**Introduction**

In June 2004, research teams at the NASA Ames and Langley Research
Centers conducted a joint human-in-the-loop simulation investigating the
feasibility and operational benefits of a free flight concept under
consideration by NASA's Distributed Air Ground Traffic Management
(DAG-TM) effort. The goal of DAG-TM is the expansion of airspace
capacity and, to this end, several concepts have been developed and
evaluated as part of this effort. The concept evaluated in this
simulation was called En Route Free Maneuvering. The premise was that
greater efficiency and capacity could be gained through a redistribution
of roles and responsibilities, and attendant decision-making, to the
aircraft operators (e.g., flight crews) and the air traffic management
system (e.g., air traffic controllers). The envisioned solution requires
new human-centered operational paradigms enabled by advances in decision
support tools: information sharing; communication, navigation, and
surveillance; air traffic management technologies; and procedures
supporting distributed separation responsibilities.

**Simulation Overview**

The simulation investigated the feasibility of an en route free
maneuvering concept with respect to traffic scalability and airspace
mixture. The free maneuvering concept assigned the en route autonomous
aircraft two tasks. First, autonomous aircraft had the task of
maintaining separation from other aircraft. Second, autonomous aircraft
arrivals were responsible for arriving at a TRACON meter fix at a
required time of arrival. Scalability refers to the ability to increase
the density of en route aircraft by adding autonomous aircraft, while
airspace mixture refers to the mixing of autonomous and managed aircraft
in the same airspace. Autonomous aircraft are aircraft flying according
to Autonomous Flight Rules (AFR) designed specifically for free
maneuvering aircraft, while managed aircraft are aircraft flying
according to standard Instrument Flight Rules (IFR) by which aircraft
fly today. The simulation was manned by a team of certified professional
controllers and commercial-rated pilots who operated within a simulated
airspace similar to the northwest portion of the Fort Worth Air Route
Traffic Control Center (ZFW). Traffic scenarios were created to simulate
realistic traffic flow into and out of the Dallas-Ft.Worth (DFW)
airport, and also overflights through the ZFW airspace. Varying traffic
volume between scenarios was accomplished by altering only the number of
overflights, and thus Scalability was manipulated only for en route
(non-arrival) flight. The arrival problem, while demanding, remained
relatively constant throughout all scenarios, but was impacted by the
increased overflights passing near and around the arrival stream.

The DAG-TM simulation environment was distributed across two NASA
facilities and several laboratories. At the Ames Research Center, the
Airspace Operations Laboratory (AOL) provided aircraft target generation
and ran the human-in-the-loop air traffic control (ATC) part of the
simulation. The AOL utilized professional air traffic controllers and
also provided "pseudopilots," who were given specialized tools that
allowed them to simultaneously control multiple background IFR aircraft.
The use of IFR pseudopilots was necessary because the volume of traffic
was such that it was impossible to assign one person for every aircraft
in the airspace.

AFR human-in-the-loop flight deck simulation and evaluations were
conducted at the NASA Ames Flight Deck Display Research Laboratory and
the NASA Air Traffic Operations Laboratory. Both centers were
responsible for gathering data on professional pilots serving as
participants. Also, as with the AOL, these laboratories provided
pseudopilots to control background AFR traffic.

The Ames and Langley AFR flight decks and pseudopilot stations were
different and flew in separate areas of the airspace. Only the Ames AFR
flight decks are examined in this report.

The entire scope of the DAG-TM En Route Free Maneuvering Simulation is
far too large to describe in the present conference report. Those
interested in greater detail are referred to the Joint NASA Ames/Langley
Joint Simulation Final Report (Raytheon ATMSDI Team, 2004). The present
report focuses solely on selected aspects the NASA Ames Flight Deck
simulation evaluations.

