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Arnold Winkelried
07-20-2001, 05:00 PM
When I fly from the USA to visit the family in Europe, I notice that the plane usually travels at a height of between 9,000 to 10,000 metres. Is this because:

international airspace law requires that height;
this is the height that gives the best aerodynamicity/fuel efficiency;
it would be more fuel-efficient to fly higher, but more dangerous;
it would be less fuel-efficient to fly higher, but less dangerous;
engineers just haven't built an airplane to fly higher than that;
other reasons?

Another question: Do longer flights (e.g. from Toronto to Sydney) go higher than that? I realize that if I'm flying from Los Angeles (California, USA) to San Francisco (California, USA) it's probably not worth climbing all the way to 10,000 metres, but on the other hand the difference in time between a 14-hour flight and a 20-hour flight probably doesn't mean that the plane is going to spend more time "climbing".

I looked up some technical specifications on jets:

Boeing 727 (http://www.simviation.com/rinfo727.htm)
Cruising Altitude: 30,000 to 40,000 feet (9,144 to 12,192 m)
Boeing 747 (http://www.simviation.com/rinfo747.htm)
Service Ceiling: 41,000 ft (approx. 12,500 m)

I assume that Airbus planes have approximately the same ceiling.

Race Bannon
07-20-2001, 05:27 PM
B (is this a quiz?)

Simple explanation:
The cruise altitude for a turbine powered airliner is a trade-off between aerodynamic drag and propulsive efficiency. Generally, you'd like to fly as high as possible to reduce drag - thinner air means less drag (to a point). But the engines need air to burn, so you can't go too high. Modern turbine engines are capable of sucking up a lot of air, so airplanes can fly much higher than you could survive in the open air (cabins are pressurized with air from the engines).

Distance of the trip does figure in, when the trip is so short that it doesn't make sense to climb so high, smaller turbo-props are more economical. Your trans-america example is, however, certainly long enough to justify a climb to 10000 meters.

Duck Duck Goose
07-20-2001, 05:33 PM
My impression was that the reason to fly so high is that that's how high you need to go to get away from atmospheric turbulence. Not to mention all those tacky commuter planes, getting in everybody's way. :D

http://www.boeing.com/companyoffices/gallery/images/commercial/K60646.html
A higher maximum cruising altitude means passengers can enjoy a smoother ride and airlines benefit by being able to fly above bad weather, congested routes and less capable airplanes.

Some of these Boeings have cruising altitudes of between 15,000 and 40,000 feet.

http://www.aviation-history.com/boeing/707.html

Broomstick
07-20-2001, 06:30 PM
Why do jets fly at the altitude they do?

Well, there's a bunch of factors here. Kellymccauley is right that the higher you fly the less drag you have, resulting in a faster groundspeed for less fuel, with a trade-off involved with less oxygen available to burn in the engines. Passenger jets are capable of significantly higher speeds than they normally fly, but only if you're willing to burn significantly more fuel.

There is also a set of rules involving direction and height, so, for instance, all the east bound traffic will be flying at one altitude and all the westbound traffic at another altitude, so the traffic is in overlapping layers.

Because there is a considerable difference between the pressure inside and the pressure outside the airplane this imposes stress on the aircraft. The higher you go the bigger the difference and the more stress. This stress is also increased by the number of times an aircraft goes through the up-and-down cycle. This is another reason for short-haul flights to stay relatively lower. The stress on the structure from going up and down 12 times on a 1000 mile route is greater than the stress of going up really high just once on one 12,000 mile trip. Since a passenger jet can last decades if properly cared for, and these stresses add up, the length of route and maintenance costs can also be a factor in choice of altitude.

Yes, flying higher gets you above turbulence - usually. But most turbulence is under 10,000 feet. Actually, it's usually under 2500 feet in my experience but then I usually fly only on really nice days. But turbulence can exist quite high up as well - either due to very high mountains, jet stream interactions, or extremely large storms.

Which brings up the point about "flying over" storms. Thunderclouds can extend up to 50,000 feet, possibly higher. With a service ceiling of "only" 40,000 feet, a Boeing 7x7 will not be able to fly over such a storm. And going through the top of a storm can be just as hazardous as going through the bottom.

