ACAS II operations in the European RVSM environment ACAS ...

ACAS II operations in the European RVSM environment 03-08-2001ACTOR/wp1/BROC/D Version 1.0

Eurocontrol ACAS Programme – Project ACTOR /41

ACAS II operations in the European RVSM environment

ACAS training for operations in RVSM environment

Project ACTOR



NOTE

This document is designed specifically for the

training of people involved in the implementation

and the use of the Airborne Collision Avoidance

System (ACAS II). In particular, it addresses

ACAS II operation use and performance in

European Reduced Vertical Separation Minima

(RVSM) airspace. However, it is not, per se,

designed for the complete training of controllers

or pilots. For a deeper knowledge, the reader is

advised to refer to ICAO and RTCA

documentation listed in the bibliography section.

John LawACAS Programme ManagerEUROCONTROL21 August 2001



TABLE OF CONTENTS

1. INTRODUCTION.........................................................................................................................................................4

2. BACKGROUND............................................................................................................................................................5

2.1. ACAS II MANDATE IN EUROPE................................................................................................................................52.2. RVSM IMPLEMENTATION.........................................................................................................................................62.3. EXPERIENCE OF TCAS II IN NAT RVSM ..............................................................................................................7

3. TCAS II DESCRIPTION............................................................................................................................................8

3.1. TCAS II PRINCIPLES ..................................................................................................................................................83.1.1. Main functionalities........................................................................................................................................83.1.2. Triggering of the advisories..........................................................................................................................9

3.2. IMPROVEMENTS PROVIDED BY TCAS II VERSION 7.0.........................................................................................12

4. EXPECTED TCAS II ADVISORIES IN RVSM AIRSPACE....................................................................... 14

4.1. INTRODUCTION.........................................................................................................................................................144.2. TRAFFIC ADVISORIES...............................................................................................................................................144.3. RESOLUTION ADVISORIES.......................................................................................................................................154.4. RATIO BETWEEN TAS AND RAS.............................................................................................................................154.5. CONCLUDING REMARKS..........................................................................................................................................15

5. COMMON ENCOUNTERS IN THE RVSM ENVIRONMENT IN EUROPE........................................ 17

5.1. INTRODUCTION.........................................................................................................................................................175.2. STANDARD RVSM VERTICAL SEPARATION .........................................................................................................185.3. MAIN CRITICAL CONFIGURATIONS EXPECTED......................................................................................................20

5.3.1. Vertical offset................................................................................................................................................ 205.3.2. Oscillation ..................................................................................................................................................... 235.3.3. Turbulence .................................................................................................................................................... 265.3.4. 1000 ft level-off............................................................................................................................................. 295.3.5. Specific case of low closure rates ............................................................................................................. 32

6. OPERATIONAL IMPLICATION........................................................................................................................ 35

6.1. CONTROLLERS...........................................................................................................................................................356.1.1. Number of TCAS II advisories ................................................................................................................... 356.1.2. Triggering of TAs ......................................................................................................................................... 356.1.3. Triggering of RAs......................................................................................................................................... 35

6.2. PILOTS........................................................................................................................................................................366.2.1. Number of TCAS II advisories ................................................................................................................... 366.2.2. Expected behaviour..................................................................................................................................... 366.2.3. Recommendation for 1000 ft level-off encounters.................................................................................. 37

7. CONCLUSION ........................................................................................................................................................... 38

8. REFERENCES ........................................................................................................................................................... 39

9. ACRONYMS ............................................................................................................................................................... 40

10. BIBLIOGRAPHY ………………………………………………………………………………………..41



1. Introduction

This brochure is part of a training packagethat addresses ACAS II (Airborne CollisionAvoidance System) performance in theRVSM (Reduced Vertical SeparationMinimum) environment, both from a pilotand a controller perspective.

For clarity, the brochure will only deal withTCAS II (Traffic alert and CollisionAvoidance System) and more precisely withTCAS II version 7.0 even if it should beACAS II, except for the description of theEuropean ACAS II mandate.

The objective of the brochure is to enablepilots and controllers to:

• understand the behaviour of TCAS II inthe RVSM environment;

• know what to expect from TCAS II in theRVSM environment;

• better manage advisories whether they areperceived necessary or not.

This package also includes:

• a set of slides highlighting the main pointsdescribed in the brochure;

• a quiz aiming to verify that the basicknowledge of TCAS II operations in theRVSM environment has been acquired.

The brochure first describes somebackground about the ACAS II mandate inEurope, the RVSM implementation and theexperience of TCAS II in the North AtlanticRVSM airspace.

The second part describes the principal andessential technical features of TCAS II,specifically for RVSM airspace, tounderstand the principle of advisorytriggering. This part of the brochure alsodescribes the improvements brought byTCAS II version 7.0 in comparison withversion 6.04a for the RVSM environment.

The core of the brochure, which presents theexpected TCAS II and RVSM interaction inEurope, is divided into two parts:

• the main results of the study on theTCAS II and RVSM interactionperformed within the EUROCONTROLco-ordinated ACASA project. Theyinclude some statistics about the expectedadvisories in the European airspace (e.g.number, duration, induced deviations)and a comparison with TCAS IIperformance in the current FL250-FL290altitude band.

• the description of the standard RVSMvertical separation, the main criticalconfigurations expected (i.e. imperfectaltitude keeping, which includes verticaloffsets, oscillations and turbulences, and1000 ft level-off) and the specific case oflow closure rate encounters. The TCAS IIbehaviour is explained for eachconfiguration and operationalrecommendations are provided.

The last part summarises the operationalimplication of TCAS II in the RVSMenvironment separately for controllers andpilots.



2. Background

2.1. ACAS II mandate in Europe

In 1995, the ECAC (European Civil AviationConference) States agreed a common ACASpolicy and implementation schedule for themandatory carriage of an ACAS II in Europe.This policy was confirmed by the ECACTransport Ministers in 1997. The approvedpolicy requires that:

• from 1st January 2000, all civil fixed-wing turbine-engine aircraft having amaximum take-off mass exceeding15000 kg or a maximum approvedpassenger seating configuration of morethan 30 will be required to be equippedwith ACAS II, and

• from 1st January 2005, all civil fixed-wing turbine-engine aircraft having amaximum take-off mass exceeding 5,700kg or a maximum approved passengerseating configuration of more than 19will be required to be equipped withACAS II.

ACAS II can issue two types of advisories:

• Traffic Advisories (TAs), which aim atalerting the crew to be ready for apotential Resolution Advisory and athelping the crew in the visual search forthe intruder aircraft;

• Resolution Advisories (RAs), which areavoidance manoeuvres in the verticalplane recommended to the pilot.

TCAS II version 7.0 is the only equipment,which complies fully with ACAS IIStandards And Recommended Practices(SARPs) published by the InternationalCivil Aviation Organisation (ICAO).Therefore TCAS II version 7.0 is requiredto meet the ACAS II mandate.

TCAS II version 7.0 has not beenspecifically developed to address the issueof TCAS II version 6.04a and RVSMoperational incompatibility, which has beenidentified with the NAT (North Atlantic)RVSM implementation. The overallobjective of this new version is to improvethe general TCAS II performance (e.g.better compliance with ATC (Air TrafficControl) procedures, interference limiting,limitation of the altitude deviations inresponse to RAs, etc.).

Practical implementation issues, involvingsupply, installation and certification ofACAS II equipment were identified duringthe implementation phase. To permitresolution of these difficulties ACAS IItemporary exemptions were available until30 September 2001 in order to providerespite to some operators that were unableto equip some of their aircraft with ACASII in time because of technical or supplyproblems.



2.2. RVSM implementation

RVSM is an approved ICAO concept toreduce aircraft separation minimum from2000 ft to 1000 ft from FL290 to FL410inclusive. The purpose of RVSM is toincrease airspace capacity and provideairspace users with more flight levels andthus optimised flight profiles.

The NAT Region was the first one in whichRVSM was implemented on 27 March 1997,initially from FL330 to FL370. On 8 October1998 NAT RVSM operations were expandedto FL310-FL390 inclusive. On 24 January2002 RVSM in the NAT Region is plannedto be further expanded to FL290-FL410inclusive, so that the RVSM operations willbe harmonised with the European (EUR)Region [RVSM].

