Institutes of Health. That lack of oxygen can occur at higher altitudes due to a decrease in oxygen pressure, according to the FAA. As the plane ascends, the level of oxygen decreases, which can cause rapid decompression for an aircraft that is not pressurized in the same way as a commercial airplane.

What about helicopters? Choppers are mainly designed to fly short distances and typically fly much lower than airplanes, normally at under 10,000 feet. They are also unable to ascend to the same height an airplane can because instead of wings, helicopters fly by rotating blades.

So how does the height compare to obstacles in the sky? Do birds ever get in the way?

Birds are most likely to obstruct planes at lower altitudes, and can present problems during takeoff and landing. The extreme case of that would be the U.S. Airways aircraft that landed on the Hudson, but that’s not typical, Jorgenson says.

But when planes are at cruising altitude, experts say birds are no longer a threat. So once the seatbelt signs goes off, you can relax and enjoy the flight.

Contact us at editors@time.com.

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