With a service ceiling of 70,000 feet (maybe higher - I don't have the exact figure) the Concorde can jump over all weather - but it drinks fuel like a fish. Trade offs again.

I dunno about the "avoiding congested routes part" - some days they're all congested. And there are plenty of business jets and small charters that can reach 40,000 feet, and fly at equal or faster speeds than the airlines.

Which is just a long winded way to say that there's not just one reason for choosing (or being assigned) a particular altitude.

Johnny L.A.
07-20-2001, 07:40 PM
Aircraft have a "service ceiling". At some point there isn't enough air to allow the aircraft to fly higher with the maximum available engine power. Some aircraft like the TR-1 (U-2) can fly very high indeed. But at maximum altitude the maximum speed and stall speed (the speed at which the wing does not develop enough lift to sustain flight -- the smooth flow of air becomes seperated from the wing) is only about five knots apart. Most aircraft have a wider margin.

As has already been stated, the main reson for choosing a high altitude is economy -- getting the most speed for the lowest fuel burn. Often aircraft will fly at an altitude to take advantage of favourable winds or to avoid unfavourable ones.

FWIW: "Odd fellows fly east." That means that east-bound traffic flies at odd-thousand feet (or for VFR, odd-thousands plus 500 feet) and west fly at even-thousands (or even+500).

flyboy
07-20-2001, 08:29 PM
Can't add much here, except:
In my plane, (EP-3), it was my understanding that the turboprop engine actually functioned more efficiently the higher the altitude. (I always assumed it applied to any turbine engine, although obviously there'd be a point of decreasing returns.) So, we'd climb as much as we could after takeoff, level off for an hour or so... fuel burns, plane gets lighter, speed increases... then we'd climb another couple grand, and repeat until we reached our max altitude.

I sometimes feel the pilots climb (mid-flight) when I fly commercial; I can only assume they're doing something like the above, avoiding turbulence/weather they think they're going to hit, or looking for more favorable winds.

Btw, airspace has nothing to do with it. The airspace airliners fly in goes up to flight level 600, or about 60,000 ft.

Sam Stone
07-20-2001, 08:44 PM
Well, it's not necessarily true that overall drag decreases as you go higher. Parasitic drag goes down as the air gets thinner, but induced drag may increase because indicated airspeed goes down. In essence, the airplane thinks its going slower for the same groundspeed as it flies higher. If the indicated airspeed drops below best L/D, drag will start to increase. At the highest altitudes, some aircraft are flying only a few knots faster than their stalling speed! This is the infamous 'coffin corner'.

The way to reduce induced drag is to have long, high aspect-ratio wings. But those are harder to to build, take up more space on the ground, weigh more, etc. So there's a tradeoff involved. In general, airplanes that are designed to fly very high will have long, thin wings. Airplanes that fly low will have short, stubby wings.

So when you're designing the aircraft, you'll have to trade off service ceiling with aircraft weight, ease of manoevering on the ground, low-level drag, structural integrity, etc. When all that comes out of the wash, you'll have a fixed optimum cruise altitude at which the airplane is most efficient.

But wait, there's more! Airplanes don't always fly at the same weight, so the optimum altitude will vary. The air is not still, so the optimum altitude will vary (winds generally get stronger as you go higher, so if you have a tail wind you'll usually want to climb high and take advantage of it. Conversely, if have a headwind, you'll often want to fly lower.

But wait, there's more! The certification requirements change if you want to fly really high. Above certain altitudes which I can't remember offhand, both crewmembers must be on oxygen at all times, and the consequences of an unintended decompression become more severe. At some point, a decompression becomes non-survivable even with oxygen masks, because the partial pressure of oxygen around you is so low that your lungs won't function properly. So there's a pretty hard limit at to how high a conventional airliner can fly.

But wait, there's more! Engines are designed to be efficient at certain altitudes. Above or below that, efficiency drops off. In the case of propeller aircraft, long thin props are more efficient at high altitudes than short stubby ones. But long thin props are a bugger to deal with. You need longer landing gear for prop clearance, which adds weight and complexity which may overshadow prop efficiency gains. And that long, thin prop might not have as much efficiency at sea level as a fatter, stubbier one. So climb efficiency decreases.

