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Similarly, the flight crews were modeled in detail only for those aircraft that experienced an encounter with turbulence, the safety issue of concern in the experiments.  Those aircraft that did not encounter turbulence were included in the simulation in a simplified form that served only to ensure a realistic level of workload for the controller and to provide appropriate constraints on the ability of aircraft to change altitude or speed.
A second issue relates to the geographical extent of the airspace being modeled in the simulation.  Depending on the research issues involved, it may be necessary to model several air traffic control sectors or even adjacent facilities, in order to ensure that the simulation experiments model given aircraft over a long enough period of time to capture the relevant interactions.  To the extent that the performance of the system is likely to vary with different airspace configuration and traffic patterns, it will generally be necessary to repeat the experiments for a range of different geographical regions in order to understand how the airspace configuration and traffic pattern affects the outcome of the simulation.  This suggests that it may be useful to define a set of standard geographical regions, that can be used to study how the results of particular changes vary across the system and for which expansion factors have been developed that can be used to extrapolate the results of simulation experiments to NAS-wide values.
A third issue concerns the management of the resulting information to be recorded in the course of the simulation.  Apart from considerations of the computational effort involved in modeling the activities of a large number of aircraft and associated air traffic control facilities, such simulations have the potential to generate a very large amount of output data describing the events occurring in the simulation.  Careful thought needs to be given to how these data are to be used to develop measures of the safety performance of the system.  In the simulation experiments reported here, the RFS measurement agents were tailored to extract and summarize selected measures of system performance during the course of the simulation.

III. THE CLEAR-AIR TURBULENCE SENSOR SCENARIO

The work performed during the current phase of research extended the demonstration of fast-time simulation techniques to aviation safety issues that was begun in the previous phases (Bobick et al., 1999; Abkin et al., 2000).  This previous work concentrated on a runway incursion scenario, chosen (amongst other things) for the ease with which the scenario could be modeled in the relatively structured airport environment.  This enabled the focus of the effort to be the integration of the fast-time simulation with human performance elements rather than the intricacies of the scenario design.  The resulting integration was achieved to a static level (see Abkin et al., 2000).  In the current phase of the research, however, it was desired that a full dynamic linkage between a fast-time simulation and human performance model be investigated in the context of a safety issue with system-wide implications.  In order to meet this higher-level program objective, while also leveraging technical expertise from other areas of the NASA Aviation Safety Program, the chosen safety issue was modified to a clear-air turbulence sensor technology scenario.  This scenario was chosen for its relevance to NAS safety and its requirement to model human performance and interaction issues in a CAT exposure or avoidance situation.  This chapter describes the development of the scenario and the experimental design that was developed to define how to represent the behavior of the air traffic controllers and pilots in the study.
Definition of the Problem
Clear-air turbulence (CAT) is a suitable safety issue to focus upon for its importance to both safety and overall system efficiency within the NAS.  Except for major accidents involving hull losses, atmospheric turbulence (both convective and clear air) is the leading cause of injuries to airline passengers and flight attendants.  In an average year, 17 US-based aircraft encounter turbulence severe enough to cause injuries, and from 1981 to 1997 over 750 minor injuries, 80 serious injuries, and three fatalities were attributed to turbulence [see http://www.faa.gov/apa/TURB/Facts/fact.htm].  Due to the dangers associated with turbulence, exposure to or avoidance of areas of turbulence have significant system-wide implications regarding the emergency handling of aircraft with injured parties onboard or the more routine handling of aircraft deviating around turbulence.
　
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