Conference PaperPDF Available

Are You Clear About “Well Clear”?

Are You Clear About “Well Clear” ?
Guido Manfredi1and Yannick Jestin2
Abstract Regulations from the ICAO use the term Well
Clear without defining it. Now, this definition is needed to
design air traffic Detect And Avoid systems. A definition is
currently discussed at the ICAO level, with work on the
associated Remain Well Clear (RWC) function underway at
standardisation bodies level (RTCA, EUROCAE). But many
members of the communities impacted by these works are not
well aware of their state. To address this lack of awareness, this
paper provides three contributions. First, it derives from ICAO
texts the components of a RWC function: boundaries, alerts and
guidances. These are linked to essential elements required to
define the Well Clear term: a start and end, the actors involved,
and the expected actions. Second, it summarizes the current
regulatory efforts in RTCA, EUROCAE and ICAO regarding
the Well Clear and Remain Well Clear notions. Third, it
proposes discussion topics to move forward. From a DAA
perspective, the notion of Well Clear is key to unlock RPAS
full integration, i.e. operation in all classes of airspaces. Though
existing works make good progress, the resources engaged on
this topic seem insufficient when compared with the complexity
and importance of the task at hand.
ATCO Air Traffic Control Operators.
CA Collision Avoidance.
CoC Clear of Conflict.
DAA Detect And Avoid.
LoWC Loss of Well Clear.
MOPS Minimum Operational Performance Standards.
NMAC Near Mid-Air Collision.
OSED Operational Service and Environment Description.
RegWC Regain Well Clear.
RoW Right of Way.
RP Remote Pilot.
RPA Remotely Piloted Aircraft.
RPAS Remotely Piloted Aircraft Systems.
RWC Remain Well Clear.
SARP Standard and Recommended Practice.
VLL Very Low Levels.
WC Well Clear.
Though mentioned ten times in ICAO’s Rules of the
Air, the term “Well Clear (WC)” is never defined in the
current ICAO regulation [1]. Until recently, this was not a
problem since the interpretation of this term was left to the
appreciation of highly trained Air Traffic Control Operators
1ENGIE Ineo - Groupe ADP - Safran RPAS Chair, Ecole Na-
tionale d’Aviation Civile, Universite de Toulouse, Toulouse, France
2ENGIE Ineo - Groupe ADP - Safran RPAS Chair, Ecole Na-
tionale d’Aviation Civile, Universite de Toulouse, Toulouse, France
TABLE I: Main differences between Collision Avoidance
(CA) and Remain Well Clear (RWC) functions. CoC is for
Clear of Conflict; NMAC is for Near Mid Air Collision.
Decision factors Safety Safety, acceptability, strategic
Responsibility Pilot Depends on airspace
(can be shared with pilot’s)
Contact ATC? If time allows Yes, notably if under clearance
Start/ Collision hazard/ Conflict/
End NMAC or CoC Collision hazard or CoC
Time horizon Tens of seconds Few minutes
Manoeuver Strong Smooth
Manoeuver None Right of Way
constraints rules, clearance
(ATCO) and pilots. However, with the ongoing effort to
integrate Remotely Piloted Aircraft Systems (RPAS) in non-
segregated airspaces, providing an objective definition of WC
has become urgent.
In many types of operations, the integration of an RPAS in
the existing traffic is likely to require the Remotely Piloted
Aircraft (RPA) to be equipped with an air traffic Detect
And Avoid (DAA) system. Depending on the operational
and regulatory environment, the DAA system should provide
one or both of the following functions: Collision Avoidance
(CA) and Remain Well Clear (RWC). The former is a
last minute manoeuver to avoid imminent collision with
air traffic and reach a safe state. The latter consists in
smooth manoeuvers considering multiple factors (e.g. safety,
operational, mission) to avoid conflicting traffic. The main
differences between CA and RWC are highlighted in Table
The notion of CA is built around the precise definition
of Collision Volume as defined in [2]. However, there is no
such objective definition of a WC volume, thus preventing
the definition of a RWC function. In fact, numerous research
works proposed methods capable of providing a RWC func-
tion [3], [4], [5], [6], but with no formal definition of the
WC volume, they cannot be applied to actual operations.
