IAPA Project Interim Report - Phase 1 page i · 2004. 2. 17. · EATM Infocentre ref. : 040123-01 EDITION DATE: November 2003 Abstract ... 12-02-2004 Director ATM Programmes George

IAPA Project Phase I Report 05-11-2003IAPA/WP0/032/D Version 1.0

EUROCONTROL ACAS Programme - CENA, EEC, QinetiQ & Sofréavia - IAPA Project

ACAS PROGRAMME

IAPA Project Phase I Report

Implications on ACAS Performancesdue to ASAS implementation

ACAS/04-005

Edition : 1.0Edition Date : November 2003Status : Released IssueClass : EATM


DOCUMENT IDENTIFICATION SHEET

DOCUMENT DESCRIPTION

Document Title

ACAS PROGRAMMEIAPA Project Phase I Report


PROGRAMME REFERENCE INDEX: ACAS ref.: ACAS/04-005 - Version 1.0

EATMInfocentre ref. :

040123-01

EDITION DATE: November 2003Abstract

The Implications on ACAS Performances due to ASAS implementation (IAPA) projectinvestigates the potential issue of airborne collision avoidance system (ACAS) and airborneseparation assistance system (ASAS) interaction in the ECAC airspace. It is focused on identifyingpotential operational issues, and providing recommendations, related to the potential interactionbetween the ACAS logic and future ASAS application procedures. Phase I of the IAPA project isnow completed: an initial, yet substantive, analysis of the potential ACAS / ASAS interaction issuewas undertaken with an ASAS 'Package 1' application, and the framework has been established foran in-depth investigation within Phases II and III.

Keywords

ACAS ASAS TCAS

CONTACT PERSON: John Law TEL: 32 2 729 37 66 UNIT: SAF/ACASProgramme

Authors: Béatrice Raynaud & Thierry Arino

DOCUMENT STATUS AND TYPE

STATUS CLASSIFICATION

Working Draft � General Public �Draft � EATM �Proposed Issue � Restricted �Released Issue �

ELECTRONIC BACKUP

INTERNAL REFERENCE NAME:

HOST SYSTEM MEDIA SOFTWAREMicrosoft Windows Type: Hard disk

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____________________________________________________________________________________________

EUROCONTROL ACAS Programme - CENA, EEC, QinetiQ & Sofréavia - IAPA Project

DOCUMENT APPROVAL

The following table identifies all management authorities who have successively approved thepresent issue of this document.

AUTHORITY NAME AND SIGNATURE DATE

ACAS ProgrammeManager

John Law

06-02-2004

Head of SafetyEnhancement Business

Division

Erik Merckx

12-02-2004

Director ATMProgrammes

George Paulson

12-02-2004


EUROCONTROL ACAS Programme – CENA, EEC, QinetiQ & Sofréavia – IAPA Project Page 1/68

IAPA Project Phase I Report


IAPA Project

Drafted by: Béatrice Raynaud & Thierry Arino

Authorised by: Thierry Arino on 05-11-2003



RECORD OF CHANGES

Issue Date Detail of changes

0.1 08-09-2003 Draft skeleton

0.2 23-09-2003 Draft report combining material from the variousdeliverables of IAPA Phase I

0.3 10-10-2003 Major changes following comments from final ProgressMeeting of Phase I (all sections);

Draft version of the executive summary

0.4 21-10-2003 Revised executive summary (and corresponding textsin the main body of the report) following comments

from EUROCONTROL and partners;

Revised sections 2.3 and 3.4 following updates indeliverables from the corresponding Work Packages

1.0 05-11-2003 Editorial corrections and inclusion of acronyms andreferences

Version delivered to EUROCONTROL

IMPORTANT NOTE: ANY NEW VERSION SUPERSEDES THE PRECEDING VERSION, WHICHMUST BE DESTROYED OR CLEARLY MARKED ON THE FRONT PAGEWITH THE MENTION OBSOLETE VERSION



EXECUTIVE SUMMARY

E.1. Overview

E.1.1. The IAPA (Implications on ACAS Performances due to ASAS implementation)project consists of the investigation of the potential airborne collision avoidancesystem (ACAS) and airborne separation assistance system (ASAS) interactionissue, and is focused on operations in the ECAC area.

E.1.2. Scope and purpose

E.1.2.1. The project ascertains:

� not only whether there are any significant operational implications for ACAS IIperformance due to possible ASAS implementation in the ECAC area; but also

� whether the benefits expected from ASAS could be compromised due to theoperations of ACAS II.

E.1.2.2. Focus is on identifying potential operational issues and providing recommendations,which are related to the potential interaction between the ACAS logic and the ASASprocedures.

E.1.2.3. The IAPA study relies on the tools which were developed, and the methodologywhich was established, for the Full System Safety Study and the ACAS / RVSMinteraction study completed within the framework of the ACASA project.

E.1.2.4. The study comes within the scope of the EUROCONTROL ACAS Programme and isalso of particular interest for several areas dealing with ASAS development. Themain objective is to provide guidelines on any identified ACAS and ASAS interactionissue for the development of future ASAS applications in Europe.

E.1.3. Project breakdown

E.1.3.1. The project is composed of three main steps:

� Phase I (November 2002 / October 2003) defines the initial scope of theACAS and ASAS interaction issue;

� Phase II (November 2003 / October 2004) consists in conducting a full set ofsimulations; and

� Phase III (October 2004 / June 2005) draws conclusions and summarises thework performed during the previous phases.

E.1.3.2. Phase I of the IAPA project is now completed. This phase has consisted in selectingan ASAS application of interest for the project, performing a preparatory analysis ofthe potential ACAS / ASAS interaction issue, and establishing the frameworkrequired for an in-depth investigation within Phases II and III.



E.2. Selecting a challenging ASAS application

E.2.1. In the context of the IAPA project, the following aspects were considered in theselection of the most relevant ASAS application:

� Challenging application: an application with the potential for studying amaximum of significant and realistic issues from an ACAS safety andoperational performance perspective;

� Scope and applicability: the larger the scope, the more interesting the ASASapplication since it potentially addresses a wide range of operations;

� Maturity: Airborne Surveillance applications proposed for early implementationwithin Europe (Package I) were of particular interest, as well as extensions ofthese applications into airborne separation applications (Package II).

E.2.2. Preparatory analysis of ASAS applications of potential interest

E.2.2.1. To support the selection of the most relevant ASAS application, a preparatoryanalysis of potential interaction with ACAS was performed for a set of potentialASAS applications of interest for the IAPA study, which consisted of the following:

� Package I/ASPA-S&M Enhanced sequencing and merging operations:- Two encounter types, corresponding to the merging and the in-trail

phases of the ASPA-S&M application, were selected for further analysis.In addition, the encounter dealing with the in-trail phase was proposed toinclude aircraft turns.

� Package I/ASPA-C&P: Enhanced crossing and passing operations:- Three encounter types, corresponding to the lateral crossing, the vertical

crossing and the lateral passing procedures, were selected for furtheranalysis.

E.2.2.2. For each ASAS application of interest, an initial analysis of a set of qualitativeencounters was performed based on the Guidance Material associated with theACAS II minimum requirements defined in ICAO Annex 10, Volume IV. Theobjective was to identify the set of encounter parameters (e.g. encounter geometry,flight parameters, spacing values at CPA) that have the potential to trigger anACAS II alert.

E.2.2.3. In a second step, specific encounters consisting of two aircraft trajectories weresimulated, and TCAS II logic simulations were performed. These TCAS IIsimulations were focused on the worst-case scenarios identified in the initialanalysis.

E.2.2.4. It should be noted that both the analysis based on ACAS II SARPS and the TCAS IIlogic simulations were performed assuming perfect CNS performances.

E.2.3. Main outcomes of the preparatory analysis

E.2.3.1. According to this preparatory analysis, no interaction with ACAS is anticipated forthe following ASAS applications:

� ASPA-S&M: Enhanced sequencing and merging operations. For the in-trailphases, whatever the altitude layer, and as far as the Wake Vortex separationminima are preserved; and



� ASPA-C&P: Enhanced crossing and passing operations: For the lateralpassing situations, whatever the altitude layer, since the lateral spacingvalues required to trigger an ACAS II alert during slow convergence situationsare of the order of the ACAS II minimum protection distance parameter(DMOD), e.g. 1.3NM for a Traffic Advisory above FL200. Such lateral spacingvalues would not be operationally acceptable.

E.2.3.2. Some interaction with ACAS potentially exists for the ASPA-S&M: ‘Enhancedsequencing and merging operations’ for the merging phases, but only duringmarginal situations. In particular, some merging encounters with required spacing atWPT close to the radar separation minimum in TMA, i.e. 3 NM, may trigger a TA.However, such spacing values between aircraft in sequence are unlikely to occurduring typical merging situations.

