may be in position as well, although this setup is not a normal (planned-for) configuration. The lead controller is in charge of communicating with pilots, monitoring the radar screen to maintain safe separation, coordinating with other controllers, and other services only provided by the R-side controller. When working alone, the lead controller also has the responsibility to receive and process flight-plan information and to plan and organize the flow of traffic within the sector, which are considered D-side services. When an associate controller is present, the D-side services are no longer the responsibility of the lead controller. The addition of the second controller therefore frees up more time for the lead controller to focus attention on R-side services. This division of responsibilities allows for more traffic to be handled by a two-controller team than by a single controller.

The Center for Advanced Aviation System Development (CAASD) task load model was originally developed for the purpose of assessing the maximum number of flights that can safely traverse a sector during a time period. For the reasons explained earlier, a two-controller team can handle more traffic than a single controller because the lead controller can devote all of his or her controlling time to the R-side tasks that accompany all flights. Thus for assessing the maximum throughput capacity of a sector, it is necessary to assume that two controllers are in position—one handling exclusively R-side tasks and the other handling exclusively D-side tasks. Given this assumption, it is not necessary to model the D-side task load to assess sector traffic capacity. Traffic capacity is simply a function of the controlling time available to the lead controller to perform more R-side tasks. Once the lead controller’s time is fully occupied with R-side tasks, the sector will have reached its maximum traffic capacity.

The assumption that two controllers are in position and that traffic capacity is solely a function of the controlling time available to the lead controller caused CAASD to structure the task load model so that it only estimates R-side task load. This modeling limitation, as will become evident later, has important implications for the model’s ability to predict PTT.


Figure 2-1 shows the basic structure of the task load model. Box 1 in the figure lists eight of the nine major R-side tasks in the model. To determine when these tasks must be performed—or when they are triggered—the

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