On Wednesday, October 2, 2019 at 6:17:16 AM UTC-5, Tango Eight wrote:
On Tuesday, October 1, 2019 at 3:44:11 PM UTC-4, RR wrote:

The legalitys are not so clear [...]

FAR 23.1447 spells it out (for certificated aircraft). Above 18K the requirement is for a mask that covers nose and mouth.

The fact that some people can get away with a cannula at 26K (once) does not mean that this is a sound practice. We've had two hypoxia incidents (that I know of) at Mt Washington due to pilots emulating certain old timers. Physiology varies. Physical fitness doesn't seem to be a factor (in fact casual observation suggests the reverse :-)).

T8

Yes, this applies to required aircraft equipment, not pilot use under part 91.
BUT -- everyone is different.
There was a study done with *young, fit* pilots showing that the Mtn High canula *could* keep O2 sats above 90% to FL035.
But the older and un-fitter we get, and bearing in mind the large individual differences,
The only safe thing, if you're flying above FL012, is to spend $180 or whatever and buy a good wris****ch-style oximeter and take the reading from a warm finger -- and *verify* that your oxygen supply is at least getting to your finger.
And remember, the normal, physiological hyperventilation of low pressure altitudes can decrease brain perfusion by about 20% while finger perfusion remains normal.
Bottom line: if you feel stupid, you are stupid, and it's time to descend no matter what the oxygen system is doing and no matter what the oximeter says. Been there.
DrDan