If the cabin depressurises, a little oxygen mask pops down from the top. You have a few seconds to put this on your face before you're unconscious¹, and a few minutes after that for somebody else to put it on your face before you're dead. Try doing that while inside a tent.
(I'm don't know whether this conclusion is statistically valid, considering how few non-Boeing's have been depressurising v.s. the proportion of the population with COVID-19, but it feels intuitively right, and that's how these decisions have been made lately.)
¹: read "not usefully conscious"; you might still be awake, but your brain won't be doing much thinking or hand-moving.
If the cabin depressurizes, the pilots immediately fly the plane down at a 45 degree angle to quickly move to an altitude with more hospitable air pressure. People who don't put on masks (and most people don't) regain consciousness after a couple minutes.
The real safety hazard is anything that makes it harder to evacuate the plane, such as bulky clothing or eye-wear that make it harder to see in the dark or through smoke.
The point where hypoxia can set in varies from person to person, esp. when accounting for people with respiratory disorders. 10,000 feet is often considered a rough line where the altitude is no longer safe for everyone (that is, particularly susceptible people may begin to show symptoms), but the FAA progressively requires pressurization or supplemental oxygen from 12,500 to 15,000 feet. Altitude sickness (different from hypoxia but sometimes also quite severe) will occur below 10k in some people if there has been a rapid change in altitude, but fortunately it is much more survivable than hypoxia.
There are some regions, like the Tibetan Plateau where the average elevation is actually higher than 10k feet, meaning an nosedive could technically have you hit a mountain.
That would be why most airlines don't fly over the Tibetan Plateau :)
If you really want nightmares about flying in that region, I recommend looking at the approach to Paro airport. 7.5k elevation, surrounded by 18k peaks, horrifying turbulences.
IIRC till a few years ago only 4 pilots had authorization to land there. I was more scared while landing in Boston for the first time. It seemed to me that the aircraft was landing on the water, was not expecting that.
There are a few "av-geek" videos that cover operations of airlines in the vicinity of the Tibetan plateau, and they talk about how the pilots have to be qualified and trained for the fact that the plane is never at an altitude where people breathe easily. Even railroad operations require things like the availability of supplemental oxygen for passengers and crew.
This is also an issue for planes landing at the South Pole, where the plateau can have a pressure altitude of up to 12500ft. If you arrive in a Hercules (as opposed to a DC3/Basler which is unpressurised) they have to vent the cabin just before landing.
They’re going to be descending whether atc gives them permission or not. Keep in mind, Southwest has TCAS and already knows of nearby aircraft to a near certainty. The only real goals are get to breathable air and don’t hit anything. Passengers have minutes before they run out of air.
It is true that you would max-rate descend to get to breathable altitudes, but FYI that will not be flying at a 45 degree angle lol.
If you did try to fly an airliner down at a 45 degree angle, you would either have to do it with a falling-leaf stall after slowing to about 150kts (I doubt this is possible in an airliner?) or you would quickly break the speed of sound and the aircraft would disintegrate as the wings and stabilizers were ripped off. A max rate descent feels steep, but it's not that steep lol.
They have spoilers which do allow a fairly steep angle, combined with other techniques can get you down very quickly, though not quite 45 dergees IIRC.
Some transport aircraft like the C-17 can deploy thrust reverses in flight to achieve those attitudes. I'm told the people riding in back don't enjoy it though.
I think the biggest issue is that the relationship between pitch attitude (angle) and rate of ascent/descent in aircraft is tenuous at best. During landing, for example, it is typical to have a healthy descent rate with the nose slightly up from level. I don't think I've ever seen any procedure with a fixed pitch angle, it pretty much always depends on the gross weight anyway.
Emergency descent maneuvers in the C-172 and I believe many other small aircraft actually involve a 45 degree roll angle, and whatever pitch angle achieves a desired speed (e.g. 120 kias in the C-172). This produces a tight downward spiral, the spiral helps because it moves lift off the vertical axis which lets you descend more quickly at a fixed speed - otherwise you'll speed up until the wings fall off. I suspect the 45 degrees above may have come from confusion over which axis it was around. I'm not sure what the exact justification is for the choice of 45 degrees but 60 degrees roll angle is usually considered sort of the upper limit (beyond that is technically aerobatics) so 45 is pretty commonly considered the "steep" extreme.
It looks like the procedure is different for the big boys. A quick google shows that the emergency descent procedure for the 747 is thrust closed, speedbrakes at flight position, gear down, make 320 kias. Pitch angle that produces would depend on weight but it's not going to be 45. No spiraling involved, but then 747 seems to (unsurprisingly) have a lower max bank angle (really max load factor) than smaller planes.
The point is you wouldn't even really pay attention to the pitch attitude, you'd pay attention to airspeed. The answer to "what should the pitch be" is pretty much always "whatever results in the airspeed you want."