Hence the current efforts, reported in this paper, to agree
on a definition for WC and then RWC. The notion of
RWC originates from regulatory requirements of the ICAO
Annex 2 Rules of the Air [1]. It is strongly linked with
the notions of collision risk and right-of-way rules. Hence,
coming up with a definition is no easy task. On top of an
effort from the ICAO’s Standard and Recommended Practice
(SARP) panels, two standardization bodies have taken up
the challenge: EUROCAE and the RTCA, along with helper
The goal of this paper is threefold. First, based on existing
definitions from ICAO documents, an operational decompo-
sition of the RWC function is proposed and linked to the
definition of WC. Second, an overview of current efforts on
the definition of the RWC function is provided. Third, topics
are provided to fuel discussions about the possible evolutions
around the WC and RWC notions. This paper first describes
the RWC function in more details. Then, the current efforts
deployed to tackle the WC and RWC definition problems
in the ICAO, EUROCAE and RTCA are described, with a
stress on the particular contributions and hypotheses of each
entity. A discussion is proposed about the paths not being
currently explored. This work concludes on the short term
objective of current works and ways to go.
As mentioned in the previous section, the notion of RWC
is directly linked to ICAO’s Rules of the Air [1], “An aircraft
shall not be operated in such proximity to other aircraft as
to create a collision hazard.” So the primary objective of
a RWC function is to prevent collision hazards. Moreover,
according to ICAO’s Manual on RPAS, RWC is “the ability
to detect, analyse and manoeuver to avoid a potential conflict
by applying adjustments to the current flight path in order to
prevent the conflict from developing into a collision hazard”
[2]. Meaning that the RWC function should start when a
conflict is detected, and finish when the conflict is solved
(RWC succeeded); or when it developed into a collision
hazard (RWC failed), in which case the CA function shall
engage, if available. Finally, though this is not clear from the
ICAO Annex 2 [1], it seems reasonable to assume that the
right-of-way rules apply as soon as a conflict exists.
The previous RWC definition relies on the term “conflict”
as a starting point and in its relationship with the right-of-
way rules. But the term “conflict” is not defined in these
documents. The rest of this paper uses the definition of
conflict from ICAO’s Air Traffic Services Planning Manual
[7] and formulated as a “Predicted converging of aircraft in
space and time which constitutes a violation of a given set
of separation minima”. By relying on the term “conflict”,
the definition of RWC asks for the definition of separation
These minima are materialised by boundaries which sep-
arate the airspace in volumes where different rules apply.
Considering there is a Remote Pilot (RP) in or on the
loop, such boundaries need to be associated with alerts and
guidances. An important thing to consider when reading
this section is that WC and RWC are different concepts.
WC is an aircraft state influencing the application of the
right-of-way rules, while RWC should be understood as a
separation minima and the RWC functions is a function
aimed at ensuring that the RPAS stays out of the RWC
A. Boundaries
When a conflict arises between two aircraft, both need to
follow the Right of Way (RoW) rules. According to the RoW
rules, the possible manoeuvers of two aircraft involved in
Fig. 1: The Remain Well Clear threshold and Remain Well
Clear volume as defined in the ICAO RPAS Manual. The
collision avoidance threshold and collision volume are not
considered here as they are not related to the RWC function
but rather to the CA function. (image from the ICAO’s
RPAS Manual). For the ACAS community, note that the
intruder and threat notions used for RWC are different from
the ones in ACAS documents where intruder means under
surveillance, and threats require RAs.
an encounter will be gradually constrained as the encounter
evolves. To determine the constraints applicable to each
aircraft, two questions need to be considered: “who has the
RoW?” and “are the two aircraft WC?” The aircraft which
has the RoW has to maintain heading and speed for the rest
of the encounter, but it will not have any further constraints.
