Human Centered Decision Support Tools for Arrival Merging and Spacing

Vernol Battiste, Everett Palmer, Walter Johnson, Nancy Smith

NASA Ames Research Center

Tom Prevot, Joey Mercer, Stacie Granada, Nancy Johnson, Quang Dao, Paul
Lee

San Jose State University

NASA Ames Research Center

A simulation of terminal area merging and spacing with air traffic
controllers and commercial flight crews was conducted. The goal of the
study was to assess the feasibility and benefits of ground and
flight-deck based tools to support arrival merging and spacing
operations. During the simulation, flight crews arrived over the
northwest and southwest arrival meter fixes and were cleared for the
flight management system arrivals to runways 18 and 13 right. The
controller could then clear the aircraft to merge behind and space with
an aircraft on a converging stream or to space behind an aircraft on the
same stream of traffic. The controller remained responsible for aircraft
separation. Empirical research was performed to assess air and ground
tools and the effects of mixed equipage. During the all tools
conditions, 75% of the arrivals were equipped for merging and spacing.
All aircraft were ADS-B equipped and flew charted FMS routes which were
coordinated based on wake turbulence separation at the arrival runway.
The aircraft spacing data indicate that spacing and merging were
improved with either air or ground based merging and spacing tools, but
performance was best with airborne tools. Both controllers and pilots
exhibited low to moderate workload and both reported benefits from the
concept.

Introduction

At the core of the concept of Distributed Air-Ground Traffic Management
is the idea that National Airspace System (NAS) participants can be
information suppliers and team members who collaborate at all levels of
traffic management decision making (Raytheon ATMSDI, 2003). One such
concept and the focus of this paper is Concept Element 11 (Terminal
Arrival Self-Spacing for Merging and in-Trail Separation).

Sorensen (2000) characterizes the CE 11 approach process as involving
one of three operational modes. Each mode possesses potential benefits
but also presents significant operational and technical challenges.
These modes are: Free Maneuvering, Merging, and Spacing. During *Free
Flight Maneuvering*, equipped aircraft can design their own direct path
within a defined approach corridor (not under investigation in this
study). *Merging* occurs when an equipped aircraft is delegated the
responsibility for adjusting in-trail position behind the designated
lead aircraft approaching from another stream; finally, the *Spacing*
concept is one in which an *equipped aircraft is cleared to maintain a
specified temporal position from a designated lead aircraft*.

The objective of CE 11 is to minimize the in-trail spacing buffers
between terminal area arriving aircraft flying under instrument
meteorological conditions (IMC). CE 11 utilizes time-based, in-trail
spacing to take advantage of the natural spacing compression of arriving
aircraft as they decelerate in preparation for landing (Abbott, 2002).
To support the transition of responsibility for maintaining the desired
spacing interval, from the controller to the flight crew, advanced ATM
technologies (decision support tools -- DST) were developed for both
controller and flight crews (Granada, Dao, Wong, Johnson, Battiste,
2005).

In a previous study of merging and spacing, NASA ARC researchers
employed a human-in-the-loop simulation with pilots and controllers, and
tested time-based merging and spacing. Results of this study highlighted
the need for clear delegation of responsibilities and unambiguous
procedures under a variety of operational scenarios. Specifically,
controllers were unclear about pilots' separation responsibilities. This
ambiguity was particularly apparent when aircraft were spacing less than
the assigned interval but still further than the legal separation
requirement. Results of a follow-up study at NASA ARC reflected the
progress made through the development of tools and procedures. When
given the choice of issuing a spacing clearance to equipped aircraft,
the TRACON controllers opted to provide the clearance about 85 percent
of the time. This finding suggests that controllers were comfortable
with the tools and procedures, and confident with the ability of pilots
to accurately self-space (Lee, et al., 2003).

During an operational evaluation of in-flight spacing and merging,
display integration was identified by flight crews as an issue when
spacing information was presented on the NAV Display (ND). The FAA Safe
Flight 21 operational evaluation data collected from flight crews
identified display integration, clutter, and heads-down time as
important display integration issues (Cieplak, Hahn, and Olmos, 1999).

