**Active Final Approach Spacing Tool (AFAST)**

# General Description {#general-description .unnumbered}

# January 31, 2002 {#january-31-2002 .unnumbered}

# NAS2-98005 {#nas2-98005 .unnumbered}

# Technical Research in Advanced Air Transportation Technologies {#technical-research-in-advanced-air-transportation-technologies .unnumbered}

Final

General Description

Active Final Approach Spacing Tool (aFAST)

CDRL Item 5e

# Prepared under {#prepared-under .unnumbered}

# RTO - 62 {#rto---62 .unnumbered}

# AATT Operational Concept for ATM -- Year 2002 Update {#aatt-operational-concept-for-atm-year-2002-update .unnumbered}

#### January 31, 2002 {#january-31-2002-1 .unnumbered}

#### Prepared For: NASA Ames Research Center {#prepared-for-nasa-ames-research-center .unnumbered}

#### Moffett Field, CA 94035-1000 {#moffett-field-ca-94035-1000 .unnumbered}

#### Prepared By: Titan Systems Corporation -- SRC Division {#prepared-by-titan-systems-corporation-src-division .unnumbered}

#### Billerica, MA 01821-4134 {#billerica-ma-01821-4134 .unnumbered}

![](media/image1.jpeg){width="5.416666666666667in" height="0.75in"}

**ACKNOWLEDGEMENT**

The majority of the descriptions presented in this document have been
taken directly from NASA documents shown in the bibliography. Minor
modifications have been made to the text to provide readability. The
aFAST Flow chart has however been independently developed for this
description. This approach to the development of this document was taken
in order to remain faithful to the efforts that are presently being
undertaken by the NASA AATT Project Office, the aFAST Tool Developers
and the associated NASA AATT contractors.

**Table of Contents**

1.  **Description 1**

2.  **Operational Concept 4**

3.  **Functional Flow 9**

# List of Figures {#list-of-figures .unnumbered}

1.  **aFAST System Overview 4**

2.  **PGUI for DFW Arrivals 6**

3.  **Speed Advisory Using Color 7**

4.  **Heading Advisory Using Color 8**

5.  **Combined Speed and Heading Advisory Using Color 9**

6.  **Trajectory Quick Look Using Color 10**

7.  **aFAST Functional Flow 11**

8.  **aFAST Software Architecture 13**

# Appendicies {#appendicies .unnumbered}

### Bibliography

## AATT ACTIVE FINAL APPROACH SPACING TOOL (aFAST) {#aatt-active-final-approach-spacing-tool-afast .unnumbered}

## GENERAL DESCRIPTION {#general-description-1 .unnumbered}

**1. DESCRIPTION**

**Purpose**

-   Help terminal area controllers optimize traffic flow to runways to
    minimize delay and increase throughput.

-   Compute an efficient landing sequence, approach spacing and runway
    assignment for arrival aircraft.

**Users**

-   Terminal area air traffic controllers

-   Terminal area traffic management coordinators (TMCs)

**Operational Results**

-   In operational testing at DFW, passive FAST increased landing rates
    by 9-13%.

-   The FAA estimates that passive FAST saves airlines US\$10 million
    per year at DFW.

-   aFAST is not operational however benefits analysis indicates a
    potential savings of over \$300 M/year for 10 major airports (see
    #35 Bibliography).

**Overview**

The Final Approach Spacing Tool (FAST) is a decision support tool for
terminal area (TRACON) air traffic controllers. The TRACON typically
encompasses the airspace within approximately 40 miles of a major
airport. TRACON air traffic controllers manage arrival aircraft which
enter their airspace from adjacent ATC facilities or internal airports.
The controllers are responsible for assigning an appropriate runway and
landing sequence to each aircraft and maintaining safe separation.

FAST assists air traffic controllers by providing its advisory
information on the radar planview displays. Additionally, FAST assists
traffic management coordinators by providing schedule information on
auxiliary timeline displays.

Early in the development of FAST, its functionality was divided into two
parts: Passive and Active. Passive advisories consist of runway
assignments and landing sequences to increase the efficiency of runway
usage. Active advisories consist of turn and speed commands to increase
the precision of final approach spacing.

In general, arrival aircraft are vectored from en-route airspace into
terminal airspace over metering fixes. Aircraft are assigned to a
default runway, which is typically the closest runway to their
respective metering fix. A controller occasionally reassigns aircraft to
alternate runways to reduce delay on a particular runway or to balance
delay and aircraft among multiple runways. Ideally, a controller would
prefer to finalize an aircraft\'s runway assignment as early as possible
(i.e. near the metering fix), but uncertainties in the traffic situation
often require later reassignments.