Every time I right one of these paragraphs, I think of more factors that come into play. In the end, the aircraft designer has to juggle a million different configurations and come up with the one that represents the best collection of tradeoffs for the mission the aircraft is being designed for.

Duck Duck Goose
07-20-2001, 09:23 PM
I sense that Arnold may be feeling sorry he ever asked...

:D

Robot Arm
07-20-2001, 09:58 PM
Originally posted by Johnny L.A.
FWIW: "Odd fellows fly east." That means that east-bound traffic flies at odd-thousand feet (or for VFR, odd-thousands plus 500 feet) and west fly at even-thousands (or even+500).

It's been a while since I had to know this, but I thought that only applied up to 18,000 feet. Above that (and this thread was originally about airliners) different rules and terminolgy come into play.

Johnny L.A.
07-20-2001, 11:04 PM
It's been a while since I had to know this, but I thought that only applied up to 18,000 feet.
Could be. I don't fly jets. Of course everyone knows that helicopter pilots get nosebleeds above 2,000 feet.

Dr. Lao
07-20-2001, 11:28 PM
Originally posted by Sam Stone
But wait, there's more! The certification requirements change if you want to fly really high. Above certain altitudes which I can't remember offhand, both crewmembers must be on oxygen at all times, and the consequences of an unintended decompression become more severe. At some point, a decompression becomes non-survivable even with oxygen masks, because the partial pressure of oxygen around you is so low that your lungs won't function properly. So there's a pretty hard limit at to how high a conventional airliner can fly.Well, pilots could start wearing space suits. According to the FARs there isn't an altitude where both pilots must wear oxygen masks, but at least one must wear one above 41,000. Part 121.333, paragraph c:(c) Use of oxygen masks by flight crewmembers. (1) When operating at flight altitudes above flight level 250, each flight crewmember on flight deck duty must be provided with an oxygen mask so designed that it can be rapidly placed on his face from its ready position, properly secured, sealed, and supplying oxygen upon demand; and so designed that after being placed on the face it does not prevent immediate communication between the flight crewmember and other crewmembers over the airplane intercommunication system. When it is not being used at flight altitudes above flight level 250, the oxygen mask must be kept in condition for ready use and located so as to be within the immediate reach of the flight crewmember while at his duty station.

(2) When operating at flight altitudes above flight level 250, one pilot at the controls of the airplane shall at all times wear and use an oxygen mask secured, sealed, and supplying oxygen, in accordance with the following:

(i) The one pilot need not wear and use an oxygen mask at or below the following flight levels if each flight crewmember on flight deck duty has a quick-donning type of oxygen mask that the certificate holder has shown can be placed on the face from its ready position, properly secured, sealed, and supplying oxygen upon demand, with one hand and within five seconds:

(A) For airplanes having a passenger seat configuration of more than 30 seats, excluding any required crewmember seat, or a payload capacity of more than 7,500 pounds, at or below flight level 410.So, one pilot must wear a O2 mask at all times above 25,000, unless he has a quick donning mask (which all airlines do) then it is above 41,000.

This regulation and the nature of pilots is probably the reason few airlines are designed to fly above 41,000. Pilots hate to wear masks (especially on a long flight where one would fly high enough to require one). If airline manufacturers made a plane that could fly higher, likely pilots would never want to fly it higher than 41,000. So the extra expense of such a plane would be wasted. However, if someone made a plane that benefited greatly from higher altitude flight, attitudes would probably change.

The Ryan
07-21-2001, 12:31 AM
Sam Stone
At some point, a decompression becomes non-survivable even with oxygen masks, because the partial pressure of oxygen around you is so low that your lungs won't function properly. So there's a pretty hard limit at to how high a conventional airliner can fly.
If you're getting your oxygen from the mask, why does the ambient partial pressure of oxygen matter?

