My first real flight with a pilot's license was Sunday, up to Marysville Airport to visit nearby Beale Air Force Base. Beale is home to the U-2 and RQ-4 drone; the 9th Medical Group supports the U-2 pilots with high altitude medical expertise. Once a month they offer abbreviated high altitude training to civilian pilots.
Hypoxia is a real threat to general aviation pilots. Our planes only go up to 15,000 feet or so, but they're almost always unpressurized and often don't have any oxygen on board. Legally I can fly up to 12,500 feet without any supplemental oxygen. That may not be wise, particularly since one of the symptoms of hypoxia is euphoria; you may not even recognize a problem.
Our training started with four hours of ground briefing. Then the good part, an hour in the high altitude simulation chamber for a "flight". The hardest part for me was the half hour of pure oxygen through a mask at sea level: a bit of suffocation anxiety. Once we'd lowered our nitrogen concentration in the blood we climbed up to the real test, taking the mask off at 25,000 feet pressure altitude to enjoy our 3–5 "minutes of useful consciousness".
The briefers gave us a list of about 10 different symptoms people experience during hypoxia: headache, dizzyness, gasping for air, hot flashes, turning blue, etc. Apparently you get the same symptoms every time. The game is to recognize them in yourself, then turn on your own oxygen. I was doing just fine for about a minute until suddenly wham my vision shut down, everything going grey and collapsing to a narrow tunnel. Scared the heck out of me, I was quick to put my mask back on and everything was back to normal within thirty seconds. Some of the other folks delayed putting on their masks for several minutes, either wanting to get the most from the experience or out of pure belligerence (another hypoxia symptom).
Overall it was a good trip and I'm thankful to the Air Force for providing such expert training to GA pilots. I think I know more about handling rapid decompression than the creeping hypoxia that's the real threat to me, but hopefully I'll know now to recognize when my vision starts failing or my fingernails turn blue.