I’m flying on Thursday. Got me wondering.
Why is the 32,000- 37,000 feet altitude the normal cruising altitude for commercial jet airliners? Is there something about the air ( or lack of thereof ) at that altitude that makes it easier to fly?
Cartooniverse
Air is less dense at altitude. So there is less resistance and better fuel economy. I’m sure they have to take into account lift and oxygen levels and such, but that’s the short answer.
The simple answer is that jet engines get more efficient the higher you go. More distance covered for less fuel burned.
The 30,000-ft altitude range is popular because the engines are not the only thing you have to worry about - that wing out there also has to be efficient. Wings are designed as compromises between high-speed/high-altitude efficiency and slow-speed/low altitude functionality. As wing design has improved, so has the high-alititude cruising capability of airliners.
There are other factors involved as well: the gross weight of the airplane affects how high you can climb (heavier = lower). When jets first appeared they were hailed as allowing you to fly “above the weather!!” While not technically true (thunderstorms can get as high as 50,000+ feet), the amount of weather you must avoid at FL 370 vs FL 250 is significantly less.
This simplified answer was brought to you buy someone who flies the things.
We have people here who design them, so I’d expect a more detailed answer from them!
And the maximim altitude is set because at high altitudes people can’t survive even breathing oxygen because there is insufficient pressure for the lungs to exchange gasses through the membranes. In case of accidental, rapid decompression of an airliner at, say 50000 ft., the drop down oxygen masks would be of no avail and all would die unless the plane were rapidly brought to lower altitudes.
I’m not sure but I think the maximum altitude that doesn’t require pressurization is around 40,000 ft. And that is for people in good physical condition which probably doesn’t describe the average passenger very well.
That’s pretty much got it. The higher you go, the less fuel you burn, but it’s mainly because of reduced drag on the airplane. Airliners don’t go higher than that because they can’t - you also have to have enough air to create enough lift to match your weight. Modern jets have similar levels of technology and similar limitations. There are, however, some business jets that can get up to the 50,000 foot range, and the Concorde could get to around 60,000 feet.
Cruising altitudes vary over that range because of air traffic control procedures - planes are spaced vertically as well as horizontally to prevent collisions. **pilot141 ** will correct me if I have this backward, but planes traveling generally east are assigned even-number altitudes (32,000 or 34,000 feet), while westbound planes get odd-numbered ones.
Interestingly, these altitudes aren’t the actual heights above sea level; they’re “pressure altitudes”. They would be the actual heights if the local sea-level pressure were the standard-average 29.92 inches of mercury, but, rather than constantly make adjustments during the flight to account for local weather, pilots will just assume average sea-level pressure everywhere and let their actual altitudes vary accordingly. The pilots or autopilots just have to hold a particular outside air pressure to hold a constant “pressure altitude.” Since they’re all doing the same thing up there in always-controlled “high altitude airspace”, it all works out.
“Odd fellows fly east.”
Warning: The OP has already been answered, but for further clarification:
East = Odd, West = Even for altitude assignments. So FL 190 is an eastbound level, while FL 200 is a westbound level. The Flight Levels alternate in 1,000-ft increments up to and including FL 290, where they switch to 2,000-ft increments. This means that FL 290 is east, and the next available FL (310) is westbound. From FL 290 through FL 450 there are no even-numbered Flight Levels that get assigned. So: FL 290, FL 330, FL 370, etc = eastbound. FL 310, FL 350, FL 390, etc = westbound.
Of course, all of this was designed when jets and radar were fairly new. RVSM (Reduced Vertical Separation Minima) is coming to the US, which will reduce the high-altitude separation from 2,000 ft to 1,000 ft. This will allow things such as FL 300, FL 320, etc. IIRC it will only apply up to FL 410, where the 2,000 ft rule will remain in place.
RVSM has been in use for years over the North Atlantic Tracks (a non-radar environment) and in Europe, so using it over the contiguous US (with excellent radar coverage) will not reduce safety at all.
