EUROPEAN ORGANISATION FOR THE SAFETY OF AIR NAVIGATION
EUROCONTROL
EUROCONTROL EXPERIMENTAL CENTRE
European ACAS Operational Evaluation Second Interim Report
EEC Report No. 300
EEC Task I03 EATCHIP Task CAS.ET1.ST12
Issued: July 1996
The information contained in this document is the property of the EUROCONTROL Agency and no part should be reproduced in any form without the Agency’s permission. The views expressed herein do not necessarily reflect the official views or policy of the Agency. REPORT DOCUMENTATION PAGE
Reference: Security Classification: EEC Report No. 300 Unclassified Originator: Originator (Corporate Author) Name/Location: EEC - ATM EUROCONTROL Experimental Centre (Air Traffic Management) B.P.15 F - 91222 Brétigny-sur-Orge CEDEX FRANCE Telephone : +33 1 69 88 75 00 Sponsor: Sponsor (Contract Authority) Name/Location: EATCHIP Implementation Directorate EUROCONTROL Agency DEI.2 Rue de la Fusée, 96 B -1130 BRUXELLES Telephone : +32 2 729 9011 TITLE: European ACAS Operational Evaluation Second Interim Report
Author Date Pages Figures Tables Appendix References Marc Bisiaux 7/96 xii+38 15 21 - 11
EATCHIP Task EEC Task No. Task No. Sponsor Period Specification I03 CAS.ET1.ST12 - 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 the appraisal 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 events had been established under the auspices of ICAO to support and complement the development of Standards and Recommended Practices for ACAS.
The statistical presentations and the analyses of the collected data are focussed on the events recorded within the ECAC Airspace.
Although the introduction of TCAS II has raised a number of technical an operational problems, sufficient acceptance of the system and confidence has been gained that it will enhance the safety of the ATM system.
Some of the identified issues have received appropriate treatment and will be assessed in future updates of this evaluation reports.
iii This document has been collated by mechanical means. Should there be missing pages, please report to:
EUROCONTROL Experimental Centre Publications Office B.P. 15 91222 - BRETIGNY-SUR-ORGE CEDEX France
iv EEC Report N° 300 EEC Task I03 EATCHIP Task CAS.ET1.ST12 Date of Issue: July 1996
European ACAS Operational Evaluation Second Interim Report
Marc Bisiaux
EUROCONTROL Experimental Centre
Summary
The second Interim Report on the TCAS II operational evaluation in Europe provides a status of the appraisal 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 events had been established under the auspices of ICAO to support and complement the development of Standards and Recommended Practices for ACAS.
The statistical presentations and the analyses of the collected data are focussed on the events recorded within the ECAC Airspace.
Although the introduction of TCAS II has raised a number of technical an operational problems, sufficient acceptance of the system and confidence has been gained that it will enhance the safety of the ATM system.
Some of the identified issues have received appropriate treatment and will be assessed in future updates of this evaluation reports.
This report has been compiled and was reviewed in close cooperation with a group of experts actively involved in the ACAS Operational Evaluation .
v TABLE OF CONTENTS
LIST OF ILLUSTRATIONS ix LIST OF TABLES ix EXECUTIVE 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 3 3.3.1 Pilot reports 3 3.3.2 Controller reports 4 3.3.3 Flight recorded data 4 3.3.4 Mode-S and ground radar data 4
3.4 Current TCAS II Operations & Scope of the Report. 4 3.4.1 Event processing and analysis procedures 5
4. RESULTS OF THE EUROPEAN OPERATIONAL EVALUATION 6
4.1 The European Data Collection of TCAS Events 6 4.1.1 Progress of The Data Collection 6 4.1.2 Data Sources 7 4.1.3 Contributors 9
4.2 Statistical description of TCAS Events 11 4.2.1 Type of Reported Events 11 4.2.2 Location of Events Reported 12 4.2.3 Altitude of reported events 12
4.3 Analysis of Resolution Advisories 13 4.3.1 Type of resolution advisories 13 4.3.2 Pilot reactions to Resolution Advisories 14 4.3.3 Deviations following Resolution Advisories 15 4.3.4 Aircraft proximities 16
4.4 Configuration of events 17 4.4.1 Flight Phase 17 4.4.2 Evidence of aircraft manoeuvres 18 4.4.3 Conflict geometries 18
4.5 Assessment of TCAS 19 4.5.1 Controllers’ assessment 19 4.5.2 Pilots’ assessment 20
5. ANALYSIS OF EVENTS 22
5.1 General appraisal mechanism 22
5.2 Identification and Analysis of Operational Issues 22 vii 5.2.1 Technical issues 23 5.2.2 Altitude station keeping 24 5.2.3 Bump-up scenarios 24 5.2.4 Military traffic 24 5.2.5 Holding pattern operation 26 5.2.6 Large Horizontal Miss Distances (HMD) 26 5.2.7 Airport Operations 27 5.2.8 Low altitude advisories 28 5.2.9 Reduced separation with 3rd a/c 28 5.2.10 TCAS Coordination 29 5.2.11 Large altitude displacements 29 5.2.12 Visual acquisition 29
5.3 Incident reports associated with TCAS alerts 30
5.4 Analysis of Traffic Advisories 30
6. CONCLUSIONS 32
GLOSSARY 34 REFERENCES 35 Traduction en français du sommaire 37
viii LIST OF ILLUSTRATIONS
FIGURE 1 : NUMBER OF EVENTS PER YEAR 6 FIGURE 2 : SEASONAL VARIATION OF THE DATA COLLECTION 7 FIGURE 3 : DISTRIBUTION OF EVENTS BY ALTITUDE BAND 13 FIGURE 4 : TYPES OF RA 13 FIGURE 5 : ALTITUDE FREQUENCY OF RA BY TYPE 14 FIGURE 6 : SENSE OF RA 14 FIGURE 7 : PILOT REACTION TO CORRECTIVE RAS 15 FIGURE 8 : PILOT REACTION AND VISUAL ACQUISITION 15 FIGURE 9 : VERTICAL DEVIATIONS 16 FIGURE 10 : AIRCRAFT ATTITUDE 18 FIGURE 11 : DISRUPTION CAUSED TO CONTROLLERS BY TCAS EVENTS 19 FIGURE 12 : DISRUPTION TO ATC & CONTROLLER WORKLOAD 19 FIGURE 13 : PILOT ASSESSMENT OF RAS 20 FIGURE 14 : REACTIONS TO RAS AND PILOT ASSESSMENT 21 FIGURE 15 : OVERALL APPRAISAL OF EVENTS 22
