Why do airplanes pressurize their cabins?
Because if they didn't, most of the passengers and crew would die.
They have to, so that the air inside is dense enough to provide breathable (Oxygenized) air for the passengers. It costs money to heat this outside air from ambient temperature to cabin temperature, so the airlines routinely "recycle" most of the air, forcing passengers to breathe air that is loaded with environmental pollutants and pathogens.
See what happened to Payne Stewart. That's why planes are pressurized for those that don't have portable oxygen tanks next to them.
I flew in a non-pressurized cabin once. Detroit to chicago; at about 10 minutes into the flight they told us they couldn't get the cabin to pressurize. They said they'd be flying a lot lower to try to help and to let them know (as if they could do something about it) if they were any problems.
I figure they only flew at say 14-16k feet, which i think would be about 21inHg. (so about a 27-28% drop in pressure?)
Wasn't so bad. Did the opposite ears-popping thing, other than that it was fine. I wonder if they turned up the O2 or anything. I don't think anyone on board complained at all.
I'm in pretty good shape, but when I went skydiving, my instructor said I was turning white and gave me oxygen. I think a rapid ascent from sea level to 13,500 feet can be a strain.
Even ascending a mile-high mountain can stress people if the climb is steep and they are not in shape, so a rapid change to nearly 3 times that altitude in an unpressurized plane should have some effects on some folks, especially if your pumped about the prospect of jumping out of the thing and your metabolic rate is a bit high.
SOP for us was to stay below 10,000 ft without supplemental oxygen. I believe most emergency procedures call out the 10,000 ft altitude as the level to get below asap.
Oxygen must be used by a pilot any time above 14,000 feet and above 12,500feet in excess of 30 minutes. (What about the passengers?).
A full fitting face mask must be used above 18,000 feet. The percentage of oxygen increases with altitude. It's 100% at around 43000 feet.
Above 43,000 feet, the pressure is too low for even 100% oxygen to do the job, so a pressurized system is required (suit or cabin).
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