The strength of an automation system such as FAST is its ability to
assign runways based upon accurate estimations of delay savings and
workload benefits early in the arrival process. The FAST runway
allocation algorithm attempts to meet four primary objectives: making an
early and accurate decision, reducing overall system delay, increasing
overall system throughput and reducing controller workload.

During each scheduling cycle, FAST builds a trajectory for each aircraft
from its current position to the runway threshold. The FAST sequencing
algorithm uses these trajectories to systematically order aircraft on
common trajectory paths and to merge aircraft on different trajectory
paths. Fuzzy reasoning is used to model the controllers\' cognitive
processes related to determining an efficient landing sequence.

Using the relative sequences of aircraft on each trajectory path, FAST
performs conflict prediction and resolution in order to achieve a
conflict-free arrival plan. The criteria considered during conflict
prediction are wake vortex minimum separation, custom runway specific
separation and custom flight-specific separation. When a conflict is
predicted, it is resolved by adding delay to the aircraft\'s
trajectories in the form of vectoring and speed control.

**Future Research**

While the current concept for aFAST is focused on generating speed and
heading advisories, consideration is also being given to adding altitude
advisories. There are numerous instances where altitude is a critical
component of the arrival controllers overall plan. However, adding
altitude advisories to aFAST sequencing increases complexity, requiring
very accurate altitude information. Because very accurate altitude
information would be a requirement, potential research may be dependent
upon receiving direct altitude updates from the aircraft FMS via
Datalink.

The integration of FMS and CTAS information exchanged via Datalink has
been a topic of consideration for many years. Aircraft equipage has been
a stumbling block, but there are other complicating human factors issues
surrounding the integration of FMS and the use of Datalink for aFAST
advisories.

Dynamic spacing is a candidate for aFAST related research. In the
current operational environment, the minimum separation allowed behind
aircraft is static, and set by the FAA. Controllers comply with a
structured matrix defined in the 7110.65 (e.g. 4 miles for a heavy jet
following a heavy jet). This matrix is based upon the "worst case
scenario" for wake turbulence. It has been postulated that in many
instances, these wake turbulence minimums can be safely reduced. In
order to reduce separation, controllers would need a tool like aFAST to
assist them in dynamically spacing the arrivals. The feasibility of this
concept is another area for aFAST research.

Another area where aFAST may provide potential benefits is in the
environmental impact arena. Nominal routes avoiding noise-sensitive
areas can be incorporated into aFAST. The possibility of dynamically
altering routes based on conditions (e.g. time of day or weather
conditions) is a potential area of research.

Finally, there are numerous integration issues to be explored between
aFAST and surface automation. Active FAST can provide a surface
automation tool with the specific gaps between arrivals allowing for
better departure queue management. Surface automation tools can provide
aFAST with surface conditions allowing for a negotiation between the two
systems.

A research plan has been established which will address preliminary
human factors issues such as use of color, information format, and
timing of advisories. Human factors research will continue, as the
algorithms are refined. Algorithms must first be validated in
closed-loop simulations. This will ensure that the system will calculate
optimized conflict free trajectories. In addition,
controller-in-the-loop simulations must be conducted to validate
accuracy and stability of advisory information, usability, suitability,
and controller acceptance of advisory presentation. These simulations
will be conducted initially at NASA-Ames, but will eventually transition
to the FAA Technical Center. Field-testing is also essential to the
development of aFAST. The current plan calls for conducting the field
test activities at DFW TRACON. It is anticipated that there will be
color workstations available at TRACON arrival positions prior to the
onset of field evaluations.

**2. OPERATIONAL CONCEPT**

Active FAST is designed to deal with the complexities of inter-arrival
spacing within the TRACON (particularly on the final approach path).
Active FAST generates "control instruction" level advisories whereby
controllers issue specific speed and heading instructions based upon the
advisories. Advisories will be displayed to controllers via their
standard terminal color displays. Dedicated aFAST displays will be
provided for TMCs in the ARTCC and TRACON. These displays will be used
for strategic planning. Displays will also be available for ATCSs in the
Tower. The Tower displays will provide enhanced situational awareness.

