EUROCONTROL...European ACAS Operational Monitoring 2000 Report EUROCONTROL Project SAF/ACAS - EEC...

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EUROPEAN ORGANISATIONFOR THE SAFETY OF AIR NAVIGATION

EUROCONTROL

EUROCONTROL EXPERIMENTAL CENTRE

EUROPEAN ACAS OPERATIONAL MONITORING2000 REPORT

EEC Report No. 373

Project SAF/ACAS

Issued: August 2002

REPORT DOCUMENTATION PAGE

Reference:EEC Report No. 373

Security Classification:Unclassified

Originator:EEC – ATM(Air Traffic Management)

Originator (Corporate Author) Name/Location:EUROCONTROL Experimental CentreCentre de Bois des BordesB.P.15F – 91222 Brétigny-sur-Orge CEDEXFRANCETelephone: +33 (0)1 69 88 75 00

Sponsor:EATCHIP Implementation DirectorateDEI.2

Sponsor (Contract Authority) Name/Location:EUROCONTROL AgencyRue de la Fusée, 96B –1130 BRUXELLESTelephone: +32 2 729 90 11

TITLE:

EUROPEAN ACAS OPERATIONAL MONITORING2000 REPORT

AuthorsD. Powell and Tim Baldwin

Date

8/2002

Pages

viii + 38

Figures

9

Tables

14

Annexes

1

References

-

Project

SAF/ACAS

Task No. Sponsor

-

Period

2000

Distribution Statement:(a) Controlled by: Head of ATM(b) Special Limitations: None(c) Copy to NTIS: NO

Descriptors (keywords):

ACAS, TCAS, Monitoring, Evaluation, Reporting

Abstract:

This report describes an operational monitoring of TCAS II conducted by EUROCONTROL under theauspices of ICAO during 2000.

The data collection procedures & monitoring methods are explained, and both statistical and operationalanalysis are provided.

The analysis shows the continuing need to monitor closely the operational effects of ACAS systems inEurope after the first phase of ACAS implementation.

European ACAS Operational Monitoring 2000 Report EUROCONTROL

Project SAF/ACAS - EEC Report No. 373 v

SUMMARY

The Airborne Collision Avoidance System (ACAS) is a generic term coined by ICAO for anairborne collision avoidance system that alerts and advises pilots about other aircraft (the intruders)entering a restricted volume of airspace around own aircraft.

Since 1991, EUROCONTROL has helped to organise the operational monitoring of ACAS in theEuropean airspace and for European carriers. This work was conducted under the auspices ofICAO; in parallel, the UK and France conducted their own monitoring for their respective airspacesand this forms an integral part of the European monitoring of ACAS.

The Traffic Alert and Collision Avoidance System, TCAS II is the only system commerciallyavailable that corresponds to the requirements for ACAS II. The current version of TCAS II isVersion 7. From late 2000 onwards the new version of TCAS II progressively replaced V6.04A in allaircraft. This new version complies fully with the ACAS II requirements and includes significantsafety, operational and technical improvements beyond the previous version.

From 1 January 2000, but subject to a transition period, ACAS II (i.e., TCAS II version 7.0) becamemandatory in Europe. In practice, however, the transition from TCAS version 6.04a to version7.0 often took longer than originally anticipated and exemptions were allowed throughout the year.

TCAS II is an independent airborne system based on SSR technologies. It interrogates and receivesvia the aircraft transponder Mode C and Mode S messages. The interrogation rate is about 1Hz andthe derived range and range rate and the received altitude from Mode C and or Mode-S replies areused to track other aircraft in the vicinity and the logic tests for a potential threat based on this data.

Once a potential threat is detected the logic presents it to the pilot as a Traffic Advisory (TA). If thethreat becomes imminent then TCAS II proposes an avoidance manoeuvre to the pilot in the verticalsense: this is a Resolution Advisory (RA).

The main source of information used in compiling this report was provided by pilot reports andcontroller reports. Different questionnaires were distributed to airline companies and ATC centresand the return of these reports are used to populate the main body of the database. Additionally, aprocedure was in place requesting the controllers to secure radar data recordings for all reportedevents these have been analysed and results added to the database.

Some of the key statistics are provided from the reported events in 2000 however it should be notedthat not all the reported events were RAs. Out of 3439 events, 2902 were reported as RA,s, 238 asTA's only and 299 were not known.

The number of events by flight phase detailed below is based mainly on pilot reports.

Take off 14 (0.7%), Climb 716 (34.8%), Cruise 716 (34.8%), Initial descent 672 (32.7%), Hold 24(1.2%), Approach 160 (7.8%), and Final 22 (1.1%).

When we look at the distribution of RA events by altitude for the European airspace peaks areevident in altitudes between FL80-130 and FL200-300, reflecting mainly the airspace organisation atthese levels. It is the high vertical rates in standard level-off procedures with clearances giving 1000ftseparation causing the majority of RA's (approximately 81%).

In the majority of reported cases (95% - same as the previous year) the pilot follows an RA. Pilot'sdecision not to follow an RA is almost always based on additional information either visual with theintruder, traffic information and or ATC avoidance being issued.

EUROCONTROL European ACAS Operational Monitoring 2000 Report

vi Project SAF/ACAS - EEC Report No. 373

When pilots did follow RA's about 54% deviated 500 ft or less and about 13% deviated more than1000 ft.

When controllers were asked only in 21% of cases (compared with 23% the previous year) did thecontroller express the opinion that the pilot's action in response to an RA was justified. Thecontroller's opinion about the disruptive effect of TCAS alarms reported when occupied with normal aworkload was 29%, (10% down on the previous year), and was most disruptive when occupied witha high workload 53%, (6% down on the previous year).

When Pilots gave their opinion of events 45% were a nuisance 40% useful and 16% necessary. Inonly 4% of cases was an airprox was filed.

The European Event Analysis Group categorised the events where operational issues were a factorthroughout the evaluation period. As a result the biggest issue (81%, a 22% increase compared withthe previous year) remains the nuisance level offs, which are highly correlated with high verticalrates and reversed vertical rates. Another significant issue is large vertical deviations in reaction tosome RAs which was 22%, almost double the proportion observed in the previous year.

Identified misuses of TCAS and RAs not being followed, suggests that training remains a highpriority for pilots.

The monitoring shows that TCAS improves the safety of the airspace when advisories are followedcorrectly.

The Annex A to the report gives some de-identified examples of ACAS events to assist training andawareness in the aviation community.


