In attempting to measure the hazards of hypoxia, we progress through a series of rough approximations. Pressure altitude is a very approximate surrogate for the amount of oxygen in the blood; the relationship varies greatly among different individuals. Oxygen in the blood — which can be measured in flight with a cheap gadget called a fingertip oximeter — is in turn a surrogate for performance, but even at the same level of oxygen saturation, different people experience different degrees of impairment. And impairment itself is difficult to evaluate; a highly skilled pilot, impaired, may outperform a mediocre one at the top of his form.
Yet another step is required to relate impairment to safety. It’s easy to say that none should be acceptable, but in fact people fly all the time with all kinds of impairments due to fatigue, minor illnesses, life stresses, circadian rhythm, medications permitted or proscribed — all factors that the NTSB has at one time or another associated with accidents. Since most of these are unavoidable, it is assumed that pilots’ skills are adequate to allow them to perform their tasks even when they are not in peak condition. Differences in ability and performance are taken for granted; one does not have to be Charles Lindbergh to earn a pilot’s license.
A typical altitude-chamber ride takes you up to 25,000 feet and has you do sums and sign your name while you’re there. When you return to ground pressure, you discover that your work product resembles a fourth-grader’s. But the purpose of the ride is to scare you into following the rules. It happens that 25,000 feet is close to the level at which the effects of hypoxia are quite marked and cease to be extremely variable among individuals; in fact, most people will eventually pass out at 25,000 feet. A chamber ride to 15,000 feet might be less dramatic, but it might be more representative of the everyday experience of pilots.