# Why do airplanes have to fly at 30,000+ft. **URL:** \ **Category:** Factual Questions\ **Created:** [November 1, 2008, 8:33am UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559 "2008-11-01T08:33:32Z")\ **Posts on this page:** 11\ **Page:** 3
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [November 2, 2008, 6:01am UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/41 "2008-11-02T06:01:09Z")
> [@aerodave](#): > > The stratosphere starts at around 30,000 ft, and airliners routinely fly in the stratosphere. And the tropopause (the border you mention) isn’t the lower boundary of a sharp increase in temperature. It’s the transition point from a gradual positive lapse rate (decrease in temperature with increasing altitude) to a gradual negative lapse rate (increasing temperature). You are of course correct. [Minimal research](http://en.wikipedia.org/wiki/International_Standard_Atmosphere) would have allowed me to make a more accurate post. I guess it is time to stop trusting my memory. The ICAO atmosphere model does use constant T (zero lapse rate) from 36,000’ to 65,000’, which was the dim memory I based my post on. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 2, 2008, 7:15am UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/42 "2008-11-02T07:15:52Z")
> [@Staggerlee](#): > > My ignorant disbelief was more of an invitation for proper scientists to explain why a satellite can’t orbit an earth-sized (gasless) planet at 50ft, more than anything. There is no reason you could not orbit something 50’ off the ground assuming no atmosphere. Basic orbital mechanics. Just need to get the object into freefall. Imagine throwing a ball. If you trace its trajectory you see it makes an arc. You throw it and it curves into the ground some distance ahead. Throw it harder and the arc extends before the ball hits the ground. Now throw it _really_ hard such that the arc matches the curvature of the sphere you are standing on. The ball will be continuously falling to the ground but the ground will be contonuously curving away from the ball. That is freefall and your ball is now in orbit some few feet above the ground. As long as there is nothing to slow it down or things in its way to hit you are good to go. > [@](#): > > As for your point about relatively gasless bodies; is the mass of a solid object the \*only \* determinant of it’s atmosphere’s mass? Probly not.. Oddly there seems to be little rhyme or reason to which planetary bodies will have an atmosphere. The only planets (not counting the gas giants) that have an atmosphere are Venus, Earth and Mars. Some Jovian moons have an atmosphere such as Europa which is smaller than our Moon. Lots of other moons (e.g. our moon) have none. ---
**Author:** ![Sublight](https://avatars.discourse-cdn.com/v4/letter/s/439d5e/32.png) [@Sublight](https://boards.straightdope.com/u/Sublight)\ **Post date:** [November 2, 2008, 5:04pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/43 "2008-11-02T17:04:39Z")
> [@aerodave](#): > > So you may ask yourself, “Why not go even higher?” Surely if higher is better, than flying at 70,000 ft must be better than 35,000 ft. While thinner air is a benefit for flying more efficiently, there is such a thing as too thin. Wings need a certain amount of air mass to generate lift. Engines need enough air to push out the back and create thrust. And that’s why all air-breathing aircraft have a “service ceiling”. It’s the height above which one of those two things (the wings or the engines) just won’t work any more. Even getting right up to the service ceiling becomes harder and harder the closer you get to it. So most aircraft are going to stay well below it. For a given airplane, there’s a particular altitude with an optimum balance of air that’s thin enough to cause low drag, but not too thin to fly in. And somewhere near there is where you’ll find your typical passenger jet. And for a good example of what can happen when you go too high, there’s the Pinnacle Airlines crash in 2004, in which two pilots in an empty plane tried to push the envelope just for fun and ended up getting killed. [Wiki](http://en.wikipedia.org/wiki/Pinnacle_Airlines_Flight_3701) [Darwin Awards](http://darwinawards.com/darwin/darwin2004-11.html) ---
**Author:** ![Canadjun](https://avatars.discourse-cdn.com/v4/letter/c/76d3ee/32.png) [@Canadjun](https://boards.straightdope.com/u/Canadjun)\ **Post date:** [November 2, 2008, 5:25pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/44 "2008-11-02T17:25:28Z")
[Quote=Darwin Awards article] Little did they know that their fun was doomed when they set the auto-pilot for the impressive climb. They had specified the _rate_ of climb rather than the _speed_ of the climb. The higher the plane soared, the slower it flew. The plane was in danger of stalling when it reached 41,000 feet, as the autopilot vainly tried to maintain altitude by pointing the nose up. [/quote] What’s the difference between rate of climb and speed of climb? ---
**Author:** ![diggleblop](https://avatars.discourse-cdn.com/v4/letter/d/ac91a4/32.png) [@diggleblop](https://boards.straightdope.com/u/diggleblop)\ **Post date:** [November 2, 2008, 5:31pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/45 "2008-11-02T17:31:22Z")
Maybe it was mentioned, I didn’t read every word in the thread. But I also heard that it wasn’t as bumpy that high, either. Thereby, reducing lots of air pockets and turbulence. Though I’ve been about 35,000 feet on a few planes and it was still bumpy as shit up there. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [November 2, 2008, 6:37pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/46 "2008-11-02T18:37:28Z")