LIST OF TABLES
TABLE 1 : NUMBER OF EVENTS BY AIRSPACE 6 TABLE 2: REPORTS ON EUROPEAN EVENTS BY DATA SOURCE 8 TABLE 3: CORRELATION OF DATA SOURCES 9 TABLE 4 : NUMBER OF PILOT REPORTS PROVIDED BY AIRLINE 10 TABLE 5 : NUMBER OF ATC REPORTS PROVIDED BY STATE 11 TABLE 6 : NUMBER OF EVENTS PER ADVISORY TYPE 12 TABLE 7 : LOCATION OF EVENTS 12 TABLE 8 : VERTICAL DISTRIBUTION OF EVENTS BY STATE 12 TABLE 9 : DISTRIBUTION OF RAS IN DISTANCE & HEIGHT 17 TABLE 10 : RAS BY FLIGHT PHASE 17 TABLE 11 : CONFLICT GEOMETRIES 18 TABLE 12 : CONTROLLERS’ ASSESSEMENT OF PILOTS’ REACTION 19 TABLE 13 : APPRAISAL OF EVENTS FOR SPECIFIC ISSUES 23 TABLE 14 : LOCATION OF EVENTS ASSOCIATED WITH MILITARY TRAFFIC 25 TABLE 15 : FEATURES OF MILITARY EVENTS 26 TABLE 16: DEVIATION CAUSED BY LARGE HMD RAS 26 TABLE 17 : APPRAISAL OF LOW ALTITUDE RAS 28 TABLE 18 : FEATURES OF LARGE VERTICAL DEVIATIONS 29 TABLE 19 : INFLUENCE OF VISUAL ACQUISITION ON ASSESSMENT OF RAS 30 TABLE 20: PERCENTAGE OF AIRMISS FILED PER CATEGORY OF ADVISORY 30 TABLE 21 : FEATURES OF TRAFFIC ADVISORIES 31
ix EEC Report N° 300 EEC Task I03 EATCHIP Task CAS.ET1.ST12 Date of Issue: July 1996
EXECUTIVE SUMMARY
The Traffic Alert and Collision Avoidance System (TCAS II) was introduced in operational evaluation in the context of the development by ICAO of Standards and Recommended Practices for the AIrborne Collision Avoidance Systems (ACAS). An operational evaluation of the system was based on the collection of reports on TCAS II related events from pilots and controllers as well as from data recordings.
TCAS II will, if the system calculates a risk of collision with an intruder aircraft, which is equipped with an altitude reporting transponder or TCAS, recommend avoidance manoeuvres in the vertical plane 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. A forum 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 Evaluation and 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 by all national and foreign registered passenger aircraft, of more than 30 seats, flying in US airspace. Many non-US operators subsequently equipped more of their aircraft with TCAS II than those which operate solely to the USA.
Over the 4 years of the data collection, more than two thousand events were reported to the European ACAS Evaluation, of which nearly three quarters occurred in the ECAC Airspace. The number of event reports steadily increased throughout the period.
Many TCAS users contributed to the operational evaluation. Fifty one airlines and eighteen ATC services reported using an internationally established channel. A high number of these reports confirmed specific problems identified initially by the equivalent US programme. Additional information was provided by Mode-S radar recordings and airborne recordings.
There was a significant concentration of reported TCAS encounters in the altitude band Flight Level 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 the high density traffic areas of western European airspace.
A high proportion of 84% Resolution Advisories were Corrective RAs, whilst only 16% were Preventive 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 of the xi intruder. In following corrective Resolution Advisories, larger trajectory deviations than those expected by design of ACAS were occasionally observed, which were often not coordinated with ATC 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 were considered as “Nuisance”. It was generally possible, through a detailed investigation on each event, to correlate these “Nuisance” RAs with operational and technical issues.
Operational experience, and events analysis, has shown that TCAS II provides an enhanced level of flight safety, despite the defficiencies of the earlier TCAS II software versions, ie: up to and including Version 6.02. Operational problems identified worldwide by the operational evaluations lead to mandating the implementation of an enhanced TCAS Version 6.04A which has alleviated several of the earlier operational problems. These include, for instance, a significant reduction in the number of Nuisance Advisories. 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.
xii EUROPEAN ACAS OPERATIONAL EVALUATION Second Interim Report
1. INTRODUCTION
An ACAS Operational Evaluation took place between 1991 and 1995 to assess the performance of TCAS II in the European operational environment. This report presents the results of the evaluation, covering the period 1991-1994.
The analyses carried out, within the framework of the Operational Evaluation, have confirmed that TCAS II is an effective collision avoidance system. However several operational acceptability problems were identified, which generally related to the integration of TCAS II with certain elements of the Air Traffic Management System.
This report summarizes the statistical and operational findings on the ACAS events reported over the ECAC States.
The analyses have been based on the reports provided by pilots and controllers, together with supplementary information from radar data recording and TCAS event simulations.
TCAS events have been categorised after analysis and discussion in the EEC ACAS Evaluation Group.
