Introduction to TCAS II V7

8/9/2019 Introduction to TCAS II V7

1/45


2/45

Preface

This booklet provides the background for a better understanding of the Traffic Alert and Collision Avoidance System (TCAS II) by personnel involved in the implementation and operation of TCAS II.This booklet is an update of a similar booklet published in 1990 by the Federal Aviation Administration (FAA). This update describes TCAS II Version 7.


3/45

Table of Contents PREFACE


4/45


5/45

After many years of extensive analysis,development, and flight evaluation by the

Federal Aviation Administration (FAA),other countries Civil Aviation Authorities(CAAs), and the aviation industry, a solutionhas been found to reduce the risk of midaircollisions between aircraft. This solution isknown as the Traffic Alert and CollisionAvoidance System or TCAS. In theinternational arena, the system is known asthe Airborne Collision Avoidance System orACAS.

TCAS is a family of airborne devices thatfunction independently of the ground-basedair traffic control (ATC) system and providecollision avoidance protection for a broadspectrum of aircraft types.

TCAS I provides traffic advisories (TA) andproximity warning of nearby traffic to assistthe pilot in the visual acquisition of intruderaircraft. TCAS I is mandated for use in theUnited States for turbine-powered, passenger-carrying aircraft having more than 10 and lessthan 31 seats. TCAS I is also used by anumber of general aviation fixed and rotary

wing aircraft.

TCAS II provides traffic advisories andresolution advisories (RA), i.e.,recommended escape maneuvers, in thevertical dimension to either increase ormaintain the existing vertical separationbetween aircraft. Airline aircraft, includingregional airline aircraft with more than 30seats, and general aviation turbine-poweredaircraft use TCAS II equipment.

The TCAS concept uses the same radarbeacon transponders installed on aircraft tooperate with ATC ground-based radars. Thelevel of protection provided by TCASequipment depends on the type of transponderthe target aircraft is carrying. The level of protection is outlined in Table 1. It should benoted that TCAS provides no protection

against aircraft that do not have an operatingtransponder.

Table 1. TCAS Levels of ProtectionTable 1. TCAS Levels of ProtectionTable 1. TCAS Levels of ProtectionTable 1. TCAS Levels of Protection

Own Aircraft EquiOwn Aircraft EquiOwn Aircraft EquiOwn Aircraft Equippppmentmentmentment

TCAS ITCAS ITCAS ITCAS I TCAS IITCAS IITCAS IITCAS IIMode AXPDRONLY

TA TA

Mode Cor MODES XPDR

TA TA andVertical RA

TCAS I TA TA andVertical RA

T a r g e t

A i r c r a f

t E q u

T a r g e t

A i r c r a f

t E q u

T a r g e t

A i r c r a f

t E q u

T a r g e t

A i r c r a f

t E q u

i p m e n

t

i p m e n

t

i p m e n

t

i p m e n

t

TCAS II TA TA andCoordinatedVertical RA

Based on a Congressional mandate (PublicLaw 100-223), the FAA has issued a rule thatrequires all passenger-carrying aircraft withmore than 30 seats be equipped withTCAS II.

Since the early 1990s, an operationalevaluation, known as the TCAS TransitionProgram (TTP), has collected and analyzed a

significant amount of data related to theperformance and use of TCAS II in both theU.S. National Airspace System (NAS) and inother airspace worldwide. As a result of theseanalyses, changes to TCAS II have beendeveloped, tested, and implemented. Thelatest changes, collectively known as TCASII Version 7, were certified in early 2000 andare now being implemented by the industry.

TCAS II Version 7 is the only version of TCAS II that complies with the ICAOStandards and Recommended Practices(SARPs) for ACAS II. As such, Version 7 iscurrently being mandated for carriage incertain countries or regions, e.g., Europe,Australia, and India, and has been mandatedfor carriage in 2003 by the International CivilAviation Organization (ICAO).


6/45

Background Background Background Background

The development of an effective airbornecollision avoidance system has been a goal of the aviation industry for a number of years.As air traffic has continued to grow over theyears, development of and improvements toATC systems and procedures have made itpossible for controllers and pilots to copewith this increase in operations, whilemaintaining the necessary levels of flightsafety. However, the risk of airborne collisionremains. That is why, as early as the 1950s,the concept and initial development of anairborne collision avoidance system, acting asa last resort, was being considered.

A series of midair collisions that occurred in

the United States, has been the impetus forthe development and refinement of anairborne collision avoidance system. Thesetragic milestones included the followingcollisions:

In 1956, the collision between twoairliners over the Grand Canyonspurred both the airlines and theaviation authorities to initiate systemdevelopment studies for an effectivesystem.

In 1978, the collision between a lightaircraft and an airliner over San Diegoled the FAA to initiate thedevelopment of TCAS.

Finally, in 1986, the collision betweena DC-9 and a private aircraft overCerritos, California, resulted in aCongressional mandate that requiredsome categories of American andforeign aircraft to be equipped withTCAS for flight operations in U.S.airspace.

In parallel to the development of TCASequipment in the United States, ICAO hasbeen working since the early 1980s todevelop standards for ACAS . ICAOofficially recognized ACAS on 11November 1993 . Its descriptive definitionappears in Annex 2 of the Convention on

International Civil Aviation and its use isregulated in Procedures for Air NavigationServices ----- Aircraft Operations (PANS-OPS)and Procedures for Air Navigation Services----- Rules of the Air and Air Traffic Services(PANS-RAC). In November 1995, the

SARPs and Guidance Material for were approved, and they appear in Annex 10of the Convention on International CivilAviation.

