oxygen rules

[thx1138]

According to an article I am reading "research shows that the maximum safe altitude at which an aircraft cabin can decompress and the crew remain conscious, even with the use of oxygen masks is 48,000 feet. Safely surviving decompression at such high altitude requires a pressure suit" Why doesn't the Oxygen mask work? What do pilots at 51,000 feet or the concorde do?

[Turnup]

I'm no medic, but it is the partial pressure of oxygen that is important to sustaining life. Think about it this way:

The O2 you breathe through a mask is at the same pressure as the cabin - if it were at any higher pressure it would inflate your lungs like a balloon - and with the same consequences if the over pressure were too great. Divers have to be very careful of this when ascending.

As the cabin pressure reduces, so does the pressure of the O2 from the mask. Less pressure means that you are inhaling less oxygen per lungful. Also less pressure means that the chemical processes that absorb O2 into the blood work less well (in fact at some critical pressure they actually reverse and you start to lose O2). As the pressure reduces there will come a point that the human physiology cannot oxygenate the blood sufficiently quickly and cogent brain function is the first to go.

Imaging breathing pure O2 at a pressure only fractionally above zero and its intuitive that there is very little actual O2 being inhaled.

Hope this helps.

[boofhead]

Breathing oxygen allows the body to function at a higher altitude than breathing normal air since it maintains the partial presure of oxygen needed by the blood. Flight crew oxygen masks are automatically programmed to supply the right mix of oxygen and air as the cabin altitude increases during climb, and if a depressurisation occurs will shift to 100% oxygen. Of course the mask will not do any good if it is not being worn, and it is assumed the pilots have 5 seconds of useful consciousness above 35,000 feet in which to grab the mask and put it on. But at some altitude above 30,000 feet the partial pressure of oxygen even with 100% flow is equivalent to an altitude above 10,000 feet, and the normal person will begin to suffer from hypoxia. In order to maintain enough oxygen to reach the blood at a level that will allow normal function above that altitude it must be fed to the lungs under pressure, which can be achieved by using a pressurised mask; most airline masks are capable of that (Emergency flow) until, above around 42,000 feet, a pressure vest or suit is needed to hold the chest tight so that oxygen can be fed in under high pressure. If the suit is not worn, the pilot cannot breath out against the pressure. Even at lower altitudes where the suit is not needed, it is weird to use a pressure mask since the pilot relaxes to take in oxygen and has to force it out consciously and with effort. Breathing is a chore. Not doable for passengers. If depressurisation occurs at Concorde altitudes an immediate descent must be made, because the passengers do not have a pressurised mask or pressure suit. At the lower altitudes flown by conventional airplanes it is not so important to start down as quickly, since the flight crew can survive for extended periods using the oxygen masks provided, and most passengers, even if not using oxygen, will not suffer long term effects so long as they are not kept at cruising altitudes for longer than four minutes or so. If they are breathing oxygen from the drop down masks they also could manage for as long as the oxygen lasted, even though their masks are not as sophisticated, but there is always one or two who do not know how to use the mask. Of course if they follow the normal airline safety brief, where the main aim of the flight attendant is to avoid mussing his/her hair, they will never learn how.
I am not sure of the exact altitudes for the situations I have given above, anyone who has accurate information can flame me freely.

[Genghis the Engineer]

From memory, not having done a refresher course for a few years:-

At FL100 the partial pressure of oxygen in the air (that is, number of molecules per unit volume) is enough to allow more-or-less undegraded performance of a fit human being. (Hence, the working ceiling for most light aircraft)

At FL250, 100% oxygen at the "local" pressure gives the same partial pressure of Oxygen. So this is the limit for unpressurised cabins with oxygen masks. (Hence the ceiling for aircraft such as the Tucano).

Up to FL500, it is safely possible to pressure breath in the event of a cabin pressure failure. Pressure breathing means high positive pressure of O2 through the mask. The effect of this is that you have to force air out, and relax to allow air in - uncomfortable and requires training, but not particularly difficult. (Can leave your chest muscles a bit bruised). This is therefore the limit for "normal" pressurised cabins. It is accepted that pressure breathing is useable by crew but not pax - they will lose consciousness, but recover without ill effects once below FL100 if a controlled rapid descent is made.

Above FL500 pressure breathing alone is inadequate. There are three solutions, which are:-

- Approach used in the Lightning, which is a pressurised pilots jerkin, counteracting his chest's natural tendency to try and explode.
- Approach used in the SR71, Canberra, etc. a pressure suit (not quite a space-suit, but pretty close).
- Approach used in Concorde, very small windows, very powerful cabin compressors, and the ability to maintain a survivable cabin pressure (that is below FL250 equivalent) despite a given number of failed windows.


The exact heights vary according to authority, the above were being used last time I did a B-cat refresher at BDN about 7 years ago, but I'm fairly certain are still the standard RAF and UK CAA figures. I think the USAF and FAA use FL120, FL240 and FL480, but all pretty much in the same ballpark.

G

[thx1138]

Thank you for the replies. I had thought that with 1800 psi in the tank and regulated down to 80 or so psi on our guages that no matter what the pressure was outside, as long as you had the mask sealed to your face especially with emergency flow, you could suck in plenty of oxygen. Perhaps someone on a cargo plane could be tempted to continue after a depressurization to their destination.

>- Approach used in Concorde, very small windows, very powerful >cabin compressors, and the ability to maintain a survivable cabin >pressure (that is below FL250 equivalent) despite a given >number of failed windows.

According to the article that started my first question...a transport category cabin must be pressurized so that it maintains an altitude of no more than 8,000 feet. For certification for operation above 25,000 feet, any probable failure can not allow the cabin equivelant altitude to exceed 15,000 feet For any other failure that leads to decompression that is not shown that is not shown to be extremely improbable, cabin altitude must not exceed 25,000 feet for two minutes and cannot exceed 40,000 feet at any time.