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Airborne Collision Avoidance System (ACAS)

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Airborne Collision Avoidance System (ACAS)


The Airborne Collision Avoidance System II (ACAS II) was 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.

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 (Traffic alert and Collision Avoidance System).

Information Provided by ACAS

Two types of alerts can be issued by ACAS II - TA (Traffic Advisory) and RA (Resolution Advisory). The former is intended to assist the pilot in the visual acquisition of the conflicting aircraft and prepare the pilot for a potential RA.

If a risk of collision is established by ACAS II, an RA will be generated. Broadly speaking, RAs tell the pilot the range of vertical speed at which the aircraft should be flown to avoid the threat aircraft. The visual indication of these rates is shown on the flight instruments. It is accompanied by an audible message indicating the intention of the RA. A "Clear of Conflict" message will be generated when the aircraft diverge horizontally.

Example of ACAS II traffic display, indicating a "Climb" RA with a vertical speed of 1500 ft/min.

The vertical sense (direction) of the RA is coordinated with other ACAS II equipped aircraft via a mode S link, so that two aircraft choose complementary manoeuvres. RAs aim for collision avoidance by establishing a safe vertical separation (300 - 700 feet), rather than restoring a prescribed ATC separation.

ACAS II operates on relatively short time scales. The maximum generation time for a TA is 48 seconds before the Closest Point of Approach (CPA). For an RA the time is 35 seconds. The time scales are shorter at lower altitudes (where aircraft typically fly slower). Unexpected or rapid aircraft manoeuvre may cause an RA to be generated with much less lead time. It is possible that an RA will not be preceded by a TA if a threat is imminent. The effectiveness of an RA is evaluated by the ACAS equipment every second and, if necessary, the RA may be strengthened, weakened, reversed, or terminated.

A protected volume of airspace surrounds each ACAS II equipped aircraft. The size of the protected volume depends on the altitude, speed, and heading of the aircraft involved in the encounter. See illustration below.

A protected volume of airspace surrounds each ACAS II equipped aircraft

RAs can be generated before ATC separation minima are violated and even when ATC separation minima will not be violated. In Europe, for about two thirds of all RAs, the ATC separation minima are not significantly violated.

Types of RAs (TCAS II version 7.0)

Types of RA

Complying with RAs

Pilots are required to immediately comply with all RAs, even if the RAs are contrary to ATC clearances or instructions.

If a pilot receives an RA, he/she is obliged to follow it, unless doing so would endanger the aircraft. Complying with the RA, however, will in many instances cause an aircraft to deviate from its ATC clearance. In this case, the controller is no longer responsible for separation of the aircraft involved in the RA.

On the other hand, ATC can potentially interfere with the pilot’s response to RAs. If a conflicting ATC instruction coincides with an RA, the pilot may assume that ATC is fully aware of the situation and is providing the better resolution. But in reality ATC is not aware of the RA until the RA is reported by the pilot. Once the RA is reported by the pilot, ATC is required not to attempt to modify the flight path of the aircraft involved in the encounter. Hence, the pilot is expected to “follow the RA” but in practice this does not yet always happen.

Some States have implemented “RA downlink” which provides air traffic controllers with information about RAs posted in the cockpit obtained via Mode S radars. Currently, there are no ICAO provisions concerning the use of RA downlink by air traffic controllers.

International Standard

The International Civil Aviation Organization (ICAO) is responsible for the global standardisation of ACAS.

ACAS equipment is available from three vendors. While each vendor’s implementation is slightly different, they provide the same core functions and the collision avoidance and coordination logic contained in each implementation is the same. In order to be certified, ACAS equipment must meet the Minimum Operational Performance Standards (MOPS) laid down set in RTCA and forthcoming EUROCAE documents.

The equipment which meets the ACAS II Standards and Recommended Practices (SARPs) is known as TCAS II, version 7. A joint RTCA/EUROCAE working group has finalised amendments to the MOPS, addressing three specific safety improvement changes related to the collision avoidance logic; these new MOPS will form TCAS II version 7.1 and are published as RTCA DO-185B and EUROCAE ED-143.

Types of ACAS

  • ACAS I Gives Traffic Advisories (TAs) but does not recommend any manoeuvres. The only implementation of ACAS I concept is TCAS I. ICAO Standards and Recommended Practices (SARPs) for ACAS I are published in ICAO Annex 10, volume IV but are limited to interoperability and interference issues with ACAS II. ACAS I is mandated in the United States for certain smaller aircraft
  • ACAS II Gives Traffic Advisories (TAs) and Resolution Advisories (RAs) in the vertical sense (direction). ACAS II SARPs are published in ICAO Annex 10. The only implementation of ACAS II concept is TCAS II Version 7.0. TCAS II Version 7.0 is mandated in Europe.
  • ACAS III Gives TAs and RAs in vertical and/or horizontal directions. Also referred to as TCAS III and TCAS IV. Not currently implemented and unlikely to be in the near future. ICAO SARPs for ACAS III have not been developed. Currently, there are no plans to proceed with such a development.

Equipment Requirement

In Europe, as of 1 January 2005 all civil fixed-wing turbine-engined aircraft with a maximum take-off mass over 5,700 kg, or capable of carrying more than 19 passengers, must be equipped with TCAS II version 7.0. Additionally, many State and business aviation aircraft are also equipped.

Elsewhere in the world, equipage requirements vary and different standards may be required. Refer to national Flight Information Publications for details.

Safety Benefits

The safety benefits delivered by ACAS are usually expressed in terms of the risk ratio (does ACAS make safety better or worse?). For Europe, ACAS is estimated to reduce the risk of mid-air collision by a factor of about 5 (i.e. a risk ratio of 22%).

The importance of correctly following RAs is illustrated by the fact that a pilot who never follows RAs faces three times the risk that is faced by a pilot who always follows RAs. The human is the weakest element in the ACAS control loop; without “human in the loop” the risk ratio would improve by a factor of 10.

Related Articles

Related Media

Further Reading

EUROCONTROL For further information, visit ACAS website:

Bureau d’Enquêtes et d’Analyses (BEA)


  • ICAO Annex 10 Volume IV Chapter 4 and Attachment A ;
  • ICAO Doc 4444: Procedures for Air Navigation Services (PANS-ATM) Chapter 15, Section 15.7.3

European Regulatory Documentation:

  • EU-OPS 1.668: requirement for aircraft to be equipped with ACAS
  • EU-OPS 1.398: use of TCAS
  • EU-OPS 1.420: requirement to report TCAS RAs
  • JAA TGL 11: Guidance for operators on training programmes for the use of ACAS