Project SAF/ACAS - EEC Report No. 373 vii

TABLE OF CONTENTS

LIST OF ANNEXES..................................................................................................................... VIII

LIST OF FIGURES...................................................................................................................... VIII

LIST OF TABLES........................................................................................................................ VIII

1. INTRODUCTION ..................................................................................................................... 1

1.1. OBJECTIVES................................................................................................................... 1

2. BACKGROUND ...................................................................................................................... 2

2.1. DEFINITION OF TERMS .................................................................................................... 22.2. TECHNICAL DESCRIPTION OF TCAS II ............................................................................. 32.3. THE ACAS STANDARDISATION PROCESS ........................................................................ 3

3. ACAS IN EUROPEAN AIRSPACE.......................................................................................... 4

4. DATA COLLECTION METHODS............................................................................................ 5

5. STATUS OF THE DATA COLLECTION.................................................................................. 6

6. STATISTICAL DESCRIPTION OF EVENTS ......................................................................... 10

6.1. PHASE AND ALTITUDE DISTRIBUTION OF EVENTS............................................................ 106.2. PILOT REACTIONS TO RAS............................................................................................ 116.3. CPA MISS DISTANCE DISTRIBUTION ............................................................................... 16

7. TCAS ASSESSMENT ........................................................................................................... 17

7.1. CONTROLLER ASSESSMENT .......................................................................................... 177.2. PILOT ASSESSMENT ..................................................................................................... 187.3. ASSESSMENT WITH RADAR DATA ................................................................................... 18

8. IDENTIFICATION AND ANALYSIS OF OPERATIONAL ISSUES ........................................ 19

9. CONCLUSIONS .................................................................................................................... 22

10. ACRONYMS & GLOSSARY.................................................................................................. 23

FRENCH TRANSLATION ............................................................................................................ 25

Green pages: French translation of the summary, introduction, objectives and conclusions.

Pages vertes: Traduction en langue française du résumé, de l'introduction, des objectifs et desconclusions.


viii Project SAF/ACAS - EEC Report No. 373

LIST OF ANNEXES

Annex A: Examples of TCAS issues ..........................................................................................31

LIST OF FIGURES

Figure 1: Monthly variation in number of RAs ..............................................................................7Figure 2: Hourly variation in number of RAs ................................................................................7Figure 3: Altitude distribution of RAs..........................................................................................11Figure 4: Diagram showing additional information available to the pilot when an RA

was not followed ........................................................................................................12Figure 5: Intruder information graphic presentation....................................................................13Figure 6: Magnitude of deviations as a function of altitude.........................................................14Figure 7: Encounter geometry, own aircraft against intruder ......................................................15Figure 8: Number of aircraft at a given miss distance at CPA ....................................................16Figure 9: Disruption caused by TCAS events as a function of workload.....................................17

LIST OF TABLES

Table 1: Reported events in the European Database...................................................................6Table 2: Advisory Type of reported events...................................................................................6Table 3: Number of reports per data source ................................................................................8Table 4: Proportion of events with data from each data source....................................................8Table 5: Source of Radar Recordings and ATC Reports..............................................................9Table 6: Controller Reported Events by Operational Phase .......................................................10Table 7: Pilot Reported Events by Flight Phase .........................................................................10Table 8: Pilots reporting following an RA ...................................................................................11Table 9: Deviation from Clearance as a Function of Altitude Band.............................................13Table 10: Controller Assessment of Pilot Action ........................................................................17Table 11: Pilot Event Appraisal ..................................................................................................18Table 12: Proportion of Events with an Airprox ..........................................................................18Table 13: Assessment with Radar Data.....................................................................................18Table 14: Appraisal of Events by Issue......................................................................................20


Project SAF/ACAS - EEC Report No. 373 1

1. INTRODUCTION

In order to implement ACAS in ECAC airspace in a safe and operationally acceptable way, there is aneed to monitor ACAS events occurring within that airspace and to provide feedback to operationaland technical staff working with ACAS.

This report describes and analyses the monitoring carried out during 2000. It analyses data to verifythat ACAS continues to provide a system that improves safety and is acceptable operationally, andtechnically. By doing this it provides useful operational and technical lessons about ACASperformance.

With reference to previous results, possible trends are also identified.

Chapter 2 of this report provides some background information - including technical principles ofTCAS and the International ACAS Standardisation and Implementation Process.

Chapter 3 gives a brief technical description of the TCAS II system.

Chapter 4 gives an overview of monitoring methods and data collection procedures.

Chapter 5 gives the status of the operational monitoring during 2000.

Chapter 6 provides a statistical description of the TCAS events.

Chapter 7 presents a statistical description of pilot and controller assessments to TCAS events.

Chapter 8 describes the main operational issues.

Chapter 9 draws some conclusions and makes recommendations.

Appendix A gives some de-identified examples of ACAS events to assist training and awareness inthe aviation community.

1.1. OBJECTIVES

The purpose of the CAS domain is to ensure the harmonised introduction and safe operation ofAirborne Collision Avoidance Systems (ACAS) in the ECAC area.

The purpose of the ACAS Monitoring Project is to provide information to the ACAS implementationprogramme that allows verification of the safe operation and harmonised introduction of ACAS.

A secondary purpose is to detect any anomalies in the safe operation of ACAS and to provide anyinformation that will help their correction.


2 Project SAF/ACAS - EEC Report No. 373

2. BACKGROUND

2.1. DEFINITION OF TERMS

The Airborne Collision Avoidance System (ACAS) is a generic term coined by ICAO for anairborne collision avoidance system that alerts and advises pilots about other aircraft (the intruders)entering a restricted volume of airspace around own aircraft.

Three levels of ACAS have been described by ICAO: ACAS I, ACAS II and ACAS III.

• ACAS I is an ACAS which provides information as an aid to "see and avoid" action butdoes not include the capability for generating Resolution Advisories (RAs).

• ACAS II is an ACAS which provides vertical resolution advisories (RAs) in addition to trafficadvisories (TAs).

• ACAS III is an ACAS which provides vertical and horizontal resolution advisories (RAs) inaddition to traffic advisories (TAs). Currently no ACAS III systems have been developed.

The Traffic Alert and Collision Avoidance System, TCAS II is the only system commerciallyavailable that corresponds to the requirements for ACAS II.

It is a system based on Secondary Surveillance Radar (SSR) technology that uses the altitude fromthe Mode C or Mode-S replies of the transponders of adjacent aircraft. Based on the horizontal andvertical closing rates TCAS calculates dynamic protective volumes around the own aircraft. If theclosing intruder becomes a threat, a TCAS II system proposes an RA to the pilot as a VerticalAvoidance Manoeuvre. The system co-ordinates its RA with the intruder aircraft, if it too cangenerate an RA, so that the manoeuvres are complementary.

The current version of TCAS II is Version 7.

From late 2000 onwards the new version of TCAS II progressively replaced V6.04A in all aircraft.This new version complies fully with the ACAS II requirements and includes significant safety,operational and technical improvements beyond the previous version.

