FOR INSTRUMENT FLIGHT PROCEDURES: AT NON-RADAR …€¦ · FOR INSTRUMENT FLIGHT PROCEDURES: AT NON-RADAR AIRPORTS Sheila R. Conway Langley Research Center National Aeronautics and

A METHOD OF SEPA_TION ASSU_NCE

FOR INSTRUMENT FLIGHT PROCEDURES:

AT NON-RADAR AIRPORTS

Sheila R. ConwayLangley Research CenterNational Aeronautics and Space AdministrationHampton, VA

Maria ConsiglioICASE

Universities Space Research AssociationHampton, VA

Abstract

A method to provide automated air trafficseparation assurance services during approach toor departure from a non-radar, non-towered airportenvironment is described. The method is

constrained by provision of these services withoutradical changes or ambitious investments incurrent ground-based technologies. The proposedprocedures are designed to grant access to alarge number of airfields that currently have no or

very limited access under Instrument Flight Rules(IFR), thus increasing mobility with minimalinfrastructure investment. This paper primarilyaddresses a low-cost option for airport andinstrument approach infrastructure, but isdesigned to be an architecture from which a moreefficient, albeit more complex, system may be

developed.A functional description of the capabilities in

the current NAS infrastructure is provided.Automated terminal operations and proceduresare introduced. Rules of engagement and theoperations are defined. Results of preliminarysimulation testing are presented. Finally,

application of the method to more terminal-likeoperations, and major research areas, includingnecessary piloted studies, are discussed.

1 Introduction

In the past few years the limitations of theexisting air transport system have become

obvious. Frequent flight delays and cancellations,with all the attendant stress and disruption, arenow a familiar part of air travel in the UnitedStates. Many of these difficulties result from thedominant hub-and-spoke model that requiresconcentration of a large percentage of air traffic at

a few airports. Despite the implementation of

improved air traffic management tools andconstruction of new runways, it has become clearthat significant increases in overall air traffic willsoon make demands on these airports that simplycannot be satisfied by increasing capacity. 1Moreover, the current focus on capacity problems

at hub airports has tended to obscure anotherfundamental deficiency in the hub-and-spokesystem: the need to change planes. No onetraveling from Milwaukee to Shreveport reallywants to drag their carry-on luggage through theterminal at O'Hare; they do so because there is nodirect flight available at reasonable cost. Finally,

though most people live in large metropolitanareas serviced by large international airports,there are also many people who would find travelto a small or metro-satellite airport moreappealing. Unfortunately at these smaller airports,viable, cost competitive air transportation is notavailable.

Because increasing capacity alone does notappear to provide a long-term solution to theproblem of delays or satisfy the demand for moredirect flights, another line of research hasemerged which is aimed at increasing the"mobility" of the system, meaning its ability to

accommodate larger numbers of on-demand,point-to-point IFR operations between smallerairports. This approach complements efforts toincrease capacity by promoting more evenlydistributed air traffic and reducing congestion atlarge hub airports. A number of technologies

show promise in increasing mobility: The use ofAutomatic Dependent Surveillance-Broadcast(ADS-B) data for surveillance, though still in itsinfancy, is being proven in the Bethel region ofAlaska where ADS-B targets are being used byATC in conjunction with radar returns to provideseparation services 2. The availability of ADS-B

data also under-lies development of self-

1American Institute of Aeronautics and Astronautics

https://ntrs.nasa.gov/search.jsp?R=20030009795 2020-06-30T06:46:47+00:00Z

separationtoolssuchasCockpitDisplayofTrafficInformation(CDTI)andairborneConflictDetectionand Resolution(CD&R) functionsthat arefundamental to most distributed trafficmanagementproposals.Methodsof augmentingGPSsuchas WideAreaAugmentationSystem(WAAS)promiseto provideapproachcapabilityatairportscurrentlylackingground-basedapproachfacilities. Finally, developmentof aircrafttechnologiessuchassmall,fuel-efficientturbofanengines,advancesin aerodynamics,anti-icingmethods,and avionicshas resultedin a newgenerationof smallaircraftwithseat-milecostsapproachingthatoftransport-categoryaircraft.

Unfortunately,manyof the airportsreapingbenefitsfrom theseemergingtechnologieslieoutsideofexistingATCradarcoverage.Providingconventionalair traffic separationservicesattheseairportswouldrequireoneof thefollowing:1)a dramaticexpansionoftheATCsurveillanceradarnetworkand/orADS-Bdata-linksystemsalong with a concomitantincreasein ATCpersonnel,or 2) relianceon workload-intensivenon-radarapproachprocedures. The firstapproachis likelyto beverycostly,in termsofrequiredinvestmentsin both infrastructureandpersonnel,but it wouldpermitrelativelyhigh-densityoperationsovera widearea.Thesecondoption would require less investmentininfrastructure,but it wouldstill requireadditionalATC personnel,and it wouldbe restrictedtorelativelylow-densityoperationsbecauseof thelimitationsinherentin non-radartypeoperations.Sinceneitherof theseoptionsseemsparticularlydesirable,automatedmeansfor providingIFRseparationwithoutdirectATC interventionarebeingexplored.1.1 Conventional Non-Radar IFR Procedures

Because newly introduced procedures mustfunction within the existing National AirspaceSystem (NAS), their architecture mustcomplement, and to the largest extent possible,make use of existing infrastructure andprocedures. Before addressing proposals for

automation of non-radar approach and departureseparation services, we will review these existingprocedures.

