**Distributed Air / Ground Traffic Management En route Free
Maneuvering**

# Rules of the Road: Requirements and Implementation for a Simulation of En-route Self-Separation

Nancy H. Johnson

San Jose State University

NASA Ames Research Center

Riva Canton

Raytheon-QSS Corporation

NASA Ames Research Center

Vernol Battiste

Walter Johnson

NASA Ames Research Center

National Aeronautics and Space Administration's (NASA) Distributed Air
Ground Traffic Management (DAG-TM) program has recently investigated a
concept called "En-route Free Maneuvering" as a proposed solution for
expanding airspace capacity limits. A critical element for this concept
is conflict detection and resolution (CD&R) using the 3D cockpit
situation display (CSD). The only fielded system performing some of this
function is the Traffic Alert & Collision Avoidance System (TCAS), a
radar-based alerting system used by most commercial aircraft for
collision detection and avoidance. TCAS is inappropriate for an en-route
self-separation application due to its reactive nature, and inherent
lack of flexibility. Therefore, a new system was designed with improved
intent information in the form of 4D flight plans, broadcast and shared
amongst en-route aircraft, which in turn allowed these aircraft to
detect and resolve conflicts well in advance of a projected conflict. A
key element in this approach is ensuring that *burdening*, the
assignment of final responsibility for conflict resolution is clearly
assigned to the aircraft not in right-of-way. The basis for this
burdening is called the *rules-of-the-road* (ROR), a term taken from the
rules designed for guiding collision avoidance in VFR (visual flight
rules). Given the potential complexity of determining burdening
assignment, the automation described herein computes assignment using
these rules, and then notifies the crew if it has the right-of-way or is
burdened to resolve the conflict.

## Introduction

DAG-TM is a proposed solution for expanding airspace capacity limits. It
alters the roles and responsibilities of the stakeholders -- airlines
and air-traffic control - to permit more user-preferred routing,
increased flexibility, increased system capacity, and improved
operational efficiency. It is based on the fundamental premise that the
National Airspace System (NAS) participants can be information suppliers
and users, thereby enabling collaboration at all levels of traffic
management decision-making. The success of this proposed, future
environment may depend greatly on new human-centered operational
paradigms enabled by technological and procedural innovations (Raytheon
ATMSDI Team. 2004).

Air travel has advanced from an uncontrolled "see and avoid"
environment, to vastly increased numbers of aircraft tightly controlled
by ground facilities. As it moves on into the next generation, one that
supports an increase in capacity and free flight, distributed control is
suggested as a viable air traffic management (ATM) model. Distributed
control refers to the delegation of responsibility between the air
traffic service provider (ATSP) and the flight crew, defining whom
maintains separation assurance. In this environment, the old
rules-of-the-road that supported aircraft in a visual-only airspace will
no longer work effectively to ensure safety of flight. A new set of
rules for 'autonomous aircraft' that share separation responsibilities
in a free flight environment are required.

###### Current Operations

In current operations, often-used flight rules are virtually second
nature to the pilot who is being managed by air traffic control. Pilots
flying commercial class aircraft are less likely to need to refer to the
visual flight rules although there are rules that become ingrained much
in the same manner as a driver interacts with traffic laws while driving
a car. Specifically, visual flight rules are a set of regulations that a
pilot may operate under when weather conditions meet certain minimum
requirements. Under VFR, the pilot controls the attitude of the aircraft
by relying on what can be seen out the window, although this may be
supplemented by referring to the instrument panel. A pilot flying under
VFR is usually required to stay a specified distance away from cloud
formations and remain in areas where the visibility meets minimum
requirements. In VFR, the pilot is responsible for seeing and avoiding
other aircraft, terrain, and obstructions such as buildings and towers.
Being in contact with air traffic control is optional in most airspace,
and the pilot is usually allowed to select the course and altitude to be
flown based on VFR direction of flight and altitude rules
([http://www.fact-index.com](http://www.fact-index.com/)). The pilot may
also choose to navigate by reference to visual landmarks and/or utilize
electronic navigation aids. In a distributed control environment, the
pilot would also be responsible for maintaining separation from other
aircraft.

Current commercial transport operations utilize a collision avoidance
system called Traffic Alert & Collision Avoidance System (TCAS). TCAS
scans radar information of proximal traffic to determine distance and
closure rate. If TCAS detects that an aircraft's distance and closure
rate are potentially threatening to Ownship, it will generate a traffic
advisory (TA) or a resolution advisory (RA) to the crew. Both advisories
are displayed on the TCAS display screen and are accompanied by an
auditory alert. If necessary, TCAS will compute a pitch command to avoid
collision. At this time, TCAS is limited to vertical guidance and cannot
coordinate aircraft performance standards into the resolution
advisories. With TCAS II, the pitch commands are coordinated with the
other conflicting aircraft -- up to three - to avoid escape maneuvers in
the same direction.