On the other hand, the aircraft which does not have the RoW
will be constrained depending on whether it is WC or not
from the first aircraft. If WC, it can move freely, if not WC
it will be constrained according to rules detailed in the RoW
So, for a given aircraft, depending on whether it has
the RoW or not, two different conflict evolution lines are
possible, as illustrated in Figure 2. Each line has three states:
when the aircraft are well separated and not in conflict (RoW
rules do not apply), when the aircraft are in conflict but
still WC (some RoW rules apply), and when the aircraft are
in conflict and WC is lost (more RoW rules apply). Each
of these states is associated with a bounded volume, thus
yielding three volumes. In Figure 2, the leftmost volume
is called the conflict-free volume. According to the ICAO’s
RPAS Manual [2], the two following volumes are called,
from left to right: the RWC threshold, and the RWC volume
(see Figure 1). Depending on whether an aircraft has the
RoW or not, the presence of an aircraft in one of these
volumes will constraint more or less its manoeuvers.
Recent discussions led to a proposition to rename the
“Remain WC volume” into “Regain WC volume” to stress
the necessity to be WC at any time. This proposition comes
from communities which envision DAA systems with no CA
capabilities. In this case the RWC function is the only safety
Fig. 2: Top line, the aircraft has the right-of-way: if there is no conflict it can move freely (clear background); if there is a
conflict, it is constrained to maintaining its heading and speed (dark background). Bottom line, the aircraft does not have the
right-of-way: if there is no conflict it can move freely, if there is a conflict the constraint will depend on whether it is well
clear or not. If it is well clear, it can move freely, if not it is constrained according to rules described in the right-of-way
rules (darkest background).
layer and should continue performing as long as possible.
This is similar to a CA function integrated in the RWC
function. For simplicity, in the rest of this paper we consider
that a separate CA function always exists and will not use
the term “Regain WC”. The boundaries of these volumes
can be defined using time and/or distance measures (e.g.
slant range, time to Near Mid-Air Collision (NMAC)). These
values should be chosen carefully as the volumes they define
must ideally remain larger than the CA volumes so as to
infringe them before any CA volume and ensure that the RP
and/or conflicting traffic pilot are not upset by a sudden high
priority alert.
The previous boundaries separate the different steps of an
evolving encounter depending on the constraints on the RPA
motions. With these boundaries defined, the system needs
to communicate to the RP the state of the conflict as well
as the level of maneuver available. This is done through
appropriately timed alerts.
B. Alerts
Through the boundaries, the RP can monitor the evolution
of the encounter. However, the RP is already concerned
with numerous tasks (aviate, navigate, communicate) and
the navigation is not his/her first priority. That is why
well located alerts are required to draw the RP attention
to the right information at the right time. As proposed by
Veitenburger [8], four types of alerts can be considered for a
RWC function: information, advisory, caution, and warning.
The information alert requests awareness, though no action is
needed. The advisory alert requests awareness and possible
action. The caution alert requires immediate awareness and
possible corrective or compensatory action. The warning
alert requires immediate awareness and immediate corrective
or compensatory action. Special care should be taken to have
coherent alerts between the RWC and CA functions with CA
alerts having a higher level of priority than RWC alerts. Their
evolution should be progressive along time and contradictory
alerts of the same level at the same time should be avoided.
The minimum number of alerts required and their timing is
still open to discussions.
To be most useful, alerts should come at optimal times to
ensure boundaries are not violated. They should be accom-
panied with relevant information on the situation and, in the
case of a RWC function, with avoidance information, it is to
say a guidance.
C. Guidances
Depending on the type of information provided, the guid-
ance belongs to one of four categories: informative, sug-
gestive, directive, automatic. Informative guidances provide
situational awareness; suggestive guidances limit the set of
possible actions (“don’t go there”), e.g., headings, altitudes
and speeds to avoid. Directive guidances provide a limited
set of actions to execute (“go there”), e.g., a 3-D trajectory,
a vertical direction and/or a heading along with a manoeu-
ver strength. Finally, with automatic guidances, the system
informs the RP of its intent and executes a manoeuver (“I go
there”). The RP monitors it and can inhibit the manoeuver
at any time. The guidance type should be chosen depending
on the alert level and the efficiency of the system to solve
the task at hand. In the particular case of the RWC function,
directive only guidances should be avoided. Indeed, a RWC
maneuver should take into consideration numerous factors
(traffic, weather, clearance, etc.) when the system might only
be aware of part of them and thus propose a sub-optimal
With a general idea of how the fundamental elements of
a RWC function are defined, the next section introduces
the state of current efforts to provide a definition for the
RWC function. The scope is on the contributions of ICAO
to provide a WC definition, and EUROCAE’s and RTCA’s
in defining the minimum requirements for a RWC function.
