Breathe the Thin Air

Pressure chamber training can help prevent the hazards of high-altitude flight. Hint: Hypoxia is just part of the problem.


By Ray Leis

We were at 23,000 feet, and had been there for about 10 minutes. Jake was in the left seat for this leg. He had his mask off, but hanging ready, if needed, as did I. The bird had suffered a loss of cabin pressurization, and 23,000 feet seemed to be OK for a while, even without oxygen.

We were pretty sure one or the other of us had somehow missed setting the cabin pressure controller, perhaps by not programming it for high-altitude cruise. We started running through the checklist, item by item. Then we got a call from San Antonio departure control:

Raytheon 672 X-ray, San Antonio approach, I have a relay message from your operations. They have been unable to contact you. Are you ready to copy?

I started to hunt for my pencil. Somehow they always slide away when you need them. Jake handed me his.

San Antonio Approach, this is Raytheon 672 X-ray. Ready to copy.

Raytheon 67 X-ray your operations requests that you pick up two passengers at Midland, off-load 3 marked boxes of cargo for Wilton Industries and on-load one 200-pound crate for El Paso.

Did you get all that, Jake? Who do we drop at El Paso?

Jake nodded. Two passengers for El Paso. Acknowledge.

San Antonio Approach, this is 672, we have all the information. Thanks.

I looked for Jakes pencil. I had dropped it near the left rudder pedal. I bent down to get it. Then suddenly a strange voice barked in my headset.

Catch number 5 and 6 and get them back on 100 percent oxygen now.

I felt my oxygen mask being pushed into place. After a few breaths, the lights blazed up around me. In a couple of more seconds things began to clear up. Jake was coming around a little slower, wide-eyed and surprised.

Two pressure chamber instructors were standing next to us. Memory came back rapidly. We were at Brooks Air Force Base, in San Antonio, Texas, and a dozen faces, wearing oxygen masks, were turned toward us.

Feel OK now? one of the pressure chamber instructors asked.

Fine. A little confused, but fine. Jake agreed.

Take a look at the messages on your kneeboard, the instructor said.

I looked and the pad was filled with a few unrelated figures, some unidentifiable scrawls. It was hard to believe. I was sure that everything had been going along well at the time. He noted my head-shaking surprise.

Hypoxia, the pressure chamber instructor commented, up close and personal.

I still have the page of gibberish I had written on the kneeboard. What if I had really been at 23,000? What would have happened next? Graveyard spirals and unexplained airframe break-ups come to mind.

Getting a rude awakening in this way, in the safe environment of a pressure chamber, is a big improvement over gambling your life against high-altitude flight. The FAA course objective is to make the deterioration of human performance caused by reduced barometric pressure (hypoxia) understood by all pilots.

The pressure chamber is the star of the FAAs physiological training course, but the one-day course is also filled with extremely useful information on all aspects of the human being as airborne pilot. For any pilot who plans to fly at or above 10,000 feet, this course is one you need.

Most pilots feel that they know their personal limits at altitude. The accidents that continue to show up in the NTSB reports say otherwise. Ground school you can get almost anywhere. In the pressure chamber you will be able to identify your own personal symptoms of hypoxia and experience a rapid decompression.

What you can experience and learn there can save your life – and that of your passengers.

The Atmosphere
The physiological zone (where humans can best survive) is the altitude from sea level to 12,000 feet. Once you go beyond this limit, you venture into the physiological deficit zone, which is from 12,000 to 50,000 feet. In most of this zone humans need supplemental oxygen to survive. One-half of earths air is below you when you are at 18,000 feet.

The average time before you lose consciousness at a specific altitude without supplemental oxygen is called your effective performance time. Predictably, the higher you go the shorter the EPT. At 25,000 feet the average EPT is 3 to 5 minutes. After about 20 minutes without supplemental oxygen, you will be pronounced dead.

By the time you get to 35,000 feet, EPT drops to 30 to 60 seconds and it falls to nine to 15 seconds at 45,000. Dont leave home without a full pressure suit at 63,000 feet; your blood boils.

These are average EPT times, and each person is different. An individuals time can be less or more, and a single persons time can vary from day to day, depending on such factors as diet, alcohol use, smoking, fatigue, stress and other physiological factors.

Hypoxia is a shortage in the oxygen available to the bodys tissues. While all cells require oxygen, some need more than others. The nervous system (the spinal cord and brain) uses the largest amount, with the brain consuming 20 percent of the oxygen you breathe. While most cells are capable of storing some oxygen, the brain and spinal cord cant store it at all.

There are four types of hypoxia:

Histotoxic hypoxia is a condition where the use of oxygen by the body is blocked, or interfered with, as with various over-the-counter and prescription medications, alcohol or narcotics.

Stagnant hypoxia is caused by circulation problems or excessive G forces.

Hypemic hypoxia develops when the oxygen-carrying capacity of the blood is reduced. Often caused by carbon monoxide, anemia or blood donations.

Hypoxic hypoxia occurs when there is not enough oxygen in the air, usually because of the lowered air pressure at altitude.

