Enhancing Pilot Ability to Perform CDA with Descriptive Waypoints
1ENHANCING PILOT ABILITY TO PERFORM CDAWITH DESCRIPTIVE WAYPOINTSMichael LaMarr & Dr. Nhut Ho, California State University Northridge, CaliforniaDr. Walter Johnson & Vernol Battiste, NASA Ames, Moffett Field, CaliforniaJoe Biviano, Lockheed Martin, Palmdale, CaliforniaAbstractAircraft noise is a burden on people livingaround airports and is an impediment to the growth ofair transportation. Continuous Descent Approach(CDA) is an approach that reduces noise impact onthe ground by keeping the aircraft at a higher altitudelonger than standard approaches and by keepingengines idle or near idle. However, CDAimplementation requires controllers to add largeseparation buffers between aircraft because aircraft ofdifferent sizes and weights descend at different rates,consequently creating uncertainty in separationbetween aircraft. This paper proposes a viable near-term solution to allow pilots perform CDA moreconsistently through the use of DescriptiveWaypoints (DWs), or checkpoints in terms ofaltitudes and speeds along the CDA path that providethe pilot targets and feedback along the CDA path. Ahuman in the loop study was conducted to developand determine the effectiveness of using DWs toimprove flight performance during CDA procedures,and provide recommendations on DW design andintegration into existing CDA procedures.Twelve instrument rated commercial pilots flewthree different wind conditions using one, three, orfive DWs. Dependent variables included: deviationfrom DW target altitude and Indicated Airspeed(IAS), average power usage, perceived workload, andpilot acceptance of DWs. Objective and subjectivedata were also collected to obtain pilot feedback onthe design of DWs and their integration into CDAprocedures, and determine pilot strategies whileflying CDA with DWs.The results showed that as the number of DWincreases, mean altitude deviations from the DWtargets decreased from 922 feet to 196 feet andstandard deviations from 571 feet to 239 feet with aslight increase in perceived workload and one percentincrease in power usage and. The pilots commentedthat the DWs provide useful feedback to strategizehow to make corrections to the altitude deviation inthe vertical path, and that they would feelcomfortable having the DW integrated in the flightchart or shown as a vertical view on a display. Thepilots also provided a number of recommendationsfor integrating the DW into Jeppesen charts and rolesfor the pilot flying and the pilot not flying.These results imply that DWs can be used as aneffective cuing system to enhance pilot ability toperform CDA, and that they are a potential choice fornear to midterm implementation in improving theeffectiveness of CDA procedures.BackgroundContinuous Descent Approach BenefitsNoise and emissions produced by aircraft whenlanding are a burden on people living around airportsand is an impediment to the growth of airtransportation. The produced noise limits how manyaircraft can land at night and the ability to expandmore runways or build new airports in populatedareas. Currently, aircraft descend and land atdifferent speeds based on their size and weight,making it difficult to predict their future trajectory.Air Traffic Controllers (ATC) compensate fordissimilar aircraft performance by creating anapproach pattern in which all aircraft fly time-consuming and fuel-burning stair-step flightsegments at the same speeds as they enter theterminal area [1,2,3]. This practice makes itmanageable for the ATC to separate aircraft;however, it creates a significant noise impact on thelocal community. The noise is most profound inareas where the aircraft have to fly at low altitudes
2near the runway because of the existing navigationconstraints. Specifically, aircraft land by using aninstrument landing system (ILS) glide slope tointercept the glide path at the correct descent angle tothe runway (see figure 1). The ILS provides the pilotwith lateral and vertical guidance to maintain thecorrect approach orientation for landing. This isaccomplished by leveling off at an altitude thatallows the aircraft to intercept the glide slope frombelow. If the aircraft flies above the glide slope itmay intercept a false glide slope and come into theFigure 1: Conventional Approach and Continuous Descent Approachairport at an incorrect landing angle. To operatewithin the navigation constraints of the ILS andreduce noise impact, noise abatement approachprocedures have been developed and implemented.One such procedure is Continuous Descent Approach(CDA). CDA also offers other benefits such as fuelsavings and lower emissions impact by using an idleor near idle power and by decelerating the aircraft ata higher altitude longer than the standard landingprocedure without reverting to level flights (SeeFigure 1). The benefits of CDA have shown in manystudies and demonstrations, such as the CDA flightdemonstration study conducted in Louisville,Kentucky with UPS Boeing 767-300 aircraftequipped with the Pegasus flight management system(FMS) . It was shown that CDA can reduce