During the late 1950s and early 1960s,collision avoidance development effortsincluded an emphasis on passive andnoncooperating systems. These conceptsproved to be impractical. One majoroperational problem that could not beovercome with these designs was the need for

nonconflicting, complementary avoidancemaneuvers that require a high-integritycommunications link between aircraftinvolved in the conflict.

One of the most important developments inthe collision avoidance concept was thederivation of the range/range rate, or tau,concept by Dr. John S. Morrell of Bendix.This concept is based on time, rather thandistance, to the closest point of approach inan encounter.

During the late 1960s and early 1970s,several manufacturers developed aircraftcollision avoidance systems based oninterrogator/transponder and time/frequencytechniques. Although these systemsfunctioned properly during staged aircraftencounter testing, the FAA and the airlines

jointly concluded that in normal airlineoperations, they would generate a high rate of unnecessary alarms in dense terminal areas.This problem would have undermined thecredibility of the system with the flight

crews. In addition, each target aircraft wouldhave to be equipped with the same equipmentto provide protection to an equipped aircraft.

In the mid 1970s, the Beacon CollisionAvoidance System (BCAS) was developed.BCAS used reply data from the Air Traffic


7/45

Control Radar Beacon System (ATCRBS)transponders to determine an intruders rangeand altitude. At that time, ATCRBStransponders were installed in all airline andmilitary aircraft and a large number of general aviation aircraft. Thus, any BCAS-

equipped aircraft would be able to detect andbe protected against the majority of otheraircraft in the air without imposing additionalequipment requirements on those otheraircraft. In addition, the discrete addresscommunications techniques used in theMode S transponders then under developmentpermitted two conflicting BCAS aircraft toperform coordinated escape maneuvers with ahigh degree of reliability.

TCAS II development TCAS II development TCAS II development TCAS II development

In 1981, the FAA made a decision to developand implement TCAS utilizing the basicBCAS design for interrogation and tracking,but providing additional capabilities.

TCAS is designed to work autonomously of the aircraft navigation equipment andindependently of the ground systems usedto provide ATC services. TCAS interrogatesICAO-compliant transponders of all aircraftin the vicinity and based on the replies

received, tracks the slant range, altitude(when it is included in the reply message),and bearing of surrounding traffic. Fromseveral successive replies, TCAS calculates atime to reach the CPA (Closest Point of Approach) with the intruder, by dividing therange by the closure rate. This time value isthe main parameter for issuing alerts. If thetransponder replies from nearby aircraftincludes their altitude, TCAS also computesthe time to reach co-altitude. TCAS can issuetwo types of alerts:

TAs to assist the pilot in the visualsearch for the intruder aircraft and toprepare the pilot for a potential RA;and

RAs to recommend maneuvers that willeither increase or maintain the existingvertical separation from an intruder

aircraft. When the intruder aircraft isalso fitted with TCAS II, both TCAScoordinate their RAs through theMode S data link to ensure thatcomplementary resolution senses areselected.

TCAS II is designed to operate in trafficdensities of up to 0.3 aircraft per squarenautical mile (nmi), i.e., 24 aircraft within a 5nmi radius, which is the highest trafficdensity envisioned over the next 20 years.

Development of the TCAS II collisionavoidance algorithms included thecompletion of millions of computersimulations to optimize the protectionprovided by the system, while minimizing thefrequency of unacceptable or nuisanceadvisories. In addition to these computersimulations, early versions of the collisionavoidance algorithms were evaluated via pilotin the loop simulations and during theoperation of prototype equipment in FAAaircraft throughout the NAS.

Extensive safety studies were also performedto estimate the safety improvements thatcould be expected with the introduction of TCAS into service. These safety studies havebeen continuously updated throughout therefinement of the collision avoidancealgorithms. The safety studies have shownthat TCAS II will resolve nearly all of thecritical near midair collisions involvingairline aircraft. However, TCAS cannothandle all situations. In particular, it isdependent on the accuracy of the threataircrafts reported altitude and on theexpectation that the threat aircraft will notmake an abrupt maneuver that defeats theTCAS RA. The safety study also shows thatTCAS II will induce some critical near midair

collisions, but overall, the number of nearmidair collisions with TCAS is less than 10%of the number that would have occurredwithout the presence of TCAS.

Extensive studies were also carried out toevaluate the interaction between TCAS and


8/45

ATC. The analysis of ATC radar datashowed that in 90% of the cases, the verticaldisplacement required to resolve an RA wasless than 300 feet. Based on these studies, itwas concluded that the possibility of theresponse to a TCAS RA causing an aircraft to

infringe on the protected airspace for anotheraircraft was remote. However, operationalexperience has shown that the actualdisplacement resulting from an RA responseis often much greater than 300 feet, andTCAS has had an adverse affect on thecontrollers and the ATC system. Because of this operational experience, Version 7contains numerous changes and enhancementsto the collision avoidance algorithms, theaural annunciations, the RA displays, andpilot training programs to minimize thedisplacement while responding to an RA.

In In In In- -- -Servi Servi Servi Service Operational ce Operational ce Operational ce Operational Evalu Evalu Evalu Evalua aa ations tions tions tions

To ensure that TCAS performed as expectedin its intended operational environment,several operational evaluations of the systemhave been conducted. These evaluationsprovided a means for the pilots using TCASand the controllers responsible for providingseparation services to TCAS-equipped

aircraft to have a direct influence on the finalsystem design and performance requirements.

