Traffic collision avoidance system

A traffic collision avoidance system or traffic alert and collision avoidance system (both abbreviated as TCAS, and pronounced ; "TEE-kas") is an aircraft collision avoidance system designed to reduce the incidence of mid-air collisions between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air collision (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 or authorized to carry more than 19 passengers. 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 signals, but operates independently of ground-based equipment to provide advice to the pilot on potentially conflicting aircraft.
In modern glass cockpit aircraft, the TCAS display may be integrated in the Navigation Display (ND) or Electronic Horizontal Situation Indicator (EHSI).
In older glass cockpit aircraft and those with mechanical instrumentation, such an integrated TCAS display may replace the mechanical IVSI (which indicates the rate with which the aircraft is descending or climbing).
Impetus for a system and history.
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 were spurred into action by the 1956 Grand Canyon mid-air collision.
It was not until the mid-1970s, however, that research centered on using signals from ATCRBS airborne transponders as the cooperative element of a collision avoidance system. This technical approach allows a collision avoidance capability on the flight deck, which is independent of the ground system. In 1981, the FAA announced a decision to implement an aircraft collision avoidance concept called the Traffic Alert and Collision Avoidance System (TCAS). The concept is based upon agency and industry development efforts in the areas of beacon based collision avoidance systems and air-to-air discrete address communications techniques utilizing Mode S airborne transponder message formats.
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 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 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.
Later versions of TCAS II manufactured by 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
Incidents.
The implementation of TCAS added a safety barrier to help prevent mid-air collisions. 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 the:
Overview.
System description.
TCAS involves communication between all aircraft equipped with an appropriate transponder (provided the transponder is enabled and set up properly). Each TCAS-equipped aircraft interrogates all other aircraft in a determined range about their position (via the 1030 MHz radio frequency), and all other aircraft reply to other interrogations (via 1090 MHz). This interrogation-and-response cycle may occur several times per second.
The TCAS system builds a three dimensional map of aircraft in the airspace, incorporating their range (garnered from the interrogation and response round trip time), altitude (as reported by the interrogated aircraft), and bearing (by the directional antenna from the response). Then, by extrapolating current range and altitude difference to anticipated future values, it determines if a potential collision threat exists.
TCAS and its variants are only able to interact with aircraft that have a correctly operating mode C or mode S transponder. A unique 24-bit identifier is assigned to each aircraft that has a mode S transponder.
The next step beyond identifying potential collisions is automatically negotiating a mutual avoidance manoeuver (currently, manoeuvers are restricted to changes in altitude and modification of climb/sink rates) between the two (or more) conflicting aircraft. These avoidance manoeuvers are communicated to the flight crew by a cockpit display and by synthesized voice instructions.
A protected volume of airspace surrounds each TCAS equipped aircraft. The size of the protected volume depends on the altitude, speed, and heading of the aircraft involved in the encounter. The illustration below gives an example of a typical TCAS protection volume.
System components.
A TCAS installation consists of the following components:
Operation.
The following section describes the TCAS operation based on TCAS II, since this is the version that has been adopted as an international standard (ACAS II) by ICAO and aviation authorities worldwide.
Operation modes.
TCAS II can be currently operated in the following modes:
TCAS works in a coordinated manner, so when an RA is issued to conflicting aircraft, a required action (i.e., "Climb. Climb.") has to be immediately performed by one of the aircraft, while the other one receives a similar RA in the opposite direction (i.e., "Descend. Descend.").
Alerts.
TCAS II issues the following types of aural annunciations:
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.
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.
On the other hand, ATC can potentially interfere with a pilot's response to RAs. If a conflicting ATC instruction coincides with an RA, a pilot may assume that ATC is fully aware of the situation and is providing the better resolution. But in reality, ATC is not aware of the RA until the RA is reported by the pilot. Once the RA is reported by the pilot, ATC is required not to attempt to modify the flight path of the aircraft involved in the encounter. Hence, the pilot is expected to "follow the RA" but in practice this does not always happen.
Some countries have implemented "RA downlink" which provides air traffic controllers with information about RAs posted in the cockpit. Currently, there are no ICAO provisions concerning the use of RA downlink by air traffic controllers.
The following points receive emphasis during pilot training:
An RA occurs on average every 1,000 flight hours on short/medium-haul aircraft and every 3,000 hours for long-haul aircraft.
In its December 2017 ACAS guide, Eurocontrol found in about 25% of the cases, the pilots follow the RA inaccurately.
Airbus offers the option of an autopilot/flight director TCAS for automatic avoidance maneuvers.
Safety aspects.
Safety studies on TCAS estimate that the system improves safety in the airspace by a factor of between 3 and 5.
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).
