Thin Air

They say altitude is your friend, but once you get above 10,000 feet, the greatest potential danger isn’t ice or turbulence, it’s thin air.

  • On September 29, 1993, a Piper PA-24-250 broke up over Akron, Ohio during a VFR cross-country flight, killing all three people on board. The pilot was in contact with Cleveland Center when he reported an altitude of 16,500 feet (and that he was IMC… ahem). The NTSB report’s probable cause stated “continued flight by the pilot to an altitude that was not appropriate without supplemental oxygen, which resulted in a loss of aircraft control due to hypoxia and subsequently exceeded the design stress limits of the aircraft.” (Of course, he had no business going IMC.)
  • On October 25, 1999, golfer Payne Stewart’s Lear 35 (with two pilots) departed Orlando for Dallas. Instead, it flew 1,400 miles, crashing in South Dakota when the fuel ran out. The last message from the plane was an acknowledgment that they had been cleared to climb to 39,000 feet. The plane failed to pressurize, but climbed as high as 51,000 feet during its flight. No one was heard talking on the 30-minute cockpit tape (probably because they’d been dead for three hours).

The average human body has a limited ability to function above 10,000 feet, because there is less oxygen in the air and there is less pressure to force that oxygen through the lungs. (Airliners are pressurized so that the cabin never feels higher than about 8,000 feet even if the aircraft is flying much higher. Still, some people find it easy to sleep.)

Staying Oxygenated: Almost all of the oxygen your body needs is carried by hemoglobin molecules in red blood cells. The whole process hinges upon the partial pressure of oxygen — which is a function of atmospheric pressure (which varies with altitude) and how much oxygen is in the air we breathe (which is constant at about 21%… up to about 70,000 feet, anyway). As far as your lungs are concerned, the partial pressure of oxygen — even at sea level — is less than 21%. The “back-pressure” of carbon dioxide given up by blood, as well as that of water vapor; puts the useable figure closer to 14%.

Staying Functional: How well we function doesn’t depend on how much we breathe, but rather upon how well oxygen diffuses across semi-permeable membranes from the tiny sacs in our lungs and onto those hemoglobin molecules. While air (and oxygen) pressure decreases a bit less than linearly with altitude, the ability of hemoglobin to hold oxygen and transport it within your body actually drops off rapidly along an S-shaped curve, after about 20,000 feet. But there’s danger well before you get that high.

If you’re in good health, right now your arteries are carrying blood that’s about 97% oxygen-saturated. At 10,000 feet your blood is 90% saturated. If you climbed to 14,500 feet at the top of Pike’s Peak (as I did some time ago), your blood is carrying at about 80% of its capacity. At this point you would notice that “Hey! What happened to my legs?!” feeling, just as I did, when I bounded… OK, dragged… up the steps to the visitor’s center. Up this high, most people would begin to notice vertigo, nausea, weakness, rapid breathing, impaired coordination, slowed thinking, vision irregularities (it feels like your blinks are lasting longer than your lids are closed) and increased heart rate. At 25,000 feet, a non-acclimated person would soon lose consciousness. (The partial pressure of oxygen in your lungs at 25,000 feet is slightly less than that of the aforementioned carbon dioxide back pressure.)


  1. Hypoxic hypoxia is actually what most people think of: not enough oxygen in the air, usually because of altitude.
  2. Hypemic hypoxia, describes the condition when the blood’s oxygen carrying capacity is compromised (whether due to carbon monoxide, blood donations, anemia, etc.)
  3. Stagnant hypoxia, relates to circulatory impairments (including excessive ‘g’ forces).
  4. Histotoxic hypoxia is when utilization of oxygen within the body is interfered with — such as by alcohol or narcotics.

Note: One’s perceptions at the onset of CO poisoning are about the same as those of hypoxia. (Remember this as you read on.)