E.2.3.3. Finally, the results of the preparatory analysis show that some interaction with ACASpotentially exists for the ASPA-C&P: ‘Enhanced crossing and passing operations’. Inparticular, the following encounters are likely to trigger TAs:

� For the lateral crossing situations, typically with angles of convergencegreater than 90 degrees and Horizontal Miss Distance at CPA close to theapplicable radar separation minima, i.e. 3 NM in TMA and 5 NM in en-routeECAC airspace; and

� For the vertical crossing situations, typically during level-off encounters atthe applicable vertical separation minima, i.e. 1000 ft below FL415, and2000 ft above, in the ECAC airspace, with operationally realistic vertical rates.Such encounters are common events, in particular between arrivals anddepartures in TMA.

E.2.3.4. Such 1000 ft level-off encounters may even trigger “undesirable” ResolutionAdvisories when significant, but realistic, relative altitude rates are observed close tothe cleared flight levels. This issue has already been identified for current ATMoperations, and is not specifically linked to the introduction of ASAS operations.

E.3. Selected application: ASAS lateral crossing procedure

E.3.1. Based on the agreed selection criteria, including the results of the preparatoryACAS / ASAS interaction analysis, the ASAS lateral crossing procedure wasselected for further investigation within IAPA.

E.3.2. An additional interest in working on the Package I/ASPA-C&P, lateral crossingapplication is that it will also be possible to address the issues related to the initialphase of the Package I/ASPA-S&M, merging application.

E.3.3. To support the future simulations to be conducted within IAPA Phase II, the mainassumptions related to the operational environment and the ASAS lateral crossingprocedure were further developed.



E.3.4. Operational purpose

E.3.4.1. The purpose of the ASAS lateral crossing procedure is to provide a new air trafficcontrol procedure, allowing one ASAS equipped aircraft to cross a designatedaircraft.

E.3.4.2. To allow for investigation of a wider range of operations, the IAPA operationalenvironment envisages the ASAS application within the scope of the followingoptions:

� Option 1: Airborne spacing applications, where separation minima areunchanged (i.e. applicable radar separation minima in the IAPA environment)and spacing minima depend on aircraft capabilities;

� Option 2: Airborne separation applications, where the separation tasks aretransferred to the flight crew for the duration of the ASAS lateral crossingprocedure and airborne separation standards are defined. These will includeairborne separation minima applicable by the flight crew.

E.3.5. Operational procedure and conditions of use

E.3.5.1. The air traffic controller can instruct a flight under his control to perform an ASASlateral crossing procedure if certain general conditions are met, which are intendedto ensure the compatibility of the ASAS application with the provision of separationby ATC.

E.3.5.2. The ASAS lateral crossing procedure can be accepted by the flight crew of acontrolled flight if certain general conditions are met, which allow for the safe andefficient execution of the procedure.

E.3.5.3. Two different types of operation (i.e. “pass behind” and “pass in-front” operations)are distinguished, each of which results in a heading alteration by the aircraftperforming the ASAS lateral crossing procedure.

E.3.5.4. The minimum applicable spacing (Option 1) / separation (Option 2) during the ASASlateral crossing procedure was set to 4 NM. This value is considered to be theminimum applicable between two RNP-1 compliant aircraft, assuming perfectsurveillance and communication performances.

E.3.5.5. At the “Clear of Traffic”, which corresponds to the time/location when the risk ofinfringement of applicable separation is over, the aircraft performing the ASASlateral crossing procedure can resume its navigation direct to track.

E.4. Case study of the ASAS lateral crossing procedure

E.4.1. During the specific ACAS / ASAS interaction analysis of the ASAS crossingprocedure, TCAS II logic simulations were performed on a set of qualitativeencounters involving two aircraft, one of which was performing an ASAS “passbehind” or “pass in-front” manoeuvre.



E.4.2. Looking into the parameters of the TCAS II version 7.0 logic enabled anunderstanding of the influence of various encounter parameters on TA and RAcharacteristics. Encounter parameters of interest included:� the angle of convergence between both aircraft (in between 30 and 150

degrees);� the aircraft performances (i.e. turboprop or jet aircraft);� the encounter flight level (i.e. FL80, FL140, FL240 or FL330), because the

ACAS envelope is altitude dependent;� the applicable spacing (Option 1) / separation (Option 2) value at CPA (around

the default value of 4 NM);� the vertical profiles of both aircraft, and the resulting Vertical Miss Distance at

CPA.

E.4.3. For all the encounters, the simulated trajectories were ideal trajectories without anywind effects, and any trajectory blunders or track deviations due to navigation errorsor uncertainties.

E.4.4. Main ACAS / ASAS interaction features

E.4.4.1. With regard to TAs, the main ACAS / ASAS features underlined during the casestudy of the ASAS lateral crossing procedure include the following:

� The likelihood of a TA is strongly dependent on the angle of convergence andthe aircraft speed, which itself depends on the flight level and the aircraft type.The higher the resulting closing speed, the higher the likelihood of a TA;

� No TAs were generated during encounters occurring at FL80 (low flight level)and for encounters involving aircraft with an initial angle of convergence of 30°(low angle of convergence);

� The “pass behind” manoeuvre is more likely to trigger a TA because itincreases the initial rate of convergence, while the “pass in front” decreases it;

� For both the “pass behind” and “pass in front” manoeuvres, the greater thespacing value at CPA, the less likely a TA would occur, especially in the caseof a “pass in-front” manoeuvre. In addition, when a TA is triggered, its durationdecreases with the spacing value at CPA;

� TAs only occurred during the first phase of the ASAS lateral crossingmanoeuvre, i.e. before the «resume track» phase, despite the selectedheading of 45° when flying direct to track.

E.4.4.2. Further, assuming perfect aircraft navigation (i.e. with neither blunders nor trackdeviations) as well as perfect ACAS surveillance, it is unlikely that TCAS II logicwould trigger a RA during an ASAS lateral crossing procedure, thanks to the “MissDistance Filter” feature of TCAS II logic version 7.0.

E.4.4.3. Nevertheless, it should be mentioned that the “Miss Distance Filter” might not beeffective in cases where the TCAS II logic detects an intruder manoeuvre, even ifthis detection results from the aircraft navigation performance, e.g. cross-trackdeviations.

E.4.4.4. In conclusion, the case study has confirmed that the ASAS lateral crossingprocedure is a challenging application with respect to the interaction with ACAS.



E.5. IAPA simulation framework

E.5.1. To support further investigation of the ACAS / ASA interaction within IAPA Phase II,a common simulation framework has been defined, which consists of:

� a set of scenarios “with and without ASAS”, which supports the assessmentof ASAS lateral crossing operations in comparison with conventional ATMoperations before the use of ASAS; and

� a comprehensive set of “ACAS / ASAS interaction indicators”, whichsupports the assessment of the potential improvements or drawbacks for bothACAS and ASAS operations.

E.5.2. These scenarios were defined taking into account the airspace and ASASapplication characteristics defined in the IAPA operational environment. Allscenarios (with and without ASAS) include the ACAS component. Indeed, ACAS isan essential element of the ATM system, and it is not envisaged that its role shouldbe put in question as a result of the introduction of ASAS operations.

E.5.3. ACAS / ASAS scenarios

E.5.3.1. Three different scenarios related to ASAS operations will be investigated:

� Scenario “Mix of pass behind/in-front”: Both “pass in-front” and “passbehind” procedures will be applied to ASAS equipped aircraft, in accordancewith their respective conditions of use. Further, preference will be given to the“pass behind” procedure, as far as the procedure is applicable in accordancewith its conditions of use;

� Scenario “Pass behind”: Only “pass behind” procedures will be applied toASAS equipped aircraft, in accordance with their conditions of use; and

� Scenario “Pass in-front”: Only “pass in-front” procedures will be applied toASAS equipped aircraft, in accordance with their conditions of use.

E.5.3.2. Scenario elements of particular interest include the following:� ASAS application characteristics (e.g. targeted horizontal spacing/separation

value at CPA) and conditions of use;� Characteristics of pilot behaviour during an ASAS application, as well as in

response to ACAS II alert;� Level of aircraft equipage (e.g. ACAS, ASAS/ADS-B equipage); and� Level of aircraft navigation capabilities (e.g. RNP-1, RVSM).

E.5.3.3. Unless otherwise required to perform a sensitivity study, and in order to investigateACAS / ASAS interaction on the most challenging bases, the default value for thetargeted horizontal spacing/separation value at CPA with ASAS will be 4 NM.

E.5.4. ACAS / ASAS interaction indicators

E.5.4.1. Although a single indicator (e.g. number of RAs) could be used to assess the impactof ASAS on ACAS performance, or vice-versa, an attempt is made to define distinctsets of indicators allowing for the assessment of the potential ACAS / ASASinteraction from different perspectives.



