TCAS

TCAS (Traffic Collision Avoidance System) is an aircraft-based system that operates independently of ground-based ATC to prevent mid-air collisions. TCAS interrogates the transponders of nearby aircraft and calculates their range, bearing, altitude, and closure rate.

• TCAS I: Provides Traffic Advisories (TAs) — alerts to nearby traffic
• TCAS II: Provides TAs plus Resolution Advisories (RAs) — specific climb/descend commands to avoid collision

When a TCAS RA is issued, pilots must follow it immediately, even if it conflicts with ATC instructions. TCAS II is mandatory for all aircraft above 5,700 kg or with more than 19 passenger seats. TCAS has prevented numerous mid-air collisions since its introduction.