Editor’s Note: This article first appeared in Twin & Turbine, Sep. 2004, and is reprinted here by permission.
On June 3, 1999, a pilot was seriously injured and his Cessna T310Q was destroyed during a forced landing in West Laurens, N.Y.He had just purchased the turbocharged twin the day before, and was in the process of flying it home. According to ATC transcripts, about two hours into the flight, at 21,000 feet, the pilot began acting strange. He was having trouble maintaining altitude, and failed to acknowledge radio calls. About three hours, 50 minutes into the flight, while descending, the engines started to surge and subsequently lost power. The pilot landed in an open field. Examination of the airplane revealed the fuel tanks were empty except for unusable fuel. The pilot and a witness reported the airplane departed with full tanks. A check of the pilot’s flight planning revealed sufficient fuel was available for the flight if the engine fuel mixtures were properly leaned.
The airplane was equipped with a 76.6 cubic foot, constant-flow oxygen system. According to the pilot, the system was full when he departed, and he had checked that it was operational on the acceptance flight made the day before. According to the duration chart in the owner’s manual, a full tank (1,800 psi) would supply oxygen to one pilot for over 10 hours at 22,000 feet. The system is turned on by pulling a knob to an extended position, and turned off by returning the knob to the retracted position. The FAA inspector checked the oxygen system a few days after the accident, and found it empty.The NTSB determined the probable cause to be the pilot’s impairment due to hypoxia, which resulted in reduced situational awareness and a power loss due to fuel exhaustion.According to the Aeronautical Information Manual, hypoxia can impair judgment, memory, alertness, coordination, and the ability to make calculations. Most of us have heard that before, but unless you’ve experienced it yourself, it’s difficult to imagine how you might forget to do routine things like leaning the mixture or responding to ATC calls. Hypoxia is incredibly dangerous for pilots because it’s insidious. Things are going wrong and getting worse, but you feel great.After the T310Q pilot recovered at the hospital, he filed a report describing his version of the flight. Reading it provides a glimpse into the mind of a pilot confused by hypoxia:
Etymology: Latin: insidiosus, from insidiae, ambush
1: awaiting a chance to entrap : TREACHEROUS
2: harmful but enticing : SEDUCTIVE
“… At 21,000 feet, shortly after Dubuque VOR, the autopilot shut off and I couldn’t process how to locate and reset necessary switches to reactivate it. This was my initial indication that I was ‘impaired.’ I continued flight on J94. In retrospect … I lost all track of time and can only recall bits and pieces. I found myself misplacing items (chart, pens, sunglasses) required in flight. At one point I removed my shoulder harness to retrieve items on the floor. The right engine alternator light was on, and I was unable to execute resetting the circuit breaker, obviously due to confusion, since I normally know where it is located.”Air traffic control dictated frequency changes and I was mixing up the numbers, so I kept requesting them to repeat. I noticed I would transmit but they couldn’t hear me and at times they were ‘breaking up’ as well. I cannot recall at what point in the flight this was occurring, since I lost all perception of time. Next there was a change in the engine sound (a surging). I recall switching both tanks back to the mains as I could not discern which engine was doing it.”This is the point where I lost my conviction about effectively controlling the systems in the airplane. I requested assistance in getting to the nearest airport. I had no real awareness of my location, but knew I needed to get the aircraft down immediately. I executed the controllers instructions as best I could. My remaining focus was concentrated on controlling the airspeed and keeping the wings level to assure a safe landing. When I descended through a broken ceiling at about 1,000 feet, I knew I had to get the aircraft down quickly as my thought processing was diminishing. Luckily I was in farm country and I spotted a field in which could safely set down the aircraft without any threat of injuring anyone. I successfully brought the plane down ‘gear up'”
The airplane was destroyed and the pilot was seriously injured during the landing, but the fact that he ran out of fuel early and was forced to put the airplane down probably saved his life. Had he stayed at altitude, the lack of oxygen might have caused him to fall asleep or lose consciousness. With no one in command, the aircraft likely would have followed its own course, eventually run out of fuel, and violently crashed. You may remember the October 1999 flight of the Learjet 35 chartered by professional golfer Payne Stewart, which unfortunately demonstrated a similar conclusion and killed all six aboard.
