Stefan wrote:
Tom L. schrieb:

I don't know of a equirement for using it, but there is a reequirement
for having it available on board. See FAR 23.1447.

Thanks, found it.

In simple words: FAR 91.211 requires the use of oxygen above some
altitude, FAR 23.1447 requires that the equipment be a mask above 18'000
feet.

No, it says if the aircraft is equipped with oxygen installed and
certified for flight above 18000 feet then, a mask is required.