A Pilot’s Guide to Altitude and Hypoxia

by HPA · February 3, 2012

About Oxygen in Aviation

By Hank Gibson, Gold Seal, CFI, CFII, MEI

Last Monday, I went with a group of pilots to NASA to take a high altitude training course dealing with altitude in aviation. Though I was unable to participate in the altitude chamber flight at the end of the day, it was still a valuable refresher for me on the effects of high altitude. Hopefully, after you read this article, you will be more educated on the affects and dangers that high altitudes have on pilots, making you safer. (All information that follows was taken from NASA’s High Altitude Training Seminar, unless otherwise noted).

What is Hypoxia?

Hypoxia, defined straight from NASA, is “a state of oxygen deficiency in the blood, tissues, and cells sufficient to cause an impairment of mental and physical functions.” Basically, it ain’t good. How does this oxygen deficiency occur in aviation, you ask? Good question.

The higher a pilot goes in the earth’s atmosphere, the atmospheric pressure and the partial pressure of oxygen in the air both decrease. At sea level (I’m going to use the units that NASA gave us), the atmospheric pressure is 760mm Hg (equivalent to 29.92in Hg) and the partial pressure of oxygen is 100mm Hg. The oxygen saturation of the body is at 98%.

As altitude increases, both atmospheric pressure and the partial pressure of oxygen decrease, measuring 522.6mm Hg and 60mm Hg at 10,000 feet, respectively. Up at that altitude, the oxygen saturation of the body is down to 87%. At sea level, medically, when a person’s body gets down to 87%, doctor’s give that person an oxygen canister to carry around. You see lots of older folks with them. In aviation in an unpressurized cabin, this is the level a pilot’s body is at. Mental and visual capability is impaired as well as other bodily functions. That’s only 10,000 feet!

Father and Son with Pilots

Different Types of Hypoxia

NASA told us about four different types of hypoxia in aviation, each affecting a different system of the body. The first is hypoxic hypoxia, a lack of oxygen to the lungs. This would be the most common form of hypoxia for pilots due to breathing air at a reduced atmospheric pressure. The cause could be anything from improper equipment usage to a rapid decompression.

Hypemic hypoxia is any condition that messes with the blood’s ability to carry oxygen. Where this pops up in aviation is with carbon monoxide poisoning. CO poisoning can either come from a leak in the exhaust system creeping into the heater or smoking. Our NASA instructor said if you smoke, that automatically put’s your body at 8,000 feet, even at sea level. If you don’t smoke, don’t pick it up. If you do smoke, please, for your sake, look into a program to stop smoking.

Histotoxic hypoxia prevents the cells from using the oxygen that is already there as they normally would. The cause? Alcohol and drugs. Alcohol and certain drugs inhibit the cells from utilizing oxygen rich blood. Don’t drink and fly (FAA regs) and be extremely careful with over the counter medication. If it makes you drowsy, don’t fly after taking it. Most prescription drugs don’t mix well with pilots either.

Finally, stagnant hypoxia affects blood circulation. You have probably experienced this when your arm or your foot have “fallen asleep.” Other causes of stagnant hypoxia that can be more dangerous in aviation are hyperventilation, g-forces experienced in aerobatic training, or even extreme hot or cold temperatures.

How to Identify Hypoxia

Airline Exec about to hyperventilate

The folks at NASA were very specific when they were teaching us pilots how to identify hypoxia in someone. There are signs (everybody would experience these) and symptoms (each individual would get different symptoms). Our instructor also told us that a person’s symptoms changed with age, so mine would be different in twenty years.

Here is a list of hypoxic signs that would show up in everyone:

  • Increase rate of depth and breathing
  • Cyanosis (blueing of the lips and fingernails)
  • Slurring of speech
  • Poor coordination and mental confusion
  • Euphoria
  • Belligerence
  • Becoming lethargic
  • Unconsciousness

Not everyone will experience all of these signs. Euphoria usually doesn’t change to belligerence, but these are what to look for in yourself and others. NASA also gave us a long list of possible symptoms, ranging from numbness and fatigue to nausea, but the instructor was very specific in stating that symptoms are different for different pilots. Basically, a symptom is anything that would be abnormal for you, personally.

Hypoxia Remedies

So what do you do if you discover you are becoming hypoxic? The FAA and NASA both emphasize the time of useful consciousness. Basically, that is the amount of time at certain altitudes that a pilot will remain conscious in a hypoxic state before he passes out. It ranges anywhere from 20-30 minutes at FL 180, to 9-12 seconds at FL 430 for a slow decompression. For rapid decompressions, the time is much shorter. So, act quickly.

Stewardess giving passenger briefing

The first step is to get on 100% oxygen. Check your supplemental oxygen gear to make sure it’s sealed properly. Then, control your rate and depth of breathing and get down below 10,000 feet as quickly as possible. If ATC asks, tell them it’s an emergency. Do not take this lightly because it can turn in to a dangerous situation very quickly.

