Table Of ContentUSE OF TRAFFIC INTENT INFORMATION BY AUTONOMOUS AIRCRAFT
IN CONSTRAINED OPERATIONS
David J. Wing
NASA Langley Research Center, Hampton VA
Bryan E. Barmore
Karthik Krishnamurthy*
Titan Systems, Hampton VA
Abstract Introduction
This paper presents findings of a research study A significant research activity within the NASA
designed to provide insight into the issue of intent Advanced Air Transportation Technologies project is
information exchange in constrained en-route air-traffic focused upon far-term operations of the National
operations and its effect on pilot decision-making and Airspace System (NAS). A general description of the
flight performance. The piloted simulation was activity is Distributed Air/Ground Traffic Management
conducted in the Air Traffic Operations Laboratory at (DAG-TM). NASA has developed a high-level concept
the NASA Langley Research Center. Two operational of operations for DAG-TM consisting of 15 elements
modes for autonomous flight management were spanning gate-to-gate operations 1. One particular
compared under conditions of low and high operational concept element, developed to address the en-route
complexity (traffic and airspace hazard density). The flight regime, 2 has the potential to increase capacity,
tactical mode was characterized primarily by the use of flexibility, and robustness of the NAS by distributing
traffic state data for conflict detection and resolution responsibility for (1) separation assurance, and (2)
and a manual approach to meeting operational conformance with local traffic flow management
constraints. The strategic mode involved the combined (TFM) constraints between airborne and ground-based
use of traffic state and intent information, provided the systems. In this concept element, pilots of aircraft
pilot an additional level of alerting, and allowed an designated as "autonomous" have the authority to
automated approach to meeting operational constraints. generate and implement new trajectories at their
Operational constraints applied in the experiment discretion in order to meet individual, company (if
included separation assurance, schedule adherence, applicable), and/or system-level goals. With this
airspace hazard avoidance, flight efficiency, and authority for autonomous operations comes the
passenger comfort. responsibility for separation assurance and compliance
with local TFM constraints established by the ground-
The strategic operational mode was found to be
based air traffic service provider (ATSP). Aircraft not
effective in reducing unnecessary maneuvering in
operating as autonomous aircraft are designated as
conflict situations where the intruder's intended
"managed aircraft," and similar to current operations,
maneuvers would resolve the conflict. Conditions of
their flight crews comply with clearances provided by
high operational complexity and vertical maneuvering
the ATSP, who maintains responsibility for their
resulted in increased proliferation of conflicts, but both
separation assurance and flow management
operational modes exhibited characteristics of stability conformance.
based on observed conflict proliferation rates of less
than 30 percent. Scenario case studies illustrated the Research Focus
need for maneuver flight restrictions to prevent the
creation of new conflicts through maneuvering and the Flight-deck Information Requirements
need for an improved user interface design that
A predominant research focus in the Free Flight
appropriately focuses the pilot's attention on conflict
community has been on the type of information
prevention information. Pilot real-time assessment of
required on the flight deck of autonomous aircraft to
maximum workload indicated minimal sensitivity to
enable their pilots to ensure separation from other
operational complexity, providing further evidence that
aircraft. Accurate detection of "conflicts" or predicted
pilot workload is not the limiting factor for feasibility
loss of separation between aircraft is a key requirement
of an en-route distributed traffic management system,
for autonomous aircraft operations. At issue are the
even under highly constrained conditions.
relative utility and requirement for inter-aircraft
information exchange of the current "state" (three
*Member, AIAA dimensional position and velocity vector) and "intent"
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of eachaircraft(flight plan);this surveillance
(notwithstanding runway availability limitations), and a
informatiofnormsthebasisfortrajectorpyredictions
concept that does not address capacity limitations is of
usedin automatecdonflictdetectionA. dditionally,
little practical interest. It is in the more highly
relatehdumafnactorisssueesxists,uchasdetermining
constrained conditions that operations will be found to
howpilotswouldusethesurveillanicneformatioann,d
be either fragile or robust to real-world system demands
howthisinformatiosnhoulbdepresentoendtheflight
and variability.
deckdisplaysc,onsiderinugsabilityd,isplaydesign
precedenacned,integratiownithotheprilottasks. Operational constraints can generally be expressed
in four categories. Flow management constraints are
Previousresearchhas indicatedthat, in
restrictions that must be imposed to maintain high
unconstrained operations (no schedule constraints or traffic flow while ensuring safety. Flow management
airspace restrictions to the route of flight), the exchange constraints in a future system may include such
of state information between aircraft is sufficient to crossing restrictions as a "required time of arrival"
safely enable airborne self separation in the en route (RTA) assignment at a terminal boundary for inbound
domain 3. A state-only system has the potential to autonomous aircraft. Airspace hazard constraints are
significantly reduce bandwidth requirements for future present when certain regions of airspace are inadvisable
surveillance systems such as Automatic Dependent for entry. Examples of such airspace hazards are active
Surveillance - Broadcast (ADS-B), and it reduces the special-use airspace (SUA) and convective weather
complexity of conformance monitoring and conflict cells. Performance constraints include restrictions
alerting logic. To address the conflict alerts missed by based primarily on the operating limitations of the
not using intent information, Hoekstra 3developed and aircraft. Restrictions such as maximum operating
tested a predictive airborne separation assurance system altitude, speed, or climb/descent rate govern the degrees
(PredASAS) that calculates potential off-trajectory of freedom available for conflict resolution maneuvers.