TCAS II has been mandatory in the US airspace since 1986. ACAS II became mandatory in Europeon the 1st of January 2000 although there was a transition period until 31st March 2001. ACAS II willhave a world-wide ICAO mandate from 1 January 2003.

This paper reports the results of TCAS II operation in the ECAC airspace.



2.2. TECHNICAL DESCRIPTION OF TCAS II

TCAS II is an independent airborne system based on SSR technologies. It interrogates and receivesvia the aircraft transponder Mode C and Mode S messages. The interrogation rate is about 1 Hz andthe derived range and range rate and the received altitude from Mode C and or Mode-S replies areused to track other aircraft in the vicinity and the logic tests for a potential threat based on this data.

Before TCAS II V7 information on the bearing of the intruder was only used in the pilot display.

Once a potential threat is detected the logic presents it to the pilot as a Traffic Advisory (TA). If thethreat becomes imminent then TCAS II proposes an avoidance manœuvre to the pilot in the verticalsense: this is a Resolution Advisory (RA).

Tracked distance and closure rate - in the slant range and vertical sense - permits the logic toestimate the time TAU until collision. The critical TAU thresholds depend on altitude and vary for theTA between 20 s and 48 s and for the RA between 15 s and 35 s. The defined TAU thresholdsprovide variable protective volumes for each altitude band.

If both aircraft involved in an encounter are TCAS equipped, there is an exchange of messages inthe Mode S band to ensure that the RAs are complementary.

RAs issued to the pilot are either Preventive RAs, instructing the pilot to maintain his presentvertical rate or indicate which vertical rates he should avoid or Corrective RAs which propose to thepilot an avoidance manœuvre, based on a pilot model. This model assumes that the pilot reactswithin 5 s after receiving the RA and accelerates with 0,25 g into either a climb or descend attitudeuntil he reaches a vertical speed of (± 1500 ft/min or, if that is not sufficient, (± 2500 ft/min. Thisvertical speed should be maintained until the RA is altered or the Clear of Conflict is announced. Inthe majority of cases, the deviation caused by such a manœuvre is less than 500 ft. It should bepointed out that the logic aims for a Vertical Miss Distance at the Closest Point of Approach in theorder of 400 ft (extrapolated trajectories (± standard pilot reactions). The pilot reaction shouldtherefore be prompt and precise to avoid reducing this safety margin. However an overreaction ordisregarding a weakening RA does not provide additional safety but may lead to an excessivedeviation or at worst conflict with another aircraft.

2.3. THE ACAS STANDARDISATION PROCESS

On the 11 of November 1993, ICAO endorsed ACAS. PANS-OPS (Procedure for Air NavigationServices - aircraft OPerationS) and PANS-RAC (Procedure for Air Navigation Services - Rulesof the Air and air traffiC services) have been modified and contain the new procedures for pilots(PANS-OPS) (Doc. 8168) and phraseology to be used and procedures to be observed by controllers(PANS-RAC, Doc. 4444). These modifications have been approved by ICAO and have been appliedfrom November 1996 onwards.



3. ACAS IN EUROPEAN AIRSPACE

Since 1991, EUROCONTROL has helped to organise the operational monitoring of ACAS in theEuropean airspace and for European carriers. This work was conducted under the auspices ofICAO; in parallel, the UK and France conducted their own monitoring for their respective airspacesand this forms an integral part of the European monitoring of ACAS.

As a consequence, the 'ACAS Policy Task Force' (APTF) was founded and charged at the end of1994 with developing a common policy for the introduction of ACAS in Europe. This policy:

• endorsed the mandatory carriage and operation of an airborne collision avoidance systemconforming to ICAO SARPs in the airspace of ECAC Member States;

• adopted, in principle, an implementation schedule for mandatory carriage and operation ofACAS II such that:

− with effect from 1 January 2000, all civil fixed-wing turbine-engined aircraft having amaximum take-off mass exceeding 15,000 kg or maximum approved passengerseating configuration of more than 30 will be required to be equipped with ACAS IIand;

− with effect from 1 January 2005, all civil fixed-wing turbine-engined aircraft having amaximum take-off mass exceeding 5,700 kg or maximum approved passenger seatingconfiguration of more than 19 will be required to be equipped with ACAS II.

The policy proposal was accepted by the EUROCONTROL Committee of Management and furtherproposed for adoption by ECAC (European Civil Aviation Conference). At the same time ACAS wasestablished in the European Air Traffic Control Harmonisation and Integration Programme(EATCHIP) as an independent domain and the ACAS Implementation Group (AIG) was created tocontrol and co-ordinate the implementation activities.

Subsequently, ACAS has become a programme within the EUROCONTROL Air TrafficManagement Programme (EATMP). It is steered by the ACAS Programme Steering Group (APSG).



4. DATA COLLECTION METHODS

The main source of information was provided by pilot reports and controller reports. Differentquestionnaires were distributed to airline companies and ATC centres and the return of these reportsmakes up the main body of the database.

Additionally, a procedure was in place requesting the controllers to secure radar data recordings forall reported events. Additional sources of data are Mode S research stations in France, the UK andGermany. Unfortunately, these stations are not in operational use and work only intermittently andtherefore the database Mode-S information is limited in time and to the cover of the station.

Incoming pilot reports and controller reports are transcribed and entered in the appropriate tables inthe database. Possible correlation with existing events is checked. The event synopsis is enteredinto the database, and events are categorised with respect to event issues, alert type, and geometry.

Radar recordings in many different formats are collected and processed by the EUROCONTROLTCAS simulation tool that allows the simulated replay of the event for a detailed analysis. Illustratedencounter reports for the reconstructed events are produced and distributed to the ATC centre andairlines implicated in the event.

Several TCAS recorders are owned by EUROCONTROL. They are kept in case special technicalinvestigations are required. These recorders were not used during 2000.



5. STATUS OF THE DATA COLLECTION

The European Evaluation collected not only data of TCAS events occurring in the Europeanairspace but also data of TCAS events occurring elsewhere in the world.

From 1 January 2000, but subject to a transition period, ACAS II (i.e., TCAS II version 7.0) becamemandatory in Europe. In practice, however, the transition from TCAS version 6.04a to version 7.0often took longer than originally anticipated and exemptions were allowed throughout the year.Furthermore, the exact date of transition to version 7.0 in any given aircraft was not often transmittedto EUROCONTROL, making it difficult to distinguish which version of TCAS was in use. Only29 TCAS events were confirmed to be version 7 events in 2000, of which 26 occurred in Europe,although the real figure is likely to be higher. Given the small sample of confirmed version7.0 events, it is not possible to make a statistical comparison of version 7.0 with version 6.04a in thisreport.

