Nobody’s blood is going to boil. Sure, if you had an open wound that was gushing blood that blood might start to boil. But nobody’s going to explode. What will happen is that you’ll pass out, since the partial pressure of oxygen is so low that oxygen will diffuse out of your blood into the air every time you try to breathe.
[pedant]That’s the definition of boiling, no?[/pedant]
No. As long as your blood is inside the vessels, the added pressure from the vessel walls will keep it from turning from liquid to gas at body temperature, even if the pressure outside your body is low enough to make water boil at that temperature.
Yes. What I mean is, when people talk about blood boiling they’re imagining the blood inside their body boiling and you explode. That won’t happen because your skin will prevent it from happening.
To take a familiar example, water boils at 100C. Except in a seal pressure cooker the water can get much hotter than 100C without boiling. So what keeps the water from boiling? The pressure vessel. So even if water/blood boils at a certain pressure or temperature it won’t boil unless it’s exposed to that pressure/temperature. And inside your body your blood won’t boil unless exposed to the outside pressure.
Wow, what a couple of knuckleheads. They certainly paid for their sins. I live in Missouri but I just don’t remember that incident. Wow.
This group are planning to fly to 90,000 feet with trials in July 2016 … Airbus Perlan Project (using a pressurised glider) … http://www.perlanproject.org/ … and are aiming for 100,000 by 2019.
The details of the aircraft on the website indicate the requirements accounted for to allow the pilots to survive.
The Wiki article did include this passage:
"…The crew ended the descent when they had reached 230 kt but neither engine core (N2) ever indicated any rotation during the entire descent. Since they were too high for an APU start, the ram air turbine (known as an “Air Driven Generator” on Bombardier products) was deployed to power the aircraft, and the crew donned oxygen masks as the cabin slowly depressurized due to loss of pressurization air from the engines…
Emphasis added.
Not as realistic as LSLGuys story, but I tried something similar in the F-16 simulator. I worked on a lot of the F-16’s software back in the day, and am fairly skilled at flying it (simulator). FWIW these sims have pretty good aero and reference models (I should know:)), and can mimic the forces and controls up to the max numbers the code can crunch. On a lark, I took the sim to 50,000 and found the same thing. It wallowed as though it were barely able to stay aloft. On a bet, the guys set the altitude to 70,000 and when I said “Go” started dynamic flight. It fell like a rock. As near as I could tell, the controls had no effect.
Whole lotta partial & confused info up-thread by various well-meaning folks.
Cabin pressurization comes from running engines continuously pumping large volumes of air into the cabin. Meanwhile a valve exhausts cabin air overboard. If you pump air in faster than you let it out, the pressure increases and the cabin “descends” to a lower equivalent altitude. And vice versa.
The exhaust valve(s) (“cabin outflow valves” in the argot) can’t close to 100% closed. So if for any reason the incoming airflow is interrupted, you’ve started a one-way process to vent all the pressurization out, with the cabin eventually settling at external ambient pressure and altitude.
How long is “eventually”? Big airliner, maybe 3 minutes, maybe 6. Bizjet probably less.
Side questions: All engines are available to pump cabin air in normal operations. For two-engine airplanes both are used routinely to provide instant backup and plenty of fresh air for comfort. Three and four-engine airplanes may not use all engines for pressurization in cruise. Though the unused engines can be connected to provide pressure if one of the others quits.
At maximum normal altitude for the aircraft you would expect that if one pressurization system quit the remaining one(s) could not maintain complete cabin pressure. You’d be dealing with a slow depressurization. Slow enough that it would not be a crisis. You’d need to descend, but only to a middle cruising altitude. Odds are the passengers would never recognize anything was wrong.
At maximum normal altitude for the aircraft you would expect that if one *engine *quit the remaining one(s) could not maintain cabin pressure nor aircraft altitude. You’d be dealing with a slow depressurization and with the airplane descending whether you liked it or not. As with the pressurization failure, the descent is slow enough that it (in itself) would not be a crisis. Depending on aircraft type & current weight you’d end up a low-middle cruising altitude or at very worst still plenty far up to clear mountains other than the Himalayas. The issue then becomes finding an airport within your reduced range since fuel mileage will be less at this forced lower altitude.