Dr. Lao
07-21-2001, 12:43 AM
Originally posted by The Ryan
If you're getting your oxygen from the mask, why does the ambient partial pressure of oxygen matter? Actually, it is not the partial pressure of the oxygen around you that is a problem, it is the total pressure. Eventually the pressure gets so low that it is difficult to force air into the lungs long enough for it to be absorbed. I don't know what altitude this is an issue, but above it you need a pressure suit in order to breath. SR-71 and U-2 pilots wear pressure suits, so their flights must be above that critical altitude.

flyboy
07-21-2001, 06:45 AM
Sorry, Arnold

Originally posted by Robot Arm
It's been a while since I had to know this, but I thought that only applied up to 18,000 feet. Above that (and this thread was originally about airliners) different rules and terminolgy come into play.

From my trusty FAR/AIM: VFR on-top only goes up to FL290. IFR cruising altitudes, though (what airliners use), are addressed in three chunks: up to 18k, at or above 18k-FL290, and at/above FL290 (this last chunk being 4,000 ft increments, thus giving 2,000 ft of separation instead of only 1,000 ft).

Arnold Winkelried
07-21-2001, 11:08 AM
Duck Duck Goose, you may be right. Whatever happened to the simple, one-line explanations? ;)

Thank all of you experts for your answers. (I'm not going to acknowledge everyone individually, but I appreciate all of you taking your time to provide the "straight dope".) From reading all the excellent information presented, it seems that the answer is "with airplanes currently being built, that's the highest altitude at which it's convenient to fly."

Next question would be - are jet aircraft engineers busy trying to design planes that will fly even higher, or do they focus their energies elsewhere? I know that it's been in the news recently that Airbus wants to build a supersize jet that will hold about twice the amount of people of a Boeing 747. Does this mean that going higher than the current ceiling is not being researched? Some people mentioned that going higher might require the pilot to wear an oxygen mask, which pilots hate, but that's only a question of regulations, which could easily be changed.

Dr. Lao
07-21-2001, 12:32 PM
Originally posted by Arnold Winkelried
Next question would be - are jet aircraft engineers busy trying to design planes that will fly even higher, or do they focus their energies elsewhere? I know that it's been in the news recently that Airbus wants to build a supersize jet that will hold about twice the amount of people of a Boeing 747. Does this mean that going higher than the current ceiling is not being researched?In aviation R and D new planes that can fly into new places almost always require new engines. Just about every revolutionary new airplane was built around a revolutionary engine. The turbofan jet engine has been around for close to 20 years and modern airplanes fly about as fast and high as they did 20 years ago.

Of course, engine designers are always working on new engines. However, until a engine is designed to be much more efficient than modern engines when flown at higher altitudes there are not going to be any aircraft designs for any high flying planes.Some people mentioned that going higher might require the pilot to wear an oxygen mask, which pilots hate, but that's only a question of regulations, which could easily be changed.I don't think that is such a good idea. The regulations are there for a reason. At 40,000 ft a person has about five seconds of useful consciousness. In the event of an explosive decompression, that is precious little time to put on an oxygen mask. The higher one goes, the less time there is. Therefore, I think it would be better for pilots to get used to the masks if they fly higher than 40,000, rather than change the regulations.

Johnny L.A.
07-21-2001, 01:15 PM
At 40,000 ft a person has about five seconds of useful consciousness. In the event of an explosive decompression, that is precious little time to put on an oxygen mask. The higher one goes, the less time there is.
When I went through the alititude chamber at Edwards AFB, they said that in case of a rapid decompression above 50,000 feet a person's blood would start to boil without a pressure suit.

Sam Stone
07-21-2001, 02:01 PM
I hate to say this, but I think the guys at Edwards were wrong. Water in an open container might boil at 50,000, but the body acts as a pressure vessel, so your blood won't boil.

The main problem is that there is not enough pressure around you to force the air in your lungs to be absorbed into the body. So you have to surround your entire body with a higher pressure, i.e. a pressure suit.

