EUROCONTROL · for the safety of air navigation The information contained in this document is the...

EUROPEAN ORGANISATIONFOR THE SAFETY OF AIR NAVIGATION

The information contained in this document is the property of the EUROCONTROL Agency and no part should be reproduced in any formwithout the Agency’s permission.

The views expressed herein do not necessarily reflect the official views or policy of the Agency.

EUROCONTROL EXPERIMENTAL CENTRE

European ACAS Operational EvaluationSecond Interim Report

EEC Report No. 300

EEC Task I03EATCHIP Task CAS.ET1.ST12

Issued: July 1996

EUROCONTROL

REPORT DOCUMENTATION PAGE

iii

Reference:EEC Report No. 300

Security Classification:Unclassified

Originator:EEC - ATM(Air Traffic Management )

Originator (Corporate Author) Name/Location:EUROCONTROL Experimental CentreB.P.15F - 91222 Brétigny-sur-Orge CEDEXFRANCETelephone : +33 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 9011

TITLE: European ACAS Operational Evaluation

Second Interim Report

AuthorMarc Bisiaux

Date

7/96Pages

xii+38Figures

15Tables

21Appendix

-References

11

EATCHIP TaskSpecification

CAS.ET1.ST12

EEC Task No.

I03

Task No. Sponsor

-

Period

1991 to 1994

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

Descriptors (keywords): Airborne Collision Avoidance - ACAS - TCAS

Abstract:

The second Interim Report on the TCAS II operational evaluation in Europe provides a status of theappraisal of the Airborne Collision Avoidance System for the period covering 1991 to 1994.

The framework under which was conducted the collection of operational reports on TCAS II relatedevents had been established under the auspices of ICAO to support and complement the development ofStandards and Recommended Practices for ACAS.

The statistical presentations and the analyses of the collected data are focussed on the events recordedwithin the ECAC Airspace.

Although the introduction of TCAS II has raised a number of technical an operational problems, sufficientacceptance of the system and confidence has been gained that it will enhance the safety of the ATMsystem.

Some of the identified issues have received appropriate treatment and will be assessed in future updatesof this evaluation reports.

iv

This document has been collated by mechanical means. Should there be missing pages, please report to:

EUROCONTROL Experimental CentrePublications Office

B.P. 1591222 - BRETIGNY-SUR-ORGE CEDEX

France

v

EEC Report N° 300EEC Task I03

EATCHIP Task CAS.ET1.ST12Date of Issue: July 1996

European ACAS Operational EvaluationSecond Interim Report

Marc Bisiaux

EUROCONTROLExperimental Centre

Summary

The second Interim Report on the TCAS II operational evaluation in Europe provides a status of theappraisal of the Airborne Collision Avoidance System for the period covering 1991 to 1994.

The framework under which was conducted the collection of operational reports on TCAS II related eventshad been established under the auspices of ICAO to support and complement the development ofStandards and Recommended Practices for ACAS.

The statistical presentations and the analyses of the collected data are focussed on the events recordedwithin the ECAC Airspace.

Although the introduction of TCAS II has raised a number of technical an operational problems, sufficientacceptance of the system and confidence has been gained that it will enhance the safety of the ATMsystem.

Some of the identified issues have received appropriate treatment and will be assessed in future updates ofthis evaluation reports.

This report has been compiled and was reviewed in close cooperation with a group of experts activelyinvolved in the ACAS Operational Evaluation .

vii

TABLE OF CONTENTS

LIST OF ILLUSTRATIONS ixLIST OF TABLES ixEXECUTIVE SUMMARY xi

1. INTRODUCTION 1

2. BACKGROUND ON ACAS 2

3. THE OPERATIONAL EVALUATION OF TCAS II 3

3.1 The ACAS Standardisation Process 3

3.2 ACAS within Europe. 3

3.3 Data collection procedures 33.3.1 Pilot reports 33.3.2 Controller reports 43.3.3 Flight recorded data 43.3.4 Mode-S and ground radar data 4

3.4 Current TCAS II Operations & Scope of the Report. 43.4.1 Event processing and analysis procedures 5

4. RESULTS OF THE EUROPEAN OPERATIONAL EVALUATION 6

4.1 The European Data Collection of TCAS Events 64.1.1 Progress of The Data Collection 64.1.2 Data Sources 74.1.3 Contributors 9

4.2 Statistical description of TCAS Events 114.2.1 Type of Reported Events 114.2.2 Location of Events Reported 124.2.3 Altitude of reported events 12

4.3 Analysis of Resolution Advisories 134.3.1 Type of resolution advisories 134.3.2 Pilot reactions to Resolution Advisories 144.3.3 Deviations following Resolution Advisories 154.3.4 Aircraft proximities 16

4.4 Configuration of events 174.4.1 Flight Phase 174.4.2 Evidence of aircraft manoeuvres 184.4.3 Conflict geometries 18

4.5 Assessment of TCAS 194.5.1 Controllers’ assessment 194.5.2 Pilots’ assessment 20

5. ANALYSIS OF EVENTS 22

5.1 General appraisal mechanism 22

5.2 Identification and Analysis of Operational Issues 22

viii

5.2.1 Technical issues 235.2.2 Altitude station keeping 245.2.3 Bump-up scenarios 245.2.4 Military traffic 245.2.5 Holding pattern operation 265.2.6 Large Horizontal Miss Distances (HMD) 265.2.7 Airport Operations 275.2.8 Low altitude advisories 285.2.9 Reduced separation with 3rd a/c 285.2.10 TCAS Coordination 295.2.11 Large altitude displacements 295.2.12 Visual acquisition 29

5.3 Incident reports associated with TCAS alerts 30

5.4 Analysis of Traffic Advisories 30

6. CONCLUSIONS 32

GLOSSARY 34REFERENCES 35Traduction en français du sommaire 37

ix

LIST OF ILLUSTRATIONS

FIGURE 1 : NUMBER OF EVENTS PER YEAR 6FIGURE 2 : SEASONAL VARIATION OF THE DATA COLLECTION 7FIGURE 3 : DISTRIBUTION OF EVENTS BY ALTITUDE BAND 13FIGURE 4 : TYPES OF RA 13FIGURE 5 : ALTITUDE FREQUENCY OF RA BY TYPE 14FIGURE 6 : SENSE OF RA 14FIGURE 7 : PILOT REACTION TO CORRECTIVE RAS 15FIGURE 8 : PILOT REACTION AND VISUAL ACQUISITION 15FIGURE 9 : VERTICAL DEVIATIONS 16FIGURE 10 : AIRCRAFT ATTITUDE 18FIGURE 11 : DISRUPTION CAUSED TO CONTROLLERS BY TCAS EVENTS 19FIGURE 12 : DISRUPTION TO ATC & CONTROLLER WORKLOAD 19FIGURE 13 : PILOT ASSESSMENT OF RAS 20FIGURE 14 : REACTIONS TO RAS AND PILOT ASSESSMENT 21FIGURE 15 : OVERALL APPRAISAL OF EVENTS 22

LIST OF TABLES

TABLE 1 : NUMBER OF EVENTS BY AIRSPACE 6TABLE 2: REPORTS ON EUROPEAN EVENTS BY DATA SOURCE 8TABLE 3: CORRELATION OF DATA SOURCES 9TABLE 4 : NUMBER OF PILOT REPORTS PROVIDED BY AIRLINE 10TABLE 5 : NUMBER OF ATC REPORTS PROVIDED BY STATE 11TABLE 6 : NUMBER OF EVENTS PER ADVISORY TYPE 12TABLE 7 : LOCATION OF EVENTS 12TABLE 8 : VERTICAL DISTRIBUTION OF EVENTS BY STATE 12TABLE 9 : DISTRIBUTION OF RAS IN DISTANCE & HEIGHT 17TABLE 10 : RAS BY FLIGHT PHASE 17TABLE 11 : CONFLICT GEOMETRIES 18TABLE 12 : CONTROLLERS’ ASSESSEMENT OF PILOTS’ REACTION 19TABLE 13 : APPRAISAL OF EVENTS FOR SPECIFIC ISSUES 23TABLE 14 : LOCATION OF EVENTS ASSOCIATED WITH MILITARY TRAFFIC 25TABLE 15 : FEATURES OF MILITARY EVENTS 26TABLE 16: DEVIATION CAUSED BY LARGE HMD RAS 26TABLE 17 : APPRAISAL OF LOW ALTITUDE RAS 28TABLE 18 : FEATURES OF LARGE VERTICAL DEVIATIONS 29TABLE 19 : INFLUENCE OF VISUAL ACQUISITION ON ASSESSMENT OF RAS 30TABLE 20: PERCENTAGE OF AIRMISS FILED PER CATEGORY OF ADVISORY 30TABLE 21 : FEATURES OF TRAFFIC ADVISORIES 31

xi

EEC Report N° 300EEC Task I03

EATCHIP Task CAS.ET1.ST12Date of Issue: July 1996

EXECUTIVE SUMMARY

The Traffic Alert and Collision Avoidance System (TCAS II) was introduced in operationalevaluation in the context of the development by ICAO of Standards and Recommended Practicesfor the AIrborne Collision Avoidance Systems (ACAS). An operational evaluation of the systemwas based on the collection of reports on TCAS II related events from pilots and controllers aswell as from data recordings.