What if you don’t have any supplemental oxygen? Well, what were you doing above 10,000 feet? Get your butt down to a lower altitude ASAP and think a little bit more when picking your altitude next time.

Altitude in aviation can be dangerous if you aren’t prepared. For those unpressurized, un-oxygenated pilots, keep it below 10,000 feet. Your brain will work better. You don’t want to sound like this guy.

How it Started

One of my best friends in high school, (Doug Gray) was a private pilot. He offered to take me up for a flight in a 1967 Cessna 150, N6228S. We took off from Calhoun, Georgia, and he took me on a scenic tour of the area, I was hooked. I later found out that my English teacher, (Jan Haluska) was also a flight instructor and the school was offering a ground school course the next year, which he taught, along with flight training with the goal of becoming a private pilot. I managed to talk my dad into funding the training, at the time the total cost was right around $500. Cessna 150 rental rates where $12 an hour, and the 172 we used for cross country was $15 an hour including fuel.

Training Begins

It started August of 1977. The fall semester rolls around, and I am enrolled in ground school, and if memory serves me, we met twice a week. First came the paperwork for my student pilot certificate, which at the time was included with the 3rd class medical. I loved ground school, especially learning navigation, plotting courses on the sectional, again this was before iPads and GPS, so we did a lot of dead reckoning and VOR navigation for cross countries. At the end of the course, I made an 83% on the Private Pilot written exam, remember this was before we had the question-and-answer books that on future test I would consume before taking a written.

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Flying

The fun really began on August 22, 1977, it was my first flight. To my surprise we used the same Cessna 150, N6228S that my friend Doug Gray had taken me for a ride in. The Cessna 150 had a standard VFR instrument panel, with one NAV/COM, and one VOR CDI. No intercom, so no headset, at the time I didn’t know what I was missing. I didn’t get my first headset until I started my instrument training in 2003. I was a big guy, I was 6’ 4” and weighed 245 lbs. but I do not remember being uncomfortable in the 150 even with the instructor in the right seat.

First flight lasted .8 hours, and we accomplished orentation, shallow turns, stability, and effects of flight controls. Over the next few months we added steep 720’s, slow flight, stalls, turns around a point, S-turns, emergencies, landings, (short field, soft field and normal).

Learning

At this point I want to talk about a training experience that still stands out. We were close to solo and were practicing takeoffs and landings. Turning base to final Jan got very upset at the way I was cross controlling the aircraft. Cross control is when you are in, say a left turn, and use opposite rudder to line the nose up with the runway. So, he had me depart the pattern and head east to the practice area, and climbing up to 5500 feet MSL, he had me slow down to just above stalling speed, start a left shallow turn and add right rudder. As the airplane stalled I had the strangest sensation, no roller coaster has ever come close, instead of blue sky in the windscreen I was looking at brown ground, and as far as I could tell we were upside down, through the terror of the moment Jan talked me out of the spin, controls neutral, oposite rudder, pull slowly out of the dive. As we leveled off he asked me what altitude we were at, as I remember it was around 2,800 feet MSL. Then he asked me what would happen if I experenced this on base to final in the traffic pattern. The answer was obvious, I would be a pile of wreckage off the end of the runway with a very short-lived aviation career. Needless to say this cured my cross-control tendencies.

Another training event the sticks out in my mind was the first time we did takeoffs and landings at the High School runway. The runway was 1,500 feet long, not sure of the width, but it seems like we had about 5 feet on each side of the wheels when on the center line of the runway. Landing from the south you also had to go between a cutout in the trees to be able to stop in time on the runway. After getting confortable landing here every runway since seemed huge. I remember landing at Chattanooga (KCHA) on a night cross country and commenting to Jan, that I felt like I could of landed sideways on the runway, it felt that large.

Very soon after this I started wearing old shirts to all my flight lessons, the reason for this occurred on February 8, 1978. The lesson that day was stalls, takeoffs and Landing. As we were taxing back in Jan told me it was time for my first solo. I was very excited, and after some last minute advice from Jan, including watch for floating on landing the plane will be light with him not in the right seat, complete 3 takeoffs and landings to a full stop. It was a blast, and at the end of my shirt was shorter in the back because Jan cut the tail out of it signed and dated my solo [an aviation tradition]. Total flight time accumulated on the day of my solo was 18.1 hours. I was now officially a pilot with solo priveleges.

In my next article I start cross country training, when Jan decided that it would be best accomplished this in the 172. Checking out in N5970R was like moving into a 747, this started a love affair with one of my faviriote planes to date. Since this time I have flown over 900 hours in many different models of the 172 and feel like I am stepping into an old friend every time.

More about Randy here: https://www.flyhpa.com/team/randy-delong/

 

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