Hypoxia: Sneaky assassin


Considering the alternatives, hypoxia is probably quite a pleasant way to die. When the brain isn’t getting enough oxygen, there’s often an onset of euphoria, a gradual decline in visual and mental sharpness, and increasing drowsiness, then just a gentle fade to black.

The thing about hypoxia is that its symptoms are so insidious that pilots may not know anything is wrong until it’s too late. Aside from some high-profile cases of obvious pressurization system failure, notably the Payne Stewart Lear Jet 35 crash in 1999 and the Helios crash in Greece in 2005, experts are reluctant to speculate just how many fatal accidents could possibly be blamed on hypoxia.

Those two dramatic cases, although they involved sophisticated systems and professional pilots, offer a wake-up call to the rest of us in aviation. National Transportation Safety Board (NTSB) Senior Investigator Robert Benzon, who has been involved in some of the highest-profile aircraft accident investigations since the agency’s earliest days, pointed out that there’s a real lesson to be learned from what he calls “the Flying Dutchman accidents.”

Referring to the ghost ship of legend, he suspected that the occupants of both airplanes were already gone, hundreds of miles before the jets ran out of fuel and crashed. Controllers knew disaster was inevitable and were helpless. According to Benzon, the pilots surely tried to trouble-shoot, and lost consciousness before they could locate the malfunction. His lesson is this: “If you even suspect something is wrong with the pressurization system, the first step is to put on oxygen. Those pilots didn’t, and 127 people died.”

Although civilian pilots are not required to don oxygen masks until 12,500 feet and higher, an altitude easily attained by modern GA airplanes, many healthy people exhibit a decline in their decision-making skills, manual dexterity, and mental acuity by 10,000 feet. Disorientation can sneak up rapidly, and it may be hard to decide what’s wrong. By 18,000 feet, an altitude that requires oxygen masks, not nasal cannulas, the time of useful consciousness may be 15 or 20 minutes. At 25,000 feet, it’s more like four minutes.

Night vision begins to deteriorate due to hypoxia at 5,000 feet. Any pilot who flies higher than that at night can attest that a few minutes on oxygen before descending to the destination airport will dramatically “turn up the lights” on the ground.

Individuals vary in their sensitivity to hypoxia, and it also depends on a person’s physical fitness, age, weight, tobacco exposure, fatigue, cockpit temperature, and stress.

I fly a turbocharged Mooney, and from time to time find myself using oxygen at altitudes where the typical time of useful consciousness, should something go wrong, isn’t very long. How soon would I notice the effects of altitude without supplemental oxygen? Hoping to find out, I made an appointment with the altitude training experts at Langley AFB in Hampton Roads, Virginia.

462 I took my seat in a classroom with 15 military personnel and government contract pilots, as a succession of Air Force instructors covered aviation physiology, the quickie intro course. There was lots of good background information, much of which was familiar to any diver. We were reintroduced to the gas law guys and the dangers of decompression sickness, and the disabling pain of trapped gases expanding in blocked body cavities, like ears and sinuses. We covered visual acuity issues and dark adaptation. Good stuff to know.

Then it was time for the pressure chamber experience. Unfortunately, while I was welcome as a journalist on assignment to sit through the lectures and observe the pressure chamber in operation, I wasn’t permitted to actually experience the low pressure environment…something about the FAA’s agreement with the Air Force having not been renewed.

That was disappointing, but perhaps even better, I got to watch through the pressure chamber windows, undistracted by my own body’s reactions, as my classmates discovered their limits.

482 There was a thorough briefing, a practice run at ambient pressure. The participants donned their oxygen masks, checked their systems, and hit the switches to begin the oxygen flow. They removed the masks, reset the switches, and then it was time for the “real thing.” The airman in charge of the pressure chamber’s operation counted air pressure levels in thousands of feet, as sea-level air was sucked out of the chamber. At 25,000 feet pressure altitude, the students removed their oxygen masks. An instructor inside the chamber and her two assistants monitored the chamber’s occupants for signs of discomfort or incapacitation.

524 The participants were invited to pick up a clipboard and begin filling in answers on a simple worksheet. They worked quickly and confidently, at first. Soon their concentration seemed to wane, and one or two, complaining of dizziness, reached for their masks. Some yawned, looked around, began to breathe more rapidly and fidget. Was it getting cold in there? More masks went on. One man said he felt his visual field narrowing, and reached for his mask, fumbling with the equipment.

When asked what he was feeling, another guy, still holding out, said, “I’m suddenly really hungry,” eliciting a general laugh. He shook his head to clear the cobwebs, and strapped his mask on.

Finally there was only one left, a young woman, who had run into a snag on her worksheet. As she squinted at the question, trying to focus, an airman asked, “How are you doing?”


"I feel good! Really good!"