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2. BACKGROUND ON ACAS
Airborne Collision Avoidance System (ACAS) is a generic term for Airborne Collision Avoidance Systems. ACAS is intended to improve air safety by providing a ‘last resort’ method of preventing collisions, or near collisions.
By utilising Secondary Surveillance Radar (SSR) technology, the ACAS equipment operates independently of ground-based aids and ATC by monitoring other aircraft, in the vicinity, and assessing the which a/c represents potential collision threats by interrogating the transponders in these aircraft. However, non-transponding aircraft can not be detected.
The Traffic Alert and Collision Avoidance System was developed, principally in the US and is currently the only available implementation of an ACAS.
Three levels of TCAS are envisaged, namely TCAS I, TCAS II and TCAS IV (to meet ACAS III requirements). 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 pilot that an aircraft presents a potential threat by generating a Traffic Advisory (TA). However TCAS I does not provide collision avoidance advice. • TCAS II will, if the system calculates a risk of collision with an intruder aircraft, which is equipped with an altitude reporting transponder or TCAS, recommend avoidance manoeuvres in 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 providing horizontal resolution advisories in addition to the vertical resolution advisories provided by TCAS II.
Only TCAS I and TCAS II are commercially available. TCAS II is the system with which this report is principally concerned.
A significant proportion of European, and foreign registered, passenger carrying aircraft operating in ECAC airspace are now equipped with TCAS II. It is recognised that these aircraft are operating at an enhanced level of flight safety.
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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, to support the development of ACAS Standards and Recommended Practices (SARPs) by the SSR Improvements and Collision Avoidance Panel (SICASP). European States, together with Eurocontrol, have made a significant input to the evaluation.
SICASP has stated that an “ACAS cannot resolve all possible collisions and may cause some risk of collision“ but the “use of ACAS is expected to reduce significantly the risk of collision with aircraft operating altitude reporting transponders, provided that carriage of such transponders is sufficiently widespread.“.
ICAO ACAS operating procedures and ATC provisions for ACAS equipped aircraft are contained in PANS - OPS (Doc.8168) and PANS - RAC (Doc.4444) respectively. SARPS for ACAS I & II are contained in Annex 10.
In addition SICASP has also produced documentation ( State Letter AN 7/11.12-94/62 ) to assist States 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 Evaluation Co-ordination Group (AECG) to co-ordinate the ACAS Operational Evaluation within European airspace .
The AECG was established to develop data collection procedures, to monitor and report the progress of 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 Federal Aviation 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 in parallel with those in the airspace of the ECAC States.
The operational evaluation began officially on March 1, 1991, although the first report of a TCAS event, 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 be collected and dispatched to the regional ACAS Evaluation Centres, as soon as possible, so that the recorded radar data of the event, which is normally retained for only a short period of time, could acquired 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 a copy of the report, as soon as possible, to the regional Evaluation Centre for the airline’s State. In cases where the event occurred in the region of another Evaluation Centre a copy of the event report was also forwarded to the relevant Evaluation Centre.
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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 their attention.
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 and to 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 distribute these amongst several participating aircraft operators.
Eight recorders, and recorder downloading systems, were purchased and these were initially placed with DELTA, FINNAIR and IBERIA.
The TCAS recorder manufacturer provided training, in the use and configuring of the recorders and the downloading systems, for the European aircraft operators.
The downloading system comprised an adapted PC, an interface to the recorder, a high density storage cassette drive (at least 80 Mb) and the software required to configure the recorder to record appropriate 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 to a high density storage cassette and to dispatch this to the regional ACAS Evaluation Centre for decoding 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 basis of 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 process the data and analyse the results were made available at the EEC Evaluation Centre for those States unable 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 by all national and foreign registered passenger aircraft, of more than 30 seats, flying in US airspace.
For reasons of operational flexibility and fleet engineering commonality, many non-US operators are equipping more of their aircraft with TCAS II than those which operate solely to the USA. As a result, it is estimated that more than 20% of the large commercial aircraft flying in European airspace are equipped with, and operating, TCAS II.
In September 1991 a proposal was agreed to review each TCAS RA event systematically. An ACAS Events Analysis Group (EAG)was created with representatives of the Civil Aviation Authorities of France, Germany and the UK.
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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 RAs iii) 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 and identify 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 the evaluation TCAS II modifications were introduced. The current Version 6.04A software has alleviated several of the previously identified operational problems, and the future Version 7 software, due for implementation in 1998, will further improve the operational interface. This report adresses issues raised between 1991 and 1994, before the 100% implementation of Version 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.
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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 end of 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 outside the 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 airspace Europe USA & Canada Other Airspaces Total 1468 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 was the deadline for the implementation of TCAS II on board European carriers to comply with US Regulations.
Similarly an increase in returned questionnaires was also observed in 1994.
900 874
800
700
600
500 447
400
300
200
112 100 35
0 1991 1992 1993 1994 Figure 1 : Number of Events per Year
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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 data collection.
120
100
80
1991 1992 60 1993 1994
40
20
0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 2 : Seasonal Variation of the Data Collection
Any attempt to directly correlate this information with traffic data, or conflict statistics, will probably be 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. The number 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 direct technical insight into system performance.
For economic reasons, as well as installation complexity, only 8 recorders were purchased to equip selected aircraft.
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The airborne equipment included the necessary functions to download the recorded data at three different sites in Europe: IBERIA in Spain, FINNAIR in Helsinki, Finland, and at the European base of DELTA 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 to provide statistics for the RA rates and characteristics, within the limited area of the coverage of the Mode ‘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 their Mode ‘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 the collection of recorded radar data,associated with the relevant events, from the relevant ACC recording systems.
The ACAS data received by Mode ‘S’ stations has been used in completing the analysis of 225 TCAS events.
Pilot and Controller Reports
Forms were designed, at the start of the evaluation, to gather information and subjective assessments of TCAS events from both Pilots and Controllers.
