By its nature, a Airborne Collision Avoidance System (ACAS) RA is a warning of an immediate risk of collision. It is therefore essential that all activities connected with response to a TCAS RA should make a positive contribution to collision avoidance. For this reason, RTF phraseology associated with TCAS RAs must be clear, precise and unambiguous.

RTF phraseology has developed following the introduction of TCAS in response to emerging problems. This has resulted in extensive revision of procedures contained in ICAO Doc 4444: PANS-ATM and ICAO Doc 8168: PANS-OPS. The current phraseology is detailed below. (ICAO Doc 4444 PANS-ATM, Chapter 12, Para 12.3.1.2)