Roles and Responsibilities

The DAG En Route Free Maneuvering concept placed a large number of new
responsibilities on the AFR pilots, responsibilities that have
previously been reserved to air traffic control. First, and foremost,
the AFR pilots were assigned the responsibility for maintaining
separation from all IFR (ATC managed) aircraft. In addition, they were
responsible for maintaining separation from other AFR aircraft according
to a set of "rules-of-the-road" (c.f., Johnson, Canton, Battiste, &
Johnson, 2005). When responsible, or "burdened," AFR pilots were
required to resolve any impending loss-of-separation (LOS) prior to two
minutes to the predicted time of the LOS. In addition, when attempting
to resolve a conflict, AFR pilots were not allowed to create a predicted
LOS that was less than four minutes in the future. Some AFR flights were
solely overflights, but others flew arrivals. These AFR arrivals were
given the responsibility for meeting a required-time-of-arrival (RTA)
(+/- 15s) for the DFW Meter Fix BAMBE. The arrivals began the simulation
during cruise \~200 NM from BAMBE, and were assigned their RTA when they
were approximately 160 nm (\~20 min) from BAMBE. In addition to the RTA
restriction, AFR aircraft were required to cross BAMBE at 11000 ft (+/-
300 ft) and 250 kts (+/- 10 kts). If these crossing restrictions could
not be met, then the TRACON controller could refuse the AFR aircraft
permission to cross into the TRACON, or, at the controller's discretion,
new crossing restrictions could be given. At BAMBE, the status of
aircraft transitioned back to that of an IFR aircraft.

NASA Ames Flight Deck Implementation

The NASA Ames flight deck implementation for the DAG-TM En Route Free
Maneuvering work is based on the Airborne Management of En route
Separation Display (AMES Display). The AMES Display was developed
between 1995 and 2000 to support initial examinations of Free Flight by
NASA's Advanced Aeronautical Transportation Technologies Project
(Johnson, Battiste, Delzell, Holland, Belcher, & Jordan, 1997; Johnson,
Battiste, & Holland, 1999). The AMES Display was not only a display, but
was an integrated approach to the display, manipulation, and management
of flight path information required for free maneuvering in the en route
environment. As such, a free maneuvering concept was actually embedded
in its design, although the present DAG-TM work considerably surpasses
this initial work in scope and requirements. Nevertheless, important
design decisions made during the development of the AMES Display
continued to inform the DAG-TM work. Perhaps the most important of these
concerned the role of intent information.

[Intent Information]{.underline}: Free maneuvering responsibilities
could not be met using today's flight deck resources. Specifically,
conflict detection and resolution (CD&R) capabilities are required. In
order to predict the "when" and "where" of conflicts, it is necessary to
predict the flight paths of the surrounding aircraft, and this, in turn,
requires information about the future flight path. There are two general
approaches to this problem. One approach bases conflict predictions on
the broadcast of aircraft state information (current position and 3-D
velocity), or state information plus Mode Control Panel (MCP) commanded
intent (in this case, the location at which an aircraft automatically
levels off during a climb or descent also is broadcast). In other words,
this first approach bases predictions solely on current-state variables.
However, in order to insure detection of conflicts prior to a minimum
time to LOS, this approach also requires restricting an aircraft from
changing its flight path (i.e., state) until the pilot assures that it
will not create a near-term conflict with other aircraft. If a pilot was
trying to follow a flight plan using this approach, it would require the
flight crew to monitor and ensure the safety of maneuvers at every
trajectory change point. This could substantially increase the mental
workload of the flight crew. The second general approach is to share
intent information based on flight plans (e.g., broadcasting state plus
future trajectory change points in the flight plan). Recently, the
broadcast of partial flight plan information, in the form of the next
four trajectory change points, has been embraced by the RTCA (2002).
However, if flight intent information is represented by a finite number
of trajectory change points (e.g., as in the four points suggested by
the RTCA), the time and area represented by the information could vary
greatly (from representing a great amount of time/airspace to
representing very little amount of time/airspace). Thus new conflicts
could appear with little warning. Therefore, a different approach was
utilized in the development of the AMES Display. Specifically, the AMES
Display uses information regarding the *entire* flight plan. By
incorporating the entire flight plan, the information presented on the
AMES Display ensures (at the design level) that the flight crew has an
adequate look-ahead time and eliminates abbreviated flight plans as the
cause of alerts that "pop-up" with insufficient time to resolve.