The Flight Deck Display Research Group at NASA Ames has designed a suite
of tools which should enable operators to safely and efficiently perform
the necessary merging and spacing tasks essential to the success of the
concept. In this report, we focus mainly on the evaluation of the flight
deck DST. However, some discussion of the controller tools and tasks are
necessary to set the context in which the flight deck tools were
evaluated. The cockpit situation display (CSD), which is presented on
the ND, includes a 3-D cockpit display of traffic information (CDTI),
and the merging and spacing tools (FDDRL, 2004). The CSD integrates
information derived from the spacing algorithms with traffic position,
aircraft identification and intent to present a display of the current
and predicted traffic situation (see Figure 1). Armed with this
information and tools, flight crews were allowed to perform airborne
merging and spacing operations when cleared to do so by the controller.
This paper also examines the feasibility of the merging and self-spacing
concepts from the flight deck perspective under mixed traffic
conditions, where only some of the aircraft were equipped for
self-spacing and merging. See Callantine, Lee, Mercer, Prevot and Palmer
(ATM-2005) for CE-11 ground side results.

![](media/image1.png){width="2.571527777777778in"
height="2.640277777777778in"}

Figure 1: 3-D Cockpit Situation Display

Methods

Pilot Participants

Nine air transport and/or commercial rated pilots and four certified
professional controllers participated in the study. Pilots had an
average of 10,405 flight hours and 3,912 hours in glass cockpits. All
flight crew members were familiar with the advanced 3-D CDTI display
system and received 2 days of training on the merging and spacing task
and procedures. Four full performance level controllers with TRACON
experience manned the feeder and final control positions in dual TRACON
operations.

Experimental Conditions

Four experimental conditions were created to examine pilot and
controller performance: No Tools, Ground Tools only, Air Tools only, and
Air & Ground Tools. Data was collected from thirty two trials, with
eight trials per condition. To assess the operational feasibility of the
concept from the flight deck perspective, the following items were
assessed: assigned vs. achieved inter-arrival spacing,
usability/usefulness, flight crew workload, and safety. Additionally,
pilots were asked to provide comments on the issue of call sign
confusion when multiple aircraft IDs (call signs) are used in a single
transmission. Post run and simulation questionnaires were used to assess
concept feasibility and display usability.

![](media/image2.png){width="3.0993055555555555in"
height="1.9527777777777777in"}

Figure 2: DFW TRACON Airspace.

# 

# Airspace and Controller Tasks

Controllers pairs (feeder and final) managed the western portion of the
Dallas Fort Worth TRACON airspace. The feeder controller initially
cleared the aircraft for either the Fever or Bambe FMS arrival, and if
applicable, to follow a lead aircraft to 18R (see Figure 2). The Final
controller managed the merge between the two arrival streams, which were
procedurally separated by 1,000 feet at the GIBBI intersection.

## 

## Controller Display and Tasks

Controllers utilized a wake-vortex aware arrival schedule, which
computed estimated times of arrival for runway 18R. In the conditions
with ground tools, merging and spacing information was incorporated into
each aircraft's data tag. For example, as illustrated in Figure 4,COA
538, a B733, landing 18R, assigned to follow BAW 601 80 seconds in trail
and is currently 69 seconds in trail. Additionally, the spacing circle
provides relative information about the spacing goal (see Figure 3).

## ![](media/image3.png){width="2.301388888888889in" height="2.433333333333333in"}

## Figure 3: Controller Display with merging and spacing tools.

## 

## Roles and Responsibilities

Controllers were responsible for separation at all times. Flight crews
could be cleared to merge behind then follow a lead aircraft on a
conjoining route or to follow an aircraft on the same route. Controllers
could cancel a spacing clearance at any time.

![](media/image4.jpeg){width="2.18125in" height="2.5590277777777777in"}

## Figure 4: FMS transitions to runway 18R - Streams merged at GIBBI. 

Procedures

Each aircraft started the scenarios 15 to 40 nm from the BAMBE or FEVER
meter fixes. Upon entry, pilots were cleared to fly an FMS arrival route
(see Figure 4) and were instructed to allow their aircraft to fly and
descend along the FMS arrival path, even if Ownship seemed to follow
another aircraft too closely -- i.e., they did not adjust speed or
altitude unless commanded by the air traffic controller (ATC). Pilots
checked in with controllers when they received a data link clearance or
at 5 nm from the meter fix. Pilots were instructed to expect spacing
clearances any time after reaching the meter fix. Controllers issued
clearances to merge and follow or follow behind a designated lead
aircraft. Controllers utilized normal controller procedures -- radar
vectors, "direct to", speed and altitude -- to manage the unequipped
aircraft. The pilots utilized the airborne spacing tools and procedures
to implement the assigned spacing command.

