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COST-BENEFIT STUDY OF FREE FLIGHT WITH
AIRBORNE SEPARATION ASSURANCE
Mario S.V. Valenti Clari1
Rob C.J. Ruigrok2
Jacco M. Hoekstra3
National Aerospace Laboratory NLR, P.O. Box 90502, 1006 BM Amsterdam, The
Netherlands
Hendrikus G. Visser4
Delft University of Technology, P.O. Box 5058, 2600 GB, Delft, The Netherlands
Presented as Paper 2000-4361 at the AIAA Guidance Navigation and Control Conference, Denver, CO, 14-17
August 2000. Copyright © 2001 by National Aerospace Laboratory. Published by the Air Traffic Control
Quaterly, with permission.
1 M.Sc., Research Engineer at Flight Simulation Department, [email protected]
2 M.Sc., Research Engineer at Flight Mechanics Department, [email protected]
3 Ph.D., Research Engineer at Human Factors Department, [email protected]
4 Ph.D, Lecturer at Faculty of Aerospace Engineering, [email protected]
2
ABSTRACT
The research presented in this paper is based on the Free Flight with Airborne Separation Assurance concept
that has been developed for the 1997 NASA Free Flight project, conducted at the National Aerospace
Laboratory NLR in Amsterdam. As a follow up to the 1997 project this paper first focuses on the issues of fuel
and time efficiency of the conflict resolution manoeuvres in Free Flight using the Airborne Separation
Assurance concept as a baseline. An analysis is made of the fuel and time efficiency of the possible conflict
resolution manoeuvres (heading change versus altitude change) on a small scale by conducting so-called one-
on-one simulation experiments.
The next step is a cost-benefit analysis of Free Flight on a large scale by simulating a mixed-equipped
traffic environment over a specified area in European Airspace. These large-scale Monte-Carlo simulation
experiments have been set-up with Free Flight (equipped) traffic flying direct routes and non-equipped traffic
flying along specified ATC routes to their destinations. The analysis is aimed at getting more insight in the costs
and benefits of direct routing combined with the airborne separation assurance responsibility as is assumed in
the NLR concept for Free Flight.
INTRODUCTION
Due to the continuous growth of air traffic over the last decades, the current Air Traffic Control (ATC) system is
reaching its flow capacity limits. The removal of constraints upon traffic flow could allow a more efficient user-
preferred routing and the removal of all constraints may eventually lead to realisation of a Free Flight Air
Traffic Management (ATM) system. Free Flight has been proposed as a new concept for a future ATC system
that can cope with the ongoing congestion of the current system and, moreover, has the potential to offer great
economic benefits. The Radio Technical Commission for Aeronautics (RTCA) states that any activity that is
aimed at removal of constraints can be regarded as move towards Free Flight (RTCA,1995). But the questions
remains: "what is Free Flight really?". The research presented in this paper will assume that the removal all
constraints of current ATC will imply also a complete shift of the separation assurance responsibility from the
ground to cockpit. In other words, pilots will not only be allowed to freely select and fly their routes but will
also have to perform additional tasks related to separation assurance; this ultimate form is often referred to as
Free Flight with Airborne Separation Assurance.
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FREE FLIGHT WITH AIRBORNE SEPARATION ASSURANCE
In 1997, the National Aerospace Laboratory (NLR) in Amsterdam started a project to study the human factors
issues of Free Flight in co-operation with NASA, the Federal Aviation Authority (FAA) and the Dutch Aviation
Authorities (RLD). This project was initiated as part of the Advanced Air Transportation Technologies program
(AATT) and has ever since been related to the Distributed Air-Ground Traffic Management (DAG-TM) work
co-ordinated by NASA. The first two years of the project encompassed off-line simulations to define a base-line
Free Flight concept, an ATM safety analysis of the concept and a Human-in the-Loop simulation experiment to
investigate the impact of this new concept on human factors. The studies resulted in the development of a
concept of Free Flight with Airborne Separation Assurance (van Gent et. al., 1997 & 1998), moreover, the
concepts were implemented in a real-time simulation environment. This developed Airborne Separation
Assurance System (ASAS) consists of the conflict detection, resolution and prevention modules (CDR&P) and
Cockpit Displays for Traffic Information (CDTI).
The overall conclusion of the studies was that the feasibility of Free Flight with Airborne Separation Assurance
could not be refuted if all aircraft were fully equipped with proper conflict detection and resolution tools.
Nevertheless, as with all research, the experiments also raised some key questions. For example, it was noticed
during the experiments that pilots preferred to resolve conflict situations by manoeuvring horizontally; meaning
they preferred executing a heading (track) change above executing an altitude or speed change to resolve
conflicts with other aircraft. This is somewhat remarkable because, when heading is used to resolve a conflict,
the aircraft will often need to manoeuvre more than when using an altitude (vertical speed) change.
In the experiment debriefings, pilots explained that they avoided vertical manoeuvres because they thought it
would have a negative impact on both,
the fuel efficiency of the flight (economic aspects)
the passengers perception of the ride quality (passenger comfort aspects)
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The option of using speed changes for conflict resolution was used even more seldom, because pilots thought
that the available (operational) "speed window" in cruise flight would not allow this kind of conflict resolutions.
It was concluded that an additional study was needed to give more insight in the costs and benefits of the
conflict resolution manoeuvres (heading change, altitude change and speed change) in Free Flight with Airborne
Separation Assurance.
