High-flight hazards

October 1, 2001

Bruce Landsberg has served as executive director of the AOPA Air Safety Foundation since 1992.

Before you knock the lowly catfish, remember that we are all bottom dwellers. The fish lives at the bottom of a river and we live at the bottom of an ocean of air known as the atmosphere. When taken from the bottom of our ocean we suffer the same fate as the fish removed from water. Aircraft can take us out of the livable portion of the atmosphere quickly. At high altitudes without oxygen or pressurization the outcome is obvious, but in the transition levels the onset of hypoxia is a bit more subtle.

The loss of a Learjet 35 with golfer Payne Stewart on board was a strong reminder to everyone about the importance of oxygen. Briefly, the Learjet was cleared to Flight Level 390 and the first officer acknowledged as the flight was passing 23,200 feet. Air traffic control attempted contact six minutes and 20 seconds later as the flight passed FL360. There was no response. The Learjet climbed to 48,000 and, despite numerous attempts to contact the crew—and a fighter intercept to check out the situation—the flight continued in a straight line for four hours. After fuel exhaustion, the Learjet crashed into a South Dakota field. The crew had become incapacitated because of some pressurization problem, but the exact reason is still unknown.

As you will see later, this time period was more than adequate for the crew to become incapacitated. This accident bears great similarity to the loss of a Cessna Conquest II that disappeared over the Atlantic Ocean with a Louisiana college football coach after departing from Texas in the late 1970s. The aircraft was not recovered and no probable cause was ever determined.

With many light aircraft such—as turbocharged Mooneys, Beech Bonanzas, Socata TBM 700s, Cessna T182s and Centurions, Piper Malibus, and Turbo Arrows, and all manner of twins—capable of high flight pilots need more than a cursory knowledge of high-altitude physiology. But, many of us have flown at high altitude, so what's the big deal?

At sea level, oxygen makes up 21 percent of the atmosphere, at a pressure of 14.7 pounds per square inch. Our lungs transfer oxygen to the blood, which carries it to the brain, which is addicted to the stuff. At 20,000 feet, the percentage of outside oxygen is exactly the same but the pressure is about half of the sea-level value. The body's internal oxygen pressure is higher than the outside pressure, and so the oxygen transfer is reversed. The brain begins to object—strenuously. Nasty things occur, such as the inability to reason, loss of motor skills, unconsciousness—and, ultimately, death—so pilots should have more than a casual interest.

The fundamental issue is how to increase the amount of oxygen to the lungs, to the blood, and then into the brain. There are only two ways to increase the amount of oxygen transfer: increase the percentage of oxygen or increase the pressure to force more oxygen into the body's tissues. The regulations require pilots to start using oxygen at 12,500 feet if they are going to be above that altitude for more than 30 minutes, and at all times above 14,000 (see " Pilot Counsel: Supplemental Oxygen," June Pilot).

The simplest method is a continuous-flow system where oxygen from a tank is supplied through a cannula, a tube that fits under the nostrils, or through a mask. One disadvantage of the continuous-flow system is that it leaks oxygen at a fixed rate, whether the pilot needs it or not, and thus tends to be somewhat wasteful. However, it is simple and inexpensive. Cannulas are fine up to FL180, and then the rules require a mask. As one goes higher and the outside pressure drops, the continuous flow systems, while delivering more oxygen, can't push enough into the blood so a pressure demand system, using a more sophisticated mask and delivery system, is needed.

The most elegant solution, and naturally the most expensive, is to pressurize the airplane and, by packing more air into the cabin, the pressure is increased enough to force more oxygen into the lungs. Many of today's jets can maintain a cabin altitude of 8,000 feet at FL410. Life is good until the pressure system fails. Then that comfortable shirt-sleeve environment can become hostile in moments with a fast leak, or insidiously if the leak is slow.

I had read pamphlets and magazine articles about hypoxia but had never been through an altitude chamber. The University of North Dakota's Aerospace Foundation offers a two-day corporate physiology course, and last February seemed like a great time to visit Grand Forks. The ground school includes the theory behind why all this is important, as well as the human physiology that hinders pilots at altitude.

