The Airborne collision avoidance system tracks aircraft in the surrounding airspace through replies from their air traffic control transponders. If the system diagnoses a risk of impending collision, it issues a resolution advisory (RA) to the flight crew, which directs the pilots how best to regulate or adjust their vertical rate so as to avoid a collision. Experience, operational monitoring and simulation studies have shown that when followed promptly and accurately, the RAs issued by ACAS II significantly reduce the risk of mid-air collision.
ACAS/TCAS indications are intended to help pilots avoid potential collisions. For the system to achieve its intended safety benefits, pilots must operate the system and respond to resolution advisories. For Europe, ACAS/TCAS is estimated to reduce the risk of mid-air collision by a factor of about 5.