Partial Pressure: Your Mileage May Vary

I spend most of my flying life bumping around down low in the course of instruction. Entire days spent in the pattern at a thousand feet AGL, or perhaps a few hours doing maneuvers at three or four thousand AGL, and the occasional cross country at five or six thousand feet. I spent a number of years in a pressurized airplane flying in the lower flight levels, fifteen thousand up to the low twenties. It has been, however, not so often lately that I have had occasion to take a non-pressurized airplane up to the ten-thousand foot range for an extended period of time. Sure, I have flown Mooneys and light twins around the country, to Canada and back numerous times, across the country a few times, up and down the coast a host of times, back and forth to New England repeatedly, but those flights are scattered over decades. If I were to go back in my logbook I would probably find a couple of thousand hours or so in that region, but not so very often lately. But the fact is that many normally aspirated piston airplanes do very nicely at ten, eleven, twelve thousand feet where the true airspeed gains are significant, the air is smoother, much weather lies below, there is less traffic, lean-of-peak engine settings are effective and fuel economy can be maximized. Here’s the problem of which I am reminded … On this round-the-world excursion we are fortunate to have a turbonormalized aircraft, with a large Continental engine boosted at altitude to maintain near-sea-level manifold pressures well up into the flight levels. Nevertheless, for reasons having to do with European regulations, leg length, fuel economy — this amazing Tornado Alley Bonanza conversion burns fuel in the climb like a prairie fire — we find ourselves often in the eight to twelve thousand foot range, where oxygen is not required, at least in the U.S., but where I have come to believe it is warranted.

I have many pilot friends who tell me, “I fly at twelve thousand all the time. I feel fine. Don’t need oxygen.” (We are speaking, of course, of supplemental oxygen, but we all know that’s what we mean, right?) Frankly, I am dubious. I have long noticed that, unpressurized, without oxygen, above roughly eight thousand feet, I am not firing on all eight cylinders mentally. I can function well enough, I suppose, but I am not at my best. One might well ask, how good can my best be? I’m not young anymore. There’s a lot of wear and tear evident. But, for someone of my age I am in reasonably good condition, probably in a high percentile when it comes to those things that govern cardiac and respiratory function. And I definitely sense a dimunition of mental function and an increase in fatigue and I think I see it in others. I have also had the experience many times – and again on this journey – of feeling that way and then strapping on supplemental oxygen and enjoying a rapid A/B comparison, without and then with additional oxygen. Not only can I sense an almost immediate difference in my energy, my attention, my ability to reason and calculate, my night vision – that is often striking – and my level of fatigue, but I can measure the effect with a pulse oximeter. At eleven thousand feet, my O2 sats will run in the mid to upper eighties, which most medical writings consider to be hypoxic. With supplemental oxygen, they will immediately rebound to the mid-nineties. I have verified that several times on this trip.

I also suspect that hypoxia plays an unappreciated or unrecognized role in some puzzling general aviation accidents. I read about too many accidents that seem inexplicable. In the aftermath, our pilot’s friends, family, pilot acquaintances and instructors tell us that our pilot was careful and thorough and cautious and maintained his airplane impeccably, took no chances, remained current, flight planned diligently, and flew capably – and yet the evidence suggests that our pilot went out and tried to fly five hours on four hours of fuel. Or something similar.

One possible explanation: hypoxia typically makes you stupid and then it makes you not care very much. Or vice versa. It can make you think that four hours of fuel has magically become five hours of fuel. Even if you are diligent, capable, and careful at sea level.

I know you have been flying for a long time. You answered the FAA questions about hypoxia and passed all of your knowledge tests. And you are not much interested in wading through tedious texts about partial pressures of gases and hemoglobin bonding and probably have not felt the need to revisit the subject. Up to you.

Hypoxia affects each of us differently and to a different extent. As they say, your mileage may differ. But you might think seriously about at least measuring your mileage. These days you can buy a functional, if not scientific-grade, pulse oximeter for less than the cost of filling up a Skyhawk. Or borrow one from a friend. We fly with one aboard. You are welcome to borrow it. But not for a few more weeks. I’m wearing it.

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