Thoughts on falling from 100K. …
If we solve the life support issues and have an aircraft that has unpowered flight controls so we can steer once we have enough atmosphere to work with, falling from 100K might work or it might not.
There are three risks that I see: Going too fast, tumbling, and deep stall. In order:
- Too fast: If we simplify by assuming there’s zero atmosphere from 100K ft. down to 50K ft. we get that an object dropped from 100K ft will be doing about 1100 knots or 1200mph when it’s fallen the ~10 miles to 50,000 ft. In Mach terms for the actual 50K atmosphere that’s about M1.8. And the corresponding “indicated airspeed”, a measure of how strong the wind blast “feels” corrected to sea level is ~550 knots.
These are speeds that modern fighters can deal with. But no bizjet or airliner is going to survive going that fast even pointed properly into the relative wind. It’ll be shedding parts then going ass over teakettle & breaking up. Even if it was aligned properly with the relative wind earlier on, weird stuff happens to static stability at speeds that high and aircraft not designed for that weird stuff will lose stability & get sideways to the wind. And promptly shatter.
In reality, it isn’t a vacuum from 100K down to 50K. So that won’t be a drag-free freefall. But it’ll be close enough that although the speed passing through 50K ft. will be less, it will still be unsurvivably fast.
2. Tumbling: Whatever boosted us up there can’t have been perfectly aligned with the CG.As a result we’ll have some tumbling motion at the apex at 100K ft. If we imagine instead being dropped from a balloon it’s better. But no aircraft is perfectly balanced or perfectly straight. As it falls it’ll begin to tumble slowly at first. Whatever assymetries exist will probably reinforce this tumbling as it interacts with the slowly thickening atmosphere. You can get some insane rates, hundreds of degrees per second pretty readily if this builds up for any amount of time. The free-fall time from 100K to 50K is about 1 minute.
The flight controls are designed and sized to manage the kinds of forces and rates encountered in normal flight. If you get a large mass rotating madly, even full opposite controls may take many minutes to provide enough force to retard the rates to something reasonable. Meantime you’re accelerating like mad (ref #1 above), and the atmosphere is getting thicker quickly.
The odds are very good you’ll end up sideways to the relative wind at speeds high enough to tear off big pieces long before you can settle the tumbling.
3. Deep stall. If we assume the speeds don’t get insane AND we assume we avoid tumbling, you’ve still got a problem.
You’ll have been boosted upwards in a nose high attitude, or maybe dropped from a balloon in a level attitude. Either way, you’ll be descending very steeply but probably not pointed that way. The so called “angle of attack”, between the longitudinal axis of the wing, and of the airflow will be very large.
Once that angle gets big enough then conventional aircraft flight controls are ineffective at rotating the nose downward to align with the wind. You’re stuck falling with the nose up and no way to get it aligned to regain normal flight.
This is termed “deep stall”. T-tailed aircraft are generally more susceptible to entering this state (more accurately they have a lower threshold to get trapped in it), but most any high speed aircraft can be deep-stalled if you try hard enough. This was the Chuck Yeager NF-104 scenario mentioned by several people above. There have been others involving aircraft of all types and sizes.
The fix here for #3 is to drop out aircraft from a balloon pointed straight down. But old #1 and #2 above are still out there.
Obviously flying machines can be designed to do this maneuver. But ones not designed for it won’t do well at it.
Two things I’m not sure about.
I know that at maximum altitude the engines don’t work at full thrust. Planes routinely get close to that high, but they keep flying for many hours, and they simply don’t have that much fuel. So is the engine thrust limited by air density so that each engine has to run at over 50% of maximum power for that altitude? If not, then one engine would be able to keep the plane flying at that altitude.
Dropping to reach so much speed that it rips the plane apart requires two things: an atmosphere idle enough to let the plane fall without much resistance, and an atmosphere dense enough to cause damage rather than let the plane be controlled. Obviously, there is a gradual transition from one state to the other. I would assume that this is gradual enough to make the control surfaces to their job and slow everything down so the high speed / dense atmosphere combination can be avoided.