RVSM has also been implemented in thePacific Region since 24 February 2000,between FL290 and FL390, within theOakland Oceanic FIR and Anchorage FIR.

On 12 July 2001 the EUROCONTROLProvisional Council confirmed theimplementation of RVSM in the Europeanairspace (shown on figure 1) with effectfrom the 24 January 2002, between FL290and FL410 (inclusive). Except wheretransition tasks are carried out, onlyaircraft, which are RVSM approved, will beauthorised to operate within the EURRVSM airspace.

Figure 1: European RVSM and RVSM transition airspace



An early introduction of RVSM has started inIreland, the United Kingdom, Germany andAustria. This early introduction is not basedon the procedures and requirements in placefor the full European RVSM implementation.The early introduction of RVSM by Germanyand Austria consists of the application byATC of a 1000 ft vertical separation aboveFL290 between RVSM approved aircraft ona tactical basis. In the United Kingdom andIreland RVSM levels can be requested on theflight plan. Until 24 January 2002, non-RVSM approved aircraft are not excludedfrom the airspace in which this earlyapplication is in use.

The mandatory carriage of ACAS II inEurope and the implementation of RVSM arenot linked even if there is an interactionbetween them. ACAS II is not, itself, aprerequisite for RVSM. The ACAS IImandate does not imply that all aircraft in theRVSM airspace will be equipped withTCAS II version 7.0, in particular before 1stJanuary 2005. Some of them (e.g. certainbusiness jets) can either still be equippedwith TCAS II version 6.04a or not equippedat all.

2.3. Experience of TCAS II in NATRVSM

Since 27 March 1997, RVSM has beenimplemented progressively in the NATairspace. Aircraft equipped with TCAS IIversion 6.04a have flown in this airspacesince this date. As TCAS II version 6.04a hasbeen developed on the basis of a 2000 ftvertical separation above FL300, it is notadapted to RVSM airspace. Furthermore inthe NAT airspace, aircraft

follow the same tracks for several hourswith close ground speed values, which wasa new operational configuration forTCAS II.

Prior to the implementation of RVSM inthe NAT airspace, a study [NAT] showedthat TCAS II version 6.04a in RVSMairspace would generate some potentialissues:

• undesirable TAs would be triggeredbetween level aircraft separated by1000 ft and they can last severalminutes and be repetitive;

• undesirable RAs could be triggeredbetween aircraft separated by 1000 ft;

• large vertical deviations (e.g. severalthousands of feet) in response to RAscould occur despite appropriate pilots’reactions.

In the first weeks following theimplementation of RVSM in the NATairspace, a large number of complaintsemerging from pilots were identified aboutundesirable long duration TAs betweenaircraft at adjacent flight levels. Such TAswere regularly issued by TCAS IIversion 6.04a. No operational procedurelike a navigational offset or a slight speedmodification has been approved toeliminate the recurring or long durationTAs, a lateral offset being only allowed toavoid wake vortex.

Since the beginning, a few undesirable RAshave been reported. However none of themhas induced any large vertical deviation.The RAs were triggered mainly because ofturbulence (wake vortex or meteorologicalturbulence) or imperfect altitude keeping.



3. TCAS II description

3.1. TCAS II principles

3.1.1. Main functionalities

TCAS II is an on-board system designed toimprove air safety by acting as a 'last-resort'method of preventing mid-air collisions ornear mid-air collisions by recommending topilots manoeuvres in the vertical plane whena risk of collision is detected.

TCAS II is designed to work bothautonomously and independently of anyaircraft navigation equipment and groundsystems. TCAS II monitors other aircraft inthe vicinity by interrogating their transponderand assesses the risk of collision.

When the intruder aircraft is also fitted witha TCAS II, both TCAS II units co-ordinatetheir RAs in order to select complementaryresolution senses. This feature makes itvery important that pilots should followtheir RAs as accurately as possible.Furthermore it is hazardous tomanoeuvre in the opposite sense to theRA, as shown on figure 2.

Closest Point of Approach(CPA)

“Climb, Climb”

Prohibitedmanoeuvre

“Descend, Descend”

Standardmanoeuvre

Standardmanoeuvre

Figure 2: Co-ordinated RAs in TCAS-TCAS encounters



If for any reason, a pilot cannot follow a“Climb” RA triggered by his TCAS II, heshould at least remain level and not initiate adescent.

The TCAS II logic is based on two mainconcepts:

• the warning time, which is based on thetime-to-go (and not distance-to-go) to theClosest Point of Approach (CPA);

• the sensitivity level, which is a functionof the altitude and which defines thelevel of protection by assigning largerthreshold values for higher altitudebands.

To better understand the main criticalconfigurations between TCAS II and RVSM,the next part describes briefly the basicprinciples for the triggering of TAs and RAs.However, it addresses specifically thesensitivity level value ranging from FL200 toFL420, which includes the RVSM airspacealtitudes.

For a more detailed description of TCAS II,readers should refer to the ACAS brochuredeveloped by the EUROCONTROL ACASProgramme [ACAS].

3.1.2. Triggering of the advisories

TCAS II triggers an advisory when a rangetest and an altitude test are both satisfied.These tests are performed on each altitude-reporting target, every second.

The principle of the range test is to computethe time-to-go to the CPA by dividing thedistance between aircraft by the closure rate.The range test is satisfied if this time is lowerthan a threshold value, i.e. 48 s for TAs and35 s for RAs.

To address the specific case of very lowclosure rates, a protection distance has beenintroduced to avoid an intruder comingvery close in range without triggering anyadvisories. For this type of encounter, therange test is satisfied if the distancebetween aircraft is lower than thisprotection distance, i.e. 1.3 NM for TAsand 1.1 NM for RAs.

The principle of the altitude test is tocompute the time to reach co-altitude bydividing the relative altitude betweenaircraft by the relative vertical speed. Thealtitude test is satisfied if this time is lowerthan a threshold value, i.e. 48 s for TAs and35 s for RAs. If the own aircraft is level,the time threshold value for RAs is reducedto 25 s in order to detect a possible level-offmanoeuvre of the intruder and thus to avoidthe triggering of the RA.

To address the specific case of level aircraftor aircraft with a very low relative verticalspeed, the altitude test is satisfied if therelative altitude between aircraft is lowerthan a fixed vertical distance threshold, i.e.850 ft for TAs and 700 ft for RAs.

It should be noted that the altitude of theintruder used by TCAS II is received byinterrogating its transponder. Therefore thisaltitude is encoded with either a 100 ft or a25 ft quantization. It means that the altitudeof the intruder seen by TCAS II can beslightly different than the real one. Incontrast, TCAS II sees the altitude of theown aircraft with a 1-ft quantization.

Figures 3 and 4 illustrate the range and thealtitude tests, with the associated thresholdvalues for the altitude band including theRVSM airspace.



ANDRange test Altitude test

OR

Timecriteria

< 48 s

Intruder

TCAS

Distancecriteria

1.3 NM

Intruder

TCAS

(very lowclosure rate)

< 48 s

Intruder

TCAS

850 ft

850 ft

TCAS

Intruder ⇒ RA

Intruder ⇒ no RA

Figure 3: TA triggering by TCAS II version 7.0 in RVSM airspace



ANDRange test Altitude test

OR

Timecriteria

< 35 s

Intruder

TCAS

Distancecriteria

1.1 NM

Intruder

TCAS

(very lowclosure rate)

< 35 s

Intruder

TCAS

(< 25 s if TCASaircraft is level)

700 ft

700 ft

TCAS

Intruder ⇒ RA

Intruder ⇒ no RA

Figure 4: RA triggering by TCAS II version 7.0 in RVSM airspace



When an RA is triggered, TCAS II computesthe most appropriate vertical manoeuvre inorder to achieve a target vertical missdistance, which is equal to 600 ft.

TCAS II tries to prevent as much as possiblethe triggering of RAs that induce a verticalcrossing of the trajectories. Some very strongbiases against crossing RAs have beenimplemented. This type of RA is rare.