These questionnaires have provided the main source of information on TCAS encounters to the Evaluation. 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 the recordings 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 Source Data Source Total Pilot source 911 Controller source 601 Radar recording 642 Mode S recording 225 Airborne recording 42
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Correlation of Data
The correlation of the various data sources is shown in Table 3. In this table, the possible combinations of presence of information from either of the sources are listed in lines for Controller and Pilot sources, and in columns for Airborne and Radar sources. The percentage of reports for each 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 ATC reports but occasionally with Airborne or Radar data.
The rather low correlation of pilot and controller information may be questionned, while the reporting procedure requested that the pilot contact ATC to advise that a TCAS Resolution Advisory has required a deviation from the current ATC clearance.
Doing a similar analysis on the basis of corrective RAs only, the proportion of events not supported 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 complete the report form.
Table 3: Correlation of Data Sources Airborne source NO NO YES YES Radar source YES NO YES NO Total Controller Pilot source source 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%
4.1.3 Contributors
The TCAS users have been actively involved in providing the data used in this report. The list of contributors 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 also indicated. A very high proportion of these reports were provided by British Airways and KLM, who had 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 with specific TCAS equipment installation problems.
It is also apparent that a very low number of reports have been received from North American airlines, 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 TCAS Transition Programme (TTP), which requested reports on TCAS events considered to be significant rather than the rules adopted for the European Evaluation which requested reports on all TCAS Resolution Advisory events if possible.These reports should have been forwarded at the level of Regional Evaluation Centres.
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Table 4 : Number of pilot reports provided by Airline Operator name Number of events reported AIR 2000 28 AIR FRANCE 83 AIR MADAGASCAR 1 AIR PORTUGAL 3 AMBASSADOR AIRWAYS 1 AMERICAN AIRLINES 5 AUSTRALIA ASIA AIRLINES 1 AUSTRIAN AIRLINES 7 AVIANCA DE COLOMBIA 1 AVROL INT'L AEROSPACE 2 BALAIR AG 4 BRITANNIA AIRWAYS 137 BRITISH AIRWAYS 410 CALEDONIAN AIRWAYS 25 CANADIAN AIRLINES INT'L 3 CATHAY PACIFIC 41 CAVEI AVIR LEMITANIM 1 CHINA AIRLINES 1 CONDOR FLUGDIENST GMBH 18 CONTINENTAL AIRLINES 4 DELTA AIRLINES 14 DEUTSCHE LUFTHANSA AG 24 FINNAIR 24 FORD AIR TRANSPORTATION 3 HAPAG-LLOYD FLUG 8 IBERIA 10 IRISH AIR CORPS 6 JAPAN AIRLINES 2 KIMBERLY CLARK CORP 1 KIRK AS ALCATEL 1 KLM ROYAL DUTCH AIRLINES 450 LAUDA AIR 7 LOT - POLISH AIRLINES 1 LUFTTRANSPORT 27 MARTINAIR 73 MONARCH AIRLINES 37 PETROLAIR S.A. 1 QANTAS AIRWAYS 20 SAAB SCANIA 1 SABENA 14 SCANDINAVIAN AIRLINES SYSTEM 77 SINGAPORE AIRLINES 3 SPANAIR 1 SURINAM AIRWAYS 2 SWISSAIR 97 TRANSPORTES AEROS DE CABO VERDE 1 TRANSWEDE AIRWAYS 5 TRINIDAD-TOBAGO BWI INT'L 1 UNITED AIRLINES 3 UNITED ARAB EMIRATES 1 VIRGIN ATLANTIC AIRWAYS 94
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Table 5 : Number of ATC reports provided by State State Number of ATC Reports Austria 3 China 1 Czech Republic 2 Denmark 1 France 56 Germany 78 Hong Kong 34 Hungary 1 Ireland 6 Italy 8 Maastricht UAC 66 Netherlands 34 Spain 11 Sweden 9 Switzerland 13 United Kingdom 296 United Arab Emirates 1 USA 1 Total 621
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 the data 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 only provided 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 the European airspace of the ECAC States.
As not all fields were completed in many questionnaires, statistical statements will refer to different sample sizes, which depend on the availability of all the relevant information and on the data 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 gather information on the TCAS II events which generated Resolution Advisories (RAs).
However, not all reports deal with RAs. In some cases reports dealt, specifically, with Traffic Advisories (TAs), while in other cases it is not possible to identify if the report was made as a result 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.
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Table 6 : Number of Events per Advisory Type Event Type Count RA 1219 TA Only 204 Undefined 45 Total 1468
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-Route phase of flight a breakdown between the reports at high and low level , across European airspace, is shown in Table 8. An arbitrary level of 1500 ft has been selected to distinguish between high/low TCAS events
The proportion of events in lower aispace represents about a third of the entire sample.This is typical of reported TCAS events in Europe. Whilst the US TTP data collection exercise has shown a higher proportion of TCAS events reported in the lower airspace.
Table 7 : Location of Events Table 8 : Vertical distribution of events by State Total State AUSTRIA 20 State LOW HIGH Total BELGIUM 31 AUSTRIA 1 19 20 BULGARIA 4 BELGIUM 2 26 28 CANARIES 19 BULGARIA 0 44 CYPRUS 2 CANARIES 12 618 CZECH REPUBLIC 1 CYPRUS 1 12 DENMARK 9 CZECH Republic 0 11 FRANCE 82 DENMARK 3 710 GERMANY 134 FRANCE 16 58 74 GREECE 67 GERMANY 16 97 113 HUNGARY 11 GREECE 13 52 65 IRELAND 11 HUNGARY 0 11 11 ITALY 45 IRELAND 2 911 NETHERLANDS 70 ITALY 1 43 44 NORWAY 1 NETHERLANDS 34 34 68 POLAND 11 NORWAY 1 01 PORTUGAL 7 POLAND 0 11 11 ROMANIA 2 PORTUGAL 3 47 SPAIN 40 ROMANIA 0 22 SWEDEN 13 SPAIN 9 27 36 SWITZERLAND 36 SWEDEN 4 711 TURKEY 19 SWITZERLAND 9 25 34 UK 520 TURKEY 0 19 19 YUGOSLAVIA 13 UK 204 275 479 YUGOSLAVIA 1 12 13
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 .