The ground training covers gas and how it equalizes itself inside and outside the body. There are four places where gas can be an issue: the middle ear, the sinuses, the teeth (a poorly filled cavity or an abscess), and the gastrointestinal tract. Consuming carbonated beverages or a Mexican dinner with refried beans may cause significant pain in an unpressurized environment if one is unable to equalize it through obvious, if impolite, measures. At 25,000 feet, gases expand to three times their volume at sea level. This is why an unopened potato-chip bag inflates as the aircraft climbs. It is also why one should never open a sealed thermos abruptly, even at a relatively low altitude. It is a mistake most pilots only make once. Loosen the cap to allow the pressure to equalize before takeoff.

The altitude chamber itself is the size of a small tractor-trailer with a few thick windows installed for instructors and onlookers. Myriad large pipes suck air into and out of the box while oxygen lines feed the 16 training stations inside. Benches line the walls, with oxygen masks at each station, and there's an outside control console where the operator can pump the chamber up or down. Each mask is equipped with a cloth helmet headset and a microphone so everyone can communicate.

The initial chamber ride consists of three parts: a sinus check, the hypoxic experience at 25,000 feet, and a second shot without oxygen down at 18,000. The sinus check is an important part of the process, as anyone who has ever flown with a bad head cold will verify. On the way up, as pressure is decreasing outside the body, the higher pressures inside may not readily equalize. Air trapped inside the sinuses or middle ear may not get out and your head feels like it might explode. Reverse the problem on the way back down—substitute vise for explosion. If you can't open the equalizing Eustachian tubes through the Valsalva maneuver (pinching the nose and blowing out with the mouth closed) a phenomenal headache may result, and it can be debilitating. For the chamber flight it could be a bad trip to pop up to FL250 and then back down with a clogged head, so they always check.

Most of us have had exposure to low-grade hypoxia after a day of flying at 10,000 to 11,000 feet. My symptoms are a dull headache and fatigue. I believe that a significant number of unexplained general aviation accidents, where a normally capable pilot either exercises poor weather judgment or is unable to fly an instrument approach, may be attributable to some hypoxic degradation. It can be subtle. A friend sent me a pulse oximeter, which is a device that measures the oxygen content in the blood by clipping it to a finger. A baseline reading is established at your home field. When the oxygen drops to about 86 to 88 percent, that's the beginning of hypoxia. The company recommends that pilots should go on oxygen when the reading drops 5 percent and must when it drops by 10 percent. My baseline is 93 percent. I typically cross the 88-percent line around 8,500 feet. These devices are available for sale and provide a fairly accurate measurement of an individual's physiological altitude.

The "best" part of the ride is experiencing severe hypoxia. A University of North Dakota aviation student and the instructor were my companions this day as we climbed at 3,000 feet per minute up to 25,000 feet on oxygen. When the chamber stabilized at FL250 the instructor asked us to remove our masks and perform some simple tasks. Studying other people as they get hypoxic is much more fun than going to a bar to watch drunks. They misbehave in similar ways and the results happen quickly—you don't have to buy several rounds or tip the bartender. By the way, overindulgence of alcohol results in histotoxic hypoxia, where the cells are poisoned by a lack of oxygen intake. There's a useful bit of drinking trivia.

Basic tasks quickly illustrate when a pilot is losing it. The time of useful consciousness or effective performance time is defined as "the time from loss of sufficient oxygen until the subject is no longer able to perform the task in a safe or efficient manner." At 25,000 feet this is estimated at three to five minutes. Various tasks include paper-and-pencil quizzes, sorting of playing cards into suites, and other manners of diversion. I chose the monstrously complex Fisher-Price block cube that any 3-year-old can conquer. It has various shapes that must be dropped through an appropriately shaped opening.

It was no problem at first, and my initial reaction was that this oxygen business might be overrated, but then my peripheral vision became a bit fuzzy, I had hot flashes, and my fingers failed to follow instructions. The cube suddenly had so many complex shapes and holes. Was that square really a polygon? Concentration was slipping fast. At one minute and 37 seconds it was apparent that the observers would be highly amused if I persisted, so the mask went back on and in a few seconds I was ready, once again, to take on any 3-year-old in the place.

My young partner working a simple paper-and-pencil test was starting to lose it as well. I asked how he was feeling and the response was that everything was "just great." He put the pencil down on the clipboard, sat there for a moment, reached for the mic switch, but didn't push it. His lips and fingernails were bluing rapidly. The clipboard started to slide off his lap and he just looked at me. The lights were on but nobody was home. Now the instructor was speaking to him, telling him to turn on the oxygen and put on his mask. Very slowly he reached around, donned the mask, and the instructor twice advised him to switch on the regulator that would start the lifesaving flow. He'd been without oxygen for four minutes and was essentially gone.