The lack of oxygen at high altitudes is not the main cause of hypoxia. Its the lowered air pressure. The percentage of oxygen in the air doesnt change as you climb to altitude, but the air pressure does. Without adequate air pressure – or more specifically adequate partial pressure of oxygen – insufficient amounts of oxygen pass through the membranes in the lungs and into the blood.

The effects of hypoxia on a pilot depend on how severe the oxygen deprivation is and how long it has been going on, as well as the health of the pilot. Hypoxia has a wide range of effects, including loss of night vision (starting at 5,000 feet). Disorientation, poor judgment, lack of coordination, unconsciousness and death can also be induced by hypoxia.

For some pilots and passengers, even short periods of hypoxia can cause some serious after-effects. They resemble any form of oxygen deprivation, resulting in sluggishness, lack of coordination, severe headache and other symptoms. Hypoxias aftereffects are much like a severe alcohol hangover or carbon monoxide poisoning. Everyone who is exposed to inadequate oxygen pressure is going to suffer from hypoxia.

Most pilots will experience some mild symptoms of hypoxia at some point in their careers, such as after even short periods above 10,000 feet. The retinas show oxygen deprivation very early, with most people experiencing a loss of night vision above 5,000 feet. At 10,000 feet, pilots have given away most of their night vision. At this point, performance and judgment begin to get into a fuzzy area, and various symptoms and headaches start to appear.

You risk rapidly losing judgment, memory and thought at 14,000 feet, where the oxygen level declines to a danger level. You may feel fine – euphoric, even. You certainly become disoriented and disabled if you venture much higher.

While federal regulations provide some guidance, the medical reality is far from cut and dried. The FAA says that any aircraft occupant flying with a cabin altitude of more than 10,000 feet is a potential hypoxia case waiting to happen unless supplemental oxygen is available.

While the FARs require pilots to use supplemental oxygen when above 12,500 feet for more than 30 minutes and at all times above 14,000 feet, the Air Force is more conservative. Air Force pilots use oxygen above 10,000 feet and from the ground up at night.

Perhaps the greatest danger of hypoxia is its ability to rob you of your senses without you recognizing there is a problem. The pilot of a Cessna 340A was flying solo at 25,000 feet when he decided he needed to heed natures call. He left the cockpit for the rear of the aircraft.

To load the dice, the 340A was operating with a known cabin pressurization deficiency that limited the airplane to maintaining a 10,000-foot cabin pressure when the airplane was at 17,000 feet. After two hours and 10 minutes, ATC declared the flight to be no radio. Nearly four hours after departure, the Cessna 340A was plotted on radar in a descending left turn to ground impact. The wreckage indicated the left engine lost power due to fuel starvation while the right engine generated power to impact. The pilots body was found in the aft cabin area.

Explosive and Rapid Decompression
Of course, not all pressurization problems are insidious; some are attention-getters. Visualize going from a cabin altitude of 8,000 feet to 25,000 or 35,000 feet in half a second.

The air suddenly is filled with fog and you are confused and shocked. Unless you understand what has happened, you dont have much time to take some life-saving action.

Explosive decompression was a hard lesson learned early in the era of pressurized airliners. The early British Comets were pressurized to sea level for passenger comfort. On one flight, the airplane was cruising at altitude when a small fatigue spot in the fuselage blew out. The hole instantly widened to about 3 feet square.

Everything in the cabin that was loose – people, seats, baggage and whatever else – departed the aircraft through that opening. Investigators spent months fishing it all out of the Mediterranean Sea and put all the pieces back together. Besides providing lessons on how to improve the structure of a pressure vessel, the accident also led designers to reduce the amount of pressure the cabin needed to contain, with cabin altitudes now usually 8,000 to 10,000 feet.

The lower level of pressurization means explosive decompression now would not be as disastrous as it was on the Comet. Even so, it can still be pretty scary for the unsuspecting passengers. Quick action is required: oxygen masks on in 5 seconds or less.

Rapid decompression – which takes place over about 5 seconds – is very similar. In either case, the pilot needs to get on oxygen and make an emergency descent to a lower altitude. On the way, the crew makes sure oxygen masks are on and working for passengers and crew. A lack of masks or a depleted oxygen supply is a direct indictment of the pilot in command, who is responsible for the preflight.

Flying in the oxygen altitudes is challenging, but adds great utility to turbocharged airplanes. The oxygen system becomes another aspect of the airplane to inspect and maintain. One way of ensuring proper operation is to add PRICE to the preflight checklist. The acronym covers oxygen Pressures and quantities, Regulators, Indicators, Connections and tank security, and Emergency operation.

Also, make sure your passengers know how to find and don masks or cannulas, particularly if the airplane is pressurized. There have been cases in which pilots passed out from hypoxia and passengers struggled to find oxygen supplies that could revive the pilot.

Other Breathing Problems
Following a rapid decompression – and assuming the people on board are on oxygen – the next problem that usually will crop up, and a real danger – is hyperventilation. Rapid breathing, especially in an oxygen environment, causes too much carbon dioxide to leach out of the bloodstream. This causes blood vessels to constrict, which serves the dual purpose of actually reducing oxygen to the brain and restricting blood flow through the lungs.