The initial operational evaluation of TCASwas conducted by Piedmont Airlines in 1982.Using a TCAS II prototype unit manufacturedby Dalmo Victor, Piedmont flewapproximately 900 hours in scheduled,revenue service while recording data on theperformance of TCAS. These recorded datawere analyzed to assess the frequency andsuitability of the TAs and RAs. During thisevaluation, the TCAS displays were notvisible to the pilots, and observers from theaviation industry flew with the aircraft tomonitor the system performance and toprovide technical and operational commentson its design.

In 1987, Piedmont flew an upgraded versionof the Dalmo Victor equipment forapproximately 1200 hours. During thisevaluation, the TCAS displays were visible tothe pilots and the pilots were permitted to usethe information provided to maneuver the

aircraft in response to RAs. This installationincluded a dedicated TCAS data recorder sothat quantitative data could be obtained on theperformance of TCAS. In addition, pilot andobservers completed questionnaires followingeach TA and RA so that assessments could bemade regarding the value of the system to theflight crews.

This evaluation also provided the basis for thedevelopment of avionics certification criteriafor production equipment, validated pilottraining guidelines, provided the justificationfor improvements to the TCAS algorithmsand displays, and validated the pilotprocedures for using the equipment.

Following the successful completion of thesecond Piedmont evaluation, the FAAinitiated the Limited Installation Program(LIP). Under the LIP, Bendix-King andHoneywell built and tested commercialquality, pre-production TCAS II equipmentthat was in compliance with the TCAS IIMinimum Operational PerformanceStandards (MOPS). Engineering flight testsof this equipment were conducted on themanufacturers' aircraft, as well as FAAaircraft. Using data collected during theseflight tests, together with data collectedduring factory and ground testing, bothmanufacturers equipment was certified via aSupplemental Type Certificate (STC) for usein commercial, revenue service.

The Bendix-King units were operated byUnited Airlines on a B737-200 and a DC8-73

aircraft. Northwest Airlines operated theHoneywell equipment on two MD-80 aircraft.Over 2000 hours of operating experiencewere obtained with the United aircraft andapproximately 2500 hours of operatingexperience were obtained with the Northwestinstallations.


9/45

The experience provided by these operationalevaluations resulted in further enhancementsto the TCAS II logic, improved testprocedures, and finalized the procedures forcertification of production equipment. The

most important information obtained from theoperational evaluations was the nearlyunanimous conclusion that TCAS II was safe,operationally effective, and ready for morewidespread implementation.

With the successful completion of these earlyoperational evaluations, there was a highdegree of confidence that a system withsufficient maturity was available to meet theCongressionally mandated implementation of TCAS II in U.S. airspace.

As part of this mandated implementation , thelargest operational evaluation of TCAS,known as the TTP, was initiated. The TTPbegan in late 1991 and has continued throughthe initial implementation, the mandatedupgrade to Version 6.04A Enhanced, and isstill active as Version 7 enters operation. Inconjunction with the TTP in the U.S.,EUROCONTROL has conducted extensiveevaluations of TCAS operations in Europe,and the Japan Civil Aviation Bureau (JCAB)has conducted similar assessments of TCAS II performance in Japanese andsurrounding airspace. Other countries alsoconducted operational evaluations as the useof TCAS increased during the past 10 years.

The system improvements suggested as aresult of these TCAS II evaluations led to thedevelopment and release of Version 6.04AEnhanced in 1993. The principal aim of thismodification was the reduction of nuisancealerts, which were occurring at low altitudesand during level-off encounters, and the

correction of a problem in the altitudecrossing logic.

After the implementation of Version 6.04AEnhanced, operational evaluations continuedwith the same objective, and proposedperformance improvements led to the

development of Version 7. The MOPS forVersion 7 was approved in December 1997and Version 7 units became available forinstallation in late 1999. Version 7 isexpected to further improve TCAScompatibility with the air traffic control

system throughout the world.

Toward a Requirement for Toward a Requirement for Toward a Requirement for Toward a Requirement for Worldwide Carriage Worldwide Carriage Worldwide Carriage Worldwide Carriage

The United States was the first member of ICAO to mandate carriage of an airbornecollision avoidance system for passengercarrying aircraft operating in its airspace.

Because of this mandate, the number of long-range aircraft fitted with TCAS II andoperating in European and Asian airspacecontinued to increase, although the systemcarriage and operation were not mandatory inthis airspace. As studies, operationalexperience, and evaluations continued todemonstrate the safety benefits of TCAS II,some non-U.S. airlines also equipped theirshort-haul fleets with TCAS.

In 1995, the EUROCONTROL Committee of Management approved an implementationpolicy and schedule for the mandatory

carriage of TCAS II in Europe. The EuropeanAir Traffic Control Harmonization andIntegration Program (EATCHIP) ProjectBoard then ratified this policy. The approvedpolicy requires the following:

rom 1 January 2000, all civil fixed-wing, turbine-powered aircraft having amaximum take-off mass exceeding15,000 kg, or a maximum approvedpassenger seating configuration of more than 30, will be required to beequipped with TCAS II, Version 7; and

From 1 January 2005, all civil fixed-wing, turbine-powered aircraft having amaximum take-off mass exceeding5,700 kg, or a maximum approvedpassenger seating configuration of more that 19, will be required to beequipped with TCAS II, Version 7.


10/45

Because of delays in obtaining Version 7equipment, a number of exemptions to the1 January 2000 date were granted byEUROCONTROL. Each of the exemptionsgranted have a unique end date for the

exemption, but all exemptions will expire on31 March 2001.

Other countries, including Argentina,Australia, Chile, Egypt, India, and Japan,have also mandated carriage of TCAS IIavionics on aircraft operating in theirrespective airspace.