## 

## Pilot Clearance and Tasks

ATC provided clearances such as "Continental 538, merge behind then
follow Speedbird 601-- 80 seconds in trail," or "Continental 538, follow
Speedbird 601 -- 80 seconds in trail." Pilots read back the clearance
and engaged self-spacing; see flight deck procedures below. If the
algorithms did not command appropriate speeds based on the spacing
setting, pilots were asked to disengage spacing and inform ATC that they
were unable to space.

# Tools for Merging & Spacing Operations

If a merging and spacing clearance was assigned, the flight crew
followed the steps listed below using a mouse to position the curser:

1\) Pilots first clicked on the Spacing button on the CSD tool strip.

![](media/image5.jpeg){width="2.8208333333333333in"
height="0.26666666666666666in"}

![](media/image6.jpeg){width="1.2041666666666666in" height="1.5in"}2)
Pilots then selected the assigned lead aircraft by clicking on its
symbol within the CSD. In this case, TWA79 was selected.

3\) The spacing interval specified by ATC was then entered. To increase
the spacing interval, pilots right-clicked on the seconds (**Sec:XX**)
button; to decrease the interval pilots left-clicked the seconds button.

![](media/image7.jpeg){width="2.75in" height="0.26805555555555555in"}

![](media/image8.jpeg){width="0.6326388888888889in" height="1.05in"}

4\) Pilots then clicked the start button on the CSD tool strip, which is
located next to the "seconds" button in the figure above. Pilots were
informed they would need to wait for the spacing algorithms to
initialize. When the spacing algorithm was initialized (i.e., ready to
engage spacing) the upper left corner of the CSD displayed a message
indicating the spacing status. Also, the lead aircraft became
highlighted in orange.

![](media/image9.jpeg){width="1.5979166666666667in"
height="0.5159722222222223in"}

5\) Finally, to engage the auto throttles,\
pilots selected the SPC button on the MCP.\
This activated the algorithm to begin commanding\
the proper speeds (via the auto throttles) to move the aircraft towards
the spacing goal.

![](media/image10.jpeg){width="2.4192913385826773in"
height="1.4610236220472441in"}

6\) When the spacing is engaged and active, feedback is provided at the
upper left corner of the CDTI.

![](media/image11.jpeg){width="1.4729166666666667in"
height="0.46805555555555556in"}

![](media/image12.jpeg){width="1.0047244094488188in"
height="1.355511811023622in"}![](media/image13.jpeg){width="1.0881889763779529in"
height="1.35in"}![](media/image14.jpeg){width="1.0645669291338582in"
height="1.35in"}

Visual feedback regarding Ownship spacing status was provided via a
color-coded "spacing box." The color and location of the spacing box
reflected Ownship position relative to the assigned temporal spacing
value. That is, if Ownship was given an assigned spacing value of 100
seconds and was more than 10 seconds ahead (e.g., the aircraft is
currently at 83 seconds), the spacing box was depicted as yellow and
Ownship appeared slightly ahead of the box. When Ownship was less than
10 seconds ahead or less than 20 seconds behind the assigned spacing
value, the spacing box was depicted as green, and Ownship appeared
inside the box. Finally, if Ownship was more than 20 seconds behind the
assigned spacing value, the spacing box was depicted as white and
Ownship appeared behind the box.

Simulation environment

The simulation study was conducted utilizing three fully integrated NASA
ARC research laboratories/facilities: the Airspace Operations Laboratory
(AOL), Flight Deck Display Research Laboratory (FDDRL), and Crew Vehicle
Systems Research Facility (CVSRF). See DAG-TM, 2003 for a full
description of each laboratory.

Results

This section presents the results of the Merging and Spacing operation
at the 80 and 100 second intervals. Additionally, data on the efficiency
of the merging and spacing operation, flight crew workload, safety and
acceptability are described. Participating flight crews conducted 256
total approaches, 32 in each condition.

