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* [[1996 Charkhi Dadri mid-air collision]] accident over [[New Delhi]]
* 1999 Lambourne near-collision, involving a [[Boeing 737 Classic#737-300|Boeing 737-300]] and a [[Gulfstream IV]]. The airspace above [[Lambourne]] is the waiting zone for [[Heathrow Airport|Heathrow]]. The event is notable as both planes entered the zone from different directions leading to an imminent head-on collision (one o'clock position). The traffic advisory (amber mark) almost immediately turned into a resolution advisory (red mark) with a projected time for collision of less than 25 seconds.<ref>{{cite web|url=https://www.gov.uk/aaib-reports/boeing-737-300-d-abek-and-gulfstream-iv-n77sw-26-february-1999|title=Boeing 737-300, D-ABEK and Gulfstream IV, N77SW, 26 February 1999|publisher=[[Air Accidents Investigation Branch]]|archive-url=https://
* [[2001 Japan Airlines mid-air incident]], in which the Captain of Japan Airlines Flight 907 (a [[Boeing 747-400]]), 40-year old Makoto Watanabe ({{langx|ja|渡辺 誠|translit=Watanabe Makoto|label=none}}), chose to descend, ordered by the air traffic controller, when TCAS told the flight crew to climb, nearly colliding with the descending JAL Flight 958 [[McDonnell Douglas DC-10|DC-10]] en route from [[Gimhae International Airport|Busan]] to Tokyo's [[Narita International Airport|Narita Airport]].
* [[2002 Überlingen mid-air collision]], between a [[Boeing 757]] and a [[Tupolev Tu-154]], where the Tupolev pilots declined to follow their TCAS resolution advisory (RA), instead following the directions of the air traffic controller, while the Boeing pilots followed their TCAS-RA, having no ATC instruction.
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[[File:ACAS Protection Volumes Diagram.svg|TCAS Volume|center]]
{| class="wikitable"
|+ Symbols used to depict nearby traffic on cockpit displays
|-
| style="color: aqua" | ◇
| Distant traffic
|-
| style="color: aqua" | ◆
| Traffic within 6 NM horizontally and 1200 feet vertically
|-
| style="color: gold" | ●
| Traffic close enough to trigger TA, within 40 seconds of potential collision
|-
| style="color: red" | ■
| Traffic close enough to trigger RA, within 25 seconds of potential collision
|}
=== System components ===
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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|url-status=dead|title=BFU Investigation Report AX001-1-2/02|archivedate=January 23, 2007}}</ref>
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
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|url-status=dead|title=Comment Response Document (CRD) to NPA 2010-03|archivedate=October 9, 2010}}</ref> Separately, a proposal has been made to amend the [[ICAO]] standard to require TCAS II Version 7.1 for compliance with ACAS II SARPs.
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* TCAS is not fitted to many smaller aircraft mainly due to the high costs involved (between $25,000 and $150,000). Many smaller personal business jets for example, are currently not legally required to have TCAS installed, even though they fly in the same airspace as larger aircraft that are required to have proper TCAS equipment on board. The TCAS system can only perform at its true operational potential once all aircraft in any given airspace have a properly working TCAS unit on board.
* TCAS requires that both conflicting aircraft have transponders. If one aircraft doesn't have a transponder, then it will not alert TCAS as there is no information being transmitted.
* Military aircraft may not be using TCAS. They could be operating with their transponders off based on their mission requirements.
To overcome some of these limitations, the FAA is developing a new collision avoidance logic based on dynamic programming.
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|Europe ([[European Aviation Safety Agency|EASA]])
|All civil turbine-powered transport aircraft with more than 19 passenger seats (or MTOM above 5,700 kg)<ref name="eurocontrol" />
|ACAS II (Effectively TCAS II Version 7.1)
|1 March 2012
|-
|Europe ([[European Aviation Safety Agency|EASA]])
|All civil turbine-powered transport aircraft with more than 19 passenger seats (or MTOM above 5,700 kg)<ref name="eurocontrol" />
|ACAS Xa<ref>{{cite web|title=European legislation clears aircraft to fly with ACAS Xa collision-avoidance systems|url=https://www.flightglobal.com/safety/european-legislation-clears-aircraft-to-fly-with-acas-xa-collision-avoidance-systems/162070.article|date=2025-03-05|archiveurl=https://archive.today/20250305110955/https://www.flightglobal.com/safety/european-legislation-clears-aircraft-to-fly-with-acas-xa-collision-avoidance-systems/162070.article|archivedate = 2025-03-05}}</ref> or ACAS II (Effectively TCAS II Version 7.1)
|10 March 2025
|-
|Australia ([[Civil Aviation Safety Authority|CASA]])
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* [https://web.archive.org/web/20100421004001/http://www.eurocontrol.int/msa/public/standard_page/ACAS_Upcoming_Changes.html TCAS II Version 7.1]
* [http://www.allstar.fiu.edu/aero/TCAS.htm Discussion of TCAS] {{Webarchive|url=https://web.archive.org/web/20050831201200/http://www.allstar.fiu.edu/aero/TCAS.htm |date=2005-08-31 }}
* {{usurped|1=[https://web.archive.org/web/20070209222607/http://www.airsport-corp.com/adsb2.htm Critical discussion of TCAS using hypothetical abuse/exploit scenarios of TCAS usage]}}
* [http://www.faa.gov/documentLibrary/media/Advisory_Circular/TCAS%20II%20V7.1%20Intro%20booklet.pdf Introduction to TCAS II Version 7.1]
* [https://web.archive.org/web/20110612131435/http://www.eurocontrol.int/msa/gallery/content/public/documents/SIRE+_WP7_69D_v1.2.pdf Decision criteria for regulatory measures on TCAS II version 7.1]
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