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The time of useful consciousness (TUC) is the length of time an average pilot, breathing ambient-pressure air without supplemental oxygen, is capable of functioning usefully at a given altitude. For instance, at FL180 (approximately 18 kilofeet AMSL), the TUC is 20-30 minutes for a steady ascent to this pressure altitude, or 10-15 minutes following a rapid or explosive decompression; at FL280 (~28 kft AMSL), TUC is 3-5 minutes for a gradual ascent (and half as long for a rapid/explosive decompression), and, at FL400 (~40 kft), TUC is only 15 to 20 seconds, even for a steady ascent (and a mere 7 to 10 seconds following a sudden decompression).1

However, healthy humans are capable of a very considerable degree of acclimatisation to high pressure altitudes (and resultant low partial pressures of oxygen); humans have demonstrated the ability to acclimatise to, and live and function indefinitely at, altitudes up to 19.5 kft, and long-term acclimatisation is thought to be possible up to at least 24-26 kft (although complete acclimatisation takes an increasingly long time - up to weeks to months in the upper reaches of this range - at very high altitudes).2 As extreme examples, a pilot living in La Rinconada (elevation 16.7 kft AMSL) could have an indefinite TUC as high up as FL180 (where, as mentioned above, the average pilot would be able to function without supplemental oxygen for at most half an hour), and someone just back from climbing Mount Everest sans oxygène supplémentaire could potentially be able to maintain an indefinite TUC up to at least FL220 or so (where an average pilot would have a TUC not exceeding ten minutes or so).

A high-altitude-acclimatised pilot would obviously have considerably longer TUCs for a given altitude than a sea-level pilot (even at altitudes where even a highly-acclimatised pilot cannot function indefinitely); how does a pilot acclimatised to high altitudes adjust the times in the standard TUC table to account for this?


1: Despite the 50% reduction in TUC following a rapid or explosive decompression, these, paradoxically, tend to be less dangerous (except in the rare cases where the decompression is sufficiently violent, or occurs from a sufficiently inconvenient part of the aircraft, to cause severe damage to aircraft structure and/or critical systems, or to physically suck one or more of the occupants from the aircraft), due to their violently obvious nature, than gradual decompressions (where the aircraft occupants can easily become incapacitated by hypoxia before they even realise that they have a pressurisation problem).

2: Beyond 24-26 kft AMSL (in what is aptly known as the Death Zone), long-term acclimatisation is mostly thought to be beyond the human body's capabilities; however, some degree of incomplete acclimatisation (allowing one to survive and usefully function at these altitudes without supplemental oxygen for considerably - sometimes very considerably - longer than an unacclimatised person would be capable of, although not indefinitely) is possible (which is how it's possible for a sufficiently-acclimatised person to climb Mount Everest without using any supplemental oxygen at all), and some scientists (from Bolivia, appropriately enough) dispute that there is an upper limit at all (or, at least, one below 29 kft AMSL).

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    $\begingroup$ This research highlights the importance of latitude and season (it isn't just an AMSL value), which helps with the Everest climbs -- latitude affects the barometric pressure by as much as 30 mmHg (RE 2nd footnote). (La Rinconada is close to the equator.) Also acclimatization is subjective (humans don't respond the same to the same acclimatization), given that, I don't think there is one answer. $\endgroup$ – ymb1 Mar 21 at 21:48
  • $\begingroup$ Given the TUC at some altitudes is seconds, acclimatization is going to give you negligible increase in conciousness $\endgroup$ – SSumner Mar 22 at 17:44
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The simple answer is "they don't". Training will dictate that at altitude, they need to be able to don masks within a few seconds, regardless of an individuals tolerance to altitude. The problem isn't that you have X seconds of full-cognition and then you pass-out, your brain function degrades during that period. It's best to get your mask on and oxygen flowing before doing anything else, and this is how pilots train.

