Pilots' Spatial Mental Models for Memory of Heading and Altitude

Susanne Delzell, San Jose State University, Moffett Field, California

Walter W. Johnson, NASA Ames Research Center, Moffett Field, California

Min-Ju Liao, National Research Council, Moffett Field, California

# Abstract

A study was conducted that examined areas of a Cockpit Display of
Traffic (CDTI) committed to memory during a task that required
monitoring of aircraft separation. The data showed that pilots tended to
recall aircraft primarily as a function of spatial range and bearing,
and not as a function of temporal range.

# Introduction

Researchers are currently developing design guidelines for cockpit
displays of traffic information (CDTIs) to support cockpit-based
separation from other aircraft. Although air-transport pilots have some
experience monitoring traffic and potential conflicts on Traffic
Collision Avoidance Display (TCAS), displays, self-separation is a new
task. Therefore, the information needed by the pilots\' in order to
accomplish this task, and how they will use that information, are not
well understood.

A cognitively based approach to this problem is to describe the relevant
information, and how a pilot, in terms of a mental model uses it. A
mental model is a cognitive construct that reflects a combination of
predispositions (biases) and situation specific learning (see Tversky,
1991, for a discussion of spatial mental models). A description of this
model can aid the development of good displays in two ways. First, it
allows a designer to match the information and format of the display to
the mental model when the mental model is appropriate. Second, it allows
a designer to create a display that compensates where the mental model
is incomplete or incorrect.

One method of gathering information about a mental model is to examine
the contents of long-term memory (LTM). The information represented in
LTM is determined (at least in part) by what a person attends to, and
how permanently it is encoded. This, in turn, is determined by
predispositions and situation specific factors. This study examined
potential spatio-temporal determinants of the mental model that guide
attention and retention of information when pilots are evaluating a
self-separation task. Specifically, it focused on how the mental model
was related to the x-y positions on the display.

Among the most critical elements of traffic information are distance of
other aircraft to Ownship in space (spatial proximity), the absolute
angular displacement of an aircraft relative to Ownship path (relative
bearing), and the time it would take an aircraft to reach Ownship if
that aircraft immediately turned onto a collision course (temporal
proximity). These measures form the bases for three corresponding mental
models that may guide a pilot's self-separation activity.

The predictions of these models are illustrated in Figure 1. The Spatial
Proximity Model simply predicts that aircraft closer to Ownship will be
better attended and encoded, and therefore better remembered. The
Relative Bearing Model predicts that aircraft with smaller relative
bearings will be remembered better. The Temporal Proximity Model
predicts that aircraft that could reach Ownship in less time will be
better remembered. For an airspace in which other aircraft are flying at
approximately the same speed as Ownship, this corresponds to a set of
nested centered in front of Ownship.

These three models represent different levels of sophistication with
respect to traffic dynamics. The Spatial Proximity model is least
sophisticated in that it only takes current separations into account.
The Temporal Proximity model is more sophisticated, parsing the x-y
space in terms of time needed for loss of separation. The Relative
Bearing model reflects differences in closure rates (i.e., the fact that
aircraft in front of Ownship tend to have higher temporal proximities
than do aircraft to the side of Ownship).

In this experiment pilots were presented with a set of traffic scenarios
on a CDTI and asked to evaluate their freedom to maneuver. The purpose
of this evaluation task was to encourage the pilots to evaluate the
display with respect to separation. After viewing and evaluating the
scenario the pilots were interrupted to displace the contents of
short-term memory. Following this, pilots were asked to recall as much
information as possible about the traffic aircraft. These recall data
were then used to determine which model or combination of models pilots
likely use.![](media/image3.wmf){width="2.0145833333333334in"
height="2.2993055555555557in"}

# Methods

## Participants

Twelve right-handed male participants with normal or corrected-to-normal
vision were paid for their participation. All participants were active
airline pilots experienced with glass cockpit instrumentation and with
TCAS.

## Apparatus

Scenarios were run on a 200MH Pentium Pro computer with an Omni Comp 300
SX OpenGL 3d accelerator graphics card. The monitor was 17 inches
diagonal with 1600 horizontal by 1200 vertical resolution. Mouse and
keyboard were standard. The size of the CDTI image on the monitor was
7\" x 7\". The display was static over the period of monitoring time
given.

## Design

All pilots viewed 24 scenarios, with 12 aircraft presented on the CDTI
during each scenario, for a total of 288 aircraft. The locations and
headings of the aircraft were randomly generated subject to the
following constraints:

-   The 288 aircraft located by randomly assigning six aircraft that fit
    within the parameters of each of the 48 predefined and approximately
    equally sized x-y bins that covered the surface of the display.

-   Within each bin, two of the six aircraft were located at each of the
    three altitudes: 31,000, 33,000 and 35,000 feet.