I’m trying to understand why there would be a valve at all, if it’s not meant to ever close 100%. Why not just have a 3 cm (or whatever size) hole in the belly of the plane and be done with it?
I imagine the concern is that valves are so prone to failure that a cabin outflow valve might get stuck closed and cause the plane to overpressurize and pop, or the passengers to die of CO2 buildup, or some such. Apparently there is no equivalent concern that the pressurization system(s) would fail, probably because they are redundant and one can descend to lower altitude, etc.
But if that’s the case, again, why have a valve at all if they can’t be trusted?
Maybe I’m misunderstanding the purpose of the valve. I’ve been thinking that it’s meant to hold air in the cabin. Is the cabin outflow valve actually used the other way, to dump air fast in an emergency? Like smoke or fumes filling the cabin? Maybe you always want a 3cm hole in the fuselage, but occasionally you really want a 50cm hole?
Immediately Googled that upon reading the OP, of course I’ve been beaten but here’s the link: https://en.wikipedia.org/wiki/Pinnacle_Airlines_Flight_3701.
Though if they’d given up on trying to restart the engines/trying to hide their screwup, they had plenty of time to recover and glide to a safe landing.
Modern fighter jets (optimized for speed and altitude over over passenger planes optimized for fuel economy, because speed is life and altitude is speed) are rated up to 65k feet/20km in a shirtsleeve environment (with supplemental oxygen). A stripped-down F-15 zoom climbed (get going really fast at airliner altitude and then pull up to trade that speed for height) up to 102k feet/30km+ with no ill effects during Cold War dickwaving contests.
Governments pretty much gave up trying to break the records after that, preferring instead to keep their fighters’ capabilities secret for obvious reasons.
Because the pressure coming out of the engines (pressurization air is usually a takeoff from the compressor section of the engine, in modern turbine-engined aircraft) isn’t constant, nor is the desired pressure inside. A one-size-fits-all nonmoving valve would either overpressurize the cabin on the ground or not make enough pressure up high. You want to dump almost all the bleed air overboard when the engines are running hard on takeoff to stay around atmospheric pressure, but hold in all you can when the engines aren’t much above idle while cruising at 30k feet.
But surely that regulation is done in the input side, before the air enters the cabin. If the flow into the cabin is unregulated, coming from a variable compressor, and the difference is made up for by throttling the cabin outflow valve, that seems like a worse situation than having a regulator between the compressor and the cabin in the first place.
While definitely their best bet to escape a crash, gliding a jetliner to a safe landing is no slam dunk; a ton of things can still go wrong. The pilots of the Gimli glider were very skilled, and very lucky. It helped that one of them was also a glider pilot.
You’d probably find, due to the available air, that the stall speed exceeded the cruise speed. I remember reading that the U2 had a cruise speed something like 5 kts above it’s stall speed at altitude, and it was designed for high altitude.
As you suggest, much of the regulation is done on the input side.
Typically the regulation hits the stops at the extremes, so air conditioner output would be constant between say 1/3rd of max power and 90% of max power. But at 100% of max you get extra flow beyond the control range, whereas at idle you get something less than the minimum control range. This last effect is why most jets before the latest and greatest didn’t really have adequate air conditioning on the ground. Idle engines just didn’t put out enough oomph.
But we also regulate the output side.
The reason for this two-ended control is pretty simple.
We have to inject air in at a rate that keeps enough fresh air flowing for however many hundred people. If the cabin is neither climbing nor descending then we have to let it out at exactly the same rate we let it in. That flow rate is a lot more than a small opening can flow. Airliner outflow valves are holes about a foot square and are usually open about halfway. Widebody aircraft often have more than one that size.
As a separate matter on the ground we want to keep plenty of airflow coming in, but we don’t want any pressure build-up at all. If we did have any pressure build-up on the ground we couldn’t get the doors open. Which might complicate an emergency evacuation. So we need valves big enough to keep the differential pressure at zero at a time when the engines are making their max possible output: take-off at sea level.