TCAS II will, if the system calculates a risk of collision with an intruder aircraft, which is equippedwith an altitude reporting transponder or TCAS, recommend avoidance manoeuvres in the verticalplane to the pilot by generating a Resolution Advisory (RA).

The present report is focusing on TCAS II events occuring during the period 1991 to 1994. Aforum for reviewing ACAS issues in Europe was established during 1991 by EUROCONTROL,under the auspices of ICAO. This forum co-ordinated the European ACAS Operational Evaluationand permitted to develop the data collection procedures, to monitor the progress of the evaluation,and to discuss any problems arising from TCAS II operations.

Since 30 December 1993, US legislation has required that TCAS II be carried and operated byall national and foreign registered passenger aircraft, of more than 30 seats, flying in USairspace. Many non-US operators subsequently equipped more of their aircraft with TCAS II thanthose which operate solely to the USA.

Over the 4 years of the data collection, more than two thousand events were reported to theEuropean ACAS Evaluation, of which nearly three quarters occurred in the ECAC Airspace. Thenumber of event reports steadily increased throughout the period.

Many TCAS users contributed to the operational evaluation. Fifty one airlines and eighteen ATCservices reported using an internationally established channel. A high number of these reportsconfirmed specific problems identified initially by the equivalent US programme. Additionalinformation was provided by Mode-S radar recordings and airborne recordings.

There was a significant concentration of reported TCAS encounters in the altitude band FlightLevel 250 to Flight Level 300, which may be attributable to the high percentage of ‘short haul’flights operating at these levels, within the European States generally and more particularly in thehigh density traffic areas of western European airspace.

A high proportion of 84% Resolution Advisories were Corrective RAs, whilst only 16% werePreventive RAs. In 70% of the reported events the pilot followed the Corrective RA. In themajority of cases, where the RA was not followed, the pilot reported visual acquisition of the

xii

intruder. In following corrective Resolution Advisories, larger trajectory deviations than thoseexpected by design of ACAS were occasionally observed, which were often not coordinated withATC and caused considerable concern to operational ATC staff. Pilots found that RAs were“Necessary” in 28% of cases and “Useful” in 22% of cases. However, 50% of the RAs wereconsidered as “Nuisance”. It was generally possible, through a detailed investigation on eachevent, to correlate these “Nuisance” RAs with operational and technical issues.

Operational experience, and events analysis, has shown that TCAS II provides an enhanced levelof flight safety, despite the defficiencies of the earlier TCAS II software versions, ie: up to andincluding Version 6.02. Operational problems identified worldwide by the operational evaluationslead to mandating the implementation of an enhanced TCAS Version 6.04A which has alleviatedseveral of the earlier operational problems. These include, for instance, a significant reduction inthe number of Nuisance Advisories. However, as the system continues to evolve, the futureVersion 7 software, due for introduction in 1998, is expected to improve the operationalacceptability still further.

EUROPEAN ACAS OPERATIONAL EVALUATION Second Interim Report

July 1996 Page 1

1. INTRODUCTION

An ACAS Operational Evaluation took place between 1991 and 1995 to assess the performanceof TCAS II in the European operational environment. This report presents the results of theevaluation, covering the period 1991-1994.

The analyses carried out, within the framework of the Operational Evaluation, have confirmedthat TCAS II is an effective collision avoidance system. However several operational acceptabilityproblems were identified, which generally related to the integration of TCAS II with certainelements of the Air Traffic Management System.

This report summarizes the statistical and operational findings on the ACAS events reported overthe ECAC States.

The analyses have been based on the reports provided by pilots and controllers, together withsupplementary information from radar data recording and TCAS event simulations.

TCAS events have been categorised after analysis and discussion in the EEC ACAS EvaluationGroup.


July 1996 Page 2

2. BACKGROUND ON ACAS

Airborne Collision Avoidance System (ACAS) is a generic term for Airborne CollisionAvoidance Systems. ACAS is intended to improve air safety by providing a ‘last resort’ method ofpreventing collisions, or near collisions.

By utilising Secondary Surveillance Radar (SSR) technology, the ACAS equipment operatesindependently of ground-based aids and ATC by monitoring other aircraft, in the vicinity, andassessing the which a/c represents potential collision threats by interrogating the transponders inthese aircraft. However, non-transponding aircraft can not be detected.

The Traffic Alert and Collision Avoidance System was developed, principally in the US and iscurrently the only available implementation of an ACAS.

Three levels of TCAS are envisaged, namely TCAS I, TCAS II and TCAS IV (to meet ACAS IIIrequirements). These implementations correspond to the ICAO ACAS levels l, ll and lll:

• TCAS I provides a flight deck display of traffic in the vicinity, and, if appropriate, alerts the pilotthat an aircraft presents a potential threat by generating a Traffic Advisory (TA) . HoweverTCAS I does not provide collision avoidance advice.

• TCAS II will, if the system calculates a risk of collision with an intruder aircraft, which isequipped with an altitude reporting transponder or TCAS, recommend avoidance manoeuvresin the vertical plane to the pilot by generating a Resolution Advisory (RA) .

• TCAS IV is still under development and is intended to include a capability for providinghorizontal resolution advisories in addition to the vertical resolution advisories provided byTCAS II.

Only TCAS I and TCAS II are commercially available. TCAS II is the system with which this reportis principally concerned.

A significant proportion of European, and foreign registered, passenger carrying aircraft operatingin ECAC airspace are now equipped with TCAS II. It is recognised that these aircraft areoperating at an enhanced level of flight safety.


July 1996 Page 3

3. THE OPERATIONAL EVALUATION OF TCAS II

3.1 The ACAS Standardisation Process

Under the auspices of ICAO, an international ACAS Operational Evaluation was undertaken, tosupport the development of ACAS Standards and Recommended Practices (SARPs) by the SSRImprovements and Collision Avoidance Panel (SICASP). European States, together withEurocontrol, have made a significant input to the evaluation.

SICASP has stated that an “ACAS cannot resolve all possible collisions and may cause some riskof collision“ but the “use of ACAS is expected to reduce significantly the risk of collision withaircraft operating altitude reporting transponders, provided that carriage of such transponders issufficiently widespread.“.

ICAO ACAS operating procedures and ATC provisions for ACAS equipped aircraft are containedin PANS - OPS (Doc.8168) and PANS - RAC (Doc.4444) respectively. SARPS for ACAS I & IIare contained in Annex 10.

In addition SICASP has also produced documentation ( State Letter AN 7/11.12-94/62 ) to assistStates in the introduction of ACAS, addressing, in particular, the areas of safety benefits,pilot/controller training and aspects of implementation.

3.2 ACAS within Europe.

During 1991 EUROCONTROL, under the auspices of ICAO, established the ACAS EvaluationCo-ordination Group (AECG) to co-ordinate the ACAS Operational Evaluation within Europeanairspace .

The AECG was established to develop data collection procedures, to monitor and report the progressof the evaluation, and to discuss any issues arising from TCAS II operations.

The AECG meetings also provided a forum for the ARINC Research Division and the USA FederalAviation Administration (FAA), to report on the progress of the US TCAS Transition Programme (TTP),which includes evaluations of TCAS performance in US airspace, which have been conducted inparallel with those in the airspace of the ECAC States.

The operational evaluation began officially on March 1, 1991, although the first report of a TCASevent, in European airspace, arrived in February 1991.

3.3 Data collection procedures

Procedures were developed to ensure that TCAS events data, from the various sources, would becollected and dispatched to the regional ACAS Evaluation Centres, as soon as possible, so that therecorded radar data of the event, which is normally retained for only a short period of time, couldacquired to support the analysis of the particular event.

3.3.1 Pilot reports

Pilots were requested to complete a report for each TCAS event and the airline then dispatches acopy of the report, as soon as possible, to the regional Evaluation Centre for the airline’s State. Incases where the event occurred in the region of another Evaluation Centre a copy of the event reportwas also forwarded to the relevant Evaluation Centre.


July 1996 Page 4

For events of particular interest, which occurred in airspace where recorded radar data was available,this data was requested by the EEC Evaluation Centre for analysis.

3.3.2 Controller reports

ATC controllers were also requested to complete reports for TCAS events, which came to theirattention.

