ACAS II

The Airborne Collision Avoidance System II (ACAS II) has been introduced in order to reduce the risk of mid-air collisions or near mid-air collisions between aircraft. It serves as a last-resort safety net irrespective of any separation standards.


ACAS II is an aircraft system based on Secondary Surveillance Radar (SSR) transponder signals. ACAS II interrogates the Mode C and Mode S transponders of nearby aircraft (‘intruders’) and from the replies tracks their altitude and range and issues alerts to the pilots, as appropriate. Non-transponding aircraft are not detected.

ACAS II works independently of the aircraft navigation, flight management systems, and Air Traffic Control (ATC) ground systems. While assessing threats it does not take into account the ATC clearance, pilot’s intentions or autopilot inputs.

Currently, the only commercially available implementations of ICAO standard for ACAS II (Airborne Collision Avoidance System) are TCAS II version 7.0 and 7.1 (Traffic alert and Collision Avoidance System).

What are the safety benefits of TCAS II?

The safety benefits delivered by TCAS II are usually expressed in terms of the risk ratio: a comparison of the risk with and without TCAS (i.e. does TCAS II make safety better or worse?) – a risk ratio of 0% would indicate an ideal system (the risk is eliminated) and a risk of 100% would indicate an ineffective system (the risk is unaltered). Real systems have a performance somewhere between these extremes. It is important to remember that risk ratio is a relative measure expressing the improvement in safety rather than the absolute level of safety.

For Europe, ACAS is estimated to reduce the risk of mid-air collision by a factor of about 5 (i.e. a risk ratio of 22%).

International Standard

The International Civil Aviation Organization (ICAO) is responsible for the global standardisation of ACAS.

Standardization organisations - RTCA and EUROCAE (European Organisation for Civil Aviation Equipment) are responsible for the development of Minimum Operational Performance Standards (MOPS) for ACAS. The latest MOPS have been developed jointly by RTCA and EUROCAE. For the current TCAS II version (7.1) the Standards have been published in RTCA document DO-185B and EUROCAE document ED-143. In order to be certified, any ACAS II equipment must meet the standards specified in the MOPS.

ACAS equipment is available from three principle vendors, all of them based in the USA. Systems by other manufacturers may become available. While each vendor’s implementation is slightly different, they provide the same core functions and the collision avoidance and coordination logic contained in each implementation is the same.

This dossier provides a technical overview of ACAS II and relevant documentation: bulletins and safty messages, training materia, studies, regulatory framework.

In this dossier

ACAS II bulletins A series of ACAS II Bulletins has been...
Welcome to the ACAS (Airborne Collision Avoidance System)...
History The development of version 7.1 was initiated by...
Traffic Advisories and Resolution Advisories Two types of...
Types of ACAS Three types of ACAS have been specified in...
Here you can find a list of documents covering the topic of...
Current European ACAS II equipage requirements In Europe,...
The International Civil Aviation Organization ( ICAO ) is...
15.7.3.5 ACAS can have a significant effect on ATC...
United States Federal Aviation Administration - FAA...
Early developments 1956 The first conceptual research of an...