Euphoria and a false sense of ability is a side-effect of altitude sickness/hypoxia. There are numerous videos out there showing altitude chamber tests where individuals are given simple tasks as the oxygen saturation decreases. These tasks can be as simple as those infant "put the shape in the hole" boxes, and it is surprising to see how well the people think they are doing versus how poorly they are actually performing. These two things combine for a very dangerous situation and you, as a pilot, need to recognize that this can happen and to mitigate that as quickly as possible, without trying to take into account your own tolerance (which can change day-to-day) for altitude.

Check this one out, this is a military pilot getting cards wrong, this is a stark demonstration of the effects. These symptoms happen in seconds. Watch how fast his cognition comes back when given oxygen.

Or this one which demonstrates euphoria and the inability to follow directions or perform basic actions.

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I can second Ron's answer (you don't) and add a personal anecdote; back in the mid 80s I was invited to join some skydivers taking a High Altitude Indoctrination Course at the Defense and Civil Institute of Environmental Medicine in Toronto when they had an open spot. A jumper had to have taken the course to be able to jump from altitudes above 12000 feet in Canada. I was no longer jumping but a chum invited me along to fill in a seat opened by a cancellation.

The chamber ride was only to 25000 because the intent was only to experience hypoxia symptoms, not decompression events, and especially to learn what your unique hypoxia-onset symtoms were.

We all sat in the chamber (a yellow steel box with military transport style seats - I think there were about 6 or 8 of us) with oxy masks on, each with clip board/paper with vertical rows or columns of boxes next to geometric shapes. After the air was slowly pumped out until we were at 25000 pressure altitude, we were told to disconnect the mask hose at the seat base and start drawing matching shapes of each shape in the box. When we felt symptoms coming on, were to observe them to the extent we could, then reconnect the oxy mask hose before being incapacitated if possible (there was a guy in the chamber to do it for you if you were).

My observations, albeit a distant memory:

  • When I disconnected the oxygen source in the thin air, I couldn't tell the difference at all, sitting stationary. The air doesn't feel thin. You would have to be exerting yourself to become short of breath.

  • When I was drawing my shapes, by about the 2rd column, a minute or two in, I could tell that my shapes were starting to get mis-shapen, but I didn't think it was a big deal. I remember thinking my shapes were fine although they were lopsided.

  • My chum was a couple seats down and was a smoker. He went into giggling mode in maybe 1 to 2 minutes, laughing like he was on Nitrous Oxide. The smoking cut his tolerance time down a LOT.
  • My first obvious symptom was tunnel vision (a good thing - if your first symptom is euphoria like my friend had, that's bad). I remember my peripheral vision vanishing (it's impossible to describe) and only being able to see the clipboard, my knees and my hands.
  • I think I then started to enter euphoria stage, but for me the sensation came on really quickly, like a shot of alcohol right to the brain. If you've ever had Nitrous at the dentist, it is very similar. It came on so fast it startled me, and that plus the tunnel vision prompted me to reconnect the hose (you're already primed for action by the briefing and it can be enough to get you to act even though you are mentally compromised, which was the point of the training). If it had come on gradually, I probably would have done nothing and enjoyed the high until I passed out.
  • My smoker friend had to have his hose reconnected for him as he started to fade (everybody noticed the giggling stopping), quite a bit sooner than the non-smokers.
  • When I reconnected the hose, the tunnel vision and drunk sensation vanished immediately after the first or second breath. It was like going from dunk to instantly un-drunk in a heartbeat. Very strange experience in itself.
  • I looked at my shapes; first column nice, second column, slightly off, third column like a House of Mirrors of distorted shapes.
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    $\begingroup$ While certainly interesting, I fail to see how this answers the question asked by the OP. $\endgroup$ – dalearn Mar 22 at 17:01
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    $\begingroup$ Yes I did. "You don't" is basically the answer. In any case, do we have to enforce the posting rules to this level of anal retentiveness? $\endgroup$ – John K Mar 22 at 17:44

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