Such reports were made independently of any existing standard incident reporting procedures.

Participating State authorities were asked to collect and validate the controller TCAS event reports andto forward these reports to their regional Evaluation Centre.

3.3.3 Flight recorded data

EUROCONTROL agreed to purchase a limited number of TCAS flight recorders and to distributethese amongst several participating aircraft operators.

Eight recorders, and recorder downloading systems, were purchased and these were initially placedwith DELTA, FINNAIR and IBERIA.

The TCAS recorder manufacturer provided training, in the use and configuring of the recorders andthe downloading systems, for the European aircraft operators.

The downloading system comprised an adapted PC, an interface to the recorder, a high densitystorage cassette drive (at least 80 Mb) and the software required to configure the recorder to recordappropriate event data; to download the recorded data and to clear down a recorder for re-use.

Aircraft operators were requested to download the recorders on a regular basis, to transfer the data toa high density storage cassette and to dispatch this to the regional ACAS Evaluation Centre fordecoding and analysis.

3.3.4 Mode-S and ground radar data

It was anticipated that some State Aviation Authorities would be able to extract radar data on the basisof reported TCAS events, or of TCAS event data downlinked via Mode-S.

A common format, for the exchange of recorded radar data, was adopted and the facilities to processthe data and analyse the results were made available at the EEC Evaluation Centre for those Statesunable to carry out TCAS simulations

3.4 Current TCAS II Operations & Scope of the Report.

Since 30 December 1993, US legislation has required that TCAS II be carried and operated byall national and foreign registered passenger aircraft, of more than 30 seats, flying in USairspace.

For reasons of operational flexibility and fleet engineering commonality, many non-US operatorsare equipping more of their aircraft with TCAS II than those which operate solely to the USA. As aresult, it is estimated that more than 20% of the large commercial aircraft flying in Europeanairspace are equipped with, and operating, TCAS II.

In September 1991 a proposal was agreed to review each TCAS RA event systematically. AnACAS Events Analysis Group (EAG)was created with representatives of the Civil AviationAuthorities of France, Germany and the UK.


July 1996 Page 5

The Terms of Reference of the EAG were agreed and require the group to:

a) Review and analyse reports from pilots,controllers and airborne or ground based recorded data

b) Classify encounters in a manner consistent with other international agencies.c) Identify :

i) geographical areas of concern;ii) operational procedures which produce an excessive number of RAsiii) possible additional training requirements.

d) To support any other analysis procedures or recommendations from the AECG.

The event analysis includes :

a) Synthesis of the various data sources;b) Transcription of the original reports;c) Review of events at the EAG meetings;d) Request further supporting data as necessary;e) Categorise events as issue or alert;f) Conclusions & Recommendations;g) Report and close event.

The EAG reports regularly to the AECG.

One of the main purposes of the current ACAS Operational Evaluation has been to assess andidentify the operational effects, of TCAS II upon both the ATC System and flight operations.

As a result of operationally unacceptable TCAS II performance characteristics identified by theevaluation TCAS II modifications were introduced. The current Version 6.04A software hasalleviated several of the previously identified operational problems, and the future Version 7software, due for implementation in 1998, will further improve the operational interface. Thisreport adresses issues raised between 1991 and 1994, before the 100% implementation ofVersion 6.04A.

3.4.1 Event processing and analysis procedures

Each TCAS event report, received at the EEC Evaluation Centre, is processed systematically.

The report is entered into the database system. This phase involves :

a) Entry of report data, from any source, into the appropriate database table;b) Determine, if necessary, the latitude and longitude position of the event;c) Correlation, if any, of the new event with those already in the database;d) Cross refer correlated events;e) Enter event synopsis into database;f) Classify encounter geometry and enter into database.


July 1996 Page 6

4. RESULTS OF THE EUROPEAN OPERATIONAL EVALUATION

4.1 The European Data Collection of TCAS Events

The European ACAS Evaluation started to receive data on TCAS events in early 1991.

This report summarises the results of the data collected on TCAS events occurring up to the endof 1994.

Over these 4 years, data and reports were collected on TCAS events which occurred worldwide.Most of the data collection has been provided by aircraft operated by carriers registered outsidethe USA and Canada. Some additional European events were reported by USA carriers.

The information was complemented by reports received from ATC Centres in a variety of States.

Table 1 : Number of events by airspaceEurope USA & Canada Other Airspaces Total1468 439 456 2363

The total database comprised 2363 events of which 1468 occurred in ECAC Airspace.

4.1.1 Progress of The Data Collection

The carriage and operation of TCAS was not mandatory in Europe, but it’s use was permitted.

The number of event reports collected steadily increased towards 31 December 1993, which wasthe deadline for the implementation of TCAS II on board European carriers to comply with USRegulations.

Similarly an increase in returned questionnaires was also observed in 1994.

35

112

447

874

0

100

200

300

400

500

600

700

800

900

1991 1992 1993 1994

Figure 1 : Number of Events per Year


July 1996 Page 7

The graph above shows the progress of the data collection, by year, over the evaluation period.

The seasonal variations in event reports are shown in Figure 2, for each year of the datacollection.

Any attempt to directly correlate this information with traffic data, or conflict statistics, will probablybe misleading since reports were not received for all the events which actually occurred .

4.1.2 Data Sources

The data collection was based upon five different sources:

a) Pilot reports;b) Controller reports;c) Airborne recordings;d) Mode-S air-ground data link;e) Ground radar data.

Ground radar data was provided as supporting information to other sources when possible. Thenumber of reports provided by each of these sources is summarized in Table 2, below.

Airborne Recordings

Airborne recordings of ACAS data, using dedicated recorders, were expected to provide directtechnical insight into system performance.

For economic reasons, as well as installation complexity, only 8 recorders were purchased to equipselected aircraft.

0

20

40

60

80

100

120

Jan Feb Mar Apr May Jun Jul Aug Sep O ct Nov Dec

1991

1992

1993

1994

Figure 2 : Seasonal Variation of the Data Collection


July 1996 Page 8

The airborne equipment included the necessary functions to download the recorded data at threedifferent sites in Europe: IBERIA in Spain, FINNAIR in Helsinki, Finland, and at the European base ofDELTA Airlines in Frankfurt.

This system provided encounter recordings on a low number of 42 TCAS events, over the four years.

Mode ‘S’ data link to ground stations.

The recording and collection of TCAS II alerts from Mode-S radar stations, is automatic and is able toprovide statistics for the RA rates and characteristics, within the limited area of the coverage of theMode ‘S’ radar station.

This system has been developed and used in the UK, at their Experimental Mode ‘S’ stations.

During the evaluation period France also implemented procedures to receive TCAS data from theirMode ‘S’ station but no data was available during this reporting period.

This data can be used to initiate both the collection of controller reports, from ATC Centres, and thecollection of recorded radar data,associated with the relevant events, from the relevant ACC recordingsystems.

The ACAS data received by Mode ‘S’ stations has been used in completing the analysis of 225 TCASevents.

Pilot and Controller Reports

Forms were designed, at the start of the evaluation, to gather information and subjective assessmentsof TCAS events from both Pilots and Controllers.

These questionnaires have provided the main source of information on TCAS encounters to theEvaluation. The EEC Evaluation Centre received 911 pilot reports and 601 controller reports.

Ground Radar Data.

The procedure established for the data collection allowed, when feasible, for the recovery of therecordings of radar tracks of aircraft involved in TCAS events.

This source of information was available for 642 of the reported European TCAS events

Table 2: Reports on European events by Data SourceData Source TotalPilot source 911Controller source 601Radar recording 642Mode S recording 225Airborne recording 42


July 1996 Page 9

Correlation of Data

The correlation of the various data sources is shown in Table 3. In this table, the possiblecombinations of presence of information from either of the sources are listed in lines for Controllerand Pilot sources, and in columns for Airborne and Radar sources. The percentage of reports foreach combination of sources are given at intersecting cells of the table.

In the table, ‘Radar Source’ includes data from both Mode ‘S’ reports and Radar Recordings.

This table shows that 46% of events have been described by pilot reports not correlated with ATCreports but occasionally with Airborne or Radar data.

The rather low correlation of pilot and controller information may be questionned, while thereporting procedure requested that the pilot contact ATC to advise that a TCAS ResolutionAdvisory has required a deviation from the current ATC clearance.

Doing a similar analysis on the basis of corrective RAs only, the proportion of events notsupported by controller reports is even higher at 51%

This may be because the pilot made no report to ATC, the controller did not observe the event,considered the deviation from the ATC clearance was insignificant or was too busy to completethe report form.

4.1.3 Contributors

The TCAS users have been actively involved in providing the data used in this report. The list ofcontributors and the various data sources are shown in Table 4 & Table 5 below.