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There is a significant concentration of reported TCAS encounters in the altitude band Flight Level 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 the high density traffic areas of western European airspace.
450
400
350
300
250
200
150
100
50
0 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 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.
Preventive 16%
C o rrective 84%
Figure 4 : Types of RA
There were 1065 reported events, where both altitude information and an indication of the type of RA were available. Analysis shows the high proportion of 84% were Corrective RAs, whilst only 16% were Preventive RAs (Figure 4).
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300
250
200
Corrective 150 Preventive
100
50
0 0-50 50-100 100-150 150-200 200-250 250-300 300-350 350-400 400-450 Figure 5 : Altitude frequency of RA by Type
It has been found , as shown in Figure 5 above, that Preventive RAs occur predominantly in the altitude layer Flight Level 250 to Flight Level 300.
Similarly the large majority of TCAS events reported at low level are Corrective.
448 450
400
350
300
250 229
200
150
100 62 65
50
0 Climb Reduce Descent Descend Reduce Climb Figure 6 : Sense of RA
The chart (Figure 6 above) shows the distribution of the RA Sense for all the reported Corrective RAs, where detailed RA information was available. This shows that a higher proportion of RAs instruct the pilot to Climb.
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4.3.2 Pilot reactions to Resolution Advisories
Not Followed 30%
Followed 70%
Figure 7 : Pilot Reaction to Corrective RAs
In 70% of the reported events the pilot followed the Corrective RA.
250 235
200 181
150 132 Not Visual Visual
100
50 45
0 Not Followed Followed
Figure 8 : Pilot Reaction and Visual Acquisition
In the majority of cases, where the RA was not followed, the pilot reported visual acquisition of the intruder.
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4.3.3 Deviations following Resolution Advisories
The initial design of TCAS II algorithms were such that aircraft’s vertical deviation in response to a corrective RA would seldom exceed 300ft.
Figure 9, below, shows the pilot’s estimations of their vertical deviations from their ATC Cleared Flight 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 vertical displacement was more than 300ft.
Although sometimes required by TCAS, large deviations of more than 1000ft with some very large displacements of up to 4500ft, which could be attributed to inadequate TCAS training were also reported.
Such large deviations, in response to corrective RAs, which are not coordinated with ATC caused considerable concern to operational ATC staff.
250
200
150
212 100
167
50 97
30
0 0-300 300-600 600-1000 >1000
Figure 9 : Vertical Deviations
4.3.4 Aircraft proximities
Details on the proximity of the ‘intruder’ when the RA was first generated, were available for 681 reported TCAS events, where both the range and relative altitude of the threat were known or estimated.
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.
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Table 9 : Distribution of RAs in Distance & Height Relative Altitude (ft) Range (NM) 0 - 300 300 - 600 600 - 1000 1000 - 2000 > 2000 Total 0-2 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%
It must be stressed that this information is based on snapshot of separations at the time of the RA and give no indication of the evolution of the event or the separation at the Closest Point of Approach.
4.4 Configuration of events
4.4.1 Flight Phase
Table 10 : RAs by Flight Phase Flight Phase Number of % of sample events 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%
The data on the phase of flight was extracted from 721 pilot reports. The table above indicates that the largest number of TCAS events occured in Cruise, Climb or Initial descent phases. Only 11.4% occured in the approach segment or on final approach.
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4.4.2 Evidence of aircraft manoeuvres
Descent 25%
Level 52%
Climb 23%
Figure 10 : Aircraft Attitude
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 the scatter 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 being either level or climbing towards the equipped aircraft to level off.
This last geometry has triggerred numerous unnecessary “Nuisance” RAs. As a result of the data from the European Evaluation, and the US TTP, Change Proposals were developed to modify the TCAS II software to reduce the frequency of alerts with these geometries. Table 11 : Conflict Geometries Intruder Profile Own Aircraft Profile C CL DDLL Total C 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%
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4.5 Assessment of TCAS
4.5.1 Controllers’ assessment
Table 12 : Controllers’ assessement of Pilots’ reaction Justified pilot action Corrective Preventive (Undefined) Total No 234 13 48 295 Yes 65 1 23 89 Total 299 14 71 384
ATC reports were provided for 601 TCAS events in European airspace. Only 384 reports included responses to the question on justification of the pilot’s action. In 295 cases the controller’s opinion was that the pilot’s action was not justified.
Disruptive 33%
Non disruptive 67%
Figure 11 : Disruption caused to Controllers by TCAS events
In 544 reports, in which comment on the level of distraction or disruption to the controller’s current
200 186 180
160
140 125 120
Not disruptive 100 92 Disruptive
80
60
40 32 27 28
20
0 Low Normal High
Figure 12 : Disruption to ATC & Controller Workload
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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 of RAs, 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 low workload conditions, when the events were generally considered to be non disruptive.
4.5.2 Pilots’ assessment
Useful 22% Necessary 28%
Nuisance 50%
Figure 13 : Pilot assessment of RAs
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.
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160 150
138 140
125
120
100
81 Not followed 80 Followed
60
40 37
20 14
0 Necessary Nuisance Useful Figure 14 : Reactions to RAs and pilot assessment
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 was followed or ignored is generally consistent.
However , 54% of RAs followed were “appraised” as being in the “Nuisance” category.