Separation services can be classified as eitherradar (target-to-target) or procedural (target-to-airspace). The latter is used when accuratesurveillance data is not available, (generally non-

radar environments), or the intent of a target isunknown (VFR targets or IFR operations inuncontrolled airspace). There are also hybrid

techniques, utilizing both local target separationand more general airspace structure to keep non-participatory aircraft separated from IFR

operations. One example is a block of highaltitude airspace, known as a "wave window", thatallows gliders not equipped with transponders tooperate in Class A Positive Control Airspace. AllIFR traffic is separated from the block of airspace,and within the window, the gliders use acombination of see and avoid and specialized

rules of the road to maintain their own separationfrom each other. ATC is responsible for theformer, the pilots the latter. Similarly, the structureof ICAO-defined airspace types serves a similarpurpose: to minimize the mixing of different typesof traffic where possible, and ensuring the

compatibility of mixed traffic by mandatingequipment appropriate to each type of airspace.

In a non-radar airport environment, separationservices are often provided to IFR flights byensuring that airspace around the airport has noother IFR flights within it, i.e. the airspace is

"sterile". Additional requests for operations at theairport are postponed until the IFR arrival ordeparture is complete, hence the name "one-in/one-out". For departures, pilots are restricted toa specified departure window known as aclearance void time, during which the airspacefrom the departure airport to the point at which

radar contact is expected or position reporting willcommence is guaranteed by ATC to be sterile. Ifthe departure window is missed, a new clearancerequest must be made to gain entry to controlledairspace. For arrivals, the same principles apply,though the "one-in/one-out" window is typically

defined by the loss of radar contact (common indescent) and the pilot's action of closing their flightplan. If, for example, an arriving aircraft requestsan approach while another aircraft was landing,the arriving aircraft will likely be required to hold ata location at a safe distance and altitude from the

active approach path. They will be given anExpect Further Clearance (EFC) time when theycan expect to receive clearance to initiate theirapproach. This procedure permits ATC toseparate multiple flights in the absence ofsurveillance data (radar) without requiringexcessive position reporting.

Many people, including licensed pilots, aresurprised to learn that many airports in the US withno radar coverage or control tower haveinstrument approach procedures with finalapproach segments in uncontrolled (class G)airspace, where the pilot, not the controller, isresponsible for traffic avoidance. Since the floor of


controlledairspacenearinstrumentairportsintheUSis typicallyonly700'AGL,ATCcanprovideadequateseparationbetweenIFR arrivalsanddepartures,but the possibilityexiststhat VFRtrafficof whichATChasno knowledgemaybepresent in the airport vicinity below 700'.RegulationspermitVFR flightwithin classGairspaceinthevicinityofanairportas longastheaircraftcanremainclearof cloudsandmaintainflightvisibilityof 1 mile. Becausethereis nopracticalwayfor an arrivingIFRaircraftto see-and-avoidVFRtrafficoperatingnearthebaseofa700' ceiling, the only means for ensuringseparationbetweenIFRandVFRtrafficin thesecircumstancesis mutualuse of the CommonTrafficAdvisoryFrequencyfor positionreporting.If the VFR aircraft is not radio-equipped,separationis simplyleftto chance.Fortunately,the lowvolumeof VFRoperationsat airportsinextremelymarginalmeteorologicalconditionshasmadetrafficconflictsofthistyperare.1.2 Automated Instrument Procedures

There have been a number of studies aimed

at developing efficient instrument access to non-

radar facilities through the automation of approachand departure traffic separation procedures. Mostof these attempts depend on 4-D (lateral, verticaland time determinate) flight path prediction.Tobias and Scoggins 3 attempted to automatetraditional IFR services by building off Tobias'earlier work, 4 describing prediction of conflict-free

approach paths. Using conventional separationstandards, they generated airport-relative altitude,azimuth and range data from the beacontransponder systems (Mode A/C). From thesedata, the ground-based automation assignedroutes of flight designed to provide sequence and

maintain separation. A synthesized voice system

transmitted these clearances to the participatoryaircraft via VHF radio. Morgenstern and Telsch _described a similar system intended for VFRadvisories.

The most significant obstacle to deployingsuch a full-scale automated approach and

departure system is the challenge of legallycertifying it. The air traffic control system is aninherently conservative institution: evenincremental changes to ATC procedures requirelengthy regulatory processes. Moreover, thesystem has evolved though many hundreds ofthousands of hours of service in all kinds of

conditions, and represents the distillation of oftenbitter experience with a myriad operationaldifficulties and mishaps. The safety record of the

modern ATC system is exemplary, and theprocedures upon which this record is basedshould not be altered lightly. With this in mind, we

propose a much less ambitiousapproach/departure automation system than hasheretofore been advanced: one that is based on

the existing, well-proven non-radar "one-in/one-out" IFR procedures described in Section 1.1.While this method is neither the most efficient use

of the airspace, nor the most convenient for the

pilot, it does have the virtues of safety, simplicityand relatively low cost. While our proposedsystem is limited to relatively low-densityoperations, it represents architecture from whichmore efficient, albeit more complex, full-serviceautomated systems can be developed after

operational experience has been accumulated andas demand for point-to-point operations increases.

