411A:
Further to my last posting I have had some further thoughts on the subject and would like to make the following comments:
The first time I ever got involved with an oxygen system was more than 40 years ago when I was on advanced flying training in the Royal Air Force flying the DH Vampire. This consisted of a simple constant-flow system with the option of a 100% setting. The oxygen mask was an "H" Type which had a vent plug on the side.
Now then, we used to crawl up to 38,000 feet with this pretty basic apparatus with the purpose of making "high speed runs".
None of us had any difficulty with this and I cannot believe that we were being asked to do something with fatal consequences.
The first pressure demand system that I came across used the American A12A regulator. This was a great improvement over the constant flow system and, incidentally, when fitted to one of my gliders for the purpose of mountain wave flying, at least doubled the endurance of a 750 litre O2 bottle!
I have in front of me Volume 6 of the RAF's AP3456 entitled Aviation Medicine and Survival. In Part 1 Section 1 Chapter 2 which deals with the Physiological Effects of Altitude and at the end of Paragraph 19 it states:
"The oxygen concentration required at an altitude of 34,000 feet in order to maintain an alveolar PO2 (oxygen pressure) of 100 mm Hg is 100%.
20. Ascent to altitudes above 34,000 feet, even whilst breathing 100% oxygen, results in the alveolar PO2 falling below that produced by breathing air at ground level, ie PO2 of 100 mm Hg, breathing 100% oxygen at an altitude of 40,000 feet produces an alveolar PO2 of about 60 mm Hg ie an intensity of hypoxia equivalent to that produced by breathing air at an altitude of 8,000 - 10,000 feet. Ascents to altitudes higher than 40,000 feet breathing 100% oxygen gives rise to significant hypoxia. As indicated by the corresponding alveolar PO2's, the intensity of the hypoxia produced by breathing 100% oxygen at 45,000 feet is slightly more than the hypoxia produced by breathing air at 18,000 feet. The maximum altitude at which it is acceptable to fly an unpressurized aircraft, considering hypoxia alone, when oxygen is breathed at ambient pressure, is 40,000 feet. In the event of decompression of a pressurized aircraft when rapid descent is initiated immediately the cabin pressure falls, breathing 100% oxygen at ambient pressure will provide adequate protection against severe hypoxia at cabin pressures up to 43,000 feet. Severe hypoxia can only be avoided on exposure to altitudes above 40,000 feet by increasing the total pressure of the gasses in the lungs above the pressure of the environment, a technique termed positive pressure breathing- usually abbreviated to "pressure breathing".
As far as I am aware no airliner is equipped for the wearing of pressure breathing equipment by the crew. Most are equipped with a pressure demand system with Normal, 100% and Emergency Flow options.
So if we were (in the interests of flight safety) to summarise the preceding information I think it would be reasonable to assume that most systems are adequate to sustain life at a maximum height of 34,000 feet. Therefore, if you regularly fly above 40,000 feet then you should be very aware that you should get down to at least 34,000 feet pronto.
Personally speaking, I have not been above 40,000 feet for 20 years. In fact, I have not been above 35,000 feet for 15 years.
This takes me back to the last line in my original posting in which I said:
"Happiness is freight - I can stay up there without a single worry for I shall run out of fuel long before I run out of oxygen!". I stick by this statement but I should have added "on the current equipment that I operate".
In any event, there should be no reason on earth for a freighter to have to descend to 10,000 feet for they do not have a rubber jungle in the back and that was my original point!
Lou Scannon has also raised a very interesting and important side issue about the fire suppression issue.
No doubt you will spend your Christmas Day scanning the pages of pprune trying to figure out just who you are going to attack next so let me take this opportunity to wish you a Merry Christmas and a Happy New Year.
PS. We are still waiting to hear how your Oakland chappie managed to try to start all 4 engines on a DC-6 at once when it appears from the DC-6 chaps that this is impossible.
PPS. Was his Chief Pilot Bill K...... by any chance?
PPS. Knowing what a stickler for accuracy that you are the dead centre of town on this side of the Pond is a "cemetery" not a "cemetary".