By lex, on February 16th, 2004
As you ascend in altitude, whether as a mountain climber or a fighter pilot, the atmospheric partial pressure decreases – the proportion of oxygen in the air you breathe remains constant, but the number of molecules are reduced.
In addition to the quadrennial water survival qualifications *, touched on here , there is also the requirement to attend aviation physiology training. Part of this is exposure to a hypobaric chamber. Exposure to a hypoxic environment is designed to teach aviators their “personal” hypoxia symptoms, which vary from individual to individual. You always share these rides with several other folks.
Routinely, a surgical glove is tied off inside the chamber when you’re pressurized to sea level. As the atmosphere is evacuated during your ascent, the glove will expand, graphically depicting the change in partial pressure – since the air within the sealed glove is still at sea level, the gases within expand in proportion to the ambient atmospheric pressure.
Something similar will be going on inside your personal anatomy – gases in solution, or suppressed by the normal atmospheric pressure, will leave solution, and find the most convenient aperture to equalize the imbalance. All this will come to a head about the same time you will be asked to take off your oxygen mask, and breathe deep of the now attenuated air that your chamber mates have made positively noxious through their personal contribution to the hypobaric leveling process.
The good news is that once you start back down again, the increase in barometric pressure will ensure that any internal gasses that were not voided, are expressed through the increase in atmospheric pressure upon your diaphragm. It almost doesn’t seem fair that you get hit coming and going, but there it is.
While at altitude, and with your O2 mask off, you’ll be variously tasked to do some simple math problems, write notes or play “paddy-cake.” The first few times through the trainer, the average naval aviator will try to show how tough he is by waiting until the last possible moment to bail out of the test, and put his mask back on. After all, everything is a competition, isn’t it?
Some of course will overshoot, and fall senseless to the floor, while specially trained hospital corpsmen stand by, waiting to place the oxygen masks back on their now insensate forms. I suspect this sort of thing is the subject of much jocosity in the corpsman locker room, after the ride.
It’s good training though – and I learned that the first thing that happens to me when I get hypoxic is that I lose color perception (the cones in your eyes are very sensitive to oxygen deprivation), I feel a buzzing in my cheeks, and a heaviness in my head. I get foggy.
I was flying from Meridian, Mississippi to San Diego as a student in a T-2 Buckeye, our basic jet trainer back in the 80′s. My instructor in the front cockpit filed our flight plan, which took us through Davis-Monthan Air Force Base, in Tucson, Arizona, elevation 4000 feet above mean sea level. We stopped for gas before our last leg to San Diego.
Now, the T-2 was not what one would call a long-legged jet. Five hundred miles on a bag of gas was as much as one could hope for. In fact, I learned just how big Texas was, flying T-2′s from Meridian to San Diego. You stopped twice for gas along the way, both times in Texas.
But our route had taken us a different path this time, through Arizona. We would need to stay at altitude for the final leg to California, since jet engines are more efficient at higher altitude (for the same reason as the partial pressure discussion above: the absolute ratio of fuel to air in combustion is constant, but since the air is thinner, the gas can be leaned out as well).
We got our gas, climbed in, and attempted to crank the engines – they wouldn’t light, and eventually we figured out that the higher elevation at Tucson, in combination with the Air Force fuel (of a different flash point than Navy gas) was preventing the igniters from sustaining ignition.
Fortunately, we knew where the fuel control unit was on the T-2, and after opening up the engine bay, rotated the knob to reflect the USAF fuel requirements. We didn’t bother to get out of our flight gear doing so, with the result that I laid upon, and cracked, my oxygen mask, without realizing it.
Once at altitude, the O2 bleeding past the crack in my mask became an issue, and it became clear watching the usage rates that we would not have enough O2 to carry us all the way to the west coast. We couldn’t descend to a lower altitude, since the fuel utilization would rise unacceptably, there were no acceptable diverts between us and San Diego, and so we had to remain at altitude.
My instructor came up with a bright idea: we would take turns on the oxygen. When one of us got to feeling hypoxic, he’d put the mask on, and the other would take his off. In this fashion, we’d make it safely all the way.
What could possibly go wrong?
We’d been airborne for a bit more than an hour or so, and I’m off the O2, feeling fine. Feeling really good, in fact. WHEE! But there’s a buzzing in my cheeks, my head feels funny, and the colors are starting to grow a little dim. S’OK, though – probably because it’s dark outside!
My instructor asks me to figure out our remaining time to San Diego, as well as our fuel on deck upon arrival. I did some back of the envelope math, and came up with, “Two and a half hours, and we’ll have 5000 pounds of gas on arrival.” There was a long, pregnant pause from the front seat. Problem was, we’d been airborne for an hour and fifteen minutes, and the longest we could have stayed aloft was about 2.5 hours. I’d just calculated a time on deck that was 75 minutes longer than we’d have gas. Oh, and by the way, the T-2 carried about 4000 pounds total. My hypoxic math had just magically transferred another 1000 pounds over capacity into the aircraft, after we’d already used up about 2000 pounds.
“Time for you to put the mask back on,” my IP said gravely. “I’ll go back to cabin atmosphere.” When I put the mask back on, I was amazed at how quickly the world came back into focus again – how the instrument panel lights were red – actually red! Where they had been shades of grey only moments before. I reworked the math on our arrival time and fuel on board, and passed that to my IP. Who was sounding very, very sleepy. Fortunately, we were closing in on the coast, and started our let down, just at the moment when I had become concerned that he would never answer up to my queries on the intercom, and I’d have to land from the back seat with an unconscious instructor in the front. It all worked out for the best though. We landed, and slapped high fives, went for a beer and learned about flying from that. And in later years, when the hypobaric chamber corpsmen would try to test us to see how long we could stay off the O2 in the chamber before passing out, I’d disappoint them by taking my mask off, breathing deeply once or twice, and putting the mask back on again.
Let the younger guys test their limits – I know mine.
** 05-12-20 Link added – Ed.