E.5.4.2. More precisely, four distinct sets of ACAS / ASAS interaction indicators are definedto highlight potential improvements or drawbacks in terms of:

� ACAS safety performance: From an ACAS perspective, priority is given tothe assessment of the safety benefits provided by ACAS with and withoutASAS. The aim would be to answer the question: “what is acceptable from anACAS standpoint?”. The ACAS safety indicators are typically related to:- effective RAs during non-nominal operations.

� ASAS performance: From an ASAS perspective, the purpose is to assess thepotential impact of ACAS on the expected benefits from the ASAS application.However, It is not intended to assess the impact on the ASAS applicationperformance from an overall ATM perspective. The assessment will be limitedto the impact of ACAS on the applicability of the ASAS lateral crossingprocedure. Hence, the associated ACAS / ASAS interaction indicators aretypically related to:- the likelihood of an ACAS alert during the execution of the (selected)

ASAS application.

� Pilot acceptance: The objective is to assess from a pilot’s perspective theacceptability of ACAS during ATM operations with and without ASAS through:- the issuance of appropriate ACAS II alerts (typically, RAs) during non-

nominal operations; and- the extent to which undesirable ACAS II alerts (both TAs and RAs) are

limited during nominal operations.

� ACAS / ASAS compatibility: The objective is to assess the impact of ASASon ACAS, and vice-versa, from an overall ATM perspective, i.e. thecompatibility of ACAS with ATM operations with and without ASAS, includingthe extent to which:- disruptive ACAS II alerts (typically, undesirable RAs), and deviations

resulting from compliance with RAs, are limited during operations.

E.6. Simplified modelling of the ASAS application

E.6.1. The simplified model developed during Phase I is intended to simulate the behaviourof the selected ASAS application, i.e. the ASAS lateral crossing procedure. It willsimulate the effect of the ASAS application on the aircraft trajectories taking intoaccount the most relevant parameters of the ASAS application.

E.6.2. The approach adopted when developing this simplified model was sustained by thefollowing principles:

� to simulate the nominal effects of the ASAS application (i.e. assumingperfect ASAS performance) starting from an encounter involving conflictingaircraft trajectories (with no ATC intervention); and

� to modify the trajectory of the aircraft performing the ASAS application,rather than to implement an airborne ASAS logic that would support theexecution of the ASAS application in real-time, but with potentially limitedperformances due to the simulated airborne surveillance and separationprocessing functions.



E.6.3. Within Phase II, the simplified model of the ASAS application should be used within:

� the study based on the ASAS encounter, to derive a set of ASAS encountersfrom a set of encounters (without ATC) generated from the ATM encountermodel;

� the study based on modified radar data, to generate a set of ASAS encountersfrom a set of encounters extracted from radar data recordings and modified toremove ATC intervention, when appropriate; and

� the study based on data extracted from fast-time simulations, to generate aset of ASAS encounters from encounters issued from fast-time simulationsbased on European flight plan data.

E.7. Towards an ASAS encounter model

E.7.1. Within IAPA Phase I, the initial work has developed the specification of an ATMencounter model for the airspace associated with the selected ASAS application.This work builds on the specification of an ATM encounter model in the ACASSARPs and the specification of the European safety encounter model in the ACASAproject.

E.7.2. The objective within IAPA Phase II will be to derive an ASAS encounter model fromthe ATM encounter model. The ASAS encounter model is intended to model anairspace in which the selected ASAS application is used by ATC according to theconditions of applicability and operational use defined in the IAPA operationalenvironment.

E.7.3. General features

E.7.3.1. The IAPA encounter model can be used to generate an arbitrarily large set ofartificial encounters whose properties are characteristic of a given airspace.

E.7.3.2. These encounter properties are specified by appropriate parameters, which valuescan be different in each encounter and are determined by being selectedstochastically from a distribution of probabilities representative of the consideredairspace.

E.7.3.3. Each encounter consists of a series of positions of two aircraft at regular intervalsthroughout a time window of 8 minutes duration. This window is centred upon theinstant of “closest approach”, which is defined so as to take account of the differentscales of horizontal and vertical ATC separation.

E.7.4. Encounter properties

E.7.4.1. The altitude at which each encounter occurs is a dominant feature of the model. Theairspace is divided into a number of altitude layers whose boundaries have beenchosen to reflect the differing characteristics of air traffic and ATC procedures atdifferent altitudes. Most of the distributions within the encounter model have adependency on the particular layer to which an encounter has been assigned.



E.7.4.2. The model includes eight aircraft performance classes based on engine type andairframe size. These are broad classes intended to reproduce the typicalperformance limitations of groupings of aircraft rather than reproduce the preciseperformance of any particular types. Limitations on altitude, speed and vertical rateare taken into account when the distributions within the model are sampled.

E.7.4.3. In those encounters in which it is adjudged that separation is being provided in agiven dimension (i.e. horizontal or vertical) at closest approach, the aircraft profilesof the two aircraft in the other dimension are independent – in those encounters inwhich it is adjudged, if that separation is not being provided in a given dimension theaircraft profiles in the other dimension are correlated.

E.7.4.4. The vertical and horizontal profiles of each aircraft are specified by the timing andmagnitude of accelerations defining aircraft manoeuvres:

� The vertical profile consists of three segments of flight at constant vertical ratebetween which there are two potential vertical manoeuvres in which thevertical rate changes. Where the vertical profiles of the aircraft need to becorrelated, this is achieved by selecting the profile types from a jointdistribution.

� The horizontal profile consists of three segments of flight on a given headingbetween which there are two potential turns. The probability of turns occurringdepends on the general trend of the vertical profile, and whether or not thehorizontal profiles of the aircraft need to be correlated.

� In addition, the speed of the aircraft may vary in an encounter. The probabilityof a change in speed and its nature is determined by the general trend of thevertical profile.

E.7.5. Encounter generation

E.7.5.1. Once the positions and velocities of the two aircraft at closest approach, and theaccelerations and timings defining the manoeuvres, have been determined it ispossible to construct the aircraft trajectories throughout the encounter window.

E.7.5.2. In some respects the encounters generated by the process described above can betoo smooth, lacking the variations around general trends found in real aircrafttrajectories. This shortcoming is overcome by introducing realistic variations in theaircraft trajectories. This process incorporates both a random component (“wobble”)and a systematic component (“modulation”) – hence it has come to be known as“wobbulation”.

E.7.5.3. Wobbulation is applied independently to both the horizontal and vertical positions ofeach trajectory. For the IAPA model, the wobbulation parameters have been chosenso that the variations in horizontal position are compatible with the aircraft havingnavigation of RNP-1 capability, and the variations in vertical position are compatiblewith the aircraft having altimetry and navigation performance that is RVSM MASPScompliant.



E.8. Main achievements of Phase I

E.8.1. Phase I of the IAPA project has first consisted in selecting an ASAS application ofparticular interest for the project, i.e. an application with the potential for studying amaximum of significant and realistic issues from an ACAS safety and operationalperformance perspective.

E.8.2. This selection was supported by a preparatory analysis of the ACAS / ASASinteraction (WP04) for a selected set of Package I Airborne Surveillance applicationspresenting the potential for an extension into airborne separation applications(Package II).

E.8.3. Following the identification of a potential interaction between ACAS and ASAS,Phase I has also established the framework required for an in-depth investigation:

� the operational environment and the selected ASAS application(i.e., ASPA-C&P, lateral crossing with “pass behind” and “pass in-front”procedures) have been defined (WP01);

� a simulation framework has been proposed involving three different scenarioswith full ASAS / ADS-B equipage and a set of ACAS / ASAS interactionindicators (WP02);

� a simplified model of the selected ASAS application has been developed(WP03); and

� an ATM encounter model has been specified, with the objective of supportingthe development of an ASAS encounter model (WP05).

E.8.3.1. This framework will be further developed within Phase II (with the ASAS encountermodel) and will support the full set of planned simulations.

E.9. Future simulations within Phase II

E.9.1. Within Phase II, the in-depth investigation of the potential ACAS / ASAS interactionwill be performed through various studies based on different sources of data:

� a study based on the ASAS encounter model (WP06);

� a study based on modified radar data (WP07);

� a study based on data extracted from fast-time simulations (WP08); and

� a study based on data extracted from real-time simulations (WP09).

E.9.2. The rationale for conducting simulations on different sources of data is tocompensate for the limitations related to anyone of them, and to identify a larger setof issues.

E.9.3. The use of a common simulation framework during the various IAPA data-orientedstudies should allow for the validation of ACAS / ASAS interaction trends identifiedwith each source of data.