noiseby 3.5 to 6.5 dBA (3 dBA is noticeable to the ear)and fuel consumption by 400 to 500 pounds.CDA Implementation Challenges & CurrentWork on Improving Performance PredictabilityCurrently implementation of CDA is notpractical in moderate to high traffic because itrequires a larger separation buffer between aircraftthan the standard landing procedure. Predictingwhere the aircraft will be is cognitively taxing on thecontroller and pilots because deceleration is non-linear and humans have a difficult time judging non-linear deceleration when speeds are constantlychanging [3, 5]. To implement CDA, ATC have toknow when aircraft are at the right distance from theairport to initiate the clearance to start the descent. Ifthe air traffic controller tells the pilot to initiate CDAtoo early, then the aircraft will arrive before therunway and will have to level out before landing.Leveling out early requires power increase, which inturn creates more noise and defeats the purpose of theprocedure. If the ATC tells the pilot to initiate theprocedure too late, then the pilot will end up makinga fast landing or have to initiate a go-around foranother approach (which produces more noise anduses more fuel, which also defeats the purpose of theCDA).To assist ATC in managing CDA, ground basedautomation is being developed. Stell et al is workingon an algorithm for ground based automation calledThree-Dimensional Path Arrival Management(3DPAM) to provide top of descent (TOD)prediction to controllers . The purpose of thisalgorithm is to predict TOD location within atolerance of 5 nm to help controllers manage CDAtraffic. This algorithm takes into account that evenwith a perfect TOD predictor the vertical profiles andpath distance must be predicted accurately as well.Kuper et al conducted a study with threedifferent ground based support tools for ATC to workAirportContinuousDescent ApproachConventionalApproach10,000feet4,000 feetILS Glide Slope
3in Super Density Operations (SDO) environment .These tools are timeline, slot marker, and speedadvisory. Timeline gives the ATC estimated time ofarrival (ETA), slot marker gives a visualrepresentation (circle) to the ATC where the aircraftshould be if on schedule, and the speed advisoryprovides controllers with air speed advisories that canbe used to correct scheduling errors. Based onsubjective feedback, ATC preferred the slot markertool.Alam proposed that dynamic CDA would allowthe FMS to create unique CDA routes based onaircraft type, size, and weight . But in order fordynamic CDA to be implemented ATC will need tohave the same information as the pilot. This can beaccomplished with real time data linking betweencontrollers and pilots with controller pilot data linkcommunications (CPDLC). The controller would beable to uplink the flight plan to the FMS to initiateCDA, and the pilot would then send the executedFMS CDA route back to the controller.Ren and Clarke developed the Tool for theAnalysis of Separation and Throughput (TASAT) topredict trajectories of different aircraft performingCDA and to determine the minimum spacing at ametering point in order to avoid separation violations. Factors that contribute to aircraft trajectoryvariations are aircraft type and weight, FMS logic,and pilot technique and winds.Other challenges to the implementation of CDAprocedures remain the difficulty that pilots have inmanaging the deceleration of aircraft in the presenceof uncertainties in pilot response time, verticalnavigation performance (VNAV - controls verticalautomation of aircraft according to flight profileprogrammed in the FMS), and wind conditions .Koeslag also identified other issues with currentCDAs . One issue is how the vertical flightprofile is managed and depends on the type of FMSinstalled. Another issue is that wind can cause theaircraft to deviate from the FMS-predicted flighttrajectory. One of Koeslag’s proposed solutions wasto have a fixed CDA vertical profile to improvearrival time predictability and add flap guidance inthe primary flight display (PFD) to correct for speeddeviations (see Figure 2).Figure 2: Flap Cues Recommended by Koeslag inPrimary Flight Display Other research efforts aiming to make CDAmore predictable are focusing on equipping FMSwith 4D guidance (x, y, z, and time). Mooreproposed 4D information with a required time ofarrival (RTA) to aid ATC in establishing a strategictime scale of CDA traffic flow . The algorithmsdesigned in this research are aimed to minimize time,fuel, and emissions produced. One problem noted isthat automation can cause the VNAV to makeoccasional thrust changes that can generate extranoise and fuel usage.Other research on RTA during CDA operationproposed to provide pilots with an energymanagement system (see Figure 3) in the navigationdisplay to minimize fuel, noise, and emissions. Thesystem provides pilots an optimal vertical path withenergy events and energy error cues for managingthrottle and drag .Figure 3: Energy Management System used byNASA Langley Research Center
