For any pilot who flies at high altitudes, hypoxia is something to be aware of and there is no better way to know what hypoxia feels like than to spend time in an altitude chamber. John Fiscus recently did just that and has written about what happened on the Cirrus Owners & Pilots Association (COPA) website.
In case you are not familiar with hypoxia, there are actually four types and the most common type that pilots are susceptible to is hypoxic hypoxia which is caused by insufficient pressure to push a sufficient amount of oxygen through the walls of the lungs and into the blood stream. In other words, oxygen deprivation which leads to impaired judgment and ultimately to unconsciousness.
To experience the full force of hypoxia, the altitude chamber that John sat in was depressurized to 25,000 feet or 7,620 meters. After taking off the oxygen mask, John says he felt weird but he set to work answering some math questions. However:
Two minutes in I was struggling to do that last math question. My handwriting was awful. I never even got to the maze. Just a little shy of 5 minutes I started seeing stars around the edge of my vision and the tunnel started to form… and slowly collapse. I took me a few seconds to realize what was happening and I’m not certain that I really knew what was coming anymore, just that I’d had enough. I pushed my mask on and flipped all the switches forward to get some O2. Everything came back at the first breath, along with a headache.
John concludes by saying that if you fly above 12,000 feet, having hypoxia training is worth it because different people will experience different symptoms. In that way, you can then recognize the symptoms and will be better prepared to deal with it the next time you are in an airplane and should it happen. Definitely good advice worth heeding.
And in case you are not able to experience hypoxia in an altitude chamber, several videos showing what happens have been posted on Youtube.
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Being both an Alpine and IFR pilots, altitudes over 10.000 are not unknown to me. Cruising at FL100 is usually not a problem, even for extended periods, but I beware of everything higher than that.
I experienced once light symptoms while flying at FL135. It took a good 30 minutes before I felt a change. This was a flight over the Alps, in the region of Montana, Jugfrau and Matterhorn. When I started to feel a bit dizzy, I descended, and the symptoms disappeared almost immediately.
Good awareness and self-observations are key factors to avoid such problems.
http://www.plasticpilot.net/blog/2008/02/22/pressurization-for-dummies-part-ii/