Traffic Collision Avoidance System (TCAS)Ecole Polytechnique de Montreal

AER3205 - Caracteristiques Del Avion

Joaquim Villen Benseny

April 8, 2015

Contents

1 Introduction and historical background 2

2 Systems related concepts background 42.1 Collision Avoidance Concepts . . . . . . . . . . . . . . . . . . 4

2.1.1 Traffic Advisory (TA) . . . . . . . . . . . . . . . . . . 52.1.2 Resolution Advisory (RA) . . . . . . . . . . . . . . . . 52.1.3 Sensitivity Level . . . . . . . . . . . . . . . . . . . . . 52.1.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.5 Protected Volume . . . . . . . . . . . . . . . . . . . . . 6

2.2 ATM and ATC background . . . . . . . . . . . . . . . . . . . 62.3 TCAS Related Aircraft Systems . . . . . . . . . . . . . . . . . 7

2.3.1 Transponder . . . . . . . . . . . . . . . . . . . . . . . . 72.3.2 Cockpit Presentation . . . . . . . . . . . . . . . . . . . 72.3.3 Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 System Operation, Performance and Versions 93.1 System Operation . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.1 Surveillance . . . . . . . . . . . . . . . . . . . . . . . . 103.1.2 Threat Detection and Display . . . . . . . . . . . . . . 103.1.3 Threat Resolution . . . . . . . . . . . . . . . . . . . . . 11

3.2 System Performance . . . . . . . . . . . . . . . . . . . . . . . 133.3 TCAS Family and Versions . . . . . . . . . . . . . . . . . . . 15

3.3.1 TCAS II Development and Versions . . . . . . . . . . . 15

4 Conclusions 17

Chapter 1

Introduction and historicalbackground

After many years of extensive analysis, development and flight evaluationby Federal Aviation Administration (FAA), other countries Civil Aviationauthorities (CAAs) and the aviation industry, developed Traffic Alert andCollision Avoidance System or TCAS to reduce the risk of mid-air collisionsbetween aircraft. In the international area, this system is known as theAirborne Collision Avoidance System or ACAS.

TCAS is a family of airborne devices that function independently of theground-based air traffic control (ATC) system and provide collision avoidanceprotection for a broad spectrum of aircraft types.

The interest in developing a collision avoidance system began at mid-1950s, after a mid-air collision between two commercial aircrafts over theGrand Canyon. During years, the Federal Aviation Administration (FAA)explored the possibility to develop a Beacon Collision Avoidance System(BCAS), a transponder-based airborne system. In 1978, another air collisionoccurred near San Diego between one commercial and one general aviationaircraft, leading to the expansion of the BCAS; in 1981 the name was changedto the Traffic Alert and Collision Avoidance System (TCAS). A third mid-aircollision in 1986 in California, prompted Congress in 1987 to pass legislationrequiring the FAA to implement an airborne collision avoidance system bythe end of 1992. The law applied to all turbine-powered aircraft with morethan 30 passenger seats in the United States. A subsequent law extendedthe original deadline by one year to the end of 1993. The first commercialTCAS systems began flying in 1990 [FAA, 1979].

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Monitoring and safety assessments led to a series of changes resulting inan international version of TCAS referred to as Version 7 or the AirborneCollision Avoidance System (ACAS). In January 2003, the International CivilAviation Organization mandated the use of ACAS worldwide for all turbine-powered aircraft with passenger capacity of more than 30 or with maximumtake-off weight exceeding 15,000 kg. In January 2005, that mandate wasextended to cover aircraft with more than 19 passenger seats or maximumtake-off weight of more than 5700 kg. Today, more than 25,000 aircraftworldwide are equipped with TCAS [Kuchar and Drumm, 2007]

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Chapter 2

Systems related conceptsbackground

Before going deep into the TCAS subject some concepts will be explainedto be able to understand how it works. First of all some collision avoidanceconcepts will be explained. Also, since TCAS is not the only factor trying toavoid mid-air collision, but there are also others as Air Traffic Management(ATM) and Air Traffic Control (ATC) procedures, a light background willbe provided. Finally some background about some TCAS key elements willbe given.

2.1 Collision Avoidance Concepts

Airborne collision avoidance is a complex problem. It has taken many yearsto develop an operationally acceptable solution and refinement of the sys-tem continues to maximize the compatibility between TCAS, ATC systemsthroughout the world, and existing cockpit procedures. The heart of colli-sion avoidance is the collision avoidance system logic, or the CAS logic. Toexplain the operation of the CAS logic, the basic CAS concepts of TrafficAdvisory (TA), Resolution Advisory (RA), Sensitivity Level (SL), tau, andprotected volume need to be understood.

