The evolution of the Airborne collision avoidance system (ACAS) and introduction of ACAS X

The international civil aviation network carries over four billion passengers around the world annually. In celebrating ICAO's 75th Anniversary Celebrations, we will be highlighting some of the crucial safety achievements that have enabled this. We hope you follow our series throughout the year, and we encourage you to use the #ICAO75 hashtag to share your thoughts and memories with us through social media.

Collisions between passenger aircraft are among the most catastrophic accidents imaginable. The first mid-air collision occurred in 1956 over the Grand Canyon, killing all 128 passengers and the crew of both airliners.  As a result, in the late 1950s, massive research was undertaken to determine strategies for avoiding collisions. Several States around the world began developing airborne collision avoidance systems, and by early 1970, the first prototypes of an airborne collision avoidance system (ACAS) were developed. It took a number of additional years to validate and further advance the development.

In parallel with the development of these systems, there were many discussions in ICAO meetings about standardizing them. During the Seventh Air Navigation Conference that was held in 1972, the conference made a total of 74 recommendations, including those related to secondary surveillance radar (SSR) and airborne collision avoidance systems (ACAS). Through the 1970s, there were extensive exchanges of views regarding the evolution of SSR and ACAS, and in 1981 the Air Navigation Commission (ANC) established the SSR Improvements and Collision Avoidance Systems Panel (SICASP) with the following terms of reference: “To undertake specific studies … with a view to developing Standards, Recommended Practices (SARPs), procedures and, where appropriate, guidance material concerning … [inter alia] collision avoidance systems.”

ICAO first published the Circular 195 Airborne Collision Avoidance Systems in 1985, with the assistance of the SICASP. In addition to the efforts made by other entities to improve ACAS performance, ICAO initiated a worldwide operational evaluation in the late 1980s and the SICASP (and its successor) was actively involved in the final evaluation, which was conducted in the early 1990s. After numerous improvements, the ICAO expert group successfully developed the ACASII provisions as described in ICAO Annex 10 Volume IV.

Today we can proudly say that air travel is incredibly safe. It has been said that “a person who flew continuously on a jet transport aircraft in today’s environment could expect to survive more than 11,000 years of travel before becoming the victim of a mid-air collision”[1]. This accomplishment has been made possible not only thanks to the deployment of ACAS, but because of several other similarly successful efforts, such as the development of appropriate operational procedures and training initiatives for flight crews and ATC.

However, we are now facing new challenges to maintain or improve upon this level of safety while supporting increased demand through more and more efficient aircraft operations. The monitoring and assessment of existing ACAS Resolution Advisories (RA) indicate that over 80% of them are triggered by the interactions between the current ACAS alerting criteria and normal ATC procedures during safe operation (where own aircraft and intruder are actually safely separated). These RAs are categorized as unnecessary or nuisance alerts. Additionally, the current ACAS logic is not sufficiently flexible to adapt to future operations that are described in the Global Air Navigation Plan.


Figure Source: U.S. Federal Aviation Administration
Figure Source: U.S. Federal Aviation Administration

The development of a new and improved ACAS, known as ACAS X, has been initiated to solve operational issues, such as unnecessary alerts, and to accommodate new procedures (such as those supporting 4D trajectory based operations).  ACAS X will also enable the use of other surveillance sources, as required to support new aircraft types such as remotely piloted aircraft.

One of the ICAO expert groups, the Surveillance Panel, the successor of the expert group mentioned above, is currently working closely with several other entities to finalize the ACAS X technical provisions for inclusion in Annex 10 Volume IV. The new provisions are based on extensive evaluations that indicated that ACAS X will provide a safety benefit by reducing the probability of certain Near Mid-Air Collisions (NMACs) scenarios by about 20%, and also significantly reducing the number of unnecessary alerts and RA reversals. ICAO continues to work with the Surveillance Panel, as well as other expert groups and entities, in order to maintain and improve the safety of the flying public.

About the Author

Ms. Mie Utsunomiya has been working as a CNS Technical Officer of ICAO HQ since 2011. Since 2015 she has been the Secretary of Surveillance Panel, which undertakes specific studies and develops technical and operational ICAO provisions for aeronautical surveillance systems, airborne surveillance systems, collision avoidance systems and their applications as outlined in the Global Air Navigation Plan. Since 2017, she has also been working in the area of spectrum management as well.

[1] James K. Kuchar and Ann C. Drumm, The Traffic Alert and Collision Avoidance System