(1) The PIC of an aircraft shall ensure that flight crew members engaged in performing duties essential to the safe operation of an aircraft in flight, use supplemental oxygen—
(a) continuously when the flight deck pressure altitude exceeds 10 000 ft for more than 120 minutes intended flight time; and
(b) at all times when the flight deck pressure altitude exceeds 12 000 ft.
(2) The PIC of an aircraft shall ensure that when a flight is conducted above FL 410, at least one pilot at a pilot station wears an oxygen mask when the other pilot leaves the flight deck for any reason.