More than you all wanted to know, probably…
ElvisL1ves,
I had answered your points, but on preview I see Pilot141 has beaten me to the the [submit], so I’ll scratch most of that part …
In addition to the 2000’ separation, as mitigated by RVSM, that Pilot141 so clearly explained, there’s another interesting exception …
In Chile and Argentina, and maybe some other countries I don’t know about, eastbound/westbound rule is rotated 90 degrees, where one set of altitudes is reserved for southbound courses from 091 to 270 degrees, and the other strata is reserved for northbound courses from 271 through 090. In those tall narrow countries where most traffic is going nearly north/south, this provides better overall separation. The east/west rule was originally designed for the USA, then propagated to the rest of the world.
David Simmons,
The rules on altitudes without pressurization and/or oxygen masks vary for civilian, airline and military use.
For US airliners the rules are that between 10,000 and 12,000 feet, the pilots must use oxygen masks after 1/2 hour above 10,000. Above 12,000 feet the pilots must be on oxygen continously. Above 15,000, everybody on board must be on oxygen continuously.
Those rules apply to the effective altitude in the cabin for pressurized airplanes, which is the same as the actual altitude for unpressurized airplanes.
So as a practical matter, 15,000 is as high as you go with passengers, because 1) they’re not going to be happy about wearing oxygen masks, and B) the passenger oxygen system only supplies a few minutes’ worth, enough for an emergency descent. It won’t work long enough to cruise anywhere.
At my carrier, the company rules prohibit ops above 10,000’ unpressurized with passengers. The only reason we’d ever operate unpressurized with passengers would be after a malfunction enroute, or if we were stuck at an out-base with no maintenance and had to get the airplane and pax to somplace where the pressurization could be repaired.
I’ve flown an unpressurized airliner exactly once on a ferry flight with no passengers and it was a pain in the ears even for pros, despite our being real ginger on climbs and descents. You wouldn’t want to ride through that in the back.
I’m not aware of a specific civilian rule on absolute max altitude for unpressurized flight. But I do know that in the US Air Force, the max authorized actual altitude for an unpressurized airrcraft is 25,000 and 50,000 is the max altitude for a pressurized aircraft without the crew wearing pressurized quasi-space suits.
So I’m pretty sure your 40,000 number is incorrect operationally, although it might be a valid number for aircraft design standards. I just drive 'em, I don’t build 'em.
Odd=east, thanks for the correction, guys, and thanks for the ATC info.
I can fill something else in - the big-iron pilots here have described altitudes as “FL320” and the like. That’s “flight level”, which is pressure altitude divided by 100. “Flight level 320” is a pressure altitude of 32,000 feet, which could be an actual altitude of, oh, 29,000 or 35,000 depending on local weather down below.
Yes, 40000 is the wrong altitude. The max altitude can be flown on just oxygen with no pressure suit, or pressure cabin, is 50000 ft. It’s a physical limit and not just an arbitrary number because without pressure you just can get enough oxygen into the blood to stay conscious above that. I checked Britannica after posting because I became unsure and was going to post a correction but you beat me to it.
Of course it isn’t a hard limit, some people, like me, couldn’t make it at 20000 ft. and others could probably go to somewhat more than 50000. I suspect all military pilots have the physical conditioning to do it, but they always wear pressure suits anyway if the plane is capable of altitudes like that.
Our group flew a series of missions at 13-14000 ft. The B-26 in the ETO had no oxygen system and it was a bitch. Any exertion, even sliding the seat back and forth, at all left us puffing like we were stacking straw in August. There was a lot of bitching about it. So the Squadron Commander, who incidently was Lucius D. Clay, Jr., called all the aircrews up to the orderly room and said something to the effect that - You guys wouldn’t have a bit of trouble at these altitudes if you were in good physical condition. So, starting tomorrow, all crews not on the mission loading list will do 1 hr. of calisthenics every afternoon and I will lead it.
Somehow the bitching stopped. Although the flights didn’t get any easier.
Make that “can’t get enough oxygen.”
Long distance jets adjust altitude, depending on direction, to either enter or avoid the jet stream. This contributes significantly to fuel efficiency.
