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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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