E.9.4. Further, it is also planned to investigate the impact of ASAS operations on the safetybenefit provided by ACAS, through a dedicated safety case based on the ED78AOperational Safety Assessment methodology (WP10).



E.10. Conclusions

E.10.1. The IAPA project is one European contribution to address the potential ACAS andASAS interaction issue. This contribution is required because of the plannedevolutions of the European ATM system through a greater involvement of the flightcrews in separation provision, which may impact the forecasted performance of bothACAS II and the ATM system change itself.

E.10.2. The main results of the preparatory analysis of the potential ACAS / ASASinteraction issue are the following:

� the likelihood and duration of TAs are highly geometry and altitude dependent;

� the “Miss Distance Filter” performance of TCAS II logic version 7.0 is critical toprevent the issuance of undesirable RAs;

� the major influencing factors on the ACAS / ASAS interaction are:- the airborne spacing values; and- the aircraft trajectory quality.

� It is not yet possible to determine the implications, due to ACAS / ASASinteraction issues, for ACAS safety performance.

E.10.3. In addition, in view of the major assumptions and limitations of the preparatoryACAS / ASAS interaction study performed within IAPA, definitive conclusions shallnot be drawn for any of the Package I applications.

E.10.4. It is emphasised that it is necessary to investigate further the set of ACAS / ASASinteraction issues identified. The work methodology and framework developedduring Phase I will facilitate this task.

E.11. Recommendations

E.11.1. The work conducted within IAPA Phase I has demonstrated that:

� ACAS / ASAS interaction issues must be taken into account when developingASAS applications envisaged for implementation;

� further in-depth analysis of the identified ACAS / ASAS interaction issuesshould be performed; and

� the impact of ASAS operations on safety benefits provided by ACAS requiresto be investigated.

E.11.2. IAPA Phase II work is commencing (November 2003). It will capitalize upon themethodology and framework which has been developed, and the preparatoryanalysis which has been undertaken, within IAPA Phase I.



LIST OF DEFINITIONS

Closest Point ofApproach (CPA)

Local minimum in the physical distance between twoaircraft (slant range).

The issuance of ACAS II alerts and the type of alertdepends on the predicted time to CPA, which iscalculated by dividing the slant range by the closure rate.

Closest Point ofPropinquity

Local minimum in the “propinquity” distance between twoaircraft.

The “propinquity” distance scales the horizontal andvertical distances between the aircraft according to therespective separation minima applicable by ATC.

The closest point of propinquity is used as the instant ofclosest approach in the IAPA encounter model.

Clear of traffic Time or location when:- Either the aircraft are diverging laterally (time to

modified CPA is negative) and the current distancebetween the aircraft is equal or superior to the valueof the applicable lateral spacing(Option 1) / separation (Option 2) [by ASAS]

- Or the aircraft are not converging vertically and thedifference in altitude is equal or superior to theapplicable vertical separation [by ATC].

Horizontal CPA Local minimum in the horizontal component of the slantrange between two aircraft.

Initial CPA Horizontal CPA without the effect of the ASASapplication.

Modified CPA Horizontal CPA with the effect of the ASAS application.

Option 1 Airborne Spacing application, where separation minimaare unchanged (i.e. applicable radar separation minimain the IAPA environment) and spacing minima depend onaircraft capabilities.

Option 2 Airborne Separation application, where the separationtasks are transferred to the flight crew for the duration ofthe ASAS lateral crossing procedure and airborneseparation standards are defined. These will includeairborne separation minima applicable by the flight crew.

Wobbulation Process to introduce realistic variations in the aircrafttrajectories by incorporating both a random component(“wobble”) and a systematic component (“modulation”).



TABLE OF CONTENTS

1. INTRODUCTION ...................................................................................................................... 18

1.1. OBJECTIVE AND SCOPE ....................................................................................................... 18

1.2. BACKGROUND AND CONTEXT........................................................................................... 19

1.3. PROJECT OVERVIEW............................................................................................................ 20

1.4. DOCUMENT OVERVIEW....................................................................................................... 21

2. PREPARATORY ACAS / ASAS INTERACTION ANALYSIS............................................ 22

2.1. APPROACH OVERVIEW........................................................................................................ 22

2.2. SELECTING A CHALLENGING ASAS APPLICATION.................................................... 24

2.3. SELECTED ASAS APPLICATION: LATERAL CROSSING OPERATIONS.................. 29

2.4. PREPARATORY SIMULATIONS AND RESULTS ............................................................. 33

3. FRAMEWORK FOR AN IN-DEPTH ACAS / ASAS INTERACTION ANALYSIS.......... 41

3.1. GENERAL .................................................................................................................................. 41

3.2. SIMULATION FRAMEWORK DEFINITION ...................................................................... 41

3.3. SIMPLIFIED MODEL OF THE ASAS APPLICATION BEHAVIOUR............................. 44

3.4. TOWARDS AN ASAS ENCOUNTER MODEL ..................................................................... 47

4. FUTURE DEVELOPMENT AND SIMULATIONS............................................................... 55

4.1. GENERAL .................................................................................................................................. 55

4.2. STUDY BASED ON ASAS ENCOUNTER MODEL.............................................................. 55

4.3. STUDY BASED ON MODIFIED RADAR DATA.................................................................. 56

4.4. STUDY BASED ON DATA EXTRACTED FROM FAST-TIME SIMULATIONS ........... 56

4.5. STUDY BASED ON DATA EXTRACTED FROM REAL-TIME SIMULATIONS .......... 57

4.6. SAFETY CASE BASED ON OSA METHODOLOGY .......................................................... 57

5. CONCLUSIONS......................................................................................................................... 59

6. RECOMMENDATIONS ........................................................................................................... 61

7. REFERENCES ........................................................................................................................... 62



7.1. IAPA REFERENCES................................................................................................................. 62

7.2. EXTERNAL REFERENCES .................................................................................................... 64

8. ACRONYMS............................................................................................................................... 66



LIST OF FIGURES

Figure 1: Example of a “Pass Behind” instruction ..........................................................................................31Figure 2: Envelope for ASAS lateral crossing manoeuvres.............................................................................32Figure 3: Traffic Advisory during an ASAS “pass behind” lateral crossing encounter.........................................34Figure 4: Traffic Advisory during an ASAS “pass in front” lateral crossing encounter.........................................35Figure 5: Not filtered RA during an ASAS “pass behind” lateral crossing encounter .........................................36Figure 6: Input/output of the simplified model of the ASAS application ............................................................45Figure 7: Illustration of a simplified modelling of the ASAS application for a “pass behind” encounter .................46Figure 8: Illustration of a simplified modelling of the ASAS application for a “pass in-front” encounter.................46

LIST OF TABLES

Table 1: Package 1 Airborne Surveillance applications ..................................................................................25Table 2: Aircraft types ................................................................................................................................27Table 3: Altitude layers in IAPA model .........................................................................................................48Table 4: Aircraft performance classes ..........................................................................................................50



1. Introduction

1.1. Objective and scope

1.1.1. The IAPA project consists in the investigation of the Airborne Collision AvoidanceSystem (ACAS) and the Airborne Separation Assistance System (ASAS) interactionissue, and is focused on operations in the European Civil Aviation Conference(ECAC) area. IAPA stands for Implications on ACAS Performances due to ASASimplementation.

1.1.2. The IAPA project aims at addressing, before any European ASAS implementation,an open issue (ACAS and ASAS interaction) never thoroughly investigated, andproviding guidelines (on the ACAS and ASAS interaction issue) for the developmentof future ASAS applications in Europe.

1.1.3. Although the focus is on the potential interaction of ACAS with Air Trafficmanagement (ATM) with and without ASAS, the IAPA project ascertains:

� not only whether there are any significant operational implications for ACAS IIperformance due to possible ECAC ASAS implementation; but also

� whether the benefits expected from ASAS could be compromised due to theoperation of ACAS II.

1.1.4. Focus is on identifying and assessing potential operational issues resulting from thepotential interaction between the ACAS logic and the ASAS procedures.

1.1.5. Phase I of the IAPA project is now completed. This phase has consisted in selectingan ASAS application of interest for the project, performing a preparatory analysis ofthe potential ACAS / ASAS interaction issue, and establishing the frameworkrequired for an in-depth analysis of the identified ACAS / ASAS interaction issues.

1.1.6. Within Phases II and III, this in-depth investigation of the potential ACAS / ASASinteraction issues will be performed through various studies based on differentsources of data:

� modified radar data;

� data extracted from real-time simulations;

� data extracted from fast-time simulations; and

� automatic artificial encounters (ATM encounter model).

1.1.7. The rationale for using different sources of data is to compensate the limitationsrelated to each source of data, and to cope with a larger set of issues. Further, IAPAwill investigate the impact of ASAS operations on the safety benefit provided byACAS.