In order to obtain more insight in the issues raised in the Human-in-the-Loop experiments, NLR started in 1998
a preliminary cost-benefit study of Free Flight with Airborne Separation Assurance in co-operation with Delft
University of Technology (DUT). The benefit study can be divided into two major parts. The first part deals
with a study of the costs and benefits of the conflict resolution manoeuvres on a small scale. The second part
zooms out, in order to compare a full scale Free Flight environment with an ATC environment like today. This
large-scale analysis is aimed at getting more insight in user-preferred routing costs and benefits of the airborne
separation assurance concept. Before these analyses will be discussed, a brief overview will be given of the
concept for Conflict Detection and Resolution (CD&R), which is an important part of a future ASAS and a base
line for the results presented in this paper.
CONFLICT DETECTION
A conflict is defined as an actual or potential intrusion of a protected zone in the near future. The task of a
conflict detection module is to predict such an intrusion of the protected zone of the own aircraft by other
aircraft (intruders). The protected zone is currently defined by enroute ATC standards as a circular zone of 5
nautical mile radius and a height of 2000 ft (altitude - 1000ft to + 1000ft). The protected zone is also often
referred to as Protected Airspace Zone (PAZ), which is a definition that originates from the RTCA. This paper
will use the wording protected zone, because it is inline with earlier publications on the same subject, but there
is no difference with the PAZ as defined in various other publications.
As part of an aircraft's ASAS system the conflict detection module only detects conflicts with aircraft for which
the intrusion of the protected zone takes place in the near future. The near future is defined by using of a fixed
look-ahead time of five minutes. In this way an alert zone is created, dependent on the aircraft’s airspeed and
direction of flight.
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The conflict detection module uses the current state (position and altitude) and trend vector (ground speed, track
and vertical speed) to detect conflicts. A new conflict is detected when, an intrusion of the protected zone is
predicted, and if the time of this intrusion is within the look-ahead time. The conflict information from the
detection module can be presented graphically to the crew by using the CDTI.
CONFLICT RESOLUTION
Once a conflict is detected it must also be resolved. Over the years various methods for resolving conflict
situations have been proposed. Some methods use force field techniques, others use genetic algorithms, or rule
based methods, or optimisation techniques; a clear overview of all the different methods for approaching a
conflict situation is given in (Kuchar and Yang, 1997).
For conflict resolution, the NLR ASAS uses the so-called Modified Voltage Potential (MVP) concept. The MVP
is based on algorithms presented in a publication of the Massachusetts Institute of Technology, Lincoln
Laboratory (Eby,1994). The MVP uses this force field algorithm for a geometric approach of a conflict
situation. Figure 1 gives an illustration how the concept works.
Figure 1 Modified voltage potential resolution method
When the conflict detection module has predicted a conflict with traffic, the resolution module uses the
predicted future position of the own aircraft (ownship) and the obstacle aircraft (intruder) at the moment of
6
minimum distance. The minimum distance vector is the vector from the predicted position of the intruder to the
predicted position of the ownship, the closest point of approach (CPA).
The avoidance vector is calculated as the vector starting at the future position of the ownship and
ending at the edge of the intruder’s protected zone, in the direction of the minimum distance vector. The length
of the avoidance vector is the amount of intrusion of the ownship in the intruder’s protected zone and reflects
the severity of the conflict. It is also the “shortest way” out of the protected zone.
The ownship should try to accomplish this displacement in the time left till the conflict; the loss-of-
separation time. Dividing the avoidance vector by the time left yields a speed vector that should be summed to
the current speed vector. The result is an advised track (heading/track change) and ground speed (speed change).
Using the three-dimensional vector an advised vertical speed (altitude change) is calculated also. In case of
multiple conflicts within the look-ahead time, the avoidance vectors are summed.
FREE FLIGHT SIMULATION TOOL: TRAFFIC MANAGER
The above-described concepts for CD&R have been tested using a tool for simulating air traffic environments,
called the Traffic Manager (TMX). With the TMX it is possible to generate a traffic environment with various
aircraft types. Both automatic and interactively controlled traffic can be generated by TMX.
For the simulation of the Free Flight traffic the TMX uses six-degrees-of-freedom models containing
auto-pilot, auto-throttle functionality, route guidance functionality and a pilot model. TMX uses the Eurocontrol
Base of Aircraft Data (BADA) aircraft performance data (Bos, 1997) and additional data from other sources for
the simulation different aircraft types. The pilot model includes a delayed reaction to conflict resolution
advisories and a delayed resuming of navigation to the aircraft’s destination, once a conflict is solved. The
resolution advisories from the conflict detection and resolution algorithms are taken over by the pilot models,
thus controlling the auto-pilot to resolve the conflict. The TMX formed the simulation tool for the research
presented in this paper.
COST-BENEFIT STUDY OF CONFLICT RESOLUTION MANOEUVRES IN
FREE FLIGHT
7
As a first step in understanding the economic aspects of conflict resolutions manoeuvring, several one-on-one
conflict situations were tested with respect to fuel and time efficiency. The one-on-one conflicts were simulated
with horizontal (heading change) and vertical resolutions (altitude change) in such a way that results could be
compared. This section deals with the set-up, results and issues of these experiments.
EXPERIMENT SET-UP
The aim of the experiments was to compare the horizontal conflict resolution (heading change only) with the
vertical conflict resolution in several one-on-one conflicts. The method used for the experiments is based on the
idea of choosing the position of a large number of experiment points in the protected zone of an intruder aircraft.
Each experiment point represents a minimum distance point for a conflict that will occur during an experiment.
The minimum distance point (i.e. the CPA of a conflict) is the most important parameter for the conflict
resolution module because it indicates the maximum amount of intrusion (0 – 5 nm). The experiment points for
the horizontal and vertical conflict experiments have been chosen as shown in Figure 2a/b.
