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Fast-Time Simulation for Air Traffic Operations Analysis
The use of fast-time simulation techniques to model the flow of aircraft through the airspace system and on the airport surface is well established, although these applications have generally addressed issues of throughput and delay rather than safety concerns.  Typically, the procedural rules that govern the way in which the flow of air traffic is managed by the air traffic control system, particularly the maintenance of safe separation between aircraft, are incorporated into the logic of the model.  This has two consequences.  The first is that any deviation of individual aircraft from the optimal spacing and flight path resulting from the variability in human performance of the flight crews and controllers involved must be specified through a combination of the model logic and the inputs to the specific model run.  This is typically handled by defining statistical distributions on the relevant parameters, which are used to vary the minimum separation between aircraft, aircraft speeds, and so forth in the course of the run.  While the parameters of these distributions can be selected so that the outcome of this process can be calibrated to the observed behavior of the system, it is difficult to anticipate how these distributions might change in response to changes in technology, procedural rules, training, or similar factors.  More important from the perspective of safety analysis, these distributions contain no feedback mechanism that alters the behavior of the system in response to an incipient violation of procedural requirements.  Either the distributions are specified in a way that implicitly ensures that no such violation can ever occur, or if such violations are allowed, then when they do occur, the simulation continues to mindlessly apply the chosen value of the distribution as if nothing untoward is happening.
The second consequence is that there is no mechanism to simulate the occurrence of human error on a realistic basis.  All the aircraft behave exactly according to the procedural rules specified in the simulation logic and input data.  The simulation can be forced to model the occurrence of an error, but the nature of the error and the response of the system to the error has to be precisely specified in advance, which defeats the purpose of the exercise.
The introduction of an explicit representation of the behavior of the flight crew and controllers that can respond to the evolving situation in the simulation offers the potential to overcome these limitations.  However, this requires a simulation capability that can model the flow of air traffic in a way that can interact with an appropriate representation of the performance of the humans involved.  Since the simulation of human performance is computationally intensive, for reasons of computational efficiency it is desirable to be able to structure the simulation so that only some of the humans involved are explicitly modeled.  For example, the behavior of the flight crew of selected aircraft might be modeled in detail while the actions of the flight crew of the other aircraft in the simulation are modeled in the usual way.
Representation of Air Traffic Operations
The representation of air traffic operations in a simulation of the current air traffic management system, or any future evolution that shares control responsibilities between ground-based and airborne components, must address both the actions of the air traffic controllers and the actions of the flight crews in response to control instructions or other events.  This requires the ability to model the flight path of each aircraft, the communications between the air traffic controllers and the aircraft under control, and the decisions by the controllers and flight crews in response to those communications and the evolving situation.  A key design issue in the simulation is the degree of fidelity with which the aircraft flight control system is represented.  In the real world, aircraft do not follow their assigned flight paths as if they are on rails.  Their flight crews monitor the flight path from their navigation instruments, make adjustments to their flight controls (or the autopilot or flight management system inputs if the aircraft is being flown by the autopilot), and the flight controls then change the dynamic forces acting on the aircraft that modifies the path of the aircraft through the air.  However, rather than model this entire feedback loop in full detail, it is often sufficient to represent the outcome of this process as if the aircraft is indeed constrained to follow its defined flight path.
　
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