Design of a Co-Planar Airborne Separation Display
Joost Ellerbroek, Student member, IEEE, Koen C. R. Brantegem, M. M. (Ren´e) van Paassen, Member, IEEE,
Abstract—This paper describes a concept for a co-planar
airborne self-separation display, that is designed to aid pilots in
their separation task, by visualizing the possibilities for conﬂict
resolution that the airspace provides. This work is part of an
ongoing research towards the design of a constraint-based 3-D
separation assistance interface that can present all the relevant
properties of the spatio-temporal separation problem. A display
concept is proposed that presents speed, heading and altitude
action possibilities in two planar projections of the maneuver
action-space. The interface also visualizes how these projections
interact with each other.
Index Terms—Ecological Interface Design (EID), Airborne
Separation Assistance System (ASAS), self-separation, situation
I. INTRO DUC TIO N
IN THE CURRENT Air-Trafﬁc Management (ATM) con-
cepts for unmanaged airspace, aircraft will ﬂy optimized,
four-dimensional trajectories, that have been determined and
coordinated completely before the actual ﬂight , . To
resolve trafﬁc (or other) conﬂicts that result from uncertainties
that arise during ﬂight (e.g., bad weather, departure delays),
automated systems will be used to detect conﬂicts, and provide
resolution advisories to the pilot.
Although automation provides the resolutions, pilots will
ultimately be responsible for the validity of that resolution.
They should therefore be able to monitor the trafﬁc situation,
and the proper functioning of the automation, and should
be able to intervene in case the automation fails. In other
words, pilots should be able to detect, and act upon very
infrequent situations that were not anticipated in the design
of the automation. It is therefore of paramount importance
for automation to be transparent and understandable to the
The work presented in this paper is part of an ongoing
study on the design of a 3-D separation assistance interface.
The study employs a constraint-based approach, inspired by
Ecological Interface Design (EID). EID is a proven design
paradigm from the domain of process control , , that has
in recent years also been applied in several aviation-related
interface concepts –. In this approach, work-domain
analysis tools such as the Abstraction Hierarchy (AH) are used
to identify relevant constraints and relations on multiple levels
Published in IEEE Transactions on Human-Machine Systems, 01/2013;
43(3):277-289, DOI:10.1109/TSMC.2013.2242888. This work has been co-
ﬁnanced by the European Organisation for the Safety or Air Navigation
(EUROCONTROL), under its Research Grant Scheme launched in 2008, and
by the National Aerospace Laboratory NLR. The content of the work does
not necessarily reﬂect the ofﬁcial position of EUROCONTROL or the NLR
on the matter.
The authors are with the Control and Simulation section of the Faculty of
Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629
HS Delft, The Netherlands. Email: J.Ellerbroek@TUDelft.nl
of abstraction , . An extensive analysis of the work
domain relevant to airborne separation was performed during
the design of previous constraint-based concepts , ,
. This paper will summarize the relevant constraints and
relations that were identiﬁed in the analysis, which stands at
the basis of the concept presented in this paper. For a more
exhaustive description of the actual work-domain analysis, the
reader is referred to the previous publications.
The aim of this study is to create an interface that realizes
proper support for airborne separation, by showing the impli-
cations of other trafﬁc for the affordances∗of locomotion, and
how they relate to limitations of the own aircraft , .
The interface presented in this study does not explicitly relate
to speciﬁc automation functions. Instead, it visualizes work
domain information, which invariably forms the premise on
which both automation and the human operator should base
their actions. By going beyond visualizations that relate only to
the automation logic, these displays help pilots gain a deeper
understanding of the functions and relations within the work
domain , , , which will be invaluable to pilots
when they need to judge the automation’s functioning .
These displays should provide support in routine as well as
unforeseen situations, where pilots may have to rely on their
own problem-solving skills to resolve a conﬂict.
This study has led to three display concepts , , .
Each of these concepts presents a planar projection of the own
aircraft’s 3-D maneuver space. All three projections represent
simpliﬁed, 2-D versions of the maneuver space. They in-
escapably discard information, a problem that is inherent to the
presentation of multi-dimensional data on a 2-D surface ,
. Our ultimate goal is therefore to ﬁnd a representation
that mitigates as much as possible the problems of presenting
multi-dimensional data on a 2-D surface.
Several studies have investigated the effects of 3-D vi-
sualization methods for airborne trafﬁc information displays
–. They compare between perspective and (co-)planar
displays that give a basic representation of trafﬁc. For the
current work, however, the focus lies not only on representing
trafﬁc, but also on what such trafﬁc means to pilots in terms
of achieving functional goals, and how it relates to other
functions and constraints in the work domain. This will pose
different demands on the method of presentation.
This paper argues for a co-planar representation, on the
basis of previous concepts and the corresponding work-domain
analysis. It will describe how to mitigate the problems that
∗James J. Gibson deﬁned affordances as opportunities for action, provided
by an object or by the environment. An affordance is considered always in
relation to the actor, and therefore dependent on the actor’s capabilities and
goals , . For instance, with respect to an engine, air affords propulsion,
but with respect to a wing, air affords lift.
arise when information which is intrinsically three dimensional
is distributed across two 2-D displays, and how to show the
interactions that can occur between the two planar presen-
tations. The decision for a co-planar display is a departure
from the aim of the previous concept , which was to
create a single, integrated perspective presentation of the
3-D constraints. The following section will therefore discuss
the rationale for choosing a co-planar display. Section III
introduces functional presentations for relevant work-domain
constraints. The fourth section describes the co-planar display
concept. A ﬁfth section illustrates how the visualizations in the
display concept link back to the work-domain analysis. The
paper concludes with a discussion of the beneﬁts, drawbacks,
and remaining challenges of this concept.
II. THR EE-D IM E NS I ONAL DATA VIS UALI ZATIO N
For the visualization of a 3-D space on a 2-D screen, two
options can be distinguished: perspective displays and co-pla-
nar displays, each with their own beneﬁts and drawbacks ,
. The decision to choose either of these two methods will
depend on the task requirements for the resulting display. A
co-planar display has uniform, undistorted axes in its viewing
planes, which beneﬁts precise position and angle judgments.
Perspective displays, on the other hand, have more “pictorial
realism”: they correspond more closely to the 3-D world ,
, . Perspective displays can also employ texture and
shading to increase realism, and improve spatial awareness
. Perspective displays might therefore be preferred when
the task requires complex shape understanding. St. John et
al. also differentiate between tasks that involve only separated
spatial dimensions, and tasks that involve integrated spatial
dimensions, where co-planar displays are better suited for the
former, and perspective displays for the latter .
A drawback of co-planar displays in the current context
is that some of the information on the interaction between
locomotion constraints is lost, when these constraints are
presented using separate horizontal and vertical projections.
Also, distributing the information across two displays requires
the pilot to mentally integrate the information from both
displays. Perspective displays, on the other hand, suffer from
perspective distortions, which can induce errors in judging
distances and angles on the display , , . The
presentation of 3-D structures also suffers from problems of
occlusion: when viewed from a ﬁxed angle, the front facing
side of the structure hides the internal details of the structure.
