14 CFR § 121.394

During passenger boarding, on each airplane for which more than one flight attendant is required by § 121.391, the certificate holder may:

Reduce the number of required flight attendants by one, provided that:

The flight attendant that leaves the aircraft remains within the immediate vicinity of the door through which passengers are boarding;

The flight attendant that leaves the aircraft only conducts safety duties related to the flight being boarded;

The airplane engines are shut down; and

At least one floor level exit remains open to provide for passenger egress; or

Substitute a pilot or flight engineer employed by the certificate holder and trained and qualified on that type airplane for one flight attendant, provided the certificate holder—

Describes in the manual required by § 121.133:

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

How other regulatory functions performed by a flight attendant will be accomplished by the substitute pilot or flight engineer on the airplane.

Ensures that the following requirements are met:

The substitute pilot or flight engineer is not assigned to operate the flight for which that person is substituting for a required flight attendant.

The substitute pilot or flight engineer is trained in all assigned flight attendant duties regarding passenger handling.

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.

The substitute pilot or flight engineer is in possession of all items required for duty.

The substitute pilot or flight engineer is located in the passenger cabin.

The substitute pilot or flight engineer is identified to the passengers.

The substitution of a pilot or flight engineer for a required flight attendant does not interfere with the safe operation of the flight.

The airplane engines are shut down.

At least one floor-level exit remains open to provide for passenger egress.

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:

The airplane engines are shut down;

At least one floor level exit remains open to provide for passenger egress; and

The number of flight attendants on board is at least half the number required by § 121.391, rounded down to the next lower number in the case of fractions, but never fewer than one.

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.

The time spent by any crewmember conducting passenger boarding or deplaning duties is considered duty time.