Tuesday, September 23, 2014

Hypoxia. A Condition To Avoid At All Costs.......

Oxygen in aviation
In revisiting the recent crash/disappearance of an almost new TBM900 many questions arise.
Perhaps the most cogent, why was there no  perception of hypoxia/anoxia on the part of the pilot? After reviewing communications between the pilot and ATC, there never was a mention of a loss of cabin pressure, which leads to hypoxia etc.  

An important question to answer is: why the pilot did not acknowledge any of the indications of a high cabin altitude that should have been available on his instrument panel? His actions indirectly confirm that he didn’t consider anoxia an immediate threat. Was all the low cabin oxygen detection equipment working properly?

A normal response by a trained, experienced pilot as he was, would have been: declaration of an emergency and extremely rapid descent to a lower altitude such as 10000 feet. Therefore one might conclude that normal thought processes were compromised. This could have been secondary to some degree of hypoxia. According to Harrison et al, writing in Principles of Internal Medicine: “When hypoxia is general, all parts of the body may suffer some impairment of function, but those parts which are most sensitive to the effects of hypoxia give rise to symptoms which dominate the clinical picture. The changes in the Central Nervous System are especially important, and here the higher centers are most sensitive. Acute hypoxia, therefore, produces impaired judgment, motor incoordination and a clinical picture closely resembling acute alcoholism.”

I remember well  an emergency decent that I  experienced as a passenger riding in the cockpit of an FAA Boeing 727 out of Oklahoma City in 1997. I was riding “up front” as a passenger while doing a summer internship at The Aero-Medical Institute. We were at approximately 30,000 feet, when the Captain announced loss of pressurization. There was a lot of hissing, oxygen masks dropped like flies and we all struggled to place the masks on correctly as rapidly as possible. The pilot advised us all through the intercom:” loss of cabin pressure, put on masks”. The quick to don masks were on the flight crew and me in seconds. That is all the time one has, in reality, before symptoms of hypoxia will appear. The actual time depends to a great deal on the altitude the decompression occurs.

The pilot deployed everything he could in seconds to get us down rapidly. First the throttles retarded. Then flaps, spoilers and gear deployed, essentially simultaneously. And we were headed down at more than 10,000 feet per minute in a continuous series of steep turns. I don’t know the actual time, as I was holding on for dear life. Finally when we reached about 10,000 feet  things slowed down. The loose papers were gathered up and we could start to take off our masks. It was a very valuable lesson for any pilot to experience.  I had been through a simulated decompression in the training area a few days previous to the flight. So I was somewhat prepared, although the physical effects of all the actions of the rapidly descending, corkscrewing airplane are hard to describe, and so much more dramatic than in the simulator.

I had one other altitude adventure in the old days. As a fairly newly licensed pilot, I was headed to Boston from Syracuse in an older Cessna 182. As I was only licensed for VFR flight, I had to plan to avoid the clouds, which were forecast to be broken along the route. By the time I was halfway to Albany, the clouds began to thicken. I had to decide whether to keep on going or keep dropping lower to get under the developing cloud layer, or turn back. I decided to keep going and therefore began to climb to get on top. That climb continued until I was well past Albany and ended up at 15,500 feet. Although I knew about hypoxia, I never had really experienced it. That was going to change on this flight. I noted that my breathing rate had increased, and that I had developed a slight headache. My ability to function seemed ok though. But I knew that I really couldn’t plan on going any higher. I probably had also reached the service ceiling of the bird I was flying anyway. If I had to continue much longer at 15,500 feet I decided that I would have to declare an emergency and do an IFR descent as I had no Oxygen aboard. Fortunately as I reached for the microphone, I noted that the clouds below starting to thin, with glimpses of the ground. I immediately started to descend and was able to land outside Boston at Hanscom Field under VFR conditions. That trip was a major stimulus for my continuing with my IFR training and getting my ticket.

In summary. Hypoxia is a very real insidious hazard that threatens every pilot. At the earliest sign of it, quickly put on your readily available O2 mask if available. If not proceed to an immediate lower altitude if possible and declare an emergency.

Happy, safe clear headed flying.

Reference: See FAR 91.211 Which goes further into anoxia and when supplemental Oxygen is to be used.