An inherent problem with TCAS is that it is reactive and involves little
planning on the part of the pilot. A resolution advisory provides the
pilot with a 25-second response time before loss of separation (LOS).
TCAS logic does not incorporate flight path intent and as a result, when
crews respond to a TCAS RA, they are instructed to perform either a
vertical maneuver or to remain at current altitude. A suggested vertical
maneuver may send the conflicting aircraft off their intended flight
path and unexpectedly into another ATC sector. And, because TCAS does
not have flight path information, false alerts are frequent in busy,
high workload terminal areas.

The TCAS system has been a tremendous asset, however, the free flight
environment may be one in which the most effective and efficient
resolutions are based on *planned* maneuvers, which require information
about aircraft intent. Therefore, in the least, TCAS will require some
form of supplement. This supplement should alleviate the problem of
radical maneuvers by providing the crew with critical time-based
information and the ability to resolve potential conflicts without
drastic maneuvers off their published flight path. When a potential
conflict is presented to the crew in a timely manner, they can resolve
it by performing the necessary deviations to the flight path that do not
compromise flight safety or integrity.

## Simulation Environment

The goal of the NASA human-in-the-loop simulation was to investigate the
feasibility and operational benefits of a concept element (CE) under
consideration as part of the DAG-TM program: *CE 5 En Route Free
Maneuvering*. The work was completed as part of the Advanced Air
Transportation Technologies (AATT) project under NASA's Airspace Systems
program. The main purpose of *En Route Free Maneuvering*, is to reduce
excessive, en route trajectory deviations that result from separation
assurance and traffic flow management (TFM) conformance by distributing
the responsibility for separation assurance. An additional benefit of
distributing responsibility is that increases in capacity can be
realized without placing an added burdening on the ATSP (Raytheon ATMSDI
Team. 2004).

The simulation environment was distributed between NASA Ames Research
Center and NASA Langley Research Center using the Aeronautical Datalink
and Radar Simulator (ADRS) processor to link the facilities. The ADRS
functions as the communication management and data distribution hub
(Prevot, Palmer, Smith, Callantine. 2002). The DAG-TM airspace was a
modified portion of the airspace in and around Fort Worth Air Route
Traffic Control Center (ARTCC) (ZFW) and Dallas/Fort Worth TRACON.
Participants consisted of seven controllers and 20 licensed pilots. All
pilots were air-transport rated and had glass cockpit experience. All of
the controllers and 10 of the pilots were located at NASA Ames.

It was important to test this concept in a mixed-equipage environment
where some of the participating traffic would be under ATC control
(labeled IFR), and others as free flight (labeled AFR for autonomous
flight rules). All the AFR pilot-stations in the simulation were
equipped with a CSD with datalink capability. Based on the distributed
control model, if an AFR aircraft was in conflict with an IFR aircraft,
the IFR aircraft always maintained right-of-way, *except* when the ATSP
assumed verbal responsibility for conflict resolution. Because of the
strategic nature of most conflicts either one or both of the two
aircraft may maneuver for resolution, although only one will be
'burdened". This procedure allows maximum flexibility but assures that
only one aircraft has the *responsibility* for resolving the conflict.

It was expected that all aircraft would remain on their broadcast
(assigned) flight path during the simulation, only deviating if
commanded by an ATSP. In addition, all AFR aircraft were required to
implement only those flight plan changes that would not conflict with
the broadcast intent of any other aircraft, IFR or ARF, well beyond the
prescribed four minute to LOS window. All burdened AFR aircraft were
expected to resolve any predicted high level conflict -- notification of
a less than four minutes to LOS alert - at least two minutes prior to
LOS.

To assist the pilots with making route changes to resolve conflicts,
their flight path can be viewed and easily manipulated on the CSD
display. Future position over time can be shown with pulse predictor's
running along planned paths of travel. Pilots can also display traffic
in a 3D perspective view (Johnson, Battiste, Granada, Johnson, Dao,
Wong, Tang. 2005). The route analysis tool (RAT) allows the flight crew
to develop, evaluate, and implement potential flight plan changes
(Granada, Dao, Wong, Johnson, Battiste. 2005).

#### 

## Conflict Resolution with CSD Tools

In the DAG-TM studies, an aircraft operating as AFR in the en-route
airspace is allowed to free maneuver. This involves the pilot generating
user-preferred trajectory changes and instructing the aircraft's Flight
Management System (FMS) to initiate the trajectory. On-board automation
broadcasts the modified trajectory using Automatic Dependence
Surveillance-Broadcast (ADS-B) to the ATSP and other aircraft. The
flight crew has the responsibility to ensure that trajectory changes do
not generate near-term conflicts (less than four minutes to loss of
separation) with other aircraft. The CD&R provides predicted conflict
alerts that require the flight crew to respond accordingly, either
taking evasive action or allowing the intruder aircraft to maneuver
depending on which aircraft is burdened to resolve (Canton, Refai,
Johnson, Battiste. 2005). In contrast to the centralized air traffic
management rules that govern IFR operations, in a free flight
environment the rules are based on a distributed-control model that
references and resolves all potential aircraft conflicts by determining
right-of-way, ensuring the pilots participation in the decision-loop in
a timely manner.

