Airborne Safety Nets (ACAS / TCAS)

Preventing mid-air collisions

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.

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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 implementation of ICAO standard for ACAS II (Airborne Collision Avoidance System) is TCAS II version 7.0 and version 7.1 (Traffic alert and Collision Avoidance System).

From 2009, a new collision avoidance system - ACAS X - is being developed. As of December 2013 the development has moved into the standardization phase. ACAS X is a family of airborne collision avoidance systems, to replace the existing TCAS II and provide the system to new classes of users.

 Addressed user needs

  • Access ACAS/TCAS training material
  • Receive EUROCONTROL ACAS Bulletins
  • Know how ACAS/TCAS operates
  • Find out more about TCAS version 7.1 training
  • Learn about the development of ACAS X and any differences between ACAS X and TCAS II


Further info and contacts

For more information about ACAS contact:

ID: A422
Air navigation service providers, Aircraft operators, Military
Function / domain