I was reading an article in the Chicago Tribune about the recent crash in Greece and was puzzled to come across the following:
Huh? Ok…I suppose they may be of use in keeping you alive but they do not prevent you from passing out? Isn’t there a danger in passing out all by itself and why can’t the airplane manufacturer manage something a little more robust? Money I suppose but would it really be all that much more expensive to provide an oxygen delivery system capable of keeping passengers conscious?
I guess the questions here are:
Is this really true?
Is this really ok (by the industry) or just one of those little tidbits they really don’t want to advertise? (I.e. Feel good about our planes…we have safety systems [sub](not that they are terribly useful)[/sub])
The masks are not intended for long use as the pilot’s priorities in a depressurization will be A: get his own mask on and B: get the plane to a low enough altitude that ambient pressure provides sufficient oxygen.
Their basic use is to enable survival until the aircrew have restored livable conditions. You can’t cover all eventualities. The idea is usually to descend to a lower flight level but if the pilots are incapacitated then no they are of limited use.
I guess I shouldn’t minimize the part about keeping you alive. If passing out is the price you pay for surviving it is a small price indeed.
Nevertheless I thought the oxygen masks aboard an airplane were sufficient to maintain consciousness in the case of a cabin depressurization. Till I read this I would positively freak if I felt myself passing out in such circumstances convinced my oxygen mask was failing me and after passing out I would die.
I still do not see why a system to provide enough air flow to sustain consciousness would be that burdensome on a commercial passenger jet. How much more air/oxygen flow is necessary between passing out but sufficient to sustain life and retain consciousness? IANADoctor but I would be surprised if the difference is all that great. The onboard systems have no trouble providing enough oxygen to the cabin in normal flight with no trouble…why so much harder if the air is redirected into the face masks?
Obviously I am not an expert and in the wisdom of airplane designers I guess this way has its reasons. Just seeking to understand it is all.
It may be that at super high altitudes the air pressure is just too low. If the air pressure is low enough, even if you breathe pure oxygen it will be too thin to sustain life. You can’t fill your lungs with significantly more air pressure than is pressing on the outside of your chest; your lungs will simply rupture; so there is in principle a limit to how much good an oxygen mask will do at high altitudes. You would probably need a pressurized suit to survive. That’s my theory, anyhow.
You can’t do that anyway unless air is forced into your lungs. Oxygen masks don’t work that way, they only provide oxygen at ambient pressure. You can briefly increase pressure in your lungs slightly by forcing your diaphragm up while holding your breath but that cannot fill your lungs, only expel the air in them.
I believe the oxygen provided in the passenger cabin is from tanks, not a scrubber system of any sort. Once the tanks are out, it’s back to ambient atmosphere to provide oxygen for your noodle.
They are, provided you don’t expect them to do so for a long time. As Padeye has noted, the pilots are supposed to promptly begin a rather hasty descent to a lower altitude. (According to a guy I know who flies them, a 747 is capable of a safe descent rate well over 60 knots - 6000’/minute.)
Saying that oxygen masks are “not powerful enough to sustain consciousness for very long” is at least a bit misleading. It isn’t the mask that sustains conciousness - it’s the oxygen flowing through the mask. And “very long” needs to be qualified - supplemental oxygen is not supposed to be needed for more than a few minutes.
AFAIK emergency oxygen in airliners is provided from cyhemical generators which release it from a catalyzed reaction. I’m not familiar with the kind used in airliners but the navy uses a similar system for breathing apparatus used by firefighters.
The altitude at which the partial pressure of pure oxygen equals that of air at sea level is around 38,000’. That’s also about the highest a B-737 would normally cruise.
It’s generally thought that 50,000’ is the limit of survivability without a pressure suit. Note that the altitude record in a sailplane (no pressurization or pressure suit) is just over 49,000’. So this has been survived at least once.
I went through the altitude chamber at Edwards AFB, and this is hte information they gave. They said that around 50,000 feet a pressure suit is required to prevent ‘the bends’.
I don’t know how commercial O[sub]2[/sub] regulators work, but in the chamber we had demand oxygen regulators. That is, the O[sub]2[/sub] was provided at positive pressure somewhat in the way a SCUBA regulator works.
