Crew O2 in the US
14 CFR 121.333 C (II)
"(3) Notwithstanding paragraph (c)(2) of this section, if for any reason at any time it is necessary for one pilot to leave his station at the controls of the airplane when operating at flight altitudes above flight level 250, the remaining pilot at the controls shall put on and use his oxygen mask until the other pilot has returned to his duty station."
Operators can also supplement this by requiring a CC seated in a jump or crew seat per approved sop
Last edited by neilki; 25th Mar 2015 at 12:24.
Reason: clarification