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* Clear of conflict
When a TA is issued, pilots are instructed to initiate a visual search for the traffic causing the TA. If the traffic is visually acquired, pilots are instructed to maintain visual separation from the traffic. Training programs also indicate that no horizontal maneuvers are to be made based solely on information shown on the traffic display. Slight adjustments in vertical speed while climbing or descending, or slight adjustments in airspeed while still complying with the ATC clearance are acceptable.<ref name=DOC9863-6>{{Cite web|url=http://www.eurocontrol.int/msa/gallery/content/public/documents/Doc9863_ACAS_Controller_Training_chp6_1.pdf|archiveurl=https://web.archive.org/web/20120305182903/http://www.eurocontrol.int/msa/gallery/content/public/documents/Doc9863_ACAS_Controller_Training_chp6_1.pdf|
When an RA is issued, pilots are expected to respond immediately to the RA unless doing so would jeopardize the safe operation of the flight. This means that aircraft will at times have to manoeuver contrary to ATC instructions or disregard ATC instructions. In these cases, the controller is no longer responsible for separation of the aircraft involved in the RA until the conflict is terminated.
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== Safety aspects ==
Safety studies on TCAS estimate that the system improves safety in the airspace by a factor of between 3 and 5.<ref name=ACASA>{{Cite web|url=http://www.eurocontrol.int/msa/gallery/content/public/documents/Safety/WP1.pdf|archiveurl=https://web.archive.org/web/20110722013223/http://www.eurocontrol.int/msa/gallery/content/public/documents/Safety/WP1.pdf|
However, it is well understood that part of the remaining risk is that TCAS may induce midair collisions: "In particular, it is dependent on the accuracy of the threat aircraft's reported altitude and on the expectation that the threat aircraft will not make an abrupt maneuver that defeats the TCAS Resolution Advisory (RA). The safety study also shows that TCAS II will induce some critical near midair collisions..." (See page 7 of Introduction to TCAS II Version 7 and 7.1 (PDF) in external links below).<ref name="TCAS70" /><ref name="TCAS71" />
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In the future, prediction capabilities may be improved by using the state vector information present in ADS–B messages. Also, since ADS–B messages can be received at greater range than TCAS normally operates, aircraft can be acquired earlier by the TCAS tracking algorithms.
The identity information present in ADS–B messages can be used to label other aircraft on the cockpit display (where present), painting a picture similar to what an air traffic controller would see and improving situational awareness.<ref>{{Cite web|url=http://www.narcap.org/articles/hci_aero.pdf|archiveurl=https://web.archive.org/web/20110717192103/http://www.narcap.org/articles/hci_aero.pdf|
== Versions ==
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TCAS II is the first system that was introduced in 1989 and is the current generation of instrument warning TCAS, used in the majority of [[commercial aviation]] aircraft (see table below). A US Airways 737 was the first aircraft certified with the AlliedBendix (now Honeywell) TCAS II system. It offers all the benefits of TCAS I, but will also offer the pilot direct, vocalized instructions to avoid danger, known as a "Resolution Advisory" (RA). The suggestive action may be "corrective", suggesting the pilot change vertical speed by announcing, ''"Descend, descend"'', ''"Climb, climb"'' or ''"Level off, level off"'' (meaning reduce vertical speed). By contrast a "preventive" RA may be issued which simply warns the pilots not to deviate from their present vertical speed, announcing, ''"Monitor vertical speed"'' or ''"Maintain vertical speed, Maintain"''. TCAS II systems coordinate their resolution advisories before issuing commands to the pilots, so that if one aircraft is instructed to descend, the other will typically be told to climb – maximising the separation between the two aircraft.<ref name="TCAS71" />
