How to adjust time of useful consciousness for pilots acclimatised to high altitudes? 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).
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.