Speaking in generalities here, not about any particular incident.
A rapid decompression at serious altitude is a very time-sensitive event. Get it right ASAP or you’ll never get it at all. And the smaller the airplane, the shorter interval for the cabin to depressurize out the hole or whatever. If there is any confusion or other distractors, and you don’t address your oxygen first, you’re screwed. Even if the airplane is trying to roll on its back just then, it’s mask first, then regain control.
Which is certain trained for, but also runs counter to substantially 100% of one’s actual malfunction experience plus most of one’s training. The time available may be just barely enough to get the mask on with little to no fumbling. A moment ago you were sipping your coffee & thinking about women and now you need to be Quick-draw McGraw. Or else.
A general issue with warning systems of any kind is how often they are tested to detect otherwise-silent failures. if you’ve got a slow leak or never pressurized from takeoff, AND the cabin altitude warning system does not sound the alarm passing 10,000 ft as it’s designed to, the crew is now driving into an exponentially decaying situation as they get cumulatively stupider and stupider at an ever-increasing rate. By the time it’s obvious to them that something is wrong, they may well lack the capacity to think of what to do, much less do it.
There was a nasty near-accident in a 727 where they took off unpressurized and got increasingly stupid during the climb. The (much younger) flight engineer just barely figured out what was going on in time; the (much older) pilots were already too loopy to think. There’s more to that particular story but the moral to me is that all the numbers one reads about mental decay times were developed working with fit 25yos, not overweight 60yo smokers.
A general issue with bizjets is how thorough, or not, crews’ preflights are. Checking that your oxygen system really works every time is more challenging discipline-wise when you fly the same jet every workday, not 3 different jets on each workday.
I went through an altitude chamber once. (Got a little card saying I’m OK to fly in a military airplane – which I never had the opportunity to do.) They did a rapid decompression, with the fog and all. We also experienced gaining altitude without oxygen. Nobody passed out, but it was surprisingly hard to do math. We we put on the masks, it was surprising that we could suddenly see colours.
I never understand this. I can hold my breath for a minute with little problem. Even old me. As a 20-something I could do 90-120 seconds (once upon a time I was in good shape and an avid swimmer). And I do not loose mental capacity doing so (or not much). Yet they tell us pilots are disabled in 30 seconds in a cabin decompression.
LSLGuy is much more knowledgable than I, but let me try to answer. (Helicopter pilots get nosebleeds over 500 feet. )
When you hold your breath, you have lungs full of air at the correct pressure. As the oxygen is used, you will need to take another breath. As you gain altitude in an aircraft, the atmospheric pressure decreases. Unlike when you hold your breath and have a ‘supply’ of oxygen, every breath you take as you climb is at a lower pressure so you can pass out very quickly if you’re high enough.
Once I had some dry ice. I put it in a cup of water, and thought it would be neat to breathe in the fog. One breath, and I felt about to pass out. Obviously CO2 is different from breathing in air at a lower pressure; but I think it illustrates how fast things can happen.
Ag lower partial pressures of oxygen your blood can’t absorb it well. As Johnny LA said above, it’s completely different than having a lung full of air that your body slowly uses.
Think about people who get choked out in a hold. Once the blood supply to your brain is cut off, unconsciousness happens in aeconds. Onset can be so rapid that above 35,000 ft the regs require that one pilot always wears a mask, or there must be two quick-donning masks available and two pilots at the controls.
Some business jets fly higher than airliners typically do. The Gulfstream 650 has a service ceiling of 51,000 ft. So has one of thr Falcon jets and the Bombardier 6000.
At 50,000 feet, the time of useful consciousness without oxygen is 6-9 seconds. By the time you realize something is wrong, you’ve probably used up half of it.
Further, in a rapid decompression, if you somehow successfully held your breath as the pressure decreased, your lungs would burst and you’d never have another breath at all.
As the air pressure in the cabin rapidly collapses from 8-10 psi absolute to 1 or 2, something between 80 to 90% of the air volume (and oxygen volume) will leave your lungs at the same rate. You can’t stop it and wouldn’t want to try. So very quickly you won’t have that reserve in your lungs.
Next factor.
When you hold your breath at sea level, there is oxygen (and CO2) exchange happening for the whole duration. As the oxygen in the lungs is depleted into the blood, the flow rate into the blood declines; it’s driven by the partial pressure differential driving simple diffusion plus hemoglobin chemistry on the blood side of the barrier. Like any system approaching equilibrium, the rate of change declines asymptotically to zero as time passes and equilibrium is approached.
The same thing happens the other way as CO2 works out from the blood into the air in the lungs. With each ordinary breath the rate of CO2 transfer starts out high and declines over time as the lung air saturates with CO2 and the partial pressures equalize. The physical / psychological sensation of needing to take a breath is almost completely driven by CO2 build-up which is in turn driven by the reduction in CO2 transfer ability while CO2 production in the body continues unabated.
At high altitudes, the partial pressure of oxygen in the ambient air is less than the partial pressure of oxygen in badly oxygenated blood, much less in fully oxygenated blood. If you breath normally, or even deeply, in a 1-2 PSI atmosphere the net flow of oxygen can be out of the blood into the air. You won’t last long actively pumping your blood oxygen overboard. That’s the thing that really seals the deal at very high altitudes and results in times of useful consciousness in the single digit seconds.
The brain has zero oxygen reserves. It goes black within a couple seconds of oxygen supply being reduced to a small fraction of oxygen demand. “Out like a light” is about right. The rest of the body does a lot better for many more seconds before it too is screwed. Unfortunately, that additional time isn’t real useful to pilots; conscious brains are a prerequisite for flying.
