14 CFR § 121.394 - Flight attendant requirements during passenger boarding and deplaning.
(1) Reduce the number of required flight attendants by one, provided that:
(iii) The airplane engines are shut down; and
(iv) At least one floor level exit remains open to provide for passenger egress; or
(i) Describes in the manual required by § 121.133:
(A) The necessary functions to be performed by the substitute pilot or flight engineer in an emergency, to include a situation requiring an emergency evacuation. The certificate holder must show those functions are realistic, can be practically accomplished, and will meet any reasonably anticipated emergency; and
(ii) Ensures that the following requirements are met:
(B) The substitute pilot or flight engineer is trained in all assigned flight attendant duties regarding passenger handling.
(C) The substitute pilot or flight engineer meets the emergency training requirements for flight attendants in evacuation management and evacuation commands, as appropriate, and frequency of performance drills regarding operation of exits in the normal and emergency modes on that type aircraft.
(D) The substitute pilot or flight engineer is in possession of all items required for duty.
(E) The substitute pilot or flight engineer is located in the passenger cabin.
(F) The substitute pilot or flight engineer is identified to the passengers.
(G) The substitution of a pilot or flight engineer for a required flight attendant does not interfere with the safe operation of the flight.
(H) The airplane engines are shut down.
(I) At least one floor-level exit remains open to provide for passenger egress.
(b) During passenger deplaning, on each airplane for which more than one flight attendant is required by § 121.391, the certificate holder may reduce the number of flight attendants required by that paragraph provided:
(1) The airplane engines are shut down;
(2) At least one floor level exit remains open to provide for passenger egress; and
(c) If only one flight attendant is on the airplane during passenger boarding or deplaning, that flight attendant must be located in accordance with the certificate holder's FAA-approved operating procedures. If more than one flight attendant is on the airplane during passenger boarding or deplaning, the flight attendants must be evenly distributed throughout the airplane cabin, in the vicinity of the floor-level exits, to provide the most effective assistance in the event of an emergency.
(d) The time spent by any crewmember conducting passenger boarding or deplaning duties is considered duty time.