-   For each pair of aircraft at each altitude within each bin, one
    aircraft was randomly assigned a heading that would eventually cross
    the path of Ownship, and one aircraft assigned a heading that would
    not cross the path of Ownship.

-   The locations and headings of the aircraft were selected such that
    their future and past trajectories never resulted in them passing
    within 5 NM of Ownship, or within 2.5 NM of each other.

-   No aircraft populated the area in front of Ownship for a ten-mile
    range (to avoid focusing attention on this area).

-   The locations of the twelve aircraft were selected such that the
    altitude tags never overlapped.

Three altitudes were used in order to place a sufficient number of
aircraft in each scenario without these aircraft being on conflicting
trajectories. Furthermore, all combinations of heading (toward and away
from Ownship path) and altitude were included within each x-y bin to
ensure that the effect of location could be evaluated with minimal
confounding effects of altitude and heading.

## Description of the CDTI {#description-of-the-cdti .unnumbered}

Figure 2 shows the CDTI used in the study. At the bottom of the CDTI
there is a filled triangular symbol with a short line segment extending
out from it. The point of this triangle corresponds to the position of
the Ownship, and the line segment is the predicted path for the next 30
seconds. The other aircraft are depicted by similar, but unfilled,
triangular symbols with short lines oriented in their direction of
travel. The traffic symbols also have associated altitude tags.
![](media/image6.png){width="3.2597222222222224in"
height="3.2597222222222224in"}

In addition to the aircraft, there are several other structural elements
on the display. There is a compass arc at the top of the display, which
shows Ownship direction. No aircraft were ever displayed beyond this
compass arc. There is also a set of range rings on the display, which
are circular arcs with the Ownship at the center. The range was set at
80 miles, with scale markers at 20-mile intervals. Speed was set at 480
knots for all aircraft, including Ownship and Ownship altitude was
33,000 feet. The depicted airspace extended 80 NM in front, 20NM behind,
and 42.5 NM to the sides of Ownship.

## Procedure and Task Description {#procedure-and-task-description .unnumbered}

It was explained to the pilots that each trial presented a different
traffic scenario. They were told that their primary task was to evaluate
how much freedom they had to safely change their heading without losing
required separation from other aircraft (defined at 2.5 miles). The
recall task was described as incidental to the primary task. It was
emphasized that pilots should not memorize information from the display,
but simply focus on the primary task as they would in a real flight
situation, attending to information based on importance to the task.
During each trial there were three separate stages which are described
below.

### CDTI Presentation and Inspection Stage:  {#cdti-presentation-and-inspection-stage .unnumbered}

At the beginning of this stage the message "Ready to Begin Next Trial"
appeared and remained on the screen until a mouse button was pressed. At
this time a CDTI similar to that depicted in Figure 2 was displayed and
remained on the screen for one minute. During this one-minute interval
the pilots were told to evaluate how much freedom they had to safely
change their heading at all three altitudes (assuming a change of
altitude) and to combine their assessments into one evaluation, which
they would be asked for at the end of the trial.

###  Interruption Task Stage:  {#interruption-task-stage .unnumbered}

At the end of the first stage, the CDTI was replaced by a "interruption"
task display (Figure 3). The purpose of this task was to interrupt
visual and verbal short-term memory for the scenario and thus to
simulate the effects of the various tasks that a pilot may do between
sampling from the CDTI. Performance on the memory task would then be a
measure of the traffic knowledge encoded in long-term memory. The
interruption task required pilots to evaluate true/false statements
about magnitude readings from four graphical "Engine Gauges" (e.g.
"Engine 4 is greater than Engine 3 and greater than Engine 2"). Pilots
were told that it was very important to be accurate, and they were given
feedback (a beep) each time they gave an incorrect answer.

### CDTI Maneuver Evaluation and Recall Stage:  {#cdti-maneuver-evaluation-and-recall-stage .unnumbered}

After pilots responded to the interruption task, the third stage of the
trial was presented. During this stage pilots rated their freedom to
maneuver and recalled the information they remembered from the display.

When rating their freedom to maneuver, pilots were asked to use a cursor
to select one of five possible ratings from \"Very Low\" to \"Very
High\" as a measure of their overall freedom to maneuver. After the
selection was made they went on to the memory recall task.

During the memory recall task pilots were asked to recall the headings
and altitudes of as many aircraft as possible. The pilots were told not
to guess at random, but that they could guess if they were fairly
confident of their memory. The memory task began with a display of the
x-y positions of the aircraft from the previous scenario. However, in
this display the triangular symbols were replaced by small circles, and
all altitude tags were removed. To the left of the display several more
response controls and buttons were shown and used to collect recall
responses using the following techniques:

Selection: The mouse and cursor could be used to select the symbol for
any aircraft. This caused an additional circle to appear around the
symbol, along with a line extending from that circle. Pilots then
proceeded to any of the four sub-tasks (listed below) which could be
performed in any order. Recalls could be revised/modified for any
aircraft as many times as the pilot wished on any trial, and pilots were
never forced to make a recall response.

