History & future of airborne collision avoidance

Early developments


The first conceptual research of an airborne collision avoidance system was initiated in 1956 by Dr John S. Morrel after a mid-air collision between a DC-7 and Lockheed Super Constellation over the Grand Canyon in the USA.


The 1978 collision between a Boeing 727 and a Cessna 172 over San Diego, California led the United States Federal Aviation Administration (FAA) to start the development of the first airborne collision avoidance system.


Eight years later, in 1986, another mid-air collision occurred over Cerritos, California – a DC-9 collided with a Piper Archer. Following this accident, the phased-in mandate of ACAS began in the USA.


The collision between a Boeing 747 and an Ilyushin 76 near New Delhi, India in 1996 triggered the process of mandating ACAS in other parts of the world.
Early versions of TCAS II (versions 6.02 and 6.04a)


Throughout the 1980s, the performance evaluations of early versions of TCAS II contributed to the gradual enhancement of the equipment and software. In September 1989 the design of version 6.02 was completed and put into operations from April 1990.


In order to determine the TCAS II system performance, ICAO commissioned a worldwide operational evaluation in the late 1980s. The evaluation was conducted in the early 1990s.


As a result of the evaluation a number of improvements were suggested. That led to the development and release of version 6.04a in 1993. The new version aimed to reduce the number of nuisance alerts, which were occurring at low altitudes and during level-off encounters.

TCAS II version 7.0

After the implementation of version 6.04a, further operational evaluations were carried out and proposed performance improvements led to the development of version 7.0. It was approved in December 1997 and became available at the beginning of 1999.

Version 7.0 further improved TCAS compatibility with the air traffic control system. The most significant enhancements were the introduction of a horizontal miss distance filter and 25-foot vertical tracking, more sophisticated multi-threat logic, compatibility with Reduced Vertical Separation Minima (RVSM) operations and the reduction of electromagnetic interference.

TCAS II version 7.1

The development of version 7.1 was initiated by EUROCONTROL following the discovery of two safety issues with the current TCAS II version. Development was undertaken jointly by the RTCA in the United States and by EUROCAE (European Organisation for Civil Aviation Equipment) in Europe with support and contributions from several other organizations, including major airlines and air navigation service providers (ANSPs).

Amendment to ICAO Annex 10 (volume 4) published in October 2010 introduced a provision stating that all new ACAS installations after 1 January 2014 shall be compliant with version 7.1 and after 1 January 2017 all ACAS units shall be compliant with version 7.1.

An Implementing Rule published by the European Commission in 2011 (and amended in 2016) mandates the carriage of ACAS II version 7.1 within European Union airspace by all civil aeroplanes with a maximum certified take-off mass exceeding 5,700 kg or authorised to carry more than 19 passengers as of 1 December 2015.



Over the years, air traffic has continued to increase. The developments of modern air traffic control systems have made it possible to cope with this increase, whilst maintaining the necessary levels of safety. Despite technical advances in ATC systems, there are cases when the separation provision fails due to a human or technical error. Any separation provision failures may result in an increased risk of a midair collision. Therefore, an airborne collision avoidance system, acting as a last resort, has been considered from the 1950s. In 1955, Dr John S. Morrel proposed the use of the slant range between aircraft divided by the rate of closure or range rate for collision avoidance algorithms (i.e. time rather than distance, to the closest point of approach). Today’s airborne collision avoidance system is based on this concept.

During the late 1960s and early 1970s, several manufacturers developed prototype aircraft collision avoidance systems. Although these systems functioned properly during staged aircraft encounter testing, it was concluded that in normal airline operations, these systems would generate a high rate of unnecessary alarms in dense terminal areas. This problem would have undermined the credibility of the system with the flight crews.

In 1978, the collision between a light aircraft and an airliner over San Diego, California led the US Federal Aviation Administration to initiate, three years later, the development of TCAS (Traffic alert and Collision Avoidance System). In 1986 the collision between an airliner and a light aircraft over Cerritos, California resulted in a US Congressional mandate that required some categories of US and foreign aircraft to be equipped with TCAS II for flight operations in US airspace.

In parallel to the development of TCAS equipment, ICAO (International Civil Aviation Organization) has developed, from the beginning of the 1980s, standards for ACAS (Airborne Collision Avoidance Systems).