As of 2006, the only implementation that meets the ACAS II standards set by [[ICAO]]<ref>{{Cite web|url=http://www.eurocontrol.int/msa/public/standard_page/ACAS_ICAO_Provisions.html|archiveurl=https://web.archive.org/web/20100421004027/http://www.eurocontrol.int/msa/public/standard_page/ACAS_ICAO_Provisions.html|
After the [[2002 Überlingen mid-air collision]] (July 1, 2002), studies have been made to improve TCAS II capabilities. Following extensive [[Eurocontrol]] input and pressure, a revised TCAS II Minimum Operational Performance Standards (MOPS) document has been jointly developed by RTCA (Special Committee SC-147<ref name=SC-147>{{Cite web |url=http://www.rtca.org/CMS_DOC/SC-147%20TOR%20Rev9%20-%20PMC%20Approved%20-%2006-10-2010.pdf |title=SC-147 Terms of Reference – Revision 9 |access-date=2011-08-28 |archive-url=https://web.archive.org/web/20110929020450/http://www.rtca.org/CMS_DOC/SC-147%20TOR%20Rev9%20-%20PMC%20Approved%20-%2006-10-2010.pdf |archive-date=2011-09-29 |url-status=dead }}</ref>) and EUROCAE. As a result, by 2008 the standards for Version 7.1 of TCAS II have been issued<ref name=SIRE>{{Cite web|url=http://www.eurocontrol.int/msa/gallery/content/public/documents/SIRE+_WP7_69D_v1.2.pdf|archiveurl=https://web.archive.org/web/20110612131435/http://www.eurocontrol.int/msa/gallery/content/public/documents/SIRE+_WP7_69D_v1.2.pdf|
TCAS II Version 7.1<ref name="TCAS71" /> will be able to issue RA reversals in coordinated encounters, in case one of the aircraft doesn't follow the original RA instructions (Change proposal CP112E).<ref name=CP112E>[http://adsb.tc.faa.gov/TCAS/CPs/CP112E.pdf Change proposal CP112E] {{webarchive|url=https://web.archive.org/web/20090109091211/http://adsb.tc.faa.gov/TCAS/CPs/CP112E.pdf |date=2009-01-09 }}</ref> Other changes in this version are the replacement of the ambiguous ''"Adjust Vertical Speed, Adjust"'' RA with the ''"Level off, Level off"'' RA, to prevent improper response by the pilots (Change proposal CP115).;<ref name=CP115>[http://adsb.tc.faa.gov/TCAS/CPs/CP115.pdf Change proposal CP115] {{webarchive|url=https://web.archive.org/web/20090109091245/http://adsb.tc.faa.gov/TCAS/CPs/CP115.pdf |date=2009-01-09 }}</ref> and the improved handling of corrective/preventive annunciation and removal of green arc display when a positive RA weakens solely due to an extreme low or high altitude condition (1000 feet AGL or below, or near the aircraft top ceiling) to prevent incorrect and possibly dangerous guidance to the pilot (Change proposal CP116).<ref name="CP116" /><ref name=EUROTCAS71>{{Cite web|url=http://www.eurocontrol.int/msa/public/standard_page/ACAS_Upcoming_Changes.html|archiveurl=https://web.archive.org/web/20100421004001/http://www.eurocontrol.int/msa/public/standard_page/ACAS_Upcoming_Changes.html|
Studies conducted for [[Eurocontrol]], using recently recorded operational data, indicate that currently{{when|date=July 2012}} the probability of a [[mid-air collision]] for each flight hour in European airspace is 2.7 x 10<sup>−8</sup> which equates to one in every 3 years. When TCAS II Version 7.1 is implemented, that probability will be reduced by a factor of 4.<ref name="EUROTCAS71" />
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TCAS IV had replaced the TCAS III concept by the mid-1990s. One of the results of TCAS III experience was that the directional antenna used by the TCAS processor to assign a bearing to a received [[Transponder (aviation)|transponder]] reply was not accurate enough to generate an accurate horizontal position, and thus a safe horizontal resolution. TCAS IV used additional position information encoded on an air-to-air data link to generate the bearing information, so that the accuracy of the directional antenna would not be a factor.