Table 1 shows the number of reported TCAS events that took place between 1 January and31 December 2000. For the remainder of this report, only European data is used in the analyses.

Table 1: Reported events in the European database

Airspace Europe Other airspace USA Total

Number 3439 37 100 3576

Not all of the events were RAs. Table 2 shows that most reported events were RAs. The followinganalyses in this report are based on RA data only, unless stated otherwise.

Table 2: Advisory type of reported events

TA RA Unknown Total

238 2902 299 3439

The number of reported events increased 57% over the previous year, with a 67% increase in thenumber of reported RAs.

The monthly variation in RAs during this period is depicted in Figure 1. The hourly variation isdepicted in Figure 2.



189

223

239224

301 304

272 273

300

246

188

143

0

50

100

150

200

250

300

350

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Nim

ber

of

even

ts

Figure 1: Monthly variation in number of RAs

0 3 1 111

47

147

169177 175 174

183 185

162

191

172

190

132

116 119

70

36

82

0

50

100

150

200

250

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Nu

mb

er o

f ev

ents

Figure 2: Hourly variation in number of RAs

Both of these figures show that the number of reported events appears to follow the general trafficfluctuation.



Table 3 and Table 4 show the data sources for reported European RA events. The numbers includeabbreviated TCAS event reports from participating States, which were generally counted ascontroller reports.

The proportion of controller reported events exceeds that of pilot reported events, for the first time.

Noticeably more events were supported by controller reports; an increase of 10% compared with theprevious year, coupled with an 8% increase in the number of radar recorded events.

Table 3: Number of reports per data source

Number of:

Pilot reports 1955

Controller reports 2098

Ground radar recordings 1735

Mode S reports 0

Airborne recordings 0

Total 5788

Table 4: Proportion of events with data from each data source

Events with:

Pilot reports 1802 52%

Controller reports 2163 63%

Ground radar recordings 1188 35%

Mode S reports 0 0%

Airborne recordings 0 0%

Standard questionnaire forms were generally used by both pilots and controllers. Beyond the basicdescription of the events, these questionnaires frequently contain pertinent and lively remarks aboutthe environmental situation and working conditions.

We are fortunate that a relatively high number of radar recordings are available for event analysis. Insome countries, the controller secures radar data when filling out the report form and this procedureis encouraged since it provides us with efficient means to analyze the event in depth. From time totime this data is sent to other European ACAS analysis centers.

Table 5 shows the number of radar recordings and ATC reports received from the participatingStates (reports and radar data received from Maastricht ATC are distributed according to the State inwhich the event occurred). These reports include both RAs and TAs.



Table 5: Source of radar recordings and ATC reports

State Number of ATC reports Number of radar recordings

AUSTRIA 12

BELGIUM 127 20

CZECHOSLOVAKIA 1

DENMARK 2

FRANCE 434 225

GERMANY 473 40

HUNGARY 7 6

IRELAND 6

ITALY 3 3

NETHERLANDS 122 116

NORWAY 29 1

SAUDI ARABIA 1

SWEDEN 118

SWITZERLAND 51 40

UK 735 1298

UNITED ARAB EMIRATES 5

Total 2124 1751



6. STATISTICAL DESCRIPTION OF EVENTS

This part of the report relies heavily on information provided in the questionnaire by pilots andcontrollers. Not all of the questions were answered in all of these forms, also some agencies havesent in questionnaires modified to meet their own requirements that are different from the proposedlayout. In addition radar information about events was available in some cases. Consequently thefollowing statistics refer to different sample sizes, which are stated in the Figures.

6.1. PHASE AND ALTITUDE DISTRIBUTION OF EVENTS

Table 6 shows the number of events by operational phase based on ATC reports i.e.:

TMA operation : Take-off, Approach, Final.

Transition Operation : Initial Climb/Descend, Hold.

Cruise Phase : When the aircraft was in level flight at higher altitudes.

Table 6: Controller reported events by operational phase

Cruise TMA operation Transition operation Unknown Total

320 93 789 916 2118

Table 7 is broadly consistent with Table 6, which shows the number of events by flight phase basedmainly on pilot reports:

Table 7: Pilot reported events by flight phase

Phase Numberof RAs

% ofsample

Take-off 14 0.7%

Climb 716 34.8%

Cruise 450 21.9%

Initial descent 672 32.7%

Hold 24 1.2%

Approach 160 7.8%

Final 22 1.1%

2058



Figure 3 shows the distribution of RA events in altitude bands for the European airspace. Peaks areevident in altitudes between FL80-130 and FL200-300, reflecting mainly the airspace organisation atthese levels, coupled with high vertical rates in standard level-off procedures (approximately 81% ofthe RAs).

The peak around FL80-130, is also the result of level-offs 1000ft apart between two aircraft, oneclimbing from, the other descending to major European airports.

729

5644

4042

5684

100108

155168

10074

5037

4740

6373

106118

110147

108119

127176

146113

1437

921

325

315

12

0 20 40 60 80 100 120 140 160 180 200

0-10

20-30

40-50

60-70

80-90

100-110

120-130

140-150

160-170

180-190

200-210

220-230

240-250

260-270

280-290

300-310

320-330

340-350

360-370

380-390

Figure 3: Altitude distribution of RAs

6.2. PILOT REACTIONS TO RAS

Table 8 shows that in the majority of reported cases (95% - same as the previous year) the pilotfollows an RA. It represents a sustained improvement (from 75%) over the European average duringthe operational evaluation of TCAS. This may show increased awareness of TCAS among theairlines reporting RAs.

Table 8: Pilots reporting following an RA

Followed RA Number

Y 2299

N 113



Figure 4: Diagram showing additional information available to the pilot when an RA was not followed

Figure 4 shows that the Pilot’s decision not to follow an RA is almost always based on additionalinformation either visual with the intruder, traffic information and or ATC avoidance being issued.

There were only 6 pilots who stated that they were not visual with an intruder had no trafficinformation and had not been given ATC avoiding action but of these 4 were in VMC conditions.

Of the two pilots who were IMC and did not follow the RA in one case the RA was a Descend RAwhich occurred at low level 1500 to 2000ft soon after take off a subsequent RA to Climb wasfollowed. The second pilot was in a high rate of climb to a cleared level and was reducing this whenthe RA to Descend occurred. The RA quickly downgraded to Monitor Vertical Speed.

There were 26 RA’s not followed where no indication of having or not having additional informationwas reported.

Figure 5 provides a graphic presentation of intruder information available when the RA was notfollowed.