Future ACAS X

The FAA has funded research and development of a new approach to airborne collision avoidance (known as ACAS X) since 2008. This new approach takes advantage of recent advances in ‘dynamic programming’ and other computer science techniques (which were not available when TCAS II was first developed) to generate alerts using an off-line optimisation of resolution advisories. It is the intention that ACAS X will eventually replace TCAS II.

It is envisaged that ACAS X will provide an improvement in safety while reducing the unnecessary alert rate. ACAS X will use the same hardware (antennas, processors, and displays) as the current TCAS II system and the same range of available RAs will be used. Consequently, pilots and controllers would perceive no change with the transition to the new system, which will be fully compatible with current TCAS II systems.

Two of the key differences between TCAS II and the current concept for ACAS X are the collision avoidance logic and the sources of surveillance data.

As well as the standard ACAS X (known as ACAS Xa) variants are under consideration to extend collision avoidance protection to situations and user classes that currently do not benefit from TCAS II:

  • ACAS Xa – The general purpose ACAS X that makes active interrogations to establish the range of intruders. The successor to TCAS II.
  • ACAS Xp – A version of ACAS X that relies solely on passive ADS-B to track intruders and does not make active interrogations. It is intended for general aviation aircraft (that are not currently required to fit TCAS II).
  • ACAS Xo – A mode of operation of ACAS X designed for particular operations for which ACAS Xa is unsuitable and might generate an unacceptable number of nuisance alerts (e.g. procedures with reduced separation, such as closely spaced parallel approaches).
  • ACAS Xu – designed for Unmanned Aircraft Systems UAS or Remotely Piloted Aircraft Systems (RPAS).

Minimum Operational Performance Standards (MOPS) are being developed for ACAS Xa and ACAS Xo within RTCA and EUROCAE standardization working arrangements. The completion is currently scheduled for 2018 and ACAS X may become operational in 2020.

For more information about ACAS X see EUROCONTROL ACAS Guide (pages 17 and further).

ACAS Milestones


  • Dr. John Morrel introduces the time-based concept of collision avoidance


  • Grand Canyon mid-air collision (DC7 and Super Constellation)


  • New York mid-air collision (DC8 and Super Constellation)


  • MITRE start development of BCAS (Beacon Collision Avoidance System)


  • Zagreb mid-air collision (DC9 and Trident)


  • San Diego mid-air collision (B727 and Cessna 172)


  • FAA pursue airborne solution and BCAS becomes TCAS


  • San Luis Obispo mid-air collision (Cessna 414 and Rockwell 112TC)


  • Cerritos mid-air collision (DC9 and PA28)


  • Salt Lake City mid-air collision (M20 and SA226)
  • US Public Law 100-223 (the Airport and Airway Improvement Act) - the basis for TCAS implementation in the United States


  • ICAO established SICASP/4 panel, world-wide evaluation


  • First operational TCAS (Version 6.02)


  • USA mandate for Version 6.02 (based on number of seats)


  • FAA AD (airworthiness directive) requiring upgrade to Version 6.04a


  • ICAO SARPs published in Annex 10


  • India mid-air collision (B747 and IL76)
  • India mandates TCAS II carriage by 1999
  • ICAO consultations about world-wide mandate


  • Namibia mid-air collision (Luftwaffe Tu154 and US Air Force C141)
  • USA mandate TCAS for military transport aircraft
  • Version 7 design finalised


  • European mandate for ACAS II based on MTOM (Maximum Take-off Mass)


  • EUROCONTROL’s ACASA (ACAS Analysis) study


  • Überlingen mid-air collision (B757 and Tu154)


  • ICAO mandate for ACAS II (Annex 6)
  • ICAO PANS-OPS modified to emphasize the need to follow the RA
  • USA mandate for Version 7 in RVSM (MTOM based)


  • Issues identified in EUROCONTROL’s ACAS monitoring lead to RTCA SC147 being reconvened


  • Extension of European mandate to smaller aircraft (above 5,700 kg MTOM or 19 passengers)


  • Brazil mid-air collision (B737 and Embraer Legacy 600)
  • EUROCAE WG75 (TCAS) formed - works with RTCA SC147


  • TCAS II version 7.1 MOPS finalized


  • ICAO Annex 10 vol. IV amended to include version 7.1 mandate


  • TCAS II version 7.1 Implementing Rule published by the European Commission