The demonstrated safety benefits of theequipment, and the 1996 midair collisionbetween a Saudia Boeing 747 and aKazakhstan Ilyushin 76, resulted in an ICAOproposal for worldwide mandatory carriage of ACAS II on all aircraft, including cargoaircraft, beginning in 2003. To guarantee theeffectiveness of this mandate, ICAO has alsomandated the carriage and use of pressurealtitude reporting transponders, which are aprerequisite for generating RAs.

After the mid-air collision between a GermanAir Force Tupolev 154 and a U.S. Air ForceC-141 transport aircraft, off Namibia inSeptember 1997, urgent consideration wasgiven to the need to equip military transportaircraft with TCAS. Although only a limitednumber of countries have included militaryand other government-owned aircraft in theirmandates for TCAS carriage, severalcountries, including the United States, haveinitiated programs to equip tanker, transport,and cargo aircraft within their military fleetswith TCAS II Version 7.

Standards and Guidance Standards and Guidance Standards and Guidance Standards and Guidance Material Material Material Material

The data obtained from the FAA and industrysponsored studies, simulations, flight tests,and operational evaluations have enabledRTCA to publish the MOPS for TCAS II.The current version of the MOPS, DO-185A,

describes the standards, requirements, andtest procedures for TCAS Version 7.

RTCA has also published MOPS for TCAS I,DO-197A, which defines the requirementsand test procedures for TCAS I equipment

intended for use on airline aircraft operated inrevenue service.

The FAA has issued Technical StandardOrder (TSO) C118a that defines therequirements for the approval of TCAS Iequipment. A draft Advisory Circularoutlining the certification requirements andthe requirements for obtaining operationalapproval of the system has been prepared andis being used by the FAAs AircraftCertification Offices (ACO) as the basis forapproving TCAS I installations andoperation.

For TCAS II, TSO C119b and AdvisoryCircular 20-131a have been published for useby FAA airworthiness authorities incertifying the installation of TCAS II onvarious classes of aircraft. Advisory Circular120-55a defines the procedures for obtainingoperational approval for the use of TCAS II.While the FAA developed these documents,they have been used throughout the world bycivil aviation authorities to approve theinstallation and use of TCAS.

ICAO SARPs and Guidance Material forACAS I and ACAS II have been published inAnnex 10. The procedures for use of ACAShave been published in PANS-RAC andPANS-OPS. These documents provideinternational standardization for collisionavoidance systems.

For the avionics, the Airlines ElectronicEngineering Committee (AEEC) has

completed work on ARINC Characteristic735 to define the form, fit, and function of TCAS II units. Similar work on the Mode Stransponder has been competed, and theresults of that work are contained in ARINCCharacteristic 718.


11/45

System components System components System components System components

Figure 1 is a block diagram of TCAS II. ATCAS II installation consists of the followingmajor components.

performs airspace

determination and selection, and generationof advisories. The TCAS Processor usespressure altitude, radar altitude, and discreteaircraft status inputs from its own aircraft tocontrol the collision avoidance logicparameters that determine the protectionvolume around the TCAS aircraft. If atracked aircraft is a collision threat, theprocessor selects an avoidance maneuver thatwill provide adequate vertical miss distancefrom the intruder while minimizing theperturbations to the existing flight path. If thethreat aircraft is also equipped with TCAS II,the avoidance maneuver will be coordinated

with the threat aircraft.

Figure 1. TCAS II Block DiagramFigure 1. TCAS II Block DiagramFigure 1. TCAS II Block DiagramFigure 1. TCAS II Block Diagram

Mode S Transponder Mode S Transponder Mode S Transponder Mode S Transponder

A single control panel is provided to allow

the flight crew to select and control all TCASequipment, including the TCAS Processor,the Mode S transponder, and in some cases,the TCAS displays. A typical control panelprovides four basic control positions:

StandStandStandStand----bybybyby: Power is applied to theTCAS Processor and the Mode Stransponder, but TCAS does not issueany interrogations and the transponderwill reply to only discreteinterrogations.

TransponderTransponderTransponderTransponder: The Mode S transponderis fully operational and will reply to allappropriate ground and TCASinterrogations. TCAS remains inStandby.

TA OnlyTA OnlyTA OnlyTA Only: The Mode S transponder isfully operational. TCAS will operatenormally and issue the appropriate


12/45

interrogations and perform all trackingfunctions. However, TCAS will onlyissue TAs, and the RAs will beinhibited.

AutomaticAutomaticAutomaticAutomatic or TA/RATA/RATA/RATA/RA: The Mode Stransponder is fully operational. TCASwill operate normally and issue theappropriate interrogations and performall tracking functions. TCAS will issueTAs and RAs, when appropriate.

As indicated in Figure 1, all TCAS controlsignals are routed through the Mode Stransponder.

The antennas used by TCAS II include adirectional antenna that is mounted on the topof the aircraft and either an omnidirectionalor a directional antenna mounted on thebottom of the aircraft. Most installations usethe optional directional antenna on the bottomof the aircraft.

These antennas transmit interrogations on1030 MHz at varying power levels in each of four 90 azimuth segments. The bottom-mounted antenna transmits fewerinterrogations and at a lower power than thetop-mounted antenna. These antennas alsoreceive transponder replies, at 1090 MHz,and send these replies to the TCAS Processor.The directional antennas permit thepartitioning of replies to reduce synchronousgarbling.

In addition to the two TCAS antennas, twoantennas are also required for the Mode Stransponder. One antenna is mounted on thetop of the aircraft while the other is mountedon the bottom. These antennas enable the

Mode S transponder to receive interrogationsat 1030 MHz and reply to the receivedinterrogations at 1090 MHz. The use of thetop- or bottom-mounted antenna isautomatically selected to optimize signalstrength and reduce multipath interference.