During the course of the encounter, theadvisory strength is continuously evaluatedand can be modified either by increasing it orreversing it if necessary, or by weakening itif the threat reduces. Weakening the RAshould reduce the vertical deviation.

TCAS II is able to handle multiple threatencounters either by attempting to resolve thesituation with a single RA, which willprovide the target vertical miss distance fromeach of the threat aircraft, or by selecting anRA that is a composite of non-contradictoryclimb and descend restrictions.

3.2. Improvements provided byTCAS II version 7.0

The development of TCAS II version 7.0 wasbased on the results of the variousoperational

monitoring programmes of TCAS IIversion 6.04a, which have identified severaloperational and technical issues. The FAA,in collaboration with RTCA SC-147,decided that version 7.0 would address onlythe items that were considered essential[TCAS7]. Among all the changes that wereincluded in version 7.0, some were made inorder to improve the TCAS II operations inRVSM airspace.

The main improvement comes from themodification of the altitude bandboundaries of the TCAS II. TCAS IIversion 6.04a defines a set of parametervalues for altitudes greater than FL300,which is compatible with the 2000 ftvertical separation applied in CVSM(Conventional Vertical SeparationMinimum) airspace but not with a 1000 ftvertical separation. To take into account theimplementation of RVSM, the FL300boundary has been modified to FL420within TCAS II version 7.0. ThereforeTCAS II version 7.0 uses the sameparameter values between FL200 andFL420, which are compatible with RVSM.

Table 1 shows the values of the mainTCAS II parameters for the advisorytriggering in the altitude band including theRVSM airspace for both TCAS II versions.

TA RA

Timethreshold

Altitudethreshold

Timethreshold

Altitudethreshold

Target verticalmiss distance

TCAS II v6.04a 48 s 1200 ft 35 s 800 ft 700 ft

TCAS II v7.0 48 s 850 ft 35 s 700 ft 600 ft

Table 1: TA and RA parameter values in RVSM airspace



The consequences of this boundarymodification are:

• a reduction of the altitude threshold forTAs from 1200 ft to 850 ft, whichprevents the triggering of TAs betweenlevel aircraft separated by 1000 ft;

• a reduction of the altitude threshold forRAs from 800 ft to 700 ft, whichdecreases the TCAS sensitivity toimperfect altitude keeping;

• a reduction of the target vertical missdistance from 700 ft to 600 ft, whichshould decrease or even remove thevertical deviations in response to RAs.

In addition, if the own aircraft is level, thereduced time threshold value is furtherreduced from 30 s to 25 s in order to providea few more seconds to detect a possible level-off manoeuvre of the intruder and thus toavoid the triggering of the RA. This shoulddecrease the number of RAs for level aircrafttriggered by aircraft levelling off 1000 ftabove or below.

TCAS II version 7.0 also introduces amechanism to weaken RAs when aircraft arediverging very slowly but not sufficiently toissue the clear of conflict. This improvementis implemented in order to avoid largedeviations resulting from long duration RAsthat are not weakened by TCAS IIversion 6.04a.

Yet another new feature of TCAS IIversion 7.0 should improve significantlyTCAS II overall performance and thereforein RVSM airspace too. A filtering of RAsin the horizontal plane, called MissDistance Filtering (MDF), has beenimplemented. It prevents RAs to betriggered if the predicted horizontaldistance at CPA is sufficient in terms ofcollision avoidance. As a consequence,some RAs can be filtered in the RVSMairspace for aircraft with a predictedhorizontal distance at CPA greater than2.2 NM under specific conditions (e.g.aircraft steady in heading). Therefore thereis a possibility that some expected RAsmay not be triggered in such situations.

Finally, the determination process of thetype of RA to be triggered during anencounter with another TCAS II equippedaircraft has been modified in TCAS IIversion 7.0. The objective is to avoid as faras possible to generate an RA, which wouldrequire a manoeuvre opposite to the currentown aircraft vertical trajectory. Forinstance, a climbing aircraft should receivean “Adjust Vertical Speed” RA requiring toreduce the rate of climb instead of a“Descend” RA. The aim is to improve thecompatibility of the triggered RA with theclearance and therefore to reduce thedisruption caused to controllers and pilots.



4. Expected TCAS II advisories in RVSM airspace

4.1. Introduction

Within the framework of theEUROCONTROL co-ordinated ACASA(Airborne Collision Avoidance SystemAnalysis) project, whose objective was toinvestigate several areas related to TCAS IIoperations in Europe, one of the mainactivity was dedicated to the study ofTCAS II and RVSM interaction incontinental Europe [ACASA].

The goal of the TCAS II and RVSMinteraction study was to perform acomparison of TCAS II performance withinthe future RVSM environment and thecurrent CVSM environment. The study wasto focus not only on technical issues, but alsoto identify potential operational issues and toprovide recommendations. In order to copewith a larger set of issues, the study wasbased on different sources of data, i.e.modified radar data, real-time and fast-timesimulations, automatic and non-automaticartificial encounters.

The main results of this study are presentedhereafter. They provide a good indication ofthe number of advisories that are expected tooccur when TCAS II version 7.0 will be inoperation in RVSM airspace. To betterassess the operational implication for pilotsand controllers, a comparison is made withwhat is currently happening in the altitudeband FL250-FL290.

4.2. Traffic Advisories

With the whole TCAS II fleet fitted withTCAS II version 7.0, the number of TAs inRVSM is expected to increase by a factor ofabout 4 in comparison with CVSMoperations. Actually, this increase is mainlydue to the low number of TAs triggered inCVSM.

However, the number of TAs triggered byTCAS II version 7.0 in RVSM airspace isexpected to be lower than the number in theFL250-290 altitude band. Nevertheless, theincreased number of TAs triggered aboveFL290 may become an operational issue ifpilots are not properly informed.

This issue is much more critical for aircraftthat will still be fitted with TCAS IIversion 6.04a: the study has pointed outthat the number of TAs triggered in RVSMshould be increased by a factor of about 25in comparison with CVSM. In this case, therate of TA triggering is disturbing forpilots. It may also have a negativeimplication on ATC if pilots request for atraffic information following a TA or eveninitiate a manoeuvre.

On a pilot perspective, it is expected that amaximum of 47 TAs per 1000 flight hoursshould be triggered by TCAS II version 7.0in RVSM (e.g. about 1 TA every 20 flighthours). This ratio is three times greater thanthe one for CVSM (17 TAs per 1000 flighthours) but it is four times smaller than theone for the FL250-290 altitude band (184TAs per 1000 flight hours). Finally it isexpected that TCAS II version 6.04a shouldgenerate 840 TAs per 1000 flight hours inRVSM, i.e. about 1 TA per flight hour.

Two specific types of TAs can be expectedin greater proportions than in CVSM:repetitive TAs and long duration TAs.

The proportion of repetitive TAs isexpected to increase compared with CVSMbut it should be close to the one alreadyexperienced below FL290 (i.e. about oneout of ten TAs). Nevertheless, repetitiveTAs may appear to be a TCAS IIoperational issue if pilots consider therepeated occurrences of the “Traffic,Traffic” aural annunciation as disruptive.



The issue of long duration TAs is notexpected to be as significant as it was withTCAS II version 6.04a during theimplementation of RVSM in the NAT. First,TCAS II version 7.0 reduces drastically thenumber of TAs, in particular by suppressingTAs between aircraft separated by 1000 ft.Then, the route structure is totally differentin the NAT and EUR Regions: the specificconfiguration that induces long duration TA(i.e. low closure rate encounters, see part 4.4)is expected to be rare in the Europeanairspace.

4.3. Resolution Advisories

With the whole TCAS II fleet fitted withTCAS II version 7.0, the number of RAs inRVSM is also expected to increase by afactor of about 4 in comparison with CVSMoperations. This increase is again due to thevery low number of RAs triggered in CVSM.

For aircraft equipped with TCAS IIversion 6.04a, pilots will receiveapproximately twice as many RAs as withTCAS II version 7.0. Nevertheless, based onthe current experience below FL290, thisnumber is expected to be operationallytolerable to both pilots and controllers.

