In last month's Aftermath, which concerned fatal accidents that the NTSB had linked to hypoxia, I was puzzled by a few that involved experienced pilots who had been flying, in one case for a rather short time, in the 12,000- to 18,000-foot altitude range without supplemental oxygen. If scrambled fighters find a maskless pilot slumped in his seat, clearly unconscious, as his airplane cruises past its destination at 25,000 feet, it’s reasonable to suspect an oxygen problem. On the other hand, if a pilot with minimal instrument experience gets tangled up in clouds at night at 15,000 feet, is hypoxia to blame? You can’t really tell, since plenty of pilots get tangled up in clouds at night at 5,000 feet with the same outcome.
Part 91 allows you, the pilot, to fly between 12,500 and 14,000 feet without supplemental oxygen for no more than 30 minutes. Pilots are required to use oxygen above 14,000 feet at all times. Passengers are permitted to gasp for breath indefinitely below 15,000 feet; above 15,000, everybody in the airplane must be provided with supplemental oxygen.
It is important to note that when hypoxia is discussed in terms of altitude, it is pressure altitude, not density altitude, that matters. There is always plenty of oxygen around — a whole atmosphere of it — but what controls the amount that reaches the brain is pressure differences across various membranes in the body. The process by which oxygen reaches the brain is affected by a great many factors that vary widely among individuals. By some definitions, normal people acclimatized to sea level begin to be hypoxic at 7,000 feet or so. Many people experience symptoms of altitude sickness at 10,000 feet; others are comfortable and seem to function normally at 15,000. The time of useful consciousness — a standard metric — at 29,000 feet is said to be around two minutes, and yet people — thoroughly acclimatized — have climbed Mount Everest without supplemental oxygen.
Some authors find Part 91 far too liberal. One doctor-pilot, Fred Furgang, declares in an essay on hypoxia that “supplemental oxygen is needed at altitudes lower than those required by the Federal Aviation Regulations. It is a good idea to consider oxygen for flights above 5,000 feet at night and about 8,000 feet during the day.” Well and good, but safe flying is about the careful management of risk, not about its complete elimination. How many pilots do you know who would turn back and land rather than cross a 10,000-foot ridge without oxygen?