Hypoxia
Just back on here after several hours. My sincere apologies if this is yet another repetition!! Before going to bed I read someone asking for something on hypoxia. From my own experience, here goes...
Hypoxia means low oxygen, a gas we all need in order to live. Whereas all pilots are required to know about hypoxia, all aircrew in the RAF and, I expect, most western-aligned armed forces, are subjected to it in a barochamber.
There is a world of difference between learning something from a book and experiencing it. Sure, civvies will react as well as military-trained colleagues in a given situation. It is in not detecting hypoxia where there is a potential for major error. Experience must trump anecdote.
An explosive decompression gets your attention, make no mistake! The first reaction, instilled many, many times in every pilot's life, is to get on oxygen. By far the most dangerous situation derives from a slow leak of cabin pressure because all the clues can be rendered subtle enough as to pass the detection threshold unnoticed.
Aircraft are designed to fly high but humans are not. Unpressurised flight would require so much oxygen for everyone to breathe that it would be impractical. Plus, the pressure changes, particularly in the descent would deliver so many nose bleeds, ear drum ruptures and blown sinuses in a sufficient number of the population that the experience would render the prospect of air travel too unpleasant.
Aircraft designed for high altitude flight are designed to operate with a cabin pressure as close as practicable to mean sea level pressure. Were they designed to operate AT mean sea level pressure, the extra strengthening would render them too heavy. And so, as with most things in aviation, we have a compromise. Airliner cabins operate at a pressure differential that the structure can cope with, while keeping the air inside at a pressure which most people can cope with.
Aircraft pressurisation systems work, not by shutting the air in, but by controlling the flow of cabin air out to atmosphere. Were something to create an extra hole where the outflow defeats the pressure regulator, the net result must be a reduction in cabin pressure.
At all altitudes where current airliners fly, the atmosphere is a gas whose composition is 78% nitrogen, 21% oxygen and 1% other gases. Air pressure reduces with altitude. In your lungs, whilst alveolar gas still contains 21% oxygen, the number of molecules decrease as the pressure decreases. Fewer molecules of oxygen deprive the brain from functioning correctly - including the ability to process information and act on it!! Yes, including the ability to recognise the symtoms of hypoxia!! Well from here it gets messy!
In a slow leak, if you're busy, there will be too many distractions for you to notice the symptoms, either in yourself or in the pilot in the seat next to you. You might grumble at the First Officer for missing a radio call. Or for fumbling a simple calculation or (and don't jump on this one because it needs to be looked at in the round) mis-setting some equipment. You feel tired. But it is 1am! You have a headache. But ATC have been a pain, the ordinarily switched on First Officer is still making mistakes!! Was that radio call for us? Where's that glass of water you asked for AGES ago?
If you were alerted to the possibility of hypoxia, you might, given adequate cockpit lighting, wonder why your First Officer's lips were blue - not that you ordinarily look at them! Or that your finger tips had a blueish tinge!
I just think that it would be useful for all airline pilots to experience hypoxia, as we did in the RAF. There IS a difference between experiencing it and reading about it. Some will argue it is a small difference. But what if the "feelings" a pilot were feeling triggered a memory? It's busy. There are distractions. It is easy to pass tiredness, making basic mistakes that everyone makes from time to time and having a headache, to perfectly reasonable causes. But hypoxia is a perfectly natural cause, too. Of course it is money!! In the end, you only get the safety you're prepared to pay for.