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system performance and pilot blunders, where human errors and equipment problems
often dominate an infrequently observed population. The Reich Model emphasizes
navigation performance. Its representation of “abnormal/unmodeled” operations, and its
empirical as opposed to predictive nature, have also been criticized. The ICAO Required
General Concept of Separation Panel (RGCSP) requires collision risk assessments as well
as operational evaluations of proposed separation standard changes.
For controlled airspace, great care must be taken in developing deviation distributions
based on empirical data. The empirical data includes the effects of air traffic control
intervention. “Factoring out” these interventions and relating them to separation minima
will require insightful and ingenious analysis.
Recent pressure with the development of the ICAO FANS concept and ICAO’s
endorsement of Automatic Dependent Surveillance and satellite navigation and
communication systems require the timely development of risk methodologies that
consider the full range of communications, navigation, and surveillance performance
factors. Methodologies that evaluate the ATC control loop, model various kinds of
airspace geometry, and are predictive in nature, minimizing the standards development
SEPARATION SAFETY MODELING
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data-taking time period (to enable the return in investments), are key to operator
investment in new technology.
Professors Simpson and Ausrotas [R5.4] outline the difficulties in evaluating risk in ATC
operations which have ground intervention. They propose a risk analysis framework for
evaluating such operations: “The risk analysis is needed to provide an indication of the
benefits of introducing intervention processes which use improved surveillance and
communications, and improved decision support for controller intervention.” They
introduce the concepts of Conformance Management and Hazard Management for
reducing the risk of loss of planned separation in an intervention environment.
The following material summarizes alternative methods and models for assessing risk in an
ATC environment with intervention. It begins with a review of hazard analysis techniques
which have been used to assess risk in complex systems, and whose application to the
ATC separation problem is just beginning. These methods should be considered as
additional tools for risk modeling, which may be viewed as complimentary to the more
typically applied collision risk modeling and simulation tools identified later in this section.
The Collision Risk Modeling (CRM) approach emphasizes empirical (statistical) modeling,
navigation performance, and spatial distributions. The Reich Model is the widely used
CRM for oceanic parallel track operations. The Hazard Analysis approach tends to have
applicability to analytic modeling, communications, surveillance and control-loop
performance, and temporal/event representations. Recent experience with collision risk
modeling for the North Atlantic has suggested an approach which begins to combine the
classical Reich approach with the hazard analysis approach [R5.5]. The Review of the
North Atlantic Lateral Collision Risk Model suggests the decomposition of the error terms
into specific error categories (which could be based on a functional hazard assessment).
The ICAO Review of the General Concept of Separation Panel (RGCSP) is developing a
Manual on Airspace Planning Methodology for the Determination of Separation Minima
[R5.6]. The Manual identifies the factors impacting separation minima (Chapter 3) and
the methodology for evaluating safety (Chapter 6). Steps in the safety evaluation process
include the proposed system definition, setting evaluation criteria, identification of
hazards, hazard frequency estimation, consequence modeling, risk estimation, risk
evaluation, risk reduction measures, and implementation/monitoring. Both collision risk
modeling and hazard/risk analysis methods are identified as risk analysis tools.
5.2 HAZARD ANALYSIS TECHNIQUES APPLIED TO COLLISION RISK
The application of hazard analysis techniques to the development and certification of
complex aircraft systems has been used by the industry for several decades. The
application of these methods to the problem of loss of separation risk in the ATC system is
now under development. RTCA SC-189/EUROCAE WG-53, Air Traffic Services Safety
and Interoperability Requirements committee/working group, has a charter to advance
CNS/ATM concepts and support operational implementation by developing guidance
APPROACHES TO COLLISION RISK ANALYSIS
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material to define safety requirements and inter-operability requirements for Air Traffic
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a concept paper for separation safety modeling(19)
