The Airborne Collision Avoidance System (ACAS) was developed as an independent, aircraft-based safety family that interrogates secondary surveillance radar transponder signals to detect conflicting traffic without relying on ground equipment. By issuing gap-filling guidance while the ground-based Air Traffic Control system remains the primary separation provider, ACAS continuously monitors airspace to reduce the likelihood of mid-air collisions and near-mid-air events. Among its variants, ACAS II refines collision-avoidance logic for a broad spectrum of aircraft types, underscoring the evolution from the original implementation - often called the Traffic Collision Avoidance System (TCAS) - into a suite of airborne safeguards that today constitute a vital aviation safety technology.
What is ACAS in aviation?
An airborne collision avoidance system (ACAS) operates independently of ground-based equipment and air traffic control, warning pilots of the presence of other aircraft that may present a threat of collision.
In aviation, ACAS is the term used by the International Civil Aviation Organization to denote ACAS II, a last-resort safety net that operates independently of ground-based Air Traffic Control. It is a family of airborne devices based on Secondary Surveillance Radar transponder signals and is being used to describe short-range systems intended to prevent actual metal-on-metal collisions. Airborne Collision Avoidance System (ACAS) was developed as a safety system that reduces the likelihood of mid-air collisions by interrogating Mode C and Mode S transponders within a nominal 14 NM range, tracking altitude and range of intruders, and issuing Traffic Advisories that indicate positions and relative altitudes of transponder-operating aircraft on a display to assist flight-crew in visual acquisition.
When risk is imminent, the system recommends a vertical maneuver and gives Resolution Advisories in the vertical sense. ICAO Annex 10, Volume IV defines ACAS standards and recommended practices, while RTCA and EUROCAE prepare Minimum Operational Performance Standards. Current equipment is available from four vendors - ACSS, Garmin, Honeywell, and Rockwell Collins - and the evolving ACAS X family is being developed to replace the current Traffic Alert and Collision Avoidance System II, offering members like ACAS-Xa (the baseline successor), ACAS-Xu for RPAS with vertical and horizontal resolution advisory manoeuvres, ACAS-Xr for helicopters, ACAS-Xp for general aviation, and ACAS-Xo as an optional extension for closely spaced parallel approaches.
How does ACAS work in aviation?
ACAS operates independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft that are equipped with SSR transponders. It interrogates the Mode C and Mode S transponders of nearby aircraft, tracks their altitude and range from the replies, and issues alerts to the pilots as appropriate. Because ACAS II is designed to work as a standalone system, it needs no connection to aircraft navigation, flight-management systems, or ATC ground radar. It generates a Traffic Advisory that gives the crew intruder range, bearing, and relative altitude, followed, if necessary, by a Resolution Advisory - a recommended vertical manoeuvre that maintains or increases vertical separation between aircraft for collision avoidance. When the intruder aircraft is also fitted with an ACAS system, the two units coordinate their RAs through the Mode S data link so that both flight crews receive compatible vertical instructions.
ACAS X follows the same airborne-only philosophy, yet replaces the deterministic logic of ACAS II with a probabilistic model. Every second it collects surveillance measurements from an array of onboard sources, then estimates the relative position and speed of nearby aircraft using tracking algorithms and a numeric lookup table optimised with respect to a probabilistic model of the airspace. This allows ACAS X to reduce the overall alerting rate by 65% on recorded radar tracks in US airspace while boosting safety by 20% on the United States encounter model. When pilots follow their RAs, ACAS Xa logic delivers an increased safety benefit of between 16% and 24%. If one pilot does not follow the advisory, the system still provides a safety benefit of about 47%. Future variants - ACAS X U for Remotely Piloted Aircraft Systems and ACAS X R for rotorcraft - will incorporate horizontal resolution manoeuvres. The system works autonomously and independently of any ground or navigation support, and pilots must operate the system and respond to Resolution Advisories.
What is the difference between ACAS and TCAS in aviation?
The difference between ACAS and TCAS is that ACAS is the basis for the design of TCAS. While often used interchangeably, TCAS refers specifically to the hardware and software configurations that comply with ACAS specifications. TCAS is the specific implementation used in most commercial and civil aircraft, whereas ACAS is the overarching ICAO Airborne Collision Avoidance System standard. TCAS is currently the only detailed specification from the industry and the only implementation of ACAS. ACAS includes TCAS I and TCAS II, which are iterations of the ACAS standard, and TCAS II version 7.1 is the current version. ACAS Xa is a next-generation system designed to replace TCAS II, providing better algorithms for conflict resolution and reduced unnecessary alerts.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Which aircraft must be fitted with ACAS II?The European Aviation Safety Agency (EASA) requires ACAS II (effectively TCAS II, version 7.1) for all fixed-wing turbine-powered aircraft that have a maximum take-off weight greater than 5,700 kg (12,566 lb) or are authorised to carry more than 19 passengers since January 1, 2005.
The International Civil Aviation Organization mandates ACAS II for civil fixed-wing turbine-engined aircraft with a maximum take-off mass exceeding 15,000 kg (33,069 lbs) or a passenger seating configuration of more than 30, effective from 1 January 2000. Where ACAS II is installed, the equipment must be compliant with version 7.1 to operate within Reduced Vertical Separation Minimum (RVSM) airspace.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