4Other flight demonstration studies found thatpilot delay in initiating the flaps had undesired effectson VNAV in that it causes the aircraft to deviate fromthe altitude programed on the FMS . Anotherproblem identified with the VNAV is that whendescending, the VNAV’s logic gives the altitudeconstraint higher priority than the speed constraint.With factors such as tail wind, the aircraft does notalways meet the speed targets. It is important to meetboth the speed and altitude constraints on the flightpath for fuel and time efficiency, and for trafficseparation.Development of Descriptive WaypointThese studies have been beneficial to CDAresearch and development, but are aiming for mid tolong term implementation; until better VNAV logicsand FMS designs can compensate for pilot delay,altitude and speed constraints, and winduncertainties, the pilot has to control the verticalprofile manually. If pilots have information to helpthem stay on the flight path and manage theiraircraft’s speed, CDA would be more feasible fordaily use. One way to aid pilots in executing CDA isto give them feedback information. Without the helpof a cuing system, pilots find it difficult to managethe aircraft energy to meet a target speed at a specificaltitude in the presence of uncertainty. According toHo et al, there are two reasons for uncertainty duringCDA . One reason is the pilot’s inability toestimate future position of aircraft because thedeceleration profile is non-linear. The second reasonis that the pilot’s projection may be incorrect becauseof wind uncertainty. They proposed to provide pilotfeedback info in terms of gates, which are an altitudeand speed target along the flight path. Forconsistency purpose, gates will be called DescriptiveWaypoints (DW) in this paper. In Ho’s study,different numbers of gate conditions (0, 1, 2, and 3)were proposed to use with a flap schedule. Each ofthese conditions also had wind uncertainty and nowind uncertainty. For the two DW condition, theDWs were located at 5000 and 3000 feet from therunway, and the three DW condition had DWs at5000, 4140, and 3000 feet (see Figure 4). The DWswere given to pilots on a cue card shown in Figure 4.Providing vertical information and DW at 7,000 feetaltitude improved pilot ability to perform CDA. Inthe three DW condition, pilots were able to achievethe target speed at a higher rate than the otherconditions.Figure 4: DW Cues with Flap, Altitude andIndicated Air Speed References Given that CDA is being considered to beinitiated at further distances from the airport (e.g.,such as 37,000 to 40,000) for fuel savings andemissions reduction, the concept of DW wasconceived and formulated as an extension of the gateconcept . The name DW was chosen becausedynamic waypoints (target altitude and speedwaypoints that can be created in real time, relying onthe existing waypoint database) focus on providingupdated waypoint information, and DW is adescription of a waypoint that could include a timetarget, flap requirement, gear deployment, targetaltitude, and speed. For this study, only altitude andIndicated Air Speed (IAS) was provided in the DW.Flap information was displayed in the PFD.Display of Descriptive WaypointsWith the description of DW defined, it isimportant to examine how to display the DWinformation to the pilot. The navigation display (seeFigure 5) is a useful way of informing the pilot of thegeneral surroundings, and is currently limited to a 2Dperspective.
5Figure 5: 2D Navigation DisplayPilots also use arrival charts when they are ontheir final approach, with the vertical information(altitude) displayed as text as in the 2D navigationdisplay, as well. Figure 6 represents the verticalprofile the pilot needs to fly with the step down fixesto intercept the glide slope and land.Figure 6: Flight Chart Final Approach Thomas and Wickens found that it is easier tomake specific and accurate judgments based onabsolute spatial information displayed in 2D with 3Dinformation . This is because 3D displays tend tomake the x, y, and z axis ambiguous, whereas 2Dinformation gives precise x and y information, butwill need the 3D information in text. There is also aproblem that occurs while using 3D views: withoutother depth cues available, the location of objectsbecome ambiguous . Even with this problem 3Dviews do have their advantages. For example, shapeunderstanding is beneficial in 3D, whereas 2D ismore accurate for precision tasks . For thecurrent study with a focus on implementation in thenear term, it makes sense to display the DWs to pilotsin 2D because pilots are only controlling theirvertical descent during CDA.Implementation of DWs into the current flightdeck system can be accomplished by displaying themin a vertical flight chart (see Figure 7). Vertical flightcharts typically display the final approach rightbefore the glide slope at approximately 3,000 to10,000 feet altitude. The initial altitude in thecurrent study starts at 23,000 feet altitude at 70 milesfrom the airport. This is motivated by the fact thatCDA procedures are being proposed to start at a veryfar distance from the airport, such as the top ofdescent location, which is typically at 37,000 to40,000ft .