Subjective symptoms, which will vary by individual, are headache, nausea, euphoria, tingling, dizziness, fatigue, vision problems, and numbness. You won't know what your symptoms are until you've experienced them. In your aircraft at high altitude is not the optimum place to learn. Observers will clearly note mental confusion, cyanosis (blue lips and nails), poor judgment, loss of muscle coordination, and—if one waits long enough—unconsciousness.

The next stop was at 18,000 feet in a darkened chamber with some dim red lighting to simulate the typical cockpit at night. Masks came off and we were told to read until we had difficulty seeing the page. I was already a believer about the problems of night vision after age 45, and with a touch of hypoxia it was clear that it was impossible to read even the big print.

When we fly on airliners there is always a briefing about what to do in the event of cabin pressure loss. What would really happen? It probably would bear little resemblance to the reasoned response and relaxed environment that is depicted on the preflight videotapes. This is defined as a rapid decompression, not explosive, and when it happens, believe me, you'll know it.

During the second chamber ride we were pressurized to a cabin altitude of 8,000 feet, similar to that of a Piper Malibu, Cessna 414, or one of the turboprops. The chamber was depressurized, in about one second, to 25,000 feet. There was a loud hiss as the air departed from both the chamber and us. The higher pressure inside the various body cavities we discussed earlier equalized right now. The chamber fogged instantly as the temperature momentarily dropped the ambient air below its dew point. The only thing to do was to put on the mask quickly, hit the regulator switch, and get on with business. This was a controlled environment and we were expecting the decompression. How would someone react in an aircraft at altitude?

At 30,000 feet, effective performance time is one to two minutes; it drops to just nine to 12 seconds at FL430. There is only time enough for the obligatory exclamation and to don the mask. Forget about troubleshooting the problem and talking on the radio. Get the mask on and start an immediate descent down to a life-sustaining environment—around 15,000 feet. Once the descent is stabilized, then call ATC to advise them that you've had a change in plans. Two things that aren't duplicated in the chamber are the tremendous wind noise and airflow, since the chamber isn't moving all that fast relative to the building, and there is no immense temperature drop. With outside temperatures of 35 to 50 degrees Celsius below zero at the high altitudes, hypothermia is a real issue and even coming down at 4,000-plus feet per minute, the passengers who can still talk will probably be asking for more heat and some blankets. Baby, it's cold outside in the flight levels, even by North Dakota standards.

The FAA takes the high harsh environment seriously in the regulations. Unless the aircraft is equipped with quick-donning masks when above FL350, one pilot is required to wear a mask even if the aircraft is pressurized. This is not required with a quick-don mask that must be able to be placed on the face using one hand, sealed, and supplying oxygen within five seconds. Naturally, most high-altitude aircraft have the quick-don masks, but if a crewmember leaves the flight deck above FL350, the other is required to don the mask. There isn't much time to respond and there are no prizes for second place. The rules also require a 10-minute supply of oxygen for pressurized aircraft flying above FL250 to provide time for an emergency descent.

FAR 61.31 requires that anyone operating a pressurized aircraft capable of flight above FL250 receive ground training on high-altitude meteorology, hypoxia, high-altitude sickness, time of useful consciousness without oxygen, and considerably more physiology than we have discussed here. Training and a flight instructor's endorsement are also required when acting as pilot in command above FL250 to ensure that pilots understand about emergency descent procedures, quick-don masks, and all the other items that go into a successful escape from a rapid decompression. When someone talks about getting high, pilots who have taken the chamber ride will really know what it means. The AOPA Air Safety Foundation strongly recommends taking a chamber ride to anyone who will be flying in the higher altitudes.

Pilots interested in altitude chamber training information, which is available at various military installations around the country, may contact the FAA at 405/954-6212; write FAA Airmen Education Branch, AAM-420, CAMI, Post Office Box 25082, Oklahoma City, Oklahoma 73125; or contact the UND Aerospace Foundation for the aviation physiology corporate aviator course by calling George LaMora at 701/777-3286 or e-mailing him.

Training is also offered by the L.B. Barometric Training Center, 1698B West Hibiscus Boulevard, Melbourne, Florida 32901; telephone 321/676-3200 .

See also the index of "Safety Pilot" articles, organized by subject.

ASF wishes to thank UNDAF for the opportunity to participate in this excellent training program and Mike Busch at AvWeb for supplying the pulse oximeter.