Airborne collision avoidance system

Helping pilots avoid potential collisions by tracks aircraft in the surrounding airspace through replies from their air traffic control 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.

Airborne Collision Avoidance System (ACAS) guide

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