A story in the Times today about the missing Malaysian airliner says that the emergency air supply for pilots lasts for an hour, but for passengers it lasts “only a few minutes.” This is kind of alarming. How long exactly does the emergency air supply last for passengers, and why is it so short? Is it just to lull us into a false sense of security?
WAG: It would be prohibitive in cost and weight to carry enough bottled air for hundreds of people. I think the idea is those few minutes allows the crew to take the aircraft to a much lower altitude, where air is plentiful.
+1. If there is a loss of cabin pressure the pilots will descend. It’s not that there’s NO oxygen… just that at higher altitudes the air pressure is lower and people don’t get enough air to stay conscious. Descending to an altitude with higher air pressure will rapidly solve that problem.
In a previous discussion someone mentioned the passenger air is generated by a chemical process and lasts about 15 minutes. That should be plenty of time for a quick descent to under 20,000 feet. Pilots need to use oxygen over 12,500 feet I think, but you would be breathing and alive at 20,000 feet. The crew have a big scuba-tank-like air bottle that’s good for much longer.
One speculation is that the Malaysia plan had a similar incident to one on an Australian airliner - the crew air bottle blew its top (the valve assembly) and shot out the side of the plane. Since the air bottle is in the same compartment as the electronics, flooding the area with pure oxygen may have started an electrical fire in the Malaysian plane (It did not in the Australian plane). The bottle then causes a hole in the hull, and depressurization. The crew dons their masks and deals with the fire, turning off all the electronics breakers and turning back to the nearest airport. By the time they realize they have no oxygen, they pass out and the autopilot carries on with the new heading, before they can start a descent.
This. As an aside, the chemical reaction in passenger O[sub]2[/sub] generators liberates a lot of heat in the process of producing breathable oxygen. That’s fine when properly installed, but in other circumstances, problematic; O[sub]2[/sub] generators that were improperly packaged for transport are what brought down ValuJet flight 592.
The pilots use oxygen from bottled sources, but passenger oxygen is created through chemical oxygen generators.
Hopefully, not too rapidly.
I worked on the US Air Force version of the DC-9-30 - the C-9A Nightingale. It had a 10 liter LOX (Liquid Oxygen) tank for the aircrew and a 25 liter tank for the passengers. That’s a lot of oxygen! However main reason for the large amount of LOX was they were hospital aircraft which often transported burn patients who needed massive amounts of oxygen.
For the curious, here’s a discussion on rapid descents that I started a few years ago. For commercial aircraft, depending on the type, it looks like the best they can do is 6000-12,000 vertical feet per minute (with most being closer to 6000). So if you’re cruising at 35K feet when you lose cabin pressure, you can get to a breathable altitude in about four minutes.
Sometimes for the rest of their lives.
Imagine this happening at altitude.
It’s more important to keep the pilots conscious than the passengers.
The emergency procedure for sudden decompression at a high altitude is a rapid descent to an altitude where the air is thick enough to support life. This should only take a few minutes, but it’s most critical to have emergency air when you’re at the highest point, that is, at the beginning of the episode. At some point during the descent you’ll pass through air thick enough to sustain life, even if not consciousness so if the emergency air has run out by then you’ll still be OK, you just won’t be awake, and then a little lower down you’ll be semi-conscious, and so on. For obvious reasons, though, the pilots not only have to be aware but clear-headed. So more oxygen for them, as their mental acuity is most critical for saving everyone aboard.
And, as noted, it comes down to cost/benefit. Emergency oxygen supplies take up room and have a certain weight. There has to be enough to be life-saving, but it also has to fit into the airplane along with everything else that needs to be there, so space is limited.
You can always cheat by cross-controlling it if there’s a need (such as a fire). It’s been done in a 767.
So, for those of us who are not familiar with this kind of thing, what exactly do you mean by “that’s a lot of oxygen”? How long is the 10 liters of liquid oxygen supposed to last, and for how many people? And how about the 25 liters? It seems like 10 liters of liquid oxygen could produce enough gaseous oxygen to let you breathe a long time…?
