A pilot discovers, close up, an insidious killer
We were at 35,000 feet, and student Number 5 had been without his oxygen mask for less than a minute. Number 5 was in trouble, but he didn't know it. The instructor tried to get student Number 5's attention. "Gang load your regulator, Number 5. Put your oxygen mask on. Number 5, gang load your regulator and put your oxygen mask to your face." Student Number 5's eyes were open, but he wasn't responding. His skin was the color of someone who had been embalmed. His hands shook, and he could no longer hold on to anything.
He was gasping convulsively and was unable to speak. The instructor gang loaded Number 5's regulator (all regulator switches to the up position in one motion) and placed the oxygen mask to his face. One hundred percent under high pressure revived him, although he remained shaky for several more minutes. Student Number 5 had little recollection of what he had just experienced. We descended to 30,000 feet, and student Number 13 was instructed to remove his oxygen mask. He was fine for a minute or so but complained of feeling very tried. Suddenly, he slumped in his seat.
Only after he heard the instructor's voice coming through the headphones was he able to rouse himself enough to get his oxygen mask back on without assistance. After a day and a half of classroom instruction on various topics related to aerospace physiology at Fairchild Air Force Base (AFB), Spokane, Washington, our training culminated in actually experiencing how hypoxia can affect not only those of us who
fly but those who fly with us. Now, after watching demonstrations, it was our turn. The chamber pressure was adjusted to 25,000 feet, and the rest of us (10 Air Force personnel and five-civilian employees from the Boeing Company) were instructed to turn off our regulators and remove our oxygen masks. We were given work sheets with math and word problems and a puzzle maze to solve.
As we completed our work sheets, we were to record how we were feeling. The purpose was not to see how long we could last without pressurized oxygen but to be able to identify two or three symptoms of hypoxia in ourselves and then take appropriate action by gang loading our regulators and replacing our oxygen masks.
For some students in our class, the symptoms of hypoxia were quite dramatic - hot and cold flashes, tingling in the hands and feet, blue fingernails, nausea, headache. My symptoms were subtle: a slight headache, a slight sensation of just not feeling well, a little tingling in the feet and legs-symptoms I would have not recognized without this training.
As pilots recall from ground school, the air we breathe is made up of 21 percent oxygen, 78 percent nitrogen and one percent other gases. As altitude increases, this mixture of gases remains the same, but atmospheric pressure decreases. Our lungs need a certain amount of atmospheric pressure for oxygen to be absorbed, and at altitudes above 10,000 feet, the pressure is not sufficient for this to take place, depriving the brain and other tissues of adequate oxygen, which leads to the condition known as hypoxia.
Each person's tolerance and reaction to hypoxia is different, but the end result is the same. As the amount of oxygen to the tissues decreases, you will remain conscious for a short time but will become less and less functional. Unless you correct the situation by breathing pressurized oxygen, if it is available, or by descending to a lower altitude, the hypoxia will lead to unconsciousness and eventually, death.
Depending on altitude, once you start becoming hypoxic, you have only a limited amount of time to correct the situation before useful function is lost. This period is referred to as "Time of Useful Consciousness (TUC). As you can see by the chart above, you may have several minutes to recognize your symptoms and take corrective action-or you may have only seconds.
In a rapid decompression situation such as might be experienced during a catastrophic event on an airliner, the times of useful consciousness, as shown on the chart, are cut in half. So, for instance, if you were to experience rapid decompression at 40,000 feet, rather than having 15 to 20 seconds to react, you would have only from 7.5 to 10 seconds of useful consciousness. This is why you are always briefed to get your own oxygen mask on before taking care of your children. If you help someone else first, you may not have time to help yourself.
Because the body functions quite nicely from sea level to 10,000 feet, pilots are not required to have pressurized oxygen available for flights at this altitude or below. However, some tissues, like those of the eye, are very susceptible to any decrease in available oxygen-just how susceptible would become graphically apparent during the night vision portion of our training.
After spending some time at ground level with lights off and oxygen masks removed, we went up to 10,000 feet. After a short time at this altitude, we were each handed a large color wheel. We were able to see the various segments on the color wheel, but the colors themselves were nearly indistinguishable, appearing only as shades of gray.
Upon command, we donned our oxygen masks. Two or three breaths of oxygen worked like magic! The colors became vividly apparent along with detail many of us had previously been unable to see. It was an impressive demonstration of how flight at 10,000 feet adversely affected our night vision even after a short amount of time.
One of the most dangerous aspects of hypoxia is that the onset of symptoms may be very subtle and may not cause discomfort. For some individuals, the symptoms can even be pleasant. If the onset of hypoxia is slow, the symptoms may be well developed before you recognize them, and by then, it may be too late to help yourself.
One of the main reasons pilots attend aerospace physiology training is to be able to experience their own symptoms of hypoxia and to witness the symptoms in others in the controlled environment of an altitude chamber.
CAMI's Aeromedical Education Division offers a 1-day aviation physiology course for FAA flight crews, civil aviation pilots, and FAA aviation medical examiners (AMEs). In addition to the basic academic contents, this course offers practical demonstrations of rapid decompression (8 to 18K feet) and hypoxia (25K feet) in a hypobaric chamber. This course is in Oklahoma City, Oklahoma and is free. To schedule you must call (405) 954-7767 / 4837. Class starts at 8 AM and is finished around 3:30 PM.
A similar aviation physiology course is offered to civil aviation pilots at USAF physiological training units across the U.S. under the USAF/FAA Physiological Training Agreement. This program has been very successful and is a good example of how government organizations can collaborate to promote safety in civil aviation. To be scheduled you must call (405) 954-6209/4837. The cost is $35 dollars.
To participate in the altitude chamber you must be 18 years of age or older, no beards, and have a current class 3 or better flight physical.
CAMI's Aeromedical Education Division also offers a survival course
for general aviation pilots at Oklahoma City, Oklahoma. This is a 8-hour introductory course that will provide the basic knowledge
and skills for coping with various common survival scenarios. This course
will teach students how to easily assemble and use a personal survival
kit. The course is free and to schedule call (405) 954-6207/4837.
By Carol Colleen
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