On a pilot perspective, it is expected that amaximum of 3 RAs per 1000 flight hoursshould be triggered by TCAS II version 7.0in RVSM (e.g. about 1 RA every 330 flighthours). This ratio is three times greater thanthe one for CVSM (1 RA per 1000 flighthours) but it is five times smaller than theone for the FL250-290 altitude band (15 RAsper 1000 flight hours). It is expected thatTCAS II version 6.04a should generate 6RAs per 1000 flight hours in RVSM, i.e.twice as many as TCAS II version 7.0.

A majority of the RAs could be considered asundesirable. Nevertheless, this number isexpected to be lower than the one currentlyexperienced in the FL250-290 altitude band,which has no adverse effect on ATCperformance.

A large proportion of these undesirableRAs are caused by 1000 ft level-offmanoeuvre, which is an already knownTCAS II issue below FL290 (cf.paragraph 5.3.4 for the description andparagraph 3.2 for the improvementsprovided by TCAS II version 7.0).

The characteristics of the RAs triggered byTCAS II version 7.0 in RVSM airspace areclose to those triggered between FL250 andFL290 in terms of duration (about 22 s),percentage of positive RAs (i.e., “Climb”or “Descend”) or deviations.

The study has also highlighted that somerepetitive RAs may be expected withTCAS II version 7.0.

4.4. Ratio between TAs and RAs

The ratio between TAs and RAs forTCAS II version 7.0 is not greatly modifiedby the implementation of RVSM. Thenumber of TAs not followed by an RA isstill significant.

This result underlines the fact that noaction (i.e. request of traffic information ormanoeuvre) should be initiated by pilots inthe event of a TA.

With TCAS II version 6.04a, the ratio isexpected to be about 4 times greater thanwith TCAS II version 7.0 because of thevery high frequency of TA triggering. Theconsequence is that TAs would not fulfiltheir role to prepare the crew for a possibleRA because of the very large number ofTAs without any RA.

4.5. Concluding remarks

TCAS II version 7.0 is compatible withRVSM. However, the number of TCAS IIadvisories above FL290 is expected toincrease with the implementation ofRVSM. Nevertheless, the expectedadvisory rate in the RVSM airspace islower than the rate currently observedbetween FL250 and FL290.



TCAS II version 6.04a is not compatiblewith RVSM for operational reasons, mainlya very high rate of TA triggering.

However, a mixed equipage with a lowpercentage of TCAS II version 6.04a (i.e.10%) among a fleet mainly equipped withTCAS II version 7.0 is not expected to havea significant implication for controllers incomparison with a full TCAS II version 7.0scenario. Nevertheless, the high rate ofadvisories triggered by TCAS IIversion 6.04a will be an operational issue forthe pilots of these aircraft.

No TCAS II safety issues related to theimplementation of RVSM have beenidentified. The preliminary safety studieshave concluded that TCAS II version 7.0provides safety benefits in an RVSMenvironment. Despite operationalcompatibility issues, TCAS II version 6.04aalso provides safety benefits.



5. Common encounters in the RVSM environment in Europe

5.1. Introduction

The most common encounter involves twolevel aircraft separated by 1000 ft. TCAS IIversion 7.0 is compatible with suchconfigurations whereas TCAS IIversion 6.04a triggers TAs.

In addition some potential criticalconfigurations between TCAS II and RVSMhave been identified. Advisories are expectedto be mainly triggered by two of them:• imperfect altitude keeping, which

includes vertical offset, oscillation andturbulence;

• 1000 ft level-off encounters.

In addition, there is the specific case ofencounters involving aircraft with a lowclosure rate. They do not by themselvestrigger any advisory but they are likely tomake the advisories caused by the criticalconfigurations last much longer (i.e. severalminutes).

All these configurations are describedseparately hereafter. However, it should benoted that several of them can occursimultaneously in a specific encounter (e.g.an aircraft levelling off below an oscillatingaircraft), thus increasing the probability of anadvisory triggering.

In the description of the criticalconfigurations, it is assumed that:• the aircraft are equipped with TCAS II

version 7.0 (a specific paragraph dealswith TCAS II version 6.04a in eachdescription);

• the aircraft are RVSM approved1;

the aircraft are converging or are close inrange.

The conventions used in the figures todescribe the configurations are as follows:

Trajectory seen by TCAS II

Flight level

Trajectory predicted by TCAS II

TCAS II threshold

1 “automatic altitude control system is required capable of controlling altitude within ±20 m (±65 ft) about theselected altitude, when the aircraft is operated in straight and level flight under non-turbulent non-gust conditions”[TGL6].



5.2. Standard RVSM verticalseparation

Description

These encounters involve level aircraft atadjacent flight levels separated by 1000 ft.Figure 6 illustrates this standard type ofencounter for TCAS II version 7.0.

Explanation for TCAS II version 7.0

In case of level aircraft, the altitude test isperformed by the comparison of the relativealtitude between the aircraft with an altitudethreshold. If the relative altitude is lowerthan this threshold, the altitude test issatisfied and an advisory is triggered.

The altitude threshold values for TCAS IIversion 7.0 are 850 ft for TAs and 700 ft forRAs. It means that when both aircraft arelevel at adjacent flight levels, the altitudetest is not satisfied and therefore neither aTA nor an RA will be triggered.

FL340

FL330

850 ft

Intruder

TCAS

1,000 ft ⇒ no TA

Figure 6: standard RVSM vertical separation



Difference with CVSM airspace

TCAS II version 7.0 behaves identically inCVSM and RVSM (i.e. no advisory betweenaircraft separated by the standard verticalseparation) because its thresholds arecompatible with a 1000 ft vertical separationbetween FL290 and FL410.

Specific case of TCAS II version 6.04a

TCAS II version 6.04a has larger thresholdvalues for altitudes corresponding to RVSM:1200 ft for TAs and 800 ft for RAs.

As a consequence, in RVSM, a TA will betriggered as soon as the range test is satisfiedbetween two aircraft because the relativealtitude is lower than the altitude thresholdfor TAs.

Nevertheless, when two aircraft are level atadjacent flight levels, TCAS II version 6.04adoes not trigger any RA. Indeed the altitudetest is not satisfied because the altitudethreshold for RAs is lower than the standardvertical separation.

In CVSM, the standard vertical separationbetween flight levels is 2000 ft, which ismuch larger than the altitude thresholdvalue, thus preventing the triggering of anyadvisory.

Pilots’ and controllers’ behaviour

With a full TCAS II version 7.0 equipage,no advisory will be triggered even with thereduced vertical separation. Therefore therewill be no implication for controllers andpilots.

Pilots of aircraft fitted with TCAS IIversion 6.04a should be aware that they aregoing to experience a large number of TAsin RVSM triggered by aircraft level andseparated by 1000 ft. Nevertheless, thosepilots should not request for a trafficinformation and they shall not manoeuvrein response to TAs only.



5.3. Main critical configurationsexpected

5.3.1. Vertical offset

Description

These encounters involve level aircraft atadjacent flight levels. Each aircraft has avertical offset towards the flight level of theother aircraft. As a consequence the relativealtitude between the aircraft is lower than1000 ft and TAs can be triggered in somerare cases. Figure 7 illustrates this type ofencounter in which both aircraft have offsets.


When each aircraft has a vertical offsettowards the other aircraft, the relative altitudeseen by TCAS II can be lower than thealtitude threshold for TAs (i.e. 850 ft).Consequently, the altitude test is satisfied anda TA is triggered.

This type of encounter can occur with RVSMapproved aircraft, i.e. with a maximum offsetvalue of 65 ft. Indeed, the altitude of theintruder aircraft seen by TCAS II can beslightly different from the real one due to thealtitude report quantization, especially with a100 ft altitude report quantization.

For instance, two 51-ft offsets induce thetriggering of a TA with a 100 ft altitudereport quantization. A level aircraft at33949 ft is seen at 33900 ft by the TCAS IIof the other aircraft (i.e. a 100 ft offset isseen).

Then if the other aircraft has also an offsetof 51 ft (i.e. the aircraft flies at 33051 ft),the relative altitude seen by TCAS II is849 ft (i.e. a 151 ft combined offset). Asthis value is lower than the altitudethreshold (850 ft), the altitude test istherefore satisfied and a TA is triggered.



