Almost 15 years ago a well-known professional golfer named Payne Stewart was a passenger in a Lear 35 that departed Orlando, Florida for Dallas, Texas. The departure was unremarkable. Three minutes after the last communications with the Lear it made a turn consistent with a human input, but just three minutes later ATC was unable to raise any response from the aircraft.
Attempts to contact it, including an F-16 that rendezvoused with the aircraft, were unsuccessful. The F-16 pilot reported that the windows were covered by condensation or frost. The Lear continued in a northwesterly direction, climbing to a high of 46,000 feet, until it ran out of fuel and spiraled into a field in South Dakota. The two pilots and four passengers were all killed. Because the aircraft was destroyed the NTSB couldn’t determine the exact reason for this tragedy. To paraphrase NTSB’s accident report, they attributed the probable cause to pilot incapacitation due to lack of oxygen. In all likelihood, they died as a result of hypoxia.
Insidious
Most GA aircraft are unpressurized and don’t operate at the extreme altitudes as the Lear. Still, even if you don’t fly in the flight levels, and you don’t fly higher than a few thousand feet, hypoxia can affect you. In fact, if you live at sea level and visit the high country, like Denver, Colorado, you can be affected by hypoxia on the ground.
Ever feel unusually tired or sleepy or have a dull headache during or after a flight? Those are two of the most obvious hypoxic symptoms. Others are light-headedness, dizziness, increased breathing rate, poor coordination, euphoria, poor night vision, impaired judgment, and sweating.
Probably the most important aspect of hypoxia is something that you should consider before flight. Hypoxia is insidious. It can affect you without your being aware. And worse, the more you’re affected, the less likely you’ll realize it. That’s probably what happened to Payne Stewart’s pilots.
In addition, hypoxia affects people differently. Those that smoke, that aren’t physically active, are older, drink heavily, have colds or respiratory problems, or that live closer to sea level are all more likely to suffer the affects of hypoxia at lower altitudes and more strongly than people that don’t fit those criteria. It’s important to remember that everyone, even a young, non-smoking, super athlete that lives high in the mountains, will be affected by hypoxia. The issue isn’t if, but at what altitude its affects will make themselves known.
The not so obvious symptoms are also insidious and can affect the safety of flight without the pilot even being aware. The FAA reports that even healthy non-smokers lose fully 25 percent of their night vision by 8000 feet. Poor night vision often manifests itself throughout the full range of vision acuity or in extreme cases, it causes tunnel vision; the darkening of peripheral vision.
A second devious symptom may be, the most dangerous. As the blood/oxygen level decreases cognitive abilities also decrease. In other words, pilots don’t think or make decisions as well. Ever had problems doing a calculation that normally would have been easy? Ever just feel generally confused or euphoric? The cause may have been hypoxia. This cognitive impairment effects judgment and might be one reason that pilots sometimes make irrational decisions that leave others shaking their heads wondering why.Hypoxia and Supplemental O2
Oxygen composes about 21 percent of the air we breathe no matter what the altitude. The problem is that atmospheric pressure decreases with altitude and it’s pressure that determines how much oxygen is assimilated into the bloodstream. The higher the altitude, the lower the atmospheric pressure, the lower the number of molecules per cubic foot, and the less oxygen is available to be absorbed into the blood. For comparison, atmospheric pressure at 18,000 feet is half that at sea level.
The solution is to breathe a higher concentration of oxygen. You can measure both pulse and blood/oxygen level with a pulse/oxymeter, a small, relatively inexpensive device that clips to a finger. It should be used by anyone that might be exposed to hypoxia. A blood/oxygen level of at least 90 percent is generally considered minimum for flying. AEROX, a leading manufacturer of aircraft oxygen equipment feels 93 percent is a safer value. Depending upon your physiology and background you might want to use an even higher blood/oxygen minimum. You certainly shouldn’t fly with a lower percentage.
FAR Part 91.211 spells out the rules regarding oxygen requirements. The short version is that at cabin pressure altitudes above 12,500 feet MSL up to and including 14,000 feet MSL, the required minimum flight crew—usually just the pilot for a GA aircraft—should use supplemental oxygen for all portions of the flight between those altitudes for longer than 30 minutes. Any time the flight is above 14,000 feet the flight crew must breathe supplemental oxygen for the entire time. Passengers are only required to have supplemental oxygen above 15,000 feet.
