All airliners, business jets and turboprops operated commercially have pressurization systems to maintain air in the cabin under pressure, as described in “Pressurization for Dummies – Part I“. Light aircrafts pilots like me fly are normally not pressurized. This is because the normally aspirated (non-turbo) engines we use are not able to climb above 15′000 feet…

So when we fly in the mountains at 10′000 or 13′000 feet (see this Matterhorn gallery, we were flying at 13′500 feet), the air pressure in our lungs is about half what it is at ground level, and less oxygen goes in blood. As the brain needs an enormous quantity of oxygen, it is the first organ to suffer from the lack of oxygen (a.k.a. hypoxia).

When preparing for theory exams, we learn about the symptoms of hypoxia:

Impaired Judgement – Headache – Tingling – Increased rate of breathing – Muscular Impairment – Memory Impairment – Visual Sensory Loss – Tunnel Vision – Impairment of Consciousness – Cyanosis – Formication – Unconsciousnes – Death

At high altitude, death can occur within minutes. So, how to cope with lack of oxygen when flying at altitude in non-pressurized airplanes ? There are two options…

Pilots flying with turbo engines can climb up to 22′000 feet, and at this altitude unconsciousness would occur after approximately 30 minutes. To survive this altitude without pressurization, the solution is to use nasal oxygen cannula. This does not increase pression, but as the pilot will then breath 100% oxygen (compared to 20% in normal air), so more will go in blood.

It is obviously important to make sure the oxygen tanks are not empty, and that nothing obstructs the cannulas. On a famous case, a light twin engine was flying from southern to northern Europe. While in cruise, the pilot engaged the auto-pilot and slid his seat backwards, unfortunately blocking oxygen supply. After a couple minutes, both fell asleep, and after hours, one of the engines ran out of fuel. The plane started a descent, and when reaching about 8′000 feet, the pilots did wake-up.

Can you imagine the situation ? They woke up in an unknown location, with one engine out ! They could manage the landing properly. What if both engines had run out of fuel at the same time ? What if they had been flying westbound and wake up 1′000 miles away, over the Atlantic Ocean ?

The second way to avoid the effects of hypoxia is simply to not fly for too long above 10′000 feet. My “personal best” is 30 minutes at 13′500 feet, in the Alps. As I knew about the symptoms, I was aware of them and monitored myself…

Hypoxia affects all of us differently, and in my case it started with dizzy feelings, and a light headache. Thinking of the symptoms probably protected me from impaired judgement, and I descended back to destination. The symptoms disappeared quickly, and I felt perfectly fit when landing…

If you have any experience with hypoxia, feel free to share it. I would also be interested in knowing what is the maximal altitude you flew in non-pressurized aircraft. Mine is 13′500 feet.
This post is the second in a series about pressurization,. Read part one here.

GolfHotelWhiskey.com has a good post about a ride in the hypoxia chamber. The author describes much more advanced symptoms. The kind you don’t want to experience at controls of a plane…