The minimum relative altitude seen byTCAS II will not be lower than 835 ftwhereas the altitude threshold for RAs is700 ft. Therefore no RA can be triggeredeven with two offsets equal to the maximumvalue compatible with RVSM approval (i.e.65 ft) and with a 100 ft altitude reportquantization.

If only one aircraft has a vertical offset, noadvisory is triggered because even with a100 ft altitude report quantization, therelative altitude seen by TCAS II will not belower than 900 ft. Therefore the altitude testwill not be satisfied.

With a 25 ft altitude report quantization,which is the most frequent case, no TA canbe triggered even with two offsets equal tothe maximum value compatible with RVSMapproval (i.e. 65 ft). Indeed, for instance

an aircraft is at 33065 ft and the otheraircraft is at 33935 ft. With the 25 ftaltitude report quantization, the altitudeseen by the TCAS II of the lower aircraft is33925 ft. Therefore the minimum relativealtitude seen by TCAS II is 860 ft. Thealtitude test is not satisfied.

As a conclusion, TAs triggered betweenlevel aircraft at adjacent flight levels withvertical offsets are rare events. They canoccur when the intruder reports its altitudewith a 100 ft altitude report quantization (asmall proportion of the fleet) and whenboth aircraft have at least a 51-ft verticaloffset.

FL340

FL330

< 850 ft ⇒ TA

Figure 7: vertical offset




In CVSM airspace, neither TAs nor RAs canbe triggered: the difference between the2000 ft vertical separation applied in thisairspace and the altitude threshold (i.e. 850 ftfor TAs) is too large to be influenced byvertical offsets of both aircraft, even if theyare at the maximum of the approved values.


TCAS II version 6.04a triggers TAs betweenlevel aircraft at adjacent flight levelsseparated by 1000 ft. Therefore it alsogenerates TAs for aircraft with verticaloffsets.

With a 100 ft altitude report quantization,the minimum relative altitude seen byTCAS II cannot be lower than 835 ftwhereas the altitude threshold for RAs is800 ft. Therefore TCAS II version 6.04adoes not trigger any RA between RVSMapproved aircraft flying with verticaloffsets.


In case of TAs, whether in an RVSMairspace or not, pilots should not request fora traffic information and they shall notmanoeuvre in response to TAs only. Themain objective of the TA is to prepare thecrew to a possible RA.

TAs are not to be notified by pilots tocontrollers (there is no standardisedphraseology to report TAs). Thereforecontrollers have no specific procedure toapply.



5.3.2. Oscillation

Description

These encounters involve level aircraft atadjacent flight levels. They have oscillationsin the altitude keeping. As a consequence therelative altitude between the aircraft can belower than 1000 ft and TAs can be triggeredin some rare cases. Figure 8 illustrates thistype of encounter in which both aircraft areoscillating respectively around FL330 andFL340.


When both aircraft oscillate around theircleared flight level, the relative altitude seenby TCAS II can be lower than the altitudethreshold for TAs (i.e. 850 ft). Consequently,the altitude test is satisfied and a TA istriggered.

This type of encounter can occur with RVSMapproved aircraft, i.e. with a maximumoscillation amplitude of 65 ft. Indeed, thealtitude of the intruder aircraft seen byTCAS II can be slightly different from thereal one due to the altitude reportquantization, especially with a 100 ft altitudereport quantization.

For instance, two aircraft oscillating with a60-ft amplitude around their cleared flightlevel can induce the triggering of a TA witha 100 ft altitude report quantization.Indeed, an aircraft oscillating around34000 ft can be seen at 33900 ft by theTCAS II of the other aircraft when itsaltitude gets lower than 33950 ft. Then if atthe same time the altitude of the otheraircraft is greater than 33050 ft, the relativealtitude seen by its TCAS II is lower thanthe altitude threshold (850 ft). Therefore thealtitude test is satisfied and a TA istriggered.

The minimum relative altitude seen byTCAS II will not be lower than 835 ftwhereas the altitude threshold for RAs is700 ft. Therefore no RA can be triggeredeven with two simultaneous oppositeoscillations equal to the maximum valuecompatible with RVSM approval (i.e. 65 ft)and with a 100 ft altitude reportquantization.



If only one aircraft is oscillating, no advisoryis triggered because even with a 100 ftaltitude report quantization, the relativealtitude seen by TCAS II will not be lowerthan 900 ft. Therefore the altitude test willnot be satisfied.

With a 25 ft altitude report quantization,which is the most frequent case, no TA canbe triggered even for two aircraft oscillatingwith the maximum value compatible withRVSM approval (i.e. 65 ft). Indeed, anaircraft can reach 33065 ft and the otheraircraft 33935 ft. With the 25 ft altitudereport quantization, the altitude seen by theTCAS II of the lower aircraft is 33925 ft.Therefore the minimum relative altitude seenby TCAS II is 860 ft. The altitude test is notsatisfied.

As a conclusion, TAs triggered betweenoscillating aircraft on adjacent flight levelsare rare events. They can occur when theintruder reports its altitude with a 100 ftaltitude report quantization (a smallproportion of the fleet) and when bothaircraft have at least 51-ft simultaneousopposite oscillations.

FL340

FL330

< 850 ft ⇒ TA

Figure 8: oscillation




In CVSM airspace, neither TAs nor RAs can betriggered: the difference between the 2000 ftvertical separation applied in this airspace and thealtitude threshold (i.e. 850 ft for TAs) is too largeto be influenced by oscillations of both aircraft,even if the amplitudes are at the maximum of theapproved values.


TCAS II version 6.04a triggers TAs between levelaircraft at adjacent flight levels separated by1000 ft. Therefore it also generates TAs foroscillating aircraft.

With a 100 ft altitude report quantization, theminimum relative altitude seen by TCAS IIcannot be lower than 835 ft whereas the altitudethreshold for RAs is 800 ft. Therefore TCAS IIversion 6.04a does not trigger any RA betweenRVSM approved aircraft oscillating aroundtheir cleared flight level.


In case of TAs, whether in an RVSM airspaceor not, pilots should not request for a trafficinformation and they shall not manoeuvre inresponse to TAs only. The main objective of theTA is to prepare the crew to a possible RA.

TAs are not to be notified by pilots tocontrollers (there is no standardised phraseologyto report TAs). Therefore controllers have nospecific procedure to apply.



5.3.3. Turbulence

Description

These encounters involve an aircraftexperiencing an atmospheric or a vortexturbulence that makes it deviate verysuddenly with a high instantaneous verticalspeed and an important acceleration towardsanother aircraft on an adjacent flight level.As a consequence there is a high verticalconvergence and pop-up RAs can betriggered on-board both aircraft. Figure 9illustrates this type of encounter in which thelower aircraft experiences a turbulence thatmakes it suddenly climb towards the otheraircraft.


TCAS II is provided every second with thealtitude of the own aircraft, from which itcomputes a vertical speed. If the aircraftexperiences severe turbulence that makes itdeviate very suddenly towards anotheraircraft, the altitude varies with an importantacceleration. Therefore, TCAS II computes ahigh vertical speed. Therefore when itperforms the altitude test with this highvertical speed, the time to reach co-altitude islower than the time threshold. The altitudetest is thus satisfied and an advisory istriggered.

Depending on the acceleration during thealtitude variation, the time to co-altitudecan become suddenly lower than the timethresholds for both TAs and RAs. In thiscase, a pop-up RA (i.e. no preliminary TA)is triggered. If it is only lower than the timethreshold for TAs, then a TA is triggered.

The behaviour of the level aircraft TCAS IIis similar. It receives the altitude of theintruder every second. When it detects thesudden altitude variation, it computes ahigh vertical speed for this intruder. Due tothis high vertical convergence, the altitudetest can be then satisfied and an advisory istriggered. Depending on the severity of theturbulence, either a TA or a pop-up RA canbe triggered.