Note that 91.211 refers to cabin pressure altitude. That’s ambient pressure for a non-pressurized aircraft and internal cabin altitude for a pressurized aircraft. Also, notice that 91.211 references MSL altitudes. If you think about the issue, it’s the actual atmospheric pressure that matters, not MSL. That means it’s density altitude in a non-pressurized aircraft and cabin altitude in a pressurized aircraft that you need to be cognizant of. On hotter days hypoxia can affect individuals, especially in non-pressurized aircraft, at much lower MSL altitudes than you might expect.
Wait a minute… we’ve already discussed the fact that the physiology of individuals is different. At 8000 feet comparing the blood/oxygen level of young healthy non-smoking triathlete living in Denver to an older overweight two pack-a-day smoker living in LA and you’ll typically see a substantial difference.
Individual Considerations
Given the potential individuals variations what’s the recommendation? Minimally, always meet the requirements of FAR 91.211. Also consider using the FAA’s recommendation to use oxygen above 8000 feet during the day and above 5000 feet at night. Then, be sure to take into account your personal history, physiology, and current health. You might want to start breathing oxygen even lower.
There are generally two methods for administering supplemental oxygen.
Aviators’ cannula are adaptations of medical cannula where a tube is worn on the face so each nostril receives a direct supply of oxygen. It should have a method of showing and regulating the oxygen flow. AEROX provides a product called Conserving Cannula that incorporates a small bladder to supply oxygen to the nose while it substantially increases the amount of time a given oxygen supply will last. I’ve been using their cannula in conjunction with a pulse/oxymeter to measure my blood/oxygen levels for 25 years and typically find the cannula comfortable and the oxymeter an absolute necessity.
My research turned up a number of publications stating that the FAA requires an oxygen mask instead of a cannula for use above FL180. But I couldn’t find actual regulations that substantiates that. I know of pilots that fly in the lower flight levels (e.g. one, up to FL220) that use cannula and a pulse/oxymeter to ensure their blood/oxygen remains above 90 percent.
The theory is that if the blood/oxygen remains at 90 percent or greater they feel cannula use is safe even in the flight levels. We’re not going to dispute that, but instead offer the following. First, AEROX strongly believes that cannula are only safe through FL180. Since they test and sell cannula, you should consider their opinion when making your own. Second, given the insidious nature of hypoxia and the shorter time of available consciousness at higher altitudes, the question you have to ask is: Is using cannula above FL180 worth the risk?
Masks come in two flavors: with or without a microphone. While they may look cool to some, my personal experience is that they can become uncomfortable during a longer flight. As a result, I prefer to remain below FL180 and use cannula.
Final Thoughts
Be aware that oxygen is an accelerant. If you combine oxygen, an oil based product, and a spark be prepared for a ferocious fire. We’re not trying to discourage you from using oxygen, on the contrary. Just be aware and make sure you turn the oxygen off when you’re not using it and don’t introduce a spark or flame in the cabin when using oxygen. In other words, NO SMOKING.
Because the time of useful consciousness decreases with altitude, a more effective applicator of supplemental oxygen like a mask becomes more critical at higher altitudes so in any event, it makes sense to use a mask above FL180.
In the years that I have been using cannula while monitoring my blood/oxygen levels I’ve learned that being a mouth breather isn’t conducive to acceptable blood/oxygen levels. If you’re a mouth breather like me, it’s a good habit to focus on closing your mouth and breathing through your nose. Not that long ago I was comfortably cruising at 17,000 feet when I noticed that my blood/oxygen level was in the low 80% levels. Closing my mouth and breathing through my nose increased the percentage about 5 percent within a minute or so. Then, taking a number of very deep breaths increased it to above 90 percent. I always try to fly with my mouth closed and take continuous slow deep breaths to keep my oxygen level high.
The lack of supplemental oxygen at night doesn’t particularly affect my night vision, but a pilot friend whose physiology doesn’t seem that much different from mine has substantially worse night vision. The use of oxygen equals us out in terms of our nighttime visual acuity.
Even if you haven’t been flying at an altitude where you think you need supplemental oxygen I’ve found that at the end of a long flight a few minutes of deep breathing oxygen before arriving at my destination reinvigorates, helps increase concentration, and makes everything seem easier. In fact, the use of oxygen can help keep you sharper throughout every phase of flight.
Joe Shelton is a commercial, instrument, multi-engine pilot who flies a Columbia 400 from a coastal California airport.
This article originally appeared in the August 2013 issue of IFR Refresher.
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