1.1.8. The IAPA project relies on the tools which were developed, and the methodologywhich was established, for the Full System Safety Study [ACA1a] and theACAS / Reduced Vertical Separation Minimum (RVSM) interaction study [ACA3a]completed within the framework of the ACAS Analysis (ACASA) project.



1.1.9. The IAPA study comes within the scope of the EUROCONTROL ACAS Programme.It is also of particular interest for several areas dealing with ASAS development, e.g.the Cooperative Action of R&D in EUROCONTROL (CARE) on ASAS, theEUROCONTROL Air-Ground Cooperation (AGC) Programme and theEUROCONTROL Automatic Dependent Surveillance (ADS) Programme. Theproject is based on a two-year-and-a-half schedule and has started in November2002. The project sponsor is EUROCONTROL HQ. The study is conducted by aconsortium of four organisations (CENA, EEC, QinetiQ and Sofréavia) with a leadingrole for Sofréavia (ATM division).

1.2. Background and context

1.2.1. The carriage and operation of the ACAS II compliant equipment TCAS II version 7.0is mandatory in the ECAC area by 1 January 2000 for all aircraft with more than 30passenger seats or more than 15,000 kg. From 1 January 2003, all aeroplanes of amaximum certificated take-off mass in excess of 15,000 kg or authorised to carrymore than 30 passengers shall be equipped with an ACAS II. With thisimplementation phase, ACAS is now part of the current European ATM System.

1.2.2. The EUROCONTROL organisation has defined a Roadmap of OperationalImprovements (OI) to be implemented as part of the overall ATM system out to2020. A significant proportion of the defined OI’s are enabled by ADS-Broadcast(ADS-B) applications. The envisaged ADS-B related applications have, forimplementation feasibility reasons, been organised into three packages. Eachpackage includes both Ground Surveillance applications (GS) and AirborneSurveillance applications (AS).

1.2.3. The ICAO Surveillance and Conflict Resolution Systems Panel (SCRSP) hasdeveloped a Circular on ASAS [ICAO ASAS], which addresses the whole range ofAirborne Surveillance applications included in Packages I, II and III.

1.2.4. Although no commitment yet exists to internationally implement ASAS, and inconformity with the vision for the potential evolution of ATM described by ICAO[ICAO OCD], the operational use of ASAS is seen as a promising option to providean increase in capacity and flight efficiency while enhancing flight safety.

1.2.5. Along the way towards a mature ASAS environment, compatibility must be assuredbetween current and future systems and procedures. Before ASAS can berealistically implemented, questions remain to be answered in several areas. Inparticular, the issue of ACAS and ASAS interaction in the ECAC area has to beaddressed.



1.3. Project overview

1.3.1. The IAPA project [IAPA/00/002] is composed of three main steps:

� Phase I (scope) defined the initial scope of the ACAS and ASAS interactionstudy. It is composed of the following Work Packages (WP):- WP01: ASAS application selection and definition. Based on agreed

criteria, including the results of WP04, the work consisted in selecting anddefining an ASAS application of interest for the IAPA study;

- WP02: Performance indicator definition. The work consisted in defining acommon simulation framework for IAPA, which consists of a set ofscenarios and indicators to assess the ACAS / ASAS interaction;

- WP03: Simplified modelling of the ASAS application behaviour; and- WP04: Case study. This work consisted in a preparatory analysis of the

potential interaction with ACAS for some ASAS applications of potentialinterest for IAPA, and in a specific analysis of the ASAS applicationselected for further investigation within IAPA.

- WP05: ASAS encounter model development. This work started withinPhase I with the specification of an ATM encounter model, and willproceed within Phase II with its derivation into an ASAS encounter model.

� Phase II (analysis) will consist in conducting the required simulations, basedon different sources of data, for analysing the ACAS and ASAS interaction. Itwill also investigate the impact of ASAS operations on the safety benefitprovided by ACAS.It is composed of WP05 and of the first tasks of WP06 to WP10 defined asfollows:- WP05: ASAS encounter model development;- WP06: Study based on ASAS encounter model;- WP07: Study based on modified radar data;- WP08: Study based on data extracted from fast-time simulations;- WP09: Study based on data extracted from real-time simulations; and- WP10: Safety case based on the ED78A Operational Safety Assessment

(OSA) methodology.

� Phase III (synthesis) will conclude by summarising the work performed duringPhase I and Phase II and delivering guidelines for the development of futureASAS applications.It is composed of the report development tasks of WP06 to WP10 and of thefinal work package:- WP11: Synthesis and guidelines.

1.3.2. All work packages taken as a whole describe the complete work programme to becarried out within the IAPA project. The data-oriented work packages (i.e., WP04,WP06, WP07, WP08 and WP09) are stand-alone studies. WP01 and WP02 definetheir common bases.



1.3.3. If the three phases are eventually conducted, the IAPA project will represent a totaleffort of more than 11 man-years, including effort related to WP00: Projectmanagement.

1.4. Document overview

1.4.1. Chapter 1 briefly introduces the objective and scope of the document, together withthe background and context of the IAPA project. Finally, it provides a projectoverview in which the various WPs and the three phases of the project aredescribed.

1.4.2. Chapter 2 describes the preparatory analysis of the ACAS / ASAS interaction issueperformed during IAPA Phase I. It discusses the methodology used to select anASAS application of interest for the project, and provides some preparatory resultsregarding the identified ACAS / ASAS interaction issues.

1.4.3. Chapter 3 describes the framework established during IAPA Phase I to allow for amore in-depth investigation of the ACAS / ASAS interaction issue. This frameworkconsists in the various scenarios, the ACAS / ASAS interaction indicators, thesimplified model of the selected ASAS application, and the ATM encounter model,which will support the simulations to be conducted within Phase II.

1.4.4. Chapter 4 introduces the further framework development and the full set ofsimulations planned for IAPA Phase II. It also explains the principles sustaining thework methodology and framework adopted for the IAPA study.

1.4.5. Chapter 5 summarises the main achievements of IAPA Phase I, It draws someconclusions about the potential ACAS / ASAS interaction for the ASAS applicationsof interest. It also emphasises the need to further investigate the ACAS / ASASinteraction issue and to proceed with the Phases II and III.

1.4.6. Based on the preparatory analysis which has been undertaken in IAPA Phase I,chapter 6 provides some general recommendations related to the ACAS / ASASinteraction issue.



2. Preparatory ACAS / ASAS interaction analysis

2.1. Approach overview

2.1.1. General

2.1.1.1. The range of ASAS applications for potential investigation within the IAPA studyinclude the Package I of AS applications proposed for early implementation inEurope.

2.1.1.2. The ASAS applications of particular interest are those with the potential forstudying a maximum of significant and realistic issues from an ACAS safety andoperational performance perspective, i.e. the most challenging ASAS applicationsin terms of potential ACAS / ASAS interaction.

2.1.1.3. To support the selection of the most relevant ASAS application, a preparatoryanalysis of potential interaction with ACAS was performed for a set of potentialASAS applications of interest for the IAPA study, which consisted of the following:

� Package I/ASPA-S&M Enhanced sequencing and merging operations;

� Package I/ASPA-C&P: Enhanced crossing and passing operations;

� Possible extension of the previous Airborne Spacing applications into AirborneSeparation applications (Package II).

2.1.1.4. Based on agreed selection criteria, including the results of this preparatoryACAS / ASAS interaction analysis, the ASAS lateral crossing procedure wasselected for further investigation within IAPA.

2.1.1.5. Therefore, the main assumptions related to the airspace, and the ASAS lateralcrossing procedure, were further developed to support future work within IAPAPhase II.

2.1.1.6. Further, a specific ACAS / ASAS interaction analysis was performed to confirm thefinal selection of this ASAS application as a challenging one from an ACAS / ASASinteraction perspective.

2.1.2. Analysis based on ACAS II Guidance Material

2.1.2.1. In a first step, for each ASAS application of interest, an initial analysis of a set ofqualitative encounters was performed based on the Guidance Material associatedwith the ACAS II minimum requirements defined in ICAO Annex 10, Volume IV[ICAO ACAS].

2.1.2.2. At this stage, potential ACAS II alerts were identified based on the ACAS IIprotection volume defined by means of the range test and the altitude test. Theobjective was to identify the set of encounter parameters (e.g. encountergeometry, flight parameters, spacing values between aircraft that have thepotential to trigger an ACAS II alert, either a Traffic Advisory (TA) or a ResolutionAdvisory (RA).



2.1.3. Preparatory analysis based on TCAS II logic simulations

2.1.3.1. In a second step, Traffic alert and Collision Avoidance System (TCAS) II logicsimulations were performed on specific encounters consisting of two aircrafttrajectories, which were generated using a basic aircraft simulator [BASILE]. TheseTCAS II simulations were focused on the worst-case scenarios identified in theinitial analysis.