5 nm 1 nm
a) Top View
Horizontal Experiment Points
b) Side View
Vertical Experiment Points
5 nm
10
00
ft
20
0 f
t
situation line a
line c
line d
vertical scale
exaggerated
line b
Figure 2 Predefined experiment points in horizontal and vertical plane
The points are defined for various amounts of intrusion with a 200 ft interval. For the horizontal resolutions, the
amount of horizontal intrusion is chosen with a 1 nm interval. For the vertical resolution method, only the
amount of vertical intrusion will be important, and for the horizontal resolution method only the horizontal
8
amount of intrusion will be important. This subdivision facilitates the task of comparing the two resolution
methods.
Horizontal Conflict Experiments
The general experiment set-up has been chosen as follows. Each experiment starts with two aircraft flying with
constant speeds and altitudes according a predefined scenario. One of the two aircraft will be regarded as the
experiment aircraft (ownship) and the other is the intruder aircraft; see for example Figure 3a.
destination
ownship
Experiment Area
b) Conflict Resolved
ownship direct route to
destination
start of conflict resolution
manoeuvre
initial position
ownship
initial position
intruder
situation d
reference flight track ownship
flight track intruder
situation d
destination
ownship
Experiment Area
a) Conflict Detected
Figure 3 Example experiment situation
9
The flight path of the own aircraft is a direct flight over 120 nm to a destination on the edge of the experimental
area. Each experiment stops when the experiment aircraft exits the experimental area. The initial position of the
intruder is chosen in a way that when a conflict is first detected during flight, the associated CPA is located at a
desired experiment point in the protected zone of the intruder.
For this purpose, the horizontal experiments have been arranged in four initial experiment situations.
All experiment situations are related to the position of the predefined points in the protected zone of the intruder
aircraft. The points are chosen on four lines (a,b,c and d) as illustrated in Figure 2a. The experiment points on,
for example, line d are related to the initial experiment situation d, which is illustrated in Figure 3.
When a conflict is detected the ownship will manoeuvre in order to resolve the conflict. The intruder will hold
his track without manoeuvring; so the own aircraft will completely have to resolve the conflict (non-nominal
case). When the conflict is resolved the aircraft will hold its flight track until it is safe to direct back to the
destination (Figure 3b).
Vertical Conflict Experiments
Vertical conflict experiments have been executed with a similar set-up as the horizontal conflict experiments. In
all the tests, the intruder aircraft was on a head-on collision course with the ownship, because only the vertical
amount of intrusion needed to be varied.
Nevertheless, only the ASAS conflict detection module (not the resolution module) has been used for
the execution of the vertical experiments; a standard altitude change procedure has been used for the vertical
conflict resolutions. The reason for this approach is to focus on the efficiency of manoeuvres, as pilots would
execute in Free Flight with airborne separation assurance. For the relevance of the study it was decided to
implement a procedural approach of resolving the vertical resolution manoeuvres in which the ownship has been
assigned to resolve all conflicts with:
a climb/descent with constant Mach number
a level-off altitude of 100ft above/below the intruder aircraft’s protected zone
a fixed vertical speed of 600ft/min
10
The vertical manoeuvre is illustrated in Figure 4.
1000 ft
1000 ft
100 ft
100 ft
min.dist
protected zoneintruder
climb600 ft/min
intruder
ownshipreference flight track
ownship
descent600 ft/min
Figure 4 Vertical conflict resolution manoeuvre
The ownship is assigned to return to the original altitude after waiting a predefined time interval when the
conflict has been resolved.
Aircraft performance validation
All results presented in this paper have been generated with a medium range twin-engine aircraft. The
performance of the used BADA aircraft model has been validated (Valenti Clari, 1998) by comparing it with the
much more sophisticated simulation model of the same aircraft used in NLR’s Research Flight Simulator (RFS)
and a comparable performance model of Delft University of Technology (DUT). The validation focussed on
fuel consumption for level changes in cruise flight, and for climb/descents to/from various cruise altitudes.
RESULTS
The complete experiment matrix of the one-on-one experiments consisted of
44 vertical resolution experiments, consisting of:
- 11 descents at FL200 and FL300
- 11 climbs at FL200 and FL300
88 horizontal resolution experiments (4 situations of 11 points at FL200 and FL300)
2 reference flights (without manoeuvring)
11
Vertical Conflict Experiments
The results for the climb manoeuvres at FL300 are presented in Figure 5 and Figure 6.
Vertical Resolution Method (Climb)
Flight Profiles
29500
30000
30500
31000
31500
32000
32500
0 20 40 60 80 100 120 140
Ground distance [nm]
Alt
itu
de
[ft
]
intrusion: +0 ft (c999)
intrusion: +200 ft (c800)
intrusion: +400 ft (c600)
intrusion: +600 ft (c400)
intrusion: +800 ft (c200)
intrusion: 1000 ft
intrusion: -800 ft (c_200)
intrusion: -600 ft (c_400)
intrusion: -400 ft (c_600)
intrusion: -200 ft (c_800)
intrusion: -0 ft (c_999)
c_999
c400
c200
c000
c_200
c_400
c_600
c_800
c999
c800
c600
M = 0.70
ISA conditions
no wind
reference altitude: 30,000 ft (FL300)
Figure 5 Flight paths for Climb manoeuvres at FL300
Vertical Resolution Method (Climb)
Comparison with Reference
-10 -5 0 5 10
-0 ft (vc_999)
-200 ft (vc_800)
-400 ft (vc_600)
-600ft (vc_400)
-800 ft (vc_200)
INTRUSION 1000ft (vc000)
800 ft (vc200)
600 ft (vc400)
400 ft (vc600)
200 ft (vc800)
0 ft (vc999)
Exp
eri
men
t p
oin
t [-
]
Compared to reference [%]
Flight time
Fuel consumed
M = 0.70
ISA conditions
no wind
reference altitude: 30,000 ft (FL300)
Figure 6 Fuel burned and time used compared to the reference flight at FL300
12
Horizontal Conflict Experiments
The results for the situation a at FL300 are presented in Figure 7 and Figure 8.