This paper will focus on describing theelements of a basic automated

approach/departure system, establishment ofoperational procedures, identification of the

equipment and systems necessary forimplementation. Consideration of the necessarynavigation signal accuracy, integrity andavailability for area-navigation-based (RNAV)instrument approaches are not addressed, asthere is a large body of work on these subjects.

2 Concept of Operations

2.1 The proposed model

The proposed model for automating non-

radar, non-tower arrivals and departures, hereafterreferred to as the Automated Airport ControlVolume (AACV) model, builds on the existingprocedural or target-to-airspace separationarchetype described in Section 1.1 and extends itsuse towards truly distributed air traffic control.During periods of IMC, a block of airspace (an

Airport Control Volume, ACV) will be establishedaround the airport and a local automation systemwill manage access so that only one aircraft will bepresent in the ACV airspace at a time. Theautomated system will further limit access toappropriately equipped aircraft that follow

specified procedures. Like the existing proceduralseparation methods on which it is based, theAACV model provides a simple, relatively low-costand extremely effective way to minimize theopportunity for traffic conflicts in the criticalapproach and departure phases of flight.

Moreover, defining the ACV as extending to thesurface can eliminate the small but finite possibilityof a conflict between IFR and VFR traffic in


uncontrolledairspace. Finally,the operationalconceptallowsfor growthby laterallowingtheparticipatingaircraftwithinthisairspacetoprovidetheirownseparationservicesusingacombinationof proceduresand specializedtools, includinglocalizedsurveillancedata.

Ourfundamentalapproachto designof theACVarchitectureandassociatedproceduresis toplace a minimumnumberof constraintsonparticipatingaircraftnecessaryforsafety.Athigh-volume terminals,optimizingcapacity is aparamountconcernandrigidconstraintsmustbeimposedontrafficbecausetheactionsofasingleaircraftcanaffectdozensof otherflights,leadingto disruptionsand delays. At the low-volumeairportssuitablefor an AACV, not only isoptimizingcapacitylessof a concern,butaircraftcan maneuverwith relativefreedomwithoutinterferingwitheachother.Thisenvironmentismoreakinto airportoperationsunderVFR,wherepilots have manydegreesof freedom,yet arelativelysimpleset of priorityrulessufficestokeeporder.Wehavethereforepursueda hybridsystemof rule-basedmaneuvering,airborneself-separationandgroundcontrolinordertoensureasafeandreasonablyefficientsystem.2.2 System components

The AACV system is comprised of four distinctcomponents we will address separately. Theyinclude 1) the Airport Control Volume airspace, 2)

protocols governing access to the ACV, 3) theautomation system and communicationsnecessary for managing the traffic flow in and outof the ACV, and 4) traffic management proceduresin the vicinity of the ACV that provide the transitionbetween AACV operations and the enroutestructure.

2.2.1 The Airport Control Volume

The Airport Control Volume is similar inconcept to a class E surface area, though the ACV

would also include the Initial Approach Fixes(IAF's) associated with approaches at the airportand would be restricted to typical initial approachaltitudes (see Figure 1). In order to accommodatethe FAA's basic RNAV approach, the ACV isdefined nominally as 12 NM from the airport, fromthe surface to 2500' AGL. As with class E

airspace, there is no reason for the ACV to beeffective when the airport weather is above VFRminima, although some of the associatedequipment may proveuseful as an aid to visual separation during

operations in VMC when the exclusionary rules donot apply.

RNAV RWY _7

Figure 1 : Airport Control Volume (ACV)

2.2.2 Access protocols

Arriving aircraft must request a clearance toenter the ACV and they must remain clear of theACV until clearance to enter has been granted.Either the pilot or the automation will initiate theclearance request based on a specific triggeringevent such as distance or time to destination.

Similarly, departing aircraft must also request adeparture clearance, and shall remain on theground, clear of active runways until a departureclearance has been granted. Aircraft can alsorequest to transition through the ACV enroute toanother destination.

Only one aircraft (referred to as the "priorityaircraft") at a time will be granted access to theACV by the automated system. Aircraft will begranted access to the ACV by the automatedsystem in order of their priority as established inaccordance with a mutual exclusion protocol that

locks all aircraft out of the approach/departureairspace while it is in use by the priority aircraft.Once the priority aircraft is clear of the ACV,sequence is reestablished among all known


requests,andthenewpriorityaircraftwill receiveanautomatedclearancemessagegrantingaccesstotheACV.

Althoughthespecificsarea matterforfurtherstudy, the protocol design will need toaccommodatespecialcircumstancesarisingfromeventssuchas abortedtakeoffsand missedapproaches.Therearea numberof waystheseeventscanbeaccommodated,butforthepresentstudyit sufficesto recognizethatthe ACVwillhave to remainlockeduntil the situationisresolved.

