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I've been reading about Time of Useful Consciousness (TUC), and it says that at FL 180, the time of useful consciousness is 20 to 30 minutes at rest.

But I've been to a trip to Tibet, and we went on a bus trip around the places, and at one point the bus came to a mountain pass around 18,500 feet, and we all got off the bus, walked around, took pictures, and even ate lunch, so it was definitely not at rest. It was about 40 minutes to 1 hour, and I only felt slightly uncomfortable/out of breath.

According to the chart, I should have passed out, or at least become "useless". Why didn't this happen? Are the charts simply very conservative and apply to, say, unfit, crumbling, old pilots?

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    $\begingroup$ If you have the opportunity at such an altitude again, using just paper and pencil (no calculator), multiply two four-digit numbers. I tried this experiment after sitting at 14,000feet for an hour in a Cessna 310 while a friend was flying the airplane on oxygen. I did three of that kind of problem. When checked with a calculator, all three of my answers were wrong. $\endgroup$ – Terry May 10 '15 at 3:57
  • $\begingroup$ I was actually on my computer programming while on the bus (which stayed above 15,000 feet for about 2 hours), and I definitely felt pretty useful, if a bit easily-exhausted. I think it's just the acclimatization. $\endgroup$ – ithisa May 10 '15 at 5:20
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    $\begingroup$ The Dunning–Kruger effect suggests that as your mental acuity declined, your effectiveness in assessing your own mental acuity probably suffered a corresponding decline. $\endgroup$ – A E May 12 '15 at 14:04
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In short: acclimatization, both chronic and acute.

During your Tibet-trip, you had probably spent days at high altitudes, allowing your body to acclimatize to the lower oxygen partial pressure (similar to how some elite endurance athletes prepare themselves for competition), by increasing your hemoglobin count (and other mechanisms), making you better prepared for the walk at 18,500 ft. This is an example of chronic acclimatization.

A normal sea-level dweller will not have had the benefit of the same acclimatization phase in a depressurization situation (albeit a pilot could probably have some degree of acclimatization from continuously being exposed to higher than sea-level cabin altitudes), and will hence have a shorter TUC. The onset of the depressurization will also play a big part in determining your TUC. A slow depressurization will allow for some acute acclimatization, whilst a rapid depressurization will allow for none. The shock of a rapid depressurization will also decrease your TUC.

The table on the Wikipedia page you quoted is the same (or very similar) to what you'll find in most Human Performance literature for airmen, and is, according the same Wikipedia page "average TUC for healthy, young military pilots".

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    $\begingroup$ That's probably true. I've stayed about a week above 10,000 feet (there's literally no place lower than 10,000 feet around) before getting to 18,500. I noticed that for about a week after returning to a sea-level place, I was able jog ridiculous distances without feeling tired. $\endgroup$ – ithisa May 9 '15 at 18:41
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    $\begingroup$ Conversely when I was a teenager I walked to the top of a 10,000 feet peak (in France): I was panting at the top, and still panting after sitting there for 30 minutes. $\endgroup$ – ChrisW May 10 '15 at 15:08
  • $\begingroup$ @user54609: Whoa that's really cool... how far were you able to jog? I'm really curious now haha $\endgroup$ – Mehrdad May 11 '15 at 10:55
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    $\begingroup$ @Mehrdad : Pretty unimpressively actually. It's just that before going to Tibet, I couldn't jog at all: I would go out of breath in about 2 minutes. I'm not exactly the most fit person on earth. After the trip, that became upwards of 15 minutes. $\endgroup$ – ithisa May 12 '15 at 19:23
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At lower altitudes, "useful" consciousness doesn't end when you are unable to act. Rather, what happens is that your ability to make sound decisions and carry them out effectively degrades: for example, instead of responding to an alarm buzzer by figuring out what's causing it, you might simply hit the "silence" button, or you might be so fixated on keeping the airplane flying straight-and-level that you don't notice the alarm, or you might be unable to concentrate on either task long enough to do anything effectively.

I'm not familiar with aviation literature on the subject, but at Mauna Kea Observatories (14,000 feet), they found that providing supplemental oxygen to people working there greatly reduced the error rate. See, for example, a Keck Observatory article on the subject, or a study on the effects of altitude for a proposed Chilean observatory that heavily references Mauna Kea.

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    $\begingroup$ After thinking for bit, perhaps this applies in a way in my case too. I am usually reasonably good with my DSLR, but all the photos I took from that pass were very poorly framed, poorly exposed, or even completely out of focus, and I didn't even notice until later. $\endgroup$ – ithisa May 10 '15 at 5:35
  • $\begingroup$ Do you have a citation on the Mauna Kea claim? $\endgroup$ – March Ho May 10 '15 at 6:25
  • $\begingroup$ @MarchHo, I've been trying to find it. I seem to recall that it was an offline source, which makes searching hard. $\endgroup$ – Mark May 10 '15 at 8:42
  • $\begingroup$ try here: "Supplemental oxygen is also a good idea for visiting scientists and engineers who have to do other thought-intensive work while on the summit. Otherwise they could find themselves more prone to errors or just plain stumped by problems that would be easy to solve at lower elevation." $\endgroup$ – Erich May 11 '15 at 23:22
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    $\begingroup$ also, this: "at high altitude[,] accurate work can be done, particularly if detailed protocols are written out, but it takes longer and more concentration is required. Problem solving is particularly difficult and this is where errors are most likely. The principal way in which it is planned to respond to the problem of reduced mental ability is to provide an oxygen enriched atmosphere in those situations where it is required" $\endgroup$ – Erich May 11 '15 at 23:32

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