2.1.3.2. At this stage, the indicators used to assess the ACAS / ASAS interaction werebasically the ratios of TAs and RAs per aircraft.

2.1.3.3. It should be noted that these ratios of TCAS II alerts depend highly on the aircraftperformance mix included in the set of simulated encounters, which were notintended to be representative of the European fleet of aircraft during thispreparatory case study.

2.1.3.4. The computed ratios only aimed at providing trends about the potential for TAs andRAs during each ASAS application of interest, and therefore, should not beconsidered as accurate metrics of the ACAS / ASAS interaction in the Europeanairspace.

2.1.4. Specific analysis of the ASAS lateral crossing procedure

2.1.4.1. In a third step, TCAS II logic simulations were performed based on a set ofqualitative encounters that included the two types of ASAS lateral crossingoperations, i.e. the “pass in-front” and the “pass-behind” operations.

2.1.4.2. The objective was to identify the scenarios (i.e. encounter geometry, type of ASASmanoeuvre and aircraft type) that have the potential to trigger TAs and RAs.

2.1.4.3. Looking into the parameters of the TCAS II version 7.0 logic enabled anunderstanding of the influence on TA and RA characteristics of the angle ofconvergence of aircraft, the aircraft type, the Horizontal Miss Distance (HMD) andthe vertical profile.

2.1.5. ACAS simulation tools and assumptions

2.1.5.1. The execution and analysis of ACAS II simulations are performed using the Off-line Simulator of Collision Avoidance Resolution(OSCAR) test bench [OSCAR].

2.1.5.2. This ACAS simulation tool consists in a set of integrated tools to prepare, executeand analyse scenarios of encounters involving TCAS II equipped aircraft. Inparticular, it includes an implementation of the TCAS II logic version 7.0 inconformity with the Minimum Operational Performance Standards (MOPS), withRequirement Working Group (RWG) approved changes 1 to 92 and 98.

2.1.5.3. During this preparatory ACAS / ASAS interaction study, all TCAS II simulationswere conducted using the following assumptions:

� Both aircraft are TCAS II version 7.0, supply their TCAS II logic with the finestown altitude quantization (i.e., one foot) and report their altitude in 25-ftquanta;

� Standard pilot behaviour is simulated in both aircraft, in response to an RAtriggered by their TCAS II equipment.



2.2. Selecting a challenging ASAS application

2.2.1. Categories of ASAS application

2.2.1.1. The ‘Principles of Operation for the use of Airborne Separation AssistanceSystems’ [PO-ASAS] aim to advance the work in the ASAS area in a cooperativemanner, taking into account US and European perspectives for global applicability.“The main guiding principle is that Air Traffic Services (ATS) can be enhancedthrough greater involvement of flight crews and aircraft systems in cooperation withcontrollers and the ATM system” [PO-ASAS].

2.2.1.2. The IAPA project uses the [PO-ASAS] definitions:

� Airborne Separation Assistance System: An aircraft system that enablesthe flight crew to maintain separation of their aircraft from one or more aircraft,and provides flight information concerning surrounding traffic.

� ASAS application: A set of operational procedures for controllers and flightcrews that makes use of the capabilities of Airborne Separation AssistanceSystems to meet a clearly defined operational goal.

2.2.1.3. Four ASAS application categories have been defined by [PO-ASAS]:

� Airborne Traffic Situational Awareness (ATSA) applications;

� Airborne Spacing (ASPA) applications;

� Airborne Separation applications;

� Airborne Self-separation applications.

2.2.1.4. The overriding consideration in defining these ASAS application categories wasthe level of responsibility and tasks delegated to the flight crew.

2.2.2. Near / medium term ASAS applications for the ECAC area

2.2.2.1. As defined in Package I [PACK I], the near / medium term Airborne Surveillanceapplications envisaged for implementation in Europe are related to the followingtwo high-level categories of applications:

� ATSA applications;

� ASPA applications.

2.2.2.2. The various ASAS applications included within Package I for these two categoriesof ASAS applications are listed in the following table:



Application description AcronymAirborne Traffic Situational Awareness applications category� Enhanced traffic situational awareness on the airport surface Package I/ATSA-SURF� Enhanced traffic situational awareness during flight operations Package I/ATSA-AIRB� Enhanced visual acquisition for see & avoid Package I/ATSA-S&A� Enhanced successive visual approaches Package I/ATSA-SVAAirborne Spacing applications� Enhanced sequencing and merging operations Package I/ASPA-S&M� In-trail procedure in oceanic airspace Package I/ASPA-ITP� Enhanced crossing and passing operations Package I/ASPA-C&P

Table 1: Package 1 Airborne Surveillance applications

2.2.2.3. The implementation of Package I is also an opportunity to build the futurepackages on experience. Some applications included in Package I are going to befurther improved in Package II for better performance through new or morestringent requirements. For those applications, Package I is seen as a first stepleading eventually to the full benefits.

2.2.2.4. It is also envisaged that Package II includes new applications belonging to the twolast categories of the PO-ASAS document (i.e. Airborne separation and airborneself-separation applications). These applications are expected to bring furtherbenefits in terms of capacity and flexibility.

2.2.3. Selection criteria and priority scheme for the IAPA project

2.2.3.1. In the context of the IAPA project, the following aspects were considered in theselection of the most relevant ASAS application:

� Scope and applicability: en route, Terminal control Area (TMA) and surface.

- The larger the scope, the more interesting the ASAS application since itpotentially addresses a wide range of operations.

� Maturity: appraised according to the level of harmonisation of conceptdefinition and the amount of progress in the validation process:- Availability of OSA documentation: Initial Operational Service and

Environment Definition (OSED) was a prerequisite. Initial OperationalHazard Assessment (OHA) work was highly recommended to support theWP10 safety case to be conducted within IAPA from an ACASperspective;

- Availability of real-time simulation data, and radar data, were of highpriority in order to support the analysis to be conducted respectively withinWP09 and WP07 of IAPA.

� Challenging application: an application with the potential for studying amaximum of significant and realistic issues from an ACAS safety andoperational performance perspective.- The degree of challenge in terms of potential ACAS / ASAS interaction

was of highest priority in the selection process: the more challenging theASAS application, the more interesting for the purposes of IAPA.



2.2.4. ASAS applications of interest for the IAPA project

2.2.4.1. Package I/ASPA-S&M, Package I/ASPA-C&P and Package II/separationapplications showed comparable results in the achievement of the criteria with:

� An advantage to Package I/ASPA-S&M for the availability of real-timesimulation data, OSED, radar data and maturity;

� An advantage to Package I/ASPA-C&P and Package II/separation aschallenging applications.

2.2.4.2. Package I/ASPA-S&M may be implemented earlier than Package I/ASPA-C&P orPackage II/separation, but it is less challenging in terms of ACAS interaction.

2.2.4.3. An additional interest in working on the Package I/ASPA-C&P, lateral crossingapplication is that it will also be possible to address the issues related to the initialphase of the Package I/ASPA-S&M, merging application.

2.2.4.4. There is also a particular interest in investigating the Package I/ASPA-C&P, lateralcrossing application because of the potential for a sensitivity study on the spacingvalues and the possibility to assess two different types of operation (i.e. “passbehind” and “pass in-front” operations).

2.2.4.5. Furthermore, the ASAS lateral crossing application can be seen as an excellentbridge between Package I and Package II and it is recognised that a Package Iapplication with the potential to become a Package II application constitutes thebest compromise.

2.2.4.6. In conclusion, the Package I/ASPA-C&P, lateral crossing application was selectedbecause of the significant advantage of the Package I/ASPA-C&P compared toPackage I/ASPA-S&M regarding the ACAS interaction criteria, the relatively lowimpact of existing differences for the other criteria, and the conclusions of thepreparatory analysis presented in the following section.

2.2.5. Preparatory ACAS / ASAS interaction analysis

2.2.5.1. Available documentation related to the potential ASAS applications of interest forthe IAPA study was reviewed in order to define the most relevant set of encountersto be analysed.

2.2.5.2. Descriptions of ASPA-S&M – Sequencing and Merging applications included:

� NUP II Cluster D Arlanda OSED [NUPII-ITS],

� NUP II Cluster D Frankfurt OSED [NUPII-FRA], and

� NUP II Cluster E Cooperative ATS OSED [NUPII-COOPATS].

2.2.5.3. From these descriptions, two encounter types, corresponding to the merging andthe in-trail phases of the ASPA-S&M application, were selected for further analysis.In addition, the encounter dealing with the in-trail phase was proposed to includeaircraft turns.