Know Thine Enemy
Hypoxia means “reduced oxygen” or “not enough oxygen,” and it can come in several forms. The most obvious is an inadequate supply of oxygen in the air. Insufficient air pressure also causes hypoxia. So can transportation problems in your bloodstream. Or oxygen can be readily available in the air and in your blood, but factors prohibit it from getting absorbed by your tissues. The various forms of hypoxia are grouped into four categories based on their causes: hypoxic hypoxia, hypemic hypoxia, stagnant hypoxia, and histotoxic hypoxia.Hypoxic hypoxia is any condition that interrupts the flow of oxygen into the lungs. This is the type of hypoxia encountered by pilots at altitude due to the reduction of air pressure. As you climb during flight, the normal flow of oxygen into your system starts to diminish because the decreasing atmospheric pressure also decreases the migration of oxygen molecules through the membranes in your respiratory system. In a sense, low atmospheric pressure due to high altitude causes the same problem for the body as a drowning or a blocked airway. In each case, the normal flow of oxygen into the bloodstream has been interrupted.Hypemic hypoxia is any condition that interferes with the ability of the blood to carry oxygen. The lungs have plenty of oxygen, but the blood won’t carry it to the rest of the body. It can be caused by a lack of blood in the bloodstream due to severe bleeding. It can also be caused by certain blood diseases, such as anemia. For pilots, the greatest danger of hypemic hypoxia comes from carbon monoxide poisoning. If an aircraft’s exhaust leaks into the cockpit, the carbon monoxide in the exhaust bonds with the pilot’s hemoglobin (the blood molecules that transport oxygen), and prevents the hemoglobin from carrying oxygen. With all the carbon monoxide circulating around the body and no oxygen, the pilot gets hypemic hypoxia.Stagnant hypoxia is any condition that interferes with the normal circulation of blood. Stagnant means “not flowing.” Your lungs and blood have oxygen, but the blood isn’t pumping around the body properly. Ever have your foot go to sleep? This is a local form of stagnant hypoxia. Imagine your brain going to sleep! Heart failure, shock, or constricted arteries can all cause it. For pilots, it can be brought on by pulling excessive positive G force. You may have heard the term “gray out.” The color vision of a pilot pulling G becomes increasingly gray and constricts into tunnel vision as they approach blackout from stagnant hypoxia.Histotoxic hypoxia is any condition that interferes with the normal utilization of oxygen in the cell. Oxygen is in the lungs, it is infused in the blood, and it is circulating around the body, but it is not being accepted once it reaches the cells. Alcohol, narcotics, and cyanide all can interfere with the cell’s ability to use the oxygen in support of metabolism and cause histotoxic hypoxia. Most of us are smart enough to stay away from narcotics and cyanide, but beware of alcohol. Research shows one ounce of alcohol equates to about 2,000 feet of physiological altitude due to its histotoxic reaction in your cells.Of the four types of hypoxia, it is hypoxic hypoxia that is the greatest danger to pilots. With decreasing air pressure as you climb, the higher you go, the more dangerous it becomes. In the case of a rapid decompression at high altitude, not only are you deprived of breathing in new oxygen, but the oxygen in your cells migrates out towards the lower pressure atmosphere too. The rate this happens depends on the pressure differential between the atmosphere and inside your body, but the combined effect can quickly cause impairment and unconsciousness.
|Altitude (ft)||Time of Useful Consciousness|
|35,000||30 seconds to 1 minute|
How fast does this happen? The time from the exposure to a low-pressure environment to the time when an individual is no longer capable of taking proper corrective and protective actions is known as the time of useful consciousness. The table at right illustrates the time of useful consciousness at various altitudes for average people.Up in the flight levels, you have precious little time to react to a rapid decompression. At jet cruising altitudes, time of useful consciousness is less than a minute — basically just enough time to put on your oxygen mask and initiate an emergency descent. For a high-performance jet flying above 40,000 feet, its only a matter of seconds. If you’re considering a single-pilot jet, or have put down a deposit for one of the high-flying very light jets under development, this is very important to keep in mind.Ultimately, the actual time of useful consciousness will depend on the individual. Some folks can last longer than others, depending on their health, heart rate, blood chemistry, and other factors. Altitude tolerance also changes from day to day in each individual. For example, if you enter the cockpit in a fatigued state, you will be less resistant to hypoxia than you would be if you were well rested. Similarly, if you have been subject to poor nutrition, you’re more prone to hypoxia because your glucose (blood sugar) is low. If you’ve been drinking alcohol, you’ll be more susceptible. Some over-the-counter medicines can cause cells not to utilize oxygen properly and therefore make you less altitude resistant. Any physical activity will reduce your time of useful consciousness too. For example, if you did ten deep knee bends at 25,000 feet with your oxygen mask off, your time of useful consciousness would be reduced by 50 percent. And finally, the faster you ascend to altitude, the shorter your time of useful consciousness becomes. With all these factors coming into play, you can see that the time of useful consciousness chart is only a guideline.The unpredictable and insidious nature of hypoxia is the reason the FAA put FAR 91.211 in place. The rules spell out the use of supplemental oxygen when flying at high altitudes. Most of us first encountered these regulations when we took our private pilot written tests. Any aircraft flying above 12,500 feet is subject to them.