One potential problem with TCAS II is the possibility that a recommended avoidance maneuver might direct the flight crew to descend toward terrain below a safe altitude. Recent requirements for incorporation of ground proximity mitigate this risk. Ground proximity warning alerts have priority in the cockpit over TCAS alerts.
Some pilots have been unsure how to act when their aircraft was requested to climb whilst flying at their maximum altitude. The accepted procedure is to follow the climb RA as best as possible, temporarily trading speed for height. The climb RA should quickly finish. In the event of a stall warning, the stall warning would take priority.
Both cases have been addressed by Version 7.0 of TCAS II and are currently handled by a corrective RA together with a visual indication of a green arc in the IVSI display to indicate the safe range for the climb or descent rate. However, it has been found that in some cases these indications could lead to a dangerous situation for the involved aircraft. For example, if a TCAS event occurs when two aircraft are descending one over the other for landing, the aircraft at the lower altitude will first receive a "Descend, descend" RA, and when reaching an extreme low altitude, this will change to a "Level off, level off" RA, together with a green arc indication directing the pilot to level off the aircraft. This could place the aircraft dangerously into the path of the intruder above, who is descending to land. A change proposal has been issued to correct this problem.
Relationship to Traffic Advisory System (TAS).
TCAS technology has proved to be too expensive for small business and general aviation aircraft. Manufacturers and authorities recognized the need for a viable alternative to TCAS, that’s why Traffic Advisory System has been developed. TAS is actually a simplified version of TCAS I. The system structure, components, operation, traffic display and TA logic are identical, but the minimum operational performance standards (MOPS) of TAS allow some simplification comparing to TCAS I:
The following documents contain all of the differences between TCAS I and TAS:
In spite of all these, most of the manufacturers do not grab the above mentioned opportunities to make simplified devices. In the consequence of market competition many TAS equipments operate just like they would be a TCAS I, with interference limiting, using TCAS I symbology etc., and can even have better surveillance performance (in range and tracked aircraft) and specifications than TCAS I.
Relationship to Automatic Dependent Surveillance – Broadcast (ADS–B).
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. ADS-B messages are also carried on a Universal Access Transceiver (UAT) in the 978 MHz band.
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. Currently UAT based traffic uplinks are provided in Alaska and in regions of the East coast of the USA.
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.
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.
Drawbacks to TCAS and ADS–B.
The major demonstrated problem of the ADS–B protocol integration is this added verbosity of the extra information transmitted which is considered unnecessary for collision avoidance purposes. The more data transmitted from one aircraft in accordance with the system design, the fewer the number of aircraft that can participate in the system due to the fixed and limited channel data bandwidth (1 megabit/second with the 26/64 data bits to packet length bit capacity of the Mode S downlink data format packet). For every Mode S message of 64 bits, the overhead demands 8 for clock sync at the receiver and Mode S packet discovery, 6 for type of Mode S packet, 24 for who it came from. Since that leaves only 26 for information, multiple packets must be used to convey a single message. The ADS–B "fix" proposal is to go to a 128 bit packet which is not an accepted international standard. Either approach increases channel traffic above the level sustainable for environments such as the Los Angeles Basin.
Versions.
Passive.
Collision Avoidance systems which rely on transponder replies triggered by ground and airborne systems are considered passive. Ground and airborne interrogators query nearby transponders for mode C altitude information, which can be monitored by third-party systems for traffic information. Passive systems display traffic similar to TCAS, however generally have a range of less than .
TCAS I.
TCAS I is a cheaper but less capable system than the modern TCAS II system introduced for general aviation use after the FAA mandate for TCAS II in air transport aircraft. TCAS I systems are able to monitor the traffic situation around a plane (to a range of about 40 miles) and offer information on the approximate bearing and altitude of other aircraft. It can also generate collision warnings in the form of a "Traffic Advisory" (TA). The TA warns the pilot that another aircraft is in near vicinity, announcing "Traffic, traffic", but does not offer any suggested remedy; it is up to the pilot to decide what to do, usually with the assistance of Air Traffic Control. When a threat has passed, the system announces "Clear of conflict".
TCAS II.
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.
As of 2006, the only implementation that meets the ACAS II standards set by ICAO was Version 7.0 of TCAS II, produced by three avionics manufacturers: Rockwell Collins, Honeywell, and ACSS (Aviation Communication & Surveillance Systems; an L3 Technologies and Thales Avionics joint venture company).
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) and EUROCAE. As a result, by 2008 the standards for Version 7.1 of TCAS II have been issued and published as RTCA DO-185B (June 2008) and EUROCAE ED-143 (September 2008).
TCAS II Version 7.1 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). 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).; 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).
Studies conducted for Eurocontrol, using recently recorded operational data, indicate that currently the probability of a mid-air collision for each flight hour in European airspace is 2.7 x 10−8 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.