The list of contributing airlines is shown in Table 4, the number of pilot reports provided is alsoindicated. A very high proportion of these reports were provided by British Airways and KLM, whohad started to equip their aircraft with TCAS at the very early stages of the evaluation.

Additionally, a high number of events, at this stage of the evaluation, were associated withspecific TCAS equipment installation problems.

It is also apparent that a very low number of reports have been received from North Americanairlines, although there was a request to provide reports of events occurring in ECAC airspace.

This shortfall in anticipated reports may be due to the different approach adopted in the US TCASTransition Programme (TTP), which requested reports on TCAS events considered to besignificant rather than the rules adopted for the European Evaluation which requested reports onall TCAS Resolution Advisory events if possible.These reports should have been forwarded atthe level of Regional Evaluation Centres.

Table 3: Correlation of Data SourcesAirborne source NO NO YES YESRadar source YES NO YES NO Total

Controllersource

Pilotsource

NO NO 10.10% 2.37% 12.47%NO YES 8.95% 37.15% 0.34% 46.44%YES NO 14.44% 10.92% 0.07% 25.42%YES YES 12.34% 3.25% 0.07% 15.66%

Total 45.83% 51.32% 0.07% 2.78% 100.00%


July 1996 Page 10

Table 4 : Number of pilot reports provided by AirlineOperator name Number of events reportedAIR 2000 28AIR FRANCE 83AIR MADAGASCAR 1AIR PORTUGAL 3AMBASSADOR AIRWAYS 1AMERICAN AIRLINES 5AUSTRALIA ASIA AIRLINES 1AUSTRIAN AIRLINES 7AVIANCA DE COLOMBIA 1AVROL INT'L AEROSPACE 2BALAIR AG 4BRITANNIA AIRWAYS 137BRITISH AIRWAYS 410CALEDONIAN AIRWAYS 25CANADIAN AIRLINES INT'L 3CATHAY PACIFIC 41CAVEI AVIR LEMITANIM 1CHINA AIRLINES 1CONDOR FLUGDIENST GMBH 18CONTINENTAL AIRLINES 4DELTA AIRLINES 14DEUTSCHE LUFTHANSA AG 24FINNAIR 24FORD AIR TRANSPORTATION 3HAPAG-LLOYD FLUG 8IBERIA 10IRISH AIR CORPS 6JAPAN AIRLINES 2KIMBERLY CLARK CORP 1KIRK AS ALCATEL 1KLM ROYAL DUTCH AIRLINES 450LAUDA AIR 7LOT - POLISH AIRLINES 1LUFTTRANSPORT 27MARTINAIR 73MONARCH AIRLINES 37PETROLAIR S.A. 1QANTAS AIRWAYS 20SAAB SCANIA 1SABENA 14SCANDINAVIAN AIRLINES SYSTEM 77SINGAPORE AIRLINES 3SPANAIR 1SURINAM AIRWAYS 2SWISSAIR 97TRANSPORTES AEROS DE CABO VERDE 1TRANSWEDE AIRWAYS 5TRINIDAD-TOBAGO BWI INT'L 1UNITED AIRLINES 3UNITED ARAB EMIRATES 1VIRGIN ATLANTIC AIRWAYS 94


July 1996 Page 11

Table 5 lists the States which have provided ATC Reports.

A high proportion of the TCAS event reports were provided by the UK/CAA, which collected thedata via the Mode-S air-ground data links at the Mode ‘S’ radar stations at Gatwick and Malvern.However, these experimental stations do not record continuously. The Mode-S station onlyprovided a small proportion of the event reports, most were from pilots and controllers.

4.2 Statistical description of TCAS Events

The report now concentrates on the analysis of TCAS events that have been reported in theEuropean airspace of the ECAC States.

As not all fields were completed in many questionnaires, statistical statements will refer todifferent sample sizes, which depend on the availability of all the relevant information and on thedata sources.

The sample sizes will be also be stated when relevant.

4.2.1 Type of Reported Events

The principal objective of the data collection, and subsequent TCAS event analysis, was to gatherinformation on the TCAS II events which generated Resolution Advisories (RAs).

However, not all reports deal with RAs. In some cases reports dealt, specifically, with TrafficAdvisories (TAs), while in other cases it is not possible to identify if the report was made as aresult of a TA or an RA, or even if the report was actually TCAS related.The spread of event types is given in Table 6, below.

Table 5 : Number of ATC reports provided by StateState Number of ATC ReportsAustria 3China 1Czech Republic 2Denmark 1France 56Germany 78Hong Kong 34Hungary 1Ireland 6Italy 8Maastricht UAC 66Netherlands 34Spain 11Sweden 9Switzerland 13United Kingdom 296United Arab Emirates 1USA 1Total 621


July 1996 Page 12

4.2.2 Location of Events Reported

Table 7 shows the location and number of the reported TCAS events, within European airspace.

In order to differentiate between events in the vicinity of Airports and those in the En-Routephase of flight a breakdown between the reports at high and low level , across Europeanairspace, is shown in Table 8. An arbitrary level of 1500 ft has been selected to distinguishbetween high/low TCAS events

The proportion of events in lower aispace represents about a third of the entire sample.This istypical of reported TCAS events in Europe. Whilst the US TTP data collection exercise hasshown a higher proportion of TCAS events reported in the lower airspace.

4.2.3 Altitude of reported events

A more detailed analysis of reported TCAS events by altitude band, for the entire ECAC sample,is shown in Figure 3 .

Table 6 : Number of Events per Advisory TypeEvent Type CountRA 1219TA Only 204Undefined 45Total 1468

Table 7 : Location of EventsState Total

AUSTRIA 20BELGIUM 31BULGARIA 4CANARIES 19CYPRUS 2CZECH REPUBLIC 1DENMARK 9FRANCE 82GERMANY 134GREECE 67HUNGARY 11IRELAND 11ITALY 45NETHERLANDS 70NORWAY 1POLAND 11PORTUGAL 7ROMANIA 2SPAIN 40SWEDEN 13SWITZERLAND 36TURKEY 19UK 520YUGOSLAVIA 13

Table 8 : Vertical distribution of events byState

State LOW HIGH TotalAUSTRIA 1 19 20BELGIUM 2 26 28BULGARIA 0 4 4CANARIES 12 6 18CYPRUS 1 1 2CZECH Republic 0 1 1DENMARK 3 7 10FRANCE 16 58 74GERMANY 16 97 113GREECE 13 52 65HUNGARY 0 11 11IRELAND 2 9 11ITALY 1 43 44NETHERLANDS 34 34 68NORWAY 1 0 1POLAND 0 11 11PORTUGAL 3 4 7ROMANIA 0 2 2SPAIN 9 27 36SWEDEN 4 7 11SWITZERLAND 9 25 34TURKEY 0 19 19UK 204 275 479YUGOSLAVIA 1 12 13


July 1996 Page 13

There is a significant concentration of reported TCAS encounters in the altitude band Flight Level250 to Flight Level 300, which may be attributable to the high percentage of ‘short haul’ flightsoperating at these levels, within the European States generally and more particularly in the highdensity traffic areas of western European airspace.

4.3 Analysis of Resolution Advisories

4.3.1 Type of resolution advisories

Resolution Advisories are qualified as follows.• Preventive : RAs which do not require a change in the current flight path;• Corrective : RAs requiring the pilot to take action and alter the vertical flight profile.

There were 1065 reported events, where both altitude information and an indication of the type ofRA were available. Analysis shows the high proportion of 84% were Corrective RAs, whilst only16% were Preventive RAs (Figure 4).

0

50

100

150

200

250

300

350

400

450

0-50 50-100 100-150 150-200 200-250 250-300 300-350 350-400 400-450

Figure 3 : Distribution of Events By Altitude Band

C o r r e c t iv e8 4 %

P r e v e n t iv e1 6 %

Figure 4 : Types of RA


July 1996 Page 14

It has been found , as shown in Figure 5 above, that Preventive RAs occur predominantly in thealtitude layer Flight Level 250 to Flight Level 300.

Similarly the large majority of TCAS events reported at low level are Corrective.

The chart (Figure 6 above) shows the distribution of the RA Sense for all the reported CorrectiveRAs, where detailed RA information was available. This shows that a higher proportion of RAsinstruct the pilot to Climb.

0

50

100

150

200

250

300

0-50 50-100 100-150 150-200 200-250 250-300 300-350 350-400 400-450

Corrective

Preventive

Figure 5 : Altitude frequency of RA by Type

448

62

229

65

0

50

100

150

200

250

300

350

400

450

Climb Reduce Descent Descend Reduce Climb

Figure 6 : Sense of RA


July 1996 Page 15

4.3.2 Pilot reactions to Resolution Advisories

Not Followed30%

Followed70%

Figure 7 : Pilot Reaction to Corrective RAs

In 70% of the reported events the pilot followed the Corrective RA.

