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Aeromedical Forum

High Altitude Flying—High Risk Flying?

It’s been a long time since we’ve talked about the medical issues of high altitude flying. Since then General Aviation pilots have been spending more and more time at higher altitudes. The ubiquity of turbocharged single-engine GA craft has made it easy to crank an unpressurized cabin up to the flight levels and get that big tailwind. Before you do that though, make sure you have the right equipment for supplemental oxygen delivery and know how to use it! Your risks of oxygen starvation or hypoxia increase exponentially with altitude. Symptoms of hypoxia vary from person to person. It’s important for you to know your reaction to drastically lowered oxygen pressure. Since it’s a really bad idea to buzz up to altitude, take off your mask and seeing how goofy you get, consider taking a ride in an altitude chamber. I flew the FAA’s chamber in Oklahoma City. It was a very informative experience undergoing rapid decompression at 27,000 feet and trying to do simple math at 17,000 without benefit of supplementary O2. It doesn’t work at all well! I also found that my response to hypoxia is an initial sudden adrenaline rush – rapid heartbeat, sweating an attempt to focus and “hang on”. Other folks do indeed get goofy – inappropriate laughter, nonchalance. Others just get solemn, silent and start looking like so many dead fish staring at the windshield. None of these reactions bode well when you’re trucking along at 200 knots. Unless supplemental oxygen is administered, the sensorium clouds, vision starts to tunnel and then you black out. Just to refresh you ground school memories, here’s a table of Time of Useful Consciousness (TUC) i.e. Time to Dead Fish Look, as a function of altitude:

Time of Useful Consciousness Altitude in feet

30 min or more 15,000

20 to 30 min 18,000

5-10 min 22,000

3 to 6 min 25,000

2.5 to 3 mins 28,000

1 to 3 mins 30,000

30 sec to 60 sec 35,000

15 to 20 sec 40,000

9 to 15 sec 43,000

6 to 9 sec 50,000

Now you might look at that table and think, “Hey at Flight Level 180 I’ve got half an hour before I have to strap on the oxygen.” Not so. By 20 to 30 minutes at 18,000 feet, a healthy non-smoking pilot at rest becomes a slobbering idiot. This is the end of a continuum of dysfunction that begins with more subtle but potentially fatal degradations of mental capacity.

So when do you turn on the O2? The Flight Review answer is, of course FAR 91.221 which states that supplemental oxygen is required for the flight crew at altitudes above 12,500 feet (MSL) up to and including 14,000 feet if you are up there more than 30 minutes. Above 14,000 feet, oxygen is required for the flight crew all the time. At cabin pressure altitudes above 15,000 feet (MSL) you’re required to provide your passengers with O2. But like so many FAR’s, these are bare bone minimums. Mental agility doesn’t just switch off at 14,001 feet. It gradually deteriorates often starting at much lower altitudes. How fast and how far depends on the individual, his/her age and experience, medical condition, medications on board, level of fatigue or stress. If you’re puffing a pack of cigarettes a day, cut those altitudes in half to get your TUC! Effects of hypoxia on vision are particularly severe during night flying and are commonly noticeable as low as 5,000 feet. So if you are flying into an unfamiliar airport at night, especially if visibility is an issue, it’s smart to use supplemental oxygen. For most GA applications the relatively simple nasal prong delivery system works well. You can pick up a set at most of the aviation supply web sites. At or above Flight Level 180 however, a full face mask is mandatory to insure adequate oxygen delivery. You do want to invest in a pulse oximeter if you are flying at or above 13,000 feet. This is a fairly simple gizmo used in hospitals to measure blood oxygen concentration. It looks like a truncated close pin which slips over your index finger. A read-out tells you what percentage of your blood cells are carrying oxygen. Healthy individuals usually exhibit oxygen saturation (SaO2) values between 97% and 99% at sea level. The percentage will normally drop off as you get to altitude. A SaO2 value below 90% indicates hypoxia. In other words, you’re in trouble! Ask ATC for lower, pronto.

Fly wisely. See you next month.

As always, comments, questions and suggestions are welcome:


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