Although ACAS III is mentioned as a future system in ICAO Annex 10, ACAS III is unlikely to materialize due to difficulties the current surveillance systems have with horizontal tracking. Currently, research is being conducted to develop a future collision avoidance system (under the working name of ACAS X).
TCAS III.
Originally designated TCAS II Enhanced, TCAS III was envisioned as an expansion of the TCAS II concept to include horizontal resolution advisory capability. TCAS III was the "next generation" of collision avoidance technology which underwent development by aviation companies such as Honeywell. TCAS III incorporated technical upgrades to the TCAS II system, and had the capability to offer traffic advisories and resolve traffic conflicts using "horizontal" as well as vertical manouevring directives to pilots. For instance, in a head-on situation, one aircraft might be directed, "turn right, climb" while the other would be directed "turn right, descend." This would act to further increase the total separation between aircraft, in both horizontal and vertical aspects. Horizontal directives would be useful in a conflict between two aircraft close to the ground where there may be little if any vertical maneuvering space.
TCAS III attempts to use the TCAS directional antenna to assign a bearing to other aircraft, and thus be able to generate a horizontal maneuver (e.g. turn left or right). However, it was judged by the industry to be unfeasible due to limitations in the accuracy of the TCAS directional antennas. The directional antennas were judged not to be accurate enough to generate an accurate horizontal-plane position, and thus an accurate horizontal resolution. By 1995, years of testing and analysis determined that the concept was unworkable using available surveillance technology (due to the inadequacy of horizontal position information), and that horizontal RAs were unlikely to be invoked in most encounter geometries. Hence, all work on TCAS III was suspended and there are no plans for its implementation. The concept has later evolved and been replaced by TCAS IV.
TCAS IV.
TCAS IV uses additional information encoded by the target aircraft in the Mode S transponder reply (i.e. target encodes its own position into the transponder signal) to generate a horizontal resolution to an RA. In addition, some reliable source of position (such as Inertial Navigation System or GPS) is needed on the target aircraft in order for it to be encoded.
TCAS IV has replaced the TCAS III concept by the mid 1990s. One of the results of TCAS III experience has been that the directional antenna used by the TCAS processor to assign a bearing to a received transponder reply is not accurate enough to generate an accurate horizontal position, and thus a safe horizontal resolution. TCAS IV uses additional position information encoded on an air-to-air data link to generate the bearing information, so 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 (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 ADS-B development started.
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 Windshear Detection and GPWS alerts and warnings have higher priority than the TCAS. The 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 updates usually happen at a much slower rate than the TCAS interrogations).
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 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.
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 will be implemented within Version 7.1 of TCAS II.
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 FAA and EASA have already published the TCAS II Version 7.1 Technical Standard Order (TSO-C119c and ETSO-C119c, respectively) effective since 2009, based on the RTCA DO-185B and EUROCAE ED-143 standards. On 25 September 2009 FAA issued Advisory Circular AC 20-151A 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 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. On 14 September 2010, EASA published the Comment Response Document (CRD) to the above-mentioned NPA. Separately, a proposal has been made to amend the ICAO standard to require TCAS II Version 7.1 for compliance with ACAS II SARPs.
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, 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.
Among the system manufacturers, by February 2010 ACSS certified Change 7.1 for their TCAS 2000 and Legacy TCAS II systems, and is currently offering Change 7.1 upgrade for their customers. By June 2010 Honeywell published a white paper with their proposed solutions for TCAS II Version 7.1. Rockwell Collins currently announces that their TCAS-94, TCAS-4000 and TSS-4100 TCAS II compliant systems are software upgradeable to Change 7.1 when available.
Current limitations.
While the safety benefits of current TCAS implementations are self-evident, the full technical and operational potential of TCAS is not fully exploited due to limitations in current implementations (most of which will need to be addressed in order to further facilitate the design and implementation of Free flight) and NextGen:
To overcome some of these limitations, the FAA is developing a new collision avoidance logic based on dynamic programming.
In response to a series of midair collisions involving commercial airliners, Lincoln Laboratory was directed by the Federal Aviation Administration in the 1970s to participate in the development of an onboard collision avoidance system. In its current manifestation, the Traffic Alert and Collision Avoidance System is mandated worldwide on all large aircraft and has significantly improved the safety of air travel, but major changes to the airspace planned over the coming years will require substantial modification to the system.
ACAS X.
A set of new systems called ACAS X will use this new logic:
The first FAA-scheduled industry meeting was held in October 2011 in Washington DC, to brief avionics manufacturers on the development plans for "ACAS X" – including flight demonstrations scheduled for fiscal 2013. The FAA says its work "will be foundational to the development of minimum operational performance standards" for ACAS X by standards developer RTCA.
It is estimated that, if ACAS X will be further developed and certified, ACAS X will not be commercially available before mid 2020s. And it is said to be unclear at this stage whether ACAS X would provide any horizontal resolutions.