Horizontal Resolution Method (Situation a)
Flight Tracks
-25
-15
-5
5
15
25
0 20 40 60 80 100 120 140
distance [nm]
dis
tan
ce
[n
m]
intrusion: 0 nm (a050)
intrusion: 1 nm (a040)
intrusion: 2 nm (a030)
intrusion: 3 nm (a020)
intrusion: 4 nm (a010)
intrusion: 5 nm (a000)
intrusion: 4 nm (a_10)
intrusion: 3 nm (a_20)
intrusion: 2 nm (a_30)
intrusion: 1 nm (a_40)
intrusion: 0 nm (a_50)
30,000 ft (FL300)M = 0.70
ISA conditions
no wind
Note: scales are not equal
a_10
a_20
a_30
a040
a020
a050
a_50
a030
a_40
a000
a010
Figure 7 Flight tracks for situation a at FL300
Horizontal Resolution Method (Situation a)
Comparison with Reference
-10 -5 0 5 10
0 nm (a050)
1 nm (a040)
2 nm (a030)
3 nm (a020)
4 nm (a010)
INTRUSION 5 nm (a000)
-4 nm (a_40)
-3 nm (a_30)
-2 nm (a_20)
-1 nm (a_10)
-0 nm (a_50)
Exp
eri
men
t p
oin
ts [
-]
Compared to Reference [%]
Flight Time
Fuel Consumed
M = 0.70
ISA conditions
no wind
30,000 ft (FL300)
Figure 8 Fuel burned and time used compared to reference flight (situation a)
13
DISCUSSION
When analysing the results it should be clear that the aim of the experiments is to get a better understanding of
the economic aspects of the resolution manoeuvres.
One of the most determining factors is expected to be the type of aircraft, because optimal flight issues are very
dependent of the type of aircraft. All experiments have been conducted with a simulation model that estimates
the behaviour of a medium range twin-engine civil aircraft. Another factor that could influence the performance
is the environmental condition (e.g. wind and weather).
When analysing and comparing the fuel consumption of all experiments it is clear that in one case the
experiment aircraft saves fuel with respect to the reference flight over the defined trajectory. This occurs when
the aircraft performs a vertical climb to resolve the conflict; as illustrated in Figure 5 and 6. Figure 6 shows that
for all experiment points (different intrusions in the protected zone of the intruder) the total fuel consumed is
less than the reference value. The low points in the protected zone show the biggest gain. This makes sense
because, for these low intrusions, the experiment aircraft has to perform a high altitude step in order to resolve
the conflict; bringing it to a more fuel efficient cruise level. This implies that after performing the altitude step it
would perhaps be even more efficient to remain at the higher level. Of course the distance to destination also
influences this decision.
When assuming a constant Mach number (and flight in the troposphere), the true airspeed (and also the
ground speed) will decrease with the increasing altitude. This means that the aircraft will arrive later on its
destination, which can also be observed from Figure 6. The amount of time lost is however very small; in the
order of a few seconds for the experiment flight over a distance of 120nm.
The vertical climb resolutions are very promising when regarding the fuel consumption figures. However, there
are some issues that could seriously constrain the vertical resolution manoeuvre. It is likely to assume that pilots,
when they are given the user-preferred routing possibility, will perform the cruise flight as close as possible to
the operational ceiling of the aircraft; especially on long routes. When the pilot wants to perform a climb in
order to resolve a conflict, the aircraft could be constrained by the operational ceiling. Other aspects, like the
14
influence of the engine spool-up noises (e.g. when performing climbs near the operational ceiling) on the
passenger comfort, could also pose a constraint on climb manoeuvres.
So, assuming for the moment that the climb manoeuvre is not always an option, this leaves the vertical
descent manoeuvre and the horizontal heading change as the possibilities to resolve the conflict. A trade-off can
be found between the advantages and disadvantages of these manoeuvres.
This trade-off between the horizontal heading change manoeuvre and the vertical descent manoeuvre
can be combined with MVP concept for the set-up of a decision model. Figure 9 illustrates the decision with the
protected zone of the intruder subdivided in zones for different resolution options.
Climb
DescentHeading Heading
R = 5 nmr2 = 4 nm
r1 = 1.5 nm
Side View Protected Zone
Figure 9 Decision model for resolution method
If the initial position of the minimum distance point is located in the upper half of the protected zone, the
vertical climb resolution is the more optimal manoeuvre. The figure illustrates in the lower half of the protected
zone the trade-off between the horizontal (heading change manoeuvre) and the vertical descent manoeuvre. The
vertical climb manoeuvre is in the lower half of the protected zone not an option, because it would go against
the MVP concept.
It can be concluded that the use of the vertical resolution method is not as bad for the fuel consumption as
thought by some of the pilots who participated in the 1997 Human-in-the-Loop experiments. The vertical climb
manoeuvre could even lead to a more efficient flight operation. However, if the climb manoeuvre is not possible
the geometry of the conflict (the position of the minimum distance points in the protected zone) can be used to
determine what is better: a descent manoeuvre or a heading change.
