What is TCAS and how does it prevent mid-air collisions?

TCAS (Traffic Collision Avoidance System) is an independent safety system that warns pilots of nearby aircraft and provides avoidance guidance. It works by: Interrogating transponders on other aircraft, Calculating range, closure rate, and relative altitude, and Issuing traffic advisories (TA) and resolution advisories (RA). TAs alert pilots to nearby traffic; RAs command vertical maneuvers (climb/descend) coordinated between aircraft to ensure separation. TCAS II provides vertical RAs and is mandatory for aircraft over 5,700 kg or 19 passengers. Pilots must follow RAs even if contradicting ATC instructions. TCAS has prevented numerous collisions since its introduction.