As arrivals enter the TRACON, they are assigned a runway and sequence
number. Active FAST builds a plan for these arrivals based on aircraft
performance characteristics, airspace constraints, and separation
requirements. A trajectory for each aircraft is created and adjusted
based on real time radar updates. These trajectory calculations include
identification of when and where each aircraft should receive speed
adjustments or headings. These speeds and headings will eventually be
able to be incorporated into future technologies such as Datalink.
However, in the near term operational environment, these advisories will
be displayed in logical increments (e.g. speeds of 210 knots and 180
knots, headings in 10 degree increments) to the TRACON arrival
controller so that they can be issued as control instructions. Active
FAST continues to monitor and update the plan based upon radar track
updates. The plan is modified when necessary, and ultimately leads to an
optimized delivery of aircraft to the runway threshold.

The primary users of the aFAST advisories are the TRACON arrival
controllers. However, many other users can benefit from the information.
Other controllers within the TRACON can view the aFAST advisories to
better understand the arrival controller's plan (e.g. a departure
controller may want to know whether or not an arrival may be instructed
to slow down or turn). TMCs in the TRACON can use the aFAST information
to make dynamic runway changes for aircraft near the TRACON boundary.
The information displayed on the Planview Graphical User Interface
(PGUI) can also help the TMCs in the TRACON and ARTCC better understand
the traffic situation inside the TRACON. Controllers in the ATCT can
also benefit from the PGUI by observing where gaps will occur in the
arrival stream (for runway crossings or departure slots).

Other potential users of the aFAST information are in the Ramp Tower and
the Airline Operations Centers. For example, the Ramp Tower can use the
information to accurately determine when an aircraft will be on the
ground, and thus need a gate. The Airline Operations Center may also
benefit from aFAST information, since it is more accurate than their
current source of information. It has been shown that CTAS information
sharing with airline operations control centers leads to an improvement
in airline operational planning. The applications of Ramp Tower and
Airline Operations Center usage are potential research areas.

![](media/image2.png){width="6.460416666666666in"
height="5.232638888888889in"}

Figure 1 aFAST System Overview

As shown in Figure 1, the aFAST system uses aircraft flight plans and
position data from FAA computers, inputs from TRACON arrival controllers
and traffic managers, and current weather information, to produce
advisories to assist controllers in managing and controlling arrival
traffic. The weather information is provided either by the Rapid Update
Cycle (RUC), or by the Integrated Terminal Weather System (ITWS). RUC
provides a weather forecast every 3 hours (80 km grid). ITWS provides a
weather forecast every 5 minutes (2 km grid).

TRACON arrival controllers interact with aFAST, both receiving
advisories and providing inputs, through standard FAA hardware. The
aFAST advisories will be displayed to TRACON controllers on FAA TRACON
display systems. Controller inputs will be made through message entry
devices. Traffic managers interact with aFAST through dedicated aFAST
displays. They provide inputs such as runway spacing requirements,
airport configuration, and airport acceptance rates. Traffic managers in
both the ARTCC and TRACON may monitor aFAST timelines to gain a more
accurate picture of the real-time operation in the TRACON.

In the current operational environment, there is only a minimal set of
automation tools to assist air traffic controllers. The most advanced
automation for controllers includes the Display System Replacement (DSR)
in the ARTCCs and Full Digital ARTS Displays (FDADs) in the TRACONs
(though the majority of TRACONs have even earlier generation displays).
These displays are monochrome, and have very limited capabilities. As a
result, controllers are forced to rely primarily on their mental skills
to solve the complex problems presented to them. While controllers have
always done an excellent job managing these complexities, some
situations are beyond the scope of their informational awareness.
Aircraft performance characteristics and wind speed/direction are two
areas where controllers often lack accurate information. Active FAST's
ability to integrate such information into its trajectory predictions
suggests that it may be a valuable resource to controllers in such
situations.

The principal techniques that controllers use for sequencing and spacing
aircraft in the TRACON are speed, heading, and altitude instructions. If
aircraft are on a straight route segment, speed is the primary degree of
freedom. On curved approaches and base turns heading is the primary
degree of freedom. On merging routes, altitude is the primary degree of
freedom.

Although much of the CHI requirements definition for aFAST is still
being investigated, it will build upon research that was conducted
during the development of the pFAST. As in pFAST, advisory information
will be displayed in the full data blocks. Additional symbology will be
drawn on the display at the position where the turn or speed reduction
should occur. It is likely that color will be used either for the
advisory, the advisory position indicator, or both.

Once the usability and acceptability of aFAST advisories have been
assessed in simulation, the operational implications must be addressed
in high fidelity simulation and limited operational testing. Since the
advisories may be displayed to controllers in both the feeder and final
sectors, the procedures required for coordinating the handoff of an
aircraft with an active advisory must be addressed.