During the No Tools condition flight crews followed ATC guidance as they
would today, thus no relative spacing and merging data are reported. Of
the remaining 128 runs in the air tools and air and ground tools
conditions controllers assigned spacing to the flight deck 116 times.

![](media/image15.wmf){width="2.8506944444444446in"
height="1.5277777777777777in"}

![](media/image16.wmf){width="2.8465277777777778in"
height="1.5277777777777777in"}

Figure 6 and 7: Initial and final spacing intervals for 80 and 100
seconds (mean and standard deviation).

Figures 6 and 7 show the spacing intervals data from the start of
spacing and merging and/or spacing until spacing was discontinued at or
near the final approach fix or by the controller. These graphs
illustrate that, overall spacing performance was improved for All Tools
condition and that performance was best in the Air Tools only condition
(mean 78.8 and 99.4, respectively), followed by Air and Ground Tools
(77.8 and 95.8), and finally Ground Tools (77.6 and 93.8). However,
these trends were not significant (p \>.05). Additionally, the expected
improvement in spacing performance with air and ground tool was not
found. However, controllers preferred to conduct spacing operations with
only ground tools. They suggested that conducting merging and spacing
operations when flight crews were managing spacing added additional
variability and made it difficult to manage unequipped aircraft.

Spacing efficiency

From the flight deck perspective, a measure of efficiency was related to
when aspacing and merging clearance was issued by the controller. If the
clearance was issued early in the approach, the flight crews had more
time to set up the systems and manage progress toward the spacing goal.
If the clearance was issued late (i.e., near the base to final leg of
flight), then this task may interfere with other tasks that require
completion before landing. A t-test was conducted to examine this
notion. The pilots' data was split into three groups; early, middle, or
late approach clearances. A one-sample t-test was used to compare the
three groups relative to the 80-second spacing goal. Results indicated
that the early or mid approach groups did not significantly differ from
the 80-second spacing goal (*p* \> .05). However, when the spacing
clearance was issued late, the spacing performance did significantly
vary from the 80-second spacing goal, *t*(22) = -3.33, *p* \< .01,
indicating a decline in spacing performance (see Figure 8).

![](media/image17.wmf){width="2.9319444444444445in"
height="2.032638888888889in"}

Figure 8: Spacing performance with early, mid and late spacing
clearance.

  ------------------- ---------- --------------- ---------------- -----------
                      **Air**    **Ground**      **Air/Ground**   **None**

                      *M* SD     *M* SD          *M* SD           *M* SD

  Peak Workload       2.56 .69   2.25 .67        2.40 .72         2.23 .61

  **Overall           2.34 .65   2.22 .67        2.28 .69         2.21 .63
  Workload**                                                      

  **ATC               2.51 .75   2.52 .61        2.31 .58         2.53 .74
  Communication**                                                 
  ------------------- ---------- --------------- ---------------- -----------

Table 1: Crew workload and communication by conditions.

Workload, Communication and Usability

After each approach, pilots entered a workload rating reflective of
their perceived workload for the run using a modified NASA Task Load
Index (TLX). There were a total of 32 trials in which the pilots
provided workload data. The TLX rating scales were modified to include a
peak workload assessment and an estimated communications workload
relative to normal operations. Additionally, each rating was based on a
Likert scale format that had "Normal Ops" as the median rating of 3 on a
scale of 1 to 5, with a rating of 5 for "High" workload. This method was
not used to suggest that "Normal Ops' represents a medium level of
workload, but it provided a familiar baseline for the participants. For
this report only the peak, overall, and ATC communication workload
values are presented.

The mean Peak Workload value was 2.45, SD = .72, the mean Overall
Workload was 2.25, SD = .66, while the mean ATC Communication Workload
was 2.39, SD = .57. Across all conditions flight crews' ratings were
relatively similar. The mean workload ratings were subsequently examined
for each of the four conditions (Air tools, Ground tools, Air and Ground
tools, and No Tools) separately. Table 1 includes the mean workload
values for Peak Workload, Overall Workload, and ATC Communication
Workload by each of the four conditions. As the table shows, flight
crews rated the workload of the merging and spacing task below that of
normal operations for all conditions (where normal operations was
represented by a value of 3). The table also shows that crews rated
communication workload lowest in the air/ground tools condition,
suggesting that when both pilots and controllers have supporting tools,
communication may be reduced.