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2.1.1 Traffic Advisory (TA)

The Traffic Advisory is one of the two alerts that TCAS can issue. This alertconsists into assisting the pilot in the visual search for the intruder aircraftand to prepare him for a potential RA.

2.1.2 Resolution Advisory (RA)

Is the second and most critical alert that TCAS can issue. This alert recom-mends maneuvers that will either increase or maintain the existing verticalseparation from an intruder aircraft. When the intruder aircraft is also fittedwith TCAS II, both TCAS co-ordinate their RAs through the Mode S datalink to ensure that complementary RAs are selected.

2.1.3 Sensitivity Level

Effective CAS logic operation requires a trade-off between necessary protec-tion and unnecessary advisories. This trade-off is accomplished by controllingthe sensitivity level (SL), which controls the time or thresholds for issu-ing Traffic Alerts (TA) or Resolutions Advisories (RA), and therefore thedimensions of the protected airspace around each TCAS-equipped aircraft.The higher the SL, the larger the amount of protected airspace is and thelonger the alerting thresholds are. However, as the amount of protectedairspace increases, the incidence of unnecessary alerts has the potential toincrease [FAA, 1979].

2.1.4

TCAS primarily uses time-to-go to CPA rather than distance to determinewhen a TA or an RA should be issued. The time to CPA is called the range and the time to co-altitude is called the vertical . is an approximationof the time, in seconds, to CPA or to the aircraft being at the same altitude.The range is equal to the slant range (NM) divided by the closing speed(knots) multiplied by 3600. The vertical is equal to the altitude separation(feet) divided by the vertical closing speed of the two aircraft (feet/minute)times 60 [FAA, 1979].

H(s) =x(NM)

VREL 3600 (2.1)

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V (s) =h(ft)

V S(fpm) 60 (2.2)

2.1.5 Protected Volume

A protected volume of airspace surrounds each TCAS-equipped aircraft. Thevertical and the fixed altitude thresholds determine the vertical dimensionsof the protected volume. The horizontal dimensions of the protected airspaceare not based on distance, but on . plus an estimate of the protectedhorizontal miss distance. Thus, the size of the protected volume depends onthe speed and heading of the aircraft involved in the encounter [FAA, 1979].

2.2 ATM and ATC background

The basic safety goal of the ATM system during en-route commercial flightsis to avoid mid-air collisions. Therefore, an analysis of the system operationduring a mid-air collision can provide insights about system weaknesses andsafety.

ATM systems are composed of many nested layers resulting in complexinteractions. Interactions occur between human operators (controllers andpilots), between human operators and procedures (flight plans, rules to definethe controlled air space, the air space sectors that must be handled by somespecific controller team and general safety rules for the control of traffic), andbetween operators and hardware/software technical systems (radar systems,computer processing of radar and flight data, aircraft navigation systems,TCAS, communication systems between controllers and pilots, flight progressstrips) [de Carvalho et al., 2009].

In an ATM control system, the safety control barriers to avoid a mid-aircollision can be summarized as:

Aircraft are under surveillance of Air Traffic Controllers which is themain responsible to provide vertical and horizontal separations betweenaircrafts.

General vertical separation 1000 ft.

In cruise altitudes, vertical separation increased by using only even al-titudes (FL 320, 340. . . ) for flights following a magnetic route between

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0 and 179o and odd altitudes (FL 330, 350. . . ) for flights tracking amagnetic route from 180o to 359o.

Well defined flight plans received before the flight.

2.3 TCAS Related Aircraft Systems

In this section a background of the most important TCAS elements (transpon-der, cockpit presentation and antennas) will be given.

2.3.1 Transponder

A transponder is an electronic device that produces a response when it re-ceives a radio-frequency interrogation. The main goal of transponders is toassist in identifying them on air traffic control radar but collision avoidancesystems have been developed to use transponder transmissions as means ofdetecting aircraft at risk of colliding with each other.

Aircraft transponders may work in different modes depending on the in-formation the user wants to provide as a reply. For civil aviation there arethree modes widely known.

Mode A only transmits an identifying code. Mode C enables the Air Traffic Control (and therefore, the TCAS) the

aircraft altitude automatically.

Mode S has altitude capability and also permits data exchange.Mode C or S equipment is a mandatory requirement for the FAA.

TCAS operation requires that both aircraft the interrogator and thetarget are equipped with operating transponders. An aircraft with TCASwill receive the following information depending on the type of transponderwith which the target aircraft is equipped.

