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High flight

Know your flight physiology

Accidents involving pressurized aircraft and total pilot incapacitation are rare.

bruce landsberg With apologies to Flight Officer John Gillespie Magee, forget the surly bonds and all that soaring and wheeling in the sunlit castles of the flight levels. It’s not a benign environment, as two accidents in the waning months of 2014 proved. A turbocharged Cirrus SR22 and a TBM 900 flown by experienced pilots went down within a week of each other. Both aircraft were intercepted, the pilots unresponsive, and flew on until fuel exhaustion. Currently there are only preliminary accident reports, and probable cause may never be determined—neither aircraft was recovered after crashing into the ocean. Hypoxia almost certainly was the cause, but we may never know why.

Accidents involving pressurized aircraft and total pilot incapacitation are rare; the last high-profile tragedy involved golfer Payne Stewart, whose chartered Lear 35 went down after its pressurization system failed. Partial incapacitation and the resulting crashes are more common but nearly impossible to document, since there are no residual markers. Good pilots make inexplicable decisions and mistakes with tragic results.

The FARs require that for personal flight (Part 91) pilots use oxygen when flying at or above 12,500 feet msl for more than 30 minutes. Apparently we’re better physical specimens and less susceptible than Part 135 pilots, who must comply with these same rules beginning at 10,000 feet.

FAR 61.31(g) requires training when operating above FL250. For the hypoxia portion, a quick and inexpensive way involves changing the gas mixture fed to the pilot. Don a mask with degraded elixir and soon you’re hypoxic. Another approach is the altitude chamber, which introduces hypoxia and pressurization challenges. Senior Editor Dave Hirschman and I flew to the National Aerospace Training and Research (NASTAR) Center in Philadelphia to take its one-day course.

NASTAR’s ground school takes about two hours. The chamber resembles a small boxcar and is built to endure massive differentials. Gigantic pipes pressurize and empty the box while complex controls manage the atmospherics without destroying the subjects (us). Rapid decompression was popularized in the 1960s movie Goldfinger when the villain was sucked out the window of a high-flying jet. Keep your seat belt fastened.

Our rapid depressurization demo was relatively gentle (8,000-foot cabin to 25,000 feet). Quicker than you can say “Aw, shucks,” the air literally rushes out of the body, the cabin fogs up immediately, and it gets really cold. The chamber’s airspeed is not 300-plus knots and the outside air temperature isn’t minus 50 degrees, so the airlines’ warning that “the masks will drop from the ceiling if there’s a change in cabin pressure” doesn’t begin to do reality justice.

But rapid decompression isn’t how hypoxia usually happens—and unpressurized aircraft can’t depressurize. Pilots slowly lose their abilities and, what’s worse, they don’t care! After a few test runs to clear our ears and make sure we had no sinus pain, we climbed to 25,000 feet and removed our masks. The time of useful consciousness is less than five minutes. Move into the high flight levels and it’s measured in seconds. That’s why jets are equipped with quick-don masks.

Unlike the unsuspecting real world, we knew what was coming. In about three minutes everyone in our group decided they’d had enough. My symptoms were light-headedness, bluing around the fingernails, and visual degradation, but your reaction may be different. Oxygen saturation drops from 100-percent oxygen to less than 80 percent in just minutes at that altitude. A pulse oximeter can be purchased almost anywhere for under $50 and is an essential piece of cross-country gear. It clips on a finger and shows oxygen saturation.

Down to 90 percent, all is good. Slip to the high 80s and it’s time to supplement. Mid-80s means oxygen is needed. Long flights above 10,000 feet are oxygen-depriving. Some high-country folks will do better, but eventually we are all susceptible.

The chamber was then blacked out and run up to 18,000 feet. In dim light we were asked to discern various colors; not so good. Oxygenated, the colors popped right out. We need it at night above 5,000 feet.

In real flight, hypoxia creeps up slowly, and stupidity strikes with stealth. Feeling a little dizzy, tired, or just really happy to be flying? Get some good air. Spending the day at 9,000 feet, as many us of do on long flights, takes its toll. Deep breathing helps a little but unless you’re prepared to do that for extended periods, which is tiring itself, just go for oxygen—or fly lower.

Flying pressurized? Take the chamber ride. There are multiple places around the country to go. Knowing your flight physiology is part of becoming a well-rounded, safe pilot.


Bruce Landsberg serves in an advisory capacity for the AOPA Foundation and Air Safety Institute. He retired from the association in 2014.

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