Why are two separate transmissions required to alert/brief aircrew and passengers during an emergency?

The requirement for two separate transmissions during an emergency is based on the technical capability of the communication systems onboard the aircraft. The interphone system, used for crew communication, and the passenger address system, which communicates with passengers, often operate independently of each other. This means that both systems cannot broadcast messages at the same time.

As a result, alerts and information must be conveyed through two distinct channels—first to the crew using the interphone system, followed by a separate announcement to passengers via the passenger address system. This protocol ensures that the crew is aware of the situation first, allowing them to prepare and manage the emergency effectively while also ensuring that passengers receive critical and appropriate information subsequently, depending on the situation at hand.

The separation of transmissions is crucial for maintaining effective communication and situational awareness during emergency scenarios.