In the majority of cases, where the RA was not followed, the pilot reported visual acquisition ofthe intruder.

45

235

132

181

0

50

100

150

200

250

Not Followed Followed

Not Visual

Visual

Figure 8 : Pilot Reaction and Visual Acquisition


July 1996 Page 16

4.3.3 Deviations following Resolution Advisories

The initial design of TCAS II algorithms were such that aircraft’s vertical deviation in response to acorrective RA would seldom exceed 300ft.

Figure 9, below, shows the pilot’s estimations of their vertical deviations from their ATC ClearedFlight Level after following a Corrective RA.

In a sample of 506 TCAS events, the vertical deviation was 300ft, or less, in only 212 cases.

The reported vertical deviations show that in more than 50% of the cases the verticaldisplacement was more than 300ft.

Although sometimes required by TCAS, large deviations of more than 1000ft with some very largedisplacements of up to 4500ft, which could be attributed to inadequate TCAS training were alsoreported.

Such large deviations, in response to corrective RAs, which are not coordinated with ATCcaused considerable concern to operational ATC staff.

4.3.4 Aircraft proximities

Details on the proximity of the ‘intruder’ when the RA was first generated, were available for 681reported TCAS events, where both the range and relative altitude of the threat were known orestimated.

In 5% of the events (34) the intruder is in close proximity, inside 2nm and 300ft vertically.

The largest percentage of intruders were within 1000-2000 ft and within 2 to 6 nm.

212

167

97

30

0

50

100

150

200

250

0-300 300-600 600-1000 >1000

Figure 9 : Vertical Deviations


July 1996 Page 17

It must be stressed that this information is based on snapshot of separations at the time of the RAand give no indication of the evolution of the event or the separation at the Closest Point ofApproach.

4.4 Configuration of events

4.4.1 Flight Phase

The data on the phase of flight was extracted from 721 pilot reports. The table above indicatesthat the largest number of TCAS events occured in Cruise, Climb or Initial descent phases. Only11.4% occured in the approach segment or on final approach.

Table 9 : Distribution of RAs in Distance & HeightRelative Altitude (ft)

Range (NM) 0 - 300 300 - 600 600 - 1000 1000 - 2000 > 2000 Total0-2 5.29% 2.20% 3.23% 4.70% 1.17% 16.59%2-4 3.52% 2.35% 6.02% 12.19% 4.26% 28.34%4-6 2.79% 3.38% 5.29% 11.01% 4.41% 26.87%6-8 2.20% 1.03% 3.52% 5.14% 3.67% 15.57%8-10 0.88% 0.59% 2.35% 1.32% 0.88% 6.02%10-12 0.59% 0.59% 2.20% 1.62% 4.99%12-14 0.15% 0.15% 0.15% 0.44% 0.88%14-16 0.15% 0.15% 0.15% 0.15% 0.59%18-20 0.15% 0.15%Total 15.57% 10.28% 20.70% 37.30% 16.15% 100.00%

Table 10 : RAs by Flight PhaseFlight Phase Number of

events% of sample

Take off 10 1.39%Final 18 2.50%Hold 26 3.61%Approach 62 8.60%Initial descent 124 17.20%Climb 175 24.27%Cruise 306 42.44%


July 1996 Page 18

4.4.2 Evidence of aircraft manoeuvres

Additional data, on altitude and vertical speed, from 934 TCAS event reports show (Figure 10)that broadly similar percentage of climbing and descending aircraft reported TCAS events.However, more than half of the TCAS event reports came from aircraft in level flight.

4.4.3 Conflict geometries

Data on the encounter geometries, extracted from various sources, has been used to show thescatter distribution and configuration of conflicting aircraft pairs in 1061 events

Table 11, below, is encoded as follows :

The attitude of the equipped aircraft and of the intruder are indicated by letters (C: Climbing, D: Descedingt, L: Level);

A second letter L indicates cases where the aircraft has levelled, after a climb or a descent.

The most frequent conflict geometries are with ‘own’ aircraft being level and the intruder beingeither level or climbing towards the equipped aircraft to level off.

This last geometry has triggerred numerous unnecessary “Nuisance” RAs. As a result of the datafrom the European Evaluation, and the US TTP, Change Proposals were developed to modify theTCAS II software to reduce the frequency of alerts with these geometries.

Level52%

Climb23%

Descent25%

Figure 10 : Aircraft Attitude

Table 11 : Conflict GeometriesIntruder Profile

Own Aircraft Profile C CL D DL L TotalC 1.90% 0.36% 4.07% 0.18% 6.32% 12.83%

CL 0.09% 0.45% 3.52% 4.79% 8.85%D 2.80% 0.18% 2.62% 0.36% 6.32% 12.29%

DL 0.81% 5.15% 0.18% 4.34% 10.48%L 6.14% 13.28% 5.06% 3.61% 27.46% 55.56%

Total 11.65% 19.06% 12.38% 7.68% 49.23% 100.00%


July 1996 Page 19

4.5 Assessment of TCAS

4.5.1 Controllers’ assessment

Table 12 : Controllers’ assessement of Pilots’ reactionJustified pilot action Corrective Preventive (Undefined) Total

No 234 13 48 295Yes 65 1 23 89Total 299 14 71 384

ATC reports were provided for 601 TCAS events in European airspace. Only 384 reportsincluded responses to the question on justification of the pilot’s action. In 295 cases thecontroller’s opinion was that the pilot’s action was not justified.

In 544 reports, in which comment on the level of distraction or disruption to the controller’s current

Non disruptive67%

Disruptive33%

Figure 11 : Disruption caused to Controllers by TCAS events

125

186

2827

92

32

0

20

40

60

80

100

120

140

160

180

200

Low Normal High

Not disruptive

Disruptive

Figure 12 : Disruption to ATC & Controller Workload


July 1996 Page 20

ATC tactical plan was made, most events (67%) were reported as being non disruptive.

The relationship between workload, as perceived by the controller, and the disruptive effect ofRAs, is shown above.

Controller comment was included in 492 of the 601 controller TCAS event reports.

The percentage of RAs which occured during periods of high workload is small.

In these cases, the TCAS events have been slightly more disruptive than in normal or lowworkload conditions, when the events were generally considered to be non disruptive.

4.5.2 Pilots’ assessment

Comments, by pilots, on the relevance of the RAs, was provided in 77% of the pilot reports.

RAs were found “Necessary” in 198 cases (28%) and “Useful” in 156 cases (22%).

However, 50% of the RAs are considered as “Nuisance”.

These ratios are similar for Corrective and Preventive RAs.

No significant variation in the ratio of pilot assessments were seen during the evaluation.

Necessary28%

Nuisance50%

Useful22%

Figure 13 : Pilot assessment of RAs


July 1996 Page 21

The relation between the pilot’s appraisal and their reaction to Corrective RAs is shown above.

The relationship between their overall appraisal of the event and the fact that the RA wasfollowed or ignored is generally consistent.

However , 54% of RAs followed were “appraised” as being in the “Nuisance” category.

14

125

37

138

150

81

0

20

40

60

80

100

120

140

160

Necessary Nuisance Useful

Not followed

Followed

Figure 14 : Reactions to RAs and pilot assessment


July 1996 Page 22

5. ANALYSIS OF EVENTS

5.1 General appraisal mechanism

To provide a consolidated view on the largest possible number of reported TCAS events amechanism was established to categorise each event, on the basis of all available informationfrom the various sources. This is particularly useful as one particular view of any event may bedistorted.

Three main categories of events have been established :

• Compatible : RA, either preventive or corrective, which reinforces the ATC clearance• Nuisance : RA, usually corrective, in controlled situation where normal separation was being

maintained• Useful : RA, usually corrective, where normal separation was significantly infringed

The evaluation of TCAS events has shown the general categories to be distributed as shownabove.

5.2 Identification and Analysis of Operational Issues

During the analysis of TCAS event reports the EAG developed a classification system to indicateareas where the operational use of TCAS II could cause problems within the established AirTraffic Management Systems,

The operational issues, which were initially detected by the US TTP, provided a basis for theidentification of both functional and system problems.

Particular operational issues were monitored throughout the period and are described brieflybelow.

Compatible29%

Nuisance52%

Useful19%

Figure 15 : Overall Appraisal of Events


July 1996 Page 23

Some encounters raised more than one issue.

Specific problem areas are identified below :

• Technical problems: linked to invalid system behaviour or system’s installation• RAs generated due to specific Altitude station keeping systems (Soft Hold)• 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 displacement 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 a/c• Visual clearance /aquisition and separation

For each of these cases, which are not necessarily mutually exclusive, the percentage of theadvisory appraisal for each category (Compatible, Nuisance, Useful) is shown in Table 13 below.This table is based on those events only which had received such a categorisation.