TCAS operation is automatically suppressedwhenever the Mode S transponder istransmitting to ensure that TCAS does nottrack its own aircraft.

The TCAS interface with the pilots isprovided by two displays ---- - the trafficdisplay and the RA display. These twodisplays can be implemented in a number of ways, including displays that incorporate bothdisplays into a single, physical unit.Regardless of the implementation, theinformation displayed is identical. Thestandards for both the traffic display and theRA display are defined in DO-185A.

Traffic Display Traffic Display Traffic Display Traffic Display

The traffic display, which can beimplemented on either a part-time or full-timebasis, depicts the position of nearby traffic,


13/45

relative to its own aircraft. It is designed toprovide information that will assist the pilotin visual acquisition of other aircraft. If implemented on a part-time basis, the displaywill automatically activate whenever a TA oran RA is issued. Current implementations

include dedicated traffic displays; display of the traffic information on shared weatherradar displays, MAP displays, EngineIndication and Crew Alerting System(EICAS) displays; and other multifunctiondisplays.

A majority of the traffic displays also providethe pilot with the capability to select multipleranges and to select the altitude band for thetraffic to be displayed. These capabilitiesallow the pilot to display traffic at longerranges and with greater altitude separationwhile in cruise flight, while retaining thecapability to select lower display ranges interminal areas to reduce the amount of display clutter.

Traffic Display Symbology

Both color and shape are used to assist thepilot in interpreting the displayedinformation.

The own aircraft is depicted as either a whiteor cyan arrowhead or airplane-like symbol.The location of the own aircraft symbol onthe display is dependent on the displayimplementation. Other aircraft are depictedusing geometric symbols, depending on theirthreat status, as follows:

n unfilled diamond ( ), shown ineither cyan or white, but not the samecolor as the own aircraft symbol, isused to depict non-threat traffic.

filled diamond ( ), shown in either

cyan or white, but not the same color asthe own aircraft symbol, is used todepict Proximate Traffic. ProximateTraffic is non-threat traffic that iswithin 6 nmi and 1200 ft from ownaircraft.

filled amber or yellow circle ( ) isused to display intruders that havecaused a TA to be issued.

A filled red square ( &) is used todisplay intruders that have caused an

RA to be issued.

Each symbol is displayed on the screenaccording to its relative position to ownaircraft. To aid the pilot in determining therange to a displayed aircraft, the trafficdisplay provides range markings at one-half the selected scale and at the full scale.Additional range markings may be providedat closer ranges, e.g., 2 nmi, on some displayimplementations. The selected display rangeis also shown on the display. The rangemarkings and range annunciation aredisplayed in the same color as the ownaircraft symbol unless the traffic display isintegrated with an existing display thatalready provides range markings, e.g., a MAPdisplay.

Vertical speed information and altitudeinformation are also provided for alldisplayed traffic that are reporting altitude.Relative altitude is displayed in hundreds of feet above the symbol if the intruder is aboveown aircraft and below the symbol if the

intruder is below own aircraft. When theintruder is above the own aircraft, the relativealtitude information is preceded by a ++++ sign.When the intruder is below the own aircraft, a------------ sign precedes the relative altitudeinformation. In some aircraft, the flight levelof the intruder can be displayed instead of itsrelative altitude. The flight level is shownabove the traffic symbol if the intruder isabove the own aircraft and below the trafficsymbol is the intruder is below the ownaircraft. If the intruder is not reporting its

altitude, no altitude information in shown forthe traffic symbol. The altitude information isdisplayed in the same color as the aircraftsymbol.

An arrow is displayed immediately to theright of a traffic symbol when the target


14/45

aircraft is reporting its altitude and isclimbing or descending at more than 600fpm. An up arrow is used for a climbingaircraft; a down arrow is used for adescending aircraft. The arrow is displayed inthe same color as the aircraft symbol.

When an aircraft causing a TA or RA isbeyond the currently selected range of thetraffic display, half TA or RA symbols willbe displayed at the edge of the display at theproper relative bearing. In someimplementations, a written message such asTRAFFIC, TFC, or TCAS is displayed on thetraffic display if the intruder is beyond theselected display range. The half symbol or thewritten message will remain displayed untilthe traffic moves within the selected displayrange; the pilot increases the range on avariable range display to allow the intruder tobe displayed; or the pilot selects a displaymode that allows traffic to be displayed.

In some instances, TCAS may not have areliable bearing for an intruder causing a TAor RA. Because bearing information is usedfor display purposes only, the lack of bearinginformation does not affect the ability of TCAS to issue TAs and RAs. When a No-Bearing TA or RA is issued, the threat level,as well as the range, relative altitude, andvertical rate of the intruder, are written on thetraffic display. This text is shown in red foran RA and in amber or yellow for a TA. Forexample, if an RA was issued against anintruder at a range of 4.5 nmi and with arelative altitude of +1200 feet anddescending, the No Bearing indication onthe traffic display would be:

RA 4.5 +12RA 4.5 +12RA 4.5 +12RA 4.5 +12

Figure 2 shows the use of the various traffic

symbology used on the traffic display.