Hypoxia has objective symptoms (what you’d see in others) including increased respiration, cyanosis (turning blue — especially lips and the skin under fingernails), poor judgement, confusion, and lack of muscle coordination. But the subjective symptoms that we encounter in ourselves are a bit more important — unfortunately, they vary with the individual and can include: headache, fatigue, dizziness, tingling, apprehension, euphoria, just feeling dopey, and more…

Grim deception: The victim often never suspects a thing. The first symptoms can be literally, intoxicating. The first effects are experienced by the brain, but these often pass unnoticed due to the associated impairment of judgement. You feel secure and nonchalant; you may become dizzy or feel tingling. Like the guy who insists on driving home from a party, you might feel a headache, but you are only half aware of it. Then your heart races; your lips and the skin under the fingernails begin to turn blue; your field of vision narrows; the instruments look fuzzy. But you feel confident that you are doing a better job of flying than you have ever done before… right up to the point when you make a fatal mistake. (This might be fuel mismanagement, navigational errors, inadvertent entry into clouds, forgetting to turn on your oxygen…) Then, it’s fade to black.

Regardless of acclimatization, endurance, or other attributes, everyone suffers the consequences when exposed to inadequate oxygen pressure. The extent of these symptoms is dependent on altitude, but even short periods above 10,000 feet will produce symptoms in the majority of pilots. Your retinas are actually the most insistent and self-centered tissues in your body when it comes to oxygen — even more so than the brain. And that’s the first thing to go, at even 5,000 feet: night vision can fade rapidly. One can misread charts, instruments, or ground features. At 10,000 feet, forget about good night vision, and your brain is receiving an absolute minimum oxygen supply. Your judgment and performance may soon become compromised, and symptoms such as tingling or headache may become apparent, dependent on your physical condition.

At 14,000 feet, oxygen saturation is at danger levels, and you risk rapid onset of degradation of judgment, memory and thought.

Unless you’ve felt it before, the impairment of judgment could leave you feeling just fine and confident. Any higher, and you can become disoriented and disabled. If you are alone, your chances of survival are decreasing rapidly. At 18,000 feet, the concentration of atmospheric oxygen is only half that at sea level, and you pass out in 30 minutes. At 25,000 feet, time of useful consciousness is three to six minutes (assuming previously normal cabin pressure).

CFR Title 14 (the ‘FARs‘) requires pilots to use supplemental oxygen after 30 minutes when above 12,500 feet MSL, and at all times when above 14,000 feet. (The Air Force is stricter: above 10,000 feet, you go on oxygen, period.) Like everywhere else in the regs, these are bare minimums. Cut yourself some slack!

Insider Insights: Sure, people live in the Andes… you don’t. The folks who live up there have acclimatized — their bodies house monster levels of hemoglobin (besides bigger lungs and better pumping hearts). Individual reactions to hypoxia vary with both the individual, but also due to the flying environment itself. Factors include altitude (of course) duration of exposure, physical activity (or increased flying workload), extremes of temperature, and personal factors such as fatigue, alcohol, and carbon monoxide — say from cigarette smoke (which combines with hemoglobin at least 200 times more readily than oxygen). The other ‘I’M SAFE’ factors (illness, medication, stress, and emotion) can also adversely affect us.

Don’t let hypoxia get a foothold. Carry oxygen and use it. Don’t gauge your ‘oxygen hunger‘ by how you feel — gauge it by the altimeter. You can also do what I’ve done (three times now). For less than the cost of an hour in a 152, you can take a ride in the altitude chamber. This will show you how your body reacts when deprived of normal levels of oxygen. You will learn flight physiology from the standpoint of the environment, the human body, as well as the mind. Plus, if you’ve had this training, you’ll also know what the onset of CO poisoning would feel like, to you.

Insider Tip: If you live anywhere near these 14 facilities around the country currently offering this training, I say go for it. The following USAF facilities offer their altitude chambers to the public: Andrews (MD), Beale (CA), Brooks (TX), Columbus (MS), Fairchild (WA), Fort Rucker (AL), Holloman (NM), Langley (VA), Little Rock (AR), Offutt (NE), Peterson (CO), Randolph (TX), Shaw (SC), and Tyndall (FL).

by contacting the FAA Civil Aeromedical Institute at the Mike Monroney Aeronautical Center, Aeromedical Education Division, AAM-400, P.O. Box 25082, Oklahoma City OK 73125. (They also offer a new survival course for GA pilots.) The telephone number is 405-954-4837 (fax -2305). The email contact for Airman Education Programs is They have a terrific web site at ‘ This training is an excellent investment. (I don’t get paid for saying that.) Try it. You won’t be disappointed!