Description

Resolution Advisories (RAs) are issued to the pilots by the Airborne Collision Avoidance System (ACAS) (also referred to as TCAS - Traffic alert and Collision Avoidance System) if a risk of collision is identified. The RA takes precedence over ATC instructions and the controller should not interfere with the resulting collision avoidance manoeuvre.

Typically, controllers only know about RAs when, and if, pilots report them. The controllers' situational awareness may diminish if an aircraft departs from its clearance as the result of an RA and the pilot does not promptly report this.