Normally an aircraft operating in IFR conditions is under the control of
an ATSP at all times, with the ATSP retaining separation responsibility.
In this study, IFR flights were managed through voice and datalink
clearances provided by the ATSP with separation responsibility being
transferred to the AFR aircraft.

One critical component of any system where control and responsibility is
distributed is a set of operating rules that govern the activities of
all participants. However, as we move from centralized to distributed
roles and responsibilities, changes will be needed to govern and guide
interaction between air and ground operators. The distributed control
required for a free flight environment requires, among other things, the
successful implementation of new 'rules-of-the-road' (ROR) to accompany
the new information provided by a CSD and its suite of automation tools.

As participants in a distributed-control environment, the role of the
pilot changes. In addition to maintaining responsibility for flying the
aircraft, the pilot must now make decisions about the flight plan. For
example, pilots will have to ask themselves questions such as "Is my
route the most efficient for my aircraft? Is it conflict-free? Is it the
best route for meeting the assigned required-time-of-arrival (RTA)?" To
aid the pilot, many of these tasks have been automated with the
integration of the CSD into the cockpit. Since this new environment is
no longer one of "see and avoid" but one of complex, articulated flight
paths, the pilot can no longer apply simple flight rules to avoid a
conflict situation. Formerly, conflict avoidance was based on the
location of the aircraft when the conflict was detected, *not* the
location of the aircraft when the conflict may occur. So, a new set of
rules, based on the following guidelines were written:

-   remove any ambiguity about who is responsible for conflict
    resolution.

-   accommodate more complex route geometries.

-   reduce the likelihood of conflicts occurring and encourage a more
    organized environment (e.g. pilots abiding by the altitude rule so
    they reduce the likelihood of being burdened in a conflict).

-   if a conflict occurs, allow for a more efficient decision strategy.

-   assign responsibility *and* create accountability for conflict
    resolution.

The CSD rules-of-the-road are listed below and are referenced by the
computer automation in the order presented:

**- IFR aircraft have the right-of-way when in conflict with AFR
aircraft**, except when the ATSP has assumed verbal responsibility.

\- **Aircraft on a flight plan *always* have the right-of-way** when in
conflict with an aircraft that is off of its flight plan, "on a vector".
An aircraft is on a vector when its broadcast flight plan does not
include a destination airport

**- Altitude Rule:** Aircraft have the right-of-way when:

**A** - Traveling EAST (based on the magnetic compass of 0 - 179 deg)
and flying at an ODD altitude level.

**B** - Traveling WEST (based on the magnetic compass of 180 -359
degrees) and flying at an EVEN altitude level.

**An AFR aircraft is burdened if not flying correct direction for
altitude**. This rule provides natural separation between level east and
west bound flights, reducing the possibility of fast closing head-on
conflicts between AFR flights.

**- Left/Right Rule (when conflict angle is \> 20 degrees):** Aircraft
on the right at the point of conflict has the right-of-way during an
encounter between two aircraft when both are level, on ascent, or
descent paths.

**- Level Flight Rule:** Aircraft in level flight have the right-of-way
over a climbing or descending aircraft (regardless of heading).

**- Descend/Climb Rule:** Descending aircraft have the right-of-way over
climbing aircraft. The decision to provide priority to the descending
aircraft was to aid flight crews arriving into busy terminals to meet
ATM arrival constraints. 

**- Overtake Rule**: When the intercept angle between two conflicting
aircraft is less than 20 degrees (in other words, they are on the "same"
path), the lead aircraft has the right-of-way. 

***Note:** When none of the rules above apply to the conflict, Ownship
assumes responsibility for resolving the conflict. The above rules
should cover all possible conflicts, this failsafe rule was added to
provide an additional layer of safety. A final failsafe in the system is
TCAS.*

Safety of flight takes precedence over all rules.

In addition to the flight rules, the pilots were also assigned specific
"roles and responsibilities". Although not embedded in the automation,
it was expected they would be followed during the simulation:

-   Aircraft ***must*** maintain a minimum separation of 5NM and 1000ft
    vertical separation from *all* aircraft.

-   AFR aircraft ***must*** resolve all conflicts for which they are
    responsible at least two minutes before LOS. If unable to do so,
    they were asked to contact the ATSP for assistance.

-   AFR aircraft may not create flight plan changes that cause a LOS of
    less than four minutes.