There are other issues with flying really high. For one, it means you have to climb and descend more. For shorter flights, that might not make a lot of sense. Second, you start to run into Mach limits. At very high altitudes, there may be only a few miles per hour difference between the stalling speed of the aircraft and the speed of sound. For example, the U-2 spy plane when at max cruising altitude had only a 7 mph difference between its stalling speed and the onset of high-speed buffet. Needless to say, it took some precision flying at that altitude (somewhere around 80,000 ft). The U-2 is a good design to look at to see what's needed to fly at high altitudes - LONG WINGS. Long, thin wings. And that's a hard structure to build. The U-2 uses a very small and light fuselage to keep the stress on the wings down, and they still flap like a duck.

So... If you want to go much higher than airliners do now, you're going to have to design a supersonic plane. And that opens up a whole new set of serious problems - inlet design on the engines, skin heating from aerodynamic friction, the list goes on and on. Supersonic aerodynamics are very different than subsonic, which is why the Concorde looks very different than other airliners. And in general, Supersonic designs are not very efficient when flying slowly, so you want to get them up high and supersonic as soon as possible. That's also why the Concorde is such a gas pig.

We're at the point in aircraft design right now where most aircraft are pretty optimal. You don't see a lot of radically different aircraft being built, and for a reason. So improvements are now incremental. New avionics, slightly bigger dimensions, slightly more efficient engines, etc.

I agree with Dr. Lao, btw. Aircraft are usually designed around the engines. It takes radical engine breakthroughs to spark radical new aircraft. If someone can design a scramjet that is efficient enough to be economical, you'll see a flurry of activity in aircraft design, and some radical new planes.

Johnny L.A.
07-21-2001, 02:36 PM
I hate to say this, but I think the guys at Edwards were wrong. Water in an open container might boil at 50,000, but the body acts as a pressure vessel, so your blood won't boil.
What about "the bends"?

Broomstick
07-21-2001, 04:50 PM
Yeah, I think it's the bends this guy was talking about. Saying your blood "boils" could be just another way of saying the nitrogen starts to form bubbles.

Sam Stone
07-21-2001, 05:01 PM
The bends are not the same thing, but I see what you're getting at. A liquid under pressure will absorb more gas into solution than a liquid that's not under pressure. When SCUBA diving, you are breathing compressed air, and it's the high pressure of the air being forced into the lungs that allows the nitrogen gas to go into solution. Remove the pressure, and the nitrogen starts to bubble. Just like popping open a bottle of coke. (In SCUBA, we learned a little ditty when studying gas laws to remember that this is a demonstration of Henry's Law: 'The best way to remember Hank, the gas dissolves like in the Coke you drank!').

I think it's the compressed air you breath that causes the bends. And yeah, the pressure of the water on your body is transferred to your lungs, which then shrink unless pumped back up with compressed air. There's a science toy called a 'Cartesian Diver' which illustrates this. You can make one yourself - find something that has a little air bubble in it and and is compressible, and weight it so that it just barely floats. Now put it in a a plastic pop bottle, and fill it with water and put the cap on. Squeeze the bottle, and the little 'diver' will sink because the pressure causes the air bubble to shrink. Release the bottle, and the diver will come up again.

Would you get the bends if you were suddenly tossed out into space? I don't know. The pressure differential is one atmosphere. That's like going from 33 ft deep in water to the surface quickly. Normally, that wouldn't cause the bends. But if it happens instantly? I dunno. An air embolism is much more likely. Remember kids - if your airplane explosively decompresses at 30,000 feet, exhale. Don't try to hold your breath!

Anyway, I don't think your blood will boil, because the body does act as a pressure vessel. It'll expand a little, and the pressure on the blood might drop a little, but I don't believe it's enough to cause it to boil. Remember 2001? There's a scene in which a guy has to leave an airlock without a spacesuit and float across open space into another one. A lot of scientists I've talked to think that that is possible. I have my doubts, because I think the brain might become oxygen starved really fast and knock you out. But if you were pulled across unconscious, and were outside for no more than a minute or two, I think you might be okay.

Broomstick
07-21-2001, 05:16 PM
Originally posted by Arnold Winkelried
Duck Duck Goose, you may be right. Whatever happened to the simple, one-line explanations? ;)

T'ain't nuthin' that simple in aviation... why just getting home from breakfast last week involved quite a bit more than I expected, but that's another story. Was a bit strange flying back with none of the electric gizmos working, though. The guy I was with was such a stitch! "What if the remaining fuel pump goes out?" Indeed :rolleyes: "Tighten your seatbelt, buddy, 'cause in that case landing is about 5 minutes away."