TCAS IV development continued for some years, but the appearance of new trends in data link such as Automatic Dependent Surveillance – Broadcast ([[Automatic dependent surveillance-broadcast|ADS-B]]) have pointed out a need to re-evaluate whether a data link system dedicated to collision avoidance such as TCAS IV should be incorporated into a more generic system of air-to-air data link for additional applications. As a result of these issues, the TCAS IV concept was abandoned as [[Automatic dependent surveillance-broadcast|ADS-B]] development started.<ref name="sci_aeronautics" /><ref name=FAA_TCAS>{{Cite web|url=http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/nextgen/research_tech_dev/eng_dev_serv/tcas/|archiveurl=https://web.archive.org/web/20110426234440/http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/nextgen/research_tech_dev/eng_dev_serv/tcas/|
== Current implementation ==
Although the system occasionally suffers from false alarms, pilots are now under strict instructions to regard all TCAS messages as genuine alerts demanding an immediate, high-priority response. Only [[Airborne_wind_shear_detection_and_alert_system|Windshear Detection]] and [[Ground Proximity Warning System|GPWS]] alerts and warnings have higher priority than the TCAS. The [[Federal Aviation Administration|FAA]], EASA and most other countries' authorities' rules state that in the case of a conflict between TCAS RA and [[air traffic control]] (ATC) instructions, the TCAS RA always takes precedence. This is mainly because of the TCAS-RA inherently possessing a more current and comprehensive picture of the situation than air traffic controllers, whose [[radar]]/[[Transponder (aviation)|transponder]] updates usually happen at a much slower rate than the TCAS interrogations.<ref name="TCAS70" /><ref name="TCAS71" />
If one aircraft follows a TCAS RA and the other follows conflicting ATC instructions, a collision can occur, such as the July 1, 2002 [[Überlingen mid-air collision|Überlingen disaster]]. In this mid-air collision, both airplanes were fitted with TCAS II Version 7.0 systems which functioned properly, but one obeyed the TCAS advisory while the other ignored the TCAS and obeyed the controller; both aircraft descended into a fatal collision.<ref name=BFUAX001>{{Cite web|url=http://www.bfu-web.de/cln_003/nn_53140/EN/Publications/Investigation_20Report/2002/Report__02__AX001-1-2___C3_9Cberlingen__Report,templateId=raw,property=publicationFile.pdf/Report_02_AX001-1-2_%C3%9Cberlingen_Report.pdf|archiveurl=https://web.archive.org/web/20070123052035/http://www.bfu-web.de/cln_003/nn_53140/EN/Publications/Investigation_20Report/2002/Report__02__AX001-1-2___C3_9Cberlingen__Report%2CtemplateId%3Draw%2Cproperty%3DpublicationFile.pdf/Report_02_AX001-1-2_%C3%9Cberlingen_Report.pdf|
This accident could have been prevented if TCAS was able to reverse the original RA for one of the aircraft when it detects that the crew of the other one is not following their original TCAS RA, but conflicting ATC instructions instead. This is one of the features that would later be implemented within Version 7.1 of TCAS II.<ref name="SIRE" /><ref>{{Cite web|url=http://www.eurocontrol.int/msa/gallery/content/public/documents/acas/TCAS-SAF-T2-D07_v1.2.pdf|archiveurl=https://web.archive.org/web/20120305182717/http://www.eurocontrol.int/msa/gallery/content/public/documents/acas/TCAS-SAF-T2-D07_v1.2.pdf|
Implementation of TCAS II Version 7.1 has been originally planned to start between 2009 and 2011 by retrofitting and forward fitting all the TCAS II equipped aircraft, with the goal that by 2014 the version 7.0 will be completely phased out and replaced by version 7.1. The [[Federal Aviation Administration|FAA]] and [[European Aviation Safety Agency|EASA]] have already published the TCAS II Version 7.1 Technical Standard Order (TSO-C119c<ref name=TSO-C119c>[http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgtso.nsf/0/95b393711426c9c78625759b006481ea/$FILE/TSO-C119c.pdf FAA Technical Standard Order TSO-C119c]</ref> and ETSO-C119c,<ref name=ETSO-C119c>{{Cite