Just curious how the stipulation for a mask above 18k would be regulated (no pun intended) given that portable equipment is generally exempted from all regs.
(In the UK, i.e. G-reg aircraft, where if the ANO prescribes carriage/usage of some portable item (in this case o2) the kit has to be
type approved, and that was the mistake in the initial o2 legislation (because the CAA o2 regs were brought in without anybody realising that no type approved o2 kits existed....) which was subsequently repealed by an AIC.)
It's an interesting scenario versus insurance. If you file a flight plan for FL200 but don't carry a mask... But then under the FAA regs (N-reg now) you always have to carry at least one mask, in case somebody gets a blocked nose.
I know of cases where US pilots wrote stories about flights on which they climbed to say 14k feet, on a flight plan stating as much, without carrying o2, and then when somebody pointed out the obvious, they edited them rather fast
