What TCAS warning is sounded when an aircraft has entered the outer protective layer?

The warning "TRAFFIC, TRAFFIC" is signaled by TCAS (Traffic Collision Avoidance System) when an aircraft enters the outer protective layer, indicating that there is another aircraft nearby that might pose a potential threat. This warning is a preliminary alert that informs the pilot to be aware of surrounding traffic and is crucial for maintaining safety in crowded airspace.

At this stage, the system has identified another aircraft in close proximity, prompting the pilot to look for visual confirmation and be prepared for possible maneuvers. The purpose of this warning is to enhance situational awareness so that pilots can take necessary precautions to prevent any risk of mid-air collisions. This fits within the operational framework of TCAS, which is designed to help mitigate the risk of collisions by alerting pilots to nearby aircraft and urging vigilance.

In contrast, other warnings like "CLIMB, CLIMB," "DESCEND, DESCEND," and "BANK LEFT, BANK LEFT" are responses that indicate specific evasive action should be taken to avoid a collision once a closer threat has been confirmed. They are triggered when a more immediate danger is assessed, thus illustrating a progression from awareness to active maneuvering in response to nearby aircraft.