conflicts and displays avoidance bands on the heading, Economic constraints include user-generated
airspeed, and vertical speed indicators. This conflict operational guidelines that must generally be met a
prevention system was designed to provide information majority of the time for a commercial aviation business
regarding which maneuvers would lead to a conflict to remain viable for the long term. Examples include
without the crew needing to "probe" or "try various fuel efficiency, schedule considerations, and passenger
maneuvers." The conclusion was made that "if all comfort.
equipped aircraft are fitted with PredASAS, there is no
When constraints of these types are considered in
longer a need to know intent information because
combination with the task of separation assurance, the
nobody will turn (or climb/descend) into a conflict." 3
type of traffic surveillance information provided to the
A study conducted at the NASA Ames Research flight crews may play amore critical role in their ability
Center suggested that pilots nevertheless preferred to be to repeatedly and reliably meet their separation
provided traffic-aircraft intent information, and the assurance responsibility than in unconstrained
preferred source was Flight Management System operations. This issue extends beyond the minimum
(FMS) flight plan data 4. In this study, flight crews were requirement for information that enables airborne
alternately provided with three types of traffic separation assurance, and it extends beyond the
information: state data, Flight Control Panel (FCP) preferences of the flight crew. The study of constrained
data, or FMS data. The flight crews were given the operations is critical to determining the overall
opportunity to use voice communication channels to advisability of exchanging intent information to enable
communicate directly with other aircraft to gather intent the NAS participants to achieve their operational
information or negotiate resolutions. Results of the objectives, meet established constraints, and operate
study indicated that pilot preferences for intent with long-term stability in the future airspace
information centered primarily on the improved ability operations.
to understand the conflict alerts. Intent information
Modes of Autonomous Operations
type (state, FCP, or FMS) was found to have no effect
on separation assurance. In the experiment, two modes of autonomous
aircraft operation, tactical and strategic, that relate to
Operational Constraints
the use of traffic intent information were tested for
Whereas these studies addressed unconstrained comparison. The two operational modes are both
operations, little research has been performed on the considered viable solutions, and they differ in several
feasibility of constrained operations. Constrained respects beyond just the level of information exchange.
operations are important to consider in concept
feasibility and viability analyses. Operational
constraints ultimately limit airspace capacity
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Tactical mode maneuvering safely to resolve the original conflict, the
pilot monitors the conflict prevention information to
Aspects of the tactical mode have been developed determine when it is safe to return to course.
and investigated over several years in batch and piloted
simulation studies by the NLR (National Aerospace Strategic Mode
Laboratory of The Netherlands) 3, and it is primarily
The strategic mode is a closed-loop (automated)
characterized by simplicity in several respects. It was
method of trajectory planning. Any trajectory changes
designed to minimize the requirements placed on
recommended to the flight crew will have been
supporting technology, including both data link and
determined a priori to meet all known constraints and
pilot decision-support automation. Broadcast data-link
optimization criteria, while both solving the current
bandwidth requirements are minimized by employing
conflict and returning the aircraft to course. This
conflict detection based only on the current aircraft
approach places greater demand on decision-support
state vector (current position, altitude, ground track,
automation in that it must generate trajectories for pilot
ground speed, and vertical speed). On-board conflict
review based on a simultaneous solution of constraints
detection algorithms deterministically compare (in the
and objectives. More information on future actions of
current implementation) the state vectors of traffic
the traffic would allow for earlier detection of many
aircraft with that of the ownship. To minimize false
conflict situations and greater flexibility and
alerts associated with extrapolation errors 5,state-vector-
acceptability of new trajectories. Therefore, the
based conflict detection is limited in its "look ahead"
intended trajectory ("intent") of each aircraft is
time horizon. The research of NLR has determined that
included in its broadcasted data-link message. This
a five-minute look-ahead horizon is sufficient for
approach places a greater demand on the data link
separation assurance.
bandwidth to accommodate the additional information.
If a conflict is detected, the pilot is alerted, and the For this experiment, the intent message was defined to
conflict resolution algorithm is automatically activated be a series of trajectory change points, although other
to calculate maneuver advisories for the pilot. These forms of intent are also being considered 7.
conflict resolution advisories are simple in that they are
Conflict detection in the strategic mode is
recommended changes to the ownship heading,
performed using a combination of traffic state and
airspeed, or vertical speed. The pilot implements the
intent information. The state-based conflict detection is
maneuver by setting targets in the FCP to match the
identical to that used in the tactical mode, and a 5-
advised settings. This procedure is comparable to the
minute look-ahead horizon was used in the experiment.
pilot's current use of the FCP to comply with a vector
The intent-based conflict detection deterministically
for traffic issued by Air Traffic Control (ATC).
