this post was submitted on 04 Aug 2025
493 points (98.8% liked)
xkcd
12541 readers
83 users here now
A community for a webcomic of romance, sarcasm, math, and language.
founded 2 years ago
MODERATORS
you are viewing a single comment's thread
view the rest of the comments
view the rest of the comments
Aircraft are typically pressurized to the equivalent of about 5000-7000 feet altitude.
They do not oxygenate the cabin. The oxygen supplies on board are usually in the form of chemical generators, sometimes known as "oxygen candles", and can only provide about 15 minutes supply. That should be plenty of time to descend below 10,000 feet, where everyone can come off oxygen. They don't "burn" those chemical generators except in actual emergencies.
Pilots and crew have a sufficient supply of bottled oxygen. Pilots and crew are required to go on oxygen if they spend more than 30 minutes above 12,500 feet cabin pressure, or any time over 14,000 feet. Passengers are required to be on supplemental oxygen above 15,000 feet cabin pressure.
Above 35,000 feet flight altitude, at least one pilot must either be on oxygen, or have a mask that can be donned in less than 5 seconds.
The oxygen requirements only come into play when you fly depressurized. The reason that passengers aren't on oxygen when you are flying pressurized at 30,000 feet (probably 5-7,000 ft pressure altitude inside the plane) is that the air they are pressurizing (thin, high altitude air) is still the same ratio of nitrogen/oxygen/CO2 even though there are smaller amounts of each.
The FAAs requirement of people being on supplemental oxygen only matters in non-pressurized flight, like small Cessnas and pressurized aircraft experience a depressurization emergency.
That is true for the 12,500, 14,000, and 15,000 feet cabin pressure numbers I gave, yes.
However, the oxygen requirement associated with the 35,000 foot number I gave applies to both pressurized and unpressurized flight. If your aircraft is at or above 35,000 feet pressure altitude, at least one of the pilots must be on oxygen, or must have a quick-donning mask available at all times.