Theprioritylist is generatedbyasequencingprotocolin thelocalautomationsystembasedonrequestsfor approach,departureor transitionoperations.Therankingalgorithmwill takeintoaccounta numberof factorsin creatingtheranking.Evenwiththe simple "one-in/one-out"operationalmodel,it is possibleto orderrequeststo makebetteruseof the airportresource.Asimplestudy of arrivalsequencingscenariosshowedas muchas a 46% decreasein thesummativedelaytakenbyallparticipatingaircraftby assigningprioritiesto all requestsreceivedwithina particularperiodon the basisof airportusageratherthanin theordertherequestswerereceived.Thetypeof operation,thepositionofarrivingaircraftin relationto the IAF's,aircraftgroundspeedandtheavailableapproachprofilesmustallbeconsideredindeterminingtherankingwhichmakemostefficientuseof the airspacebecauseeachhasaneffectonthedurationoftherequestedoperation.Forexample,it maybemoreefficientto assigna departureor a transitionrequest(whichare relativelyquickoperations)ahigherprioritythananapproachrequestthatwasreceivedearlier.

In practice,reorderingthe prioritylist in theinterestofefficiencyshould,however,belimitedtoinsertingsmall numbersof departureandtransitionoperationsbetweenpreviouslyrankedarrivalrequestsanddeferringdeparturerequestswhenpredictedholdingtimeforarrivalsbecomesexcessive;therankingof arrivalrequestsshouldnot be alteredin relationto eachother. Thereasonforthisissimple:arrivingpilotsneedsomeideawhentheycanexpecttoreceiveanapproachclearancein orderto makepotentiallycriticaldecisions regardingfuel managementanddiversionto analternatedestination.Afterpilotshavebeenadvisedofthe EFCtime,therelativeorderof arrivalsmustbepreservedin orderforthisinformationto remainvalid.

Althoughthe prioritylist is createdon thebasisofvalidclearancerequestsbyaircraftinthe

immediatevicinityof the ACV,it mayalsobeadvantageousto developforecastsfor airportdemandthat can be usedto generatedelayestimateswell in advance. Flightplan data,requiredenrouteupdatesanddatafromtheFAA'sHOST computer or other ground-basedsurveillancesystemscanbeusedbytheAACVautomationto periodicallycomputeand updatepredictionsof the sequenceof arrivingaircraft,periodswhentheairspacemaynotbeavailablefor departures,or whenoveralldemandon theairportexceedsitscapacity.Thisinformationcanthen be disseminatedin the form of pilotadvisories.

2.2.3 Automation system and supportingmessage exchange

As noted above, all automatedcommunication, sequence generation and

clearance granting functions are performed by alocal AACV data processing system. Allparticipatory aircraft will be required to be ADS-Bequipped, with a 40-mile transmission capability(although preliminary studies suggest that atransmission capability as low as 20 miles may besufficient). ADS-B will provide the primary meansof communication between the aircraft and the

AACV system, providing general data transfer andsurveillance functions. For our initial

implementation, we have attempted to use aminimal message set, assuming substantial costfor all transmitted data.

For Approaching Aircraft: A specific on-condition request report would be designated,containing Aircraft ID, intended airport facility, andrequest code (landing or transition). Thismessage could be generated by the on-boardflight planning and/or navigation avionics at a

specified point on the flight path (e.g. 20 NM todestination). "Priority granted" messages will betransmitted to the aircraft through data link and willconsist of airport ID, aircraft ID, time, IAFassignment, and a single priority bit. "Prioritydenied" messages will consist of airport ID, aircraftID, EFC time, and a single denial-of-priority bit.

The denied aircraft would re-request at the EFCtime to minimize message traffic. This time is alsouseful in the event of lost communications. An

"operations complete" message could be used tounlock the airspace. Optionally, ADS-Bsurveillance data transmitted from the priorityaircraft indicating the aircraft's velocity has

dropped below a specified threshold for aspecified duration could perform this function.State data as described by RTCA DO-2426 (Time,


Lat, Long, altitude,ground-referencedvelocityvector,ID,category,navigationaldataquality)willbesufficientinputfortheground-basedarbitratortocalculatepriority.

For Departing Aircraft. As for arrivals,departures would use the specific on-conditionrequest report, containing Aircraft ID, intendedairport facility, and request code (departure)."Priority granted" messages will be receivedthrough data link, and will consist of airport ID,

aircraft ID, time, departure runway, and a singlepriority bit. "Priority denied" messages will consistof airport ID, aircraft ID, EFC time (or equivalent,as discussed in Section 2.2.2), and a singledenial-of-priority bit. The denied aircraft would re-request at the expect further clearance time to

minimize message traffic. An "operationscomplete" message could be used to unlock theairspace or ADS-B surveillance data transmittedfrom the priority aircraft indicating aircraft positionoutside the ACV, could perform this function. Statedata will be sufficient input for the ground-based

arbitrator to calculate priority

2.2.4 Traffic sepa_tion outside the ACV

Another significant operational element

necessary for a feasible system is some form oftraffic separation in the vicinity of the ACV. Whilethe AACV concept is primarily intended tocompliment the development of self-separationcapabilities, it could be implemented within theexisting ATC architecture. In such a case, ATC

would likely provide separation outside of ACV bycontrolling arrivals and assigning each aircraft adiscrete holding area where it would be required toremain until clearance into the ACV is granted.