compared the ownship flight plan to the traffic aircraft
Concurrently, aconflict prevention system (PredASAS)
broadcast intent in a search for intent conflicts s.
monitors all possible single-dimensional maneuvers for
Whereas a 15-20 minute look-ahead horizon is thought
conflicts, and it indicates to the pilot what maneuvers
to be appropriate for intent-based conflict detection, a
would cause a new conflict, essentially a "no-go"
shorter horizon of 8 minutes was used in the current
alerting system. All maneuvers outside of the displayed
study to allow more data to be gathered during the
no-go bands are conflict-free for at least the next 5
limited availability of the subject pilots. This shorter
minutes, assuming the traffic aircraft do not maneuver
time horizon was still of sufficient length to permit
during this time.
exploration of some of the implications of differing
The resolution maneuvers in this operational intent and state time horizons.
mode are tactical in nature because they only resolve
A conflict-alerting decision algorithm was
the conflict and do not account for a return to the
developed to determine when and how to alert the pilot
original flight plan or the consideration of external
to potential and actual conflict situations. The utility of
constraints, such as RTAs or airspace hazards. This
combining state-based and intent-based conflict
highlights the primary characteristic of the tactical
detection is that the alerting system can distinguish
mode, that of the open-loop (manual) nature of meeting
between a conflict situation that requires ownship
constraints. It is hypothesized that the pilot would
action and a situation that will likely be resolved by
typically solve problems sequentially: First, resolve the
maneuvers of the traffic aircraft. The latter situation
conflict by maneuvering clear; second, avoid any
would require no ownship action but would have the
nearby airspace hazards; third, develop an efficient plan
potential for elevating to the former category should the
to return to course; and fourth, make adjustments to
traffic aircraft not maneuver appropriately. Such events
meet RTA and other ATC constraints. This approach
include failure of the aircraft to observe priority and/or
has the effect of spreading decision-making over time
maneuver flight rules (described below) or an
and possibly simplifying the maneuver decisions. After
unannounced deviation from the broadcast intentions
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(i.e.,flightplannon-conformancIteis).hypothesized
ground speed, or vertical speed that creates a near-term
thatdistinguishinwgithinthealertinglogicbetween
conflict (for the current study, within 5 minutes). The
situationthsatrequiroerdonotrequireownshiapction
pilot would meet the requirements of this role by
wouldreducuennecessamrayneuverianngdtherefore
avoiding flight in the direction of a conflict prevention
improvoeveralslystemstabilityT.healertinglogicis
band, although transition through a band is permitted.
describeindmoredetaiilnreferenc9e.
This maneuver flight role has the additional benefit of
Incontrastotthetacticaml odet,hestrategmicode providing some predictability in autonomous-aircraft
usems odemcommercaiairlcrafttechnolotgoyassisint operations, which should aid the ATSP in developing
conflictresolution.By coordinatinthgeconflict- stable strategies for managed-aircraft separation.
resolutiocnalculationwsiththeflightplanningand
A priority flight rule defines which aircraft in a
trajectorgyeneratiofunnctionosftheFMSa, complete
given conflict situation is responsible for resolving the
re-plarmionfgthelocatlrajectorcyanbeperformethdat
conflict. The tactical mode has no priority flight role in
guarantetehsatthenewtrajectoriyswithintheflight
that it assumes every autonomous aircraft shares equal
envelopeIn. additionth,eFMScanalsobeusedto
responsibility to resolve conflicts, which is prudent
closethelooponATSPconstraintSs.peeadndpath
given the limited time horizon for detecting and
strategiethsatmeeatnRTAatadownstreafimxor
resolving conflicts. The strategic operational mode also
airspaceboundarycanbe incorporateindto the
assigns equal responsibility for near-term conflicts.
proposecdonflict-resolutitoranjectory.Resolution
However, for conflicts more than 5 minutes away, the
strategiecsanalsoincorporapteredicteldocationosf
conflict geometry is used to determine who has _'right-
convectivweeathecrellsandscheduleadctivatioonf
of-way." By assigning resolution responsibility to one
SUAoranyregionthatwouldbeconsiderheadzardous
aircraft in a conflict pair, predictability should increase,
orinadvisabtoleentera,ssuminthgisinformatiownas
total maneuvering at the system level should decrease
madeavailabletotheaircraftsystems.Sincethe
(ideally by one-half since generally only one aircraft in
solutionspacethatmeetstheseconstrainwtsould
a pair would maneuver), and system-level traffic flow
normallybe large,trajectoryoptimizatiocnanbe
stability may be enhanced. For conflicts detected
performetdo achievea desiredgoal,suchasfuel
significantly far in advance (perhaps greater than 15
economay,comfortabrildee,oranearlyarrival.The
minutes - a subject of future research), the benefits of
FMScanthenbeusedtoflythecompletreesolution
assigning responsibility are likely to disappear, and
trajectoryp,otentiallryeducintghetaskloadof the
therefore priority flight roles would no longer be
flightcrew.
applied.