2.3.2 Cockpit Presentation

The TCAS interface with the pilots is provided by two displays: the trafficdisplay and the RA display. These two displays can be implemented in anumber of ways, including incorporating both displays into a single, physical

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unit. Regardless of the implementation, the information provided is identical[FAA, 1979].

2.3.3 Antenna

The antennas used by TCAS II (current version) include a directional an-tenna that is mounted on the top of the aircraft and either an omnidirectionalor a directional antenna mounted on the bottom of the aircraft. Most in-stallations use the optional directional antenna on the bottom of the aircraft[FAA, 1979].

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Chapter 3

System Operation,Performance and Versions

3.1 System Operation

TCAS processes are organized into several elements. First, surveillance sen-sors collect state information about the intruder aircraft (e.g. its relativeposition and velocity) and pass the information to a set of algorithms todetermine whether a collision threat exists and therefore it will be classi-fied. Depending on the intruders information it may be classified among 4statuses: other traffic, proximate traffic, Traffic Alert (TA) traffic and Reso-lution Alert (RA) traffic.

Once a status has been assigned to the intruder the Traffic Display willdisplay the traffic on the Navigation Display following the standards. SeeSection. If the traffic is stated as TA the system will give aural advisorieswhile if the traffic is stated as RA a second set of threat-resolution algorithmsdetermines an appropriate response. If the intruder aircraft also has TCAS,the response is coordinated through a data link to ensure that each aircraftmaneuvers in a compatible direction.

Collision avoidance maneuvers generated and displayed by TCAS aretreated as advisories to flight crews, who then take manual control of theaircraft and maneuver accordingly. Pilots are trained to follow TCAS advi-sories unless doing so would jeopardize safety [Kuchar and Drumm, 2007].

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3.1.1 Surveillance

Surveillance of the air traffic environment is based on air-to-air interrogationsbroadcast once per second from the antenna located on the TCAS aircraftusing the same frequency (1030 MHz) and waveform as ground-based air traf-fic control sensors. Transponders on nearby intruder aircraft receive theseinterrogations and send replies at 1090 MHz. Two types of transponders arecurrently in use: Mode S transponders, which have a unique 24-bit identi-fier, or Mode S address, and older Air Traffic Control Radar Beacon System(ATCRBS) transponders, which do not have unique addressing capability.To track ATCRBS intruders, TCAS transmits ATCRBS-only all-call in-terrogations once per second and then, all ATCRBS aircraft in a regionaround the TCAS aircraft reply. In contrast, Mode Sequipped intrudersare tracked with a selective interrogation once per second directed at thatspecific intruder and then only that one aircraft replies. Selective interroga-tion reduces the likelihood of garbled or overlapping replies, and also reducesfrequency congestion at 1030/1090 MHz.

Replies from most ATCRBS and all Mode S transponders contain theintruders current altitude above sea level. TCAS computes slant range onthe basis of the round-trip time of the signal and estimates the bearing to theintruder by using a four-element directional antenna [Kuchar and Drumm,2007].

3.1.2 Threat Detection and Display

TCASs complex threat-detection algorithms begin by classifying intrudersinto one of four discrete levels: other traffic, proximate traffic, Traffic Advi-sorys (TA) and Resolution Advisorys (RA). To project an aircrafts positioninto the future, the system performs a simple linear extrapolation based onthe aircrafts estimated current velocity. The algorithm then uses several keymetrics to decide whether an intruder is a threat, including the estimatedvertical and slant range separations between aircraft.

A display in the cockpit depicts nearby aircraft (See Figure 3.1), indicat-ing their range, bearing, and relative altitude; an arrow indicates whetherthe intruder is climbing or descending. Such traffic display information aidsthe pilot when attempting to visually acquire traffic out the windscreen.Distant, non-threatening aircraft appear as hollow diamond icons. Whenintruder close within certain lateral and vertical limits, the icon changes to

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a solid diamond, alerting the flight crew that traffic is proximate but is notyet a threat. If a collision is predicted to occur within the next 20 to 48seconds (depending on altitude), TCAS issues a traffic advisory (TA) in thecockpit. This advisory comes in the form of a spoken message, traffic, traf-fic. The traffic icon also changes into a solid yellow circle. The TA alertsthe pilot to the potential threat so that the pilot can search visually for theintruder and communicate with ATC about the situation. A TA also servesas a preparatory cue in case maneuvering becomes required. If the situationworsens, a resolution advisory (RA) warning is issued 15 to 35 seconds beforecollision (again depending on altitude). The RA includes an aural commandsuch as climb, climb and a graphical display of the target vertical ratefor the aircraft (See Figure 3.2). A pilot receiving an RA should disengagethe autopilot and manually control the aircraft to achieve the recommendedvertical rate [Kuchar and Drumm, 2007].