When self-separation tools such as CDTI andCD&R become available, the simplest approach

would be to require pilots to use these tools toprovide their own separation from other aircraftoperating in the vicinity of the ACV. A number ofstudies have suggested that pilots can beprovided appropriate tools to enable self-separation in these circumstances 7' 8, 9, but thematter requires careful consideration. While the

effectiveness of self-separation tools in theenroute environment has been demonstrated, the

application of these tools to a situation wheremultiple aircraft are attempting to execute holdingmaneuvers in close proximity is less wellestablished. Aircraft trying to remain close to aparticular point are constrained in a way that

aircraft enroute are not. Moreover, the pilotworkload immediately in advance of executing anapproach is significantly higher than during the

enroute phase, leaving less time to attend to trafficavoidance.

It is beyond the scope of this study to

establish specific requirements for separationassurance outside the ACV, but further research

must be undertaken to answer this questionbefore a complete description of an operationalAACV system is possible. It may turn out thatadditional constraints must be placed on self-separating aircraft outside the ACV akin to the

holding assignments used by ATC today, but to doso would be to remove some of the simplicityinherent in the AACV concept and advance thesystem significantly closer to a full-scaleautomated traffic control system.

2.3 Non-normal Operations

Existing procedures have evolved toaccommodate a wide range of non-normal

operational situations. Fortunately, most of theseprocedures translate readily into the AACVconcept. The most significant abnormal situationsare discussed below.

2.3.1 Lost Communications: AircraftAvionics Failure

As in other "lost com" situations in today'ssystem, a pilot who is unable to report an updated

estimated time of arrival (ETA) is required to try tohonor their last-reported ETA. If they are runningearly, they reduce speed to adjust their arrivaltime. If they are running late however, they maymiss their window and have no way to make upsufficient time. The AACV system would have noway to know if they were truly just late or had gone

"lost com" and are actually in the vicinity of theAACV, but not reporting. Whenever failure of anaircraft to make a mandatory report or the loss ofADS-B data suggests that an aircraft hasexperienced avionics failure, the AACV systemwould respond by locking the ACV. Of course, ifADS-B surveillance data for the Iost-com aircraft

later becomes available to the AACV systemindicating the aircraft has not yet arrived in theairport vicinity, some operations could beresumed, if it is clear they can be completedbefore the Iost-com aircraft arrives.

Federal regulations preclude extended IFRoperations without ATC communicationcapability. 1° If VMC exists, pilots are expected to

land "as soon as practicable". In the case ofavionics failure enroute to or around an AACV

airport in IMC, the following guidelines derivedfrom the existing regulations could pertain: Ifaccess to the ACV has been granted prior toavionics failure, the flight would immediately


proceedto the appropriateIAF to initiatetheapproachandlanding.Ifaccessto theACVwasdeniedpriorto avionicsfailure,the flightwouldremainoutsidethe ACV until the EFC timereceivedwith the denial message,and thenproceedto the appropriateIAF to initiatetheapproachandlanding.In eithercase,theACVwouldremainlockeduntiltheaircraftis confirmedbyATCto havelanded,divertedto anotherairportor departedthearea. TheAACVwouldthenbemanuallyresetfornormaloperations.2.3.2 Lost Communications: AACV Failure

From an aircraft's perspective, if an approachclearance request is made but no response isreceived, there may be no immediate way to tell ifthere is a communication failure in the aircraft

transceiver or a failure of the ground equipment.Before following procedures for lost

communications, it would be prudent for the pilotto try to determine if it is the airport reception ortheir transmission/reception that has failed. A testmessage could be included in the message set forthis purpose, or facilities at another nearby airportmay be of use. If the pilot can confirm that theairport facility has lost capability, an approach

under IFR using these procedures should not beinitiated. If an approach clearance has alreadybeen granted when airport automated sequencecommunication is lost, a pilot should be able tocomplete their approach assuming they still haveappropriate approach guidance signal.

2.3.3 Urgency and Emergency Situations

Extended arrival delays resulting from high

demand can cause problems for some operators.Weather and mechanical problems can causeemergencies to arise at any time. The AACV canautomatically accommodate many of thesesituations by the use of a priority word in the on-condition ADS-B message. We could use 3 bits todescribe four levels of urgency, granting clearance

according to priority first, then position/timeprecedence:0) Normal Operations. Sequence generated asdescribed in Section 2.2.2.

1) Re-request. Set only by system, not userselectable; priority supercedes initial transition anddeparture requests.2) Pilot-initiated Urgency request, e.g. Low fuelstate; requesting aircraft assigned next prioritystatus; "priority granted on basis of urgencyrequest" message transmitted to ATC.3) Emergency. e.g. Declaration of an in-flightemergency; priority status of current "priorityaircraft" rescinded if practical (for instance, priority

aircraft has not yet entered the ACV) and earliestpossible clearance of emergency aircraft for theapproach; "declaration of emergency" messagetransmitted to ATC for investigation.