Sorry, it’s not just a matter of physical conditioning - a good chunk of a person’s ability to tolerate this sort of low pressure (or not) is innate and irrelevant to their conditioning. That’s why even extremely fit mountain climbers can run into trouble when going to a peak like Everest, which puts people into that zone where even with pure oxygen exerting oneself to, say, put on one’s boots in the morning can be a trial.
There’s an interesting bit in the book Voyager, about the non-refueled circumnavigation of the planet, where they were testing the two pilots’ tolerance for just such a low-pressure environment (their airplane being unpressurized). Big, tough, in-shape, jogs-miles-a-day former fighter pilot Dick Rutan had markedly less tolerance for unpressurized high-altitudes than petite, ordinary, never-military, in-good-shape-but-not-great Jeana Yeager. Rutan was the one with all the “conditioning” and the actual experience with high altitude flight. Yeager was strictly civilian, with no prior experience at altitudes requiring oxygen. Go figger.
I gather, from what I’ve read, that no one completely understands why one person tolerates extreme low pressure and continues to function while another person doesn’t. Certainly, general state of health is a factor, but it’s not the only one.
Interesting thread. I’ve always wondered about this myself.
Are you saying that a typical airliner would be mechanically capable of flying efficiently at 50,000, but it isn’t done for safety reasons?
I do seem to remember flying somewhere once, and the captain saying that our cruising altitude would be 42,000 ft.
A typical airliner is NOT capable of flying at 50,000 ft. A few of the newer business jets can get that high, as can some military aircraft. As I stated earlier, it’s mostly wing design that limits the max altitude (although engines have something to do with it as well). Most airliners top out in the low-40s.
In addition to the wings and engines, another consideration is pressurization - making the cabin feel “normal” to the passengers. To cruise at 50,000 feet and have a cabin altitude below 10,000 feet requires a “tight” pressurization system - meaning a high psid (differential pressure) and a fuselage that doesn’t “leak”. I’m guessing, but I think the psid required would be something on the order of 10 psi or more. This can be done, of course, but it adds complexity and/or weight to the structure. Once again, a compromise is made - and so far no one has decided that the effort required to gain those extra 8,000 feet of altitude was worth it (at least on the large airliner side). This may change in the future.
I don’t design airliners, so I can’t say for sure, but there are other limitations. There is an altitude above which the wings won’t generate enough lift to counteract the plane’s weight (the “service ceiling”). Boeing does not provide information about service ceiling, unfortunately, but for airliners, I think it’s around 40,000 feet. The problem with altitudes this high is that by the time the plane has burned enough fuel to reach it, it’s time to start descending. I could see an altitude like that only used on an extremely long-haul route.
A Cessna Citation X, OTOH, has a much higher ceiling of about 50,000 feet. These planes are lighter, have a better power-to-weight ratio, and are optimized for higher cruising altitudes.
No, I’m not saying that. The question has already been answered but I’ll just add this. I don’t know what the absolute max altitude airliners are capable of but the designers know all about this physiological pressure limitation and certainly won’t add performance than won’t be used. I think as a practical matter most commercial airline flights are at not much over 40000 ft. Actual aircraft ceiling will be somewhat higher than that because at maximum operating altitude there still is need to make turns without losing altitude.
Of course, physical conditioning isn’t the whole story. And while we’re on this semi sidetrack, I saw a TV show about a gadget that mountain climbers can carry with them for use in extreme cases of altitude sickness. It was a sort of airtight sleeping bag with a foot pump. The victim is put inside, the bag is sealed and air is pumped in to maintain pressure. There is some means of exchanging the air to avoid CO[sub]2[/sub] buildup and they can then keep the victim alive until a helicopter can come. Some people can get altitude sickness as low as 8-10000 ft.
This is great !! I love you guys and gals. 
I was away from The Dope for a few days, hence my silence after posting the O.P.
Entirely fascinating stuff, well worth the price of admission. 
By all means, carry on !!
Cartooniverse, who flew a small poorly pressurized puddle-jumper from Indianapolis to Chicago/Midway today, then swapped for a nice tasty jet from C/M to Newark.