Some good avmed stuff here, and some not so good.
Alexban is correct about needing oxygen supplied at a positive pressure above about 40,000 ft. Osaka Joe gets the prize for being the first to remember the very real hazards of decompression sickness (or illness) but is not correct when he says, “ . . .emergency oxygen is exactly what you need at these altitudes.” If he’s referring to altitudes above 40,000 ft then “emergency” oxygen, (ie 100% oxygen with a slight overpressure), is not enough. We need oxygen under a higher pressure than that.
We must be careful not to confuse an oxygen mask’s ability to provide “overpressure” with a mask’s ability to provide positive pressure breathing. The mask’s “overpressure” facility, (which we normally have selected as a precaution prior to donning the mask), provides a slight overpressure to make sure we cannot breathe any fumes in from the cabin in the event of a less than perfect mask seal. This overpressure will also be provided automatically, regardless of the switch selection made by the pilot, in the event the cabin climbs above a certain altitude. Again, this is to prevent the pre-determined oxygen/air mix from being diluted with ambient cabin air due to a poor mask seal. This overpressure is not the same as the positive pressure breathing that is required for flight above 40,000 ft.
Above about 40,000 ft positive pressure has to be applied to the O2 to provide the necessary partial pressure to push it into the blood stream. This positive pressure is in a different magnitude to the slight overpressure used for fume prevention. (Mask overpressure to protect against leakage is in the order of 2 mmHg: positive pressure breathing will be in the order of 10-30 mmHg. That’s quite a difference - ask any military pilot who has practised positive pressure breathing in an altitude chamber or custom built training rig).
The 100% or emergency selection on the masks will not, therefore, protect us above 40,000 ft. (Consider this: if it could, the military wouldn’t add an additional positive pressure delivery system in their masks, would they?) If, however, your aircraft is certified to fly above 40,000 ft then there must be a facility in the mask and regulator assembly to provide you with oxygen at sufficient partial pressure to keep you functional until you have time to carry out your drills. These drills invariably involve a descent to a safe altitude. You’ll be out of positive pressure breathing territory very quickly provided, of course, you don’t screw up the drills.
You will not find any aircraft drills that recommend, under any circumstances (except temporarily for safety altitude), that you stay above 10,000 ft cabin altitude following a decompression. Even the military fighter drills tell the pilots to get below 25,000 ft. This is to protect against decompression illness - "the bends”. The military (and civil) masks can easily protect against hypoxia at 25,000 ft but they cannot protect against the bends.
(Note: it used to be 25,000 ft for the bends, then it was dropped to 22,000 ft because studies showed there is still a low risk there). We’re considered safe below 18,000 ft. Safe, that is, from the bends, but not from hypoxia if you are not wearing a functioning mask (despite contrary advice from some non-experts on other threads who think that flight above 10,000 without oxygen may be OK in some circumstances. It isn’t. Never. This is why cabin altitudes are limited to 8,000 ft and the military provide expensive bottled oxygen free-of-charge above this altitude).
Intruder wrote: “A long-distance runner, mountain climber, or a person who normally lives at a high altitude will have a longer TUC than the average Joe.”
Not necessarily. A person from Denver may not, someone from Peru might. You cannot predict a person’s TUC.
“A cold, fatigue, or other factors can significantly reduce TUC.” Well said.
In summary:
Supplementary oxygen is required above 10,000 ft (in fact it’s 8,000 ft for safety – refer to haemoglobin’s oxygen dissociation curve in your old avmed notes).
Mask oxygen concentration (and hence partial pressure) progressively increases above 10,000 ft to provide 100% O2 at 33,700 ft (100% O2 at 33k provides the equivalent O2 partial pressure of sea level).
Between 33,700 ft and 40,000 ft the partial pressure of the 100% oxygen in the mask drops to the same partial pressure of ambient oxygen at 10,000 ft (that's 103 mmHg if you’re still with me).
Above 40,000 ft positive pressure breathing is required to maintain this 103 mmHg partial pressure of oxygen. (ie Proper positive pressure breathing – the mask overpressure provided by the 100%/EMERG selections is not good enough).
At any time: slight mask overpressure is required to prevent ingress of ambient air in the case of suspected fumes or smoke. This usually needs to be selected by pilot during pre-flight checks and protects against poor mask fit.
Above about 8-10,000 ft: slight mask overpressure is automatically supplied by a barostat in the regulator to prevent mask dilution by ambient air (protects against poor mask fit).
Finally, never be complacent about the oxygen drills or your TUC. The book guidelines are exactly that – rough guidelines. That super fit, mountain climbing FO in Mensa may fare considerably worse than his fat, weedy, intellectually retarded, chain-smoking Captain.
Many posters start with, “. . . it’s been a while since my avmed training but I think I recall . . .”
In the wake of the Helios crash and some other near misses, why not get those books out now or Google the subject. But only read those reputable sources who reference their facts.
Ref: Aviation Medicine, 2nd Ed. Ernsting and King (1988) ISBN 0 7506 2275 X. Part I, Chapters 1-9.
Airbus A320/321 FCOM