Conflicrtesolutioandvisoriefosrconflictbsaseodn
validinten(ti.e.t,heintrudearircrafistdeterminteodbe
conformintogitsbroadcainsttentw) eregeneratbeyda
genetic-algorithm-boapstiemdizatiornoutin1e°. This
routinewasdesignetdoiteratetrajectorcyonstraints
withtheFMSuntilaconflict-fretreajectorthyatmeets
alladditionaclonstrain(tes.g.R, TA)is determined.
Furtheirterationasrethenperformetdooptimizea
selectepdaramet(eer.g.f,uel-bumminimizationT)h.e
trajectorwyouldnormallbyeflownbytheFMS.For
conflictsrequirinogwnshiapctionthatarebasedon
stateinformationre,solutioandvisorieisdenticatlo
thoseinthetacticamlodearepresentaesda_'quicfkix"
optionfortheflightcrew.
Twotypesofflightrulesareenvisionefodrthe
strategmicodeofoperatioena,chprovidinagdistinct
benefit.A
maneuver flight rule is one that governs
what types of maneuvers are not permissible in certain
situations. The strategic mode incorporates amaneuver
flight role that is designed to prevent near-term
conflicts from suddenly appearing. The same role was
applied to the tactical mode, as described earlier in the
Figure 1. ND as modified to present traffic,
use of the conflict prevention system. The role states conflict, and resolution information.
that an aircraft may not implement a change in track,
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Flight Deck Display Design not considered by the algorithm (such as other traffic
and airspace hazards).
A new cockpit display of traffic information (CDTI)
design concept, exercised in full in the strategic mode In the strategic mode, aproposed conflict-resolution
for this experiment, was developed to address the issues trajectory is automatically loaded as an alternate route
of effectively integrating (rather than superimposing) in the FMS and is displayed on the ND for pilot review.
state and intent information for conflict detection into a A Control and Display Unit (CDU) page was devised
single presentation. The design built on state-only and for accepting (or rejecting) the trajectory. If the conflict
intent-only display features previously developed and persists until also detected as a state conflict, tactical-
investigated by NLR 3 and NASA Ames Research maneuver options (same as those calculated in the
Center 11. The aircraft simulation used in the current tactical mode) are displayed to the pilot to permit
experiment was a representation of the MD- 11 aircraft. immediate conflict resolution for safety. These tactical
The new display features for autonomous operations advisories are shown concurrently with the alternate
were therefore integrated into the MD-11 flight-deck FMS-route advisory, providing the pilot a tactical
display suite, and existing MD-11 conventions were option to clear the conflict alert while the strategic FMS
adhered to as much as possible. The Primary Flight route is reviewed.
Display (PFD) and Navigation Display (ND) were the
only displays affected, and an ND control panel was Experiment Obiectives and Approach
added. The new display design followed the common
The experiment was conducted in the NASA
approach of superimposing traffic data on the ND. The
CDTI features are described in reference 9. The ND Langley Air Traffic Operations Laboratory, which
contains a medium-fidelity workstation simulation of
with some of the CDTI features is shown in figure 1.
airspace operations that permits pilots to interact in
Conflict Alerting proposed future ATM environments. A more detailed
description of the simulation capability can be found in
The conflict alerting follows the MD-11 aircraft
reference 5.
system alerting convention. The alerting logic is based
on three levels of alerting. The primary objective of the experiment was to
compare the two proposed operational modes
A Level 1 alert is used when a conflict situation
applicable to airborne separation assurance in a
must be conveyed to the pilot, but no action is required
constrained en-route environment. The experiment
because the intruder aircraft is expected to resolve the
focused on operational aspects that relate to
conflict. The alert is considered advisory, similar to a
commercial-transport autonomous aircraft as defined by
traffic "point out." Note that Level 1alerts only occur
the DAG-TM Concept Element 52.
in the strategic mode, since it requires priority flight
rules and/or traffic intent information to be present. The experiment focused on the operations of a
single autonomous aircraft in en-route cruise flight with
A Level 2 alert requires action by the ownship flight
variable airspace complexity (i.e., traffic density,
crew. This alert is used when a conflict has been
weather cells, SUA). Beyond the current research
detected, and it is the responsibility of the ownship
scope were direct interactions with the ATSP, managed
flight crew to resolve the situation. A reminder of
aircraft, or other piloted autonomous aircraft. The
imminent loss of separation is given to the pilot at 3
study did not address multi-person flight crews, crew
minutes to go.
resource management, or voice communications.
A Level 3 alert requires immediate action by the Climbs and descents of the ownship were not studied,
ownship flight crew. This alert corresponds to the nor were the effects of winds or failure modes of
actual loss of separation, i.e., the aircraft have decision-support automation or CNS infrastructure.
approached within the minimum legal separation These issues will be addressed in future studies.
criteria.
