What to do when the air runs out

Aviation International News » September 2007
August 30, 2007, 6:32 AM

The good news about the proliferation of new high-altitude airplanes–turbocharged piston or turbine–is that they offer users the chance to experience the increased efficiency of an engine that likes flying where the air is thin.

The downside of airplanes that perform well at high altitudes is that although turbine powerplants don’t need much air to perform at their peak, the pilots flying them still do. Altitudes as low as 10,000 feet–even 5,000 feet msl for people who smoke or who fly at night–can change how effectively the human body processes ambient oxygen. Each thousand feet a body rises above sea level makes brain functioning more difficult.

The overall ratio of oxygen to nitrogen in the air remains constant at about 21 percent oxygen to 78 percent nitrogen. The total air pressure at FL180 is about half that at sea level, which means an average person’s lungs must work much harder to take in enough air to keep the body functioning normally. Reduced air pressure at high altitudes, coupled with the inefficiency of human lungs to process the gas, makes for a deadly combination that has been known to cause a severe reduction in a pilot’s cognitive functioning. Above FL410, for example, a pilot’s time of useful consciousness is only a few seconds without a pressure-fed oxygen system.

Most pilots believe they can handle an oxygen emergency. Overall, the chances of escaping a rapid decompression are actually better than surviving a slow leak because an explosive event gives the crew a clear wakeup call. With a leak in the cabin, on the other hand, the reduction in air pressure happens slowly, so pilots might not notice that they are exhibiting symptoms of hypoxia, such as loss of comprehension and drowsiness.

The most notorious oxygen deprivation crashes were the 1999 Payne Stewart accident and the 2005 Helios Airlines Boeing 737 event. In each, the crew lost control of the airplane because of a loss of cabin oxygen. Investigators in the Stewart crash attributed the accident to an oxygen valve that had been turned off inadvertently. In the Helios crash, investigators determined that the crew confused the aircraft’s altitude horn with the stall warning.

Any pilot who has been through an initial or recurrent simulator at any of the big training providers well remembers that part involving high-altitude emergencies that quickly evolve into rapid descents back to 10,000 feet, where oxygen concerns are manageable.

The challenge with high-altitude simulator training is that the only method that exists to replicate explosive decompression is an instructor slamming a book on the table behind the flying pilot and yelling, “A window just blew out,” or something similar to frighten us into swift action. Because the pilot knows the problem is coming, he dutifully pulls the power levers back, wraps the airplane into a 30-degree bank, drops the speed brakes and waits for it to descend through 10,000 feet. None of this is terribly frightening or particularly realistic.

Thousands of pilots–certainly all who were military aviators– have experienced oxygen deprivation within the confines of an altitude chamber. FlightSafety shows all hypoxia training students videos of pilots in the chamber. What the videos demonstrate is that someone who looks normal–and believes himself to be operating at full mental capacity–is actually experiencing problems. For example, in one altitude-chamber episode a U.S. Air Force pilot was taken to 25,000 feet and asked to remove his mask. He looked fine as the instructor had him use hand signals to indicate climbs or descents related to the instructor’s orders. However, at one point the instructor told the pilot, “Pull up. You’re diving. Pull up.” The pilot looked at the instructor but was completely unable to react. “You’re going to die, sir. Pull up,” the instructor yelled. The pilot simply did not react.

Just Keep Breathing

AIN had an opportunity to attend a shortened version of FlightSafety International’s hypoxia awareness training at the DFW Learning Center. A portion of the training is conducted in the classroom, where instructors take students through a two-hour review of high-altitude physics and human physiology. Topics include gas laws, types of hypoxia, evolved gas disorders and related FAA regulations. Flight instructor Kaye Angiel, a Falcon 10 pilot when not instructing at FSI, explained that hypoxia often begins slowly but eventually expands to three phases of performance degradation.

The mental effects are sometimes hard to notice; first there is the slowing of response, followed by the onset of decreased mental acuity, leading to the individual making more errors, and finally the apathy that precedes total loss of consciousness. The physical reactions–rapid increase in respiration, mental confusion or tingling of the skin–are more apparent to some.

The real problem with a hypoxia encounter is that if it occurs, both pilots could be affected, making it impossible for either to control the aircraft. In addition, all the symptoms of hypoxia can creep up on the pilots and passengers slowly enough to be almost unnoticeable.

FSI’s hypoxia training uses a full-motion simulator working in conjunction with a mixed gas machine to allow pilots to experience the sensations of oxygen deprivation and get a first-hand look at the degradation in performance that accompanies it. The system was designed in cooperation with the Mayo Clinic in Rochester, Minn., and based upon U.S. Navy design concepts.

The system safely alters the oxygen/nitrogen mix the pilots receive in their masks to simulate high-altitude conditions. At an indicated 22,000 feet the pilots can be made to feel as though they are receiving only 8 percent of their normal oxygen flow. In addition to learning how oxygen deprivation feels, participants come to recognize how hypoxia affects them personally and learn how to take appropriate corrective actions in time.