[Flight Deck Tools]{.underline}: The tools that supported CD&R, along
with pilot situation awareness, were all integrated into a PC-based
Cockpit Situation Display (CSD) shown in Figure 1. This display was
built on the AMES Display and should be considered an extension of that
display. Many of these tools were also integrated into an aircraft
simulator based on the Multi-Aircraft Simulator (MACS) developed by the
NASA Ames Airspace Operations Laboratory (Raytheon ATMSDI Team, 2004).

The CSD was a 3D perspective display that allowed the pilot to display
the 4D flight plans of traffic, see traffic conflicts, and manipulate
the viewing angle. Graphical path replanning capabilities of the 3D CSD
were integrated with the Flight Management System (FMS), such that a
pilot could graphically design a conflict-free 3D route, then load and
execute it from the CSD interface. Thus all conflict resolutions and
replanned flight paths depended on these human-centered tools (see
Canton, Refai, Johnson, & Battiste, 2005, for a fuller discussion of the
CD&R implementation). In addition, the CSD also incorporated a tool for
the management of RTAs that allowed the pilot to enter, execute, and
monitor RTA conformance, all from the CSD. This display is described
more fully by Granada, Dao, Wong, Johnson, and Battiste (2005), and in a
users manual that can be found at:
<http://human-factors.arc.nasa.gov/ihh/cdti/download.html>.
![](media/image1.png){width="3.009027777777778in"
height="3.082638888888889in"}

The MACS simulator emulated a Boeing 757, and included an FMS emulation.
The pilot was able to use the FMS in the standard fashion to design new
routes, but these would show up on the CSD and be tested for conflicts.
If the CSD showed them to be conflict free, the pilot could then execute
these routes from the FMS. In addition to the FMS capabilities, the
pilots could take their aircraft off of the flight plan and onto a 3D
vector using the MCP. Again, the vector would be probed for conflicts,
with the pilot executing the vector only if there was at least four
minutes of clear path ahead of the aircraft. Finally, the system allowed
the pilots to use the FMS to recapture their flight plan by using the
FMS to take them directly to any subsequent waypoint in their flight
plan.

**Method**

Experimental Design

The experimental design was a comparison of four conditions (Figure 2).
These conditions compared four levels of AFR/IFR Scalability/Mixture in
the en route ZFW Center airspace. "T0" represents a traffic threshold
that approximates the current-day monitor alert parameter for the
simulated airspace. "T1" is a projected threshold above which
managed-only operations cannot be achieved. T0 and T1 levels were
determined in a prior controller-in-the-loop study at NASA Ames. "C1"
represents a high IFR traffic volume that can be handled using normal
ATC operations. "C2" is the same traffic volume but with 25% of the
aircraft AFR, and 75% IFR. "C3" maintained the same number of IFR
aircraft as in "C2", but added AFR aircraft to increase the total
traffic by an average of 60%, while in "C4" this increase was
approximately 100%. While these factors changed the density and mix of
traffic in the en route airspace, the rate/mix of arrivals into DFW
remained constant, with 20%-25% of them being AFR in conditions C2-C4.

This design allows two critical comparisons. First, by comparing C1 and
C2 we can determine if introducing mixed operations alone enhances or
degrades performance and workload/acceptability. Second, comparisons of
C2-C4 test scalability. That is, can AFR aircraft be used to increase
traffic load by approximately 50% and 100% without significantly
degrading overall operations?

Participants

Ten current or retired airline pilots flew nine Ames experimental
aircraft in the simulations. Eight flew desktop single-aircraft MACS
flight deck stations, and two served as captain and first officer in the
Advanced Concepts Flight Simulator (ACFS), a high fidelity
reconfigurable simulator with full window visuals. The total number of
flight hours for these pilots ranged from 4,500 to 22,000, with a mean
of approximately 11,000 hours. All the Ames pilots had glass cockpit
experience ranging from 85 to 8,000 hours, with a group mean of
approximately 4,000 total hours. All participants had previous
experience with the DAG-TM project, having participated in several
previous studies. ![](media/image2.wmf){width="3.0083333333333333in"
height="1.823611111111111in"}