15
The next section will try to analyse a more general Free Flight environment in which aircraft fly their routes
from airport to airport.
MONTE-CARLO FREE FLIGHT EXPERIMENTS WITH MIXED-EQUIPPED TRAFFIC
This section will present the set-up, issues and results from Monte-Carlo simulations of a full-scale Free Flight
traffic environment. The simulations have been executed in an area of European airspace with ASAS equipped
traffic flying in the same area as not-equipped traffic, flying along specified ATC routes from airport to airport.
The ultimate goal of the experiments was to find out if the “benefits” of direct routing outweighs the “costs” of
conflict resolution manoeuvres related to the airborne separation assurance concept.
MIXED EQUIPPED TRAFFIC CONTROL ENVIRONMENT
Free Flight has been proposed primarily for future application because it has the potential to cope with the
ongoing congestion of the current ATC system. Besides the foreseen increase in airspace capacity, Free Flight
could also offer great economic advantages by eliminating the costs related with the fuel wasted when flying on
non-direct ATC routes.
On the other hand Airborne Separation Assurance implies that aircraft will be responsible for their own
separation assurance. It is not yet clear to what extent the occurrence of conflicts will influence the fuel
consumption on a global level. Furthermore, the advantage of direct routing in Free Flight may be eliminated
when a great number of conflicts have to be resolved or if safety is jeopardised
The Human-in-the-Loop experiments showed that the number of conflicts that occurred in a real-flight scenario,
in an above-nominal traffic density, was not very high (average: 3 conflicts/hour in densities that are three times
the Western European traffic density in 1996, Hoekstra et. al.,1998). These experiments were executed in a full
Free Flight environment (all aircraft equipped with ASAS). The question remains if the number of conflicts will
remain low in a mixed-equipped traffic environment.
In order to compare Free Flight (direct routing traffic) with current ATC, the Monte-Carlo Free Flight
experiments simulate “normal” ATC traffic flying specified routes from airport to airport; flying in the same
16
airspace as the Free Flight traffic; which results in a so-called “mixed-equipped” traffic environment. In this
environment the traffic equipped with ASAS is not only responsible for separation assurance with the other Free
Flight traffic, but will also have to avoid the “normal” traffic flying on the ATC routes.
The conflict resolutions during the experiments were executed using NLR’s Airborne Separation
Assurance concept. The decision module, based on the results from the one-on-one experiments, was also used
in the Monte-Carlo simulation set-up. The module decided for each conflict, which manoeuvre would be best to
execute.
EXPERIMENT SET-UP FOR MONTE-CARLO LIKE SIMULATIONS
In order to simulate (using the TMX) a “realistic” mixed-equipped traffic environment, experiments have been
set-up over a specified area in (virtual) European airspace. An experimental area (440 nm x 360 nm) was
defined that included the following four European airports:
1. Amsterdam Airport Schiphol in the Netherlands (EHAM)
2. Frankfurt/Main Airport in Germany (EDDF)
3. Paris Charles-de-Gaulle Airport in France (LFPG)
4. London Heathrow Airport in England (EGLL)
Twelve ATC routes have been defined connecting these experiment airports, using official Jeppesen European
route charts and navigation database. The routes were defined from one Terminal Area (TMA) to the other,
because the terminal area manoeuvrings were not included in the experiments. Circular TMAs with a radius of
30nm were assumed for all airports. Six additional airports outside and one airport inside the experimental area
were defined for a realistic mix of high cruising, departing and arriving traffic.
All simulations were executed using the automatic traffic scenario generation functionality of the TMX, which
was also used to generate scenarios for the Human-in-the-Loop experiments. Using this functionality the TMX
can be assigned to constantly generate traffic departing from airports and en-route entering-points (traffic
sources). When an aircraft is generated at an airport, it will enter the traffic environment at a predetermined
point on the edge of the TMA. The location of this point depends on the direction of the destination with respect
17
to the origin. The departure altitude is randomised for each individual aircraft within the range of 5000 to 7000
ft. The destination of each generated aircraft is randomly selected from a user-determined list of airports. In
order to obtain a realistic mixed-equipped traffic environment the generation of aircraft during the experiments
was split in the generation of Free Flight traffic (equipped with ASAS) and the generation of non-equipped ATC
traffic, flying the defined routes from origin-TMA to destination-TMA. The selected experiment airports
(EHAM, EGLL, EDDF and LFPG) generated a specified percentage of ASAS equipped aircraft. All other
airports only generated Free Flight traffic. This implied that the traffic environment mainly consisted of Free
Flight traffic. The mix was only applied to the direct/ATC routes between the experiment airports.
EXPERIMENT MATRIX
All the simulation experiments were executed assuming that a ground ATC ideally controlled the non-equipped
traffic. When conflicts (and intrusions) did occur, they were ignored. In other words: the ATC traffic only
followed the defined route from origin to destination, without performing any conflict resolution manoeuvres.
When analysing the results this must be kept in mind, because it implies that the fuel consumption of the ATC
traffic will be optimistic; in a real flight situation ATC traffic will rarely fly routes without extra manoeuvring or
restrictions for the avoidance of conflicts.