A. Motivation for a co-planar display concept
In the design of a separation assistance display concept, the
choice between visualizations should depend on the speciﬁcs
of the separation task, and how it is performed. From previous
studies and experiments, several arguments can be found for
the use of a co-planar display. First, experiments performed in
this study, as well as other studies, showed that pilots have a
strong preference for single-axis resolution maneuvers ,
–. Second, the design of previous constraint-based
separation assistance displays illustrated that trafﬁc constraints
can become complex, yet precise judgment of these constraints
is valuable for safe and efﬁcient conﬂict resolution. They
also illustrate that the planar projections of the constraints
show an intuitive relation with the absolute geometry of
the conﬂict, which beneﬁts situation awareness. Perspective
distortion makes this relation less visible in a perspective
projection, a problem that also hampered the previous concept
for a perspective 3-D interface . Although that concept
employed constant-velocity cutting planes to reduce the com-
plexity of the constraint visualization, it did not reproduce the
intuitive visual relation with the spatial representation of the
Two of the three current constraint-based separation assis-
tance displays will be used as a basis for the co-planar concept
, , . The three current display concepts provide
three orthogonal projections of the maneuver space: a top-
down projection , a side-view projection , and a front-
facing, ego-centric equidistant cylindrical projection . The
ﬁrst two are presented on the Horizontal Situation Display
(HSD), and Vertical Situation Display (VSD), respectively.
The third concept does not have an equivalent existing display
in the cockpit. Because the ﬁrst two concepts feature the most
intuitive maneuver space projections, and as they correspond
closest to current re-planning tasks and displays, these will
be used in the co-planar display concept. Each of the two
original display concepts assumes that a trafﬁc conﬂict lies
exactly in its plane, and the challenge in the design of the
co-planar concept discussed in this paper lies in showing the
interactions that exist between the projection planes of the co-
planar display. This will be discussed in Section III.
B. Comparison with other 3-D displays
Although most current research on airborne separation as-
sistance systems focuses on the development of automated sys-
tems that assist pilots with the separation task –, there
are also several display concepts have been developed as aids
in the task of self-separation , –. There are two
distinct types of conﬂict representation that are used in these
displays: a spatial representation, which is similar to traditional
situation displays, and a maneuver-space representation, i.e.,
visualizing how proximate trafﬁc limits ownship maneuver-
ability in terms of airspeed, heading, and vertical speed. Some
displays use only one of these representations, others combine
them. An important beneﬁt of spatial representations is that
they offer an intuitive overview of the situation, familiar to
anyone who has ever used a map. Maneuver-space displays,
on the other hand, are useful because they reveal a direct
link between constraints or commands, and the applicable
A second distinctive factor between displays is whether
they show explicit (automated) commands, or constraints on
maneuvering. Beneﬁts of explicit command displays are that
they suffer relatively little from display clutter, and that
they reduce workload. Automation also has the potential of
providing the most efﬁcient conﬂict resolution options. The
most important drawback of displays that show only explicit
resolution commands is that they hide the rationale behind the
automation. These displays do not support human information
seeking, and, in case of automation failure, the pilot is not
supported in recognizing failure, nor in seeking alternatives.
In these situations, performance can even be worse than when
completely unaided .
Constraint displays, on the other hand, give a continuous
view on maneuver options and limitations, which allow pilots
to evaluate their own resolution maneuvers. Depending on
how constraints are visualized, these displays can show the
structure of, and relations within the work domain, and can
therefore provide a useful basis for illustrating automation
logic. By showing higher level information and relations, these
displays also allow pilots to investigate the validity of the
data. An important drawback of constraint displays is that
they can result in more display clutter, compared to showing
only explicit commands. According to Tufte’s views on the
use of details (“To clarify, add detail”), however, this is not
necessarily a drawback for a well designed display .
The existing concepts described in , ,  show
that the concepts presented in the current study are not the
only displays that show constraints on maneuvering instead
of only presenting explicit commands. The concept presented
in  provides a spatial representation of constraints in the
form of no-go areas on horizontal and vertical map displays.
A different approach was taken in , where constraints
are indicated with colored conﬂict-bands on the compass, the
speed tape and the vertical speed indicator on the Primary
Flight Display, the HSD, and the VSD. The concept described
in  introduces a new display that presents constraints in a
perspective, 3-D maneuver space.
The concepts in the current study aim to improve upon such
constraint-based concepts by visualizing the structure of the
work domain, and by illustrating the relations between lower-
level elements and higher-level information. The remainder of
this paper will describe how properties of the own aircraft and
the surrounding trafﬁc are related to each other, and to higher-
level constraints and functions, in a way that is made visually
apparent on the display.
III. FUN CT IONAL P RES E NTATI ON OF C ON S TR A IN TS
For airborne trajectory planning and self-separation, several
relevant constraints have been identiﬁed , , . These
constraints fall broadly into two categories: constraints internal
to the own aircraft, and constraints external to the aircraft.
The internal constraints relevant to the problem of separa-
tion relate to the various limitations on aircraft performance.
In addition to these internal limitations, the maneuver space is
further constrained by external factors such as weather, terrain,
other trafﬁc, and airspace boundaries (such as those from
restricted airspace areas). For airborne separation, the focus
obviously lies on the constraints imposed by other trafﬁc.
Functional presentations of these constraints, and the rela-
tions between these constraints, should provide a description
that is compatible to the means that are available to the con-
troller. For trajectory planning, this implies that the description
should relate the goals and affordances of the system, to inputs
that match common ﬂight practice. In cruise ﬂight, pilots
control their aircraft by manipulating velocity, track angle∗,
and altitude settings, using the autopilot or by modifying the
planned route in the Flight-Management System (FMS). A
successful separation assistance interface should relate these
control variables and their operational limits to the affordances
of the airspace.
A. Velocity action space
A modern glass cockpit supports trajectory planning through
the Horizontal Situation Display and the Vertical Situation
Display, which show horizontal and vertical projections of
task-relevant information such as the planned route, terrain,
weather, and other trafﬁc. While these visualizations clearly
identify the elements of the airspace that constrain the ma-
neuver options of the aircraft, they do not show how these
elements shape the possibilities for pilot action. Because the
operator action space is not shown in a meaningful way, it
remains difﬁcult to relate higher level goals and constraints to
the available actions and inputs.
The design philosophy employed in this study proposes to
achieve this by combining the existing spatial representation
of airspace elements, with a velocity action space, that relates
own aircraft maneuver variables velocity, track angle and
vertical speed, to the identiﬁed internal and external constraints
, . This action space is deﬁned as the reachable subset
of the 3-D vector space that contains all possible velocity
vectors (i.e., all combinations of velocity, track angle and
When zero wind is assumed, an aircraft velocity vector in
this vector space can be deﬁned as follows:
V=VT AS ·
cos (χ) cos (γ)
sin (χ) cos (γ)
where VT AS is the (true) airspeed of the aircraft, χthe track
angle, and γthe ﬂight-path angle, or climb angle of the
aircraft. Vertical maneuvering is more commonly expressed
in vertical speed, which can be derived from VT AS and γ:
V S =VT AS sin (γ). The presence of wind can be of inﬂuence
on the velocity action space representation, but only when
ownship and intruder aircraft experience signiﬁcantly different
wind conditions. It is therefore kept out of the current analysis.