The greatest effort to define objectively and precisely
the notion of RWC is currently carried out by two stan-
dardization bodies: the RTCA and the EUROCAE. Each
has different hypotheses and, though coordination is strong,
their conclusions may differ. In the following, the current
state of their efforts regarding the definition of the RWC
function is presented, a summary of the main hypotheses
and methodologies is available in Table II.
The topic of DAA is tackled by two Special Committees
(SCs) of the RTCA, the SC-228 and SC-147. The SC-228
is tasked with developing a Minimum Operational Perfor-
mance Standards (MOPS) for RPAS, including for the DAA
part. Their work considers the DAA problem as a whole
and in all operational aspects (airspace classes, equipage,
etc.). The SC-147 is tasked with defining and updating the
ACAS performance standard. The work on RPAS started
with ACAS Xu, the new generation of CA avionics for
UAS, and was then extended to the RWC function thus
describing a full DAA system. Both RTCA groups rely
on a bottom up approach with their work being based on
existing implementations, NASA’s DAIDALUS + TCAS for
SC-228 [9] and Honeywell’s ACAS Xu for SC-147 [10].
The MOPSs are written based on the capabilities of these
existing systems. In the following, we provide an overview
of the state of the work in both groups, and the harmonisation
and reconciliation efforts currently taking place.
1) SC-228: The SC-228 organised its work in successive
Phases with an increasing operational scope at each phase.
Phase I considered “en route” IFR flights in airspaces of
class D, E and G, with an altitude comprised between 1000ft
AGL and FL180, though the lower limit is likely to be
higher due to sensor limitations (e.g. around 3000ft for air-
to-air radar). Both cooperative (i.e. transponder or ADS-B
equipped) and non-cooperative intruders were considered,
equipped with any type of CA system or DAA, and with the
following maximum performances: speed 600knts, vertical
rate 5000ft/mn, horizontal acceleration 1.5g. One of the
crucial hypothesis of the SC-228 work is that an RPA might
fly without a CA function, and rely solely on a RWC
function. In this context, the DAA system was required to
have a RWC function and optionally an ACAS II system for
CA. But the focus of this work was on the RWC part. Based
on models and experiment results provided by the NASA,
AFRL and MIT, the SC-228 selected a fixed definition for the
RWC volume based on time and distance values (see Figure
4). The experiments leading to this choice are described
in detail in [11]. A RWC threshold around this volume is
considered to provide an alert when LoWC is predicted.
Because CA is optional, three alert, with two different lev-
els, have been used for the RWC function alone: preventive
(=caution), corrective (=caution) and warning; additionally,
ACAS II, if equipped, provides a second warning alert in
the form of a resolution advisory, note that traffic advi-
sories are not issued. Guidances include showing headings
leading to a loss of well clear (suggestive) and providing
instructions to regain well clear (directive) (cf. Figure 3).
Automatic execution of RWC manoeuvers are not considered
in Phase 1, though the CA maneuver can be automatic. These
manoeuvers can be performed in the vertical or horizontal
dimensions. With the start of Phase 2, efforts aim at looking
into operations in more airspaces and in cases which require
new definitions of RWC boundaries. Specifically, low cost,
size, weight, and power sensors will have limited range so
appropriate RWC boundaries should be chosen; and terminal
area operations will require reduced separation distances
asking for suitable RWC boundaries.
Because of harmonisation constraints, choices made by
the SC-228 affect the ACAS Xu definition work lead by the
SC-147. Indeed, the ACAS Xu MOPS is expected to comply
with the Phase 1 DAA MOPS, as is explained below.