2.2.5.4. Descriptions of ASPA-C&P – Crossing and Passing applications included:

� MA-AFAS lateral crossing and passing [MA-AFAS], and

� MFF A4 operational procedures (defined as airborne separation ones)[MFF-A4].

2.2.5.5. From these descriptions, three encounter types, corresponding to the lateralcrossing, the vertical crossing and the lateral passing procedures, were selectedfor further analysis.

2.2.5.6. For the various encounter sets, the preparatory ACAS / ASAS interaction analysiswas focused on nominal ASAS operations, although some non-nominal situationswith separation minima infringements were also analysed.

Qualitative encounter sets

2.2.5.7. For each ASAS application of interest, a set of qualitative encounters[IAPA/04/029] was built resulting from all possible combination of the followingrepresentative aircraft types:

A/c type ATR42/72

SAAB2000

A320 B767-300 A340 B747-400

Propulsion Type Turbo Turbo Jet Jet Jet Jet

ApproachCategory

B B C C D D

WV Category Medium Medium Medium Heavy Heavy Heavy

Table 2: Aircraft types

Note: It should be noted that the resulting set of encounters (with equal proportion ofeach selected aircraft type) was not intended to be representative of the aircraftperformance mix in Europe.

2.2.5.8. In addition to the aircraft type, other encounter parameters of interest included therelative positioning of aircraft trajectories, as well as the aircraft relative velocities,at the Closest Point of Approach (CPA).

2.2.5.9. Aircraft trajectories were generated according to aircraft performances defined inthe EUROCONTROL Base of Aircraft Data (BADA) tables [BADA]. Further, thesimulated trajectories correspond to ideal trajectories without any wind effects, andany trajectory blunders or overshoots due to navigation errors, which wasconsidered an acceptable assumption for this preparatory case study.

No anticipated interaction with ACAS under nominal ASAS operations

2.2.5.10. According to the preparatory analysis based on ACAS II Standards andRecommended Practices (SARPs) and dedicated TCAS II logic simulations, nointeraction with ACAS is anticipated for the following ASAS applications:



� ASPA-S&M: Enhanced sequencing and merging operations: For the in-trailphases, whatever the altitude layer, and as far as the Wake Vortex separationminima are preserved.

� ASPA-C&P: Enhanced crossing and passing operations: For the lateralpassing situations, whatever the altitude layer, since the lateral spacingvalues required to trigger an ACAS II alert during slow convergence situationsare of the order of the ACAS II minimum protection distance parameter(DMOD), e.g. 1.3NM for TA above FL200. Such lateral spacing values wouldnot be operationally acceptable.

2.2.5.11. It should be noted that in-trail ASPA-S&M encounters may trigger a TA during turnconfigurations, but these situations would result in a base leg length lower than theradar separation minima in TMA, i.e. 3 NM, which would not be operationallyacceptable.

Potential interaction with ACAS under non-typical ASAS operations

2.2.5.12. Some interaction with ACAS potentially exists for the ASPA-S&M: ‘Enhancedsequencing and merging operations’ for the merging phases, but only duringmarginal situations.

2.2.5.13. In particular, some merging encounters with required spacing at Waypoint (WPT)close to the radar separation minimum in TMA, i.e. 3 NM, may trigger a TA.However, such spacing values between aircraft in sequence are unlikely to occurduring typical merging situations.

Potential interaction with ACAS under nominal ASAS operations

2.2.5.14. Finally, some ACAS / ASAS interaction potentially exists for the ASPA-C&P:‘Enhanced crossing and passing operations’. In particular, TAs are likely to betriggered during the following encounters:

� For the lateral crossing situations, typically with angles of convergencegreater than 90 degrees and HMD at CPA close to the applicable radarseparation minima, i.e. 3 NM in TMA and 5 NM in en-route ECAC airspace;and

� For the vertical crossing situations, typically during level-off encounters atthe applicable vertical separation minima, i.e. 1000 ft below FL415, and2000 ft above, in the ECAC airspace, with operationally realistic vertical rates.These encounters may even trigger RAs when significant, but realistic, relativealtitude rates are observed close to the cleared flight levels.

2.2.5.15. Such 1000 ft level-off encounters are common events, in particular betweenarrivals and departures in TMA. The performance of TCAS II in such encounters,i.e. the potential for “undesirable” RAs, is a matter of concerns for an operationalstandpoint. This issue has already been identified for current ATM operations, andis not specifically linked to the introduction of ASAS operations [EMO7].



2.3. Selected ASAS application: Lateral crossing operations

2.3.1. Operational purpose

2.3.1.1. The purpose of the ASAS lateral crossing procedure is to provide a new air trafficcontrol procedure, allowing one ASAS equipped aircraft to cross a designatedaircraft.

2.3.1.2. The ASAS lateral crossing procedure aims to take account, as much as possible,of the current working methods and practices of flight crews and controllers inorder to smooth transition. The procedure is similar to the visual separationclearance except that it is designed to be applicable both under Visual andInstrument Meteorological Conditions (IMC).

2.3.1.3. Further, to allow for investigation of a wider range of ASAS operations, the IAPAoperational environment [IAPA/01/024] envisages the ASAS application within thescope of the following options:

� Option°1: Airborne spacing applications, where separation minima areunchanged (i.e. applicable radar separation minima in the IAPA environment)and spacing minima depend on aircraft capabilities;

� Option°2: Airborne separation applications, where the separation tasks aretransferred to the flight crew for the duration of the ASAS lateral crossingprocedure and airborne separation standards are defined. These will includeairborne separation minima applicable by the flight crew.

2.3.1.4. The spacing (Option 1) / separation (Option 2) tasks are delegated to the flightcrew in order to support an increase in controller availability, leading to gains inefficiency, and potential capacity within the applicable sectors, whilst maintainingor raising current safety levels.

2.3.2. Operational environment

2.3.2.1. The environmental assumptions for ASAS lateral crossing procedure aresummarised as follows:

� Fixed routes; numerous crossing points and multiple convergence. Mix ofsteady, climbing and descending aircraft;

� Controlled airspace ATS classes A, B, C, D, and E; between FL60 andFL4101;

� Lateral and longitudinal separation minima conform to [ICAO PANS]:Generally not below 5 NM for radar separation minima in en-route airspaceand 3 NM for radar separation in the terminal area;

� 1000ft vertical separation below FL415;

1 Airspace is above FL60 in order to avoid restrictions related to noise abatement constraintsthat alter the nominal climb rates. The upper limit of FL410 corresponds to RVSM upperboundary.



� Minimum applicable spacing (Option 1) / separation (Option 2) during thelateral crossing procedure: 4 NM2;

� All possible combinations of aircraft types and ASAS/ACAS equipage can flyin the dedicated airspace without any restriction;

� The ASAS lateral crossing procedure is to be used by ASAS equipped aircraftconducting routine operations in controlled airspace. The application will notbe used in the final phase of flight.

2.3.3. ASAS lateral crossing procedure

2.3.3.1. The ASAS lateral crossing procedure can be divided into the following mainphases:

� Initialisation phase: concerns the determination of appropriate configurationby ATC for ASAS lateral crossing procedure and initiation of procedure byidentification and selection by flight crew of aircraft to be crossed;

� Execution phase: concerns mainly the execution of the manoeuvres by theaircraft for the ASAS lateral crossing procedure, the spacing(Option 1) / separation (Option 2) assurance task during the procedure andthe termination of the procedure;

� Nominal completion phase: concerns the switching to conventional ATCprocedures when ATC confirms that aircraft has completed the ASAS lateralcrossing procedure;

� Abnormal Ending: represents unexpected termination of the ASAS lateralcrossing procedure prior to nominal completion phase.

2.3.3.2. Two different types of operation (i.e. “pass behind” and “pass in-front” operations)are distinguished, each of which results in a heading alteration by the aircraftperforming the ASAS lateral crossing procedure.

2.3.3.3. The Clear of Traffic (COT) situation corresponds to the time/location when the riskof infringement of applicable separation between aircraft involved in a lateralcrossing procedure is over. At this stage, the aircraft performing the ASAS lateralcrossing procedure can resume its navigation direct to track, i.e. to fly a45 degrees heading towards its initial track.

2 The proposed value for 4 NM is considered to be the minimum applicable between twoRNP-1 compliant aircraft, assuming perfect surveillance and communication performances.This value, which only takes into account aircraft navigation performances, is derived fromthe integrity requirement for positioning accuracy of 99.999 % at 2xRNP established withinthe [RNP MASPS].



Closest point of approach

Aircraft to be crossed

Aircraft performing lateral crossing

Figure 1: Example of a “Pass Behind” instruction

2.3.3.4. No manoeuvre shall be executed that would reduce the horizontal spacingbetween two aircraft to less than the minimum applicable spacing(Option 1) / separation (Option 2) in the circumstances.