Heading Recall: Recalled heading for a selected aircraft was indicated
by placing the cursor on the line extending from the circle and dragging
the line so that it aligned with the remembered ground track for that
aircraft.

Altitude Recall: Recalled altitude for an aircraft was indicated
selecting 31000, 33000 or 35000 from a menu on the right of the display.

Enter/Revise Another Aircraft: After pilots had entered altitude and/or
heading information for a selected aircraft they could select a new
aircraft or re-select a previously chosen aircraft.

Done Entering Information: When pilots decided they had entered all of
the information they could remember for a scenario, they were told to
click on a button labeled "Done Entering Information". When they
confirmed that they were sure they were finished, the next trial began.

#  Results {#results .unnumbered}

![](media/image9.wmf)Two dependent measures were analyzed using analyses
of variance (ANOVAs) and regression analyses. Altitude responses were
scored as correct if the correct altitude was chosen from the menu.
Heading was partitioned into eight equal bins and scored as correct if
the chosen aircraft heading was within 22.5 degrees of the actual
heading. All aircraft headings and altitudes not responded to were
scored as incorrect.

## Analyses of Variance: {#analyses-of-variance .unnumbered}

Prior to conducting the analyses of variance the display was partitioned
into16 regions (Figure 4, middle panel). Each region was 21 NM wide by
21 NM long, with the 16 regions covering an area bounded by -- 9 NM and
+75 NM on the vertical ('Y') display axis, and --42 NM and +42 NM on the
horizontal ('X') display axis. Within this area Ownship was located at
the (0,0) coordinate.

Two-way fully within-subject ANOVAs were conducted on both the altitude
and the heading recall probability data. Since the focus of this
analysis was not on effects of display laterality, the data were
collapsed across the display midline (e.g. combining regions 1 and 13, 5
and 9, . . ), thereby reducing the 16 regions to eight regions. These
eight regions were then divided into two levels of an X-Location factor
and four levels of a Y-Location factor.

Both the altitude recall and heading recall ANOVAs yielded statistically
significant (p \< .05) main effects of X-Location, Y-Location, and a
significant interaction of X-Location with Y-Location. The
three-dimensional bar charts in Figure 4 show the distribution of the
basic altitude and heading recall probabilities for the uncollapsed 16
regions. These charts show that best recall performance is found closest
to Ownship for both variables. They also show that altitude was recalled
better than heading.

Figure 5 shows best fitting contour plots for these same data. In both
cases the iso-memory contours appear to be semi-circular ellipses with
Ownship at the center and clearly illustrate how recall drops with
increasing distance from Ownship. The elliptical form also shows that
memory decreases less quickly along bearings closer to 0 (straight
ahead), particularly for memory for altitude. However, memory does not
appear to be especially sensitive to how quickly an aircraft could come
into conflict with Ownship. Thus, data for altitude appears to reflect
elements of the Spatial Proximity and the Relative Bearing Model, but
not the Temporal Proximity model.

## Regression Analyses: {#regression-analyses .unnumbered}

The purpose of the regression analysis was to obtain a more quantitative
test of the three models, and their corresponding measures, outlined in
the Introduction. The Spatial Proximity model predicts that recall will
be inversely related to the distance of an aircraft from Ownship. The
Relative Bearing model predicts that recall will be inversely related to
the relative bearing of an aircraft. The Temporal Proximity model
predicts that recall will be inversely related to the time needed for an
aircraft to reach Ownship. Prior to conducting the regression analysis,
the probability of recalling each of the aircraft within each scenario
was calculated by averaging across responses for all twelve
participants. Furthermore, a large (r = 0.45) intercorrelation between
the Spatial Proximity and Relative Bearing predictors (occurring because
distance represented in front of ownship was greater than out to the
sides) was removed by using only the data from aircraft within a 42.5 NM
range. Significant intercorrelations remained between the Spatial
Proximity and the Temporal Proximity predictors (r = 0.21), and between
the Relative Bearing and the Temporal Proximity (r = 0.50).

For the heading variable, Relative Bearing accounted for 7% of the
variability; Spatial Proximity accounted for 20% of the variability,
while they jointly accounted for 25% of the variability. For the
altitude variable, Relative Bearing accounted for 23% of the
variability, Spatial Proximity accounted for 22% of the variability,
while they jointly accounted for 40% of the variability. In all cases,
and despite the intercorrelations with the other two predictors,
Temporal Proximity accounted for less than 3% of the variability and was
not statistically significant. The failure of Temporal Proximity as a
predictor indicates that recall was not better for aircraft that were
capable of quickly reaching Ownship than those that would take longer.
Instead, pilots recalled aircraft better that were spatially close to
Ownship and that were closer to straight-ahead.