2) SC-147: The SC-147 focuses on the whole ACAS
family, especially on most recent one: ACAS X. Similarly
to SC-228 Phases, the development of ACAS Xu is done
iteratively with successive Runs, the current Run being Run
3. Each Run uses a newer version of the ACAS Xu library,
with added features, and optimizes its settings on simulated
data. In the RPAS case, they are in charge of writing MOPSs
for the ACAS Xu system. Though ACAS Xu was initially
planned to perform only the CA function, it evolved into a
full DAA system integrating both RWC and CA functions.
Thus, the work of SC-147 on ACAS Xu has been extended
to RWC. The work from SC-147 is different from the one
developed in SC-228, but the ACAS Xu RWC definition
strives to support SC-228 DAA MOPS requirements. The
evolution of ACAS Xu is organised around successive vali-
dation Runs (spirales) followed by tuning phases. In parallel,
the SC-147 MOPS redaction is organised in Phases, just like
for SC-228. The hypotheses of this group’s work are similar
to the ones of SC-228 except that the minimum considered
altitude is 3800ft. Though the RWC and CA functions are
mixed into a single function, we only consider here the
information relative to the RWC part of this function. The
ACAS Xu RWC function considers two boundaries: a look-
ahead boundary, whose role is similar to the RWC threshold;
and a RWC boundary, the limit of the RWC volume. Unlike
in SC-228 definition, these two boundaries are not defined
by a geometrical description but rather described in the
ACAS Xu tables and optimized with each successive Runs.
These boundaries are associated with three levels of alerts:
preventive (=advisory), corrective(=caution) and warning (cf.
Figure 3). The advisory and caution alerts are accompanied
by a suggestive guidance while the warning alert gives a
directive guidance. It is interesting to note that the warning
alert can be given both as a part of RWC or CA sub-
functions, but there is only one warning alert. In Run 3, guid-
Airspace Min. altitude Ownship Ownship Companion Methodology
classes sensors equipage projects
1000 ft (or sensors ADS-B
RTCA SC-228 D-G min. operational Active surveillance Optional CAS NASA evaluations Top-down/Bottom-up
altitude if higher) Air-Air radar
ADS-B NASA evaluations
RTCA SC-147 D-G 3800 ft Active surveillance ACAS Xu SESAR-JU evaluations Bottom-up
Air-Air radar
Cooperative + MIDCAS
EUROCAE WG-105 A-G TBD non-cooperative mandatory CAS TRAWA Top-down
(not specific) MIDCAS-SSP
TABLE II: Comparison of RTCA SC-228, SC-147, and EUROCAE WG-105 working hypotheses, support projects and
methodologies. CAS is for Collision Avoidance (CA) System.
Fig. 3: The alerting timeline has three main events: earliest time to make a Remain Well Clear (RWC) manoeuver, latest
time to make a RWC manoeuver, and earliest time to make a Collision Avoidance manoeuver. The alerts for each of the
systems envisioned by RTCA SC-228, SC-147 and EUROCAE WG-105 are distributed with respect with these milestones.
Fig. 4: The Regain Well Clear volume defined by the RTCA
ance is only in the horizontal direction. Vertical manoeuvers
are planned to be added in Run 4. In the meantime, Phase 2
will see timeline adjustments to improve the integration of
the RWC and CA functionalities. Writing of the ACAS Xu
ConUse is planned to be finished by the end of 2017 while
writing of the ACAS Xu MOPS is planned to end by 2020
with harmonisation with SC-228 Phase 2 work. Resulting
SC-147 ACAS Xu MOPS will try to comply as much as
possible with the more general SC-228 DAA MOPS.