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5. ANALYSIS OF EVENTS
5.1 General appraisal mechanism
To provide a consolidated view on the largest possible number of reported TCAS events a mechanism was established to categorise each event, on the basis of all available information from the various sources. This is particularly useful as one particular view of any event may be distorted.
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
Useful 19% Compatible 29%
Nuisance 52%
Figure 15 : Overall Appraisal of Events
The evaluation of TCAS events has shown the general categories to be distributed as shown above.
5.2 Identification and Analysis of Operational Issues
During the analysis of TCAS event reports the EAG developed a classification system to indicate areas where the operational use of TCAS II could cause problems within the established Air Traffic Management Systems,
The operational issues, which were initially detected by the US TTP, provided a basis for the identification of both functional and system problems.
Particular operational issues were monitored throughout the period and are described briefly below.
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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 the advisory 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 these categories have been further screened to identify possibly significant features.
Table 13 : Appraisal of events for specific issues Issues Compatible Nuisance Useful Technical 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 the early stages of the US TCAS Transition Programme (TTP).
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These problems included altitude corruption, phantom intruders, TCAS Failure to display and installation specific problems.
23 of these RAs were identified as false. Others were considered as misleading nuisance advisories.
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 ATC vertical separation, were reported frequently from the beginning of the European operational evaluation.
However these events were found to involve aircraft of the same type, operated by a single airline. It was established that these unnecessary advisories were induced by fluctuations of around +/-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 as altitude 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 classified as Nuisance advisories. However, 43 of these RAs were considered Useful by pilots, which may be indicating an increased situational awareness, by aircrew, of surrounding traffic.
The consolidated advisory appraisal shows that only 7 RAs could have been considered Useful in such encounters.
5.2.3 Bump-up scenarios
The “Bump-Up” case is characterized by one of the aircraft being level and the other climbing or descending to level-off 1000ft above or below .
This geometry is usually associated with the high vertical climb/descent rates of the manoeuvering aircraft.
These circumstances were identified 357 times in the civil/civil environment, with a further 27 instances of military/civil interaction.
Most resolution advisories were corrective (311 cases) and two/thirds were followed by the pilots (208 cases).
Altitude deviations of less than 900’ were seen in 158 cases, while deviations were less than 300’ 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.
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TCAS resolution advisories involving military traffic were recorded in at least 15 States, as shown below. 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 other deviations evenly spread below this figure.
In 31 of the 41 events, where there was comment, pilots considered the RAs either Useful or Necessary.
Within the overall sample, Air Traffic Controllers considered that, in 41 events, the pilot’s action was unjustified.
Table 14 : Location of events associated with military traffic State Events BELGIUM 3 CANARIES 1 CZECH REPUBLIC 1 DENMARK 1 FRANCE 15 GERMANY 10 GREECE 1 ITALY 2 NETHERLANDS 8 PORTUGAL 1 ROMANIA 1 SPAIN 3 SWITZERLAND 5 TURKEY 2 UK 30 Unknown 11 Grand Total 95
A further analysis of the pilot and controller comments, on military associated TCAS events, has shown that additional features have contributed to the “military” encounters, as summarised in Table 18.
The contributory factors are given in decreasing order of frequency. It should be noted that the largest number of Military encounters (25) involved either formation flights or patrols.
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Table 15 : Features of Military Events Feature Total Patrol /Formation Flights 25 Coordination with ATC - RA not followed 10 RT Frequency occupied unnecessarily 5 RA followed in Coordination with ATC 4 Interception 3 Head-on & co-altitude Aircraft - Not Controlled Situation 2 Intruder not seen by ATC 2 Level/Level Geometry with normal separation 2 Approach 1 Head-on co-altitude Aircraft - Controlled Situation 1 Military not following rules 1 No Clear of Conflict 1 Normal separation / Situation controlled 1 Respective Pilot/ATC roles unclear 1
5.2.5 Holding pattern operation
35 events have been reported as having occurred in Holding Patterns, or interacting with traffic in a 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; 6 cases 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).
Table 16: Deviation caused by Large HMD RAs Deviation Total 0-300 8 300-600 5 600-900 9 900-1200 3 1200-1500 0 1500-1800 1 1800-2100 1
Only 6 of these RAs had horizontal separation of between 2.5 and 5 NM, while for all the others the miss distances were greater than 5 NM. July 1996 Page 26 EUROPEAN ACAS OPERATIONAL EVALUATION Second Interim Report
Vertical deviations from the normal flight path, following these RAs, are shown in Table 16 above and 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, future versions of TCAS Logic will include a Horizontal Miss Distance filter to eliminate most of these RAs.
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 Frequency Caused by local Helicopter traffic. 3 cases led to go-around procedures. 17 Involving GA traffic or military at low altitude at airport 4 Approach not cleared - Useful RA in airmiss situation 3 Late initiation of descent at fix 3
Two specific cases of interest are summarised below.
Event Description 1946 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 a climb RA.
2076 Approach on parallel runways. This TCAS avoidance was reported by a pilot. The flight was under radar vectoring for runway 10. The plane is stable at 4000 ft, on interception with localizer 10, 180 Kts speed, when a TA is actived. The traffic is at 9 o’clock, 800 ft above descending, converging trajectories. No visual contact, it’s day but IMC flight conditions intermittently. At this moment, the pilot declares that ATC authorised descend to 3000 ft, the traffic is at +400 ft. Checking 3000 ft, the pilot 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 RA Climb. 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 the avoidance, a B737 at 9 o’clock, trajectory a little divergent towards localizer 09 of the 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. He suggested that a better partition of tasks during a TCAS avoidance manoeuvre, and better annunciations, would reduce the stress felt by the crew.
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5.2.8 Low altitude advisories
Table 17 : Appraisal of low altitude RAs EAG advisory appraisal Altitude Compatible False Nuisance Useful 0-1000 4 1 13 3 1000-2000 1 1 8 3 2000-2500 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 this threshold 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 vertical displacements 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 not followed, the other did not have an 3rd aircraft identified.