Procedure

The simulation was conducted over eight days. The first two days were
used for orientation and final training. Initial pilot training had
taken place during two previous dry run and dress rehearsal simulations.
Four simulation scenarios per day were conducted on days 3-7, with day
seven reserved to replace scenarios if there were unanticipated problems
(there were none). The eighth day was used for extensive debriefing. On
each of the days 3-6, all of the four conditions (C1-C4) were presented.
Across these days, all eight single station pilots flew two arrivals and
two overflights for each of the four conditions. The two pilots in the
ACFS Simulator always flew arrivals. For half of the scenarios, the Ames
aircraft began in the ZFW Ardmore airspace, while on the other half they
began in the ZFW Amarillo High/Wichita Falls High airspace.

The overflights had no special requirements other than to maintain
separation from other aircraft. All of the arrivals began between 160 NM
and 200 NM from the BAMBE arrival meter fix. As the arrivals approached
160NM from BAMBE, they were given RTAs by ATC. These RTAs were designed
to require a substantial delay relative to the aircraft's present
estimated time of arrival (ETA) at BAMBE. This delay was large enough
that the pilots could not accommodate it through a speed change alone,
but required the pilots to modify their flight plans by "stretching"
them using their path replanning tools. The arrival aircraft were at
various en route altitudes above FL300, and reached their top of descent
approximately 80 NM from BAMBE. Data collection ceased for an aircraft
when the aircraft reached the BAMBE meter fix (at 11000 ft).

**Results and Discussion**

*Single Pilot AFR Meter Fix Conformance*: There was no significant
difference in objectively measured performance as a function of any of
the conditions. Therefore, from the Ames flight deck perspective,
neither mixed operations, nor increased en route traffic load had any
effect on the ability of the arriving flights to meet the meter fix
constraints. Among the AFR flights, one failed to meet the speed
constraint, and one failed to meet the altitude constraint, but all met
the RTA constraint.

*Single Pilot AFR Self-Separation Performance*: There were no LOS
incidents for the Ames AFR aircraft. There were 139 conflicts resolved.
Of these, 122 were detected prior to four minutes to conflict. The
concept requires that AFR aircraft detect conflicts at least four
minutes prior to the expected LOS, that no aircraft maneuver such that
they create conflicts with expected losses of separation under four
minutes, and that the AFR aircraft resolve conflicts prior to two
minutes to LOS. Figure 3 shows 17 late alerts (less than four minutes to
LOS). Consistent with the increased traffic load, the majority (11 late
conflicts) occurred in the C4 condition. All 17 were due to an aircraft
executing a maneuver which brought about the late alert. However, a
maneuver by an Ames single piloted AFR aircraft was responsible for
these late conflicts (concept violations) in only four instances.
Furthermore, in three of the four cases, this was due to a flaw in the
conflict resolution software (indicating to the pilot that the maneuver
was conflict free, when it was not). The reason for the remaining
apparent procedural error remains to be determined.

Figure 4 shows a total of four conflicts with late resolutions (under
two minutes to LOS), with three of them due to a software flaw in which
the conflict detection did not function properly. The fourth instance
was associated with the aforementioned incident where the pilot
maneuvered into a conflict at some point between two and four minutes to
LOS, with the result being that the pilot was not able to resolve the
conflict until there was less than two minutes until the projected LOS.

Figure 5 shows the amount of time needed to resolve conflicts. While not
an integral part of the concept, the Ames AFR pilots were asked to
resolve all conflicts within two minutes (120 sec) of receiving a
conflict alert. Figure 5 shows that this criterion was met approximately
80% of the time.