Not usually the case these days. Passenger oxygen is provided by chemical generators and activated by the mask dropping out from its storage area above your head. Once activated these supply O2 until exhausted (I think about 20 mins)
The masks drop out when the cabin altitude (not a/c altitude) reaches approx 10,000 feet.
Older aircraft did have tanks which supplied passengers though.
The aircrew generally have their own dedicated supply and this is not chemically generated - it comes from a storage tank/s. Probably because they are sometimes required to put on their masks under emergency conditions or SOP’s . They can select whether to have 100% or normal O2 supplied.
After preview I can see I have been beaten to the punch but I’ll post it anyway. It took me long enough to type
I think they’re generally rather simple constant-flow systems. They usually employ a bag to act as a reservoir for oxygen that’s flowing toward you while you’re exhaling.
I perform the functional test on the passenger oxygen system of every new 737 that rolls out of the factory. I also have experience on the 757 and 777. The oxygen provided in almost all airplanes comes from chemical reaction cannisters, these provide from 8 to 10 minutes of breathable oxygen per person. The oxygen is pressurized at about 3 to 5 psi, this should be more than enough to overcome the lack of pressure at altitude. The masks are made with 2 one way valves, this insures you don’t breathe the same air you exhale. The masks also have bags that will semi inflate, these can be sqeezed to force oxygen into those having trouble breathing.
Those in the flight deck are supplied with bottled oxygen, most have at least 20 to 30 minutes worth per person. It is a positive pressure system just like those used in fighter aircraft.
Based on what we have been able to get from the news and from some internal Boeing sources that know a lot more about the crash than most, the airplane in question suffered a sudden loss of cabin pressure at about 34,000 feet. From what those at the crash scene saw, all the oxygen masks, both crew and passenger had deployed as designed. Those flying the airplane were unable to bring the airplane down to a sufficient altitude that could sustain life. Airplane oxygen systems are tested to deploy automatically at 14,400 feet, they were flying more than twice that high.
The airplane, a 737-300 is believed to have been built in 1987 and was originally an lease airplane sold to GE Aerospace, who in turn leased the airplane to a Tunisian charter airline. The plane was sold to Helios in 1999 and from reports, has has a very spotty maintainance record since. The police in Cyprus reportedly raided the airlines offices and siezed the maintainance records of all of it’s aircraft. Boeing is pretty good at keeping us employees up to date when an airplane crashes. We all recieve emails this morning telling us we will be told more when more info is available.
Is it true that the reason they tell you to put your head between your knees in the brace position during a likely crash scenario is not to save your life on impact,but in order to make identifying you via dental records easier?
With the prevalence of autopilot on all major passenger planes why isn’t there an autopilot contolled procedure that brings a depressurised plane to a safe height? Thus if the pilots are rendered unconcious the plane still has a chance of survival. If the pilot is concious he/she could over-rule the autopilot if necessary so there is no danger in having such a system built into all airliners.
The masks used by the crew are of a type that can be supplied by a pressure-demand regulator. (Military designation “A4”, IIRC) Thes CAN supply pure O2 at slightly above ammbiant at high altitude. The crewman relaxes to inhale, and must work to exhale. It is wierd at first but not difficult to get used to.
A4 (?) regulators also have “diluter demand” settings for lower altitudes, where they feed O2 only at negative pressure, much like a SCUBA setup.
IIRC pressure demand regulators are rated to around 45,000’MSL, where diluter demand systems are good to ~35,000’. It has been several years since I needed to be sure though, so will not be suprised to be corrected.
The passenger masks are known as rebreather bag types, and ARE limited to ambiant pressure. The similar systems I have used are rated to only 18,000’. I think they actually work higher, but the FAA has lots of rule changes that kick in at 18,000’, so I think the 18K rating is more for leagal reasons than for physiological reasons.
The O2 systems on passenger aircraft are meant for short term use. If activated, the crew will immediatly decend to altitude where suplimental breathing oxygen is not needed.
You don’t pass out instantly at high altitude. Private pilots are required to use O2 above 12,500’ MSL only after 1/2 hour. above 14,000’ it is required regardless of time. The military requires it above 10,000.
Hypoxia, like alchohol consumption, is not a binary condition. Imparment of judgment and motor skills is progressive, and can get pretty severe even though the victim remains concious.
