Why Don't Airplanes Fly Higher?

The average cruising altitude for commercial flights is about 10,000 meters, but depending on the aircraft's characteristics and other conditions, it ranges from 9,500 to 13,000 meters. The benefits of flying in that “band” are significant, especially in avoiding adverse weather and saving fuel. So, why not fly higher? What stops a conventional airplane from continuing to climb? At first, nothing… until it simply can't fly anymore.

Have you played Kerbal Space Program? No? Well, you should. On one hand, it's one of the best space simulators ever made, and on the other, you learn something new at every step. A good example is airplanes. First, you discover that you're terrible at designing a plane. Then, that you're terrible at flying that plane. Finally, that you can't take that plane into space… unless you turn it into a mutant hybrid with a rocket. In the style of the “Space Shuttle,” only much more DIY.

Why? I suppose that question extends to all airplanes, with special emphasis on those models that carry passengers. It's very rare to find a commercial airplane exceeding the 13,000-meter threshold. An exception to the rule was the Concorde, which had to reach 18,000 meters to shatter the sound barrier. However, there are very good reasons why an airplane cannot or rather, should not fly higher.

Efficiency: The Cruising Sweet Spot

First, efficiency. The idea is for the plane to fly faster and burn less fuel at the same time, and what you get from cruising altitude is the best balance between both factors.

Safety: The Higher You Go, the Riskier It Gets

Then comes safety. At an average of 10,000 meters, the plane manages to avoid storms, severe winds, and the rest of the air traffic, from birds to helicopters. That also translates into a wide margin of maneuver for pilots in an emergency.

Following that line, if a plane flies higher, it would take longer to return to a safe altitude, and in the case of a decompression emergency, a second can be the difference between a plane in one piece, and another in several. Add to that communication problems. The connection between commercial aircraft at high altitude and ground services would be more unstable, more complex, and more expensive.

Aerodynamics and the Thin Air Ceiling

Finally, we reach aerodynamics. A plane flying too high will end up in a zone where the air is very thin, and insufficient to maintain lift. The pressure difference across the wings is what creates lift, but at excessive altitude that difference disappears. Another serious problem is the lack of oxygen for the engines. Beyond their spectacular efficiency, they are still combustion creatures, and oxygen is not optional.

Nor should we forget weight. The higher you go, the more fuel it takes to get there, and the heavier the plane is at takeoff. An exception to this rule was NASA's Helios prototype, which reached a not-insignificant 29,000 meters in 2001, and I say “was” because two years later it disintegrated in mid-flight. In short, planes don't fly higher because it isn't safe, it isn't efficient, and the air is scarce. Want to go higher? You need a rocket.