Supplemental Oxygen Requirements
|Atlitude||Supplemental Oxygen Required|
|Less than 12,500||None|
|Above 12,500 and up to and including 14,000||For crew only if more than 30 minutes|
|Above 14,000 and up to and including 15,000||For crew only at all times|
|More than 15,000||For crew and passengers at all times|
|* Also for cabin pressure altitude in pressurized airplanes|
|Above FL250||10-minute supply for each occupant|
|Above FL350 and up to FL410||10-minute supply for each occupant. If single pilot, the pilot must wear a mask that is feeding oxygen all the time, or automatically starts when cabin pressure exceeds 14,000 feet. If two pilots are present and the aircraft is equipped with quick donning masks, wearing them at all times is not necesary.|
|Above FL410||10-minute supply for each occupant. The pilot must wear a mask that is feeding oxygen all the time, or automatically starts when cabin pressure exceeds 14,000 feet.|
Know Thine Self
The best thing you can do is to prepare and educate yourself about hypoxia. Reading articles like this, attending lectures on the topic, and practicing emergency descents in your aircraft or simulator are all very good ways to learn theories and procedures. If you have a high-altitude endorsement, you’ve probably done some of this already.But no matter how many articles you read, lectures you sit through, or simulated emergency descents you practice, you will never learn the answer to the most important question of all when it comes to hypoxia: How does hypoxia actually manifest itself in me?Obviously to answer this question is you have to get hypoxic to find out. So how do you safely do that? The FAA provides the opportunity through its Aviation Physiology training course, which is conducted by the Civil Aerospace Medical Institute in Oklahoma City, Okla, and is offered at military facilities across the United States. This is the course where you “fly” in an altitude chamber and remove your oxygen mask to experience the onset of hypoxia at high altitude in a controlled situation. These are the same altitude chambers that are used to train fighter pilots.The course starts with a preflight briefing, which takes place in a classroom and is intended to familiarize you with the flight profile and the safety aspects of the altitude chamber. Next you go to an oxygen equipment lab, where you learn about the oxygen equipment you’ll be using, how to don and fit the mask, and regulator functions.Once all the equipment is hooked up and tested, you and your classmates enter the chamber. As an additional safety measure, everyone is required to pre-breathe 100-percent oxygen for a short time before the session starts. Once everyone’s blood is well oxygenated, the chamber “pilot” takes you up at 3,000 feet per minute to a simulated altitude of 6,000 feet. This initial ascent is just an ear and sinus check done to confirm that no one has any blockages that might prohibit them from participating further. If everyone is thumbs-up, you descend at 3,000 feet per minute to a simulated altitude of 2,000 feet and everyone is checked again. If all is well, the simulation continues.
Next you are brought back up at 3,000 feet per minute to 8,000 feet, where you pause briefly before starting a simulation of a rapid decompression. The rapid decompression simulation begins at that level because 8,000 feet is a typical cabin altitude in a pressurized aircraft cruising at altitude. After you give the thumbs-up again, the chamber pilot simulates a rapid decompression by taking you from 8,000 feet to 18,000 feet in about eight seconds. If this were an actual climb, it would be at a rate of 75,000 feet per minute! Remember that rate of climb effects time of useful consciousness. To your body, a loss of cabin pressure is just like a skyrocketing climb.After holding at 18,000 for a few minutes, you might begin to feel some early signs of hypoxia. If you live at sea level, you can expect your symptoms to be more intense than those who live at higher elevations. Short spells of dizziness, some tingling, or light euphoria is common. If you look at your watch, you realize it was only moments ago that you felt completely normal. Now you put on your oxygen mask, and turn on 100-percent flow. All symptoms disappear after you take a few breaths. You realize, “Hey, this oxygen stuff really works!”Once you’re stabilized on oxygen again at 18,000 feet, you’re ready for the real hypoxia demonstration. The chamber pilot takes you up at 3,000 feet per minute to 25,000 feet. Once level, you are instructed to remove your oxygen mask. This is the peak altitude of the flight profile and where you will have the best demonstration of your personal symptoms of hypoxia. There’s sort of a nervous giddiness in the chamber as everyone removes their masks. You smile too as you know you’re about to become stupid.
Within a minute or two, everyone is acting funny. Some are smiling and giggling at everything. Others become fixated on something in the room and won’t stop staring, as if they are watching some imaginary television show. Others might be waving their arms around with their mouths open. You get a whole range of reactions, but the most common signs and symptoms of hypoxia are shown in the table at right.At 25,000 feet, there is a five-minute limit for everyone to acquire their symptoms. After five minutes, everyone must put their mask back on no matter what. Few last that long. Once you visibly show symptoms, you are told to put your mask back on and breathe 100-percent oxygen again. With a little prompting, most people recognize that they are impaired, and do so willingly. Sometimes a student might not register the requests from the chamber staff, or their impairment hinders their efforts to get the mask back on. If that happens, there is a staff member in the chamber who is always on oxygen and ready to assist. Two or three breaths later, all is back to normal again. The chamber pilot descends back to ground level at 3,000 feet per minute.With your brain working again, you can take a few minutes remember how your hypoxia symptoms came on, and how they progressed as the minutes passed. Armed with this vivid play-by-play experience, you now know how hypoxia is most likely to try and sneak up on you. If you ever start to encounter the same symptoms in flight, you’ll immediately know you’re getting hypoxic and will be able to start an emergency descent before you fall into its insidious trap.
So, You Wanna Get High?
Hypoxia is one of several topics covered in the Aviation Physiology program. To enroll, contact the FAA Civil Aerospace Medical Institute. If you’re fortunate, you’ll never need to use what you learn. But why tempt fate? Someday, it could save your life.
More about aviation medicine is available in AVweb’s Aeromedical section.