Resolution Adviso Resolution Adviso Resolution Adviso Resolution Advisory Display ry Display ry Display ry Display

The RA display provides the pilot withinformation on the vertical speed or pitchangle to fly or avoid to resolve an encounter.The RA display is typically implemented onan instantaneous vertical speed indicator(IVSI); a vertical speed tape that is part of aPrimary Flight Display (PFD); or using pitchcues displayed on the PFD. RA guidance hasalso been implemented on a Heads-UpDisplay (HUD). The implementations usingthe IVSI or a vertical speed tape use red andgreen lights or markings to indicate thevertical speeds to be avoided (red) and thedesired vertical speed to be flown (green). Animplementation using pitch cues uses aunique shape on the PFD to show the pitch

angle to be flown or avoided to resolve anencounter. HUD implementations also use aunique shape to indicate the flight path to beflown or avoided to resolve an encounter.

In general, the round-dial IVSIimplementation is used on the older nonglassaircraft. However, some operators haveimplemented this display in their glassaircraft to provide a common display acrosstheir fleet types. Some IVSI implementationsuse mechanical instruments with a series of red and green LEDs around the perimeter of the display, while other implementations usean LCD display that draws the red and greenarcs at the appropriate locations. The LCDdisplay implementations also have thecapability to provide both the traffic and RAdisplay on a single instrument.

On glass aircraft equipped with a PFD, someairframe manufacturers have implemented theRA display on the vertical speed tape; somehave elected to provide pitch cues; and otherimplementations provide both pitch cues and

a vertical speed tape.

The standards for the implementation of RAdisplays are provided in DO-185A. Inaddition to the implementations outlinedabove, DO-185A defines requirements for


15/45

implementation of the RA display via theflight director and a HUD.

Two RA displays are required ----- one in theprimary field of view of each pilot.

Figure 3 shows an RA display implementedon an LCD display that also provides trafficinformation. Figure 4 shows the two possibleimplementations on the PFD.


16/45


17/45

Mode S Surveillance Mode S Surveillance Mode S Surveillance Mode S Surveillance

Mode C Surveillance Mode C Surveillance Mode C Surveillance Mode C Surveillance


18/45


19/45


20/45

Interference Limiting Interference Limiting Interference Limiting Interference Limiting

Electromagnetic Compatibility Electromagnetic Compatibility Electromagnetic Compatibility Electromagnetic Compatibility


21/45

Sensitivity Level Sensitivity Level Sensitivity Level Sensitivity Level


22/45

Tau Tau Tau Tau


23/45

Protected Volume Protected Volume Protected Volume Protected Volume


24/45


25/45


26/45

Tracking Tracking Tracking Tracking


27/45

Traffic Advisory Traffic Advisory Traffic Advisory Traffic Advisory

Threat Detection Threat Detection Threat Detection Threat Detection


28/45

Resolution Advisory Selection Resolution Advisory Selection Resolution Advisory Selection Resolution Advisory Selection

B

A

TCAS

Threat

CPA

downward

upward

ALIMThreat

TCASCPA

ALIM

RA Climbissued


29/45


30/45

TCAS/TCAS Coordination TCAS/TCAS Coordination TCAS/TCAS Coordination TCAS/TCAS Coordination

In a TCAS/TCAS encounter, each aircrafttransmits interrogations to the other via theMode S link to ensure the selection of complementary RAs by the two aircraft. Thecoordination interrogations use the same1030/1090 MHz channels used forsurveillance interrogations and replies andare transmitted once per second by eachaircraft for the duration of the RA.Coordination interrogations containinformation about an aircrafts intended RAsense to resolve the encounter with the otherTCAS-equipped intruder. The informationin the coordination interrogation isexpressed in the form of a complement. Forexample, when an aircraft selects an upward

sense RA, it will transmit a coordinationinterrogation to the other aircraft that restrictsthat aircrafts RA selection to those in thedownward sense. The strength of thedownward sense RA would be determined bythe threat aircraft based on the encountergeometry and the RA Selection logic.


31/45

Advisory Annunciation Advisory Annunciation Advisory Annunciation Advisory Annunciation


32/45

Air/Ground Communications Air/Ground Communications Air/Ground Communications Air/Ground Communications

Traffic Advisory Display Traffic Advisory Display Traffic Advisory Display Traffic Advisory Display

Resolution Advisory Displays Resolution Advisory Displays Resolution Advisory Displays Resolution Advisory Displays

Aural Annunciations Aural Annunciations Aural Annunciations Aural Annunciations

Performance Monitoring Performance Monitoring Performance Monitoring Performance Monitoring


33/45

TCAS AdvisoryTCAS AdvisoryTCAS AdvisoryTCAS Advisory Version 7 Aural AnnunciationVersion 7 Aural AnnunciationVersion 7 Aural AnnunciationVersion 7 Aural Annunciation Existing AuralExisting AuralExisting AuralExisting Aural

AnnunciationAnnunciationAnnunciationAnnunciationTraffic Advisory Traffic, Traffic Traffic, TrafficClimb RA Climb, Climb Climb, Climb, ClimbDescend RA Descend, Descend Descend, Descend,

DescendAltitude Crossing Climb RA Climb, Crossing Climb; Climb,

Crossing ClimbClimb, Crossing Climb;Climb, Crossing Climb

Altitude Crossing Descend RA Descend, Crossing Descend; Descend,Crossing Descend

Descend, CrossingDescend; Descend,Crossing Descend

Reduce Climb RA Adjust Vertical Speed, Adjust Reduce Climb, ReduceClimb

Reduce Descent RA Adjust Vertical Speed, Adjust Reduce Descent, ReduceDescent

RA Reversal to a Climb RA Climb, Climb, NOW; Climb, ClimbNOW

Climb, Climb, NOW;Climb, Climb NOW

RA Reversal to a Descend RA Descend, Descend NOW; Descend,Descend NOW

Descend, Descend NOW;Descend, Descend NOW

Increase Climb RA Increase Climb, Increase Climb Increase Climb, IncreaseClimb

Increase Descent RA Increase Descent, Increase Descent Increase Descent, IncreaseDescent