Visual withthe Intruder

TrafficInformation

ATC avoidanceissued

Not VisualNo Traffic infoNo ATC avoidance

5

25

35

2

1

2

7

6

No information given

26



Figure 5: Intruder information graphic presentation

When pilots did follow RAs, Table 9 shows their deviation as a function of altitude band. About 54%of deviations were 500 ft or less and about 13% of deviations were more than 1000 ft, which wereslightly less favourable than in the previous year (60% and 10% respectively). Most of the largerdeviations, above 1000 ft, occurred at FL200 or above.

Table 9: Deviation from Clearance as a Function of Altitude Band

Vertical Deviation 0-100 100-200 200-300 300-400 400-500 Total

0-500 117 185 288 34 1 625

500-1000 50 99 204 24 377

1000-1500 17 28 65 4 114

1500-2000 7 12 7 1 27

2000-2500 1 2 8 3 14

2500-3000 2 1 1 4

3000-3500 1 1 2

Total 194 328 574 66 1 1163

No ATCAvoidance No Traffic

Info Traffic infoGiven ATC

Avoidancegiven

Not stated

Not Visual with intruder

Visual with Intruder

45

23

30

79 94

17 73 2

0

5

10

15

20

25

30

35

40

45

Intruder information in addition to TCAS WhenCorrective RA was not Followed



The small deviation occurring above FL400 occurred at FL410.

Deviations greater than 2500 ft occurred in 3 instances (only one supported by radar data), all atFL210-220:

• TCAS aircraft descending at very high rate towards FL240. Other traffic maintaining FL220.TCAS climb advisory; pilot overreacted by climbing at very high rate and continuingdeviation until well after clear of conflict.

• TCAS aircraft maintaining FL220. Other aircraft climbing to FL210. TCAS aircraft deviatedto FL250. Exaggerated pilot reaction.

• Climb RA during descent, due to climbing aircraft at 2 NM on right hand side. Largedeviation possibly caused by encounter geometry.

Figure 6 shows graphically the magnitude of deviations as a function of altitude band. What isnoticeable is that the proportion of large deviations increases in the higher altitude bands. This canbe at least partly explained by the nature of the TCAS algorithms, which allow for larger manoeuvresat higher altitudes.

0-500

500-1000

1000-1500

1500-2000

2000-2500

2500-3000

0-100

100-200

200-300

300-400

400-500

0

50

100

150

200

250

300

Deviation (ft)

Altitude (FL)

Figure 6: Magnitude of deviations as a function of altitude



Figure 7 shows the distribution of the aircraft attitude at the issue of TCAS RA, expanded to take theintruder’s profile into account.

The own (i.e. TCAS equipped) aircraft attitude (i.e. climb / descend / level) is the primary informationon the bottom line and matched against the same categories for the intruder.

About 29% of RAs occurred when own aircraft was in level flight, the same proportion as in theprevious year. This is less than during the ACAS evaluation period (1991 – 1996) when nearly half ofthe events were in level flight.

16

286

212213

27

110

224

205

68

0

50

100

150

200

250

300

350

Climb Descent Level

Ow n aircraft

Num

ber

of e

vent

s

Climb

Descent

Level

Figure 7: Encounter geometry, own aircraft against intruder



6.3. CPA MISS DISTANCE DISTRIBUTION

TCAS II version 6.04a triggers alarms even if the aircraft range at CPA is greater than 5 nm. In anattempt to reduce the disruption, TCAS II version 7.0 introduces improved tracking and a horizontalmiss distance filter. To assist the assessment of the HMD filter’s effectiveness, the miss distance atCPA was derived when radar data was available. The results are shown in Figure 8. The proportionsat each range are similar to the previous year.

648

541

299

119

54

15 80

100

200

300

400

500

600

700

0-1 1-2 2-3 3-4 4-5 5-6 6-7

Range (NM)

Num

ber

of e

vent

s

Figure 8: Number of aircraft at a given miss distance at CPA



7. TCAS ASSESSMENT

7.1. CONTROLLER ASSESSMENT

Controllers were asked in the questionnaire to give their opinion of the alert and to give someindications about working conditions. Table 10 shows that only in about 21% of cases (comparedwith 23% the previous year) did the controller express the opinion that the pilot’s action was justified.

Table 10: Controller assessment of pilot action

Justified pilot action Number

Y 94

N 362

Total 456

Figure 9 shows controller’s opinion about the disruptive effect of TCAS alarms and their estimate ofthe workload at the time the RA was reported.

Only when the workload is low, is the distraction caused by TCAS rarely regarded as disruptive(18%, similar to the previous year). It is often disruptive with normal workload (29%, 10% down onthe previous year), and is most often disruptive with high workload (53%, 6% down on the previousyear).

221

305

5947

127

67

0

50

100

150

200

250

300

350

Low Normal High

Not disruptive

Disruptive

Figure 9: Disruption caused by TCAS events as a function of workload



7.2. PILOT ASSESSMENT

Pilots gave their opinion of events in 3 categories (necessary / nuisance / useful), and whether or notthe RA was followed. The results are shown in Table 11.

Table 11: Pilot Event Appraisal

Pilot appraisal Followed Not followed %

Nuisance 418 45 45%

Useful 380 31 40%

Necessary 161 3 16%

959 79

This shows a slight improvement on the previous year (a 4% reduction in nuisance alerts, and a 9%increase in useful RAs). A characteristic already noted in previous operational monitoring reports, isagain evident, that when pilots expressed an opinion, 16% of events were considered necessary, yetwhen pilots or controllers reported whether or not an airprox was filed, they said there was an airproxin only 4% of events. (Table 12). The real figure is likely to be even lower.

Table 12: Proportion of events with an airprox

Events with airprox reports

Airprox : 115 4.0%

7.3. ASSESSMENT WITH RADAR DATA

Events analysed and supported with radar data were also assessed and categorised asunnecessary, useful, or compatible with respect to ATC separation norms. An additional category,false, was attributed to cases where no real traffic were in the vicinity. (These categories are definedin section 8 of this report). The results are shown in Table 13. The proportion of useful RAs is 10%greater than the previous year, with a similar reduction in the proportion of compatible RAs.

Table 13: Assessment with radar data

Assessment Number

Unnecessary 212 59%

Useful 107 30%

Compatible 28 8%

False 15 4%



8. IDENTIFICATION AND ANALYSIS OF OPERATIONAL ISSUES

Early in the TTP program, the FAA developed a system to classify particular characteristics ofTCAS II in respect of technical or operational shortcomings.

The European Event Analysis Group adopted the basic FAA categorisation but also added someadditional issue classes. However the weight of these issues changed reflecting the different ATCand ATM structure in Europe. Events, where these operational issues were a factor, werecategorised throughout the evaluation period.

Frequently, encounters raised more than one issue.

Example: a TCAS equipped aircraft climbing towards his assigned FL, 1000 ft below a level intruder,got a ‘Descend’ RA : which raised the 3 issues ‘High Vertical Rate’, ‘Level-off above/below’,‘Reversed Vertical Rate’.

