From memory, not having done a refresher course for a few years:-
At FL100 the partial pressure of oxygen in the air (that is, number of molecules per unit volume) is enough to allow more-or-less undegraded performance of a fit human being. (Hence, the working ceiling for most light aircraft)
At FL250, 100% oxygen at the "local" pressure gives the same partial pressure of Oxygen. So this is the limit for unpressurised cabins with oxygen masks. (Hence the ceiling for aircraft such as the Tucano).
Up to FL500, it is safely possible to pressure breath in the event of a cabin pressure failure. Pressure breathing means high positive pressure of O2 through the mask. The effect of this is that you have to force air out, and relax to allow air in - uncomfortable and requires training, but not particularly difficult. (Can leave your chest muscles a bit bruised). This is therefore the limit for "normal" pressurised cabins. It is accepted that pressure breathing is useable by crew but not pax - they will lose consciousness, but recover without ill effects once below FL100 if a controlled rapid descent is made.
Above FL500 pressure breathing alone is inadequate. There are three solutions, which are:-
- Approach used in the Lightning, which is a pressurised pilots jerkin, counteracting his chest's natural tendency to try and explode.
- Approach used in the SR71, Canberra, etc. a pressure suit (not quite a space-suit, but pretty close).
- Approach used in Concorde, very small windows, very powerful cabin compressors, and the ability to maintain a survivable cabin pressure (that is below FL250 equivalent) despite a given number of failed windows.
The exact heights vary according to authority, the above were being used last time I did a B-cat refresher at BDN about 7 years ago, but I'm fairly certain are still the standard RAF and UK CAA figures. I think the USAF and FAA use FL120, FL240 and FL480, but all pretty much in the same ballpark.
G