TCAS provides indications about surrounding aircraft and especially alerts about intruders that may jeopardize the safety of the flight. The indications provide the flight crew with the position of surrounding aircraft relatively to the own aircraft. The alerts are of two types:
- Traffic Advisories (TA) that inform the flight crew of the position of intruders
- Resolution Advisories (RA) that provide the flight crew with the position of threatening intruders and instructions to avoid them.
3.1.1. TCAS DESIGNATION
The ICAO Annex 10, Volume IV defines three types of ACAS functions:
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ACAS I is the first generation of TCAS. ACAS I provides Traffic Advisories (TA) and proximity warning of surrounding aircraft to assist the flight crew in the visual acquisition of intruder aircraft. TCAS I is installed in some small aircraft and helicopters in some regions in the world (e.g. aircraft with less than 31 and more than 10 passengers in USA). TCAS I is out of the scope of the present brochure.
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ACAS II is a ACAS I augmented with the capability to provide Resolution Advisories (RA) in the vertical plane. The development of ACAS II started in the early 1990s. Several standards (or Changes) have been defined and the latest one is known as TCAS II Change 7.1 or Version 7.1. The TCAS II Change 7.1 is compliant with ICAO Standards and Recommended Practices (SARPs) for ACAS II. ICAO mandated the carriage of ACAS II Change 7.0 since 2003 (since 2000 in Europe). At the time of writing the brochure, the TCAS II Change 7.1 was released. ICAO intends to mandate the TCAS II Change 7.1 from 2012 (refer to 3.1.6 – TCAS II Change 7.1).
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ACAS III is intended to provide TA and RA in both vertical and horizontal planes. At present, no ACAS III system has been developed, and none is likely to appear in the near future due to technical and operational difficulties.
The level of protection provided by TCAS depends of the transponder capability of surrounding aircraft.
Most equipment installed on AIRBUS aircraft are compliant with TCAS II Change
7. Therefore, TCAS refers to equipment compliant with TCAS II Change 7.0 for the remainder of the document, except when specified.
3.1.2. TCAS PRINCIPLE
TCAS works autonomously and independently of the aircraft navigation equipment and ATS ground systems. Therefore, to detect and track any surrounding aircraft, TCAS periodically interrogates surrounding aircraft transponders.
The interrogation principle is similar to the one for SSR.
1.
The own aircraft TCAS interrogates the surrounding aircraft transponder.
2.
The surrounding aircraft transponder replies to the own aircraft TCAS with data provided by the surrounding aircraft TCAS (e.g. RA generated by the surrounding aircraft TCAS).
11 Both aircraft must be equipped with Mode S transponders for coordinated maneuvers.
Thanks to the interrogation principle, the TCAS computes the following parameters to determine the collision threat: 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：getting to grips with surveillance(43)
