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ACAS II Overview and Principles |
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Principles |
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Traffic Advisories (TA) and Resolution Advisories (RA) |
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Types of RAs (TCAS II version 7.0) |
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Complying with RAs |
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International standard |
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Types of ACAS |
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Equipage requirement |
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Safety benefits |
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ACAS II Limitations |
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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.
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).
For more information:
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History & future of airborne collision avoidance |
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Traffic Advisories and Resolution Advisories |
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.
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Example of ACAS II traffic display, indicating a "Descend" RA with a vertical speed of 1500 ft/min.
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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 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.
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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 third of all RAs the ATC separation minima are not significantly violated.
An RA can be generated against all aircraft equipped with an altitude reporting transponder (mode S or mode A/C). The intruder does not need to be fitted with ACAS II. However, RAs are coordinated only between ACAS II equipped aircraft. Note that in the majority of cases only one aircraft will receive an RA (regardless of whether the intruder is ACAS II aircraft or not).
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Types of RAs (TCAS II version 7.0) |
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Types of RAs (TCAS II version 7.1) |
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Pilots are required to immediately comply with all RAs, even if the RAs are contrary to ATC clearances or instructions, 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.
For more information:
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ACAS II Training Material |
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RA Downlink website |
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The International Civil Aviation Organization (ICAO) is responsible for the global standardisation of ACAS.
Standardization organisations - RTCA and EUROCAE (European Organisation for Civil Aviation Equipment) are responsible for the development of Minimum Operational Performance Standards (MOPS) for ACAS. The latest MOPS have been developed jointly by RTCA and EUROCAE. For the current TCAS II version (7.1) the Standards have been published in RTCA document DO-185B and EUROCAE document ED-143. In order to be certified, any ACAS II equipment must meet the standards specified in the MOPS.
ACAS equipment is available from three principle vendors, all of them based in the USA. Systems by other manufacturers may become available. 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.
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More information: TCAS version 7.1 |
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More information: ICAO Provisions |
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Three types of ACAS have been specified in ICAO Annex 10:
- ACAS I provides information as an aid to “see and avoid” action but does not include the capability for generating RAs
- ACAS II provides vertical RAs in addition to TAs. The only implementations of ACAS II concept are TCAS II version 7.0 and 7.1.
- ACAS III provides vertical and horizontal RAs in addition to TAs.
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Although ACAS III is mentioned as a future system in ICAO Annex 10, ACAS III is unlikely to materialize due to difficulties the current surveillance systems have with horizontal tracking. Currently, research is being conducted to develop a future collision avoidance system (under the working name of ACAS X). If developed and certified, ACAS X will not be commercially available before mid 2020s. It is unclear at this stage whether ACAS X would provide any horizontal resolutions.
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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.
On 20 December 2011, the European Commission published Implementing Rule 1332/2011 mandating the carriage of ACAS II version 7.1 within European Union airspace from 1 December 2015 by all aircraft currently equipped with version 7.0 and from 1 March 2012 by all new aircraft above 5,700 kg maximum take-off mass or a maximum passenger seating capacity of more than 19.
Elsewhere in the world, equipage requirements vary and different standards may be required. Refer to national Flight Information Publications for details.
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Flight Information Publications |
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The safety benefits delivered by TCAS II are usually expressed in terms of the risk ratio: a comparison of the risk with and without TCAS (i.e. does TCAS II make safety better or worse?) – a risk ratio of 0% would indicate an ideal system (the risk is eliminated) and a risk of 100% would indicate an ineffective system (the risk is unaltered). Real systems have a performance somewhere between these extremes. It is important to remember that risk ratio is a relative measure expressing the improvement in safety rather than the absolute level of safety.
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%).
For more information:
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ACAS II Studies |
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ACAS performance and effectiveness is effected by several system, operational and performance limitations. These limitations include:
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- Non-transponder equipped aircraft, or aircraft with an inoperative transponder will not be detected.
- RAs for traffic without an altitude reporting transponder will not be issued.
- No advisories (TAs or RAs) will be issued against aircraft having vertical rates in excess of 10,000 fpm.
- ACAS II will automatically fail if the input from the aircraft’s barometric altimeter, radio altimeter or transponder is lost.
- Certain RAs are inhibited at altitudes based on inputs from the radio altimeter:
(a)1550ft (+/- 100ft) and below, increase descend RAs are inhibited;
(b)1100ft (+/- 100ft) and below, descend RAs are inhibited;
(c)1000ft (+/- 100ft) and below, all RAs are inhibited;
(d)500ft (+/- 100ft) and below all aural annunciations (warnings) are inhibited
- Climb and Increase Climb RAs can be inhibited above predetermined altitudes or in certain aircraft configurations. These inhibits are set via programme pins during installation.
- Because of the interference limiting algorithms, ACAS II may not display all proximate transponder-equipped aircraft in areas of high-density traffic.
- The bearing displayed by ACAS II is not sufficiently accurate to support the initiation of horizontal manoeuvres based solely on the traffic display. As a result, horizontal manoeuvres based solely on information displayed on the traffic display are prohibited.
- Ground Proximity Warning System (GPWS) and terrain avoidance warning system (TAWS) warnings and wind shear warnings take precedence over ACAS II advisories. When either a GPWS/TAWS or wind shear warning is active, ACAS II aural annunciations will be inhibited and ACAS II will automatically be placed in the TA-only mode.
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Last validation: 13/01/2012
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