Above a certain altitude, your thinking gets fuzzy, but you stay conscious. The regs allow private pilots to fly at 12,500 feet for up to 30 minutes without any oxygen. You won’t die if you go above that, but you won’t be thinking as clearly, which itself can be fatal if you do something dumb.
Above another altitude, your lungs won’t absorb enough oxygen from the ambient air to provide enough oxygen to keep the brain functioning. Unconsciousness and eventual death result. For a non-acclimated person, I’d guess this altitude to be somewhere around 20,000 feet.
The next threshold is the altitude at which you need to be breathing 100% oxygen to get the same partial pressure of oxygen into your lungs that you would get at sea level. That altitude is 34,000 feet. As you climb past that, your lungs will absorb less and less oxygen even though you may be breathing it pure. The oxygen absorbed drops off rapidly after that. By 40,000 ft, you’re getting the same amount of oxygen you’d be getting without a mask at 10,000 ft.
The next threshold is the altitude at which, even if you are breathing oxygen, the partial pressure will be so low that your lungs can’t absorb the oxygen you’re breathing. At these altitudes, if you’re not in a pressure suit, you die. Even if you’re breathing pure oxygen. That altitude is around 50,000 feet or so.
The next threshold is the altitude at which water vapor boils, including the water vapor in your skin. That’s 55,000 ft. If you aren’t wearing a pressure suit, you’ll get real uncomfortable real quick.
At 65,000 feet, blood at body temperature will boil. Yuck.
There are some biz jets that fly at outrageously high altitudes. Payne Stewart’s Lear 51 could cruise at 51,000 ft. As I recall, the regs required one pilot to be wearing a mask at all times at that altitude, and I imagine the procedure for an emergency decompression involved a rapid dive to a safer altitude, but I’m not sure. Large commercial jets don’t go anywhere near that high, cruising instead somewhere between 25,000 and 40,000 ft typically. At those altitudes, if there’s a decompression you have quite a bit of time if you’re wearing a mask to descend to safer altitudes.
Now, a completely different issue is what happens if you lose pressure all at once. Lots of bad stuff can happen, from the bends to an embolism that can kill you almost instantly. Plus, explosive decompression can render the aircraft unflyable. I recall a large jet that crashed after a pressure door failed and the failure caused the skin to peel back like a banana.
The first commercial jet was the de Havilland Comet. Beautiful looking thing, with one problem - its windows were designed too big. The structure around the windows wasn’t strong enough, and after enough pressurization/depressurization cycles, a crack would form, the window would blow out, and the aircraft would disintegrate.
Oh, it wasn’t just that the Comet’s windows were too big - they were square. A square hole causes stress risers, which leads to eventual cracking and failure. That’s why jet aircraft windows are round or oval.
I knew that. I just meant that someone accustomed to living in the highest inhabited altititudes would stand a better chance at 37,000 feet than a flatlander.
Ah. On a plane that could be pressurized, but just wasn’t, I bet you’re right. I was thinking of the Cessna 172s and PA-140s I used to fly. I was always warned that if the static port got clogged and you smacked open an instrument to allow the altimer to work, the lower pressure in the cabin would throw things off a bit.
I was thinking of convective currents. In my mind’s eye, I would imagine that the air on top of a mountain would be denser, because it would have cooled as much as the atmosphere would allow and settled on top of the ground (high altitude though it may be). OTOH, if a plane were in flight at the same altitude as the top of the mountain, I would imagine that the air would be less dense, due to the mix of the rising of warm air and sinking of air that was beginning to cool. That mix of warm and cool air surely can’t be as dense and the cooled air at the bottom of a convective current, can it? What do I have wrong there?
Possibly, but it also may be simpler and cheaper to have the frame and seals around the glass made from a single strip that can be wrapped around it.
The lost Comets had their crack initiation sites at the (square) corners of the ADF antenna ports, btw. The window-corner stress concentration sites were determined during cyclic fatigue testing as part of the investigation. Window size as such is not relevant; airliner windows are fairly small because multiple layers of thick glass are much heavier than single thicknesses of sheet aluminum.
That’s true, but your airspeed and altimeter readings will still be accurate enough to land with. I hope you were told to try opening the alternate static source valve first - it’s cheaper than replacing an instrument face.
Cabin pressure might be slightly lower than ambient. If you opened a vent air inlet, it might then be slightly higher. The effects would be small, though perhaps not so small as to be insignificant - at a guess, a practical max. of 100’ on the altimeter.
I’d say the only thing wrong is that, even if we postulate a large pool of unusually cold air near the top of a mountain (rather rare, I suspect), the increase in pressure (and thus in the partial pressure of oxygen) would be tiny. In fact, the increase would depend on the depth of that pool of cold air - if it were many hundreds of feet, then you could perhaps start to measure some meaningful increase in available oxygen. But then, by definition, you are well below the summit.