Figure 2 is a graphical representation of aircraft inbound to DFW
Airport. The Full Data Blocks (FDBs) for these aircraft include the
Aircraft Identification (e.g. AAL 467), runway assigned (e.g. 18R),
aircraft type (e.g. MD80), sequence number to the runway (e.g. 4). In
some cases, aFAST advisories for heading and speed (EGF 824 has a
heading advisory of 090 and a speed advisory of 190) are also displayed.
The letters associated with the FDBs (e.g. the H associated with AAL
467) indicate the controlling position within the TRACON. The controller
responsible for the final approach course to Runway 18R uses an "H"
symbol.

The underlying white video map shows the airport layout for DFW, as well
as numerous airports, navigation aids, and airspace boundaries. The
timelines to the left of the display show aircraft inbound to 3
different runways. These timelines show when specific aircraft are
estimated (left side of the timeline) and scheduled (right side of the
timeline) at each selected runway. The aircraft are color-coded
indicating which gate the aircraft entered the TRACON. The TMCs in the
TRACON and Tower can use these timelines to determine runway loading and
gaps in the arrival stream.

![](media/image3.wmf)

Figure 2 PGUI for DFW Arrivals

![](media/image4.jpeg){width="5.7362204724409445in"
height="4.040157480314961in"}

Figure 3 Speed Advisory Using Color

Figure 3 illustrates an aFAST-generated speed advisory using color.
Active FAST has generated a speed advisory of 190 knots for ASE 243.
This is denoted by the orange "190" in the third visible line of the FDB
of ASE 243. The orange dot identifies the location where the advisory
needs to be issued by the controller to the pilot. This speed of 190
knots will assist the controller in spacing ASE 243 behind EGF 695. This
speed will synchronize the arrival stream. However, as soon as AAL 1109
begins to further reduce to the final approach speed, EGF 695 and
subsequently ASE 243 will also need additional speed reductions.

![](media/image4.jpeg){width="5.7362204724409445in"
height="4.040157480314961in"}

Figure4 Heading Advisory Using Color

Figure 4 illustrates an aFAST-generated heading advisory using color.
This figure shows a base turn heading advisory of 100 degrees for ASE
243. This is denoted by the cyan "H100" in the third visible line of the
FDB of ASE 243. Heading advisory information timeshares with the
sequence number advisory information. The cyan diamond identifies the
location where the advisory needs to be issued by the controller to the
pilot. This heading of 100 degrees will assist the controller in spacing
ASE 243 precisely behind EGF 695.

Figure5 Combined Speed and Heading Advisories Using Color

In any busy air traffic control environment, workload and frequency
congestion are primary considerations. It is often necessary for
controllers to issue both speed and heading control instructions in
close proximity to one another. It is desirable to combine transmissions
in order to reduce workload and frequency congestion. Human factors
research will be conducted to determine when controllers issue combined
advisories vs. independent advisories. A different type of symbol,
representing a combined advisory may be incorporated into the symbology
if necessary.

Figure 5 illustrates aFAST-generated speed and heading advisories using
color. Active FAST has generated a speed advisory of 190 knots for ASE
243. This is denoted by the orange "190" in the third visible line of
the FDB of ASE 243. The orange dot identifies the location where the
advisory needs to be issued by the controller to the pilot. This speed
of 190 knots will assist the controller in spacing ASE 243 *precisely*
behind EGF 695. This speed will synchronize the arrival stream. Active
FAST has also generated a base turn heading advisory of 100 degrees for
ASE 243. This is denoted by the cyan "H100" in the third visible line of
the FDB of ASE 243. The cyan diamond identifies the location where the
advisory needs to be issued by the controller to the pilot. This heading
of 100 degrees will also assist the controller in spacing ASE 243
precisely behind EGF 695. Since these two advisories are in close
proximity, it is assumed that the controller will issue both during the
same transmission.

![](media/image5.jpeg){width="6.028346456692914in"
height="4.165354330708661in"}

Figure 6 Trajectory Quicklook Using Color

In order to generate advisories, aFAST calculates the trajectories of
each arrival aircraft. A byproduct of this calculation is that these
trajectories can be displayed to the controllers. In order to help
formulate a plan of action, the controller may choose to see what routes
and speeds are being calculated by aFAST.