3) Harmonisation: With the extension of ACAS Xu to
provide a RWC functionality, overlapping between the SC-
228 and SC-147 scopes became a risk and led to the creation
of three joint harmonisation groups, so called Tiger Teams,
for surveillance, threat, and metrics and modelling. These
three groups include members from SC-228 and SC-147
as well as members from EUROCAE’s WG75 and WG105
(see below) for international harmonisation. The TTs have
three goals: organise the work on the ACAS Xu MOPS, deal
with scoping and design decisions thrown by the SCs, and
deal with scoping overlap between the DAA and ACAS Xu
In EUROCAE’s history, three work groups have been
involved in the standardisation effort for DAA systems, or
parts of it. First, the WG-73 has been tasked with developing
support documents for a CA function in airspaces A-C. The
output of the group has been a CA function Operational
Service and Environment Description (OSED). The WG-73
has since been reshaped to create the WG-105, described
in the following. Second, the WG-75 focuses on ACAS
systems, and lately on the ACAS X family. Their work
is supported by external projects like Eurocontrol’s CAFE
simulator and the development of an encounter model for
CA systems evaluation. This model could later be extended
to evaluate RWC systems as well, though this is a difficult
task that will ask for time and effort. The last, and largest,
group dealing with DAA in EUROCAE is the WG-105,
a counterpart to RTCA’s SC-228. It tackles RPAS related
standardisation efforts (C3, DAA, design and airworthiness,
SORA, ERA; and more recently UTM and RPS). The DAA
focus team is separated in three sub-groups: DAA airspaces
A-C (CA only); DAA airspaces D-G (RWC + CA); and DAA
airspaces Very Low Levels (VLL). The RWC definition is
in the scope of the DAA D-G sub group, as there is no
mandatory ATC separation services in these airspaces, thus
requiring a self-separation function: the RWC function. As
opposed to the RTCA, work done by EUROCAE adopts a
top-down approach and actively seeks to avoid limiting future
implementation choices.
Though starting their work on the topic somewhat later
than the SC-228, current work from EUROCAE WG-105
tackles a broader scope than RTCAs phase 1. Indeed, it
considers “en route” phases of the flight in airspaces A-G +
Very Low Levels (VLL). Intruders hypotheses are the same
as for SC-228. For ownship, minimum equipage includes a
CA system, plus a RWC system in airspaces D-G. Even if the
CA and RWC functions belong to two different systems, it
is considered that the CA system engages as soon as WC
is lost. Pilots acceptability studies have been carried out
to propose RWC boundaries [12]. Such boundaries would
allow to provide advisory and caution alerts with continuous
guidance being displayed and updated as long as the CA
does not provide a warning, as in [13]. The hypothesis that all
RPAS will have a CA function led to the absence of warning
alert in the RWC function (cf. Figure 3). This represents
a difference with the RTCA approach, as there is no alert
when the RoW rules start constraining the RPA manoeuvers.
Current work focuses on the choice of guidance type at the
different levels of alert with the possibility to have semi-
automatic execution of the RWC guidance. More generally,
an OSED is being developed to bring forward requirements
and assumptions for the DAA system, including the RWC
The ICAO is currently pressured into proposing a useful
definition for WC. Discussions in the DAA SARP panel
are centered around the formulation of such definition. It
should be precise enough to be useful but have a large
enough scope to allow a diversity of solutions answering
the problem. Their output is crucial as this will provide
a definition on which EUROCAE and the RTCA will
continue to build their work. According to the previous
decomposition of a RWC definition, a formal definition
from their part can be expected to define the events which
start and end the RWC function (related to boundaries) as
well as the actors involved (related to alerting) and possibly
the actions expected (related to guidance). Care needs to be
taken not to enter into too much details so as to not limit
possible solutions.
On all sides, the work on defining WC and RWC is
advancing and coordination between the entities will ensure
a coherent result. The main situations are being addressed,
still some topics are not part of the discussions. The
next section mentions some of them and proposes some
This section aims at fueling discussions on some topics
which are not yet covered by existing efforts: non-binary loss
of well clear, CA vs Regain Well Clear (RegWC), RWC for
low performance RPAS.
To begin with, the Loss of Well Clear (LoWC) is currently
being considered as a binary value: an aircraft is well clear
or not. Though, metrics to measure the severity of a LoWC
have been proposed [14] and are already used in Verification
and Validation simulations [15]. Though, including them in
the current discussions would increase the difficulty of an
already difficult task, it could be beneficial for the safety of
a RWC function. Indeed, allowing low severity LoWC could
allow a RPA to lose WC momentarily in order to avoid a
potentially dangerous situation later.