-   The ATSP may verbally assume responsibility for separating an IFR
    aircraft from an AFR aircraft.

-   If the **ATSP** creates a predicted LOS that is within four minutes,
    the ATSP shall verbally assume responsibility for separating the IFR
    aircraft from the AFR aircraft.

Although the rules were consistent with those normally used by
controllers, they were new to the pilots who informally reported them as
cumbersome and difficult to remember. Also, some of the conflict
resolution logic may have been counter-intuitive to the pilots who are
accustomed to assessing conflict geometry at the time of conflict. Since
the rules needed to cover all possible conflict situations that might
occur during all phases of flight, reducing the rule set was not an
option. And, because it was not the intention of the system designers to
turn the pilots into controllers, automating the rule set seemed
reasonable.

![](media/image1.wmf){width="2.4661417322834644in"
height="1.7956692913385828in"} *burden settlement* advisory (Figure 1)
to each aircraft involved in the conflict. The burden settlement informs
the pilot which aircraft has been burdened with the responsibility to
modify their flight trajectory in order to maintain spatial separation
(5 NM lateral and 1000 ft vertical) from the conflicting traffic. Each
settlement is accompanied by a short phrase, displayed on the CSD,
citing the particular rule-of-the-road leading to the settlement
advisory. As stated earlier, with the burdening responsibility clearly
assigned, it was![](media/image2.jpeg){width="2.4993055555555554in"
height="3.071527777777778in"}

expected that the crew of the burdened aircraft would take immediate
action to resolve the conflict. Pilots were advised not to wait until
the LOS window was down to the critical two-minute warning to resolve
the conflict for fear that the un-burdened aircraft may feel obligated
to resolve the conflict due to the short time to LOS and both resolve
simultaneously towards each other. Also, waiting to resolve the conflict
could result in a less-optimal solution, increasing the probability that
the aircraft, with now limited solutions could resolve by creating a new
conflict with another aircraft. If conflicts occurred between multiple
aircraft, the burdening logic considered the most eminent conflicting
pair by time to LOS, and assigned burdening to one. Once the initial
conflict was resolved, the next conflict was considered and a new
burdening assignment given. This process continued until all of the
potential conflicts were resolved in a timely manner.

Figure 2 shows two AFR aircraft on conflicting routes, one aircraft with
an articulated flight path. At the *detection* of the conflict, Ownship
is on the right of the conflicting aircraft but at the point of expected
LOS, Ownship is on the left and therefore burdened to resolve the
conflict. Assessing each conflict situation based on flight path intent
and burdening logic does not change over time, or as a function of when
the conflict is detected, the rules correctly determine the burdened
aircraft at the point of conflict.

###### Conclusion

After the completion of the simulation, the Ames and Langley pilots
participated in separate debrief sessions. The ten Ames pilots were
asked in a post-simulation questionnaire, if they thought the
rules-of-the-road were "clear and easy to understand", ninety percent
(9/10) of the pilots responded 'yes'. When asked if they ever had to
mentally reference the rules-of-the-road, forty percent (4/10) responded
that they had. Noted observations cited the pilots trying to figure out
who would be burdened in a potential conflict before the automation
responded with a burdening statement.

The pilots were also asked if the rules were adequate for the mixed
equipage (IFR and ARF) environment they were presented, seventy percent
(7/10) felt the rules were adequate. The pilots also appreciated that it
was not necessary for them to remember the rules; when the rules were
needed, the automation supplied them. Our data indicates that the crews
were able to resolve all conflicts before any loss of separation events
were recorded.

Although the CD&R tools allow ample time to resolve conflicts before the
four-minute window, it could be problematic if multiple aircraft are
maneuvering under the four-minute window or maneuvering and creating
conflicts with less than four minutes to LOS. This circumstance could
possibly lead to decisions that create additional conflicts of less than
four minutes and therefore resolutions that are less than optimal. To
resolve this, perhaps rule-based cooperative strategies for resolution
of near-term conflict (those under four minutes to LOS) should be
explored. It also may be the case that the four-minute window should be
expanded to six minutes. Further research is needed to determine which
of these solutions is appropriate.

It is also a plus for this application that the burdening solution is
unique to the burdened aircraft; only one aircraft needs to respond with
a flight change unlike TCAS which requires both aircraft to respond.
This reduces the possibility of both aircraft responding in a manner
that jeopardizes the safety of flight. It should be noted that there
were no case in which both pilots of conflicting aircraft acted to
resolve the conflict, and there were no instances of competing
maneuvers. These performance results, in a simulated free flight
environment, suggest that the aforementioned rules-of-the-road can
adequately support self-separation. Maintaining separation requires
earlier responses than the current-day collision avoidance tools, and
therefore a system such as the one described herein may be necessary to
support free flight.

## 

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