B. Internal constraints
The reachable area that deﬁnes the velocity action space is
bounded by constraints that are internal to the own aircraft.
These constraints relate to the various limitations on the perfor-
mance of the aircraft, such as bank limits, maneuver dynamics,
available engine thrust, stall, structural considerations, buffet
characteristics, and requirements on productivity, emissions
and passenger comfort. These limitations result in several
constraints relevant to the task of trajectory planning, such
∗Track angle differs from aircraft heading in the presence of wind. Heading
indicates in which direction the aircraft nose is pointing, as indicated e.g. on
the magnetic compass. The track angle gives the direction in which the aircraft
is ﬂying. With no wind, these angles are equal. With cross-wind, however,
there will be an offset between heading and track (the drift angle).
as maximum turn rates, maximum and minimum operating
speeds, fastest and steepest steady climb and descent, and the
steepest steady climbing and descending turn. Some of these
constraints also show interactions: For example, in turning
ﬂight, increasing the bank angle will also affect the minimum
velocity and the maximum attainable climb angle .
VT AS VT AS
Fig. 1. The 3-D velocity action space illustrated in two planar projections.
(a): Horizontal maneuvering can be expressed in combinations of airspeed
(VT AS ) and track angle (χ). The minimum (Vmin ) and maximum (Vmax)
obtainable airspeeds are the main constraints for horizontal maneuvering. (b):
Vertical maneuvering can be expressed in combinations of airspeed (VTAS )
and vertical speed (V S). It is also constrained by the minimum (Vmin) and
maximum (Vmax) obtainable airspeeds. Aircraft performance with maximum
(Tmax) and minimum (Tmin ) thrust settings determine maximum steady
climb and descent, respectively.
Fig. 1(a) and Fig. 1(b) illustrate how these constraints can
be visualized in the horizontal and vertical plane, respectively.
The horizontal maneuver space is shaped by the limitations on
airspeed, which reduce it to the ring-shaped area in Fig. 1(a).
Vertical maneuvering is also constrained by airspeed limita-
tions, as well as by the steady climb and descent performance.
Fig. 1(b) shows how these constraints combine in the vertical
plane. The vertical edges of the action space result from the
limits on airspeed. The curved edges at the top and bottom
of the vertical action space visualize the maximum obtainable
steady climb and descent at each velocity, respectively. The
resulting contour is also known as the ﬂight envelope of the
aircraft (refer to  for the derivation of this contour).
RP Z hP Z
Fig. 2. A conﬂict situation. Ownship and intruder are in conﬂict when the
line that extends from the relative velocity vector (Vrel) crosses the intruder
Protected Zone (PZ). The PZ is a ﬂat, 3-D disc around each aircraft, that
should remain clear of other trafﬁc. Common dimensions for this PZ are
RP Z = 5 nmi and hP Z = 2,000 f t.
C. External constraints
In unmanaged airspace, the reachable area that is enclosed
by the internal aircraft constraints is further restricted by
several external factors, such as weather, terrain, and trafﬁc.
For a self-separation interface the focus obviously lies on the
constraints imposed by other trafﬁc. These trafﬁc constraints
are shaped by a minimum horizontal and vertical separation
between any two aircraft, that should be adhered to at all times.
With common values of 5 nautical miles horizontal, and 1,000
feet vertical separation, this results in a 3-D Protected Zone
(PZ): A ﬂat, 3-D disc around each aircraft, that should remain
clear of other trafﬁc (illustrated for the intruder aircraft in
Fig. 2) , . Intrusion of this space is referred to as a
loss of separation. A conﬂict is deﬁned as a predicted future
loss of separation, within a certain time horizon . For this
concept, the time horizon was set between 60 seconds and 10
to 20 minutes .
Fig. 3. Relation between ownship motion relative to the intruder, and the
absolute motion of ownship and intruder. Ownship and intruder are shown
at time t0and t1.xo,rel shows how ownship has moved relative to the
intruder. Line ashows that the change in orientation between ownship and
intruder along the relative path is equal to the change along the absolute paths.
Although ownship and intruder intent can inﬂuence the
constraints imposed by the intruder, the current study will only
employ the current states to derive these constraints. Previous
studies did incorporate the effect of intent on maneuver
affordances –, however, this is beyond the scope of the
current study. Under the assumption that intruder and ownship
state remain unchanged in the near future, a conﬂict can be
predicted using the speed of ownship, relative to the intruder:
Vrel,own =Vown −Vint (2)
Relative velocity vector Vrel,own indicates how ownship
moves with respect to the intruder aircraft∗, see Fig. 3. When
the relative track of ownship (the line extended from the
relative velocity vector) crosses the intruder protected zone,
a loss of separation will occur in the near future, see Fig. 2.
For any given trafﬁc geometry, a set of relative velocity
vectors VF A can be deﬁned that would result in a conﬂict
between the involved aircraft (i.e., all possible relative velocity
vectors where the resulting relative tracks cross the intruder
PZ). Fig. 4 gives an illustration of VF A . It is a construct of
two slanted cones, connected by straight sections. Both cones
have their apex at the ownship location, and their curvature is
∗Note that the relative motion of intruder symbols on a trafﬁc display is
opposite to this relative velocity: There, the own aircraft symbol is standing
still, and intruder aircraft symbols move relative to the own symbol on the
display. The current concept observes relative motion from the perspective of
ownship (Vrel,own) in order to ﬁnd constraints on own maneuvering.
Fig. 4. The 3-D forbidden area VF A consists of a conical area that is aligned
with the edges of the intruder protected zone, as seen from the ownship, and
has its apex situated at the center of ownship. Cross-section (a) shows how
the thickness of the protected zone changes the shape of the forbidden area
from what would otherwise be a pure slanted cone, to a combination of two
slanted cones, connected by a straight section.
aligned with the upper and lower circles of the intruder PZ.
Cross-section (a) in Fig. 4 illustrates the effect of the thickness
of the intruder protected zone: if the PZ had been ﬂat, VF A
would have been a single slanted cone, and cross-section (a)
would have been an ellipse. The thickness of the PZ introduces
straight sections in the shape of VF A , and in cross-section (a).
The 3-D vector set VF A marks the constraints that other traf-
ﬁc imposes on ownship (relative) motion with respect to that
intruder aircraft, and will be referred to as the 3-D forbidden
area in the remainder of this paper. This representation only
varies as a function of time†. This means that for the current
time, the 3-D forbidden area represents the complete set of
relative velocities that would result in a loss of separation.
Fig. 5. Constraints on ownship velocity, expressed in an absolute velocity
ﬁeld. (a) illustrates horizontal intruder constraints, (b) illustrates vertical
constraints. ownship and intruder are in conﬂict when the tip of the ownship
velocity vector is inside the 3-D forbidden area. The apex of the shifted
area corresponds with the intruder velocity vector, and can be used on the
display to determine for instance relative track, and intruder vertical speed.
The forbidden area bisector shows relative bearing of the intruder.