Maintain Rate RA Maintain Vertical Speed, Maintain Monitor Vertical SpeedAltitude Crossing, MaintainRate RA (Climb and Descend)

Maintain Vertical Speed, CrossingMaintain

Monitor Vertical Speed

Weakening of Initial RA Adjust Vertical Speed, Adjust Monitor Vertical SpeedPreventive RA (No change invertical speed required)

Monitor Vertical Speed Monitor Vertical Speed,Monitor Vertical Speed

RA Removed Clear of Conflict Clear of Conflict


34/45

Regulations and Operational Regulations and Operational Regulations and Operational Regulations and Operational Gui Gui Gui Guid dd dance ance ance ance

1. The responding aircraft has returned

to its assigned altitude.

2. The flightcrew informs you that theTCAS maneuver is completed andyou observe that standard separationhas been reestablished.

3. The responding aircraft hasexecuted an alternate clearance andyou observe that standard separationhas been reestablished.


35/45

FAA Order 7110.65 also references AC120-55 to provide information on thesuggested phraseology to be used by pilotsto notify the controller about a TCAS event.The suggested phraseology is discussed inthe following section, Pilot Responsibilities.


36/45

The pilot is to inform the controller aboutthe RA deviation as soon as possible. Thephraseology, to be used by pilots, is shownin Table 6. The phraseology was developedby ICAO and has been published in PANS-RAC. The FAA has incorporated theserecommendations into AC 20-155.

TaTaTaTable 6. Recommended Phraseology forble 6. Recommended Phraseology forble 6. Recommended Phraseology forble 6. Recommended Phraseology forReporting RAsReporting RAsReporting RAsReporting RAs

TCAS Climb orTCAS DescendTCAS Climb (ordescent), returningto [assignedclearance]TCAS Climb (or

descent) completed,[assigned clearance]resumedUnable to comply,TCAS resolutionadvisory

No specificphraseology isdefined


37/45

Operational Experience Operational Experience Operational Experience Operational Experience

The evaluation of TCAS II performanceduring its implementation has demonstratedthat this equipment provides an overallimprovement in flight safety. In reportedlydangerous situations, TAs have made visualacquisition of intruders possible in sufficienttime to avoid any risk of collision. In someevents, RAs have been issued that arebelieved to have prevented critical nearmidair collisions and midair collisions fromtaking place.

However, the operational experience hasindicated that some issues related to TCAScontinue to occur. These issues include thefollowing.

Pilots sometimes deviate significantlyfurther from their original clearance thanwas required or desired while complyingwith an RA. Data and simulator trials haveshown that pilots often are not aware of theRA being weakened and many pilots do not

want to begin maneuvering back towardtheir original clearance until the RA is over.To reduce the frequency of the large altitudedisplacements while responding to an RA,Version 7 introduces new auralannunciations to accompany the weakeningRAs and provides a target vertical speed onthe RA display for the weakened RA. Inaddition, the CAS logic has been modifiedto provide only one type of weakened RAand that RA is either a Do Not Climb or DoNot Descend RA. This results in theweakened RA always calling for the aircraftto be leveled after ALIM feet of separationhave been obtained.

Pilots are often slow in reporting the initialdeviation to the controller and this resultedin situations where the controller wasissuing clearances that were in the oppositesense than that directed by the RA. Thestandard ICAO phraseology is sometimesnot used and at times, the controller does notunderstand the initial RA notification fromthe pilot. In some events, this resulted indistracting dialogue between the pilot andcontroller regarding the RA.

Some pilots request information, or refuse aclearance, based upon information shown onthe traffic display. These practices are notencouraged because they can cause addedcongestion on the radio channel and mayresult in higher controller and pilotworkloads. This improper use of the trafficdisplay has been addressed via pilot trainingprograms.

Aircraft have also been observed makinghorizontal maneuvers based solely on theinformation shown on the traffic display,without visual acquisition by the aircrew.Such maneuvers may cause a significantdegradation in the level of flight safety andare contrary to a limitation contained in theTCAS Airplane Flight Manual Supplement.


38/45

Event reports also indicate that some pilotshave not reacted to RAs, when they havetraffic information from the controller, buthave not visually acquired the intruder. Thisis a potentially hazardous situation if the

ground radar is not tracking the intrudercausing the RA. In addition, if the intruder isalso TCAS-equipped, the RAs will becoordinated, and a nonresponse by oneaircraft will result in the other aircrafthaving to maneuver further to resolve theRA.

An RA is generally unexpected by acontroller and in a majority of the cases is adisruption to his or her workload. Thisdisruption is due to an aircrafts unexpected

deviation from the ATC clearance, thesubsequent discussion regarding the RA onthe active frequency, and the possibility of an induced conflict with a third aircraft.Although the latter concern isunderstandable, many controllers do notunderstand the multiaircraft logic that isprovided by TCAS so that the initial RA canbe modified if the response does result in aconflict with a third aircraft.

Operational experience has shown that theunexpected interactions between TCAS andthe ATC systems can occur under thefollowing conditions.

Aircraft leveling off at 1,000 ft above orbelow conflicting traffic that is level mayresult in RAs being issued to the levelaircraft. These RAs are triggered becausethe climbing or descending aircraftmaintains high vertical speeds whenapproaching the cleared altitude or flightlevel. The CAS logic contains algorithmsthat will recognize this encounter geometry

and will delay the issuance of the RA to thelevel aircraft by up to five seconds to allowTCAS to detect the initiation of the

level-off maneuver by the intruder. Aprevious version of the logic included thesealgorithms at lower altitudes, and these havebeen effective in reducing the frequency of this type of RA. Version 7 expands the useof this logic to higher altitudes to address the

occurrence of these types of RAs in the enroute airspace structure.

