If turbulence only occurs between 20,000 and 39,000 ft, why not fly at 40,000 rather than, say, 36,000?

Question

Got the figures from Wikipedia - I assume they're right!!

Thank you! Good to know! My question followed a 'turbulent' (gulp!) flight home last weekend, and prompted me to look into the effects of turbulence! I'm not a great air passenger at the best of times! I understand all the effects, and the statistics, but I guess I'm just always gonna be one of those 'bad' flyers, unfortunately! Not great when you live on a small island, miles from anywhere!! Seriously, thanks to all concerned for the explanation, and now I know better than to take Wikipedia's word for it!

Answer 1

Turbulence occurs when two air masses collide with each other (Cold front/Warm front, thunderstorms, etc.). As the air is denser at lower altitudes it is more noticeable. As you go up the air density decreases, thus the "turbulence" or "wind shear" is less noticeable.

Air traffic and air routes have nothing to do with weather patterns. When an airplane departs an airport it is assigned a cruising altitude that the pilot is expected to maintain. The reason is that there is other traffic flying around and assigning altitudes depending on the heading is the only way to avoid mid-air collisions.

When a pilot encounters bad weather en route, he needs to request an altitude change from air traffic. Depending on the air traffic on that sector the flight can be assigned a different altitude.

Depending on the heading, type of airplane and many other factors each plane may have different "service ceiling" which is the maximum safe operating altitude for that airplane. Not every airplane can climb to 40,000 ft.

So you see, turbulence can occur at any altitude.

Climbing above the weather is usually the best option. But is not an arbitrary decision from the pilot.

Answer 2

Where on Earth did you get the notion that turbulence only occurs between 20,000 and 39,000 feet?? The only place you're going to find practically zero turbulence is above about 55,000 or 60,000 feet. Some of the very nastiest I've ever run into was down around 7,000 (caused by wind blowing over rough terrain). And there are always a few little 'wigglies' around 15,000 to 20,000.

PCA (Positive Control Airspace) starts at 18,000 feet (FL 180) and continues up to 60,000 over the entire continental United States. Commercial air carriers are always in this range unless they're transitioning to or from an airport. (And even most of the transition areas are positive control airspaces, called TCA's or Traffic Control Areas) This is done so that commercial flights can be controlled and the possibility of collisions minimized. (There are also Military, Prohibited, Restricted, and other types of airspace that are clearly marked on air-traffic maps. Each has their own set of regulations) Also, the cruising altitudes for various courses are prescribed. For flights above 3,000 feet and below 18,000 feet, a course on a magnetic heading of 0 to 179 degrees is required to fly at an odd number of thousands plus 500 (7500, 9500, etc.) and flights on a heading of 180 to 359 are required to fly at an even number of thousands plus 500 (4500, 6500, etc.) so as to maintain a minimum 1000 foot vertical separation. This also continues into PCA up to 29,000 feet. Above that, a 4,000 foot vertical separation is enforced. Why is this important? Because every bit of that airspace is being used over most of the USA. Restricting flights to 40,000 and above would increase the lateral congestion so badly that it would be a disaster just waiting to happen. It's bad enough being 'racked and stacked' for a half hour at LA or New York without having to wonder if there's some moron on your same flight level who can't fly a simple holding pattern without getting several miles off course. Especially at night, in clouds, when you can't even see your own wingtips ☺

Those other flight levels get used a lot.

Doug

Answer 3

The air is much thinner at that altitude and jet engines function differently.

Answer 4

I think that turbulence can happen at any altitude. Where did you get that figure?

Answer 5

They used too but now they just fly through it to save fuel.

Answer 6

the higher they fly the longer it will take them to reach their destination... if that happens they might run out of fuel