More detailed explanations of these basic problems are then given. Events involved in thesecategories have been further screened to identify possibly significant features.

Table 13 : Appraisal of events for specific issuesIssues Compatible Nuisance UsefulTechnical issues 2.44% 92.68% 4.88%Altitude station keeping 60.00% 35.88% 4.12%Airport operation 0.00% 40.00% 60.00%High vertical rate (>1500 ft/min) 24.94% 69.41% 5.65%Bump-up scenario 21.81% 76.86% 1.33%Holding pattern operation 13.79% 65.52% 20.69%Large disp (>500'[<290] else >1000') 2.00% 74.00% 24.00%Large HMD 5.77% 94.23% 0.00%Local aerodrome traffic 21.43% 57.14% 21.43%Low altitude advisories 18.42% 60.53% 21.05%Military traffic 14.81% 50.62% 34.57%Non altitude reporting traffic 0.00% 83.33% 16.67%Non-airborne intruder 0.00% 88.89% 11.11%Reduced separation with 3rd a/c 0.00% 100.00% 0.00%Inadequate Pilot/controller reaction 13.51% 63.51% 22.97%Visual clearance 14.29% 42.86% 42.86%

5.2.1 Technical issues

42 RAs are considered to be associated with technical problems which were detected in theearly stages of the US TCAS Transition Programme (TTP).


July 1996 Page 24

These problems included altitude corruption, phantom intruders, TCAS Failure to display andinstallation specific problems.

23 of these RAs were identified as false. Others were considered as misleading nuisanceadvisories.

Non-altitude reporting traffic & Non-airborne intruders can also be considered in this category.

5.2.2 Altitude station keeping

TCAS events which occured when both aircraft were cruising in level flight, with standard ATCvertical separation, were reported frequently from the beginning of the European operationalevaluation.

However these events were found to involve aircraft of the same type, operated by a singleairline. It was established that these unnecessary advisories were induced by fluctuations ofaround +/-100ft in own, or intruder, aircraft's altitude. This type of event, which is caused by a“soft altitude hold” function in the FMS, has persisted throughout the evaluation.

For the period up to the end of 1994, 171 TCAS RA event reports have been identified asaltitude station keeping problems.

146 ot these cases occured in the altitude band Flight Level 200 to Flight Level 300.

The RAs were almost equally preventive or corrective. The corrective RAs were mainly classifiedas Nuisance advisories. However, 43 of these RAs were considered Useful by pilots, which maybe indicating an increased situational awareness, by aircrew, of surrounding traffic.

The consolidated advisory appraisal shows that only 7 RAs could have been considered Useful insuch encounters.

5.2.3 Bump-up scenarios

The “Bump-Up” case is characterized by one of the aircraft being level and the other climbing ordescending to level-off 1000ft above or below .

This geometry is usually associated with the high vertical climb/descent rates of themanoeuvering aircraft.

These circumstances were identified 357 times in the civil/civil environment, with a further 27instances of military/civil interaction.

Most resolution advisories were corrective (311 cases) and two/thirds were followed by thepilots (208 cases).

Altitude deviations of less than 900’ were seen in 158 cases, while deviations were less than300’ in only 34 instances.

5.2.4 Military traffic

95 TCAS encounters have involved military traffic.

There has been a steady increase in reports of this type of event during the evaluation.


July 1996 Page 25

TCAS resolution advisories involving military traffic were recorded in at least 15 States, as shownbelow. Most occurred above FL150 with only 8 “Military” events reported below this Flight Level..

The maximum vertical displacement, recorded amongst 50 events was 1500ft with otherdeviations evenly spread below this figure.

In 31 of the 41 events, where there was comment, pilots considered the RAs either Useful orNecessary.

Within the overall sample, Air Traffic Controllers considered that, in 41 events, the pilot’s actionwas unjustified.

A further analysis of the pilot and controller comments, on military associated TCAS events, hasshown that additional features have contributed to the “military” encounters, as summarised inTable 18.

The contributory factors are given in decreasing order of frequency. It should be noted that thelargest number of Military encounters (25) involved either formation flights or patrols.

Table 14 : Location of eventsassociated with military traffic

State EventsBELGIUM 3CANARIES 1CZECH REPUBLIC 1DENMARK 1FRANCE 15GERMANY 10GREECE 1ITALY 2NETHERLANDS 8PORTUGAL 1ROMANIA 1SPAIN 3SWITZERLAND 5TURKEY 2UK 30Unknown 11Grand Total 95


July 1996 Page 26

5.2.5 Holding pattern operation

35 events have been reported as having occurred in Holding Patterns, or interacting with traffic ina holding pattern.

RAs were mostly corrective (28) and were generally followed (20).

The cases were often associated with the “Bump-Up” initiated events.

The analysis of the events concluded that 19 cases were Nuisance RAs, 4 Compatible, 3 False; 6cases could be considered as Useful and 3 had not enough information to be analysed.

The action of the pilot was considered unjustified, by ATC, in 14 instances.

5.2.6 Large Horizontal Miss Distances (HMD)

52 Resolution Advisories were generated when a significant horizontal separation existed.

The most frequent geometries of these encounters are level/level (7) and climb/descent (11).

Only 6 of these RAs had horizontal separation of between 2.5 and 5 NM, while for all the othersthe miss distances were greater than 5 NM.

Table 15 : Features of Military EventsFeature TotalPatrol /Formation Flights 25Coordination with ATC - RA not followed 10RT Frequency occupied unnecessarily 5RA followed in Coordination with ATC 4Interception 3Head-on & co-altitude Aircraft - Not Controlled Situation 2Intruder not seen by ATC 2Level/Level Geometry with normal separation 2Approach 1Head-on co-altitude Aircraft - Controlled Situation 1Military not following rules 1No Clear of Conflict 1Normal separation / Situation controlled 1Respective Pilot/ATC roles unclear 1

Table 16: Deviation caused by Large HMD RAsDeviation Total0-300 8300-600 5600-900 9900-1200 31200-1500 01500-1800 11800-2100 1


July 1996 Page 27

Vertical deviations from the normal flight path, following these RAs, are shown in Table 16 aboveand indicate the significant level of disruption caused by these events.

It is expected that, as a result of the European evaluation data identifing this issue, futureversions of TCAS Logic will include a Horizontal Miss Distance filter to eliminate most of theseRAs.

5.2.7 Airport Operations

36 events were reported as having occurred in the airport traffic environment.

7 events were related to specific runway configurations.

23 events were as a result of interaction with local aerodrome traffic.

6 of the events led to ‘own aircraft’ carrying out a missed approach procedure (go-around).

Specific features of the events:

Case Description FrequencyCaused by local Helicopter traffic. 3 cases led to go-around procedures. 17Involving GA traffic or military at low altitude at airport 4Approach not cleared - Useful RA in airmiss situation 3Late initiation of descent at fix 3

Two specific cases of interest are summarised below.

Event Description1946 An airmiss situation developed just before the aircraft intercepted the ILS due to a

second aircraft converging to the same approach. The situation was resolved by aclimb RA.

2076 Approach on parallel runways. This TCAS avoidance was reported by a pilot. Theflight was under radar vectoring for runway 10. The plane is stable at 4000 ft, oninterception with localizer 10, 180 Kts speed, when a TA is actived. The traffic is at9 o’clock, 800 ft above descending, converging trajectories. No visual contact, it’sday but IMC flight conditions intermittently. At this moment, the pilot declares thatATC authorised descend to 3000 ft, the traffic is at +400 ft. Checking 3000 ft, thepilot informed ATC of the traffic detected by the TCAS at 400 ft below by mistake.The ATC corrected: «above». At this moment, a RA is actived : « Climb Climb ».After 1 or 2 seconds, the pilot switched off the auto-pilot and followed the RAClimb. The vertical speed was 1500 fpm, the traffic (red circle on ND) was at 100 ft,practically on own aircraft symbol. The pilot reports a sequence of TCAS orders as :Climb / Monitor Vertical Speed / Clear of conflict. The highest point was at 4700 ft,radio silence during all the avoidance. The visual contact was got at the end of theavoidance, a B737 at 9 o’clock, trajectory a little divergent towards localizer 09 ofthe same airport. An airmiss was filed. The subsequent approach went normally.No announce to the passengers, the avoidance was relatively soft.After landing, ATC, explained the accident by 3 contributory events:- Separation a little too small.- The TCAS aircraft late descent towards 3000 ft.- TCAS conflict with ATC clearance.The pilot expressed that stress was felt by the crew during the incident. Hesuggested that a better partition of tasks during a TCAS avoidance manoeuvre, andbetter annunciations, would reduce the stress felt by the crew.