6Figure 7: Three Descriptive WaypointsA possible solution for midterm to long termimplementation is to use a coplanar view (horizontaland vertical profiles) of the navigation with DWinformation. The vertical display would benefitpilots, enabling them to monitor the vertical profilewhen VNAV is turned off and the pilot is manuallyflying the vertical profile (see Figure 8).Figure 8: Coplanar Navigation View Another possible long term solution that canhelp the pilot perform CDA more efficiently is todisplay DWs in the Cockpit Situation Display (CSD)to provide 3D visualization of the flight plan (seeFigure 9) . CSD is a navigation aid that pilotscan use to gain information of surrounding air traffic,alert them of possible conflicts, provide spacingtools, etc. CSD eliminates 3D ambiguities byallowing the pilot to rotate the screen 360 degreesand switch to 2D anytime.Figure 9: 3D Cockpit Situation DisplayThe current study is an extension of Ho’s gatestudy of adding predictability to the pilot duringCDA. VNAV was set to off because it was shown toaffect CDA performance in previous studies. LNAVwas left on auto-pilot
7MethodDesignA 3 wind (Fast, Normal, and Slow) x 3 DW(One, Three, and Five) within-subject factorial designwas used. The wind conditions (Fast, Normal, andSlow) were based on historical data at LouisvilleInternational Airport and were chosen to producenoticeable differences. In the Fast wind condition, thewind speed started at 52.8 knots (60% higher than thenormal wind condition), the Normal wind conditionhad a starting wind speed of 33 knots, and the Slowwind condition had a starting speed of 19.8 knots(40% lower than the normal wind condition) (seeFigure 10). The One-DW, Three-DW and Five-DWconditions provided the pilots with one, three, andfive DWs respectively along the CDA path (seeFigures 12-14). The number of DWs was designed tovary the amount of feedback provided to the pilots.Dependent variables included: altitude and IASdeviation, computed as the absolute deviation fromDW target altitude and IAS. Altitude deviations andIAS deviations are metrics used to evaluate howDWs assist pilots to maintain the CDA flight profile.Power percentage usage was computed as the averagepower the aircraft uses during the CDA. Perceivedworkload, pilot acceptance of DW, pilot strategiesand other subjective data were collected in aquestionnaire (rating scales and open endedquestions) to evaluate the effectiveness of DW andobtain feedback on pilot acceptance and theintegration of DW into existing CDA procedures.Figure 10: Wind SpeedMaterialStimuli were displayed on two 19” monitors,with one monitor running the Multi Aircraft ControlStation (MACS) software which provides a dynamicinterface that allows the pilot to fly and interact withthe aircraft’s systems, such as IAS, vertical speed,flap settings, and altitude . The other monitorwas running Cockpit Situation Display (CSD) whichwas used to display the CDA flight plan on a 2Dfixed vertical view (see Figure 11) . Dependingon the condition, the DWs were displayed on a flightchart as one, three, and five DWs as shown in Figures12-14. The CDA profile used in the current studywas developed and verified by NASA in a previousstudy [20, 21]. Modifications were made to the CDAprofile by creating aircraft start and end points,removing all traffic, and by adding DW locationswith considerations for the aircraft’s kinetic andpotential energy, noise, deceleration, speed/altitudetargets, and power usage. The CDA profile flew isthe same in all conditions.Figure 11: MACS (top picture) and CockpitSituation Display (bottom picture)01020304050600 20000Altitude, [ft]FastWindNormalWindSlowWindWindSpeed,[knots]
8Figure 12: One Descriptive Waypoint ConditionFigure 13: Three Descriptive Waypoints ConditionFigure 14: Five Descriptive Waypoints Condition