Also, is pure oxygen at 1/5 of an atmosphere the same as regular air at 1 atmosphere, as far as breathing is concerned? Or is that not how it works?
No you can’t. The B767 was done at low speed, cross controlling in a rapid descent would be stupid.
Does passenger emergency air have a lower oxygen content than average? I can see why the pilots would be provided with 100% of the oxygen they need but not others; passengers and non-piloting crew only need enough oxygen to live, it’s ok if they pass out. The pilots, on the other hand, need to be conscious and alert.
So, will breathing emergency air tend to make someone woozy or such?
It shouldn’t make you woozy. It is pure O2 that is delivered as a trickle flow appropriate for the altitude and mixed with ambient air. For that reason it is of no value in a smoke/fire situation as the mask lets the smoke in. On the other hand the pilot oxygen is not just for depressurization, it is also used as protective breathing for smoke/fire. It needs to have enough to deliver 100% O2 at positive pressure in order to prevent smoke entering the mask and goggles. That is why there is so much for the pilots, because it gets used very rapidly for smoke, this translates to a long supply if used as supplementary oxygen (less than 100%).
1 liter of LOX weights 1.14 kilograms.
Tidal volume for a healthy adult is 0.5L. Normal Repiratory rate is about 15 breaths per minute. So an adult goes through about 7.5 liters of air per minute; with air being 20% oxygen, that means an adult needs to get about 1.5 liters of gaseous oxygen per minute. Gaseous oxygen has a density of about 0.001429 kilograms per liter, so that works out to a requirement 0.00214 kilograms per minute.
So in theory, a liter of LOX should be able to keep an adult in good spirits for 1.14/0.00214 = ) 532 minutes, almost nine hours.
Of course at low ambient pressure, the breathing gas will have to be much more than 20% O2 (that’s the entire point of adding O2 to the mask), and the person will probably continue breathing at ~15 breaths per minute (if only to get rid of CO2), so he’ll be wasting a lot of oxygen. But even if the whole setup is horribly wasteful, it’s hard to imagine getting less than an hour of breathing time from a liter of LOX.
That is exactly how it works. The usual phrase is “partial pressure” - ie that amount of the air pressure contributed by the oxygen. At sea level the partial pressure of oxygen is 0.21 bar (as it makes up 21% of air).
I was checking for some numbers and came across this excellent web page that covers the issue brilliantly.
A few critical points come across. Aircraft are usually pressurised to 8000 feet. This is about 0.7 atm, and the partial pressure of oxygen drops to 0.15 bar, which is actually considered marginal for normal activities. Over a prolonged flight it is a good part of why you feel wrecked at the end. Normal blood oxygen levels are 97% saturation. 93% is considered the bottom end of OK. At 10,000 feet it drops to 90%. At 25,000 feet the saturation curve has a viscous knee in it and saturation drops to 55%, you are in deep trouble and will pass out in a few minutes and die soon after.
However the absolute pressure of air at 30,000 feet is about 0.25 atm. So if you were breathing pure oxygen, you would be receiving more than sea level amounts of oxygen, and will be fine. This is fortunate. It means you will survive on pure O[sub]2[/sub] if the aircraft de-pressurises. Indeed 36,000 feet is where the cut-over point is. 41,000 feet and breathing pure O[sub]2[/sub] is about the same as 8000 feet and air. Not good for an extended period, but doable. Just.
Yes and no. IIRC, a lungful of oxygen does not all get absorbed; not even close. You can rebreathe the same lungful several times and still get some oxygen. Hence your mask is likely fed by a steady trickle, nowhere near lung full volume every breath, to maintain a high O2 partial pressure and slowly flush out any buildup of CO2.
LOX is a big problem. There is a real danger in loading liquid air into a container; splash dangers and also frozen vents. When I was in university, they evacuated the Physics and Chemistry buildings because a large container of liquid nitrogen had gotten its pressure venting clogged with water condensation and pressure was building up. (Some lucky guys got to poke the ice plug with hot steel rods until it vented). I suspect the tank is compressed oxygen, makes more sense.