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{{short description|Aircraft collision avoidance system}}
A '''traffic alert and collision avoidance system''' ('''TCAS'''), pronounced {{IPAc-en|ˈ|t|i:|k|æ|s}} {{Respell|TEE|kas}}), also known as an '''Airborne Collision Avoidance System''' ('''ACAS'''),<ref name="NBAA">{{Cite web|url=https://nbaa.org/aircraft-operations/communications-navigation-surveillance-cns/tcas/|title=Traffic Alert and Collision Avoidance System (TCAS)|website=NBAA - National Business Aviation Association}}</ref> is an [[aircraft collision avoidance system]] designed to reduce the incidence of [[mid-air collision]] (MAC) between [[aircraft]]. It monitors the [[airspace]] around an aircraft for other aircraft equipped with a corresponding active [[Transponder (aviation)|transponder]], independent of [[air traffic control]], and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of MAC. It is a type of airborne collision avoidance system mandated by the [[International Civil Aviation Organization]] to be fitted to all aircraft with a maximum take-off mass (MTOM) of over {{convert|5700|kg|abbr=on}} or authorized to carry more than 19 passengers. In the United States, [[Federal Aviation Regulations|CFR 14]], Ch I, part 135 requires that TCAS I be installed for aircraft with 10–30 passengers and TCAS II for aircraft with more than 30 passengers. ACAS/TCAS is based on [[secondary surveillance radar]] (SSR) [[transponder (aviation)|transponder]] signals, but operates independently of ground-based equipment to provide advice to the pilot on potentially conflicting aircraft.
[[File:Tcas EU-Flysafe.jpg|thumb|upright=1.14|Combined TCAS and [[Horizontal situation indicator|EHSI]] cockpit display (color)]]
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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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== Security aspects ==
TCAS was not designed with security in mind, even in its newest versions. With the rise of [[Software-defined radio|software-defined radios]], security researchers have investigated wireless attacks on TCAS. Researchers demonstrate<ref name="Usenix">{{Cite web|url=https://www.usenix.org/system/files/usenixsecurity24-longo.pdf|title=On a Collision Course: Unveiling Wireless Attacks to the Aircraft Traffic Collision Avoidance System (TCAS)|website=Usenix Security 2024}}</ref> how to take full control over the collision avoidance displays and create RAs of arbitrary aircraft on a collision course. These attacks can be launched using [[Commercial off-the-shelf|commercial off-the-shelf hardware]] and could be used to instruct the pilot to climb or descend at will. However, these attacks are only possible when the attacker is close to the victim aircraft (up to a distance of 4.2 km), limiting the risk of abuse in the real world.
== Relationship to Traffic Advisory System (TAS) ==
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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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