> [@Canadjun](#): > > What’s the difference between rate of climb and speed of climb? Rate of climb is in feet-per-minute. If you’re rate is 500 ft/min, and you’re at 10,000 feet, one minute later you’ll be at 10,500 feet. If the autopilot is trying to maintain a set rate of climb, it’ll have to pitch the nose up or increase power. According to the wiki article, that climb is outside the manufacturer’s specs above 38,000 feet. (The plane was capable of flying at 41,000 feet, but since that’s near its limit, there’s not a lot of thrust or lift left over for climbing.) The autopilot tried to maintain a climb rate to the point that the plane lost airspeed and the engines were damaged. Speed of climb probably refers to airspeed. If the pilots set the throttles and tell the autopilot to maintain 300 kts, it’ll pitch the nose up to maintain that speed. As it gets closer to it’s maximum altitude, there’ll be less excess power for climbing, the autopilot will lower the nose, and the rate of climb will decrease; all while maintaining a speed of 300 kts. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [November 2, 2008, 9:01pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/47 "2008-11-02T21:01:39Z")
> [@Rick](#): > > :dubious: > I understand that. I was agreeing with you, and providing a real world cite. > 😕 :smack: DOH! thanks and my apologies. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [November 2, 2008, 9:11pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/48 "2008-11-02T21:11:00Z")
> [@diggleblop](#): > > Maybe it was mentioned, I didn’t read every word in the thread. But I also heard that it wasn’t as bumpy that high, either. Thereby, reducing lots of air pockets and turbulence. Though I’ve been about 35,000 feet on a few planes and it was still bumpy as shit up there. 35,000 ft gets you above a lot of the crap but not all. T-storms can outclimb about any commercial aircraft. The higher the tops, the worse the storm. And you can hit clear air turbulence and boundary layers associated with the jet stream. Generally the haze level contains a lot of the low-level turbulence. Climb above that on a clear day and it smoothes out considerably. Figure 5000 to 8000 feet in the Mid-West. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [November 3, 2008, 8:16am UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/49 "2008-11-03T08:16:53Z")
> [@Robot\_Arm](#): > > Speed of climb probably refers to airspeed. If the pilots set the throttles and tell the autopilot to maintain 300 kts, it’ll pitch the nose up to maintain that speed. As it gets closer to it’s maximum altitude, there’ll be less excess power for climbing, the autopilot will lower the nose, and the rate of climb will decrease; all while maintaining a speed of 300 kts. Yes that’s right. An autopilot may have several modes in which it can climb or descend. The terminology may differ between manufacturers but it’d be something like PITCH, VS (rate of climb), and IAS (indicated airspeed.) Unlike true airspeed, the indicated airspeed at which the aircraft stalls stays roughly constant as you climb, so if you climb in IAS mode, the aircraft noses up to maintain the desired airspeed, as you get to the performance limits of the aircraft it will still maintain the airspeed, lowering the nose as necessary. By using IAS mode you are therefore protected against stalling. If you use VS mode to climb the autopilot may quite happily stall the aircraft trying to maintain the rate of climb you’ve set. For this reason our company prohibits the use of VS mode for climbing. The pilots of that flight compounded their problem later on when they delayed informing ATC of the true nature of their problem (a double engine failure) while they tried to get them started. ---
**Author:** ![iamthewalrus\_3](https://avatars.discourse-cdn.com/v4/letter/i/258eb7/32.png) [@iamthewalrus\_3](https://boards.straightdope.com/u/iamthewalrus_3)\ **Post date:** [November 3, 2008, 6:28pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/50 "2008-11-03T18:28:04Z")
> [@Sam\_Stone](#): > > Look at [this overhead picture of a Boeing 747-400](http://www.aerospace-technology.com/projects/747/7474.html). Look at how long and thin the wings are. Now compare it to a picture of something meant to go low and slow, like my old [Grumman AA1B](http://8.5.0.95/photo/Untitled/American-AA-1B-Trainer/1373533&tbl=photo_info&photo_nr=6&prev_id=1377106&next_id=1373531) Could you expand on this? Looking at the pictures, it looks like both planes have wings that are proportional to their size. The wingspan of each is about as long as the body of the plane. The thickness of the wings is about the same, proportionally, too. Where the wing attaches to the body, the wing is about 1/4 or 1/5 as high as the plane body. The big difference I see is that the Boeing’s wings are swept back and taper at the ends, while the AA1B’s wings are perpendicular to the body and end abruptly. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [November 3, 2008, 6:33pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559/51 "2008-11-03T18:33:51Z")
> [@Rick](#): > > If you look at some airline schedules from say LA to London, the trip east is about 30-45 minutes shorter than the trip west. There’s a similar difference from LAX (Los Angeles) to SYD (Sydney). Those headwinds or tailwinds can really add up on a 13 or 14 hour flight. [Previous page](https://boards.straightdope.com/t/why-do-airplanes-have-to-fly-at-30-000-ft/470559.md?page=2)