A 2-by-2-by-3 within-subjects experimental design
Conflict Resolution Advisories was used to address the research objectives. The
primary independent variables were operational mode
The detection of a conflict triggers the calculation of
(tactical and strategic) and operational complexity (low
resolution advisories by the decision support
and high). The third independent variable was the
automation. In the tactical mode, maneuver options are
conflict situation design, described below.
displayed to the pilot on the PFD and ND that permit
conflict resolution with simple maneuvers (i.e., heading Operational complexity, for the purposes of this
and vertical speed changes). These maneuver options experiment, was assumed to be a function of traffic
were considered advisories only, and the pilot was free density and airspace hazard density. Research has
to choose a different solution if desired based on factors shown that traffic density is correlated with operational
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complex1it2y.Trafficdensitaypproximatirnegcorded
into this study). A pilot who took no action would see
1997levelswasusedforthelowcomplexictyondition
these conflicts as self-resolving. A pilot who
andwastripledforthehighcomplexitcyondition.
maneuvered may also observe the intruder maneuver
Airspacheazarddensitwyasaddedasanadditional
and therefore possibly conclude the resolution was
relevanctomplexitfyactorofconstraineodperations,
shared.
althoughno conflictswithairspacheazardswere
planned. Each scenario was terminated with an RTA
constraint at a fix. The subject pilot was tasked to
Sixteenactive commerciatlransportpilots
ensure separation from the traffic aircraft and avoid
participateindthestudy.Eachflewthreescenarioins
airspace hazards while meeting the RTA constraint. In
eachofthefourconditionrespresentiendthe2-by-2
order to assess workload impact, the subject pilot was
experimendtaelsigdnescribeadboveT. helevel-cruise
given a secondary task involving periodic monitoring
scenarioesachcontaineodneofthefollowingthree
and reporting of aircraft system status. Additionally,
conflicstituationsA.n"intent-onlyc"onflicstituation
the pilot was prompted every two minutes to record a
occurrewdhenonlytheintenttrajectoritehsreatenaed
real-time assessment of workload on a seven-point
conflict.A "state-onlyc"onflictsituationoccurred
scale from very low to very high. Only the latter metric
whenonlythestatetrajectoriethsreatenaedconflict
is reported in this paper.
(andtheintenttrajectoriedsidnot). A "blunder"
conflicstituatiownassimilatrothe"state-onlcyo"nflict
Results and Discussion
situatiobnu, ttheintrudearircrafdtidnotadherteothe
plannedtrajectorychangein thebroadcasinttent Initial findings based on researcher observations of
messagaen,dthetwoaircrarfetmaineindconflict. several scenario case studies, pilot subjective ratings
and comments on the tactical and strategic operational
Becaustheeownshidpidnothaveaccestosintent
modes, and pilot subjective ratings of display design
informatioinnthetacticaml odet,hepilotwouldhave
features are presented in reference 9. This
observeaddiffereninttruder-aircbraefhtaviothranthat
complementary paper presents a partial analysis of
plannefdoreachconflicstituationT. hust,hepilot's
objective numerical trajectory and event data. The
perceptioonfsthethreeconflicstituationasbovweould
discussion of results will focus on the influence of
differinthetwomodeosfoperatiocnonsideriendthis
strategic and tactical operations and operational
experimenret,sultinignthreepilot-perceivceodnflict
complexity level on maneuvering, conflict proliferation,
types.
operational efficiency, and pilot workload.
Additionally, case studies are presented on the two
Max-alert conflicts: A blunder conflict as defined
scenarios where aloss of separation occurred.
above loses its meaning when observed in the tactical
mode, since the observer (i.e., ownship) never knew Maneuvering
about the intruder's planned TCP in the first place. The
As described earlier, each pilot was exposed to three
intruder's behavior would appear to be a "normal"
conflict types: max alert, short alert, and self-resolving.
conflict (not precipitated by any intruder maneuver or
The pilot experienced these scenarios in both tactical
lack of an expected one) detected at the maximum look-
and strategic modes and in low and high complexity
ahead time (5 minutes). For the strategic mode
environments. Only two of the three scenarios required
observer, the corresponding conflict situation would be
a resolution maneuver by the pilot; the scenario
the intent-only conflict, which again would be detected
containing the self-resolving conflicts would not
at the maximum look-ahead time (8 minutes).
because the intruder's actions would resolve the
Short-alert conflicts: The intent-only conflict conflict approximately 1.5 minutes after the alert. In
observed in the tactical mode is comparable to the addition to these planned conflict situations, the pilot
blunder conflict observed in the strategic mode, since in encountered additional conflicts (unplanned by the
both cases the Level-2 conflict alert ("action required") experimenters) as a result of their maneuver choices.
occurred well within the look-ahead horizon (in this The pilots were expected to maneuver to resolve these
study 3.5 minutes were provided by design for both conflicts as well.
conflict situations). These both could be considered
The total count of maneuvers performed in the
"pop-up" conflicts by their respective observers.
experiment is presented in Figure 2. When considering
Self-resolving conflicts: The state-only conflict type all scenarios tested, the maneuver count was relatively
can be considered directly comparable for tactical and insensitive to operational complexity in the strategic
strategic observers, since in both cases the intruder mode, but a notable increase in maneuvers was
would have appeared to resolve the conflict observed in the tactical mode as complexity increased.