For instance, a level aircraft at FL330 isexperiencing severe turbulence. Suddenly, itsaltitude varies so as its TCAS II computes arate of climb of 2,200 fpm. If there is anotheraircraft at FL340, the time to reach thisaltitude with the computed vertical speed is27 s. This value is lower than the timethreshold for TAs (i.e. 48 s) and for RAs (i.e.35 s). Therefore a pop-up RA (either an“Adjust Vertical Speed” or a “Descend”) istriggered. Supposing that the other aircrafthas a perfectly symmetrical view (this is anapproximation because of the altitude report

quantization), the time to co-altitude is alsolower than the time threshold for TAs (i.e.48 s). However it is greater than the one forRAs (i.e. 25 s) because of the reduced timethreshold for level aircraft(cf.paragraph 3.1.2). Therefore only a TA istriggered for the level aircraft.

As a conclusion, TAs or pop-up RAs can betriggered between aircraft on adjacent flightlevels because of turbulence. Neverthelessthese are rare events. They are more likelyto occur during slow overtakes like in theNAT airspace than during crossings.

Turbulence⇒ High instantaneous vertical speed⇒ TA or pop-up RA

FL340

FL330

Figure 9: turbulence




In CVSM airspace, the 2000 ft verticalseparation prevents most of the advisories tobe triggered because of turbulence except incase of very fast and long altitude variation.The computed vertical speed has to be twicegreater than in RVSM (i.e. around 4000 fpm)to induce an RA.


TCAS II version 6.04a triggers TAs betweenaircraft level at adjacent flight levelsseparated by 1000 ft. Therefore it alsogenerates TAs for aircraft experiencingturbulence

The time threshold for level aircraft beinggreater (i.e. 30 s), TCAS II version 6.04agenerates a few more pop-up RAs thanversion 7.0.


When an RA is triggered, including pop-upRAs, pilots shall comply with their airlineoperational instructions, which usuallyrecommend to follow all RAs.

To take a lateral offset because of TCAS IIadvisory (e.g. a pop-up RA) is not anapproved procedure.


When a pilot reports a manoeuvre inducedby an RA, the controller shall not attempt tomodify the aircraft flight path but shallprovide traffic information as appropriate



5.3.4. 1000 ft level-off

Description

These encounters involve at least one aircraftlevelling off at 1000 ft from another aircraft.If the vertical rate of the manoeuvringaircraft is high when close to the clearedflight level, TCAS II computes that bothaircraft will soon be at co-altitude. Thereforeit triggers an advisory, either a TA or an RA.Figure 10 illustrates this type of encounter inwhich a descending aircraft is levelling off1000 ft above a level aircraft.


During level-off encounters, the altitude testis based on the computation of the time toreach co-altitude (i.e. 48 s for TAs, 35 s forRAs for manoeuvring aircraft and 25 s forRAs for level aircraft).

TCAS II is an independent system that doesnot have knowledge of the positive intent ofeither the own or the intruder aircraft. Itassumes that the current flight profiles willbe maintained. As modern aircraft have highvertical rates even when close to the clearedflight level, TCAS II detects a high verticalclosure rate configuration and predicts thatboth aircraft will be at co-altitude in a shortterm. Therefore it generates either a TA or anRA depending on the predicted time value.

Figure 10 depicts an example where anaircraft with a 2,400 fpm rate of descent islevelling off 1000 ft above a level aircraft.

The time threshold for TAs for both aircraftis 48 s, which represents 1,920 ft with a2,400 fpm relative vertical speed. Thereforea TA is triggered for both aircraft when thedescending aircraft passes through FL349.

The time threshold for RAs is 35 s for thedescending aircraft, which represents1,400 ft with a 2,400 fpm relative verticalspeed. Therefore an RA is triggered whenthe descending aircraft is at 400 ft from theclearance (i.e. FL344).

The time threshold for RAs is 25 s for thelevel aircraft, which represents 1000 ft witha 2,400 fpm-relative vertical speed.Therefore no RA is triggered for the levelaircraft if the descending aircraft levels offcorrectly.



TCAS II version 7.0 has been designed toavoid as far as possible to trigger RAs thatrequires a vertical trajectory change (e.g.“Climb RA” for a descending aircraft). Ittriggers RAs compatible with the clearance,which require a vertical rate reduction (i.e.“Adjust Vertical Speed”).

As a conclusion, RAs can be triggered during1000 ft level-off encounters in case of highvertical rate. These RAs could be consideredundesirable by controllers and pilots.Nevertheless, if the manoeuvring aircraftdoes not level off as required, the triggeredRAs would become necessary.

Furthermore, a reduction in the vertical rateof the manoeuvring aircraft (e.g. 1000 fpmby 1000 ft from the clearance) shouldprevent RAs and minimise the number ofTAs because the altitude test is notsatisfied.

Note: If both aircraft are manoeuvring tolevel off 1000 ft apart, the relative verticalspeed is greater. Therefore it is more likelythat RAs are triggered.

FL340

FL330

RA: 25 s ⇒ 1000 ft ⇒ No RA if correct level off

2400 fpmTA: 48 s ⇒ 1920 ftRA: 35 s ⇒ 1400 ft

TA: 48 s ⇒ 1920 ft

Figure 10: 1000 ft level-off




In CVSM airspace, the 2000 ft verticalseparation prevents most of the RAs to betriggered because of a level-off manoeuvre toan adjacent flight level. RAs could betriggered for aircraft with a very high verticalrate (about 4,000 fpm), which is usuallyachievable only in descent at these altitudes.

For the same reason, the number of TAstriggered in this configuration is also lower inCVSM airspace.


TCAS II version 6.04a is expected to triggera larger number of RAs than TCAS IIversion 7.0 as many improvements have beenimplemented in the latest version: thereduced time threshold for the level aircraft, a25 ft vertical tracker, the MDF.

In the example depicted in the figure 10, thelevel aircraft should also receive an RA if itis equipped with TCAS II version 6.04a: 30 srepresents 1,200 ft with a 2,400 fpm relativevertical speed.

In addition, the triggered RAs might moreoften request the aircraft to change itsvertical trajectory (e.g. “Climb” instead of“Adjust Vertical Speed”).


To minimise the number of TAs and RAstriggered during 1000 ft level-offencounters,it is recommended that:• when an aircraft is climbing or

descending,• when reaching an altitude 1000 ft below

or above the cleared flight level,• when the crew is aware of a traffic in

the close vicinity at an altitude adjacentto the cleared flight level,

• where possible and appropriate,

pilots should climb or descend at a rate lessthan 1000 fpm until reaching the clearedflight level.

With this reduction in vertical rate beforelevel-off, no advisory would be triggeredby 1000 ft level-off encounters with onemanoeuvring aircraft and only TAs couldbe triggered if both aircraft aremanoeuvring.

As an example, the reduction in verticalrate could result from a traffic informationprovided by the controller.

When an RA is triggered, pilots shallcomply with their airline operationalinstructions, which usually recommend tofollow all RAs, whether a trafficinformation has been provided or not andwhether the intruder is visually acquired ornot.

When a pilot reports a manoeuvre inducedby an RA, the controller shall not attempt tomodify the aircraft flight path but shallprovide traffic information as appropriate.



5.3.5. Specific case of low closure rates

Description

These encounters involve level aircraft atadjacent flight levels and following the sameroute with close ground speeds. The trailingaircraft is overtaking the leading aircraft witha low closure rate. This type of encounter byitself does not trigger any advisory, but it willlikely get the advisories caused by the criticalconfigurations (i.e. vertical offsets,oscillations, turbulence and 1000 ft level-off)to last much longer. Figure 11 illustrates thistype of encounter in which both aircraft haveoffsets and one aircraft is overtaking theother aircraft with a 30 kts closure rate.


The low closure rate does not induce anyadvisory triggering. Therefore the advisoryneeds to be triggered by one of the criticalconfigurations (i.e. vertical offsets,oscillations, turbulence and 1000 ft level-off). Figure 11 depicts an example where theTA is triggered because of two verticaloffsets (cf. paragraph 5.3.1).

Generally, when the range test is satisfiedbased on the time-to-go to the CPA, theadvisory lasts about the time threshold value(i.e. 48 s for TAs)

In the case of low closure rate encounters,the range test is satisfied based on theprotection distance (i.e. 1.3 NM for TAs)because the time-to-go is very high evenwith a low range. The advisory will lastabout the time that the aircraft remainswithin the protection distance. As bothaircraft have close ground speeds, theaircraft can remain several minutes withinthe protection distance. Therefore theadvisory can last several minutes.

