Tom L. wrote:
On Tue, 30 Oct 2007 17:18:00 +0100, Stefan
wrote:

I've heard that above some altitude, the FARs require the use of an
oxygen mask rather than a cannula in US registered aircraft. Where do I
find this requirement? (FAR 91.211 doesn't mention it.)

Thanks
Stefan

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.

That reg only covers the certification requirements for aircraft with
installed oxygen systems.