approximately 1.5 minutes after the alert (as designed Note that the metric considers the turn-away and turn-
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American Institute of Aeronautics and Astronautics
backmaneuvearssonea, ndsotheeffectsshownare
not indicatingdifferencesin the maneuvering 80
procedureB.reakingthemaneuvecoruntdownby
'_ 70
conflicttype illustratesthat a morecomplex 60
50
relationshoipfmaneuveritnogoperationmalodeand
complexietyxists. 30
20
Inthemax-alerstcenarioms,aneuvecrounwt as
higheirnthestrategmicodethaninthetacticaml odea,
counter-intuitriveesultconsiderinthgatthestrategic
modedesiginntendethdeoppositeN.earlytwo-thirds
ofthestrategmicodepilot-runwsereperformewdith
partiaolrfulltacticaml aneuverienvge,nthougahpure
strategriecsolutiownasalwayosfferedA.nalysoisfthe Conflict Type
rawdataunderlyintgheplotindicatethsat,inthelow
complexictyases,everaplilotschosteosecond-guess
[] Low complexity
thestrategircesolutioonfferedby theautomation,
[] High complexity
increasinthgestrategic-momdaeneuvecorunt.This
observatioinndicatetshatthemaneuvecorunttrend
maybetheresulotfinadequaetxeperienwceithortrust
Figure 2. Quantity of maneuvers performed in
inthestrategriecsolutioandvisorieass,thepilotshad
preventing and resolving conflicts.
verylittletimetobecomfeamiliawriththeperformance
of thistoolbeforethedataruns. Additionaolr
conflicts in the strategic mode. It may be that the intent
alternativefactorsin the trendmayhavebeen information provided in the strategic mode gave the
conservativedecision-makinge,.g., providing pilots enough confidence in the future traffic state to
redundasnetparatioinnboththeverticaalndlateral forgo preventative maneuvering.
dimensionasn,dthe absencoef strategicvertical
resolutiocnapabilitays,pilotswerefrequentolybserved In self-resolving conflict scenarios, strategic mode
toaugmetnhtelaterasltrategmicaneuvweirthavertical operations appeared to be effective in reducing
tacticaml aneuverIn. thehighcomplexictyaset,he unnecessary ownship maneuvers. In this mode, the
pilots were informed of the intruder's intent to change
underlyindgataindicatethsatmorepilotsmadeearly
trajectory before losing separation, and the Level-1 alert
(pre-conflicmt)aneuveinrsthestrategmicodethanin
provided the pilot an advisory not to take action.
thetacticaml ode.It isconceivabtlheatthehigher
complexictyausepdilotstomaneuvtearcticallryather Nearly all recorded maneuvers in strategic mode
thanwaitforthesystemtoalertthemw, hichpoints occurred during this Level-1 alert, the reasons for
eithetrowardmoreevidencoefinadequatrtuestand/or which were postulated earlier for short-alert scenarios.
In most cases, the tactical-mode pilots elected to
experienwceiththesystemorpossiblaypilotneedfor
immediately resolve the conflict upon receiving the
a longerlook-aheacdonflict-detecttiiomnein high
alert. Had they waited for the 3-minute warning to
complexietynvironments.
maneuver, as they were instructed they could, the
Intheshort-alesrctenarioasm, irror-imamgeaneuver-conflict would have disappeared with no action by the
counttrendisseefnorstrategaicndtacticaml odewsith ownship. The pilots may have considered immediate
respectot anincreasine operationcaolmplexityth;e maneuvering to be the more conservative approach, and
strategimcaneuvecorundt ecreaseasn,dthetactical yet several new conflicts were created by this decision
counitncreaseIsn.thestrategic-molodwec,omplexity in the high-complexity scenario, requiring additional
scenariosse,venof themaneuveorsccurredeither maneuvering.
duringtheLevel-1aler(t_noactionrequiredo")rwhile
Conflict Proliferation
resolvintgheprimaryconflicatfterreturnintgocourse
toosoon.Bothofthesemaybeindicatoorsfalackof The creation of a new conflict as a result of
operationeaxlperiencbeu,ttheformermayindicate maneuvers to prevent or resolve a previous conflict is
conservativfleyingtendencieTsh.isdatasetalso clearly an undesired effect. Such conflicts are termed
includeodneofthetwoloss-of-separaptiiolont-runs, '_second generation" in this paper to denote their origin.
contributinfoguradditionmalaneuvetorsthecountI.n The more preferred stable traffic environment is one
thetacticaml odea,primarycontributtoortheincreasewhere conflicts do not persist or propagate between
in maneuvecrountwasmaneuveirnsresponsteo aircraft. Requiring the complete elimination of second-
proximabteutnon-conflictibnagckgroutnradffic;such generation conflicts, however, may be unreasonable.
maneuvewresregeneralnlyotobservewdithshort-alertAs operational complexity increases, it can be expected
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American Institute of Aeronautics and Astronautics
100%
80%
60%
40%
20%
0% liiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiliiiiiiiiiiiiiiiiiiiiiiiiiiiii
Strategic Tactical
Operational Mode
Strategic Tactical
[] Level, new altitude
Operational Mode and Complexity
[] Climbing /descending
[] Level, original altitude
[] Vertical resolution only
[] Lateral and vertical resolutions
[] Lateral resolution only
Figure 4. Pilot maneuvering choice in runs
involving second-generation conflicts.