2.3.4. Conditions of use

2.3.4.1. The air traffic controller can instruct a flight under his control to perform an ASASlateral crossing procedure if certain general conditions are met, which are intendedto ensure the compatibility of the ASAS application with the provision of separationby ATC.

2.3.4.2. The ASAS lateral crossing procedure can be accepted by the flight crew of acontrolled flight if certain general conditions are met, which allow for the safe andefficient execution of the procedure.

2.3.4.3. In addition to these general conditions of use, the main assumptions related to theASAS lateral crossing procedure are the following:

� Speeds are compatible with the procedure, i.e. there is no expected majorspeed change that may adversely affect the ASAS lateral crossing procedure;

� Aircraft involved in the ASAS lateral crossing procedure can be changingaltitude;

� The crossed aircraft is required to maintain a constant heading or track;

� Required heading alterations for the aircraft performing the ASAS lateralcrossing procedure remain within the following envelope:- Maximum acceptable angle of heading alteration (α) = 30°3; and

- Maximum acceptable lateral deviation (D-LAT);

3 A value of 30 degrees is the order of magnitude derived from operational experience ofATC usual values. A value of 45 degrees would be the value generally used by FlightManagement Systems (FMS) for lateral offset manoeuvres. But, ATC generally uses smallerheading alterations in order to limit the impact of potential negative effect of the manoeuvreon the overall traffic pattern.



� Angle of convergence between initial tracks is between 30 and 150 degrees;

� The aircraft with the highest speed is instructed to perform the lateral crossingwith ASAS. However, if the aircraft have the same speed, the second at trackcrossing point will be manoeuvred;

� The lateral crossing manoeuvre with ASAS is expected to begin no later than2 minutes before CPA and within a maximum look-ahead time of 4 minutes4.

Aircraft performing the lateralcrossing procedure with ASAS

Aircraft to be crossed

D-LAT

α

Figure 2: Envelope for ASAS lateral crossing manoeuvres

Note: In the previous figure, the grey area represents the aircraft trajectory envelopewithin which the aircraft performing the ASAS lateral crossing procedure is requiredto manoeuvre.

4 Although challenging, the proposed values for the start of the ASAS manoeuvre areconsidered operationally realistic within the broad range of possible ASAS lateral crossingoperations, i.e. lateral spacing (Option 1) / separation (Option 2).



2.4. Preparatory simulations and results

2.4.1. General

2.4.1.1. A set of qualitative encounters [IAPA/04/029] was built involving two aircraft, one ofwhich performed an ASAS lateral crossing procedure. Unless otherwise specified,both “pass behind” and “pass in-front” manoeuvres were created with a spacingvalue of 4 NM at CPA.

2.4.1.2. Encounter parameters of interest included:

� the angle of convergence between both aircraft (in between 30 and150 degrees);

� the aircraft performances (i.e. turboprop or jet aircraft);

� the encounter flight level (i.e. FL80, FL140, FL240 or FL330). Indeed,depending on the simulated flight level, TCAS II operates at:- Sensitivity Level (SL) 7 at FL330 and FL240;- SL6 at FL140;- SL5 at FL80.

� the applicable spacing (Option 1) / separation (Option 2) value at CPA (aroundthe default value of 4 NM);

� the vertical profiles of both aircraft, and the resulting Vertical Miss Distance(VMD) at CPA.

2.4.1.3. The aircraft trajectories for encounters involving both aircraft flying level arecreated with the OSCAR generator of artificial trajectories.

2.4.1.4. The aircraft trajectories for encounters involving one aircraft flying level and theother aircraft either climbing or descending are generated according to aircraftperformances defined in the EUROCONTROL BADA tables (version 3.4) fortypical jet and turboprop aircraft.

2.4.1.5. Further, the simulated trajectories correspond to ideal trajectories without any windeffects, and any trajectory blunders or overshoots due to navigation errors oruncertainties.

2.4.2. Results of TCAS II logic simulations

2.4.2.1. TAs were triggered for all encounters except for the following encounters:� Encounters at flight level FL80;

� Encounters at flight level FL140 involving one jet and one turboprop aircraft;

� Encounters with an initial alpha angle of 30° (for all flight levels and pairs ofaircraft);



� Encounters with an initial alpha angle of 90° and 60° for:- two jets at FL140 for both “pass behind” and “pass in front” manoeuvres ;- one jet and one turboprop at FL240 for a “pass in front” ;

� Encounters with an initial alpha angle of 120° involving two jets at FL140 for a“pass in front” manoeuvre.

2.4.2.2. The figure below shows an encounter between a jet and a turboprop aircraft bothflying level at FL240 with initial tracks converging at an angle of 90°. A TrafficAdvisory is triggered during the first phase of the “pass behind” manoeuvre.

Figure 3: Traffic Advisory during an ASAS “pass behind” lateral crossingencounter



2.4.2.3. The figure below shows an encounter between two jets both flying level at FL330with initial tracks converging at an angle of 90°. A Traffic Advisory is triggeredduring the first phase of the “pass in front” manoeuvre.

Figure 4: Traffic Advisory during an ASAS “pass in front” lateral crossingencounter

2.4.2.4. As expected from the preparatory case study [IAPA/04/005], none of the artificialencounters triggered an RA: not even those with high rates of convergence atflight levels FL330 and FL240, i.e. TCAS II Sensitivity Level 7.

2.4.2.5. To further investigate the role of the TCAS II logic version 7.0 Miss Distance Filter(MDF) [TCAS7a], which may suppress an RA that would otherwise be triggered,additional simulations of the “pass behind” ASAS lateral crossing with small cross-track deviations were performed, which resulted in some RA occurrences.



2.4.2.6. The figure below shows an encounter between a jet and a turboprop aircraft bothflying level at FL330 with initial tracks converging at an angle of 120°. An RA istriggered during the first phase of the “pass behind” manoeuvre.

Figure 5: Not filtered RA during an ASAS “pass behind”lateral crossing encounter



2.4.3. Influence of the angle of convergence

2.4.3.1. To investigate the influence of the angle of convergence during the ASAS lateralcrossing manoeuvre, analysis was focused on encounters involving two jet aircraftin cruise flight at FL330, with the same ground speed and various angles ofconvergence varying from 30° to 150°.

2.4.3.2. The results of the TCAS II logic simulations show that the higher the angle ofconvergence, the greater the likelihood that a TA will be triggered. It shouldbe noted that a “pass behind” manoeuvre increases the convergence angle andthat a “pass in front” manoeuvre decreases the convergence angle.

2.4.3.3. Further, whatever the simulated flight level, aircraft types and type of manoeuvre,no TA was generated for encounters with an initial angle of convergence of 30°.

2.4.4. Influence of the aircraft performances

2.4.4.1. To investigate the influence of the aircraft type, analysis was focused onencounters involving two aircraft in cruise flight at FL140 (i.e. an altitude where alltypes of aircraft can fly) with an initial convergence angle of 120°. Severalencounters were created so as to vary the aircraft type, i.e. jet and turbopropaircraft.

2.4.4.2. The simulated ASAS lateral crossing manoeuvre was the “pass behind”manoeuvre with a spacing value of 4.0 NM at CPA. In addition, to investigate asituation with the greatest convergence, the heading alteration angle was set to themaximum value, i.e. 30°.

2.4.4.3. The results of the TCAS II logic simulations show that the closing speed of theaircraft involved, which derives from the aircraft ground speeds, strongly dependon the aircraft types.

2.4.4.4. For a given angle of convergence, the possibility of triggering a TA increaseswith the closing speed. Therefore, it is easier to trigger a TA with an encounterinvolving two jets than with an encounter involving a jet and a turboprop.

2.4.5. Influence of the flight level

2.4.5.1. To investigate the influence of the aircraft type, analysis was focused onencounters involving two jets flying level with tracks converging at an initial angleof 150° at the same flight level. This flight level was varied within each encounter.

2.4.5.2. The results of the TCAS II logic simulations show that the potential to trigger aTA decreases with altitude. Further, when a TA is triggered, its duration alsodecreases with the altitude.

2.4.5.3. The reduced likelihood of a TA being triggered is due to the significant HMDbetween both aircraft at CPA (i.e. the targeted spacing value of 4 NM), and to thefact that the aircraft speed generally decreases with altitude.

2.4.5.4. Further, the thresholds for triggering a TCAS alert decrease with the altitude (i.e.the TA alert time threshold equals 40 seconds at SL5, 45 seconds at

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IAPA Project Interim Report - Phase 1 page i · 2004. 2. 17. · EATM Infocentre ref. : 040123-01 EDITION DATE: November 2003 Abstract ... 12-02-2004 Director ATM Programmes George