Lack of Oxygen Slows Us Down
My training took place in an ERJ 145 simulator that had the related hypoxia training equipment installed. Since I was attending without a flying partner, I joined a class that included two other corporate pilots we’ll call Phil and Travis, who were attending the full session on behalf of their common employer. The system uses a computer feed to the instructor station designed to make constant monitoring of the student easy for the instructor through a standard pulse/oxygen sensor attached to the flying pilot’s fingertip.

Only one pilot is allowed to fly using the oxygen-deprivation system at a time. Travis volunteered to go first. Phil sat in the right seat while I sat in the jump seat to observe Angiel’s monitor.

Before she fired up the RJ, Angiel explained what we would be seeing and experiencing. After the first takeoff, Travis would hand-fly the airplane to about 22,000 feet, where he would perform a number of simple mathematical calculations, in this case subtracting seven from 1,000 and successively from the remainder, “1,000, 993, 986” and so on. The exercise would demonstrate how difficult it would be for him to find the answer. It would also give Travis insight into his reaction to the oxygen loss.

After ensuring that the plastic mask from the mixed-gas simulator was attached snugly to Travis’ face, we launched for FL220, where Travis punched the autopilot. Then he began the calculations backward from 1,000. When he reached approximately 970, Angiel said Travis’s body was reacting as if he were in an airplane flying at 22,000 feet with no oxygen mask. That should have meant increased heart rate as his body worked to draw in more oxygen, or possibly a tingling sensation on the surface of his skin. Neither Phil nor I noted any difference in his behavior. Angiel asked him how he felt and he reported no initial signs for concern, such as tingling of the skin or perspiration. Angiel decided it was time to bring him down and figure out the problem.

Once back at sea level, Angiel asked Travis to keep the mask on if he felt OK. Phil began asking him a few questions, and we noticed that Travis was not responding much. “Hey. You OK?” Phil asked. Travis looked at his partner but said nothing. A moment later his head slumped toward the outside of the fuselage and leaned against the window. Angiel pulled the mask off his face; he was out cold some two or three minutes after the monitor showed his body should have thought it was back on terra firma.

About 30 seconds elapsed before Travis came to, asking what had happened. He had no recollection of passing out or even of the few minutes before. Angiel said she’d never had a pilot pass out after returning to sea level. It was a powerful demonstration.

My Turn at the Controls
After Phil and Travis left, I buckled into the left seat of the RJ as Angiel explained that I would be flying the same profile as my predecessor. For me, doing that math would probably be a challenge at sea level. Once level at 22,000 feet with the autopilot on, I began the calculations as Angiel changed the oxygen mix. About a minute or so into the process I was passing 972, and I began to feel a tingling sensation in my skin, as well as some dizziness. It was not a major distraction and I attributed it to nerves.

Angiel asked me to draw a clock face on the paper in my lap. The circle was not quite round and I was having trouble putting the numbers in the correct places. Nonetheless, I thought I felt pretty good. Then came “959” in the calculations. I was feeling lightheaded, and I said something to Angiel, who started the mixer machine back down to sea level.

After I was “on the ground” for a few minutes, we talked about the experience. At first, I thought I was fine–just slow with the arithmetic, which I had suspected would be the case before we even started. Angiel helped me understand that what I was sensing was the beginning of hypoxia. She also said I had been moving my head as if reacting to some unknown stimulus.

For the next part of the flight we returned to 22,000 feet to try maneuvering the airplane, to gauge my reactions to varying levels of oxygen.

We tried some turns, Dutch rolls, tuning a VOR frequency and holding a normal conversation. About three minutes in, I thought I felt my skin tingle once again and knew Angiel might be trying to catch me off guard.

She told me to try some more steep turns. I wrapped the airplane into a 45-degree bank, which turned into about a 60-degree bank as I quickly corrected. I looked at the EFIS and began to think the colors no longer looked as brilliant. Once again, I thought I was overreacting.

Some four minutes into the session, I knew something was wrong, but I didn’t know what. My eyes were wandering from left to right around the cockpit, not really focusing on anything; luckily I caught sight of the illuminated red “cabin altitude” light.

We were climbing through FL185 as I confirmed the problem. The cabin had begun to depressurize and the aural alarm had failed to sound. (Angiel had pulled the circuit breaker.) She told me later that during this time I mumbled “cabin altitude” a few times but took little action. She also had a different take on my reaction time; whereas I thought I had recognized the problem quickly, from her perspective it took me much longer to develop and implement the solution.

I saw the altitude problem but simply didn’t do anything about it. “Oh. I have to get down,” I finally called out. I pulled back the throttles and pulled the speed brakes. I removed my gas mixer mask and grabbed the quick don, which was the signal Angiel was waiting for. My head began to clear within 20 seconds of breathing a more normal nitrogen/oxygen mix. By the time we reached 11,000 feet, I felt back in control and landed the RJ without incident.

Every pilot regularly flying above 12,000 should take a hypoxia-training course as part of an initial or recurrent training session. Talk is cheap. Watching videos of people in an altitude chamber can be eye-opening, but some might walk away from the experience thinking they can cope when their time comes. You can’t.

Trying to fly an airplane while your brain takes a siesta is the only way to realize that hypoxia training is not just another sign-off on a training sheet.

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