2.4 Extensions Beyond the One-lnlOne-Out

Concept

The next step in the development ofautomated approach and departure procedures,

allowing more than one aircraft into the ACV at atime, depends on tools which enable pilots notonly to self-separate, but also to order and mergethemselves on or near the approach. Self-Spacing concepts under development at NASALangley may provide such aids 11. These toolsmay help pilots adjust their flight using speed and

path guidance generated from interval and target-referenced data rather than ground-referencedinstrumentation. This would allow a pilot toeffectively fly a similar guidance cue to today'scourse deviation indicator, but by maintaining bothcourse and speed guidance, they would also be

assuring separation from the reference target asconfirmed by the cue-generating automation.

2,5 Regulatory considerations

As this procedure's main purpose is to provideseparation assurance, all of the airspace within anACV would necessarily be defined as controlledairspace in order to exclude non-participatorytraffic. Mixing AACV operations with standard IFR

operations cannot occur without full surveillance ofall traffic targets, or a means to share IFR airportusage/cancellations data between ATC and theAACV. Preferably, the AACV will havesurveillance data for all traffic targets in the area.Assuming an ADS-B-based system, mandatoryuse of ADS-B transmitters in the vicinity of an

active ACV would be one solution. Alternatively, acombination of ground-based passive (or activeMode-C based) technology and airborne receptionequipment could supplement those targets notADS-B equipped. In fact, these types of systemscould be the primary means of surveillance,

though they would most likely be cost prohibitivefor many municipal facilities.

A new subset of clearance descriptions withinInstrument Flight Rules would be necessary aswell. Regulations regarding mandatory equipmentfor participating aircraft already provide for this

type of growth, as "two way radio communicationsand navigational equipment appropriate to theground facilities" are obligatory 12. Clearly, these

types of approaches may require a whole host ofnew airborne equipment, including self-separationtools (appropriate surveillance, CDTI, CD&R, etc)


anda methodto exchangedataandclearanceinformationwiththesequencegrantingauthorityattheairportfacility.

3, Prelimina_ simulation

The AACV model combines a variety of newand existing technologies and procedures, manyof which have been studied independently of theothers. There are a number of people studyingthe ability of flight crews to self-separate usingtools such as CDTI and CD&R. The use of ADS-B

for surveillance, though still in its infancy, is alsonot novel: in addition to the Capstone projectmentioned earlier, the FAA's Safeflight 21program 13 and the North European ADS-BNetwork or NEAN/NUP project 14have collected agreat deal of in-flight ADS-B performance data.There is also a lot of work on the use of GPS and

GPS augmentation schemes such as WAAS toprovide approach capability at airports currentlylacking an instrument approach. The concepts ofprocedural separation and the one-in/one-out useof a volume of airspace near an airport are wellproven in today's NAS. Since we were able to

draw on this large body of research andoperational experience, we have developed abatch simulation to validate the systemic attributesof the AACV operational concept.

The sequencer components of this batchsimulation are designed to be compatible withpilot-in-the-loop simulations developed at NASALangley _5 for study of pilot workload and the

evaluation of new procedures. These samesoftware elements can be used in conjunction withother simulation elements for pilot/controllerstudies to validate procedures developed fortransition from traditional approaches and

operations to these automated procedures. Acombination of this batch processing capabilityand piloted simulations are needed to identifyimprovements in the operational conceptnecessary for further development of the AACV.

3.1 Simulation Description

The automated airport simulation has fourprimary functions: traffic generation to introduce

aircraft into the environment with an appropriatemix of initial conditions, trajectory estimation (andtherefore calculation of time on approach), asequencer for the determination anddissemination of sequence, a delay function thatwill insert service re-requests from aircraft initiallydenied service, and data collection.

The automated airport operation system wasmodeled as a single server queue as shown inFigure 2.

Aircraft are randomly introduced to thesimulation based on an exponentially distributed

average inter-arrival time, X. Aircraft position atrequest time is assigned randomly within anannulus designated by the outer limits of the ACVplus some maneuvering space and the modeledADS-B reception limit. An appropriate preferredIAF is assigned by the simulation based ongeometric position relative to a standard T RNAVapproach as defined by the FAA. 16

Approach arrivals are assumed to fly direct tothe IAF and then initiate the approach. For ourpurpose of exploring a mix of approach speeds,three aircraft types are used; a light single enginepiston, an Eclipse Jet, and a Transport/Regional

jet. Arrivals are assigned a speed profile basedon type. The operation duration is then calculated

for each aircraft in the simulation as y(path,speed).

Sequencer events consist of aircraft approachand departure clearance requests, though onlyunscheduled approaches have been implementedto date. An arrival can find the system locked

(airport in use) or unlocked. If the system is free,requests are en-queued for a short time (a studyvariable) and one aircraft is selected for service(given priority). The rest of the requests in thequeue are rescheduled and merged with thearrival stream with a higher priority, an updated

position and a re-request time. The re-requestpositions can be modeled differently than initialrequests because delayed aircraft may be morelikely to re-request while near an IAF (altitudeseparated from the initial approach altitude) ratherthan scattered randomly about the airportperimeter. The simulation can model the re-

request arrivals either way. Figure 3 shows atypical 10 hour-aggregate traffic sample asgenerated.

