All who fly the friendly skies regularly can probably recite from memory the safety instruction that bored-appearing flight attendants are mandated to present as planes at U.S. and Canadian airports taxi for takeoff. But as a parrot ignorantly mimics its keeper, oftentimes I wonder if any of the proffered information is actually sinking into stupefied passengers?
For example, we all know from being told countless times that if there is a change in cabin pressure, oxygen masks will fall from the overhead compartments; hopefully you also recall that you’re supposed to yank on the tubing, and then put your own mask on first before helping those next to you, especially since, depending on the altitude, you might have less than 15 seconds of useful consciousness left once the masks deploy.
Why that yank? And where is the oxygen stored that will sustain a hundred or more passengers, all breathing through masks at the same time? Has anyone who is not in the aviation industry ever stopped to think about that?
Actually, there is no oxygen stored on planes. Not only would such a combustible be dangerous to have onboard, but the storage needed – either one big centralized tank, or many smaller individual ones – would take up valuable space within the limited confines of the jet fuselage.
When you pull on the tubing, the tug triggers a spring-loaded mechanism that sets off a spark inside a small generator, the size of a canister of tennis balls, that is located over each seat. The resulting spark ignites tablets of lead styphnate and tetracene, which in turn generate heat. A mixture of sodium chlorate, barium peroxide, and potassium perchlorate inside the canister, once heated, releases oxygen.
[sidebar: of course, you might also smell a faint burning odor from the spark and heat, but this is no cause for alarm in light of what else is probably ongoing at that time all around you. In fact, if the plane is actually on fire, the masks usually won’t deploy, so as not to make the fire even worse due to the addition of extra oxygen in the immediate environment]
This chemical reaction won’t last for long, and production inside the canister starts to fade after ~15 minutes. But assuming that you put on your mask as soon as it dropped from the ceiling, you should have enough oxygen from the bubbling compound overhead to sustain you until the pilot can quickly get the stricken jet below 10,000′ altitude. At that point, ambient air pressure will be high enough for relatively normal atmospheric breathing.
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