From reading all the excellent information presented, it seems that the answer is "with airplanes currently being built, that's the highest altitude at which it's convenient to fly."

Well, actually... naw, probably better to just leave it at that :)

Next question would be - are jet aircraft engineers busy trying to design planes that will fly even higher, or do they focus their energies elsewhere?

The problem isn't designing planes to fly higher - we've got those already. The problem is designing them to fly higher economically.

IIRC, planes like the SR-71 and U-2 actually leak on the ground. In order to allow for the expansion due to atmospheric friction and internal pressurization, parts of the planes have significant and visible gaps on the ground. Once up and flying various panels expand and then fit tightly together. The SR-71 had to be built of titanium to withstand friction heating. The space shuttle has to withstand enormous stresses and pressures - it's what, something like \$100,000 a pound to fly something on it, or some other awful figure?

Even a passenger jet experiences a certain amount of actual expansion in the pressurized part of the hull during flight, something like a half an inch to an inch in diameter. Which may not sound like much, but when you consider that you're talking about a metal object your life depends on doing that... I mean, my car wouldn't survive even one pressurization cycle like that.

I know that it's been in the news recently that Airbus wants to build a supersize jet that will hold about twice the amount of people of a Boeing 747.

Oh, great, so we can kill twice as many people per crash? You know, I thought I read something about them having test flown that thing.

Does this mean that going higher than the current ceiling is not being researched?

It's all a matter of cost-effectiveness. If it can be made profitable, they'll do it. Otherwise, they won't.

Some people mentioned that going higher might require the pilot to wear an oxygen mask, which pilots hate, but that's only a question of regulations, which could easily be changed.

Easy to change regulations? Ah-ha-ha-ha... Arnold, you've never dealt with the Effin'-A-A or its international counterparts. Alright, let's assume we can change this regulation... should we?

There's a saying that the regulations are written in blood. A lot of the rules are there because somebody - or several somebodies - died. The real question is, if you de-pressurize at 40,000 feet do you have time to put on a mask? The Lear jet Payne Stewart was on rapidly de-pressurized at considerably less than 40,000 feet and apparently no one managed to get a mask on. There is a case of a military pilot ejecting at high altitude, while he was already wearing a mask, and he spent part of the following fall unconcious (he also sustained some injuries from the depressurization on the rest of his body - he had to empty the nose blood out of his mask several times, he was bleeding from his eye sockets and ears, it was pretty gross - and a great deal of pain due to intestinal expansion). Get much above the ground it's a hostile environment, one that we frail, unprotected humans can't survive without considerable help. While not officially outer space, above 40k it might as well be as far as your ability to breathe is concerned. I mean, maybe we shouldn't make pressure suits mandatory on spaceships, either.

All of which is not something folks on airlines want to think about. Just an inch or two of metal and plastic between you and death. What a charming thought.

But, by God, it's beautiful up there.

Sam Stone
07-21-2001, 05:41 PM
The Concorde is 8 inches longer at cruise speed than it is on the ground, because the airframe heats up so much and expands.

If you look in the cockpit of one sitting on the ground, you can see gaps between all the panels. At cruise, there are no gaps.

Friction heating is one of the biggest limiters, BTW. At cruise speed, parts of the Concorde heat up to over 130 degrees celcius (almost 300 degrees F.), despite the outside air temperature being about -70F. The fuselage and windows on the inside of the pressure vessel will get quite warm to the touch.

The temperature increases very rapidly past those speeds. The Concorde goes about Mach 2. By Mach 3, the temperature can be over 200 degrees Celcius, and by the time you get to SR-71 speeds, the temperature will be over 300 degrees C. Those temperatures will destroy conventional metal, so you need special titanium skins and high-temp ceramics and stuff. Expensive materials, hard to work with, not very malleable, easy to crack, etc.

