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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
[[File:Tcas EU-Flysafe.jpg|thumb|upright=1.14|Combined TCAS and [[Horizontal situation indicator|EHSI]] cockpit display (color)]]
In modern [[glass cockpit]] aircraft, the TCAS display may be integrated in the
[[File:TCAS Indicator.jpg|thumb|upright=1.14|Combined TCAS and [[Variometer|VSI]] cockpit display (monochrome)]]
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Research into collision avoidance systems has been ongoing since at least the 1950s, and the airline industry has been working with the [[Air Transport Association of America]] (ATA) since 1955 toward a collision avoidance system. [[ICAO]] and aviation authorities such as the [[Federal Aviation Administration]] (FAA) were spurred into action by the [[1956 Grand Canyon mid-air collision]].<ref name=TCAS71>{{Cite web|url=http://www.faa.gov/documentLibrary/media/Advisory_Circular/TCAS%20II%20V7.1%20Intro%20booklet.pdf|title=Introduction to TCAS II Version 7.1}}</ref>
Although [[Air traffic control radar beacon system|ATCRBS]] airborne transponders were available, it
A short time later, prototypes of TCAS II were installed on two [[Piedmont Airlines]] [[Boeing 727]] aircraft, and were flown on regularly scheduled flights. Although the displays were located outside the view of the flight crew and seen only by trained observers, these tests did provide valuable information on the frequency and circumstances of alerts and their potential for interaction with the [[Air traffic control|ATC]] system. On a follow-on phase II program, a later version of TCAS II was installed on a single Piedmont Airlines Boeing 727, and the system was [[Airworthiness certificate|certified]] in April 1986, then subsequently approved for operational evaluation in early 1987. Since the equipment was not developed to full standards, the system was only operated in [[visual meteorological conditions]] (VMC). Although the flight crew operated the system, the evaluation was primarily for the purpose of data collection and its correlation with flight crew and observer observation and response.<ref name="20-151B" />
Later versions of TCAS II manufactured by [[Bendix Corporation|Bendix]]/King Air Transport Avionics Division were installed and approved on [[United Airlines]] airplanes in early 1988. Similar units manufactured by Honeywell were installed and approved on [[Northwest Airlines]] airplanes in late 1988. This limited installation program operated TCAS II units approved for operation as a full-time system in both visual and [[instrument meteorological conditions]] (IMC) on three different aircraft types. The operational evaluation programs continued through 1988 to validate the operational suitability of the systems.<ref name="20-151B" />
=== Incidents ===
The implementation of TCAS added a safety barrier to help prevent [[mid-air collision]]s. However, further study, refinements, training and regulatory measures were still required because the limitations and misuse of the system still resulted in other incidents and fatal accidents, which include
* [[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)
* [[2001 Japan Airlines mid-air incident]]
▲* [[2001 Japan Airlines mid-air incident]]; where 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.
* 2006 collision between [[Gol Transportes Aéreos Flight 1907]] (a [[Boeing 737]]) and an [[Embraer Legacy 600]]; the Embraer's transponder had inadvertently been switched off, disabling its own TCAS and rendering the plane invisible to the TCAS on board flight 1907.
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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 ===
A TCAS installation consists of the following components:<ref name="TCAS70" /><ref name="TCAS71" />
;TCAS computer unit: Performs airspace surveillance, intruder tracking, its own aircraft altitude tracking, threat detection, resolution advisory (RA) manoeuvre determination and selection, and generation of advisories. The TCAS
;Antennas: The antennas used by TCAS II include a directional antenna that is mounted on the top of the aircraft and either an omnidirectional or a directional antenna mounted on the bottom of the aircraft. Most installations use the optional directional antenna on the bottom of the aircraft. In addition to the two TCAS antennas, two antennas are also required for the Mode S transponder. One antenna is mounted on the top of the aircraft while the other is mounted on the bottom. These antennas enable the Mode S transponder to receive interrogations at 1030 MHz and reply to the received interrogations at 1090 MHz.
;Cockpit presentation: The TCAS interface with the pilots is provided by two displays: the traffic display and the RA display. These two displays can be implemented in a number of ways including displays that incorporate both displays into a single, physical unit. Regardless of the implementation, the information displayed is identical. The standards for both the traffic display and the RA display are defined in DO-185A.<ref name=DO-185>[http://adsb.tc.faa.gov/SC-147.htm FAA DO-185 Materials and RTCA SC-147 Activities] {{webarchive|url=https://web.archive.org/web/20110507002355/http://adsb.tc.faa.gov/SC-147.htm |date=2011-05-07 }}</ref>
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The following points receive emphasis during pilot training:
* Do not
* Inform the controller of the RA as soon as permitted by flight crew workload after responding to the RA. There is no requirement to make this notification prior to initiating the RA response.
* Be alert for the removal of RAs or the weakening of RAs so that deviations from a cleared altitude are minimized.
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| Shall respond immediately and manoeuvre as indicated, unless doing so would jeopardize the safety of the aircraft || Shall not attempt to modify the flight path of an aircraft responding to an RA
|- valign="top"
| Shall follow the RA even if there is a conflict between the RA and an
|- valign="top"
| Shall never manoeuvre in the opposite sense to an RA, nor maintain a vertical rate in the opposite sense to an RA || Shall acknowledge the report by using the phrase ''"ROGER"''
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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
== Relationship to Traffic Advisory System (TAS) ==
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In spite of all this, most of the manufacturers do not take the above-mentioned opportunities to make simplified devices. As a result of market forces, many TAS systems operate just like TCAS I (with interference limiting, using TCAS I symbology, etc.), with some having even have better surveillance performance (in range and tracked aircraft) and specifications than TCAS I.
== Relationship to
[[Automatic dependent surveillance – broadcast]] (ADS–B) messages are transmitted from aircraft equipped with suitable transponders, containing information such as identity, ___location, and velocity. The signals are broadcast on the 1090 MHz radio frequency.
TCAS equipment which is capable of processing ADS–B messages may use this information to enhance the performance of TCAS, using techniques known as "hybrid surveillance". As currently implemented, hybrid surveillance uses reception of ADS–B messages from an aircraft to reduce the rate at which the TCAS equipment interrogates that aircraft. This reduction in interrogations reduces the use of the 1030/1090 MHz radio channel, and will over time extend the operationally useful life of TCAS technology. The ADS–B messages will also allow low cost (for aircraft) technology to provide real time traffic in the cockpit for small aircraft.<ref>{{cite web |url=http://www.duncanaviation.aero/straighttalk/adsb/tcas.php |title=TCAS 7.1 | ADSB Straight Talk |publisher=Duncanaviation.aero |date=2012-03-01 |access-date=2013-09-22 |url-status=dead |archive-url=https://web.archive.org/web/20130516094932/http://www.duncanaviation.aero/straighttalk/adsb/tcas.php |archive-date=2013-05-16 }}</ref>
Hybrid surveillance does not make use of ADS–B's aircraft flight information in the TCAS conflict detection algorithms; ADS–B is used only to identify aircraft that can safely be interrogated at a lower rate.
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=== TCAS II ===
[[File:TCAS RA.png|thumb|An example of a resolution advisory (RA), where the pilot is requested to climb at 1,750 feet per minute to avoid nearby traffic (shown in red)]]
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" />
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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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