Specific problem areas are identified below:

• Technical problems: linked to system malfunction.

• RAs associated with Airport operations.

• RAs generated due to the High vertical rates of either aircraft.

• High energy levelling off manoeuvres by intruder aircraft, (“Bump-up”).

• Holding pattern operation.

• Large deviation of the aircraft following a resolution advisory.

• RAs generated in encounters with large horizontal miss distance.

• Local aerodrome traffic.

• Low altitude advisories.

• Military traffic.

• Non altitude reporting traffic.

• Non-airborne intruder.

• Reduced separation with 3rd aircraft.

• Visual clearance/acquisition and separation.

• Pilot training.

For each of these issues, which are normally not mutually exclusive, the number of cases for eachcategory (defined below) is shown in Table 14. This table is based on those events which hadreceived such a categorisation.

Advisory appraisal definitions:

• Unnecessary: RA is incompatible with ATC clearance and normal ATC separation wouldhave been maintained without the RA.

• Compatible: RA is compatible with ATC clearance.

• Useful: RA where, without TCAS, normal ATC separation would have been significantlyinfringed.

• False: RA target unseen by pilot and unknown to ATC.



Table 14: Appraisal of Events by Issue

Issue category Unnecessary Compatible Useful False

LEVEL OFF ABOVE/BELOW 1526 191 46 1 1764

REVERSED VERTICAL RATE 722 6 178 12 918

HIGH VERTICAL RATE (>1500 ft/min) 552 66 50 0 668

TCAS-TCAS CO-ORDINATION 410 17 70 0 497

LARGE DISP (>500'[<290] else >1000') 318 2 142 10 472

MILITARY-OTHER 70 7 90 1 168

TRAINING 108 2 42 1 153

INSUFFICIENT DATA 58 22 68 3 151

LARGE HMD 124 7 2 1 134

PHANTOM INTRUDER 0 0 0 65 65

LOW ALTITUDE RA (<2500 ft) 12 3 40 10 65

ALTITUDE STATION KEEPING 12 0 36 0 48

DUAL/CROSSWIND RUNWAYS 4 4 34 0 42

HORIZONTAL TURN 16 1 14 1 32

HOLDING PATTERN OPERATION 8 5 12 1 26

REDUCED SEPARATION WITH 3RD A/C 18 0 6 1 25

LOCAL AERODROME TRAFFIC 4 0 20 0 24

LOW ALTITUDE TA (<2500 ft) 2 0 16 4 22

VISUAL CLEARANCE 0 3 10 0 13

ATC CONFLICT 2 0 10 0 12

TCAS MUTUAL SUPPRESSION BUSINTERFACE 0 0 0 12 12

ALTITUDE CORRUPTION 6 0 2 0 8

MISSED APPROACH 2 0 4 0 6

MODE-A TRAFFIC 2 0 2 1 5

TCAS FAILURE TO DISPLAY 0 0 0 2 2

NON-AIRBORNE INTRUDER 2 0 0 0 2

Issues 5334

Clearly the biggest issue (81%, a 22% increase compared with the previous year) remains thenuisance level offs, which are highly correlated with high vertical rates and reversed vertical rates.Flight levels separated by 1000ft, with either or both climbing and descending traffic levelling off,leads to a considerable number of these cases.

Events in which RAs were coordinated, i.e., both aircraft operating TCAS, was 23%. This is amarked increase of 13% over the previous year, reflecting the overall increase in the population ofTCAS equipped aircraft.

Large vertical deviations in reaction to some RAs occurred in 22% of issues, almost double theproportion observed in the previous year. This, together with some identified misuses of TCAS andRAs not being followed, suggests that training remains a high priority for pilots.

RAs occurring despite large horizontal separation at CPA accounted for 6% of significant issues(similar to previous year). It is anticipated that the introduction of improved tracking and miss-distance filter of TCAS version 7.0 will have a beneficial effect on this problem.



Military manoeuvres accounted for about 8% of significant issues, double the 1999 figure.

Technical problems, usually manifested by phantoms, remain an issue of concern. They representabout 3% of reported RAs (similar to the previous year). Although some of these can be traced tosuppression bus or other hardware failures (done by checking for multiple reports from the sameairframe) the large majority remain unresolved.

The majority of other problems were associated with TMAs. Some events were in holding patterns,others at low altitudes, with visual clearances, with missed approaches etc.



9. CONCLUSIONS

This report makes an assessment of the 2000 ACAS II monitoring data in Europe.

The European mandate for ACAS II in 2000, may have produced an overall increase in thepopulation of TCAS equipped aircraft in Europe, and this is suggested by the significant increase inreported events in 2000. However, the small sample of confirmed version 7.0 events was insufficientto make a statistical comparison with version 6.04a.

The data suggests that the high level of response to RAs, noted in the previous year, is beingsustained. This may be linked to improved pilot acceptability of the system suggested by thereduced number of nuisance reports. Controller assessment remains mostly stable. Nevertheless,training for both military and civil pilots and controllers remains important.

High vertical rates associated with level offs remain the major source of nuisance for TCAS. Theyare a consequence of setting thrust for optimum performance when speed restrictions apply.

Although some phantom RAs can be tracked to hardware problems, most remain unresolved.



10. ACRONYMS & GLOSSARY

ACAS Airborne Collision Avoidance System

Clear Of Conflict An indication to the flight crew that an RA has terminated

Compatible RA RA in accordance with ACAS II specification, which is compatible withclearance

Coordinated RAs Compatible sense RAs, following the exchange of coordinationmessages, in an encounter between two TCAS equipped aircraft

Corrective RA RA requiring pilot action to adjust vertical speed

CPA Closest Point of Approach

Deviation Vertical displacement from flight path, in following a Corrective RA

False RA RA was based on a false track created by erroneous surveillance dataor an onboard system malfunction

FL Flight Level

Ft Feet

HMD Horizontal Miss Distance

Horizontal Miss Distance Estimated horizontal separation at CPA of a certain traffic

NM Nautical Mile

Nuisance RA RA in accordance with ACAS II specification, recommending amanoeuvre away from clearance, where normal separation was not orwould not have been significantly infringed

Phantom A non existing threat aircraft indicated by a False RA

Pilot Model Algorithm used to model pilot reaction following an RA; with parametersto model delay, acceleration, and target vertical speed

Preventive RA RA indicating vertical speeds to avoid, but not requiring pilot action

RA Resolution Advisory

Resolution Advisory An indication to the flight crew recommending either a manoeuvre toprovide separation from all threats, or a manoeuvre restriction tomaintain existing separation

TA Traffic Advisory

TCAS Traffic alert and Collision Avoidance System

Traffic Advisory An indication to the flight crew that a certain traffic is a potential threat

Useful RA RA in accordance with ACAS II specification, where normal separationwas or would have been significantly infringed

Vertical Miss Distance Estimated vertical separation at CPA of a certain traffic

VMD Vertical Miss Distance

Programme Européen de Surveillance Opérationnelle ACAS EUROCONTROL

Projet SAF/ACAS - Rapport CEE N° 373 25

TRADUCTION EN LANGUE FRANCAISE

RESUME

Le système anticollision embarqué (ACAS) est un terme générique créé par l'OACI pour désignerun système qui avertit le pilote qu'un autre aéronef (l'intrus) pénètre dans le volume restreintd'espace aérien qui entoure son propre aéronef, et le conseille sur les mesures à prendre.