Figure 3.1: TCASs Different Traffic Outline [Kuchar and Drumm, 2007]

3.1.3 Threat Resolution

Once the criteria for issuing an RA have been met, TCASs threat-resolutionalgorithms determine what maneuver is appropriate to avoid a collision.

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Figure 3.2: RAs Display Abdallah [2015]

First, the algorithm decides the vertical sense of the maneuver that is,whether the aircraft needs to climb or to descend. Second, the system fig-ures the strength of the RAthat is, how rapidly the plane needs to changeits altitude. TCAS works only in the vertical direction; it does not selectturning maneuvers, because bearing accuracy is generally not sufficient todetermine whether a turn to the left or right is appropriate.

Figure 3.3 shows a simplification of the sense-selection process. In general,two maneuver templates are examined: one based on a climb, and one basedon a descent. Each template assumes a 5 sec delay before a response begins,followed by a 0.25 g vertical acceleration until reaching a target vertical rateof 1500 ft/min. In the meantime, the intruder aircraft is assumed to continuein a straight line at its current vertical rate. The TCAS algorithm selects themaneuver sense providing the largest separation at the predicted closest pointof approach. In the situation shown in Figure 3.3, TCAS would on the basisof these criteria advise the aircraft to descend [Kuchar and Drumm, 2007].If the intruder is also TCAS equipped, the sense of the RA is coordinatedthrough the Mode S data link to ensure that both aircraft do not select thesame vertical sense. Should both aircraft simultaneously select the samesensesay, both select a climb RAthe aircraft with the lower numerical-

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Figure 3.3: How TCAS selects the maneuver sense [Abdallah, 2015]

valued Mode S address has priority and will continue to display its climbRA. The aircraft with the higher Mode S address will then reverse its senseand display a descend RA. Higher Mode S address is selected as a decisionkey since it means that this transponder is newer than the transponder witha lower Mode S address. Once the sense has been selected, the strengthof the RA maneuver is determined by using additional maneuver templates(Figure 3.4). Each template again assumes a 5 sec delay, followed by a 0.25g acceleration to reach the target vertical rate. TCAS selects the templatethat requires the smallest vertical-rate change that achieves at least a certainminimum separation. In the example shown in Figure 3.4, the TCAS aircraftis currently descending at a rate of 1000 ft/min when an RA is issued.

Five maneuver templates are examined, with each template correspondingto a different target vertical rate. The minimum-strength maneuver thatwould provide the required vertical separation of at least 400 ft would beto reduce the descent rate to 500 ft/min; the pilot would receive an auralmessage stating that instruction. Descent rates exceeding 500 ft/min wouldappear in red on the RA display. Note that in Figure 3.4 if the intruder were100 ft higher, then the selected RA would instead be dont descend. If theintruder were another 100 ft higher still, the selected RA would be climb.

3.2 System Performance

The main functions of TCAS are to identify a potential collision threat, com-municate the detected threat to the pilot, and assist in the resolution of the

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Figure 3.4: TCAS Vertical Speed Resolution Selection [Abdallah, 2015]

threat by recommending an avoidance maneuver. As an alerting system,TCAS operates quietly in the background most of the time. When the algo-rithms determine that action is needed, TCAS interrupts the flight crew tobring the threat to their attention. This interruption may be vitally impor-tant if the pilots are not aware of the threat. In some situations, however,aircraft may operate safely close together; in those cases, the TCAS alertsare more of a nuisance than a help. An example is during an approach toclosely spaced parallel runways. In good visibility conditions, pilots can begiven the authority to maintain separation from parallel traffic by monitoringnearby aircraft visually through the windscreen. TCAS, however, does notknow that visual separation is being used and may issue a TA or an RA, thusintroducing a distraction on the flight deck when pilots should be especiallyfocused on performing their approach procedures. TCAS does inhibit issuingRAs when an aircraft is less than 1000 ft above the ground, both to reducenuisances at low altitude and to help ensure that any TCAS advisories donot conflict with potential terrain hazards.

TCAS operates in a complex, dynamic environment. Each decision maker(Air Traffic Control, pilots, TCAS itself ) uses different information sourcesand operates under different constraints and with different goals. TCAS may

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have more accurate range or altitude information about an intruder thanflight crews or ATC do. But TCAS cannot observe all the factors affecting atraffic encounter, such as the location of hazardous weather, terrain, aircraftwithout transponders, or ATC instructions a major reason that TCASis certified to operate only as an advisory system. Pilots are ultimatelyresponsible for deciding on the correct course of action, weighing TCAS alertswith the other information available to them.

