Airborne collision avoidance system

Helping pilots avoid potential midair collisions by tracking other aircraft in the surrounding airspace through replies from their transponders.

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.


ACAS/TCAS can issue two types of alerts:

  • Traffic advisories (TAs), which aim to help the pilots in the visual acquisition of the intruder aircraft, and to alert them to be ready for a potential resolution advisory.
  • Resolution advisories (RAs), which are avoidance manoeuvres recommended to the pilot. An RA will tell the pilot the range of vertical rates within which the aircraft should be flown in order to avoid the threat aircraft. An RA can be generated concerning all aircraft equipped with an altitude reporting transponder (Mode S or Mode A/C). The intruder aircraft does not need to be fitted with ACAS. When the intruder aircraft is also fitted with an ACAS system, both systems coordinate their RAs through the Mode S data link in order to select complementary resolution advisories. ACAS does not detect non-transponder equipped aircraft or aircraft with a non-operational transponder.

State aircraft

The ACAS mandate was applicable only to civil aircraft. A voluntary programme for the installation of ACAS II in military transport-type aircraft by 1 January 2005 was agreed by States. One exception is Germany, where the January 2005 deadline included “military transport aircraft”, but the German authorities put in place transitional arrangements and an exemption process. Following the civil aircraft update to the new version of TCAS II software (version 7.1) the EUROCONTROL Civil Military Interface standing Committee (CMIC), at its 49th meeting in September 2016, agreed on a revised ACAS policy for military aircraft.

The voluntary upgrade of transport-type State aircraft TCAS software to Version 7.1 is recognised as the minimum for voluntary forward fit implementation for new military transport type aircraft (MTTA) entering into service or undergoing major mid-life modernisation. The adoption of this new version for transport-type State aircraft may not be seen as mandatory in regulatory terms, but it is strongly encouraged for safety reasons, including when forward-fit opportunities arise.

Revised ACAS policy for military aircraft

Equipage requirements

In 2011, the European Commission published Regulation No 1332/2011, subsequently amended by Regulation No 2016/583, mandating, from 1 December 2015, the carriage of ACAS II (TCAS II) version 7.1 within European Union airspace by all civil aeroplanes with a MTOM exceeding 5700 kg or authorised to carry more than 19 passengers.

Aircraft not referred above but which are equipped on a voluntary basis with ACAS II, must be equipped with version 7.1. The mandate does not apply to unmanned aircraft systems.

EUROCONTROL no legal provides the above-mentioned regulations. For more information, please visit the European Aviation Safety Agency (EASA) website.

ACAS Bulletin

A series of ACAS II Bulletins has been published since 2002, each focusing on a different operational theme of interest to both aircrews and air traffic controllers. You can find the latest issues below and the full list in our Library.


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