Flight above FL100
What is the hard and fast about unpressurised (or without supplemental oxygen) flight above FL100?
I understand that this is the normal limit of unpressurised flight, for good reasons, and I have searched and read quite a few threads about degraded ability at altitude etc etc.
Lets say the lower limit of an airway is FL110 and you would have to stick at this altitude for half an hour or so to use it... legal?
Cheers, CS