> During landing, for example, it is typical to have a healthy descent rate with the nose slightly up from level.
And of course the landing itself (for airliners at least, that is) will be with the main gear first and only then does the nose wheel come down as well.
Once you've internalised that, you can't help but notice that one style of pictograms typically used to designate the "Arrivals" section of an airport (e.g. https://www.alamy.com/airport-signs-departure-and-arrivals-a...) alarmingly looks rather like a crash landing than a regular landing :-)
AFAIK, passenger oxygen masks on airplanes use a chemical oxygen generator, which only works for a few minutes before it's exhausted (the pilots use a different system with oxygen cylinders, and IIRC the rest of the crew has portable oxygen bottles).
When you hold your breath, oxygenated air remains in your lungs. This allows you to maintain consciousness for potentially a minute or two.
In a decompression event, the air is sucked out of your lungs. The air that remains has a low O2 partial pressure, and so oxygen can leak from your bloodstream into your lungs. As such the body's normal mechanisms for storing oxygen simply don't work.
It's probably worse than that. When you blow out all the air from your lungs, what's left is still at atmospheric pressure. In a decompression event the pressure drops very quickly unless you plug your nose or something. A sibling comment has more concrete numbers https://en.wikipedia.org/wiki/Time_of_useful_consciousness
When learning to scuba dive, they caution you not to ever hold your breath - not when at a stable depth, not when descending, and most certainly not when ascending - because the pressure differential can cause serious injuries. (This goes to the point of "if you run out of air at the bottom and need to make an emergency ascent, hum for the entire trip up to make sure you're breathing out slowly")
Now, you have larger pressure differentials when scuba diving - every 10m of depth is an atmosphere of pressure - but the half atmosphere that you get from 10k vs 40k feet is probably still enough to cause some serious issues. (10k feet is ~0.69 atm, while 40k is ~0.19)
This doesn't really tell you much since the compulsion to breathe is caused by rising CO2 levels, not lack of oxygen. If you breathe out, the remaining air quickly saturates.
I believe it's somewhat correct, based on some random diving training.
There is a thing where you deliberately hold your breath, which starts with taking a big lungful of air, which presumably is at normal pressure and contains 21-ish percent oxygen.
Even untrained individuals can hold their breath for a while like this.
There's another thing, which is what happens if the air you're trying to breathe either is at lower pressure, or has lower oxygen proportions. Either way, each breath has fewer oxygen molecules.
If you haven't prepared for this and try to continue breathing, you can quickly become short of breath, and then cognitive abilities become impaired, hampering your attempts to rectify the situation.
I wonder if the problem is that if cabin partially depressurizes by the time the masks drop, you may no longer be able to take a deep breath and hold it while taking your time putting on a mask.
I acknowledge that I do not have training in dealing with cabin depressurization. As luck would have it, my sister has worked for Air Canada for more than 30 years, I will ask her about it.
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I see some other comments about the oxygen actually coming out of your blood into your lungs at low pressure, and the air coming out of your lungs because the cabin is lower pressure than your lungs.
Those make sense, and had I trained for summiting 8,000m peaks instead of tech diving, I would have known that!
Suppose you're 2000 feet down, breathing hydrox or hydreliox. It's mostly hydrogen, maybe some helium, and less than 1% oxygen. You suddenly ascend to the surface. Instead of changing gas mixture, you try to hold the gas in your lungs and maintain the pressure.
You won't hold it in. As the pressure drops in your lungs, the dissolved gases in your blood will escape into your lungs. That drops the oxygen content in your blood.
Unconscious is either hyperbole or gross exaggeration.
But you only have a few dozen seconds before you become an idiot incapable of helping yourself. You can survive and be conscious for minutes in this state, but its "game over", you're not going to be helping yourself put on the oxygen mask once the critical period is up.
Given the tradeoffs of the situation (ie: your neighbor will be fine for minutes, although helpless), it makes far more sense to ensure the oxygen safety of yourself before helping other people.
> Unconscious is either hyperbole or gross exaggeration.
No it isn't.
> you can survive and be conscious for minutes in this state
No you can't.
You are not going to remain conscious for more than a few seconds at 35000 or 40000 feet. You could potentially be between 15000 to 20000 feet but there's a reason oxygen supplementation is required above 10k feet in unpressurized cabins.
> It is the period of time from the interruption of the oxygen supply or exposure to an oxygen-poor environment to the time when useful function is lost, and the individual is no longer capable of taking proper corrective and protective action.
> It is not the time to total unconsciousness
35,000 feet is 30-seconds to 1-minute of total useful consciousness, half that for rapid decompression situations.