The concepts preceding the current study illustrated that
the relation between the forbidden areas and the own velocity
vector can be made visible by translating the forbidden area
and relative velocity vector by the intruder velocity vector, see
Fig. 5 , . This would be equivalent to adding Vint on
both sides of the equal sign in Eq. (2). The shifted forbidden
area represents the constraints imposed by an intruder aircraft
in a way that directly relates to the ownship maneuver options.
An added beneﬁt, illustrated in Fig. 5, is that the apex of the
shifted forbidden area corresponds with the intruder velocity
†When neither aircraft maneuver, the opening angle of the forbidden area
will expand or contract only as a function of the closing speed of the intruder
aircraft, with respect to ownship. When ownship and intruder are not on a
collision course, the orientation of the forbidden area will also change as both
aircraft pass each other.
vector, and that the direction of the bisector of the forbidden
area is equal to the relative bearing of the corresponding in-
truder, properties that can be useful when assessing a conﬂict.
Note that this derivation of the forbidden area assumes
instant state changes‡. It can be shown that this is a safe as-
sumption when a predicted conﬂict is still in the far future ,
. However, maneuver dynamics will start to play a larger
role when conﬂicts become more imminent: in the case of
tactical maneuvers (within 10 minutes of a predicted conﬂict),
unmodeled dynamics will cause signiﬁcant errors, particularly
speed maneuvers . The previous horizontal display concept
compensated for this by observing the constraint area at time
tcur +tturn. Here, tturn is the maneuver duration for the
heading solution that corresponds to the respective forbidden
area leg. A new relative position is extrapolated using the
current aircraft velocities, which in turn is used to calculate
the corrected position for the forbidden area leg. This method
can also be applied to the current concept.
Fig. 6. Translated 3-D forbidden area and horizontal and vertical projections.
F Ahor is the horizontal projection of the 3-D forbidden area, and presents
constraints for ownship track and horizontal velocity. FAv er is the vertical
projection of the same area, and presents constraints for vertical motion.
D. Planar constraint projections
There are several visualization techniques for 3-D con-
structs such as the forbidden area, each with its speciﬁc
beneﬁts and drawbacks . The drawback these methods
have in common, however, is loss of context. This, however,
is unavoidable, as some form of reduction is necessary to
present multi-dimensional data on a 2-D surface , .
The challenge is to determine crucial parts of the context, and
to ﬁnd a visualization that maintains the relevant information.
The previous horizontal and vertical constraint displays both
show an orthogonal projection of the 3-D forbidden area
(illustrated in Fig. 6). These projections take the 3-D shape,
and discard the coordinate that is orthogonal to the projection
plane. The resulting shapes are triangular, with the triangle
‡In reality, a heading or speed change that is taken to resolve a conﬂict
will take a certain amount of time. In that time, the constraint area will have
grown slightly, and it might occur that what initially seemed a valid solution,
will in fact not resolve the conﬂict, due to the expansion of the constraint
area during the course of the maneuver.
apex at the (projected) ownship position, and the triangle legs
aligned with the edges of the projected protected zone.
In Fig. 6, F Ahor is the horizontal projection of the 3-D
forbidden area, and presents the constraints on relative track
and horizontal velocity, for all values of vertical speed com-
bined. F Aver is the vertical projection of the 3-D forbidden
area, and presents the constraints on relative vertical motion,
for all values of the ownship track angle combined.
The orthogonal projections, then, provide a global contour
of the forbidden area in their respective dimensions. Visualiza-
tion of these contours has many beneﬁts. Most importantly, the
relation between the triangular contour and the geometry of
the conﬂict is easily interpretable. The triangles reveal several
key parameters of a conﬂict, such as spatial proximity, intruder
bearing, intruder heading and velocity, and relative velocity,
and how these parameters relate to each other. These cues also
help to relate the forbidden areas to the traditional intruder
symbols on the display .
However, because the projections show constraints for all
values of the orthogonal coordinate, the constraints can be
conservative when the conﬂict does not lie exactly in that
plane. For instance, a certain combination of speed and track
angle may lie in a horizontally projected forbidden area, but
can still be conﬂict-free if there is enough vertical separation,
or if vertical separation is obtained before a horizontal loss of
separation occurs. This distinction cannot be made with the
projected forbidden areas alone.
E. Interactions between projection planes
What these projections fail to show, then, is how the
orthogonal planes interact with each other. The horizontal pro-
jection does not reﬂect vertical separation and maneuvering,
and vice versa. Cutting plane visualization partly reveals this
interaction, by showing a part (a ‘slice’) of the 3-D shape,
for a given constant value of the third dimension . In
combination with the planar projection of the 3-D shape, it
reveals which part of the projection is valid, for a speciﬁc
point along the dimension that is orthogonal to the projection
plane. The result of a cutting-plane intersection will therefore
always be a subset of the planar projection of the 3-D shape.
Fig. 7 shows a horizontal cutting plane, that intersects with
the 3-D constraint area for a certain given value of ownship
vertical speed, i.e., the vertical offset of the cutting plane
is equal to ownship vertical speed. The resulting ‘slice’ of
the constraint area represents horizontal velocity constraints,
taking into account the relative vertical motion and orientation.
This reduced forbidden area is a subset of the horizontal
projection of the 3-D constraint zone (also illustrated in Fig. 7).
When ownship and intruder are not vertically separated, and
the relative vertical speed is equal to zero, the horizontal
reduced forbidden area will be equal in shape and size to the
projected horizontal forbidden area.
Fig. 8 shows a vertical cutting plane, aligned with the
ownship track, that intersects with the 3-D constraint area. The
resulting slice of the constraint area presents vertical velocity
constraints, taking into account how a conﬂict is oriented in the
direction orthogonal to the ownship track (i.e., intruder cross-
track distance and relative intruder track angle). This reduced
Fig. 7. The horizontal reduced constraint area is given by the intersection
of the 3-D constraint area, and a horizontal cutting plane, offset vertically
by the ownship vertical speed. The horizontal reduced area illustrates exact
constraints on horizontal maneuvering, also for conﬂicts with non-zero relative
vertical distances and velocities.
forbidden area will always be a subset of the vertical (side-
view) projection of the 3-D constraint zone (see also Fig. 8).
When ownship and intruder are ﬂying with zero cross-track
distance and relative track angle ∆χ= 180◦or ∆χ= 0◦
(ownship and intruder are ﬂying head-on or are overtaking,
respectively), the vertical reduced forbidden area will be equal
in shape and size to the projected vertical forbidden area.
Fig. 8. The vertical reduced constraint area is given by the intersection
of the 3-D constraint area, and a vertical cutting plane, aligned with the
ownship track. The vertical reduced area illustrates exact constraints on
vertical maneuvering, also for conﬂicts with non-zero cross-track distances
Because these reduced forbidden areas depend on the vari-
able that is perpendicular to their plane of projection, they
effectively reveal an important part of the interaction between
the planes of projection. The reduced areas are invariant
with respect to state changes that lie in their own plane
of projection, and therefore provide a consistent set of 2-D
constraints, under the assumption that the third (perpendicular)
variable is kept constant. In this case, the reduced areas only
vary with time, similar to the forbidden area projections.