TCAS is extremely successful in providing a last-resort safety net, anddoes not necessarily need to operate perfectly to be effective. Still, it isimportant to identify situations where TCAS may have difficultyand, ifpossible, modify the logic to better handle such circumstances.

3.3 TCAS Family and Versions

TCAS has been evolving during years to reduce mid-air collisions risk; there-fore there are 4 different families of the system, TCAS I, II, III and IV[Kuchar and Drumm, 2007]. .

TCAS I was the first TCAS family and only used to display traffic andgenerate collision warnings in form of TA.

TCAS II, which is the current version, uses to display the traffic andgive vertical resolution advisories.

TCAS III was envisioned as an expansion of the TCAS II concept toinclude horizontal resolution advisory capability but never came out.

TCAS IV replaced the TCAS III concept by the mid-1990s and wasexpected to give vertical and horizontal resolution advisories but theappearance of new trends in data link such as ADS-B have pointed outa need to re-evaluate TCAS in general.

3.3.1 TCAS II Development and Versions

Actually TCAS II has been the only widely TCAS family implemented duringmore than 20 years. Since aeronautics industry is always in evolution differentversions have been developed.

Version 6.0: First mandatory TCAS II version.

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Version 6.04a: Reduction of high level of nuisance alerts which oc-curred at low altitudes and during level-off maneuvers. Also a slightmodification of the altitude crossing solved was performed.

Version 7.0: Differs from Version 6.04a in the collision avoidance algo-rithms, aural annunciations, RA displays and pilot training programsto reduce the number of RAs issued and minimize altitude displacementwhile responding to an RA.

Verion 7.1differs from version 7.0 in the allowance of additional sensereversal RAs in order to address certain vertical chase geometries. AlsoAdjust Vertical Speed, Adjust message has been replaced by LevelOff, Level Off.

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Chapter 4

Conclusions

We have learned that Airbone collision avoidance is a complex problem andthe CAS logic. The CAS logic has to manage the trade-off between necessaryprotection and unnecessary advisories, a trade-off that will depend on theflights envelope. We have also learned that TCAS uses time-to-go ratherthan distance values to determine when the alert will be issued since whentwo aircraft are going to crash depends on the speed and then on the timeand not only on the distance.

Its also important to keep in mind that TCAS is the main and lastelement regarding to air collision avoidance. There are several other safetycontrol barriers as ATC surveillance, general vertical separation rules anddefined flight plans.

We have also learned that TCAS is system composed of several elements.This system is mainly a block of software code lines, but it also needs:

Transponder: to broadcast several aircraft data parameters as altitude,identification, position, speed, heading...

Cockpit display: to give visual advisories to the pilot. Antenna: to send and receiver data.The TCAS operation consists on three phases:

Surveillance: air-to-air interrogations to other aircraft transponders areperformed constantly.

Threat Detection and Display: when other air-to-air interrogations re-sult in an aircraft nearby, the aircraft is classified in 4 different kinds

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of traffic (other, proximate, TA or RA) depending on H , V . Thenthe aircraft is displayed in the designated cockpit system following aspecific color code.

Threat Resolution: Once the criteria for issuing an RA have been met,TCASs threat-resolution algorithms determine what maneuver is ap-propiate to avoid a collision. TCAS will try to avoid changing the senseof the maneuver whenever it is possible.

Finally we can conclude that TCAS has played a really important role inair collision avoidance. Since its a really critical system a lot of effort hasbeen put by Aviation Civil Authorities to develop this system and make itmandatory for all civilian aircraft. We could end this project by saying thatprobably TCAS has saved many lives and aircraft accidents.

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Bibliography

Ibrahim Abdallah. Tcas presentation aer4720: Integration des syste`mesavioniques. 2015.

Paulo Victor Rodrigues de Carvalho, Jose Orlando Gomes, Gilbert JacobHuber, and Mario Cesar Vidal. Normal people working in normal organi-zations with normal equipment: System safety and cognition in a mid-aircollision. Applied Ergonomics, 40(3):325340, 2009.

FAA. Tcas ii booklet v7.1. Technical report, Federal Aviation Authority,1979.

JE Kuchar and Ann C Drumm. The traffic alert and collision avoidancesystem. Lincoln Laboratory Journal, 16(2):277, 2007.

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