Last week we talked about the joys of operating in the flight levels and judging from the comments, the votes seemed to be overwhelmingly in favor of “bigger is better” or at least, easier. I recounted the wonders of the Cessna 441 at FL350 but there is one drawback to that magnificent machine.
The cabin altitude is about 11,000 feet and where slow-onset hypoxia should be considered. Some of the Foundation staff will claim that I’ve spent way too much time at altitude and am permanently hypoxic. (There are times I would agree.) However, this topic gets relatively little attention and even the FARs don’t even bring up the subject of oxygen impairment below 12,500 (5,000 at night). This blog is NOT about rapid decompression where the time of useful consciousness at those altitudes is measured in seconds.
A pulse oximeter is a wonderfully simple-to-use device that clips on a finger tip to instantly measure your oxygen saturation (sat) and heart rate. Ideally, at sea level, we’d all have 100% saturation and a pulse of about 60 – fat chance! Many pilots may start in the low 90s. When your sat drops below 90%, oxygen deprivation is starting to take place. Thinking slows, and heart rate increases as the brain asks for more O2 to be pumped up.
As the sats fall, so does your ability and it can be quite insidious. After a few hours at 9,000-10,000, where many light non-pressurized aircraft fly, many of us will be hypoxic. A slight headache, fatigue, and the inability to process information as quickly as normal, are all symptoms but they vary person to person. But is it operationally significant? In more than few accident cases we see where an otherwise capable pilot made a poor decision that resulted in an accident. Why? One factor that is certainly present in some cases is slow onset hypoxia but you can’t measure O2 saturation on a corpse so the causes remain elusive and speculative.
On my flight back across the country in the C441 at FL350 last week (which we did non-stop in under 6 hours incidentally) checking the sats was instructive. Both crew seats are equipped with quick-don masks and when my sat was dipping below 90% I’d take a few minutes to bring it back up to normal. During these episodes it sounded like Darth Vader’s heavy breathing (Right here above planet earth but no Death Star in sight). Sitting in the back for a bit, without the benefit of oxygen, resulted in sleepiness and my usual slight headache. The oxygen cleared it up. It’s bad enough when passengers are sleepy and a bit out of sorts. Their brains are starving for good air but for pilots this is critical.
The message is clear–if you spend much time above a cabin altitude of 8,000, your sats are going to come down—how quickly varies by individual. Healthy, fit, non-smokers will do better than others. Even if you’re not flying a turbocharged aircraft, you may want to think about buying a pulse oximeter—they’re available for $30 – $100. After that little stocking stuffer, if the aircraft isn’t so equipped, get a portable oxygen system—your brain (and other body parts) will love it! The Air Safety Institute will be reviewing pilot misbehavior in the future due to oxygen deprivation.
Anyone have a good hypoxia story?