When the perpendicular variable is varied, the reduced area
will always change in a predictable fashion. If the change in
that variable is away from the tip of the 3-D forbidden area, the
reduced area will also move away from the tip of the triangle
in its respective projection. Similarly, if the change moves the
velocity vector closer to the tip of the 3-D forbidden area,
the reduced area will move towards the tip of the triangle in
its respective projection. Also, because the planar projections
Fig. 9. Concept for a co-planar separation assistance display. This ﬁgure shows a Horizontal Situation Display (❶) and a Vertical Situation Display (❷),
with added separation assistance overlays. ❸and ❹are the horizontal and vertical State-Vector Envelope, respectively. ❺is the projected forbidden area both
on the HSD and the VSD. ❻is the reduced forbidden area on both displays. ❼Represents the ownship state vector, ❽is a TCAS-symbol indicating the
relative location of the intruder aircraft. ❾is a speed tape, showing current IAS, selected IAS, and simpliﬁed trafﬁc constraints. ❿is a vertical speed tape,
showing the current and selected vertical speed in feet per minute, and simpliﬁed trafﬁc constraints.
effectively result in a 2-D contour of the forbidden area at its
widest point, projections of the reduced areas can never extend
beyond the boundaries of the projected forbidden area.
Aside from interactions that result from shortcomings of
methods for showing 3-D data on multiple, 2-D surfaces,
interactions can also be found between several aircraft loco-
motion variables, and their limits. A very direct interaction
can be found between the aircraft’s bank angle, and its climb
performance and stall speed: Increasing the bank angle will
increase the stall speed, and reduce the maximum climb angle.
On a larger time scale, changes in altitude will have an effect
on minimum and maximum operating speeds, and on climb
and descent performance .
These interactions can partly be captured in the visualization
when the visualized constraints are dynamically calculated for
the current values of the ﬂight variables that inﬂuence it. There
are, however, situations possible where dynamically calculated
constraints do not sufﬁce. Consider for instance a situation
where a trafﬁc conﬂict is solved by assuming a vertical speed
that is close to the maximum climb performance. It can happen
that although initially this climbing solution seemed to be
a valid solution, the reduction in climb performance due to
increasing altitude causes this solution option to disappear.
This is, however, beyond the scope of the current study.
IV. CON CEP T
Fig. 9 illustrates a design concept for a separation assistance
interface, that presents separation-related affordance cues on a
co-planar display. The combination of a Horizontal Situation
Display and a Vertical Situation Display was chosen for
this co-planar display concept, because these displays are
omnipresent in the modern ﬂight deck. These two displays
also provide the most intuitive maneuver space projections,
and they correspond closest to current re-planning tasks.
In this display concept, the 3-D trafﬁc situation is visualized
in two orthogonal, 2-D views: a top-down view (❶), and a side
view (❷). Both views present an ownship-centered moving
map, that shows spatial information such as the FMS route and
intruder aircraft positions. In addition, constraints on ownship
maneuvering are shown on both displays through velocity
action-space overlays (❸,❹).
The top-down view presents information in a polar co-
ordinate system: spatial information is expressed in relative
bearing and distance, and the velocity action-space shows
constraints for combinations of track angle and airspeed.
The side view uses a Cartesian coordinate system: spatial
information is expressed in along-track distance and relative
altitude. Here, the velocity action-space shows constraints for
combinations of airspeed and vertical speed.
A. Traditional display elements
The moving-map presentations on the HSD and VSD are not
new: the HSD is present in all modern cockpits, and also the
VSD is becoming more common. To match current practice,
intruder aircraft are represented on both displays using TCAS-
like symbology∗(❽) : an unﬁlled diamond indicates a non-
conﬂicting intruder, a ﬁlled diamond indicates a conﬂicting
intruder, with more than ﬁve minutes to a loss of separation.
This is considered a low-priority conﬂict. A conﬂict is con-
sidered medium priority when a loss of separation is between
three to ﬁve minutes away, indicated with a solid circle as
intruder symbol. A high priority conﬂict is less than three
minutes away, and is indicated with a solid square.
Separation margins are indicated around each intruder on
both displays, which results in a circle on the HSD, and a ﬂat
rectangle on the VSD, see Fig. 10. On the HSD, the intruder’s
horizontal speed vector is shown with a dotted trend line. The
length of this line is scaled such, that it equals the radius of
∗Note that this is not necessarily the best intruder visualization. Intruder
symbology design, however, is beyond the scope of the current study. TCAS
symbology was therefore chosen to match current practice.
Fig. 10. Intruder aircraft symbology. Intruders are visualized using TCAS-
style symbology: unﬁlled diamonds for non-conﬂicting aircraft, ﬁlled dia-
monds for low-priority conﬂicts, ﬁlled circles for medium-priority conﬂicts,
and ﬁlled squares for high-priority conﬂicts. (a): Intruder symbology as shown
on the HSD. The TCAS symbol is shown together with the separation margin,
a speed vector, a vertical speed arrow, and a ﬂight-level offset. (b): Intruder
symbology as shown on the VSD. The TCAS symbol is shown together with
the separation margin, a vertical speed arrow, the relative bearing in hours o’
clock, and the intruder distance in nautical miles.
the separation circle if the horizontal speed of the intruder is
equal to the ownship horizontal speed.
A small up/down arrow is shown next to the intruder
symbol, when the vertical speed of that intruder exceeds 500
ft/min. A signed number below the intruder symbol indicates
the vertical offset in ﬂight levels (1 ﬂight level equals 100
feet), see Fig. 10(a). On the VSD, the intruder TCAS symbol is
accompanied by a label that shows the relative bearing in hours
o’ clock, and the distance in nautical miles. An up/down arrow
is shown to the right of the TCAS symbol when the vertical
speed of that intruder exceeds 500 ft/min, see Fig. 10(b).
In addition to the map view, the vertical display also
includes a speed tape, and a vertical speed tape. The speed
tape (❾) shows current Indicated Air Speed (IAS), selected
IAS, and simpliﬁed speed constraints in knots. The vertical
speed tape (❿) shows the current and selected vertical speed,
and simpliﬁed vertical speed constraints, in feet per minute.
B. Velocity action-space overlays
The horizontal State-Vector Envelope (SVE) (❸) shows
the affordance space for horizontal maneuvering in terms of
track angle and airspeed, see Fig. 11(a). Because a horizontal
situation display in expanded mode (as in Fig. 9) does not
show trafﬁc behind the own aircraft, the horizontal state-vector
envelope also shows only solutions with |∆χ| ≤ 90◦. Current
horizontal situation displays also have modes that show the
situation behind the ownship. In such a mode, the horizontal
state-vector envelope would be shown as a whole circle. The
vertical State-Vector Envelope (❹) is illustrated in Fig. 11(b),
and shows the affordance space for vertical maneuvering in
terms of airspeed and vertical speed.
Intruder aircraft that are within detection range will further
reduce the available maneuver space in the horizontal and
vertical State Vector Envelopes. The reduced forbidden areas
(❻), derived in the previous section, give the most precise
representation of these constraints, see Fig. 11. On the HSD,
a reduced forbidden area gives the constraints imposed by an
intruder on ownship track angle and airspeed, for the current
value of ownship vertical speed. On the VSD, a reduced
forbidden area gives intruder-imposed constraints on ownship
airspeed and vertical speed, for the current ownship heading.
