Commercial jets fly at the tropospheric level. True or false?

Hi

Various websites give various answers to this question. I would like to resolve it once and for all.

  1. Do commercial jets fly at the tropospheric level?.
  2. Do pilots prefer to fly at the stratospheric level? I look forward to your feedback.

I think you need to define troposphere. It begins at different altitudes depending on weather and temperature. Commercial jets typically fly at 30,000 to 38,000 feet, depending on weather and weight. That can easily be in the troposphere.

I have no idea where the pilots prefer to fly other than the altitude that gets them to their location on time (the quickest most likely - saves fuel) with the fewest bumps.

Jet driver here …

“Tropospheric level” is not a well-defined term. The troposphere begins at the ground and extends upwards to the bottom of the stratosphere. There is a narrow boundary layer between troposphere and stratosphere named the “tropopause”. See https://en.wikipedia.org/wiki/Tropopause.

As a general matter jet aircraft cruise nearly as high as they can given their current weight, power, & wing size. For most current production airline jet aircraft that’s somewhere in the upper 30,000s of feet to lower 40,000s of feet. Many business jets can climb higher, easily up to the high 40s and top out at 51,000 for essentially regulatory reasons.

The high 30s to low 40s happens to be near the typical tropopause height (AKA “trop height”) for the middle latitudes such as over the US.

The trop height is dynamic in several ways. See http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/tropo.html for more. It varies based on latitude, time of year, time of day, and more or less random daily weather fluctuations.

As explained in that second link there is often a jet stream just below the tropopause level. Which means both an increased chance of turbulence as well as high speed wind. Depending on how the wind direction compares to the direction you want to go there may be good reason to seek out the jet stream or equally to avoid it.

In general the turbulence surrounds the jet stream core but flow is smooth inside the jet core. So if the jet stream is going more or less where you want to go you can often absorb 5 minutes of turbulence getting centered, then cruise for a few hours in smoothness with a big speed boost, then eat a few more minutes of bumps getting back out of the core.

With modern traffic densities it’s getting more difficult to play the natural forces that way. The requirement to get along with all the other traffic overrides the opportunity to “surf the wave” so to speak.
As a rule of thumb the stratosphere tends to have little turbulence and little wind. So again depending on your direction of flight the lack of wind may be good or bad news, while the lack of turbulence is always good news. In lower latitudes, say Central America and below, almost all aircraft lack the capability to get up into the stratosphere no matter how desirable that might be.

Conversely, up in the polar regions the stratosphere is so low that substantially all cruising takes place well within it.

Late edit: After the paragraph ending “… ‘surf the wave’ so to speak” insert the following:

As such aircraft going against the jet stream will tend to fly the geographically optimal route above the tropopause if they can (by 2000+ feet), or remain well below it (4000-8000 feet) if they can’t get above it.

And conversely aircraft going with the jet stream will tend to fly the geographically optimal route at or just below the tropopause if the turbulence permits. If turbulence is too problematic they too will avoid the area vertically by climbing above the tropopause if they can, or remain well below it if they can’t get above it.

Thank you LSL Guy for that very expansive answer. I’d be interested to know why in Central America “almost all aircraft lack the capability to get up into the stratosphere no matter how desirable that might be.”

The issue isn’t Latin America as such; the situation is the same all around the globe at that latitude. Central America is just a convenient example.

The reason, as explained in my links, is simply that at those lower latitudes the bottom of the stratosphere is at an altitude above the capability of all, or nearly all, aircraft.

By the time you get all the way to the equator in summer daytime only aircraft like the U-2 could climb high enough to even get within a mile or two of the base of the stratosphere.

Thanks LSL Guy. Very helpful.

The reason you can’t fly above 51,000 feet is physics. There isn’t enough of a pressure differential for oxygen and carbon dioxide to cross the alveolar-capillary membrane even with a 100% forced oxygen mask. At that point you need a pressure suit.

If the cockpit loses pressurization, the pilot will not remain conscious long enough to get the plane down to a more favorable altitude.

Got a chance to fly on a corporate jet from French Guiana back to Washington DC just before Xmas. They had a display which showed speed, position, altitude, etc. We hit 41,000 feet and remained there for the whole trip back (with a brief stop in Puerto Rico–amazingly, there was not a soul in the customs/immigration area–you could hear a pin drop).

How high do you have to be to see the curvature of the earth? I’ve read you could see it in the Concorde, is it possible in a business jet at 50,000 feet?

I’m not sure how high the U2 was flying at the time, but James Mays reported being able to see the curvature of the Earth during his trip:

https://www.youtube.com/watch?v=jIJRoj2qwsc