Definitions

Airborne Collision Avoidance Systems (ACAS) equipment provides two types of advisories to pilots: Resolution Advisories (RAs) and Traffic Advisories (TAs). These are defined as follows:

Resolution advisory (RA) An indication given to the flight crew recommending:

a) a manoeuvre intended to provide separation from all threats; or

b) a manoeuvre restriction intended to maintain existing separation.

Corrective RA. A resolution advisory that advises the pilot to deviate from the current flight path.

Preventive RA. A resolution advisory that advises the pilot to avoid certain deviations from the current flight path but does not require any change in the current flight path.

Traffic advisory (TA). An indication given to the flight crew that a certain intruder is a potential threat.

Source: ICAO Annex 10, vol. IV (Aeronautical Telecommunications - Surveillance and Collision Avoidance Systems), Fifth Edition, July 2014 - Definitions

Further Reading

ICAO

• ICAO Annex 10 Volume IV Chapter 4 and Attachment A ;
• ICAO Doc 4444: Procedures for Air Navigation Services (PANS-ATM) Chapter 15, Section 15.7.3;

EUROCONTROL

• EUROCONTROL: ACAS Guide - Airborne Collision Avoidance Systems
• EUROCOTROL ACAS II Website
• Overview of ACAS II/TCAS II
• ACAS Bulletins
• The Assessment of Pilot Compliance with TCAS RAs, TCAS Mode Selection and Serviceability Using ATC Radar Data, EUROCONTROL, 2nd edition, March 2021
• Traffic Alert and Collision Avoidance System (TCAS) - Selected Statistical and Performance Data in Core European Space, EUROCONTROL, 19 March 2021.

IATA

• Performance assessment of pilot compliance to Traffic Alert and Collision Avoidance System (TCAS) using Flight Data Monitoring (FDM) - Guidance Material, Edition 2