I stand by my estimate for "Dozens of seconds before you're an idiot". Most people will remain conscious after this time is up: they just will have become such idiots that its unlikely they will ever be able to fix their Hypoxia condition. (Slurred speech, unable to dexterously control your limbs, etc. etc. You're conscious, but no longer capable of putting on a mask even if it is dangling in front of you)
There's also this (https://www.youtube.com/watch?v=_IqWal_EmBg) famous recording of a Learjet 25 crew suffering hypoxia at 32,000 feet. I think it demonstrates quite well that you're _not_ going to lost consciousness within a few seconds at that altitude, although the danger is not to be understated.
Unconscious might be overstating it, but there is a concept called "Time of Useful Consciousness" - which is basically the time frame where you are still able to function.
At 40,000 feet, time of useful consciousness is 15-20 seconds.
Basically the partial pressure of oxygen is low enough at altitude that the oxygen is driven out of your blood instead of in.
There are some breathing techniques that can offset that somewhat, but only if you are prepared and trained, which is unlikely in a sudden decompression event.
If I remember correctly, commercial aircraft are pressurized at about 8,000 ft. 25,000 ft is 5.45 psi, resulting in about 5.5 psi overpressure if you are depressurized. You might have a hard time, and you can injure your lungs doing that.
The time until you pass out will depend on your oxygenation situation before the loss of pressure, general health and fitness, activity level (panicking would be a bad idea), and a bunch of other things.
Breathe out completely then stop your lungs and see how long until you get a little dizzy/can't stand it.
Depressurization means oxygen is quickly escaping, but that doesn't mean that people will rationally take a giant breath of air and hold it. I imagine most people continue breathing normally, or more rapidly due to increased stress. Not realizing that they are sucking in less and less oxygen, and begin to lose consciousness.
Or your lungs might explode / rupture / tear. (I know that happens to divers who hold their breath, but 1 atm → ~0 atm might not be enough of a pressure difference to make that happen.)
Your ability to hold your breath near sea-level does not invalidate the well-studied time-of-useful-consciousness effects from a rapid decompression in aviation.
This personal anecdote doesn't really add to this conversation in a constructive way.
But, for sake of argument, it assumes a few things:
1) That your lungs are healthy enough to hold your breath for >= 1 minute (not all are)
2) You actually have the opportunity to breath in a lungful of oxygen-rich air (depending on the situation/timing, this might not be the case)
3) You have your wits about you while a very stressful situation unfolds around you (let's argue that you, personally, are cool and collected under pressure - this; however, is not universally true for all other passengers)
You can't hold your breath in a low pressure environment. Also the low partial pressure of oxygen causes it to diffuse out of your blood instead of in. Even people who can hold their breath for a couple minutes under normal circumstances will have severe hypoxia impairment in less than a minute above 30K feet. https://en.wikipedia.org/wiki/Time_of_useful_consciousness
I always assumed it was worse than this: when you breath out your lungs still contain a slightly lowered oxygen concentration at atmospheric pressure, so the partial pressure of O2 is still quite high. In a vacuum this drops to zero, meaning that your lungs become an oxygen sink.
So not only are you unable to get more oxygen, your lungs are actively sucking the remaining oxygen out of your blood.
Try it. You can go about 30seconds in a calm situation where you know what to expect before the panic overwhelms you.
With a sudden event, you’d have about 15 seconds because there’s a lot to panic about.
Edit: just tried. 25s panic started, 30s my chest was trying to inhale, 36s I had to let myself breathe. This was at my desk while calm and in control.
On a plane I’d probably panic sooner and might be able to go a full minute before actually passing out.
Edit 2: I did use to freedive as a kid and am in very good shape (45 resting heart rate) so maybe that helps
Your lungs operate based on partial pressure of each gas. So, effectively they operate in reverse in a low pressure environment.
Also, at 1ATM you have 14.7 lb/square inch in your lungs, at 30,000 feet that drops to 4.3 psi making trying to hold your breath dangerous on it’s own.
The problem is that you probably won't notice the lack of oxygen. And after 1 minute you would not have enough mental capacity to put the mask on. There are some funny hypoxia videos: https://news.ycombinator.com/item?id=24495939
You must not. One of the most important rules of SCUBA diving is to never hold your breath. A breath of air at high pressure (deeper) expands quickly at lower pressure (shallower), which could cause you some problems (a ruptured lung).
If the cabin depressurises, a little oxygen mask pops down from the top. You have a few seconds to put this on your face before you're unconscious¹, and a few minutes after that for somebody else to put it on your face before you're dead. Try doing that while inside a tent.
(I'm don't know whether this conclusion is statistically valid, considering how few non-Boeing's have been depressurising v.s. the proportion of the population with COVID-19, but it feels intuitively right, and that's how these decisions have been made lately.)
¹: read "not usefully conscious"; you might still be awake, but your brain won't be doing much thinking or hand-moving.