14 CFR § 121.393

On each airplane for which a flight attendant is not required by § 121.391(a), the certificate holder must ensure that a person who is qualified in the emergency evacuation procedures for the airplane, as required in § 121.417, and who is identified to the passengers, remains:

On board the airplane; or

Nearby the airplane, in a position to adequately monitor passenger safety, and:

The airplane engines are shut down; and

At least one floor level exit remains open to provide for the deplaning of passengers.

On each airplane for which flight attendants are required by § 121.391(a), but the number of flight attendants remaining on board is fewer than required by § 121.391(a), the certificate holder must meet the following requirements:

The certificate holder shall ensure that:

The airplane engines are shut down;

At least one floor level exit remains open to provide for the deplaning of passengers; and

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

The certificate holder may substitute for the required flight attendants other persons qualified in the emergency evacuation procedures for that aircraft as required in § 121.417, if these persons are identified to the passengers.

If only one flight attendant or other qualified person is on board during a stop, that flight attendant or other qualified person shall be located in accordance with the certificate holder's FAA-approved operating procedures. If more than one flight attendant or other qualified person is on board, the flight attendants or other qualified persons shall be spaced throughout the cabin to provide the most effective assistance for the evacuation in case of an emergency.