Pausing between breaths will return the carbon dioxide balance and help get oxygen into the bloodstream. There are also several types of oxygen masks that have a re-breather bag tied into the mask.

The symptoms vary from those of hypoxia in that they are fairly well defined and happen in the same order for most people, whereas the onset of hypoxia symptoms varies from individual to individual.

Hyperventilation is first indicated by a tingle in the lips and numbness in your fingers.

Then your heart begins to pound and you may start losing judgment as the function of your frontal lobes takes a dive. Hyperventilate badly enough and you can have a seizure.

At one time, when I was an Air Force instructor in tandem-seat jet trainers, we knew that most students exposure to jet flight was going to be stressful.

But, instructing from the rear seat, with the back part of the students ejection seat and his helmet blocking the view, it was difficult to tell (visually) how stressed the student was. It didnt take long to discover that there was an easy way.

Since we were both on interphone, and mikes in each oxygen mask were always hot (we were on oxygen from the ground up, due to cockpit carbon monoxide) the sound of the stressed students rapid breathing was an easy giveaway. The tempo of the students breathing increased at a great rate, once we started on the takeoff roll, increasing with the 5,000 foot per minute climb.

At level-off, usually 25,000 feet, I usually had the student check his oxygen equipment again, and put the auto/mix regulator to 100 percent oxygen.

After three or four deep breaths of oxygen, and some instruction on slowing the breathing rate, the student relaxed somewhat. The crisis was over, for the time being. I learned quickly that if I didnt follow this two-step recovery, I usually had a sick student and a short flight.

Another deadly danger in piston airplanes is the carbon monoxide that comes out of a poorly maintained airplane. In singles, cabin heat usually comes from a shroud around the exhaust system. Twins may have a combustion heater that is fueled by avgas.

In either case, carbon monoxide can enter the cabin if the heating system isnt up to snuff. Keep in mind that this isnt just a winter problem, either. At 10,000 feet or more, virtually any airplane will need heat even if its 80 degrees on the ground.

Carbon monoxide can disable a pilot in a very short time because it ties up the oxygen-moving hemoglobin in the bloodstream and creates hypoxia-like conditions. A pulse oximeter wont detect carbon monoxide poisoning because blood contaminated by CO is bright red and the oximeter looks for bluish blood. If you suspect CO might be present, go 100 percent oxygen if its available and land as soon as practicable.

After landing, dont try another flight until you have had medical treatment and have a mechanic go over the exhaust/heater system before another flight.

Detecting carbon monoxide is not difficult. FBOs and pilot shops sell cardboard detectors with chemical dots that turn colors when exposed to CO. These have some merit, but they also have a relatively short useful life and must be replaced regularly. On the other side of the coin, electronic detectors of various capabilities are also on the market.

Even with a detector, however, fresh air is a good idea. Generally speaking, it makes sense to dress warmly for flying and use as little heat as possible by allowing as much fresh air into the cabin as practical.

Sneaky Opponent
An accident can happen to any pilot who tries to beat the requirements of high-altitude flight. One instrument-rated private pilot and his two passengers departed Billings, Mont., in a Piper Comanche en route to Spokane, Wash.

The aircraft was in cruise flight at 14,300 feet msl when a controller noted the airplane was not maintaining course and altitude very well. The altitude varied from 14,700 to 14,000 and the aircraft was not flying the airway. At one point, the flight deviated as much as 90 degrees from its intended course.

At some point, the airplane broke up in flight and crashed near Winston, Mont., killing all aboard.

A portable oxygen bottle with its regulator broken off was found in the wreckage path. The position of the shut-off valve indicated that the valve was on at the time of the accident.

The regulator had four connections for oxygen masks or cannulas. Two of these connectors were capped with red dust covers. The other two connectors were clean and showed no visible damage. No evidence was found that masks or cannula bayonet-type connectors had been attached to the oxygen bottle regulator during the accident.

The pilots partner said the pilot felt comfortable flying at altitudes above 12,500 feet without using oxygen. The pilot had also told his partner that he had flown in Mexico at 17,000 feet without using the oxygen on board and had not felt any ill effects.

The partner also said he had flown with the pilot across the Rocky Mountains where they went to 14,000 feet on short duration flights, and the pilot had not used oxygen.

He also noted that the accident pilot had been under a lot of stress, in that a recent trip from Mexico was delayed because of mechanical problems with the airplane, delaying the departure of the accident flight. He also stated that the accident flight was an extremely important business trip, with a large construction project hinged upon its outcome.

Sometimes its easy to forget that humans werent designed to fly. Ground-dwelling creatures like low altitudes and dense oxygen. Hypoxia cuts away the margin of safety in flight to the point at which any added circumstance can lead to disaster.

Also With This Article
Click here to view “Factors in Hypoxia.”
Click here to view “Oxygen Fuel Gauge.”
Click here to view “Roll Your Own O2 System.”
Click here to view “Living With O2.”
Click here to view “Training Locations.”

Ray Leis is an ATP, CFII and FAA Aviation Safety Counselor.


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