In order to reduce the advisory duration, aspecific algorithm is implemented toterminate the advisory when the aircraft arediverging and even if the aircraft are stillslightly within the protection distance.Consequently, the range at the terminationis lower than the protection distance (i.e.about 1 NM for TAs). However, thisalgorithm may in rare cases induce thetriggering of secondary short-durationadvisories.

In the example depicted in figure 11, bothaircraft have close ground speeds (i.e. 450and 480 kts). A TA is triggered when thefastest aircraft is 1.3 NM behind theslowest one. It ends when the aircraft arediverging and the range is slightly lowerthan 1.3 NM. With a 30 kts relative groundspeed, the TA should last about 5 minutes.



RAs triggered during a low closure rate canalso last several minutes. If the pilots reactproperly to the “Climb” or “Descend” RAs,TCAS II version 7.0 will quickly weakenthese RAs into “Adjust vertical speed” RAs(i.e. the pilots have to level off) so as to limitthe vertical deviation.

As a conclusion, long duration TAs or RAscan be triggered during low closure rate

encounters if they correspond to one of thecritical configurations (i.e. vertical offsets,oscillations, turbulence and 1000 ft level-off). Nevertheless, these are expected to berare events because they are a small subsetof the advisories triggered in RVSM.Indeed, compared with the NAT airspacethe route structure in Europe seldomprovides such encounters.

FL340

FL330

T0: Start of TA T0+ ∼5 min: End of TA

1.3 NM < 1.3 NM

450 kts

480 kts

< 850 ft

Figure 11: low closure rate and vertical offsets




In CVSM airspace, TAs and RAs are muchless frequent because of the 2000 ft verticalseparation applied in this airspace. Thereforethe implication of low closure rate encountersis even less noticeable than in RVSM.


The implication of low closure rateencounters is greater with TCAS IIversion 6.04a because it triggers much moreTAs, in particular between aircraft level atadjacent flight levels separated by 1000 ft.

In addition, if TCAS II version 6.04a triggersa “Descend” or a “Climb” RA during a lowclosure rate encounter, it cannot weaken thisRA, thus leading to larger vertical deviationsthan with TCAS II version 7.0.


When an RA is triggered, pilots shallcomply with their airlne operationalinstructions, which usually recommend tofollow all RAs.


To take a lateral offset to stop a longduration TCAS II advisory (TA or RA) isnot an approved procedure.

When a pilot reports a manoeuvre inducedby an RA, the controller shall not attempt tomodify the aircraft flight path but shallprovide traffic information as appropriate.



6. Operational implication

6.1. Controllers

6.1.1. Number of TCAS II advisories

With a full TCAS II version 7.0 equipage, noadvisory will be triggered between levelaircraft at adjacent flight levels separated by1000 ft as it is the case in an RVSM airspace.

Nevertheless, the number of TAs and RAsabove FL290 is expected to increase with theimplementation of RVSM. For RVSMapproved aircraft, some TAs could resultfrom imperfect altitude keeping. In addition,some TAs and RAs could be triggered during1000 ft level-off encounters and because ofturbulence.

However, this increase of the number of TAsand RAs above FL290 should not adverselyaffect the RVSM operations as the expectedadvisory rate in the RVSM airspace shouldbe lower than the rate currently observedbetween FL250 and FL290. This statement isparticularly true if the operational proceduresrelated to TAs and RAs are properly appliedby the crews.

Actually with the implementation of RVSM,the airspace becomes homogeneous up toFL420 because the same vertical separationis applied. Therefore TCAS II is alsobehaving homogeneously below FL420whereas it had particular performance inCVSM because of the 2000 ft verticalseparation.

6.1.2. Triggering of TAs

The expected increased number of TAs inRVSM should not have any implication forcontrollers because TAs are not to be notifiedby pilots to controllers. For this reason, thereis no standardised phraseology to report TAsand controllers have no specific procedure toapply.

Nevertheless, some pilots tend to requestfor traffic information when a TA istriggered. As the number of TAs aboveFL290 is expected to increase with theimplementation of RVSM, the number oftraffic information requested by pilots isalso expected to increase. This issue isespecially relevant for aircraft equippedwith TCAS II version 6.04a because thisversion triggers a much larger number ofTAs than TCAS II version 7.0.

6.1.3. Triggering of RAs

As the number of RAs above FL290 isexpected to increase slightly with theimplementation of RVSM, the number ofpilots’ responses to RAs is also expected toincrease. However, these responses do notmean that the aircraft will deviate fromtheir clearances. Indeed, TCAS IIversion 7.0 triggers RAs that avoid as far aspossible a vertical trajectory change andthus that are compatible with the clearance.In addition, it should be noted that thenumber of RAs triggered above FL290 isexpected to be lower than in the FL250-FL290 altitude band.

As almost all aircraft should be equippedwith TCAS II in the RVSM airspace, thetriggered RAs are co-ordinated. Thereforefor the RAs requiring to deviate from theclearance, the eventual deviation of eachaircraft will be limited when both pilotsfollow properly the RAs. Theinteroperability between TCAS IIversion 7.0 and TCAS II version 6.04a in amixed equipage environment is fullyeffective. The co-ordination for thetriggering of RAs between TCAS II units,whatever the version, is defined in theICAO ACAS II SARPs.



If a pilot overreacts to an RA, any excessivedeviation resulting from the response couldbe more disruptive than in CVSM because ofthe reduced vertical separation betweenflight levels. Nevertheless, inducedencounters with a third aircraft are expectedto be unlikely, like currently in loweraltitudes. If an encounter involving threeaircraft happens to occur, TCAS II includes amultiple threat encounter logic, whichprovides RAs for several simultaneousthreats.

“When a pilot reports a manoeuvre inducedby an ACAS resolution advisory, thecontroller shall not attempt to modify theaircraft flight path until the pilot reportsreturning to the terms of the current airtraffic control instruction or clearance butshall provide traffic information asappropriate.” [PANS-RAC] Nevertheless,when providing traffic information,controllers shall keep in mind that because ofthe update rate of their radar display, theirperception of the vertical situation of theencounter is less accurate than the TCAS IIperception.

While an aircraft is deviating from theclearance in response to an RA, the controlleris not responsible for the provision ofseparation to this aircraft.

During configurations that can result in thetriggering of an RA (e.g. 1000 ft level-offencounters), controllers can provide trafficinformation if practical. It shall not preventpilots to follow an RA if it is triggered.Nevertheless, in the case of 1000 ft level-off,pilots could reduce the vertical rate whenapproaching the cleared flight level (i.e.1000 fpm by 1000 ft from the clearance) inorder to prevent the possible triggering of anRA.

6.2. Pilots

6.2.1. Number of TCAS II advisories

In an RVSM airspace, TCAS II version 7.0equipage does not trigger any advisorybetween level aircraft at adjacent flightlevels separated by 1000 ft.

Nevertheless, the number of TAs and RAsabove FL290 is expected to increaseslightly with the implementation of RVSM.For RVSM approved aircraft, some RAscould be triggered during 1000 ft level-offencounters and because of turbulence. Inaddition, some TAs could result fromimperfect altitude keeping. A preliminarystudy has identified that 47 TAs and 3 RAsper 1000 flight hours can be expected withTCAS II version 7.0. These ratios are lowerthan those for the FL250-FL290 altitudeband.

However, pilots of aircraft fitted withTCAS II version 6.04a should be awarethat they are going to experience a highnumber of TAs mainly triggered by aircraftlevel and separated by 1000 ft (about 1 TAper flight hour). In addition, the expectednumber of RAs is twice greater than withTCAS II version 7.0. Nevertheless, it isimportant that pilots follow all the RAstriggered by TCAS II version 6.04a even ifthe number is greater than with TCAS IIversion 7.0.

Pilots should be aware of the TCAS IIversion they use and they should be trainedaccording to this version. Thus pilots willknow what to expect from their TCAS II inan RVSM airspace.