Altitude crossing clearances issued by acontroller based on maintaining visualseparation may result in RAs being issued,particularly if one of the aircraft is level

Advisories issued against some categories of aircraft, e.g., aircraft operating under visualflight rules ( VFR), high performancemilitary aircraft during high g

maneuvers, and helicopters operating inthe immediate vicinity of the airport .Although minor modifications have beenmade to TCAS to address these types of RAs, these problems are related as much tothe airspace management, in general, as tothe function of TCAS II.

Training P Training P Training P Training Programs rograms rograms rograms

Many of the operational issues identified

during the initial operations of TCAS canbe traced to misunderstandings regardingthe operation of TCAS, its capabilities, andits limitations. For these reasons, it isessential that all pilots operating the systembe trained in how to use the system and thatall controllers receive training on howTCAS operates, how pilots are expected touse the systems, and the potentialinteractions between TCAS and the ATCsystem.

The FAA and the industry have workedtogether to develop and refine trainingguidelines for both pilots and controllers.AC 120-55 contains guidance for thedevelopment and implementation of pilottraining programs. While this AC is not


39/45

directly applicable to operators that aregoverned by Part 91 and Part 135 of theFederal Aviation Regulations, the trainingguidelines contained in the AC should befollowed by these operators.

The FAA has also developed and distributeda controller training program to all of itsATC facilities.

ICAO has developed guidelines for bothpilot and controller training programs, andthis information has been distributed to allICAO member countries.

Experience has shown that it is essential that

crews operating TCAS-equipped aircraftcomplete an approved pilot-training course.The proper use of TCAS II by pilots isrequired to ensure the proper integration of TCAS into the air traffic controlenvironment and the realization of theexpected improvements in flight safety. Pilottraining should include two complementaryparts as defined below.

Theory. Pilots should have anunderstanding of how TCAS works. Thisincludes an understanding of the alertthresholds, expected response to TAs and

RAs, proper use of TCAS-displayedinformation, phraseology, and systemlimitations. This training is generallyaccomplished in a classroom environment.

Simulator practice. The response to an RA

requires prompt and appropriate reactionsfrom the aircrews involved. Therefore, it isnecessary to include RA events in theroutine flight simulator training exercises, sothat pilots can experience the circumstancessurrounding an RA in a realisticenvironment. When the inclusion of TCASinto simulator training programs is notpossible, the FAA has approved the use of other interactive training devices tosupplement the classroom training.

While controllers do not use TCAS II, theyneed to be aware of its presence,capabilities, and limitations whileperforming their responsibilities. Thecontroller training should be similar to theclassroom training provided to pilots, butsupplemented with material thatdemonstrates advisories that have had bothpositive and negative impacts on the controland traffic situation.


40/45

TCAS is a last resort tool designed to prevent midair collisions between aircraft. Operationalexperience has demonstrated the utility and efficiency of TCAS. At the same time, operation of TCAS has identified areas in which the design and algorithms needed refinement or improvementto further enhance the efficiency of TCAS and its interaction with the controllers and the ATCsystem. As a result, the aviation industry has worked to develop, test, certify, and implementTCAS Version 7. Version 7 is now being introduced into service worldwide. The technicalfeatures of the system provide a significant improvement in flight safety, and this has nowattained universal recognition in the world of aviation. Many countries have mandated thecarriage of TCAS II, and ICAO has proposed a worldwide mandate of TCAS II Version 7 by2003.

However, one must be aware that TCAS is not a perfect system. TCAS cannot preclude allcollision risks and the system may, marginally, induce an additional risk. Consequently, it isessential that ATC procedures are designed to provide flight safety without any reliance upon the

use of TCAS and that both pilots and controllers are well versed in the operational capabilitiesand limitations of TCAS.

For more information on TCAS and the capabilities and requirements for Version 7, contact theAircraft Certification Office, AIR-130, 800 Independence Avenue, S.W., Washington, D.C.20591.


41/45

ACAS Airborne Collision Avoidance System

ACO Aircraft Certification Office ADC Air Data ComputerAEECAEECAEECAEEC Airline Electronic Engineering CommitteeAGL Above Ground LevelAIC Aeronautical Information CircularALIM Altitude LimitATCRBS Air Traffic Control Radar Beacon System

BCAS Beacon Collision Avoidance System

CAA Civil Aviation Authority CAS Collision Avoidance System

CPA Closest Point of Approach

DMOD Distance MODificationDME Distance Measuring EquipmentDMTL Dynamic Minimum Triggering Level

EATCHIP European Air Traffic Control Harmonization and Integration ProgramEFIS Electronic Flight Instrument SystemEICASEICASEICASEICAS Engine Indication and Crew Alerting System

FAA Federal Aviation AdministrationFL Flight Level

FMS Flight Management SystemFRUIT False Replies from Unsynchronized Interrogator Transmissionsft feetfpm feet per minute

GPWS Ground Proximity Warning System

HMD Horizontal Miss DistanceHUDHUDHUDHUD Heads Up Display

ICAO International Civil Aviation OrganizationIFR Instrument Flight RulesIVSI Instantaneous Vertical Speed Indicator

JCAB Japan Civil Aviation Bureau

KIAS Knots Indicated Airspeed

LCD Liquid Crystal Display


42/45


43/45


44/45


45/45

Documents

Introduction to TCAS II V7