Figure 3. Maneuver choice frequency in
intent-only conflict scenarios.
that the unintended creation of new conflicts will available to the pilot for decluttering the display. Both
occasionally occur, and the vast majority of these will of these features may have contributed to ade-emphasis
likely occur beyond the ability to detect at the of aircraft at different altitudes, possibly leading to a
originating maneuver time. A more rare, but still loss of awareness of the threat they posed. The fact that
possible, occurrence might arise in highly complex many of the second-generation conflicts occurred while
situations where temporarily trading a near-term climbing or descending indicates that the conflict
conflict for a longer-term conflict is the logical or prevention bands on the vertical speed indicator were
possibly only solution, given other concurrent not generally inthe pilots' scan.
constraints. Nevertheless, the nominal goal is to
To provide an indication of the stability of the
minimize the occurrence of second-generation conflicts.
strategic and tactical operations as discussed earlier, the
As expected, the preponderance of second- total conflict count in the experiment is presented in
generation conflicts in the current experiment occurred Figure 5. As indicated earlier, the majority of the
in the high-complexity environment, and for reasons second-generation conflicts occurred in the high-
unclear, over 50 percent of these occurred in the intent- complexity condition. In the strategic mode scenarios,
only conflict type (max-alert scenario in strategic mode; the 32 planned conflicts resulted in 8additional second-
short-alert scenario in tactical mode). In maneuvering generation conflicts, or a 25 percent proliferation rate.
to prevent or resolve conflicts, the pilots had the option In the tactical mode scenarios, the 31 planned conflicts
in both modes to choose between lateral, vertical, or a (1 fewer due to an unusable data run) resulted in 9
combined maneuver. Speed was also an option, but is additional second-generation conflicts, or a 29 percent
generally considered ineffective as a pure mechanism proliferation rate. Both operational modes, therefore,
for conflict resolution in short time domains. As shown are reasonably stable at this level of complexity.
in Figure 3, pilots in the intent-only conflict scenarios However, the similarity in proliferation rates between
chose pure lateral maneuvers over vertical or combined strategic and tactical modes may be a result of the
lateral/vertical more than half of the time. significant use of tactical maneuvering observed in the
strategic mode, thereby blurring the true proliferation
The second-generation conflicts in these scenarios,
characteristics of operations in which strategic
however, only occurred in pilot-runs where vertical
resolutions are the prevalent maneuver choice. Since
maneuvering was involved. As shown in Figure 4,the
data were not obtained at a third complexity level, it is
second-generation conflicts generally occurred when
not known whether the proliferation rates vary linearly
the ownship was either climbing/descending or had
with increases in complexity.
previously changed altitude. As shown in Figure 1, a
relative-altitude color-coding scheme was used in the Operational Efficiency
display of traffic. Aircraft within 1000 ft of ownship
In the absence of available fuel-bum data and given
altitude were shown inwhite and the rest were shown in
the assumptions of the experiment, excess path length
blue or green. Additionally, an altitude filter was
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American Institute of Aeronautics and Astronautics
45
40 ,_ 2.0
35
30
25 1.5
20
1.0
0.5
0
0.0
Short Self ALL
Max.
Alert Alert Resolve
Strategic Tactical
ConflictType
OperatioMnoadlaenCdomplexity
[] Strategic Mode
[]2ndGeneraticoonnflicts [] Tactical Mode
[]PlanneIDdSconflicts
Figure 6. Excess path length for data runs with
Figure 5. Planned and second-generation lateral maneuvers only and strategic resolutions
conflict count. accepted for the maximum-alert scenarios.
flown can be considered a first-order measure of changes to the trajectory to minimize pilot task load. In
degraded operational efficiency. The initial trajectory the strategic mode, the automated resolution advisory
of the aircraft in each scenario was a direct constant- generated an "optimal" trajectory (based on minimized
altitude path to the RTA fix. Any deviation from this path length) incorporating a small buffer (2 nm) to
trajectory (to resolve conflicts) may require additional ensure separation, but it involved very few pilot actions
fuel bum and could therefore be considered a reduction to implement. The pilot could choose to have the FMS
in operational efficiency. fly this trajectory or to intervene with tactical
maneuvering or a "direct to" modification to the flight
Excess path length data is presented in Figure 6.
plan once the conflict is passed. In the current ATC
The presented data only includes the pilot-runs
system, a controller would typically have the goal of
involving lateral maneuvers only. Vertical maneuvers
providing guaranteed separation with minimal
generally provide separation with significantly smaller
controller workload. Generally, this translates to large
changes in path length and would therefore skew the
data when combined with lateral maneuvers. Since the buffers and little consideration to optimizing the
aircraft' strajectory.