9FacilitiesThe study was conducted in the SystemsEngineering Research Laboratory at California StateUniversity Northridge (see Figure 15). Pilots were ina room with a one-way mirror and sat at a desk withtwo computer monitors and a flight chart, and used amouse to interact with the monitors.Figure 15: Pilot Station SetupParticipantsParticipants included twelve instrument-rated,commercial pilots (11 male, 1 female) between theages of 24 and 67 (Mean 37.64 years old) with yearsof flying between 2 and 37 years (Mean 18.75 years)and with 590 to 23,000 (Mean 7660) hours of flighttime. Two pilots have experience with CDA, and onehas experience with CDA simulation.ResultsAltitude and IAS deviations and average powerwere collected from MACS output and put into excelfor each participant for nine conditions. Data wasorganized by dependent variable, and a 3x3 (Wind xDW) analysis of variance (ANOVA) was run onSPSS version 17 for each of the dependent variables.Attitude and IAS DeviationFigure 16 shows the means and standarddeviations of altitude deviations at the DW targets 1through 5. There was a significant main effect ofDW on altitude deviation on DW targets 2 through 4,F(1.392, 15.308)= 26.364, p<0.000, etap2=1.000 andno significant main effect of Wind on averagealtitude deviation on DW targets 2 through 4. In theFive-DW condition, (mean=196.06 sd=238.90)significant differences in average altitude deviationwere found compared to the Three-DW condition(mean=516.00 sd=287.68) and the One-DWcondition (mean=922.32 sd=570.61). The differencein altitude deviation between the Five-DW andThree- DW condition was 319.94 feet, between theThree-DW and One-DW condition was 406.32 feet,and between the Five-DW and One-DW conditionwas 726.26 feet. There was no significant interactioneffect between DW and Wind on altitude deviation ofDW targets 2 through 4. There were no maineffects or interactions with IAS.Figure 16: Average Altitude Deviation Main effect on DW0200400600800100012001400160018nm to Cheri:DW Target 1Cheri:DW Target 2Alt<10:DW Target 310nm to SDF:DW Target 45nm to SDF:DW Target 51 DWCondition3 DWCondition5 DWConditionAltitudeDeviation,[Ft]
10Power UsageThere was a significant main effect of DW onaverage power percentage, F(1.928, 21.203) = 3.731,p<0.042, η=0.609, and no significant main effect ofWind on average power percentage. In the One-DWcondition, (mean=7.25 sd=2.31) significantdifferences of average power percentage were foundcompared to the Three-DW condition (mean=7.62sd=2.39) and the Five- DW condition (mean=8.35sd=3.45). The difference in average powerpercentage between the One-DW and Three- DWconditions was .37, between the Three-DW and Five-DW conditions was .73 average power, and betweenthe Five-DW and One-DW conditions was 1.1.There was no significant interaction effect betweenDW and Wind on average power percentage. Figure17 shows the mean power for different DWconditions.Figure 17: Average Power Main effect on DWSubjective Data and FeedbackPilot Strategies and Feedback on DW DesignThe majority (10 out of 12) of the pilotscommented that they used the altitude and speeddeviations at the DWs as feedback to manage thedescent. Specifically they strategized that whencoming in too fast to a DW target they would reducevertical speed, and when coming in too slow to a DWtarget they would increase vertical speed. Thisstrategy was used in combination with their rulethumb for managing vertical speed, which is forevery three miles the aircraft descends 1,000 feet.They also reported using the speed brakes as little aspossible and using the flaps only as needed.In terms of the placement of the DWs, the pilotsmentioned that they like the spacing between eachDW target and how the 10,000 feet level off section(Alt<10: DW Target 3) helped slow down theaircraft. One pilot said that it was difficult for him toslow down at 10,000 feet from 240 IAS to 160 IASuntil the end of the scenario because of the workloadof using the speed brakes and setting the flaps. Theyrecommended that the distance between two DWs bebetween 10nm to 30nm apart when they are furtheraway the airport, and 5nm to 10nm when precisionflight is needed closer to airport. Overall, the meanfor pilot rating of the effectiveness of the DW targetlocations on the flight plan were 4.67 out of six(where 1 is very uncomfortable and 6 is verycomfortable).In terms of power management, the pilots statedthat DW helps reduce power usage of the aircraft ifthe descent was planned correctly to intercept DWaltitude without leveling out. If the aircraft is early tothe target altitude before DW target, it creates leveloff altitude segments, which increases power usage.Pilots commented that with the green arc (altitudeprediction tool) in their NAV display, they would getto the DW altitude target without leveling off.Perceived WorkloadThe results from a 3x1 (DW x Wind) within-subject analysis of variance (ANOVA) revealed asignificant main effect of DW on