Note that each intruder adds a forbidden area to the available
maneuver space. These forbidden areas, however, work in a
cumulative fashion: selecting a ‘clear’ area solves all detected
conﬂicts, without creating a new conﬂict. In the current
concept, the derivation of the constraint areas uses only state
information, and will therefore only be valid when there are no
imminent trajectory changes. Although the inﬂuence of intent
information has been considered in previous concepts –
, this is out of the scope of the current study.
The reduced forbidden areas result from the intersection
between a ﬂat cutting plane, and the 3-D forbidden area: a
compound of two slanted conical shapes, aligned with the top
and bottom of the intruder protected zone. The shapes that
result from this intersection range from circles, to ovals, to
open-ended hyperbolic curves, see Fig. 11.
Ref.  describes how the triangular shapes that result
from planar projection of the forbidden area can be used to
derive several relevant cues about the spatial conﬁguration
of a conﬂict. These cues make it easier to relate forbidden
areas to intruder symbols on the map view, but also provide
more information on the urgency of a conﬂict, and how
different resolutions would affect the trafﬁc situation. In other
words, while the reduced forbidden areas provide more precise
constraint visualization, they sacriﬁce important cues that link
several display elements together. The current concept there-
fore combines the reduced forbidden areas with the outline of
the corresponding projected forbidden areas (❺). In addition to
the added situational information, these outlines also provide
an outer limit on the shape and size of the reduced forbidden
area, when a perpendicular ﬂight parameter is modiﬁed.
The current ownship airspeed, track angle and vertical speed
are presented on the two displays by two velocity vectors (❼)
that extend from the origin of the SVE to a certain point
in the velocity vector space. On both displays, the length
of these vectors is equal to the ownship airspeed. On the
vertical situation display, the ownship vertical speed results
in a rotation γ= arcsin(V S/VT AS )of the velocity vector.
Because the horizontal situation display is oriented track-up,
the horizontal velocity vector has a ﬁxed, vertical orientation.
A dot, four kts wide, attached at the tip of each velocity vector,
acts as a margin for maneuver selection .
In combination with the reduced forbidden areas, the veloc-
ity vectors show the affordance of avoidance: placing either of
the velocity vectors outside all of the forbidden areas results in
a conﬂict-free trajectory. Intruder ﬂight-path vectors are also
shown as dots at the tip of the corresponding forbidden area tri-
angle. On the horizontal display, the distance from the tip of a
triangle to the center of the SVE is equal to the airspeed of the
corresponding intruder, see Fig. 11(a). On the vertical display,
however, this distance is equal to the in-track component of the
intruder airspeed, Vver t,int =VT AS,int ×cos(χint −χown).
Here, VT AS,int is the intruder airspeed, χint is the intruder
track angle, and χow n the ownship track angle. Moving the
ownship velocity vector towards one of these dots to resolve
a conﬂict will lead to a very inefﬁcient resolution, as it will
cause ownship to ﬂy parallel to the intruder .
C. Conﬂict urgency visualization
In addition to varying intruder symbology, conﬂict urgency
is also indicated using color coding for all of the display
Fig. 11. The horizontal (a) and vertical (b) State-Vector Envelopes. The forbidden areas correspond to one intruder, who is situated below, to the front and
to the right of ownship, crossing ownship from right to left, and climbing at a shallow climb angle. (a): The circular boundaries that constrain the horizontal
maneuver space represent the aircraft minimum and maximum operating speeds. The intruder track is offset from the ownship track by ∆ψ. The triangle
apex represents the intruder velocity Vint .(b): The vertical maneuver space is bounded by minimum and maximum operating speeds, and by minimum and
maximum steady-state climb. The vertical offset of the triangle apex corresponds to the intruder vertical speed, V Sint .
elements that correspond to one intruder. This means that the
aircraft symbols on both displays, as well as the forbidden
area triangles and reduced forbidden areas on both displays
are colored according to the urgency of the conﬂict between
ownship and the corresponding intruder. Similar to the TCAS
system, four levels of urgency have been deﬁned for the
current concept . The lowest urgency level corresponds
with intruder aircraft that are currently not in conﬂict with
ownship. For these intruders, the display elements are col-
ored white. The second level of urgency corresponds with
a conﬂicting intruder, with a loss of separation further than
ﬁve minutes away. This is deﬁned as a low-urgency conﬂict,
and display elements are colored yellow. A medium-urgency
conﬂict corresponds with a loss of separation between three
and ﬁve minutes, and is colored orange on the display. A high-
urgency conﬂict indicates a loss of separation within less than
three minutes, and is colored red on the display.
D. Visual momentum
When more than one intruder aircraft needs to be shown
on the display, it becomes more important for the display
to provide ways to link the several visual elements on the
display to an intruder and to each other. Visual cues that
facilitate this link increase visual momentum: they facilitate
the integration of information across multiple displays, and
between elements on each display . This integration is
essential for proper SA, as the elements on both displays
are intrinsically tied together. Manipulation in one plane will
often affect constraints in the other plane. Visualizing which
elements belong together should aid pilots when assessing
Woods introduces functional data overlap as a method that
“improves the visual momentum across multiple displays by
‘presenting pictorially the functional relationships that cut
across display frame boundaries’ ”. In other words, visual
momentum can be improved by showing particular informa-
tion on both displays, and by visualizing relationships between
information on both displays. The color coding that is used
to indicate conﬂict urgency is an obvious way to improve
visual momentum. The shape and orientation of the conﬂict
zones, however, also provide ways to identify display elements
that belong together. Examples are the distance to an intruder,
which also determines the opening angle of the corresponding
forbidden areas, the predicted intruder ﬂight path determines
the location of the tip of the horizontal triangle, and the vertical
speed, that determines the vertical position of the tip of the
triangle on the vertical display.
E. Comparison with previous concepts
The main difference between the current co-planar display
concept and the previous separate horizontal and vertical
display concepts, is the visualization of the interactions that
can occur between the planes of projection. The horizontal dis-
play shows constraints on horizontal maneuvering, under the
assumption that intruding aircraft are ﬂying level, within min-
imum vertical separation. Similarly, the vertical display shows
constraints on vertical maneuvering, assuming zero cross-track
distance and maneuvering. These projected constraints become
increasingly conservative when conﬂicts deviate from these
assumptions. The reduced forbidden areas show more precise
constraints by taking the conﬂict orientation orthogonal to each
projection into account.
Fig. 12 illustrates how the constraints imposed by an intrud-
ing aircraft change when the corresponding conﬂict geometry
can no longer be deﬁned in a single plane of projection. All
four examples in Fig. 12 show how conﬂict constraints would
be visualized on the new display. Note that on the original
two displays the visualization would be similar, but that all
triangles would always be completely ﬁlled.
The ﬁrst conﬂict, shown in Fig. 12(a), corresponds to an
intruder that is both on the same track and the same level
as ownship, and both aircraft are ﬂying level. In this case,
neither the assumptions for the original horizontal display,
nor those for the original vertical display are violated. As a
result, the constraints imposed by the intruder are presented
as completely ﬁlled triangles on the new display, completely
identical to what the visualization for this conﬂict would be
on the original displays.