Depuis 1991, EUROCONTROL participe à l'organisation de la surveillance opérationnelle de l'ACASdans l'espace aérien européen, pour le compte des transporteurs européens. Ces travaux sontmenés sous les auspices de l'OACI. Parallèlement, le Royaume-Uni et la France mènent leurspropres travaux de surveillance dans leur espace aérien respectif, et ces travaux font partieintégrante du programme européen de surveillance de l'ACAS.

Le système d'avertissement de trafic et de prévention des abordages TCAS II est le seulsystème commercial qui répond aux exigences de l'ACAS II. La dernière version du TCAS II, laversion 7, remplace progressivement, depuis fin 2000, la V6.04A dans tous les aéronefs. Cettenouvelle version, entièrement conforme aux exigences de l'ACAS II, comporte des améliorationsnotables sur les plans opérationnel, technique et de la sécurité.

Depuis le 1er janvier 2000, l'ACAS II (c'est-à-dire la version 7.0 du TCAS II) est obligatoire enEurope, moyennant une période de transition. Dans la pratique toutefois, le passage de la version6.04a à la version 7.0 a souvent pris plus de temps que prévu au départ, et des dispenses ont étéaccordées tout au long de l'année.

Le TCAS II est un système embarqué indépendant fondé sur les technologies SSR. Il émet et reçoitdes messages Mode S et Mode C via le transpondeur de l'aéronef. La fréquence d'interrogation estde 1 Hz environ ; les distances et variations de distance ainsi calculées ainsi que l'altitude mesuréeà partir des réponses Mode S et Mode C permettent de déceler la présence d'autres aéronefs dansle voisinage. La logique détermine sur cette base l'existence d'un danger éventuel.

Lorsqu'elle détecte un danger potentiel, la logique adresse au pilote un avertissement de trafic(TA). Si la menace est imminente, le TCAS II propose au pilote une manœuvre d'évitement dans lesens vertical : il s'agit d'un avis de résolution (RA).

Le présent rapport se fonde essentiellement sur les comptes-rendus de pilotes et de contrôleurs.Différents questionnaires ont été adressés à des compagnies aériennes et à des centres ATC. Lesréponses à ces questionnaires ont servi à constituer la partie la plus importante de la base dedonnées. De plus, il a été demandé aux contrôleurs d'obtenir les enregistrements de données radarafférents à tous les comptes-rendus qu'ils ont analysés, et ces données ont été ajoutées à la basede données.

On trouvera ci-après certaines des statistiques essentielles établies à partir des comptes-rendus de2000. Il convient de noter cependant que tous les événements notifiés n'étaient pas des RA : sur les3439 comptes-rendus, 2902 portaient sur des RA, 238 sur des TA et 299 sur des événements nonidentifiés.

La ventilation des événements par phase de vol se fonde principalement sur les comptes-rendus depilote :

Décollage 14 (0,7%), montée 716 (34,8%), croisière 716 (34,8%), descente initiale 672 (32,7%),attente 24 (1,2%), approche 160 (7,8%), et phase finale 22 (1,1%).

EUROCONTROL Programme Européen de Surveillance Opérationnelle ACAS

26 Projet SAF/ACAS - Rapport CEE N° 373

Il ressort clairement de la ventilation des RA par altitude dans l'espace aérien européen que lamajorité des événements se produisent entre les niveaux FL 80 – 130 et FL 200 – 300, en raisonprincipalement de l'organisation de l'espace aérien à ces niveaux. Ce sont les vitesses verticalesélevées utilisées dans les procédures standard de stabilisation avec des clairances à 1000 pieds quiprovoquent la majorité des RA (environ 81%).

Dans la plupart des cas signalés (95 %, comme l'année précédente), le pilote se conforme à un RA.La décision du pilote de ne pas suivre un RA est presque toujours motivée par d'autresinformations : vision de l'intrus, informations de trafic ou instruction d'évitement donnée par l'ATC.

54 % des pilotes qui se sont conformés à un RA ont dévié de 500 pieds ou moins, et 13 % environont dévié de plus de 1000 pieds.

Seulement 21 % des contrôleurs interrogés (contre 23 % l'année précédente) estiment que lamanœuvre exécutée par le pilote comme suite à un RA se justifiait. 29 % d'entre eux (10 % demoins que l'année précédente) considèrent la notification des alarmes TCAS comme gênantelorsque la charge de travail est normale, 53 % (6 % de moins que l'année précédente) comme trèsgênante lorsque la charge de travail est élevée.

Selon les pilotes, 45 % des avertissements constituaient une nuisance, 40 % étaient utiles et 16 %nécessaires. Dans 4% des cas seulement, un compte-rendu AIRPROX a été déposé.

Le Groupe européen d'analyse des événements a classifié les événements liés à des questionsopérationnelles pendant toute la période d'évaluation. Il ressort des ces travaux que le problèmeprincipal (81 %, soit une augmentation de 22 % par rapport à l'année dernière) demeure celui desstabilisations inutiles, qui sont très étroitement liées aux vitesses verticales élevées et aux vitessesverticales inversées. Un autre problème majeur est celui des écarts verticaux importants commesuite à certains RA, dont la proportion était de 22 %, soit presque le double de la proportionobservée l'année précédente.

Il ressort des cas de mauvaise utilisation des TCAS et de non-respect des RA que la formation despilotes doit demeurer une priorité élevée.

En effet, la surveillance montre que le TCAS améliore la sécurité de l'espace aérien lorsque les avisde résolution sont suivis correctement.

On trouvera à l'Annexe A du présent rapport des exemples "anonymes" d'événements ACAS quipourront servir à promouvoir la formation et une plus grande prise de conscience dans lacommunauté aéronautique.

Programme Européen de Surveillance Opérationnelle ACAS EUROCONTROL

Projet SAF/ACAS - Rapport CEE N° 373 27

1. INTRODUCTION

Si l'on veut que la mise en œuvre de l'ACAS dans l'espace aérien de la CEAC se déroule dans debonnes conditions de sécurité et d'exploitation, il faut surveiller les événements ACAS quisurviennent dans cet espace aérien et relayer ces informations au personnel technique etopérationnel qui travaille sur l'ACAS.