If necessary, charted holding could be addedto the procedure to facilitate safe and easilyselectable waiting areas for delayed aircraft.

Holding has not been modeled, as our purposewas to determine the size and scope of delays wecould expect given differing rates and mixes ofarrival traffic. Human-in-the-loop studies will benecessary to determine feasibility of free flight andself separation in this region vs. a more


(Approach and

Traffic departure)Generation

New Requests ACV not Sequence ACV

locked Queue Clearance

Re-requestsMerge into thearrival stream.

Aircraft

re-request

ACV is

locked

Delay iModel

System is locked:

Requests are delayed

granted

Requests are

delayed for theestimated

operation time

Aircraft leaves I

the system.

_ Complete_ [ Data

Operat on Collection/

Reporting

:0 -5_ _ 0

15

54-

0

5

• 10

J

Initial Requests

A ReRequests

_._

,_ 5 _N

Figure 2: Sequencer Model in Simulation

queuing window, the lateral limits of the ACVboundary, the outer limits of ADS_B reception(and therefore when an approach request could

be received), the average arrival Rate (X), the timeof re-request relative to the estimated airport usetime, and the mix of aircraft (approach speeds).Independent variable levels chosen to be

representative of the design space can be seen in

Figure 3: Typical Arrivals (10 hr Aggregate)

constrained, planned hold or automated pathstretch vector assistance as others have

suggested in the references above.Provision of more information to the returning

aircraft than simply when the airport was expectedto reopen was found to decrease queue lengths(and is useful in non-normal operationalconsiderations). By providing an estimate of aspecific aircraft's usage time (implying implicitly orperhaps explicitly sequence position as well),message traffic in the form of requests and

responses was decreased dramatically, on theorder of 75%.

3.2 Experiment Design

A six factorial response surface text matrixwas generated, varying the size of the sequence

Figure 4.

IndependentVariable Units Levels

Request window min 0.1 0.7 1.1 1.4 2.C

ACV boundary NM 12.0 14.E 16.£ 17.4 20.C

ACVannulus NM 3.0 8.C 11.C 14._ 20.C

Arrival interval(,k) min 4.0 22.1 32.£ 41.E 60.C

Re-request delay min 0.0 0.7 1.£ 1.4 2.C

% GA % 1 33 51 6_ 10C

Figure 4: Variables Limits

Using a balanced central compositeexperimental design, 86 samples, each simulating10 hours of airport arrivals were taken. For eachsample, the simulation was allowed to continue torun until all arriving traffic had been serviced.Arrival data were collected for selected responsesand averaged over the sample period. A summary

of the data is shown in Figure 5.

343 Results and Discussion

Some delays are inevitable due to the nature ofthe service an automated one-in-one out airportwould provide. Before we can field such a system,we must determine what level of delay isoperationally feasible. By looking at the summarystatistics


Response Mean Median Min MaxSim Duration 616.4 600.£ 600.0 1446.9Airport Utilization 44% 43% 15% 84%New Requests 21.7 20.£ 6 129Total RepeatArrivals 28.8 11._ 1 1187Total SequenceQueue 30.0 22.£ 6 525Max SequenceQueue 2.7 2.C 1 36Total in Holding 11.4 8._ 1 120Max In Holding 3.1 2.C 1 72Highest Priority 4.0 3.£ 2 32Avg Wait 11.5 6.3 1.5 361.3MaxWait 49.2 26.5 5.5 1341.6Ave OperationDuration 13.2 13.£ 9.4 18.9

Figure 5: Summary Data

alone, one can see that the simulation predicted

large delays under certain circumstances, but thatminimal delays and few re-requests were likely formore typical, small-airport characteristics.

An ANOVA analysis of the data showedsignificant correlation between a few independentvariables (e.g. % GA) and important measured

responses (e.g. airport utilization and average waittime), but the effects of _, the inter-arrival time,were strongly significant and dominated the effectsof the other variables. This result confirms what

one might intuitively expect: as the average inter-arrival period nears the average operation

duration, the queues in the system begin to builddramatically. The relatively mild correlationbetween the other variables and the performanceof the system implies that wide latitude can betaken in the design of a particular AACV withoutadversely affecting the performance outcome. It

was also apparent from the simulation results thatthe system is robust enough to handle occasionaltraffic spikes that might be expected to occur evenat a low-use airport.

The model (see Figure 6) of Utilization=f(%GA, A, request response time) had F=82.16,p.<.0001 showing significance at the 0.05 level.

The request response time proved to be a smalleffect as compared to the environmental factors oftraffic mix and rate of arrivals.

The model (see Figure 7) Average Delay=f(%GA, A) had F=28.53, p.<.0001 showingsignificance at the 0.05 level.