6.2.2. Expected behaviour

The implementation of RVSM shall notmodify the pilots’ behaviour that isexpected currently in non-RVSM airspacewhen a TA or an RA is triggered. Thisbehaviour is the same whatever theTCAS II version and the airspace.

When a TA is triggered, pilots should notrequest for a traffic information. In additionthey shall not manoeuvre their aircraft inresponse to TAs only [PANS-OPS]. Thisstatement is valid in any airspace, i.e. in anRVSM environment too.



The main objective of the TA is to preparethe crew to a possible RA. Theresponsibilities are divided between the pilotflying and pilot not flying as follows[TGL11]:

• pilot flying should continue to fly theaeroplane and be prepared to respond toany RA that might follow;

• pilot not flying should provide updates onthe traffic location shown on the TCAS IIdisplay, using this information to helpvisually acquire the intruder.

When an RA is triggered, pilots shallcomply with their airline operationalinstructions, which usually recommend tofollow all RAs, whether a traffic informationhas been provided or not and whether theintruder is visually acquired or not. In thecase of pop-up RAs, pilots have to deal withthe lack of preliminary TAs before RAs.

During the RA, the responsibilities aredivided between the pilot flying and pilot notflying as follows [TGL11]:

• pilot flying should be responding to theRA with positive control inputs, whenrequired;

• the pilot not flying is providing updateson the traffic location, checking thetraffic display and monitoring theresponse to the RA.

Pilots shall never manoeuvre in the oppositesense to the RA. If the intruder is alsoequipped with TCAS II (i.e. the standard casein the European RVSM airspace), amanoeuvre in the opposite sense is hazardousbecause both RAs are co-ordinated in orderto select complementary resolution senses.

Pilots shall inform controllers aboutmanoeuvres in response to RAs as soon astime and workload permit using the standardphraseology.

The alteration of the flight path shall belimited to the minimum extent necessary tocomply with the RAs [PANS-OPS].

Pilots who deviate from an air trafficcontrol clearance in response to an RAshall promptly return to the terms of theprevious air traffic control instruction orclearance when the conflict is resolved andthey shall notify the appropriate ATC unitas soon as possible [PANS-OPS]. As thecontroller is not responsible for theprovision of separation to an aircraft, whichis deviating from the clearance in responseto an RA, it is important that pilots returnto the previous clearance when the RA isterminated.

To take a lateral offset to stop a longduration TCAS II advisory (TA or RA) isnot an approved procedure.

6.2.3. Recommendation for 1000 ftlevel-off encounters

To minimise the number of TAs and RAstriggered during 1000 ft level-offencounters, it is recommended that:

• when an aircraft is climbing ordescending,

• when reaching an altitude 1000 ft belowor above the cleared flight level,

• when the crew is aware of a traffic in theclose vicinity at an altitude adjacent tothe cleared flight level,

• where possible and appropriate,

pilots should climb or descend at a rate lessthan 1000 fpm until reaching the clearedflight level.

With this reduction in vertical rate beforelevel-off, no advisory would be triggeredby 1000 ft level-off encounters with onemanoeuvring aircraft and only TAs couldbe triggered if both aircraft aremanoeuvring.

As an example, the reduction in verticalrate could result from a traffic informationprovided by the controller.



7. Conclusion

TCAS II is a last resort safety net designed toprevent mid-air collisions between aircraft.The technical features of the system providea significant improvement in flight safety,which is valid in an RVSM airspace as in anyother airspace. However, pilots andcontrollers must be aware that TCAS II is nota perfect system. TCAS II cannot exclude allcollision risks and the system may,marginally, induce an additional risk.Consequently, it is essential that ATCprocedures are designed to provide flightsafety without any reliance upon the use ofTCAS II.

TCAS II version 7.0 is compatible with theRVSM environment, even if it is expectedthat the implementation of RVSM will leadto a slight increase of the number ofadvisories above FL290 in comparison withCVSM operations. Nevertheless, theexpected advisory rate in the RVSM airspaceis lower than the rate currently observedbetween FL250 and FL290.

TCAS II version 6.04a is not compatiblewith RVSM for operational reasons, mainlya very high rate of TA triggering.Nevertheless, a mixed equipage with a lowpercentage of TCAS II version 6.04a is notexpected to have a significant implicationfor controllers.

The implementation of RVSM shall notmodify the behaviour pilots and controllersthat is expected in non-RVSM airspacewhen, a TA or an RA is triggered. To getthe full safety benefit brought by TCAS II,pilots shall comply with their airlineoperational instructions, which usuallyrecommend to follow all RAs. In addition,they shall never manoeuvre in the oppositesense to the RA because of the co-ordination with the TCAS II unit of theother aircraft.

The increased precision of the horizontalnavigation combined with the increasedprecision of the vertical navigation maylead, in case of mistakes and whatever theairspace, to a higher risk of collision.Therefore TCAS II is all the moreefficient as the last resort safety net.



8. References

[ACAS]: “ACAS brochure” – ACASA/WP6.1/015 – Version 2.0 – May 2000

[ACASA]: “Final report on ACAS/RVSM interaction” – ACASA/WP-3.6/185D –version 1.0, released issue – Christian Aveneau and Béatrice Bonnemaison

[NAT]: “RVSM and Use of TCAS” – SICASP/WG2/IP2/522 – Frankfurt, October1995 – Francis Casaux and Eric Vallauri

[PANS-OPS]: Procedure for Air Navigation Services – Aircraft Operations – PANS-OPSDoc. 8168

[PANS-RAC]: Procedure for Air Navigation Services – Rules of the Air and Air TrafficServices – PANS-RAC Doc. 4444

[RVSM]: “Guidance Material for Adaptation of the Airspace for EUR RVSMImplementation” – Document ref.: RVSM/A730 – 12/10/2000 – EuropeanReduced Vertical Separation Minimum Programme – Eurocontrol

[TCAS7]: “ Preview of TCAS II version 7.0” – Air Traffic Control Quarterly, vol. 6(4)231-247 (1998) – 1998 – W. Dwight Love

[TGL6]: “Guidance material on the approval of aircraft and operators for flight inairspace above flight level 290 where a 300 m (1000 ft) vertical separationminimum is applied” – JAA – 01/07/98

[TGL11]: “Guidance for operators on training programmes for the use of AirborneCollision Avoidance Systems (ACAS)” – JAA – 01/06/98



9. Acronyms

ACAS Airborne Collision Avoidance System

ACASA Airborne Collision Avoidance SystemAnalysis

ATC Air Traffic Control

CPA Closest Point of Approach

CVSM Conventional Vertical Separation Minimum

ECAC European Civil Aviation Conference

EUR Europe

FAA Federal Aviation Administration

FL Flight Level

fpm feet per minute

ft feet

ICAO International Civil Aviation Organisation

kts knots

MDF Miss Distance Filtering

NAT North Atlantic

NM Nautical Mile

RA Resolution Advisory

RTCA Radio Technical Commission forAeronautics

RVSM Reduced Vertical Separation Minimum

SARPs Standard And Recommended Practices

TA Traffic Advisory

TCAS Traffic alert and Collision AvoidanceSystem



10. Bibliography

Annex 2 of ICAO - Rules of the Air

Annex 6 of ICAO - Parts I, II and III

Annex 10 of ICAO - Aeronautical Telecommunications - Volume IV - "Surveillance Radarand Collision Avoidance Systems"

Doc. 4444 of ICAO - PANS-RAC - "Procedures for Air Navigation Services - Rules of theAir and Air Traffic Services"

Doc. 8168 of ICAO - PANS-OPS - "Procedures for Air Navigation Services - AircraftOperations"

Doc. 7030/4, Section 16 of ICAO - Use of Airborne Collision Avoidance Systems (ACAS)- "Approval of a proposal for amendment of the ICAO Regional Supplementary ProceduresDoc. 7030 (serial N°. EUR/NAT-S 96/48 - EUR RAC/2) - ACAS Amendment Proposal" - 29October 1997

RTCA SC-147/DO-185A - Minimum Operational Performance Standards for Traffic Alertand Collision Avoidance System (TCAS) Airborne Equipment.

ACAS II operations in the European RVSM environment ACAS ...

Documents

Transcript of ACAS II operations in the European RVSM environment ACAS ...