Event Summary 80 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 the previously cleared level was coordinated with ATC.
347 A “Bump-up”(down) initiated, with own aircraft level and intruder aircraft descending to level-off at 1000’ above . The third aircraft was below in a holding pattern.
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 to ATC because loss of (standard ?) separation with a 3rd aircraft occurred.
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Event Summary 934 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 with the TCAS equipped aircraft, took initiative to descend, as the TCAS equipped aircraft had already started his descent, while additionly turning right to “avoid” the third aircraft.
1768 A crossing military formation generated an RA. Controller anticipated risk of reduced separation with opposite direction traffic at another level as ‘own’ aircraft responded 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 coordinated encounters.
5.2.11 Large altitude displacements
Statistics on excessive vertical manoeuvres are shown below in Table 18.
Table 18 : Features of large vertical deviations Level Max Deviation (ft) Average Deviation (ft) Events 0-100 1500 854. 11 100-200 2000 906. 33 200-300 4500 957. 53 300-400 2500 1588. 9 Overall 4500 984. 106
5.2.12 Visual acquisition
Table 19 below shows the correlation between Pilot Appraisal and the AEG overall appraisal of an 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 the shaded diagonal areas of the table. Nevertheless there is generally a good correlation between the pilot and general categorisations.
However, where there was visual acquisition, RAs considered as ‘Compatible’ after analysis, were widely categorised as ‘Nuisance’ RAs by the aircrew.
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Table 19 : Influence of Visual Acquisition on Assessment of RAs AEG advisory appraisal
Pilot appraisal Compatible Nuisance Useful Necessary 14 23 43
No Visual Nuisance 26 88 1 acquisition Useful 26 23 13
Necessary 26 15 58
Visual Nuisance 68 109 1 acquisition Useful 41 27 26
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 the AEG.
“Useful” RAs generated incident reports in 22% of TCAS encounters.
Table 20: Percentage of Airmiss filed per category of Advisory
EAG advisory appraisal Airmiss filed Compatible 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 has identified 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 requests were 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 these events were justified and/or co-ordinated with ATC.
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Table 21 : Features of Traffic Advisories
Feature Events ATC Avoiding action before TCAS 1 ATC coordination initiated following TCAS RA 2 Climb/Descent on basis of TA - unjustified 3 Frequency occupied disrupting other action 7 Imprecise phraseology to report an advisory 4 Intruder aircraft on ground 3 Level off on TA against military - Justified 1 Mutual information with intruder pilot on TA 1 No conflicting traffic 3 Non Mode C Traffic 17 Not TCAS related (GPWS) 1 Pilot Familiarisation with TCAS 1 Proximate advisory on maritime transponder 2 Request for Clearance to ATC 1 Request for Traffic Information on Non Mode C target 16 Speed change on basis of TA - justified 1 TA against GA intruder 1 TA against ground based test equipment 3 TA Confirming ATC Avoiding action 4 TA initiating request for ATC verification 2 TA used to acquire intruder - coordinated with ATC 3 TA used to request avoiding action - justified 2 TA used to request Traffic Information 13 TA/RA undefined 12 TCAS operated in TA only mode - Airmiss 1 Turn on basis of TA - Coordinated with ATC - Justified 2 Turn on basis of TA - Not Coordinated - Unjustified 4 Turn on Non Mode C TA 1 Unnecessary Reporting of TA information 6 Unnecessary RT communication 2
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6. CONCLUSIONS
The TCAS II equipment is a last resort means of preventing mid-air collisions, or near mid-air collisions.
The parameters of the TCAS II algorithms are not related to ATC separation minima. The system does not provide warning of the loss of standard ATC separation, nor, necessarily, prevent close encounters 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 level of 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 conflicting traffic - 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 saturation contributing to misunderstandings.
II. Airspace management constraints.
These may include:
A. Failure of Co-ordination between Air Traffic Control authorities, either civil/civil or civil/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 to both ATC and aircrew.
These include:
1) a high proportion of Nuisance Alerts, often involving encounters where the horizontal miss distance (HMD) remains close to, or in excess of, standard ATC separation,
2) excessive vertical deviation, in response to RAs,
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3) misuse of the TCAS II traffic display - for example: horizontal and/or vertical manoeuvres based on displayed traffic information only.
In addition, in some ECAC States operational experience has shown that Nuisance, or inappropiate, RAs may be generated in the following situations:
1) military or civil aircraft, maintaining standard separation from the TCAS II equipped aircraft, but performing 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: up to and including Version 6.02.
Operational reports, from pilots, generally indicate, despite the identified operational problems, a high level of acceptance of the system, through a belief that it will enhance the safety of their operations.
The benefit of improved aircrew traffic situational awareness has also been noted, particularly when operating in areas without highly developed ATC systems.
However some controller reports indicate rather less confidence in the system and concern regarding 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 be improved by providing adequate training in the capabilities and limitations of the system. In addition timely and reliable pilot responses to TCAS II advisories are essential and should be supported by flight simulator training.
