Airborne Collision Avoidance System

ACAS (Airborne Collision Avoidance System) is the ICAO standard name for systems that independently detect nearby aircraft via transponder interrogation and provide collision avoidance guidance to pilots.

TCAS II is the most common ACAS implementation. It issues Traffic Advisories (TAs) for awareness and Resolution Advisories (RAs) that command specific vertical maneuvers (climb/descend) to avoid collision.

• ACAS operates independently of ground-based ATC
• Resolution Advisories take priority over ATC instructions
• Required on all aircraft with MTOW > 5,700 kg or 19+ seats
• ACAS X is the next-generation system using probabilistic modeling