14 CFR § 135.150

A public address system which—

Is capable of operation independent of the crewmember interphone system required by paragraph (b) of this section, except for handsets, headsets, microphones, selector switches, and signaling devices;

Is approved in accordance with § 21.305 of this chapter;

Is accessible for immediate use from each of two flight crewmember stations in the pilot compartment;

For each required floor-level passenger emergency exit which has an adjacent flight attendant seat, has a microphone which is readily accessible to the seated flight attendant, except that one microphone may serve more than one exit, provided the proximity of the exits allows unassisted verbal communication between seated flight attendants;

Is capable of operation within 10 seconds by a flight attendant at each of those stations in the passenger compartment from which its use is accessible;

Is audible at all passenger seats, lavatories, and flight attendant seats and work stations; and

For transport category airplanes manufactured on or after November 27, 1990, meets the requirements of § 25.1423 of this chapter.

A crewmember interphone system which—

Is capable of operation independent of the public address system required by paragraph (a) of this section, except for handsets, headsets, microphones, selector switches, and signaling devices;

Is approved in accordance with § 21.305 of this chapter;

Provides a means of two-way communication between the pilot compartment and—

Each passenger compartment; and

Each galley located on other than the main passenger deck level;

Is accessible for immediate use from each of two flight crewmember stations in the pilot compartment;

Is accessible for use from at least one normal flight attendant station in each passenger compartment;

Is capable of operation within 10 seconds by a flight attendant at each of those stations in each passenger compartment from which its use is accessible; and

For large turbojet-powered airplanes—

Is accessible for use at enough flight attendant stations so that all floor-level emergency exits (or entryways to those exits in the case of exits located within galleys) in each passenger compartment are observable from one or more of those stations so equipped;

Has an alerting system incorporating aural or visual signals for use by flight crewmembers to alert flight attendants and for use by flight attendants to alert flight crewmembers;

For the alerting system required by paragraph (b)(7)(ii) of this section, has a means for the recipient of a call to determine whether it is a normal call or an emergency call; and

When the airplane is on the ground, provides a means of two-way communication between ground personnel and either of at least two flight crewmembers in the pilot compartment. The interphone system station for use by ground personnel must be so located that personnel using the system may avoid visible detection from within the airplane.