web|url=http://easa.europa.eu/ws_prod/g/doc/Agency_Mesures/Agency_Decisions/2009/cs_etso_5/Annex%20II%20-%20ETSO.pdf|title=European Technical Standard Order ETSO-C119c}}</ref> respectively) effective since 2009, based on the RTCA DO-185B<ref name="DO-185" /> and EUROCAE ED-143 standards. On 25 September 2009 [[Federal Aviation Administration|FAA]] issued Advisory Circular AC 20-151A<ref name=AC-20-151A>{{Cite web|url=http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2020-151A.pdf|title=FAA Advisory Circular AC 20-151A – Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II), Versions 7.0 & 7.1 and Associated Mode S Transponders}}</ref> providing guidance for obtaining airworthiness approval for TCAS II systems, including the new version 7.1. On 5 October 2009, the [[Association of European Airlines]] (AEA) published a Position Paper<ref name=AEA-PP>{{Cite web|url=http://hub.easa.europa.eu/crt/docs/viewcrdattachment/cid_41072/aid_460/fmd_3f25c67f0cd4b56f59cb31e18ad71a35|title=AEA Position Paper on TCAS Version 7.1 implementation}}</ref> showing the need to mandate TCAS II Version 7.1 on all aircraft as a matter of priority. On 25 March 2010, the [[European Aviation Safety Agency]] (EASA) published Notice of Proposed Amendment (NPA) No. 2010-03 pertaining to the introduction of ACAS II software version 7.1.<ref name=ETSO_Dev56>{{Cite web|url=http://easa.europa.eu/certification/docs/deviation/ETSO%20DevP%2057.pdf|title=EASA Deviation Request #56}}</ref> On 14 September 2010, [[European Aviation Safety Agency|EASA]] published the Comment Response Document (CRD) to the above-mentioned NPA.<ref name=CRD201003>{{Cite web|url=https://easa.europa.eu/rulemaking/docs/crd/CRD%202010-03.pdf|archiveurl=https://web.archive.org/web/20101009134616/http://easa.europa.eu/rulemaking/docs/crd/CRD%202010-03.pdf|
[[ICAO]] has circulated an amendment for formal member state agreement which recommends TCAS II Change 7.1 adoption by 1 January 2014 for forward fit and 1 January 2017 for retrofit. Following the feedback and comments from airline operators, [[European Aviation Safety Agency|EASA]] has proposed the following dates for the TCAS II Version 7.1 mandate in European airspace: forward fit (for new aircraft) 1 March 2012, retrofit (for existing aircraft) 1 December 2015. These dates are proposed dates, subject to further regulatory processes, and are not final until the Implementing Rule has been published.<ref name="EUROTCAS71" />
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|Europe ([[European Aviation Safety Agency|EASA]])
|All civil turbine-powered transport aircraft with more than 30 passenger seats (or MTOM above 15,000 kg)<ref name="eurocontrol">{{Cite web|url=http://www.eurocontrol.int/msa/public/standard_page/ACAS_Equipage_Requirements.html|archiveurl=https://web.archive.org/web/20100421004042/http://www.eurocontrol.int/msa/public/standard_page/ACAS_Equipage_Requirements.html|
|TCAS II
|1 January 2000
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|Hong Kong ([[Civil Aviation Department (Hong Kong)|Civil Aviation Department]])
|All aircraft in Hong Kong with more than 9 passenger seats (or MTOM greater than 5,700 kg)<ref name="cad">{{Cite web|url=http://www.cad.gov.hk/reports/HKAN/AN024%20%20Issue04%20I.pdf|archiveurl=https://web.archive.org/web/20070928000559/http://www.cad.gov.hk/reports/HKAN/AN024%20%20Issue04%20I.pdf|
|TCAS II Version 7.0
|1 January 2000
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|Peru ([[Directorate General of Civil Aviation of Peru|Dirección General de Aeronáutica Civil]])
|All civil turbine-powered transport aircraft with more than 19 passenger seats (or MTOM above 5,700 kg)<ref name="RAP121K">{{Cite web|url=http://www.mtc.gob.pe/portal/transportes/aereo/regulaciones/docs/rap_rev17/rap121/rap121_subparte_k_rev%2017.pdf|archiveurl=https://web.archive.org/web/20110722104600/http://www.mtc.gob.pe/portal/transportes/aereo/regulaciones/docs/rap_rev17/rap121/rap121_subparte_k_rev%2017.pdf|
|ACAS II (Effectively TCAS II Version 7.0)
|1 January 2005
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