One other observation warrants mention: the

operation time for each priority aircraft's use of theACV was estimated using a simplified pathpredicated on random position assignment andIAF selection. The only independent variabledirectly influencing the time to fly a specified path

is the assigned type (and therefore speed profile)as determined by the independent variable %GA,

Figure 6: Utilization ]'(h, %GA)

4

<<

6800

5925

50 502.10

% GA 41 76

3300 41 90 A (rrin)

Figure 1: Average Delay J(A, %GA)

yet in the analysis of the response AverageOperation Duration, A again appears significant.Though not immediately obvious, the result is dueto the system's preferential selection of shorteroperations, and the increasing availability of arange of predicted operation times as the queuesgrow with decreasing inter-arrival times (Figure 8).

'__=_'_ ._ _ .._4_,'.__ •

_S, ---£_-_S_.".._"

_n

A % _A

Figure 8:Average Operation Duration ]'(h, %GA)


4 Conclusion

Some years ago a bumper sticker reading

"THINK GLOBALLY, ACT LOCALLY" was popular.It was understood to mean revolutionary socialchange can be accomplished through action at thegrass roots level, even when attempts to imposechange from above are sure to fail. Removedfrom its political context, this notion has somerelevance to the task of revising the air transport

system. Structural changes such as significantlyincreasing the mobility of the system, or institutingtechnological innovations like distributed air trafficmanagement or automating air traffic controlfunctions represent revolutionary change; toimpose these changes on a large-scale or system-wide basis would be a difficult and risky

proposition at best. The alternative is todemonstrate new methods and procedures initiallyon a small scale, taking great care to conform tothe fullest possible extent to the existing regulatoryand procedural framework. If the new techniquesare foundtohave merit on the basis of

operational experience, they can be replicated

elsewhere in the system and extended to larger-scale applications on an incremental basis.

The AACV concept is an ideal model forimplementing a number of highly innovativetechnologies and procedures in a low-cost, safeand conservative manner. An operational AACVwould be compatible with today's ATC structure,

but it would readily accommodate the introductionof self-separation technologies and it represents aplatform upon which more sophisticatedautomated approach and departure trafficmanagement systems could be deployed.Experience with these systems in the low volumeenvironment of the AACV could be used to

validate the technology before attempts are madeto adapt these systems to higher-volume terminalareas.

Analysis of the results of a purpose-builtsimulation has validated the basic premise of an

AACV operating under one-in-one-out protocols atlow traffic volumes. The next step in thedevelopment of the AACV concept involvesadditional simulation studies to resolve a number

of issues such as traffic management outside theAACV. This research would be followed bydevelopment of a full-scale system simulation of

sufficient fidelity to permit pilot-in-the-loop studies.If the concept continues to show promise, a fieldstudy involving an operating AACV systemprototype should be undertaken at a suitableairport. All of this research can be conducted

relatively quickly and at reasonable cost becauseof the small-scale nature of the proposed system.

1 Mineta, N. Secretary DOT Testimony beforeCommerce, Science and TransportationCommittee of the U.S. Senate, Jan. 20012 FAA AUA-600 Capstone ADS-B Evaluation

Acceptability Memo, US DOT, Dec. 20003 Tobias, L. and Scoggins, J.L. Time-Based Air

Traffic Management Using Expert Systems.NASA TM-88234, Ames Research Center, April1986

4Tobias, Automated Aircraft Scheduling Methods

in the Near Terminal Area, AIAA 72-120, Jan.1972.

5 Morgenstern, B. and Telsch, R. A Description ofthe Phase I Automated Terminal Services

Concept, FAA-EM-76-6, Nov. 1976.6 RTCA Special Committee 186, Minimum AviationSystem Performance Standards for AutomaticDependent Surveillance Broadcast (ADS-B),RCTA DO-242, Feb. 1998.7 Ebi M., A Self-Organizing Approach forResolving Air Traffic Conflicts, The LincolnLaboratory Journal Vol 7, No 2, 1994.8 Hoekstra, Ruigrok, and vanGhent, Free Flight ma Crowded Airspace?, 3 rd USA/Europe Air TrafficManagement R&D Seminar, June 2000.9 Wing, D.J.et al, Airborne Use of Traffic IntentInformation in a Distributed Air-Ground Traffic

Management Concept: Experimental Design andPreliminary Results, NASA/TM-2001-21125410FAR 91.185

11 Williams, D.H. Time-Based Self-Spacing

Techniques Using CDTI During Approach ToLanding in a Terminal Area VectoringEnvironment, NASA-TM 84601, April 200012FAR 91.205 (d) (2)13Cargo Airline Association and FAA Safeflight 21

Office. Phase 1 Operational Evaluation FinalReport, Wilmington, Ohio. April 200014 North European ADS-B Network Update

Program, NUP. h_:!/v,_,v.nu#.nu15 Peters, M. and Ballin, M. A Multi- Operator

Simulation for Investigation of Distributed AirTraffic Management Concepts AIAA-2002-4596,Aug. 200216Federal Aviation Administration, Terminal Arrival

Area Design Criteria, FAA 8260.45A, July, 2000


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FOR INSTRUMENT FLIGHT PROCEDURES: AT NON-RADAR …€¦ · FOR INSTRUMENT FLIGHT PROCEDURES: AT NON-RADAR AIRPORTS Sheila R. Conway Langley Research Center National Aeronautics and