perceivedworkload, F(1.496, 16.451)=77, p<0.000,etap2=1.000. In the one DW condition, (mean=2.58sd=0.67) significant differences in workload werefound compared to the three DW condition(mean=3.25 sd=0.75) and the five DW condition(mean=4.17 sd=0.72). The difference between theone three DW condition was 0.67 perceivedworkload, between the three one DW condition was0.92 perceived workload, and between the five DWand one DW condition was 1.59 perceived workload.Figure 18 shows the mean workload for each DWcondition. A likert scale was used to assess workload(where 1 is very low and 6 is very high).0.002.004.006.008.0010.0012.0014.001 DW 3 DW 5 DWAveragePowerPercentage
11Figure 18: Average Perceived Workload Maineffect on DWFeedback on Flight Chart, Vertical View onCSD , and CDA scenariosThe pilots rated the usefulness of a using avertical flight chart integrated with DW informationwith a mean score of 4.42 out of 6 with a standarddeviation (SD) of 1. Most pilots liked the providedDW flight chart, and commented that they would likethe DW information much better if used incombination with the Jeppesen chart, and preferred tohave the distance to next DW and descent angles(based on ground speeds) be inserted into DW flightchart.In terms of the utility of the vertical view on theCSD for maintaining the flight plan, the pilots ratedits usefulness with a mean score of 3.75 out of 6 witha SD of 1.22. They suggested a number of additionalfeatures and information to be added to the CSD,including thicker flight path, known verticaldeviation from flight path, a green arc ("banana" foraltitude confirmation), and an option to zoom in andout. A Boeing 757 pilot said that his cockpit setup(vertical NAV screen is next to the regular NAVscreen on the right) is similar to what was done in thestudy and has an energy management arrow (similarto Figure 3) which is helpful for flying CDA.For the CDA scenario designed in the study, thepilots thought that scenarios were realistic and thatthey would have flown better with the help of a pilotmonitoring (PM) and with tools such as the green arc.They suggested that the roles of the pilot flying (PF)would be able to fly vertical speed and IAS, makedecisions for meeting DWs, and vocalize plan andcallouts to the PM. As for the roles of the PM, theysuggested that the PM monitor targets, air speed,IAS, and DWs; do all calculations for the PF; setaltitudes, MCP; work the gear and flaps; crosscheckaltitude inputs; and communicate with ATC.ConclusionsIn this paper, descriptive waypoints wereproposed as a viable near-term method to assist pilotsin performing CDA procedures that start further awayfrom the airport than CDAs used in current practice.It was hypothesized that the pilot performance wouldimprove as the number the descriptive waypointincreases, and the results in the experimentcorroborate this notion. As indicated in theexperimental results and pilot feedback, thedescriptive waypoints act as a feedback mechanismthat help “reset” the system by allowing the pilots tomake adjustments based on the altitude deviationsfrom the targets at the descriptive waypoints. TheFive-DW condition (the condition with the mostnumber of descriptive waypoints) resulted in onlyone percent more in power than the other conditionsand with a slight increase in workload. Withautomated assistance (such as the green arc), thepilots believed that they would use less power byintercepting the DW altitude target without resortingto level flight. The potential small increase inworkload could be addressed by developingprocedures that assign the pilot monitoring the task ofmonitoring the deviations at the DW targets andvocalizing plans to meet the next targets.In combination, proper design of the descriptivewaypoints’ locations and targets and of theapproach’s vertical and speed profiles allowed pilotsto fly the approach consistently, which will help ATCwith aircraft separation and ultimately make CDAmore feasible in high traffic conditions. For futuredevelopment, the DWs can be implemented into thecurrent flight charts for near term implementation,and for midterm, they can integrated into moreadvanced software that would provide powermanagement and dynamically update DW altitudeand IAS targets based on wind speed and constraintson arrival time.0123451 DW 3 DW 5 DWAveragePerceivedWorkload
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