(b) A required flight crewmember may leave the assigned duty station -
(1) If the crewmember's absence is necessary for the performance of duties in connection with the operation of the aircraft;
(2) If the crewmember's absence is in connection with physiological needs; or
(3) If the crewmember is taking a rest period, and relief is provided -
(i) In the case of the assigned pilot in command during the en route cruise portion of the flight, by a pilot who holds an airline transport pilot certificate not subject to the limitations in
§ 61.167 of this chapter and an appropriate type rating, is currently qualified as pilot in command or second in command, and is qualified as pilot in command of that aircraft during the en route cruise portion of the flight. A second in command qualified to act as a pilot in command en route need not have completed the following pilot in command requirements: The 6-month recurrent flight training required by
§ 121.433(c)(1)(iii); the operating experience required by
§ 121.434; the takeoffs and landings required by
§ 121.439; the line check required by
§ 121.440; and the 6-month proficiency check or simulator training required by
§ 121.441(a)(1); and
(ii) In the case of the assigned second in command, by a pilot qualified to act as second in command of that aircraft during en route operations. However, the relief pilot need not meet the recent experience requirements of
§ 121.439(b).
Added para (b) for completeness. And this isn't the reason why we have an FA in the cockpit when one of us leaves, getting up to let the other guy in could be a duty in connection with operating the aircraft. These rules predate 9/11. These rules are for duty rest and lav breaks, that is it. The security theater, with locked doors, code words and all the secure info we won't disclose on a public website is in a different place.