strategic resolution algorithm only provided lateral
solutions due to FMS limitations in the current Pilot Workload
simulation software, the lateral maneuver data is of
As shown in Figure 7, the maximum pilot rating for
most interest for comparing strategic and tactical
workload (acquired 8times during the run on a scale of
modes. The strategic, maximum-alert data also
l=very low, 7---very high) averaged for the same data
excludes one particular data point that involved an
set shown in Figure 6was essentially equivalent in the
unusually large lateral deviation compared to the others.
strategic and tactical modes. This finding is consistent
The reason for the large maneuver is not known, but
with the observation that the primary contribution to
may be related to a known guidance problem in the
pilot workload was not conflict resolution, but rather
aircraft performance model.
scanning and manipulating the ND for traffic
The data in Figure 6 indicate that the strategic and information. This task was essentially equivalent inthe
tactical operational modes offered no significant strategic and tactical modes, except that the strategic-
differences in operational efficiency as measured by mode pilots were also cognitively processing the
excess path length. Both modes provided an available intent information. There may be little
opportunity to minimize the excess path length in benefit, and possibly some detriment, to a pilot
conflict resolution over the current ATC system. In the continuously scanning for possible traffic conflicts
tactical mode, the pilot could choose to make continual when an automated conflict detection system is also
fine-tuning adjustments to minimize flight path performing this task. It may be possible to further
deviation from the direct routing, essentially skirting reduce pilot workload by eliminating the display of
the edge of the required separation criteria, or non-threatening aircraft from the ND. A prototype of
alternatively the pilot could choose to make fewer gross
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American Institute of Aeronautics and Astronautics
7 7
_ 6 _ 6
_ L_5 iiiiiiiiiiiii_iiiii_iiiiiiiiiiiiiiiiiiiiiiiiii_iiiii_iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii_iiiiiiiiiiiiiiiiiiiiiiiiiiii_iiiiii_iiiiiiiiiiii
o 4
_ _ 3
_1
Max. Short Self ALL Max. Short Self ALL
Alert Alert Resolve Alert Alert Resolve
Conflict Type Conflict Type
[] Strategic Mode [] Low Complexity
[] Tactical Mode [] High Complexity
Figure 7. Maximum pilot workload assessments for Figure 8. Sensitivity of pilot workload self-assessment
data runs with lateral maneuvers only and strategic to operational complexity.
resolutions accepted for the maximum-alert scenarios
such a system will be tested in an upcoming ownship were to take no action. In the tactical mode
experiment. for this conflict type, the ownship pilot has no
knowledge this will happen until the intruder initiates
The sensitivity of maximum pilot workload
the maneuver, which occurs approximately 1.5 minutes
assessment on operational complexity is shown in
after the conflict alert. This case study will illustrate
Figure 8. The data indicate minimal sensitivity of
the utility of traffic intent information, the effect of
workload to operational complexity. This finding is
inexperience, and the importance of maneuver flight
consistent with the findings of Hoekstra 3, providing
restrictions.
further evidence that an en-route distributed traffic
management system may not be limited by pilot Synopsis:
workload, as is generally considered to be the case in
The ownship pilot was observing traffic with the
the current centralized system with controller workload.
ND range set to 80 nm. The four aircraft that were
Loss of Separation visible were all converging toward the area in front of
the ownship. On the left, one aircraft was 4700 ft
Of the 192 scenarios of data acquired in this
below ownship and level, and the other was descending
experiment, two occasions of loss of separation (LOS)
through ownship's altitude. The two aircraft on the
occurred, one each in the tactical and strategic
right were co-altitude with ownship. When the conflict
operational modes. An LOS for this experiment was
alert appeared for FX689, the planned intruder
defined as coming within 5 nautical miles and 1000 feet
approaching from the right, the descending aircraft was
of another aircraft, and the conflict resolution advisories
2800 ft below. See Figure 9. Although the other
and CDTI were designed to assist the pilot in avoiding
approaching aircraft on the right, WN145, was
LOS. It is interesting to note that both LOS situations
producing a large CP band for right turns greater than
occurred under conditions of low complexity, which
30 degrees, the ownship pilot turned 20 degrees in that
indicates that traffic and airspace hazard density were
direction, possibly as a turn away from the descending
not likely principal causal factors. It is also interesting
aircraft, TW499. This maneuver momentarily resolved
to note that the background traffic aircraft were
the conflict. The ownship was now navigating towards
identical for the two LOS scenarios, although the
a 23-degree opening between CP bands. Less than a
intruder aircraft were different. The actions of one of
minute later, two events occurred nearly
these background aircraft contributed to both LOS
simultaneously. FX689 started a left turn and WN145
occurrences.
started a right turn, both in the direction of ownship.
Case 1. Tacticalmode The turns of both aircraft were part of their un-
broadcasted intent, and the result was simultaneous
The designed conflict for this scenario was a state-
conflicts with the two aircraft. See Figure 10. The 23
only or self-resolving conflict, which means that the
degree CP-band opening had collapsed within 25
intruder's actions would resolve the conflict if the
seconds. The pilot turned right and then corrected hard
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