I'm currently using the two add-on 172s and hopping around South America.  I'm way up in the mountains and (though I think I'm answering my own question) how high could a 172 go before everyone on board ran out of air?

Depends on their physiology. Someone who lives at the top of a mountain can go a bit higher than someone who lives on the beach because they're already adapted to breathing less air. Legally speaking, you need to be on oxygen if you're above 12,500 feet for more than half an hour, and if you're above 14,000 feet you need to be on oxygen immediately (not that that last matters much, because the service ceiling of the 17x series is 14k, if I remember right).

 

Edited April 20, 20215 yr by eslader

It also depends on age and whether you smoke or have other lung issues.

A good article:

 

https://abcnews.go.com/Business/FlyingHigh/story?id=1936770&page=1#:~:text=At 10%2C000 feet above sea,atmospheric pressure at any altitude).

Ultimately, you're going to be limited by the service ceiling of the aeroplane rather than the supply of air, although that ceiling is of course as a result of the thinner air.

The Cessna 172, assuming it is in top notch condition, can make it up to its service ceiling of 13,500 feet (possibly a bit more depending on the fuel, payload and air temperature), and whilst that is an altitude where you could and almost certainly will be affected by the thinner air to some degree, that's not likely to be fatal for someone in good health, although it will almost certainly affect their judgement to some extent.

Pilots in the latter part of WW1 regularly patrolled at altitudes way higher than that without oxygen. The service ceiling of the Sopwith Camel from 1917 for example, is 19,000 feet and you can frequently read combat reports from pilots on the Western Front where they are scouting about at 15-17,000 feet without regarding this altitude as being especially remarkable, since there were German reconnaissance aeroplanes at the time which could make it up over 20,000 feet, which made them fairly invulnerable to most British and French aeroplanes, although pilots frequently comment that they had a severe headache after such a patrol at altitude and those high flying recon crews were supplied with oxygen.

There are no reliable statistics on what percentage of losses to such personnel were related to being up at that altitude, but we can surmise that at least some losses were as a result of being up at that altitude, not least because it would certainly affect one's tactical judgement. Having said that, humans have climbed Mount Everest without oxygen, and that's over 29,000 feet, so I daresay the guys who did that would probably think oxygen levels at 13,500 feet were pretty comfortable by comparison when they were on the way down.