# Discussion

The mental model implied by the regression analysis of the recall data
is primarily based on a combination of Spatial Proximity and Relative
Bearing. The iso-memory contours are showing somewhat better memory for
aircraft to the front of Ownship, than to the side. Notwithstanding this
asymmetry, the near-circular form indicates that the effect of spatial
proximity is dominant.

This can be contrasted with the forms that could be expected if the
Temporal Proximity variable, or the Relative Bearing variable, dominated
(Figure 1). The elliptical form does suggest, however, that the pilots'
memories were not totally insensitive to the dynamics of the traffic
situation. It shows that memories were at least somewhat better for the
traffic in front of Ownship than to the sides of Ownship. The traffic in
front includes the temporally most proximate aircraft, meaning, that
they have the fastest potential closure rate (if they turned onto a
conflicting course with Ownship). Still, a large emphasis is given in
memory to aircraft that appear to be of small practical consequence
(i.e. those directly to the sides and behind Ownship).

There may be several reasons for these findings. First, pilots were
asked to evaluate their freedom to choose alternative headings, not just
to scan for threat along their current heading. The task was meant to
encourage a comprehensive encoding and evaluation of traffic. That is,
the maneuver evaluation task required a pilot to compare the
trajectories of a large number of other aircraft with that of Ownship.
If the pilots considered a ±180-degree range of potential course changes
equally, then our findings would not be surprising (i.e. memory would be
as good in all directions from Ownship). However, the debriefing
comments of the pilots argue against this interpretation. Eleven of the
pilots stated that they only evaluated course changes of less than 30
degrees, and the twelfth pilot stated that he did not go beyond 40
degrees. Thus their statements indicated that they did not evaluate the
more extreme maneuvers.

A second possibility is that the effects are task artifacts related to
task demands. The memory task was designed as an incidental recall task,
and not one on which the pilots should focus their attention or efforts.
However, they clearly knew that it was of interest, and they would most
likely have wanted to do as well as they could on this task. Therefore,
they could have adopted a strategy to enhance recall, and the results
could reflect that strategy and not one conducive to determining the
basic memory model. However, again the debriefing comments of the pilots
argue against this interpretation. All of the pilots indicated that they
did not attempt to remember aircraft in order to meet the demands of the
recall task. Most of them insisted that they remembered aircraft based
on their performance of the primary task. This does not rule out the
possibility, however, that task demands impacted their behavior at a
subconscious level.

A third possibility is that the format of the display, or the nature of
the task itself, may have influenced the salience of individual
aircraft. In particular, since the CDTI is primarily an ego-centered
display (in contrast to an air traffic controller's perspective-free
display); it may have compelled attention to nearby aircraft, or to
those closer to the immediate path. Research shows that memory for
spatial information is affected by the observer's viewpoint, and that an
ego-centered viewpoint holds a special perceptual status (Franklin,
Tversky, and Coon, 1992). Thus these effects may have occurred despite a
pilot's conscious intent. Comments by two of the pilots support this
interpretation. These pilots said that they often \"couldn\'t help
remembering\" aircraft behind them despite knowing that these aircraft
could not possibly reach them.

The results of this study may be compared with studies of the memory of
air traffic controllers in tasks requiring the monitoring of aircraft
separation (e.g. Endsley and Rogers, 1996; Gronlund, 1997; Mogford,
1997). These studies also examined memory for aircraft information (e.g.
recalled heading, altitude, and x-y location), sometimes relating it to
whether aircraft were in conflict. These investigators focused upon
potential relationships between memory and air traffic control
performance. However, the researchers all noted that it is unclear what
part of the information used to perform air traffic control is reflected
in recall.

This caveat is certainly true for the present study. However, unlike the
above studies, the point of the present study was to determine how
spatial location affected the pilots' attention and encoding of
information (although the relative sensitivity of these two processes to
spatial location was not explored). Mental model structure was not
related to the role of stored information in managing aircraft
separation. Instead, this study used recall to uncover the spatial
determinants of what is attended and encoded. This is based upon the
premise, elaborated by Endsley (1995), that situational awareness, due
to the degree to which it is attended and encoded should be reflected in
recall.

The results indicate that pilots tend to pay undue attention to
irrelevant aircraft when they are close to Ownship. This suggests that
display designers need to find ways to focus pilots' attention on the
most relevant aircraft.

# References

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Endsley, M. R. (1995). Measurement of situational awareness in dynamic
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Franklin, N., Tversky, B., and Coon, V. (1992). Switching points of view
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Gronlund, S. D., Dougherty, M. R. P., Ohrt, D. D., Thompson, G. L. &
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