Home Travel Why Do Planes Fly At 35,000 Feet?

Why Do Planes Fly At 35,000 Feet?

by SharonKurheg

“Ladies and gentlemen, we’ve just reached our cruising altitude of 35,000 feet. I’m going to shut off the seatbelt sign and you’re free to walk around the cabin. But please keep your seatbelt fastened while in your seat, in the event of some unexpected turbulence.”

How many times have you heard that announcement or one similar to it? Ever wonder why that cruising altitude tends to be somewhere around 35,000 feet? I did. Here’s what I found out…

If you think back to the science classes you took in high school, or if you’ve gone mountain climbing, or even if you’ve gone to Denver (“the mile-high city”), you may remember that the higher the altitude you are, the thinner the air is. The thinner the air is, the less resistance/drag there is on the plane, so less fuel is needed for the plane to hit the same speed (the weight of the plane also helps to conserve fuel, which is why airlines have figured out all sorts of crazy ways to decrease a plane’s weight), and therefore there’s more energy efficiency.

There’s a bad point, though – there are fewer oxygen molecules at higher altitudes, which means less combustion, which means less power is generated when you’re so high up. Going up to a higher altitude also means a higher climb, which means the plane burns up more fuel to get to its cruising altitude.

Many factors go into exactly what that cruising altitude will be – the flight’s direction and duration and turbulence in the area. But the sweet spot allows for less drag and more fuel efficiency, but still enough oxygen to allow combustion? Between roughly 30,000 and 40,000 feet.


The European Alps, as seen from an easyJet Airbus A319-100

A few more fun factoids

  • Due to COVID, more planes were, at one point, flying closer to 40,000 feet than 35,000. Here’s why.
  • Flying at 35,000+ feet helps you avoid stuff. Most rain and wind top off at about 36,000 feet. So if you’re flying just a bit higher than that, you avoid these weather issues. You also won’t find birds, bugs, drones, light aircraft or helicopters at that height
  • The cardinal direction altitude rule stipulates that planes traveling in different directions will travel at different altitudes. Commercial planes traveling northeast, east, southeast and one degree short of south always fly at odd altitudes (33,000 feet, 37,000 feet, etc.). Those traveling in all other directions fly at even ones (32,000 feet, 38,000 feet, etc.). Planes traveling in the same direction fly 2,000 feet above or below other planes near them, to ensure they don’t collide or even get near each other.
  • Pilots don’t determine altitude. Airline dispatchers do. Airline dispatchers are airline employees who work at airline headquarters; they keep track of where the planes are, where they’re going and what each plane’s weight vs. fuel is. With that information, they calculate what a plane’s ideal altitude should be, and then request it from Air Traffic control.
  • The Concorde was built to fly at higher altitudes. It usually flew at 50,000 to 60,000 feet. The record for altitude is 123,520 feet – that was a Soviet MiG-25M military plane.

And now you know 😉

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LAURAPDX July 2, 2019 - 1:02 pm

Thanks for the post, I wondered about it too.

Vee Jay, JD (@veejangl) July 2, 2019 - 1:57 pm

Also, in the event of engine failure, it gives enough time to recover.

JASON MORGAN July 2, 2019 - 2:45 pm

Wouldn’t planes flying in the same direction fly 2,000 feet below or above others flying in same direction instead of 1,000, since they are either at odd or even?

Marc Farkouh November 28, 2020 - 4:10 pm

From a turbine engine standpoint, thinner air isn’t a combustion issue due to less oxygen but rather a compressor efficiency issue due to less mass flow. From mean sea level to the tropopause, outside air temperature decreases (which improves engine performance). However, air density also decreases (which is detrimental to engine performance). The effects of these two factors essentially cancel each other out until we reach the boundary seperating the troposphere and the stratosphere (i.e. the aforementioned tropopause). At this altitude, air temperature ceases to decrease (on average, approx. -60 degF) and is no longer negating the performance deficit caused by ever diminishing air density. This being said, the simple yet effective explanation regarding resistance/drag is spot on. Cheers!

debit November 28, 2020 - 4:15 pm

It’s also a good altitude to be able to see your destination. Why do you think there are two pilots? One has his head out of the window telling the other to make right or left turns.

Any lower and you wouldn’t be able to see the destination. Any higher and the guy might bump his head on one of the stars.

aireye onu November 28, 2020 - 4:58 pm

“Pilots don’t determine altitude. Airline dispatchers do.” Well, not exactly. It’s a team effort. The dispatchers game plan the optimum route and altitude, amount of fuel, passengers, weight & balance, etc. for all given conditions. The pilot is the final authority to accept and/or modify in real time, before and during the flight.

jt@avgeek.com November 28, 2020 - 11:22 pm

The record is 123,523


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