July 1996 Page 28

5.2.8 Low altitude advisories

Table 17 : Appraisal of low altitude RAsEAG advisory appraisal

Altitude Compatible False Nuisance Useful0-1000 4 1 13 31000-2000 1 1 8 32000-2500 0 0 2 2

Results in Table 17 show the appraisal of events having been observed below 2500ft.

Several events concerned interaction with other low altitude traffic.

TCAS normally inhibits RAs at levels below 1000ft. However , some RAs reported within thisthreshold were considered Useful.

One occurrence of a descent RA soon after take off was reported .

Two events were triggerred by the interraction between aircraft landing and taking -off.

5.2.9 Reduced separation with 3rd a/c

Reduced separation, with a third aircraft, as a result of RAs, was reported in 9 TCAS events.

These RAs were all ‘corrective’. The reports indicate that, in the majority of cases, the aircrew’assumed’ that the TCAS II equipment only considered “own” aircraft and the first “intruder”.

However, in all these events other operational issues were also raised. The large verticaldisplacements required induced the feeling of increased risk in both the aircrew and controllers.

A summary of 7 of these events is given below; among the two remaining cases one RA was notfollowed, the other did not have an 3rd aircraft identified.

Event Summary80 The 3 a/c involved in this event were all subject to normal separation standards.

The “Bump-Up” was initiated , which reduced separation with the third party.The final altitude margin was 1200ft between the affected aircraft. Return to thepreviously cleared level was coordinated with ATC.

347 A “Bump-up”(down) initiated, with own aircraft level and intruder aircraftdescending to level-off at 1000’ above . The third aircraft was below in a holdingpattern.

736 The pilot “claimed” that an RA was initiated against two aircraft.

743 An exagerated displacement of 700’, in response to an RA, caused concern toATC because loss of (standard ?) separation with a 3rd aircraft occurred.


July 1996 Page 29

Event Summary934 ATC intervened to separate a third aircraft from an aircraft making an excessive

climb in response to a “Bump-up” type RA. .

971 Discussion on RT about a Descent RA. A third aircraft, having visual contact withthe TCAS equipped aircraft, took initiative to descend, as the TCAS equippedaircraft had already started his descent, while additionly turning right to “avoid” thethird aircraft.

1768 A crossing military formation generated an RA. Controller anticipated risk ofreduced separation with opposite direction traffic at another level as ‘own’ aircraftresponded to the RA. However the RA timing was correct and no problem occurred.The controller reported that he was was distracted by the event.

5.2.10 TCAS Coordination

In 28 TCAS encounters, in European airspace, two TCAS equipped aircraft were involved.

Most of these RAs were followed; 7 were considered Useful.

The analysis of these events has not revealed any problems of incorrectly coordinatedencounters.

5.2.11 Large altitude displacements

Statistics on excessive vertical manoeuvres are shown below in Table 18.

Table 18 : Features of large vertical deviationsLevel Max Deviation (ft) Average Deviation (ft) Events0-100 1500 854. 11

100-200 2000 906. 33200-300 4500 957. 53300-400 2500 1588. 9Overall 4500 984. 106

5.2.12 Visual acquisition

Table 19 below shows the correlation between Pilot Appraisal and the AEG overall appraisal ofan event, specifically in relation to the visual aquisition, or not, of the intruder.

In a perfect correlation these appraisals would provide the maximum possible numbers in theshaded diagonal areas of the table. Nevertheless there is generally a good correlation betweenthe pilot and general categorisations.

However, where there was visual acquisition, RAs considered as ‘Compatible’ after analysis, werewidely categorised as ‘Nuisance’ RAs by the aircrew.


July 1996 Page 30

5.3 Incident reports associated with TCAS alerts

A total of 84 Airmiss Reports were associated with TCAS events.

Table 20 below, indicates the percentage of Airmiss reports for each event categorised by theAEG.

“Useful” RAs generated incident reports in 22% of TCAS encounters.

Table 20: Percentage of Airmiss filed per category of Advisory

EAG advisory appraisal Airmiss filedCompatible 2.30%Nuisance 3.10%Useful 22.38%Undefined 3.58%

5.4 Analysis of Traffic Advisories

204 TCAS event reports were classified as “TA Only”.

Most of these TA reports were issued by ATC. Not all of them can be positively identified as TAs,due to lack of data or the use of incorrect terminology in the reports.

Analysis of the majority of these reports has shown how TAs are used operationally and hasidentified some related problems, which are summarized in Table 21 .

Many of the TAs led the pilot to request “traffic information” from ATC. Most of these requestswere fully justified as they related to non Mode ‘C’ targets.

Several reports of avoiding action based exclusively on a TA have been received. Some of theseevents were justified and/or co-ordinated with ATC.

Table 19 : Influence of Visual Acquisition on Assessment of RAsAEG advisory appraisal

Pilot appraisal Compatible Nuisance UsefulNecessary 14 23 43

No Visualacquisition

Nuisance 26 88 1

Useful 26 23 13

Necessary 26 15 58

Visualacquisition

Nuisance 68 109 1

Useful 41 27 26


July 1996 Page 31

Table 21 : Features of Traffic Advisories

Feature EventsATC Avoiding action before TCAS 1ATC coordination initiated following TCAS RA 2Climb/Descent on basis of TA - unjustified 3Frequency occupied disrupting other action 7Imprecise phraseology to report an advisory 4Intruder aircraft on ground 3Level off on TA against military - Justified 1Mutual information with intruder pilot on TA 1No conflicting traffic 3Non Mode C Traffic 17Not TCAS related (GPWS) 1Pilot Familiarisation with TCAS 1Proximate advisory on maritime transponder 2Request for Clearance to ATC 1Request for Traffic Information on Non Mode C target 16Speed change on basis of TA - justified 1TA against GA intruder 1TA against ground based test equipment 3TA Confirming ATC Avoiding action 4TA initiating request for ATC verification 2TA used to acquire intruder - coordinated with ATC 3TA used to request avoiding action - justified 2TA used to request Traffic Information 13TA/RA undefined 12TCAS operated in TA only mode - Airmiss 1Turn on basis of TA - Coordinated with ATC - Justified 2Turn on basis of TA - Not Coordinated - Unjustified 4Turn on Non Mode C TA 1Unnecessary Reporting of TA information 6Unnecessary RT communication 2


July 1996 Page 32

6. CONCLUSIONS

The TCAS II equipment is a last resort means of preventing mid-air collisions, or near mid-aircollisions.

The parameters of the TCAS II algorithms are not related to ATC separation minima. The systemdoes not provide warning of the loss of standard ATC separation, nor, necessarily, prevent closeencounters between aircraft.

This is the reason for the percentage of alerts which have no value in collision avoidance terms,because standard separation is maintained, or only marginally lost.

Operational experience, and events analysis, has shown that TCAS II provides an enhanced levelof flight safety, particularly in the following circumstances:

I. Human error, either on the part of controllers or aircrew.

These may include:

A. controller - provision of incorrect clearances - late, or non, detection of conflictingtraffic - misunderstanding of RTF messages

B. aircrew - incorrect execution of ATC clearances, including altitude busts -

misunderstanding of RTF messages

Both A) and B), above, may be caused or exacerbated by RTF saturationcontributing to misunderstandings.

II. Airspace management constraints.

These may include:

A. Failure of Co-ordination between Air Traffic Control authorities, either civil/civil orcivil/military

B. Airspace where a mix of IFR and uncontrolled VFR traffic may operate.

C. Airspace where STCA is not available or is impractical.

III. Technical failures

These may include autopilot and FMS malfunctions, or ATC system failures.

There is a significant overall safety benefit to be gained from the use of the TCAS II system.

Nevertheless, operational problems have been identified, which have been, at times, disruptive toboth ATC and aircrew.

These include:

1) a high proportion of Nuisance Alerts, often involving encounters where the horizontal missdistance (HMD) remains close to, or in excess of, standard ATC separation,

2) excessive vertical deviation, in response to RAs,


July 1996 Page 33

3) misuse of the TCAS II traffic display - for example: horizontal and/or vertical manoeuvresbased on displayed traffic information only.

In addition, in some ECAC States operational experience has shown that Nuisance, orinappropiate, RAs may be generated in the following situations:

1) military or civil aircraft, maintaining standard separation from the TCAS II equipped aircraft, butperforming high energy manoeuvres,

2) aircraft interceptions, when the intercepting aircraft is Mode ‘C’ altitude reporting, 3) when only one aircraft in an 'intruder' formation is squawking normal.

This underlines the requirement to maintain efficient co-ordination between civil and military ATC.

To date, the majority of evaluation reports are related to earlier TCAS II software versions, ie: upto and including Version 6.02.