The air on the input side coming off the engines into the air conditioners is stupid hot and at stupid high pressures. Like 700C and 250psi. The cabin outflow air is, by definition, at shirtsleeve pressure & temperature. It turns out to be muuch easier to design high precision sensors and valves to operate in the latter environment than in the former. So the broad-brush control is handled at the upstream end, trying to just keep a constant flow through the A/C system, and the fine tuning control happens at the exhaust end.
That’s a little hard for me to parse out what you really meant, but I’ll give it a whack.
An engine is a machine that processes air by the pound. Thrust is more or less proportional to how many pounds of air you can run through the machine per unit time. At high altitude there’s less pounds of air in any volume of air. So thrust diminishes with altitude.
As a separate matter, jet engines are most efficient at nearly full throttle. Ideally you run them as close to max power as engine durability permits. That gets you the most power per pound of fuel.
Putting those two ideas together, the ideal flight profile is to climb at full power until full power has declined to where it’s just barely enough to keep you in the air at cruise speed. Then cruise that way to near the destination. As you burn off fuel and lose weight the same thrust & wing size will let you very slowly climb to ever higher altitudes. To complete the ideal flight profile, when you’re the right distance from the runway you reduce power to idle and glide down to a touchdown also at idle. In a truly ideal scenario you’d turn the engines off and coast down to the runway burning zero fuel all the way.
In the real world we never achieve that exact ideal, but that’s the general outline of the target we’re aiming for.
Meanwhile the gross design goal of an aircraft is to make the wings & engines the right size to be able to lift the desired weight just high enough that the engines run out of oomph about the same altitude where the wing does. And shape the wings for least drag at the speed that corresponds to how fast that top-of-climb thrust will push that much airplane against that much drag.
A corollary of “climb until full power has declined to where it’s just barely enough to keep you in the air at desired cruise speed” is that if you do have an engine failure you now have either 50% too little thrust (2 engine aircraft), or 25% too little thrust (4 engine aircraft) to stay where you are. In either case you’re stuck descending and/or slowing down until once again engine thrust available matches thrust required to sustain level flight at a plausible speed.
For some real world numbers the 767 can maintain about 22,000 feet on one engine at maximum possible takeoff weight. At more common cruise weights it can be as high as 35,000 feet. From a pure aerodynamics & engine POV we’d also choose a slower than normal speed for greater aero efficiency which might give us a higher altitude too. In the real world there are other competing practical and regulatory interests.
LSLguy, thanks for your excellent responses.
Yeah, as far as I can tell that’s pretty much the definition of service ceiling, the altitude at which the wings can no longer fly at the thrust the engine can provide. But still, most aircraft are designed to be fairly easy to recover from a stall, and at 100k feet, you’ve got a decent amount of time to work things out. The U-2 rides on a razor-edge of stalling, true, but that’s because the wings will come off in the process of recovering from the stall. Most airliners and all modern fighter jets are more solidly built.
Or you could go the other way and add more thrust instead of more wing, as in the Sr-71, which replaced and then was replaced by the U-2. I’m no aeronautical engineer, but I’m pretty sure stalling ain’t even a thing when you’ve got 1:1 or better thrust/weight – hell, look at the F-15. The wings obviously aren’t doing much for lift when you’re accelerating straight up on a pillar of fire.
Jetliners, being built for efficiency, have a pretty good glide ratio. The Pinnacle 3701 pilots just spent all their time and effort trying to cover up their mistake instead of actually flying the airplane:
[quote]
The crew glided for several minutes and then tried to restart engines using the APU at 13,000 ft. This was again unsuccessful. They then declared to Air Traffic Control (ATC) that they had a single engine flameout. At this point they had four diversion airports available to them. They lost considerable altitude while continuing unsuccessfully to attempt to restart both the left engine (two times) and the right engine (two times) for over 14 minutes, using the emergency restart procedure.[/qoute]
Also, your sports metaphor is bad. Pinnacle 3701 was a slam-dunk, the Gimli Glider was more an over-the-shoulder buzzer-beater from half court. 