Fig. 12(b) shows how the constraints change when the
intruding aircraft starts to climb. Because the intruder is still
on the same track as ownship, the assumptions for the original
vertical display still hold, and the presented vertical constraints
are still identical to how they would be presented on the
original vertical display. The horizontal constraints, however,
change as a result of the vertical maneuver of the intruder.
Where the original horizontal display would show a conﬂict,
Fig. 12. Example scenarios. An overview of each scenario is given on the
left. The black aircraft symbols represent ownship, the gray symbols represent
the intruder. (a) Ownship is behind and overtaking the intruder, both are ﬂying
level, at equal altitude. (b) Intruder is climbing. (c) Both are ﬂying level, but
the intruder is to the left of ownship, crossing to the right. (d) Intruder is to
the right of ownship, crossing to the left, descending from a higher altitude.
the reduced forbidden areas reveal that ownship would have
to accelerate to get into conﬂict with the intruder.
Fig. 12(c) shows how the vertical constraints change when
the intruder is on a different track than ownship. Similar to
the situation in Fig. 12(b), the original vertical display would
show a conﬂict, while in reality the intruder passes in front
of ownship before they get too close. Fig. 12(d) shows that
both the horizontal and the vertical presentation of constraints
change when the intruder is both off level and off track. In
this example ownship and intruder are still in conﬂict. The
presentation in Fig. 12(d) shows that maneuvers exist that
solve the conﬂict, while still being in both the horizontal and
vertical triangles. Such solutions would be impossible to derive
from the original two displays.
V. RE LATIO NSH IP S B ETW EEN T H E AH A ND THI S CO NCE PT
The constraint-based approach that was adopted in this
study used work-domain analysis tools such as the Abstraction
Hierarchy to identify constraints and relationships on multiple
levels of abstraction , . Although the work-domain
analysis for this concept has not changed signiﬁcantly since
the previous concept, looking at how the visualizations in
the current concept are related to that work-domain analysis
can provide a useful review of the concept, as well as
relevant insights for future design iterations and experiments.
This section will therefore brieﬂy describe how the different
elements of the display link back to the functions, constraints
and relationships in the abstraction hierarchy (Fig. 1 in ).
Ref.  describes the work-domain analysis in more detail.
The velocity action-space overlays proposed in the display
concept form the main additions to the horizontal and vertical
situation displays. These overlays give a consistent view on the
relations between locomotion inputs and the primary functions
of productivity, efﬁciency and safety, and they show how these
relationships are inﬂuenced by several identiﬁed constraints.
Together with the velocity vectors, the horizontal and
vertical state-vector envelopes relate to the safety goal, by
showing how internal constraints (available power, structural
limits, ...) limit possible velocity vectors. In combination with
the horizontal FMS track on the map display, the horizontal
SVE relates to the production goal through the destination
approximation constraint (deviations from track that are larger
than ∆χ= 90◦move the aircraft away from its destination).
The reduced forbidden area relates to the safety goal by
showing the affordance of conﬂict, (the own velocity vector
inside a reduced forbidden area indicates a conﬂict). Together
with the internal maneuvering constraints from the SVE, it
shows the affordance of avoidance: any vector within the SVE
that is not inside any reduced forbidden area is a possible
solution to a conﬂict. The reduced forbidden area also relates
to the efﬁciency goal, through the ‘shortest way out’ principle
. The smallest vector change out of a reduced area will also
result in the smallest path deviation. Note, however, that path
deviation in the horizontal plane does not directly compare to
path deviation in the vertical plane.
The forbidden area outlines link lower-level elements to
higher level constraints. Together with the intruder symbols
on the map displays, they link conﬂict and separation on the
abstract function level to obstruction (motion), relative motion,
and trafﬁc location on the generalized and physical function
levels. The tip of the triangle conveys intruder motion (the tip
is offset from the center of the SVE by the intruder velocity
vector), and the triangle bisector communicates intruder rel-
ative bearing, and therefore helps to link forbidden areas to
their respective intruders.
VI. PR AC TI C AL A PPL ICATI ON
An important argument in the current study is that in order
to support operators in unforeseen situations, displays should
go beyond visualizations that relate only to the automation
logic. The interface should provide a window to the reasoning
and functioning of the automation, by visualization of the
affordances of the work domain, and by making clear how
these affordances relate to the actions and advisories of the
automation. The appropriateness of these displays for real-
world applications, however, also depends on how well the
concepts extend to complex situations, such as multiple in-
truder conﬂicts, complex trajectories, and of course situations
where the automation is failing. This has also been considered
for preceding concepts, and many of the properties illustrated
in those studies apply to the current concept as well , .
Van Dam, for instance, illustrated that the forbidden areas
work in a cumulative fashion . Because each forbidden
area reveals absolute maneuvering constraints, imposed by
an intruder aircraft, a combination of forbidden areas from
multiple aircraft, superimposed onto each other, will represent
the set of states that would lead to a conﬂict with any one
(or more) of these aircraft. As a result, any state outside
of this combined constraint area is a solution to all of the
current conﬂicts. This property is inherent to the presentation
of constraints in an absolute velocity space.
Ellerbroek illustrated that conﬂicts can be solved in se-
quence, when the priority of each conﬂict is visualized using
color coding of each forbidden area . The current concept
uses single colors for each forbidden area, where the color
corresponds to the time to loss of separation, given the current
state of ownship and intruder. A possible improvement could
make use of the fact that every point in a forbidden area
corresponds to a certain state vector, which in turn corresponds
to a certain time to loss of separation. This way, each point
in a forbidden area can be colored individually. In addition
to visualization of the priorities of conﬂicts given the current
state, the interface can then also reveal the viability of possible
intermediate solutions in complex trafﬁc situations.
One of the issues with co-planar displays is that the operator
has to combine information from two displays to obtain a
complete mental picture of the situation. Although the pro-
jected forbidden areas already provide strong links between
the different elements on the displays, a crowded airspace can
still make it difﬁcult to make these links, especially when
workload is already high. One of the techniques Woods pro-
poses to improve this, is to provide across-display perceptual
landmarks . A common color, for instance, can provide a
perceptual link between items on different displays that belong
together. The priority color coding already partly fulﬁlls this
function, but can be improved with a selection system, where
the different display elements that correspond to one intruder
are highlighted upon selection.
The current concept employs the current states to derive the
constraints imposed by other trafﬁc. This method holds under
the assumption that ownship and intruder state remain constant
in the near future. When this is not the case, the affordance
space will change as a function of space and time due to
Trajectory Change Points (TCP), and other changes of state
or intent. Several studies have illustrated methods to visualize
intent in the forbidden areas –. Each of these methods
makes use of the fact that the dimension along the bisector of
the triangular forbidden area is related to the time at which
the closest point of approach with the respective intruder will
occur, with the triangle origin representing tCP A =∞. A
change in state at t=tT CP will therefore result in a change in
the forbidden area at the point where tC P A =tT C P . A similar
method can be used to include intent in the current concept,
by extending one of these methods to three dimensions.