Operational reports, from pilots, generally indicate, despite the identified operational problems, ahigh level of acceptance of the system, through a belief that it will enhance the safety of theiroperations.

The benefit of improved aircrew traffic situational awareness has also been noted, particularlywhen operating in areas without highly developed ATC systems.

However some controller reports indicate rather less confidence in the system and concernregarding the effects of TCAS II operations upon the ATM system.

This confidence and the appropriateness of pilot and controller reaction to TCAS II events can beimproved by providing adequate training in the capabilities and limitations of the system. Inaddition timely and reliable pilot responses to TCAS II advisories are essential and should besupported by flight simulator training.

Nevertheless it is now accepted that TCAS II will enhance the safety of the ATM system, andthere have been specific instances where controllers have reported that TCAS has alerted bothcontrollers and aircrew to potentially hazardous conflict situations.

TCAS II OPERATIONS IN 1995

Initial assessments of the currently mandated TCAS II Version 6.04A software indicate that thechanges have alleviated several of the earlier operational problems. These include, for instance,a significant reduction in the number of Nuisance Advisories.

In line service Version 6.04A software is providing an improved and effective airborne collisionavoidance system. However, as the system continues to evolve, the future Version 7 software,due for introduction in 1998, is expected to improve the operational acceptability still further.


July 1996 Page 34

GLOSSARY

ACAS Airborne Collision Avoidance SystemAECG ACAS Evaluation & Coordination GroupATC Air Traffic ControlATM Air Traffic ManagementEAG Event Analysis GroupECAC European Civil Aviation ConferenceFMS Flight Management SystemGA General AviationGPWS Ground Proximity Warning SystemHMD Horizontal Miss DistanceIFR Instrumental Flight RuleRA Resolution AdvisoryRT Radio TelephonyRTF Radio Telephony FrequencySARPs Standards and Recommended PracticesSICASP SSR Improvement and Collision Avoidance Systems PanelSSR Secondary Surveillance RadarSTCA Short Term Conflict AlertTA Traffic AdvisoryTCAS Traffic Alert and Collision Avoidance SystemTTP TCAS Transition ProgrammeVFR Visual Flight Rule


July 1996 Page 35

REFERENCES

[1]: RESULTS OF THE TCAS II TRANSITION PROGRAM (TTP)INTERIM REPORTFAA TCAS Program OfficeDecember 31, 1991

[2]: Minimum Operational Performance Standards For Traffic Alert And Collision AvoidanceSystem (TCAS) Airborne Equipment (2 volumes).Document no. RTCA/DO-185Radio Technical Commission For Aeronautics.September, 1983

[3]: Introduction To TCAS IIUS Department of TransportationFAAMarch, 1990

[4]: Procedures To Be Adopted For A Widescale Operational Evaluation of The AirborneCollision Avoidance System, TCASIISSR Mode-S Working GroupWorking paper CE 89/162/144November 2, 1989

[5]: An Interim Report on The Analysis of TCAS II Simulations Based on Radar Data CollectedIn The French Airspace.EUROCONTROL Experimental CentreEEC Note 15/1992

[6]: Justification And Rationale For Version 6.04 of The TCAS Ii Logic.The MITRE CorporationMemorandum no. F046-M-0775March 27, 1992

[7]: First Interim Report on the TCAS II European EvaluationEEC Note 17/92

[8]: Interim Report on the UK TCAS Operational EvaluationCAA / CS Report 9415 / S. Owens, C. Smith, A. Street

[9]: TCAS II Simulations based on radar data from the Maastricht UACEEC Note N° 23/94 - May, 1994 - M. Bisiaux

[10]: Results of TCAS II Simulations Based on French Samples.EEC Note 32/94 - December, 1994 - M. Bisiaux

[11]: Operational Evaluation of TCAS II in France Version 1.0CENA/R95-04 - January 1995 - Eric Vallauri


July 1996 Page 36

INTENTIONALLY LEFT BLANK

Sommaire du Rapport CEE N° 300Tâche CEE I03

Tâche EATCHIP CAS.ET1.ST12Date : juillet 1996

Traduction en français du sommaire.

Le TCAS II, (“Traffic Alert and Collision Avoidance System”) a fait l’objet d’une évaluationopérationnelle dans le contexte du développement par L’OACI de standards et recommandationsd’emploi pour les Systèmes Anti-Collision Embarqués (ACAS). L’évaluation opérationnelle dusystème s’est appuyée sur la collecte de rapports d’évènements en relation avec l’utilisation duTCAS II, déposés par les pilotes et les contrôleurs.

Le TCAS II recommande au pilote, dans le cas où le système estime un risque de collision avecun aéronef intrus, muni d’un système de report d’altitude ou d’un autre TCAS, des manoeuvresd’évitement dans le plan vertical sous la forme d’Avis de Résolution (RA).

Ce rapport se concentre sur les évènements TCAS survenus pendant la période d’évaluations’étendant de 1991 à 1994. Un forum européen a été créé en 1991 par EUROCONTROL, sousl’égide de l’OACI, afin de permettre le suivi des problèmes ACAS en Europe. Ce forumcoordonna l’évaluation opérationnelle de l’ACAS en Europe et permit de développer lesprocédures de collecte de données ainsi que d’assurer le suivi de l’évaluation et de discuter desproblèmes posés par l’emploi du TCAS II.

Depuis le 30 décembre 1993, la législation des Etats-Unis d’Amérique a rendu onligatoirel’emport et l’utilisation du TCAS II par tous les aéronefs de plus de 30 sièges, qu’ils soientnationaux ou étrangers, et volant dans l’espace aérien des USA. De nombreuses compagniesaériennes non américaines ont également équipé des appareils qui ne sont pas nécessairementexploités en direction des Etats-Unis.

Au cours des quatre ans de cette collecte de données, plus de deux mille évènements ont étérapportés dans le cadre de l’Evaluation Européenne de l’ACAS, dont près des trois quart se sontproduits dans l’espace aérien de la CEAC. Le nombre de rapports a augmenté de façon continuependant cette même période.

De nombreux utilisateurs du TCAS ont contribué à son évaluation opérationnelle. Cinquante etune compagnies aériennes et dix huit services de contrôle aérien ont fourni des rapportsd’évènements par le biais d’une procédure d’information internationalement reconnue. Unegrande proportion de ces rapports a permis de confirmer les problèmes spécifiques identifiésinitialement par le programme d’évaluation équivalent aux Etats-Unis. Des informationscomplémentaires furent fournies par des enregistrements de radar Mode-S et desenregistrements effectués à bord des aéronefs équipés.

Une densité significative de rencontres TCAS a été notée dans les bandes d’altitudes situéesentre les niveaux de vol 250 et 300, ce qui est attribuable au grand nombre de vols courts à cesniveaux de vol, en particulier dans les états d’Europe et dans les zones de forte densité de traficde l’espace européen.

Une forte proportion d’Avis de Résolution (84%) étaient de nature “Corrective” alors que 16%seulement étaient de nature “Préventive“. Dans 70% des avis correctifs, le pilote a appliquél’indication du TCAS. Pour la majorité des cas où l’avis de résolution n’a pas été suivi, le pilote amentionné qu’il avait acquis visuellement l’intrus. L’application des avis de résolution a provoquédans certains cas, des déviations de trajectoires plus grandes que celles prévues lors de laconception de l’ACAS ; ces déviations n’étaient souvent pas coordonnées avec le contrôle aérienet représentent un sujet d’inquiètude pour le personnel opérationnel. Les pilotes ont indiqué que28% des RAs étaient “Nécessaires” et que 22% des cas étaient “Utiles”. Toutefois, 50% desavis de résolution observés ont été considérés comme “Dérangeants”. Il a été possible dans laplupart des cas, par une investigation détaillée de chaque évènement, de corréler ces avis“Dérangeants” avec des problèmes opérationnels et techniques.

L’expérience opérationnelle et l’analyse des évènements ont montré que, malgré les déficiencesdes premières versions du logiciel TCAS II, jusqu’à la Version 6.02 incluse, TCAS II permetd’accroître le niveau de sécurité des vols. L’identification des problèmes opérationnels au niveaumondial par les évaluations opérationnelles a conduit à rendre obligatoire l’emport d’une Versionaméliorée du TCAS II (Version 6.04A) qui a supprimé plusieurs des problèmes opérationnelsinitiaux. Ces améliorations incluent par exemple une réduction significative du taux d’alertes“Dérangeants”. Toutefois, dans le cadre de l’évolution du système, la Version 7 du logiciel TCAS,attendue pour 1998, permettra encore d’en améliorer l’acceptabilité opérationnelle.

EUROCONTROL · for the safety of air navigation The information contained in this document is the...

Documents

Transcript of EUROCONTROL · for the safety of air navigation The information contained in this document is the...