The experiments have been executed for five different levels of equipment percentage on the routes:
1. 0% equipped with ASAS; 100% not-equipped; (complete ATC environment)
2. 25% equipped with ASAS; 75% not-equipped;
3. 50%equipped with ASAS; 50% not-equipped;
4. 75% equipped with ASAS; 25% not-equipped;
5. 100% equipped with ASAS; 0% not-equipped (complete Free Flight environment)
EXECUTION OF SIMULATION EXPERIMENTS; AUTOMATIC SCENARIOS
Within the scenario, TMX constantly generated traffic (according to the desired equipment percentage) with
approximately a 2-minute take-off interval on the mentioned airports. The aircraft departing from the
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experiment airports all used the same aircraft model. These generated aircraft were all instructed to fly to a
destination within the experimental area along a direct route, or along the defined ATC route. Aircraft that were
generated at the other airports were instructed to fly Free Flight to all other airports (also the airports specified in
the experimental area).
The experiment aircraft that were generated at the five specified airports in the experimental area were used for
the measurement of relevant parameters. Each time when such an aircraft reached its destination, it was deleted
from the simulation and a set of parameters was sampled (e.g. fuel consumed, flight time). To obtain a
representative measurement the sampling of parameters was not started until the number of aircraft present in
the experimental area stabilised. Each time an experiment was started the number of aircraft flying in the area
increased over the first hours of simulation. This build-up of aircraft slowly decreased after a few hours of
simulation. Each experiment (for every percentage of equipment) lasted five hours and was repeated four times.
This means that for every equipment percentage a total of twenty hours of sampling was reserved. Figure 10
below illustrates the traffic build-up in the experimental area, and the point at which the simulation sampling
started.
Number of Aircraft
during Mixed Equipped Traffic Simulations
0
50
100
150
200
250
300
350
0 5000 10000 15000 20000 25000
Time [s]
Nu
mb
er
of
Air
cra
ft [
-]
example
Sampling started
after 9550 seconds
Figure 10 Example of traffic build-up in the experimental area
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RESULTS
The experiment data was gathered and processed in order to obtain an insight in to how the direct-routing
advantage of the Free Flight aircraft compares to the aircraft flying ATC (conflict free) routes. The most
important results will be shown in this section.
Figure 11 illustrates the total flight fuel consumption averaged over all flights (and all routes). It gives a
global comparison of the efficiency of the simulated Free Flight traffic with the simulated ATC traffic.
Monte-Carlo Simulations of a mixed equipped traffic environment
Total Fuel Consumed averaged over all flights
0
200
400
600
800
1000
1200
1400
1600
0 25 50 75 100
Precentage of equipped traffic [%]
Fuel
consum
ed [
kg]
ATC traffic test 1
ATC traffic test 2
ATC traffic test 3
ATC traffic test 4
Free Flight traffic test 1
Free Flight traffic test 2
Free Flight traffic test 3
Free Flight traffic test 4
Averaged Standard Deviation
1 432 1 432 1 432 1 432
1 432 1 432 1 432 1 432
ATC ATC ATC ATC
FF FF FF FF
Figure 11 Global view of the total fuel consumption for Free Flight and ATC traffic
The figure shows the averages of all the experiments in a way that the results of various test-runs can be
compared easily. In order to illustrate the scatter of the samples, the averaged standard deviation is shown for
the second experiment series. Figure 12 shows the average number of conflicts encountered during flight for
ATC and Free Flight traffic.
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Monte-Carlo Simulations of a mixed equipped traffic environment
Total Number of Conflicts averaged over all flights
0
1
2
3
4
5
6
7
8
9
0 25 50 75 100
Precentage of equipped traffic [%]
Num
ber
of
Confl
icts
[-]
ATC traffic test 1
ATC traffic test 2
ATC traffic test 3
ATC traffic test 4
Free Flight traffic test 1
Free Flight traffic test 2
Free Flight traffic test 3
Free Flight traffic test 4
Averaged Standard Deviation
1 432
ATC
1 432
ATC
1 432
ATC
1 432
ATC
1 432
FF
1 432
FF
1 432
FF
1 432
FF
Figure 12 Global view of the number of conflicts for Free Flight and ATC traffic
DISCUSSION
The goal of the full-scale Monte Carlo simulations was to make an estimation of the global performance of Free
Flight traffic (flying along direct routes) compared to normal ATC traffic (flying along predefined routes, free of
conflicts). The role of a mixed-equipped traffic environment was also investigated.
The experiment set-up favoured the traffic flying along ATC routes. There were no delays on the routes and the
conflicts with other traffic were neglected, assuming an ideal traffic flow management. This leads on forehand
to the assumption that there would be no great differences between the performance of the Free Flight traffic
and the ATC traffic. The only constraints that were experienced by the ATC aircraft were the constraints
imposed by the route.In spite of this, the results favoured the Free Flight traffic. Figure 11 shows that the overall
fuel consumption of the Free Flight aircraft, averaged over all flights, is significantly lower than the overall fuel
consumption of the ATC traffic. The results of the separate tests are all in the same range indicating the
reliability of the test results. The increase in equipment percentage shows a slight decrease of overall fuel
consumption.This can be explained as follows. An increasing percentage of equipment increases the chance that
when a conflict occurs, the conflict will be between equipped aircraft, because there are less not-equipped
21
aircraft in the traffic environment. This implies that the aircraft can use smaller manoeuvres to resolve conflicts,
leading to lower fuel consumption.
Figure 13 shows that the number of conflicts that occur during flight is very widely scattered. The averages are
about the same value for all the tests, however, the average standard deviation shows that the measurements are
extremely spread. When analysing the data it is found that some of the flights experience lots of conflicts and
others have practically no conflicts at all. The extremes go to over thirty conflicts (per flight) on some
occasions.