Although current ATM concepts for unmanaged airspace
suggest a trafﬁc display to be used as a situation awareness
tool for automated self-separation systems, constraint-based
displays are not limited to this level of support. Because the
displays visualize work-domain constraints and relations, they
support the pilot on multiple levels of control, from fully
automated conﬂict resolution, to manual pilot decision making.
VII. DI SC USS ION
The work presented in this paper is part of an ongoing study
on the design of a trajectory planning aid. The goal is to obtain
a graphical interface that supports pilots in their new task of
airborne reconﬁguration of a pre-planned trajectory, in case
of trafﬁc conﬂicts in unmanaged airspace. The current work
focuses on ways to visualize 3-D data on a 2-D display. A
co-planar display concept has been proposed, that is based on
the previous top-down and side-view display concepts.
There are several reasons why a combination of a Horizontal
Situation Display (HSD) and a Vertical Situation Display
(VSD) was chosen for the co-planar display concept. First,
these two displays provide the most intuitive maneuver space
projections, and support the most straightforward resolution
strategies, such as single-axis maneuvers, and combinations
of speed and heading or vertical speed. A practical factor is
also that these displays are already available on a modern ﬂight
deck, and they correspond closest to current re-planning tasks.
In previous research, the visualization of constraints on the
display implicitly assumed that conﬂict geometries were ﬂat:
the constraints shown on the horizontal interface assume zero
vertical separation and no vertical maneuvering, and the side-
view display assumes that there is no cross track separation
with intruder aircraft. When these assumptions are violated
these displays will present overly conservative constraints.
Simply combining these displays, therefore, is not sufﬁcient
to create an effective co-planar solution. Aside from the fact
that good visual momentum demands visual cues that link
both displays together, each individual display also requires
modiﬁcations so that the presented constraints remain valid
when a conﬂict can no longer be deﬁned in just one plane. A
co-planar display should reveal how individual planes interact
with each other, and provide pilots with a consistent and
intuitive view on what can be a complex, 3-D trafﬁc situation.
The current concept, therefore, re-implements as much as
possible the strong points of the previous, single-plane dis-
plays. The triangular shapes that result from planar projection
of the 3-D trafﬁc constraints provide strong and intuitive cues
about the conﬂict geometry, reveal how different elements
on the display belong together, and can help pilots keep an
overview in complex trafﬁc situations with multiple intruders.
These projections are complemented with precisely derived
constraints, that are also valid in combined cross-track and
off-level conﬂict situations.
Although full, simultaneous 3-D maneuvering is still not
consistently supported, it can be argued that this is a minor
sacriﬁce when choosing a co-planar display over a perspective
display: Several studies indicate that pilots prefer single-axis
maneuvers , –. Also, the beneﬁts (e.g., in terms
of efﬁciency) of three-axis maneuvers over two- or one-axis
maneuvers are rarely ever signiﬁcant.
A possible downside of the constraint-based presentation in
this concept is that in a densely populated airspace, the state-
vector envelope can become crowded with forbidden areas,
making it less suitable (or unsuitable) as a situation awareness
tool. Although this is an undesired situation, a de-cluttering
algorithm will increase automation complexity, and reduce
transparency of actions towards the operator. This will be the
topic of a future study.
Current ATM concepts for the future of the structure of
the airspace suggest a highly optimized, and –in certain parts
of the airspace– decentralized system, with a high degree of
automation. In the decentralized parts of airspace, aircraft will
ﬂy optimized, predetermined trajectories, where automation
will be used to resolve problems that result from uncertainties
during the ﬂight. The concepts suggest that a display of trafﬁc
information should be added to help the aircrew judge these
kinds of situations, and solve problems when they arise. The
current study uses a constraint-based approach to design an
interface that supports trafﬁc situation awareness.
When used in combination with an automated system that
provides explicit resolutions, such a display should improve
operator trust and understanding of an automated resolution,
by helping him understand how constraints shape possible
resolutions. Note that this visualization is independent of the
speciﬁc implementation of conﬂict resolution automation. In-
stead, it visualizes work domain information, which invariably
forms the premise on which both automation and the human
operator should base their actions. This method of visualiza-
tion also provides an opportunity to create a visualization that
is consistent across different levels of automation.
This method of display design, however, also implies that
there are certain demands on the design of the automation. The
interaction between automation and the human actor requires
transparent functioning of the automated system. When a
resolution advisory cannot consistently be explained by the in-
formation on a display, for instance because it takes additional
(hidden) constraints into account, a pilot can hardly be asked
to judge the ﬁdelity of this resolution. Consistency between
interface and automation, therefore, requires a common model
of the work domain, from which the automation derives a
resolution, and which the interface visualizes to the operator.
This consistency will be crucial for human actors to share their
cognition and decision-making with the automation.
VIII. CON CLU SIO NS
A separation assistance display was proposed, that presents
constraints on horizontal and vertical maneuvering, in a ve-
locity action space that is overlaid on both a horizontal and
a vertical situation display. A 2-D co-planar presentation
was chosen because it offers an intuitive, undistorted and
precise view on the constraints and the trafﬁc situation. It also
corresponds more closely to current-day ﬂight-deck interfaces,
as well as to pilot resolution preferences.
A follow-up paper (this issue) will present a set of exper-
iments that evaluate this concept in terms of safety, perfor-
mance, and situation awareness, in manual conﬂict resolution
tasks. Future design iterations will focus on display clutter, in-
tent, and on the interaction with different automated resolution
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Joost Ellerbroek received the M.Sc. degree in
aerospace engineering from the Delft University of
Technology, The Netherlands, in 2007, where he
is currently working toward the Ph.D. degree. His
Ph.D. work concentrates on the design and validation
of an interface that supports interaction with airborne
separation automation. The research presented in this
paper is part of his thesis.
Koen C. R. Brantegem received the M.Sc. degree
(cum laude) from the Delft University of Technol-
ogy, The Netherlands, in 2011. He graduated within
the control and simulation section on his thesis enti-
tled “Ecological 2-D Coplanar Airborne Separation
Assurance System”. The results of his work are
incorporated in this paper. He is currently working
towards obtaining a commercial pilot license.
M. M. (Ren´
e) van Paassen received the M.Sc.
degree (1988, cum laude) from the Delft University
of Technology, The Netherlands, and a Ph.D. (1994),
on the neuromuscular system of the pilot’s arm.
He thereafter was a Brite/EuRam Research Fellow
with the University of Kassel, and a post-doc at
the Technical University of Denmark. Currently, he
is associate professor at the faculty of Aerospace
Engineering, Delft University of Technology. His
work ranges from studies of perceptual processes
and manual control to complex cognitive systems.
Max Mulder received the M.Sc. (1992) and Ph.D.
degrees (1999, cum laude) from the Delft University
of Technology, The Netherlands, for his work on
the cybernetics of tunnel-in-the-sky displays. He is
currently Full Professor and Head of the Control and
Simulation Section, Faculty of Aerospace Engineer-
ing, Delft University of Technology. His research
interests include cybernetics and its use in modeling
human perception and performance, and cognitive
systems engineering and its application in the design
of “ecological” human-machine interfaces.