Some approaches do not consider a CA function, but they
do consider a RegWC sub-function. The difference between
RegWC and CA is not clear, so we propose three differentiat-
ing elements. First, some regulatory bodies ask for the RWC
and CA functions to be independent, which would not be the
case in a RWC/RegWC system. Second, the RWC/RegWC
system implies that a LoWC would immediately trigger the
RegWC, which would not be necessarily the case for a CA
function which could trigger some time after LoWC. Third,
the end of a CA event is indicated by a Clear of Conflict
(CoC) related to a collision volume, the end of a RegWC
event would be linked to a WC volume. From a coordination
point of view, it is not clear if a RegWC manoeuver would
be required to be interoperable with existing ACAS systems.
As the RegWC solutions is being pushed, a clear definition
of RegWC will become necessary.
Current definitions associated to the notion of RWC are be-
ing developed for integrated airspace operations with RPAS
performances at least close to those of General Aviation
(GA). However, as lower performances RPAS will ask to
enter integrated airspace, there will be a need to adapt
existing definitions for their performances in terms of RPA
dynamics and sensors capabilities. To allow low performance
RPAS into airspace will ask for a delicate balance between
safety and performance requirements.
Many RPAS integration efforts are currently focused
on quick wins, mainly integration in controlled airspace.
However, complete integration will only be accomplished
when RPAS will be able to fly in any class of airspace.
For this purpose, the definition of WC and its associated
DAA functionality, the RWC, are crucial elements. Current
efforts from RTCA and EUROCA, with helper projects from
NASA and SESAR-JU, are progressing in this aspect, but the
available resources does not seem to match the complexity
of the task at hand. In the meantime, work from the ICAO’s
SARPS, generic as it may be, will provide strong foundations
on which regulators will be able to lean when defining
country specific rules. Even if the basis for nominal operation
are not established yet, one can see that there is still more
work to do e.g. for closely spaced operations and terminal
manoeuvering areas. From the point of view of our work,
future leads include gathering elements to propose a formal
definition of the RegWC function and assessing low power
sensors capabilities for RWC.
This work was funded by the Engine Ineo - Groupe ADP -
Safran RPAS Chair led by ENAC. The authors would like to
thank Catherine Ronfle-Nadaud, Alain Vallee, Eric Thomas,
Bengt-Goran Sundqvist and Ted Lester for the precious
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... However, the standardization of the RWC distance in UASs has attracted much interest, and some committees have presented detailed RWC proposals [6][7][8][9]. Nonetheless, the application of the RWC concept to unmanned aircraft is still the subject of some debate [10]. ...
... HAE H size TSE v HSM (10) where RAE is the radius of the aircraft envelope, R size is the radius of the aircraft perimeter in the horizontal plane, TSE h is the total horizontal system error, and RSM is the radius of the SM boundary as defined the following. The corresponding magnitudes for the vertical plane are H size , TSE v , and HSM, respectively. ...
Risk assessment is a key issue in enabling the use of unmanned aircraft systems (UASs) in nonsegregated areas, especially in very low-level airspace and urban areas. The specific operations risk assessment (SORA) methodology represents an important milestone in performing the risk assessments required by aviation safety agencies to UAS operators in specific operations. However, the SORA is a qualitative method used for UASs operating inside a well-bounded operational volume. This paper proposes a quantitative method that can not only be used in a closed volume but also in an airspace shared by several UAS missions and even general aviation. The basis for this is providing a separation volume (a “bubble”) to prevent collisions that is calculated using a risk-based approach. The method consists of setting a target level of safety, which is accomplished using a tradeoff between strategic and tactical mitigations. A probabilistic methodology for performing quantitative risk assessment of strategic and tactical mitigations is provided, and the dependence of the separation distance is carefully analyzed. All factors affecting the separation distance are identified, and their contributions to collision risk are probabilistically estimated. The method takes into account specific factors relating to UASs such as trajectories, separation methods, and performance. As a result, the method allows numerical determination of a separation distance for a given target level of safety and an operational scenario.
... Whereas the innermost layer will be modeled according to the Near Mid Air Collision concept, as a circle with radius: R N MAC = 2 × Maximum Wing Span + Total System Error (TSE). The self-separation can be calculated by (10). ...