The explanation of this phenomenon is that in some occasions Free Flight aircraft get stuck in a conflict
situation involving more aircraft; especially when one of those aircraft is not equipped. It sometimes occurs that
an aircraft resolves a conflict with an aircraft by changing its heading slightly because this is the best way to
resolve the situation according to the decision model (small intrusions). Nevertheless, this resolution causes a
manoeuvre that induces a new conflict with another aircraft. Again it is decided that the conflict is best resolved
by slightly changing the heading. This manoeuvre, when it is in the opposite direction as before, could cause the
reoccurrence of the old conflict. In case that this old conflict was with an ATC aircraft, it will likely be a more
urgent conflict than before, because it was assumed that ATC will not manoeuvre for conflict resolution.
This problem situation cannot be blamed on the MVP resolution but is a result of the simplicity of the
decision model in the resolution module and especially its lack of human situational awareness. A human pilot,
with the help of proper CDR&P tools and CDTI (increasing situational awareness), could have easily resolved
the problem situations. The described problem situation occurs several times in all the experiments. In spite of
this, the average of the results is hardly influenced by the peak values; only the standard deviation indicates its
presence.
It can be concluded that a decision model solely based on one-on-one conflict encounters is not enough
for efficient Free Flight. When resolving a conflict it should first be determined if the resolution causes new
conflicts (or worse: old conflicts). This additional conflict prevention task could part of the (automated) conflict
resolution calculations, but could also be regarded as an additional task of the crew. Each manoeuvre should be
checked for traffic (by using conflict prevention tools on the CDTI) before initiating it, similar to the checks
crews have to perform in order to avoid "conflicts" with terrain and weather in the aircraft proximity.
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CONCLUSION
The research presented in this paper was aimed at making an inquiry into the issues raised in the 1997 NLR
Human-in-the-Loop study. This has been done by first observing Free Flight on a small scale, by conducting
one-on-one conflict experiments. Subsequently, using the knowledge of these experiments, large-scale Monte-
Carlo tests have been conducted with Free Flight aircraft flying in a “mixed-equipped” traffic environment.
With respect to the small-scale one-on-one experiments the following conclusions can be made:
It is refuted that the vertical resolution manoeuvre always has a negative impact on fuel efficiency.
The vertical climb manoeuvre is a very fuel-efficient way of resolving conflicts.
A trade-off has been found between the vertical descent manoeuvre and the track change manoeuvre with
respect to fuel consumption and time.
With respect to the large-scale Monte Carlo simulation experiments the following conclusions can be made:
The direct-routing benefits of Free Flight outweigh the costs related to the Airborne Separation Assurance
concept.
A simple decision model has been developed that combines the results of the small-scale experiments with
the MVP concept.
In some complex conflict situations (often involving mixed-equipped traffic) the decision model lacks the
human-like anticipation for a more efficient (fast) resolution of conflicts.
The average number of conflicts encountered by Free Flight aircraft in the mixed-equipped environment is
very low, nevertheless, the results are widely scattered, which is due to the simplicity and lack of human
situational awareness of the implemented decision model
The MVP algorithms work in a mixed-equipped traffic environment.
Finally, the results of the Monte-Carlo study were beyond expectations. It can be concluded that the direct-
routing aspects of Free Flight provide great potential to reduce the fuel-related costs in the traffic environment.
23
This conclusion combined with the conclusion of the preceding NLR Free Flight studies, encourage further
studies before introduction of Free Flight in a future traffic control environment.
REMAINING ISSUES:
CURRENT AND FUTURE STUDIES
A remaining issue with respect to the study presented in this paper is the issue of competitiveness between users.
To what extent does competitiveness between users effect the cost-benefit aspects of airborne separation
assurance. The results presented in this paper were all based on experiments without actual human interaction;
all conflict resolutions were executed by pilot (decision) models flying simulated aircraft. A critical objection to
the results could be that in real-life pilots would not be so anxious to resolve all occurring conflicts, but would
anticipate the manoeuvres of the other aircraft involved.
Another issue that remains is the effect of the performance of the system providing the traffic information to the
users of the ASAS CD&R. Free Flight with Airborne Separation Assurance assumes that traffic information is
available via a datalink system such as ADS-B. Nevertheless, the performance of the used datalink system could
have impact on several aspects of the system. An obvious field of interest with respect to ADS performance
issue will be human factors: what is the minimum required (and preferred) update rate of the CDTI displays in
the Free Flight cockpit.
Currently the NLR studies are focussing on the above-mentioned issues. In the spring and summer of 2000 NLR
(under contract by NASA, FAA and RLD) will execute the so-called Human Interaction Experiments (HIE) in
which “pilots” from all over the world can participate in Free Flight simulation experiments via the Internet. The
earlier described TMX will be used as Free Flight simulation environment that will be linked to a great number
of desktop Free Flight aircraft simulation applications (FreeSim) using the TCP/IP protocol
Future work of NLR will be focussed on expanding the application of the state-based Free Flight concept in the
aircraft operational envelope. The ASAS will be tested in a more complete flight envelope from TMA to TMA.
Future experiments will include transition from Free Flight Airspace to Managed Airspace and vice versa; the
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goals for the near future is expanding the knowledge of application of Free Flight with Airborne Separation
Assurance in the complete operational envelope.