Le présent rapport comprend une description et une analyse de la surveillance exercée en 2000,ainsi qu'une analyse de données visant à déterminer si le système ACAS continue d'améliorer lasécurité et est acceptable sur les plans technique et opérationnel. Il fournit donc des enseignementsopérationnels et techniques utiles au sujet des performances ACAS.

Les tendances observables par rapport aux résultats précédents y sont également présentées.

2. CONCEPT DE DELEGATION

Le domaine CAS vise à assurer la mise en œuvre et le fonctionnement des systèmes anticollisionembarqués (ACAS) dans la zone de la CEAC dans de bonnes conditions d'harmonisation et desécurité.

Le projet "Surveillance de l'ACAS" a pour objectif de fournir au programme de mise en œuvre del'ACAS des informations permettant de vérifier si ces objectifs sont atteints.

Il vise également à détecter toute anomalie dans le fonctionnement de l'ACAS et à fournir desinformations propres à contribuer au redressement de la situation.

3. CONCLUSIONS

On trouvera dans le présent rapport une évaluation des données 2000 de surveillance de l'ACAS IIen Europe.

Le fait que, depuis 2000, les aéronefs évoluant en Europe doivent impérativement répondre auxnormes ACAS II est sans doute à l'origine de l'augmentation du nombre d'aéronefs équipés dusystème TCAS, comme le suggère l'augmentation significative du nombre de comptes-rendus en2000. L'échantillon limité d'événements confirmés comme étant liés à la version 7.0 est cependantinsuffisant pour qu'une comparaison statistique avec la version 6.04a soit possible.

Il ressort des données que le niveau de réaction aux RA reste élevé, comme les annéesprécédentes. Cette tendance peut s'expliquer par le fait que les pilotes acceptent mieux le système,comme le suggère la diminution du nombre de comptes-rendus de nuisance. L'évaluation descontrôleurs demeure, en grande partie, inchangée. Toutefois, la formation des pilotes et descontrôleurs militaires et civils reste importante.

Les vitesses verticales élevées associées aux stabilisations demeurent la source principale denuisance pour le TCAS. Elles sont dues à la poussée exercée en vue d'une optimalisation desperformances malgré les limitations de vitesse en vigueur.

Bien que certains RA fantômes puisent être attribués à des problèmes de matériel, la plupart d'entreeux demeurent inexpliqués.

EUROCONTROL Programme Européen de Surveillance Opérationnelle ACAS

28 Projet SAF/ACAS - Rapport CEE N° 373



ANNEX



ANNEX A: EXAMPLES OF TCAS ISSUES

This appendix contains a set of TCAS events from 2000 that illustrate some of the TCAS issuesdescribed in the report. The following topics are covered:

Event Number Issue

8683 TCAS RAs in multi-aircraft encounter despite large horizontal miss distances

9613 An example of TCAS saving the situation

9739 Conflict with 3rd aircraft due to pilot not reacting to RA weakening

9768 TCAS responding to an altitude bust

9906 Phantom RA - TCAS responding to surveillance errors

9918 TCAS RA with military aircraft

10023 Level off manoeuvre with 1000 ft separation – excessive deviation

Most of these reports show simulations performed using the InCAS tool. For further informationon how this is used to analyze radar data and produce these reports, please look athttp://www.eurocontrol.fr/ba_saf/acas/InCAS/Index.htm.

The event number used in the table above refers to our own reference for each event and has nosignificance in itself.



EVENT 8683 - TCAS RAS IN MULTI-AIRCRAFT ENCOUNTER DESPITE LARGE HORIZONTAL MISS DISTANCES

Event synopsis

AC3 & AC1 on heading. Separation more than 5 nm. Positive RAs in both aircraft; neither followed. AC1 turned left ± 20 degrees after TCAS alert.Unnecessary turn by AC1.

AC3 & AC2 on heading. Positive RAs in both aircraft. AC3 followed RA. Separation almost 5 nm.

Simulations with TCAS version 7.0 produce similar RAs, despite its general improvements in tracking and use of a miss-distance filter.



EVENT 9613 - AN EXAMPLE OF TCAS SAVING THE SITUATION

Event synopsis

ATC instructed AC1, descending, inbound to take up heading 050, but AC1 mistakenly followed heading 005.

AC2, climbing, outbound got TA followed by climb RA. AC1 got preventive monitor vertical speed RA.

CPA separation was 0.4 nm horizontal, 900 ft vertical.



EVENT 9739 – CONFLICT WITH 3RD AIRCRAFT DUE TO PILOT NOT RESPONDING TO WEAKENING RA

Event synopsis

AC1 descending on top of AC2 caused descend RA in latter. AC2 initially followed TCAS correctly, but did not respond to weakening RA or clear of conflictadvisories, and came into conflict with AC3. ATC instructed AC2 to climb back to FL260, at about the same time that AC2 got a descend RA, and AC2followed the ATC instruction instead of TCAS. AC3 got a climb RA, and climbed.

This case illustrates the risk of ignoring or manoeuvring in an opposite sense to TCAS guidance; particularly when both aircraft are operating TCAS.In cases of conflicting TCAS / ATC advisories, pilots should always follow the TCAS advisory.



EVENT 9768 – TCAS RESPONDING TO AN ALTITUDE BUST

Event synopsis

AC1 cleared FL270, but understood FL370. AC1 got descend RA and descended, achieving 760 ft vertical separation. Without TCAS, vertical separationwould have been only 100 ft.



EVENT 9906 – PHANTOM RA – TCAS RESPONDING TO SURVEILLANCE ERRORS

Event synopsis

The pilot of AC1 reported RA on opposite traffic 300 ft above.

At the time of AC1’s RA, the 3 other aircraft in the vicinity were all non-intruding targets, at least 9 nm distant from AC1.

Apparently a false ‘phantom’ target.



EVENT 9918 – TCAS RA WITH MILITARY AIRCRAFT

Event synopsis

AC1, maintaining FL330 on heading 105. Two military aircraft flying exercises, came within 5 nm of AC1, and both became RA targets.

AC1 got climb RA and deviated 600 ft.

CPA separation between AC1 and two military aircraft was 4.2 nm and 4.6 nm.



EVENT 10023 – LEVEL-OFF MANOEUVRE WITH 1000 FT SEPARATION – EXCESSIVE DEVIATION

Event synopsis

AC2 climbing to level below AC1.

AC1 followed climb RA, but deviated 1100 ft due to excessive vertical rate, and delayed response to weakening RA.

A simulated standard pilot reaction produces a deviation of only 400 ft.

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