Finally, Figure 6 shows the number of times Ownship and Intruder
resolved the conflict as a function of burdening. While the IFR aircraft
almost never resolved a conflict (it was never burdened), it is
noteworthy that almost 1/3 of the non-burdened aircraft resolved the
conflicts in AFR-AFR conflicts.
![](media/image25.wmf){width="3.063888888888889in"
height="2.1958333333333333in"}

*Subjective Assessments*: The subjective assessment of pilot workload
was measured following each run using the Modified Cooper Harper (MCH)
workload scale. The MCH allows for ratings between 1 (Very easy/workload
insignificant) and 10 (Impossible/task abandoned, unable to apply
sufficient effort). Pilot responses across all simulation trials ranged
from 1 to 6. However, approximately 98 percent of responses ranged from
1 to 3. In order to receive a rating between 1 and 3, it must be
possible to complete the task, and workload must be perceived as
tolerable and satisfactory. Ratings from 4 to 6 suggest that task
workload is high but not high enough to impact performance on the
primary task.![](media/image26.wmf){width="3.290277777777778in"
height="2.2604166666666665in"}

Figure 7 shows the average workload ratings of arrivals and overflights
in each condition (±1 SD). Not surprisingly, workload ratings were
higher for the arrival flights than for overflights in all conditions.
Pilots also responded unanimously with the lowest possible workload
rating (1) for the managed overflight runs (C1). In addition, Figure 7
shows an increase in perceived workload from all managed (C1) to mixed
(C2-C4) operations for both flight types, but workload remained
acceptably low.

Nine of the ten pilots completed post-simulation questionnaires that
elicited their opinion of the viability of the DAG-TM Free Flight
concept and of the implementation. Figure 8 shows that the majority
preferred AFR (Free Flight) for Safety, Workload, Ease of Meeting RTA,
and Situation Awareness.

**Conclusions**

The results support the viability of the En Route Free Maneuvering
concept. Not only were the concept requirements met, but the subjective
data show that the implementation resulted in low workload, and a high
degree of pilot acceptance. While these results must be considered as
very preliminary, especially given the near perfect nature of the
information exchanged between aircraft. Also, the lack of disruptive
events (e.g., weather and emergencies) also limits the generality of the
study. ![](media/image27.wmf){width="3.0118055555555556in"
height="1.9909722222222221in"}![](media/image28.wmf){width="2.9694444444444446in"
height="1.7069444444444444in"}

That said, the excellent performance of the pilots, along with the low
workload, suggests that the design of the tools and procedures was very
successful, both in terms of software design and human factors design.
The high ratings given to situation awareness and ease in meeting RTA
also bear this out. That this was achieved while intimately involving
the pilot in the resolutions of conflicts, and in other critical
decision-making, shows the viability of a human-centered approach to
free flight.

**References**

Canton, R., Refai, M., Johnson, W. W., Battiste, V. (2005). Development
and integration of human-centered conflict display and resolution tools
for airborne autonomous operations. *Submission to the 2005
International Symposium on Aviation Psychology,* Oklahoma City, OK

Granada, S., Dao, A., Wong, D., Johnson, W.W., Battiste, V. (2005).
Development and integration of a human-centered volumetric cockpit
situational display for distributed air-ground operations. *Submission
to the 2005 International Symposium on Aviation Psychology,* Oklahoma
City, OK

Johnson, N. H., Canton, R., Battiste, V., and Johnson, W. W. (2005).
Rules of the Road: Requirements and Implementation for a Simulation of
En-route Self-Separation. *Submission to the 2005 International
Symposium on Aviation Psychology,* Oklahoma City, OK

Johnson, W. W., Battiste, V., and Holland, S. (1999). A cockpit display
designed to enable limited flight deck separation responsibility.
*Proceedings of the 1999 World Aviation Conference*, Anaheim, CA.

Johnson, W., Battiste, V., Delzell, S., Holland, S., Belcher, S., and
Jordan, K. (1997). Development and demonstration of a prototype free
flight CDTI. *Proceedings of the SAE/AIAA World Aviation Congress*,
Anaheim, CA.

RTCA. (1995). *Free flight implementation* (RTCA Task-Force 3 Report).
Washington, D.C.: RTCA Inc.

RTCA. (2002). *Minimum Aviation System Performance Standards for
Automatic Dependent Surveillance Broadcast*. RTCA/DO-242A. RTCA, Inc.
Washington, DC.

Raytheon ATMSDI Team. (2004). *Joint NASA Ames/Langley DAG-TM Simulation
Final Report* NASA Ames Research Center. Moffett Field, CA.