Nevertheless it is now accepted that TCAS II will enhance the safety of the ATM system, and there have been specific instances where controllers have reported that TCAS has alerted both controllers 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 the changes 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 collision avoidance 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 33 EUROPEAN ACAS OPERATIONAL EVALUATION Second Interim Report
GLOSSARY
ACAS Airborne Collision Avoidance System AECG ACAS Evaluation & Coordination Group ATC Air Traffic Control ATM Air Traffic Management EAG Event Analysis Group ECAC European Civil Aviation Conference FMS Flight Management System GA General Aviation GPWS Ground Proximity Warning System HMD Horizontal Miss Distance IFR Instrumental Flight Rule RA Resolution Advisory RT Radio Telephony RTF Radio Telephony Frequency SARPs Standards and Recommended Practices SICASP SSR Improvement and Collision Avoidance Systems Panel SSR Secondary Surveillance Radar STCA Short Term Conflict Alert TA Traffic Advisory TCAS Traffic Alert and Collision Avoidance System TTP TCAS Transition Programme VFR Visual Flight Rule
July 1996 Page 34 EUROPEAN ACAS OPERATIONAL EVALUATION Second Interim Report
REFERENCES
[1]: RESULTS OF THE TCAS II TRANSITION PROGRAM (TTP) INTERIM REPORT FAA TCAS Program Office December 31, 1991
[2]: Minimum Operational Performance Standards For Traffic Alert And Collision Avoidance System (TCAS) Airborne Equipment (2 volumes). Document no. RTCA/DO-185 Radio Technical Commission For Aeronautics. September, 1983
[3]: Introduction To TCAS II US Department of Transportation FAA March, 1990
[4]: Procedures To Be Adopted For A Widescale Operational Evaluation of The Airborne Collision Avoidance System, TCASII SSR Mode-S Working Group Working paper CE 89/162/144 November 2, 1989
[5]: An Interim Report on The Analysis of TCAS II Simulations Based on Radar Data Collected In The French Airspace. EUROCONTROL Experimental Centre EEC Note 15/1992
[6]: Justification And Rationale For Version 6.04 of The TCAS Ii Logic. The MITRE Corporation Memorandum no. F046-M-0775 March 27, 1992
[7]: First Interim Report on the TCAS II European Evaluation EEC Note 17/92
[8]: Interim Report on the UK TCAS Operational Evaluation CAA / CS Report 9415 / S. Owens, C. Smith, A. Street
[9]: TCAS II Simulations based on radar data from the Maastricht UAC EEC 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 1.0 CENA/R95-04 - January 1995 - Eric Vallauri
July 1996 Page 35 EUROPEAN ACAS OPERATIONAL EVALUATION Second Interim Report
INTENTIONALLY LEFT BLANK
July 1996 Page 36 Sommaire du Rapport CEE N° 300 Tâche CEE I03 Tâche EATCHIP CAS.ET1.ST12 Date : juillet 1996
Traduction en français du sommaire.
Le TCAS II, (“Traffic Alert and Collision Avoidance System”) a fait l’objet d’une évaluation opérationnelle dans le contexte du développement par L’OACI de standards et recommandations d’emploi pour les Systèmes Anti-Collision Embarqués (ACAS). L’évaluation opérationnelle du système s’est appuyée sur la collecte de rapports d’évènements en relation avec l’utilisation du TCAS 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 avec un aéronef intrus, muni d’un système de report d’altitude ou d’un autre TCAS, des manoeuvres d’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’évaluation s’étendant de 1991 à 1994. Un forum européen a été créé en 1991 par EUROCONTROL, sous l’égide de l’OACI, afin de permettre le suivi des problèmes ACAS en Europe. Ce forum coordonna l’évaluation opérationnelle de l’ACAS en Europe et permit de développer les procédures de collecte de données ainsi que d’assurer le suivi de l’évaluation et de discuter des problèmes posés par l’emploi du TCAS II.
Depuis le 30 décembre 1993, la législation des Etats-Unis d’Amérique a rendu onligatoire l’emport et l’utilisation du TCAS II par tous les aéronefs de plus de 30 sièges, qu’ils soient nationaux ou étrangers, et volant dans l’espace aérien des USA. De nombreuses compagnies aériennes non américaines ont également équipé des appareils qui ne sont pas nécessairement exploités en direction des Etats-Unis.
Au cours des quatre ans de cette collecte de données, plus de deux mille évènements ont été rapportés dans le cadre de l’Evaluation Européenne de l’ACAS, dont près des trois quart se sont produits dans l’espace aérien de la CEAC. Le nombre de rapports a augmenté de façon continue pendant cette même période.
De nombreux utilisateurs du TCAS ont contribué à son évaluation opérationnelle. Cinquante et une compagnies aériennes et dix huit services de contrôle aérien ont fourni des rapports d’évènements par le biais d’une procédure d’information internationalement reconnue. Une grande proportion de ces rapports a permis de confirmer les problèmes spécifiques identifiés initialement par le programme d’évaluation équivalent aux Etats-Unis. Des informations complémentaires furent fournies par des enregistrements de radar Mode-S et des enregistrements effectués à bord des aéronefs équipés.
Une densité significative de rencontres TCAS a été notée dans les bandes d’altitudes situées entre les niveaux de vol 250 et 300, ce qui est attribuable au grand nombre de vols courts à ces niveaux de vol, en particulier dans les états d’Europe et dans les zones de forte densité de trafic de l’espace européen.
Page 37 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 appliqué l’indication du TCAS. Pour la majorité des cas où l’avis de résolution n’a pas été suivi, le pilote a mentionné qu’il avait acquis visuellement l’intrus. L’application des avis de résolution a provoqué dans certains cas, des déviations de trajectoires plus grandes que celles prévues lors de la conception de l’ACAS ; ces déviations n’étaient souvent pas coordonnées avec le contrôle aérien et représentent un sujet d’inquiètude pour le personnel opérationnel. Les pilotes ont indiqué que 28% des RAs étaient “Nécessaires” et que 22% des cas étaient “Utiles”. Toutefois, 50% des avis de résolution observés ont été considérés comme “Dérangeants”. Il a été possible dans la plupart 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 déficiences des premières versions du logiciel TCAS II, jusqu’à la Version 6.02 incluse, TCAS II permet d’accroître le niveau de sécurité des vols. L’identification des problèmes opérationnels au niveau mondial par les évaluations opérationnelles a conduit à rendre obligatoire l’emport d’une Version améliorée du TCAS II (Version 6.04A) qui a supprimé plusieurs des problèmes opérationnels initiaux. 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.
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