ISTR from the altitude chamber training that you need a pressure suit above 50,000 feet. (Obviously, that’s without being in a pressurised environment.)
The “pressure suit above 50K” is a USAF requirement. Which did apply to aircraft like the U-2 and SR-71 which are pressurized. Just not to a real high differential, and with their tiny cockpit volumes would vent to zero very rapidly if a failure occurred for whatever reason.
The FAA and Europeans never had such requirements, as a little thinking about Concorde operations will demonstrate.
At and below FL410 quick-donning supplemental oxygen must be available to all pilots.
Above FL410 one pilot must actively wear their mask at all times.
Above FL350 if one pilot leaves the cockpit the other must wear the mask while they’re gone. (Which altitude was changed during COVID from the historical FL250 to 350 which had the effect of greatly reducing shared use of masks.)
FL250 is the altitude where it’s generally agreed that even breathing pure oxygen will not keep your blood properly saturated; we need to pressure-feed the oxygen into you. So above FL250 is also where a canula or medical type mask would only be a little helpful and only for an emergency descent, not for anything else.
I’m not an expert on locating the FAA design standards or interpreting them, but this is interesting, especially as applied to the Gulfstream. eCFR :: 14 CFR 25.841 – Pressurized cabins.
“Extremely improbable” is a term of art with some defined number of 9s of unlikelihood. But however many 9s it represents, it still does happen sometimes. Ouch.
In terms,of regs, the only thing I could find was the quick-donning oxygen masks regulation for flying above Fl410. The Concorde used to fly at 60,000 ft and didn’t require pressure suitss for the flight crew.
It could be that you would need a pressure suit for sustained existence at greater than 50,000,ft, but that for the shorter time needed to get a depressurized plane down to a safe altitude a pressure-fed mask with pure oxygen is good enough.
You aren’t going to fly unpressurized over 50,000 ft for very long if you have a pressurization issue. You’re probably going to descend to a safer altitude where mask-only is fine in a minute or two.
@LSLGuy: What would your vertical deacent rate be if you were doing an emergency descent after a depressurization event?
Seconding this. You people are making me sound a lot smarter to my students when I rattle off something I just learned the day before in this thread (and others.)
A 727 could plummet out of the sky at about 6000 feet per minute and cruised in the low 30s. so 3-4 minutes to get to 10,000 where passengers would no longer need oxygen.
A 737 MAX emergency descends at more like 3500 feet per minute and can cruise up to FL410. So from that max altitude more like 8 minutes to the bottom, and 6 minutes from a lowish but still commonplace cruise altitude.
The procedural requirement is to get below the oxygen required altitude within 12 minutes from the onset of the problem. That ensure we get down before the passenger oxygen generation modules are depleted.
That is surprisingly low to me. What limits it? I would think that with spoilers out, engines at idle and transsonic speeds you would go down faster than that.
I didn’t test an emergency descent in my plane, but extrapolating from the glide tests I did do I estimate my RV would descend at 5000+ FPM with idle power and VNE. Hold off to VNO and it is still around 4000. (1850FPM was my highest measured descent, but I was trying to find best glide, not fastest descent).
I can’t speak to the 737 Max, but the A320 will do 7000 fpm at max speed with spoilers out. Passenger jets are pretty good gliders and need a lot of drag to get down quickly.
Airliners are by design very low-drag machines. They glide swell. A no-engine 737 glides a lot better than a 727 did. The 737 nose is old, but the wings and the MAX tail are remarkably modern & sleek / efficient; a bunch more so than the NG.
The MAX especially but the NG as well has real high idle thrust such that even though engines shut off is a pretty good rate of descent, engines at idle is much flatter. The 737 MAX at idle semi-glides far better than a 727 semi-glides with its much lower idle thrust.
Last of all a 727 comes screaming out of the sky at M 0.85 then at lower altitudes 350 knots IAS. The 737 comes down at M 0.82 and 320 knots IAS. Drag being proportional to the square of speed, he 737’s lower max speed translates into ~85% of the speed-driven drag of the faster 727.
Bottom line the 737 glides better than the 727 without engines, better^2 with engines, and can’t really dive nearly as steeply without over-speeding. And just going slower in general means the vertical component of the total velocity vector is less.
Finally, the 737 has a problem where the horizontal tail is in the wake of the speedbrakes which limits how big/effective they can be without damaging the tail. The 727 with a T-tail did not have that issue.
The end result being the speedbrakes on a 737 are smaller and deploy to a much smaller angle in flight than do those on a 727. So those are less effective at keeping the 737 from exceeding its already modest max dive speed. Full speedbrakes vs no speedbrakes is a dramatic difference in a 727. In a 737 it’s a bunch of vibration and unexciting but still noticeable changes in descent rate & angle.
The overall outcome being that an emergency descent in a 737 NG or worse yet a MAX is a decidedly leisurely affair. It’s definitely more aggressive than a routine arrival into JFK. But it doesn’t feel like a last-ditch balls-out Git 'Er Done redneck maneuver. It’s … genteel.
More on this case from Aviation Week’s “intelligence network”, their more or less real-time newsfeed:
So in ~100 feet of water & witnesses reported a steep dive. Which explains the lack of survivors and of surface wreckage despite eyewitnesses presumably in a position to drive their boats promptly to the accident site.
We also learn they were cruising at more or less 1,000 ft. Plenty low enough to encounter a drone or a big bird, but also plenty high enough to ensure that any uncontrolled fall is universally fatal.
This in a nutshell. In a small plane you could cross control the rudder and ailerons and drop it like a rock but that’s a lot of stress on the tail planes.
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