To assess the safety for advanced (new) operations, NLR-ATSI has developed a number of models and techniques. In many instances, the problem is too complex to oversee, let alone to find direct solutions. In such cases it can be helpful to break down the problem into smaller pieces, which can each be solved individually. This is exactly what is done in safety modelling. Our philosophy in the development and application of safety models is that these should be

• Valid
• Transparent
• Reproducible

Important is also the understanding that a model can never be exactly the same as reality. It is therefore particularly important to have a full understanding of the systematic and stochastic deviations of the model results from reality. A bias and uncertainty analysis is an integral part of all our methodologies.

Which modelling technique is applied depends on the problem. Therefore we have developed (and continue to develop) a suite of modelling tools. To assess the safety for advanced (new) operations, the safety institue has developed a number of models and techniques ranging from generic to specific. Below, a few of these models and techniques are described.

TOPAZ
TOPAZ (Traffic Organisation and Perturbation AnalyZer) is a safety/capacity assessment tool for evaluation of new route structures in combination with new ATM concepts for a multitude of ATM applications. TOPAZ evaluates the collision risk between aircraft within an arbitrary network of lane segments by calculating the expected number of aircraft collisions on the basis of a generalized version of the Reich collision model. Collisions are determined as events in which simultaneous loss of lateral, vertical, and longitudinal separation occurs between aircraft flying within a network of lane segments in 3D airspace. Thus all types of collisions are accounted for. TOPAZ simulates the probability density functions along the 3D route structure rather than the individual aircraft trajectories and evaluates the collision risk between aircraft as a function of traffic flow. Maximum capacity is determined as that where the risk coincides with the selected target value.

Causal models of aviation safety
In order to support decision-making it is necessary to build risk models that contain the main decision parameters - and that thus show at least in global terms how the risk of different consequences depends on factors that can be influenced by decisions. Existing risk models typically do not show the dependence of risk on many different influencing factors, and the statistics collected by operators and regulators may not even include all such relevant factors. In order to create a causal model it is therefore necessary to build and quantify a dependency model that is consistent with the overall risk. Bayesian Networks are used to build the causal model, and these are quantified by a mix of operational data and expert judgement.

WAVIR
With the increase in air traffic, wake vortex safety aspects are becoming more important, especially at the busiest airports where incidents attributed to wake encounters are reported by pilots. NLR-ATSI has developed a Wake Vortex Induced Risk assessment (WAVIR) tool, based on a workflow concept, to support the efficient use of existing and newly proposed wake alleviation technologies. WAVIR supports the evaluation of wake vortex safety aspects and required separation distances for:

• Air Traffic Management warning and avoidance procedures;
• On-board wake detection, warning and avoidance instrumentation;
• Advanced aircraft wing technology operations; and
• New designed high capacity aircraft.