ATC Clearances

+-------------------+----------------------------------------+---------+
| **Acceptable**    |                                        |         |
+-------------------+----------------------------------------+---------+
|                   | Merging and spacing task               | 4.8     |
+-------------------+----------------------------------------+---------+
|                   | Head-down time                         | 4.0     |
+-------------------+----------------------------------------+---------+
|                   | Display symbols                        | 3.4     |
+-------------------+----------------------------------------+---------+
|                   | Symbol Color                           | 4.3     |
+-------------------+----------------------------------------+---------+
| **Useful**        |                                        |         |
+-------------------+----------------------------------------+---------+
|                   | Information in aircraft data tag       | 4.0     |
+-------------------+----------------------------------------+---------+
|                   | Accept spacing clearance based on CDTI | 4.8     |
|                   | data only                              |         |
+-------------------+----------------------------------------+---------+
|                   | Accept visual approach clearance based | 3.7     |
|                   | on CDTI only data                      |         |
+-------------------+----------------------------------------+---------+
| **Safety**        |                                        |         |
+-------------------+----------------------------------------+---------+
|                   | CDTI improves flight safety            | 4.3     |
+-------------------+----------------------------------------+---------+
|                   | Enhances safety of merging and spacing | 3.8     |
+-------------------+----------------------------------------+---------+
| Table 2: User     |                                        |         |
| Feedback on       |                                        |         |
| display, tools    |                                        |         |
| and concept       |                                        |         |
|                   |                                        |         |
| (N=10; 1 = not    |                                        |         |
| acceptable,       |                                        |         |
| useful and safe,  |                                        |         |
| 5 = very          |                                        |         |
| acceptable,       |                                        |         |
| useful and safe   |                                        |         |
| scale).           |                                        |         |
+-------------------+----------------------------------------+---------+

An issue, which has stimulated considerable discussion over the past few
years, has been the potential call sign confusion that may result in a
DAG-TM environment. Specifically, the DAG-TM environment requires the
use of two aircraft call signs in a single voice transmission. The
concern has been that pilots may become confused by the use of two call
signs and, at a minimum, may need to ask ATC to repeat the clearance. In
a worst case scenario, the potential confusion could result in a pilot
accepting a clearance that was meant for another aircraft. Of course,
this worst-case scenario could lead to an accident or incident. An
important finding in the present study was that, of 323 spacing and
merging clearances, neither pilots nor controllers reported a single
instance of "call sign confusion." Flight crews reported that with the
inclusion of flight ID and the pulse predictor (c.f., Granada et.al.,
2005) on their CDTI, they were able to identify their prospective lead
aircraft and to anticipate the ATC clearance.

As Table 2 shows, flight crews found the tools, display features and the
concept acceptable, useful and safe. Also, these ratings suggest that
the flight crews may be willing to take on additionally responsibility.

## Discussion and conclusions

Based on flight crew and controller performance, comments and also their
interactions with the tools and procedures, the concept of merging and
self-spacing during arrival and approach seems feasible. Pilots
consistently rated the flight deck tools favorably in terms of
usability, usefulness, and rated the CSD favorably in terms of situation
awareness. Generally, pilot and controller workload ratings were
moderately low during spacing and merging operations. Workload
differences between tools conditions were relatively small for pilots,
and when spacing clearances were issued early or at the mid point of the
approach, pilots had little difficulty achieving the spacing goal. In
this study, pilots and controllers generally disagreed as to the best
time for the spacing clearances to be issued; however, the controllers
were only beginning to develop strategies for how to best utilize this
new tool. Finally, this study did identify a number of issues from the
flight crews' and controllers' perspectives that need to be addressed in
future research.

References

Abbott, T. (2002). *Speed Control Law for Precision Terminal Area
In-Trail Self Spacing.* NASA Technical Memorandum 2002-211742. Hampton,
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FDDRL. (2004). 3D-CDTI User Manual.
[http://human-factors.arc.nasa.gov/ihh /DAG_WEB/
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Lee, P., Mercer, J., Prevot, T., Smith, N., Battiste, V., Johnson, W.,
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Raytheon ATMSDI Team. (2003). *NASA Ames Research Center DAG-TM CE 6/11
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Sorensen, J. (2000). *Terminal Arrival: Self-Spacing for Merging and
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