Airborne Collision Avoidance System (ACAS)

The Airborne Collision Avoidance System (ACAS) is an aircraft system that monitors nearby aircraft using transponder signals and provides guidance to the crew to avoid mid-air collisions. ACAS II, implemented as TCAS II, is required on most commercial transport aircraft worldwide.

How ACAS Works

ACAS interrogates the transponders of nearby aircraft and calculates their range, bearing, and altitude. It issues two types of alerts: Traffic Advisories (TAs) — alerting pilots to traffic — and Resolution Advisories (RAs) — providing specific vertical manoeuvre commands. RAs are coordinated through the transponder datalink so each aircraft receives a complementary instruction, ensuring both manoeuvre away from each other.

RA Compliance

When an RA is received, flight crew must comply immediately, regardless of ATC instruction. ACAS/TCAS RA takes priority over ATC. After complying, notify ATC of the manoeuvre as soon as possible.

On the TC Written Exam

For CPL/ATPL: know the difference between TA (advisory) and RA (command); know that RA takes priority over ATC; know that both aircraft receive coordinated, complementary RAs.

Related Terms

See also: Transponder, Separation.