The Traffic Alert and Collision Avoidance System (TCAS) is an airborne system that interrogates transponders in other aircraft. From the replies received, the system analyzes the intruding aircraft’s altitude (if altitude reporting), closure rate, and projected flight path to predict a penetration of the airspace about your aircraft. The system then displays the other aircraft visually and aurally to alert you to any potential threats of airspace conflict. The system can report a resolution advisory (RA) only if the other transponder is transmitting an altitude.
TCAS creates a “bubble” of protected airspace around your airplane. At approx 50 NM out TCAS begins to “track” other transponder equipped aircraft. At approx 40 NM out other transponder equipped aircraft can be displayed on the active PFD/MFD. At 30 approx NM a data-link is established with other Mode S transponders to begin conflict resolution planning. At 20 to 48 seconds prior to CPA a TA is issued. AT 15 to 35 seconds from CPA an RA is generated. If the RA is ignored, the crew response is too slow, or the conflict traffic alters its course a corrective RA is issued. When the traffic is no longer a threat TCAS announces “CLEAR OF CONFLICT”.