The woman looked up, and with a sunny smile, said, “Just fine. I feel good. Really good!” The seconds ticked by as she struggled with the basic arithmetic problem, obviously unable to collect her thoughts. She resisted the suggestion that maybe it was time to put on the mask, insisting with a giggle that all was well.

Finally, the airman tapped her on the shoulder and, addressing her by name, said, “It’s time. Put on your mask now. Anybody who passes out flunks the course.”

“See?” offered Benzon. “She was going to hold out, be a hero, she was going to win. But she was probably 15 to 30 seconds from being unconscious.”

In the controlled environment of the pressure chamber, with subjects who are prepared and primed for the experience, knowing exactly what to expect and when, who have just moments earlier drilled the appropriate actions they must take, few people lose consciousness. In a cockpit, dealing with stress, fatigue and distractions, intent on troubleshooting some blinking light, and not expecting hypoxia, not recognizing euphoria or noticing any particular symptoms, the outcome can be catastrophic.

How many pilots flew into a mountainside because, a little short on oxygen saturation, they dozed off? How many VFR pilots flying at night didn’t see the clouds until they were in them and became hopelessly disoriented, too dazed to interpret the instruments? How many became anxious about an increasingly annoying headache, and began to hyperventilate until they fainted? How many felt so happy and confident that the idea of needing oxygen didn’t occur to them? There’s probably no way of knowing, but certainly some very competent high-time pilots have died in crashes that otherwise defy understanding.


Just before reaching 25,000 feet — note the high-tech baro device hanging from the ceiling. The balloon was fist-sized at sea level.

Nathanial Daggs, who runs the altitude chamber at Arizona State University, said he’s had several graduates of his program call to thank him. Thanks to their training experience with hypoxia, they caught cabin pressurization problems by recognizing their own symptoms, putting on their oxygen masks, and beginning an emergency descent. One told Daggs he believed, in retrospect, that he and his captain were both about to lose consciousness. “Pressure system malfunctions happen every year,” Daggs said. “If you’ve had the training, you’re more likely to know what the problem is and what to do next.”

Accident investigator Benzon noted, “The vast majority of pilots will never experience real hypoxia.” It happens though, and without education, “pilots might think they can beat the laws of physiology.” That’s a bet, he said, that overconfident aviators will lose.

How to minimize your hypoxia risks

If you’re planning a flight where oxygen will be required, always use your checklist to make sure your oxygen supply and pressurization systems are properly set, that you have enough O2 for all passengers and crew to complete the flight, and that your oxygen bottle is full of oxygen. “I’ve seen oxygen bottles filled with breathing air,” said Benzon. “That’s not good at high altitudes, and you wouldn’t even know it.”

En route, check your connectors, tubing, and flow meters frequently. Connectors pop out, delicate plastic tubes kink, and you may not notice. As Benzon repeatedly said, “hypoxia is a killer because it’s so subtle.”

At night, if you find yourself turning up the panel lights or if you notice lights on the ground growing dim, it may be time to plug in the nose-hose. A little supplemental O2 can be dramatic — like somebody turned up the rheostat.

At the first indication of any type of pressure-system problem or oxygen system difficulty, get lower immediately. Tell ATC what you’re doing. They’ll clear the way for you. If you have a system with an oxygen mask, get it on immediately, and troubleshoot after you’re established on the way down, once you have a clear head. Your time of useful consciousness in the flight levels could be as little as three minutes.

Help beat hypoxia

A number of issues make us more susceptible to hypoxia, and the Air Force offers some measures to counteract them.

  1. Get more exercise. Both anaerobic exercise like weight-lifting and aerobic exercise like walking and running increase your body’s ability to use the oxygen it has more effectively.
  2. Get enough rest. Fatigue is a significant contributor to hypoxia. Sleep well before a flight, avoid alcohol and caffeine at bedtime, and take naps when you can.
  3. Minimize stress. Mental, emotional, and physical strain increases the effects of hypoxia on the body.
  4. Avoid or dress for possible temperature extremes. If you’re too cold, your body will decrease blood flow to your extremities, including your head. On the other hand, heat stroke isn’t conducive to successful flight outcomes, either.

Try it yourself

Want to see what your own reaction to hypoxia is like? The FAA highly recommends altitude training for civilian pilots, and offers the sessions free of charge at the Mike Monroney Aeronautical Center in Oklahoma City. Appointments are scheduled in order of voice-mail requests, beginning on the first day of each month. Registration information can be found here: FAA.gov

While no U.S. military installations offer the service to civilians at this time, several university flight departments and private institutions offer altitude training and hypoxia simulation. While most have traditional pressure chambers, some use a normobariatric approach, where oxygen scrubbers remove oxygen from the chamber’s atmosphere, and some offer real-world high altitude training in the school’s aircraft. The prices for the basic courses run about $500.

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