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Brief history & future of airborne collision avoidance |
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Early developments |
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TCAS II version 7.0 |
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TCAS II version 7.1 |
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History of European ACAS Implementation Policy |
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Future ACAS |
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ACAS Milestones |
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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.
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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.
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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.
In December 2011, the European Commission published an Implementing Rule mandating the carriage of ACAS II version 7.1 within European Union airspace earlier than the dates stipulated in ICAO Annex 10: 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.
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History of European ACAS Implementation Policy |
In November 1995, the European Air Traffic Control (ATC) Harmonisation and Integration Programme (EATCHIP) Project Board agreed a common policy for the mandatory carriage and operation of ACAS for defined civil aircraft when flying in the airspace of the European Civil Aviation Conference (ECAC) States. It was confirmed, in February 1997, at the fifth meeting of ECAC Ministers on the Air Traffic System in Europe (MATSE 5), when it was emphasised that the maintenance of aviation safety must remain of fundamental importance in guiding future ATM developments.
The TCAS II version 7.0 implementation was divided into two phases. In Phase 1 (with the deadline initially set for 1 January 2000 and subsequently extended till 30 September 2001 due to limited availability of compliant equipment), some 10,000 aircraft belonging to over 2,000 operators were equipped.
Phase 2 of ACAS II implementation was completed on 31 March 2006 and involved some 900 aircraft. It extended the carriage requirement to all civil, fixed-wing turbine-engined aircraft having a maximum take-off mass exceeding 5700 kg or a maximum approved passenger seating configuration of more than 19.
As of 2010, some 11,000 aircraft in Europe are equipped with TCAS II version 7.0.
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The operational and technical requirements for an ACAS III (which provides horizontal in addition to ACAS II vertical resolution avoidance manoeuvres) have not been established. In addition, there are no ICAO SARPs for ACAS III, nor plans for their development.
Earlier development work on an ACAS III was terminated, not only because the active interrogation technology available was not able to provide sufficient bearing accuracy, but also because the additional benefit from an RA horizontal manoeuvre was assessed to be marginal. Statistics showed that 90% of encounters were resolved more efficiently with a vertical manoeuvre, with less than 10% which would have been resolved equally well with vertical or horizontal RAs, and 2 to 3% resolved more efficiently by horizontal RAs only; but even for those the vertical RA would still have worked satisfactorily. Therefore, the cost versus safety benefit is considered unfavourable for development of an ACAS III.
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.
A future airborne collision avoidance system may employ the ADS-B technology. The topics under consideration include:
- The degree to which ACAS or another future collision avoidance system should remain relatively independent of separation assurance mechanisms;
- The use of ADS-B data by ACAS;
- The degree to which airspace users will need to retain Mode A/C/S transponders to support ACAS interrogations and potential time frames for equipage changes;
- Coordination with other avoidance systems (e.g general aviation, UAS) or unequipped threats.
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1955
Dr. John Morrel introduces the time-based concept of collision avoidance
1956
Grand Canyon mid-air collision (DC7 and Super Constellation)
1960
New York mid-air collision (DC8 and Super Constellation)
1974
MITRE start development of BCAS (Beacon Collision Avoidance System)
1976
Zagreb mid-air collision (DC9 and Trident)
1978
San Diego mid-air collision (B727 and Cessna 172)
1981
FAA pursue airborne solution and BCAS becomes TCAS
1984
San Luis Obispo mid-air collision (Cessna 414 and Rockwell 112TC)
1986
Cerritos mid-air collision (DC9 and PA28)
1987
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
1989
ICAO established SICASP/4 panel, world-wide evaluation
1990
First operational TCAS (Version 6.02)
1991
USA mandate for Version 6.02 (based on number of seats)
1994
FAA AD (airworthiness directive) requiring upgrade to Version 6.04a
1995
ICAO SARPs published in Annex 10
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ICAO Provisions |
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1996
India mid-air collision (B747 and IL76)
India mandates TCAS II carriage by 1999
ICAO consultations about world-wide mandate
1997
Namibia mid-air collision (Luftwaffe Tu154 and US Air Force C141)
USA mandate TCAS for military transport aircraft
1997
Version 7 design finalised
2000
European mandate for ACAS II based on MTOM (Maximum Take-off Mass)
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ACAS II Equipage Requirements |
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2001
EUROCONTROL’s ACASA (ACAS Analysis) study
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ACAS II Safety Studies |
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2002
Überlingen mid-air collision (B757 and Tu154)
2003
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)
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ICAO Provisions |
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2004
Issues identified in EUROCONTROL’s ACAS monitoring lead to RTCA SC147 being reconvened
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