And BTW, the Concorde is so expensive to fly that even though it's often full, and a ticket on it costs thousands of dollars, the fleet loses money hand over fist. The British and French governments subsidize the Concorde as a point of national pride, because those two governments worked together to build it, and there was too much political pressure to let the program die when it became clear that the thing would never make money.

OxyMoron
07-21-2001, 06:30 PM
Originally posted by Broomstick
I know that it's been in the news recently that Airbus wants to build a supersize jet that will hold about twice the amount of people of a Boeing 747.

Oh, great, so we can kill twice as many people per crash? You know, I thought I read something about them having test flown that thing.

Actually, no; although Airbus has greenlighted the process, a the Airbus website says that a prototype won't be ready until 2004. Also, the BBC (http://news.bbc.co.uk/hi/english/business/newsid_1386000/1386405.stm) reported that the A380 will have a 3-class capacity of 550; the current 747 has a capacity of 412. It's possible, of course, that future versions might have an increased capacity - but the current 747-400 carries about the same number as did the 747-100 33 years ago. The advances have been in efficiency and range.

The monster you may have been thinking of is the Antonov 225, the world's largest aircraft, which made a much-delayed appearance at Le Bourget (http://news.bbc.co.uk/hi/english/world/europe/newsid_1317000/1317779.stm) this year. Six engines, 250-tonne freight capacity, etc.

Johnny L.A.
07-21-2001, 07:30 PM
When I was learning to fly I was also taking SCUBA lessons. My instructor told me that it's okay to fly and then dive, but never dive and then fly. Now when you're diving, the air is carried compressed in tanks (normally about 3,000 psi when full). At depth, your body is compressed with one atmosphere of pressure at 33 feet as Sam Stone says. There is no way your muscles can inhale at even a few feet underwater because the pressure is so great. Try it in a swimming pool with a six-foot hose as a snorkel. The regulator delivers the air so that you can breathe underwater. The pressure differential in your lungs, however, remains the same as if you are at the surface. Note: I'm saying the differential and not the absolute pressure. (Oh, by the way... I'm going on memory and common sense here, but I'm sure any misstatements will be corrected.)

But the "bends" isn't cause by the air in the lungs. The bends is caused by the gasses dissolved in the body. These gasses were accumulated under pressure. If the pressure is suddenly removed it is like popping the top off of a bottle of Coke. The gasses are no longer dissolved in the body and escape. When I went through the alitude chamber at Edwards (BTW: I just read that it's been closed) we had to "prebreathe" pure oxygen for 45 minutes before simulating altitude. This is to purge the body of accumulated nitrogen. We only "went up to" 35,000 feet and they were taking these precautions. It seems to me that the problem would be greater in a sudden decompression at 50,000 feet (that's about 17,000 metres to you, Arnold ;) ).

I think that it is very likely a passenger, who is not breathing 100% O2, would get the bends in a rapid decompression.

What about "boiling blood"? Well, I was going on memory. Sam says the body acts like a pressure vessel so it won't happen. But the body is very flexible. Ever had gas? You can expand quite a bit. And it hurts. If the body acted as a pressure vessel, why not just breathe pure oxygen and go out into space without a pressure suit? (I love hyperbole.) But let's see if we can turn to an authority. Here's an artical called Life Support: Flight Physiology (http://nasaui.ited.uidaho.edu/nasaspark/safety/lifesupport/physiology.html) from the University of Idaho:
... the drastic physiological changes begin at about 50,000 feet. At 63,000 feet, the atmospheric pressure is so low that fluids will boil at body temperature. Imagine your blood boiling!
So I was wrong about 50,000 feet, but it looks like 63,000 will do the trick.

Sam Stone
07-21-2001, 07:51 PM
Well, it says that fluids will boil at that pressure - it doesn't say that blood in your veins is AT that pressure.

I'll see if I can do some digging and come up with the straight dope on this.

Well, I'm back, and it turns out the master himself had something to say about this: http://www.straightdope.com/classics/a3_147.html

And it looks like the truth is somewhere in between. Apparently the blood will boil at some point in the process, but not immediately.

Send questions for Cecil Adams to: cecil@straightdope.com