Figure 6 illustrates a trajectory being displayed for AAL 1808. The
route in white shows the aFAST-calculated two-dimensional path. The
orange advisories along the path (S210 and S190) indicate where the
speed instructions should be issued. The cyan advisories along the path
(H360 and H090) indicate where the heading instructions should be
issued. This trajectory may change as the data on this aircraft and
other aircraft are updated via radar. Once advisories are displayed,
they are "frozen" and cannot be changed. However, non-displayed advisory
points can be varied.

3.  **FUNCTIONAL FLOW**

Figure 7 illustrates the detailed functional flow of aFAST operating in
the TRACON and depicts the functional interfaces with external data
sources. The input/output sources, appearing in double boxes, are shown
in the figure. The major functions within aFAST are:

-   Data Acquisition and Processing

-   Data Formatting and Distribution

-   Trajectory Generation

-   Arrival Planning

-   Advisory Generation

-   Conflict Detection

-   Display Message Processing

The narrative contained in the following section provides adequate
description for the aFAST functional flow (see Figure 7 ) and for the
aFAST Software architecture (see Figure 8 ).

![](media/image6.wmf){width="7.625in" height="4.402777777777778in"}

**Figure 7 aFAST Functional Flow**

The external interfaces required for aFAST include:

-   Host Computer System - To receive flight data and radar tracks from
    the ARTCC\'s HCS via an interface device.

-   Terminal Automation System -- To receive flight data, radar tracks,
    and controller entries from the Terminal Automation System via an
    interface device. This includes interfacing with the terminal color
    display hardware.

The additional external interfaces that are preferred for aFAST include:

-   Integrated Terminal Weather System (ITWS) - To receive low altitude
    wind and storm motion data from ITWS via an interface device.

-   Surface Decision Support Tools -- To exchange information with
    surface decision support tools in order to optimize the queuing of
    departures.

-   Other atmospheric data - To receive high altitude wind, temperature,
    and air pressure data as a function of position and altitude.

The aFAST system is divided into nine software modules: the Input Source
Manager (ISM), the Communications Manager (CM), the Weather Data
Acquisition Daemon (WDAD), the Weather Data Processing Daemon (WDPD),
the Route Analyzer (RA), the Profile Selector Active (PFSA), the
Trajectory Synthesizer (TS), the Timeline Graphical User Interface
(TGUI), and the PGUI. The ISM controls the flow of information from all
external ATC data sources, specifically the en-route Host Computer
System (HCS) and the Automated Radar Terminal System (ARTS) computers.
The CM controls the flow of information between the aFAST modules and
manages system start-up and shutdown procedures. The WDAD and WDPD
retrieve and process weather information for both ARTCC and TRACON
airspace. Weather information provided by RUC is sufficient for
scheduling aircraft. However, for tactical advisories such as aFAST,
higher resolution wind information is required. ITWS provides an
appropriate resolution of weather information. The RA generates and
analyzes predicted possible routes for each aircraft involved in the
scheduling process. The TS computes predicted 4D trajectories by
combining these aircraft routes, the current weather information, and
the aircraft's performance model. The RA, in conjunction with the TS,
produces 4D trajectories and ETAs spanning a range of possible paths for
each aircraft. From this set of possible flight paths for each aircraft,
the PFSA generates an efficient conflict-free schedule and the
corresponding advisories required to meet this schedule. The PGUI
provides the TRACON traffic managers with a comprehensive view of the
traffic control information being generated by aFAST and being displayed
to the TRACON controllers. Similarly, the TGUI provides the TRACON
traffic managers with the means to impose traffic management constraints
on the system, based on aFAST-generated information.

The PFSA module is the most significant difference between aFAST and
pFAST. In pFAST, there is a similar profile selector module (PFS).
However, PFS does not guarantee "conflict-free" trajectories. Passive
FAST schedules aircraft such that an efficient flow of traffic is
balanced between the adapted runways. In doing this, pFAST attempts to
ensure a conflict-free trajectory. In rare cases where the most
efficient plan has unresolved conflicts, pFAST will not attempt to
recalculate a new conflict-free plan. It is assumed that controllers
will be able to resolve individual conflicts by using speed, heading,
and altitude instructions. Active FAST is more tactical than pFAST.
Since speed and heading advisories are generated, safety requires that
these advisories be conflict free (in order to validate this, Active
FAST must be able to perform efficiently and conflict-free in a closed
loop environment). In addition, the selected software architecture
allows all aircraft trajectories and schedules to be updated within the
radar update cycle.

![](media/image7.wmf)

Figure 8 aFAST Software Architecture

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