Full-text available
Safety is the primary concern when it comes to air traffic. In-flight safety between Un-manned Aircraft Vehicles (UAVs) is ensured through pairwise separation minima, utilizing conflictdetection and resolution methods. Existing methods mainly deal with pairwise conflicts, however,due to an expected increase in traffic density, encounters with more than two UAVs are likely tohappen. In this paper, we model multi-UAV conflict resolution as a multiagent reinforcement learningproblem. We implement an algorithm based on graph neural networks where cooperative agentscan communicate to jointly generate resolution maneuvers. The model is evaluated in scenarioswith3 and 4 present agents. Results show that agents are able to successfully solve the multi-UAVconflicts through a cooperative strategy. (PDF) applsci-12-00610-v2. Available from: [accessed Jan 10 2022].
... In the absence of a human controller, a new separation model, with different minima for any pair of aircraft, is possible, thanks to the capabilities of the unmanned aircraft and U-space automation and communication. New separation minima adapt to each aircraft and its individual performance model [19]. The separation between any two aircraft is determined by creating a virtual bubble around each aircraft and ensuring that these bubbles never overlap. ...
Full-text available
Opening the sky to new classes of airspace user is a political and economic imperative for the European Union. Drone industries have a significant potential for economical growth according to the latest estimations. To enable this growth safely and efficiently, the CORUS project has developed a concept of operations for drones flying in Europe in very low-level airspace, which they have to share that space with manned aviation, and quite soon with urban air mobility aircraft as well. U-space services and the development of smart, automated, interoperable, and sustainable traffic management solutions are presented as the key enabler for achieving this high level of integration. In this paper, we present the U-space concept of operations (ConOps), produced around three new types of airspace volume, called X, Y, and Z, and the relevant U-space services that will need to be supplied in each of these. The paper also describes the reference high-level U-space architecture using the European air traffic management architecture methodology. Finally, the paper proposes the basis for the aircraft separation standards applicable by each volume, to be used by the conflict detection and resolution services of U-space.
Conference Paper
This paper proposes a bidirectional spline-RRT∗ path planning algorithm for fixed-wing unmanned aerial vehicles (UAVs). The proposed algorithm combines the basic RRT∗ algorithm with the spline method to produce smooth paths, which are essential for fixed-wing UAVs. The proposed bidirectional spline-RRT∗ algorithm generates paths satisfying approach direction constraints on both start and goal points, which makes it significantly different from typical path planning algorithms. Further, the algorithm is asymptotically optimal as the cost of the path found decreases as the number of vertices increases, with the path found finally converging to the optimal path. The proposed algorithm also considers the aerodynamic characteristics of the target aircraft. As a result, the paths produced are within load factor and thrust-to-weight ratio limits, are geometrically and dynamically feasible, sub-optimal, and satisfy approach direction constraints. The results of several simulations conducted confirm these properties of the bidirectional spline-RRT∗ algorithm. In particular, the results of a nonlinear six degree-of-freedom simulation verify the aerodynamic characteristics for the target aircraft, and show that the proposed bidirectional spline-RRT∗ algorithm can be effectively utilized in path planning problems for fixed-wing UAVs.
An overbundance of warning, caution, and advisory alerts will exist in the cockpits of future commercial transport aircraft if current cockpit design trends are not altered. Coupled with this proliferation of alerts is a lack of correlation between alert-type applications and significance. The potential for pilot saturation and/or confusion exists with these alerts. A study was performed for the FAA to identify these problem areas and to develop design guidelines for alerting systems in new aircraft. Recommendations resulting therefrom include: 1) improve pilots' audio/visual environment by minimizing exposure to unnecessary alerts; 2) incorporate central alphanumeric alert readout devices; and 3) improve categorization and/or prioritization of alerts.
Piloted well clear performance evaluation of detect and avoid systems with suggestive guidance
  • E Mueller
  • C Santiago
  • S Watza
E. Mueller, C. Santiago, and S. Watza, "Piloted well clear performance evaluation of detect and avoid systems with suggestive guidance," NASA technical report, 2016.
  • Icao
ICAO, Air Traffic Services Planning Manual. ICAO, 1984, vol.