REFERENCES
RTCA (1995), “Report of the Radio Technical Commission for Aeronautics (RTCA) Board of Directors’ Select
Committee on Free Flight”, Washington D.C., USA
Krozel, J., Peters, M. (1997), “Conflict Detection and Resolution for Free Flight”, Air Traffic Control
Quarterly, Volume 5 (3), USA
Kuchar, J.K., Yang, L.C. (1997), “Survey of Conflict Detection and Resolution Modeling Methods”,
Proceedings of the AIAA Guidance, Navigation and Control Conference, New Orleans, USA
Gent, R.N.H.W. van, Hoekstra, J.M., Ruigrok, R.C.J. (1997), “Free Flight with Airborne Separation
Assurance”, Proceedings of the Confederation of European Aerospace Societies (CEAS) 10th European
Aerospace Conference, Amsterdam, the Netherlands
Gent, R.N.H.W. van, Hoekstra, J.M., Ruigrok, R.C.J. (1998), “Conceptual Design of Free Flight with Airborne
Separation Assurance”, AIAA-98-4239, Proceedings of the Guidance, Navigation and Control Conference,
Boston, USA
Eby, M.S. (1994), “A Self-Organizational Approach for Resolving Air Traffic Conflicts”, The Lincoln
Laboratory Journal Vol.7, Nr.2
Hoekstra, J.M. (1998), Gent, R.N.H.W. van, Ruigrok, R.C.J., “Man-in-the-Loop Part of a Study Looking at a
Free Flight Concept”, Proceedings of the Digital Avionics System Conference, Seattle, USA
Valenti Clari, M.S.V. (1998), “Cost-Benefit Study of Conflict Resolution Manoeuvres in Free Flight”, Delft
University of Technology, M.Sc thesis, Delft, the Netherlands
25
Bos, A. (1997), “User Manual for the Base of Aircraft Data (BADA) Revision 2.6”, Eurocontrol Experimental
Centre Note 23/97, Bretigny, France
ABBREVIATIONS & ACRONYMS
AATT Advanced Air Transportation Technologies
ADS-B Automatic Dependent Surveillance Broadcast
ASAS Airborne Separation Assurance System
ATC Air Traffic Control
ATCo Air Traffic Controller
ATM Air Traffic Management
CDTI Cockpit Display of Traffic Information
DAG-TM Distributed Air Ground Traffic Management
FAA Federal Aviation Authority
FreeSim Free Flight Desktop Simulation
HIE Human Interaction Experiment
HMI Human Machine Interface
IFR Instrument Flight Rules
IP Internet Protocol
MVP Modified Voltage Potential
NASA National Aeronautics and Space Administration
NLR Nationaal Lucht- en Ruimtevaart-laboratorium (National Aerospace Laboratory)
RFS Research Flight Simulator
RLD Rijks Luchtvaart Dienst (Dutch Aviation Authority)
RTCA Radio Technical Commission for Aeronautics
TCP Transmission Control Protocol
TMA Terminal Manoeuvring Area
TMX Traffic Manager
DUT Technische Universiteit Delft (Delft University of Technology)
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AUTHOR BIOGRAPHIES
Mario Valenti Clari is a research engineer at the National Aerospace Laboratory in Amsterdam, the Netherlands.
He joined the NLR Flight Simulation Department after his graduation (M.Sc.) from Delft University of
Technology, in August 1998. The research presented in this publication has been part of his graduation
assignment. Since joining the NLR he has been involved in several international research projects dealing with
Free Flight and future Air Traffic Management systems (INTENT, Mediterranean Free Flight, NLR/NASA Free
Flight). Besides this main interest, Mario Valenti Clari has also performed research on the topics of Wake
Vortex Encounter Simulation and Simulator Motion Cueing.
Rob Ruigrok is a research engineer at flight mechanics department of the National Aerospace Laboratory in
Amsterdam, the Netherlands. He graduated in 1992 at Delft University of Technology on the design and
analysis of automatic landing systems. After graduation he joined the flight simulation department of NLR in
which he participated in various simulation project, studying Controller-Pilot Datalink Communication
(CPDLC), future Flight Management System (FMS) concepts, windshear detection and alerting systems and
free flight with airborne separation assurance (in co-operation with NASA). In 1998 Rob Ruigrok transferred to
the flight mechanics department of NLR in which he continued to work on the NLR/NASA Free Flight project.
His current position is project co-ordinator of the INTENT project and projectleader of the Mediterranean Free
Flight project.
Jacco Hoekstra graduated in 1991 at the Delft University of Technology, Faculty of Aerospace Engineering. He
joined NLR at the Flight Simulation and Handling Qualities department. He worked on model development,
display design, handling qualities, human factors studies and accident analysis. Topics include Head-Up
Displays, Flight Control Systems, Flight Management Systems, Controller Pilot Data Link and Free Flight. In
the beginning of 1998, he transferred to the human factors department and is now project manager of the
NLR/NASA Free Flight project. In the fall of 2001 he obtained the doctoral degree at Delft University of
Technology on the conceptual design of Free Flight. Dr. Hoekstra is also head of the Civil Aviation group that is
a part of the Human Factors department.
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Hendrikus ("Dries") Gerrit Visser received Engineering Degrees in Aeronautics from Delft University of
Technology, Delft, the Netherlands in 1981 and 1982, respectively, and a Ph.D. in Aerospace and Ocean
Engineering from Virginia Polytechnic Institute and State University in 1985. In 1983 he was a Visiting Research
Associate at the Israel Institute of Technology, Haifa, Israel. From 1985 to 1987 he worked as a system engineer at
Fokker Space & Systems, Amsterdam, the Netherlands. In 1987 he joined the Delft University of Technology,
where he is currently a lecturer in the Faculty of Aerospace Engineering. His interests include flight operations, air
traffic control, flight mechanics and optimisation theory. He is an Associate Fellow of the AIAA.
