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In November 2010, Intelligent Automation, Inc. (IAI) delivered a software tool to NASA Langley that models Merging and Spacing (M&S) for arrivals and departures in the Airspace Concepts Evaluation System (ACES) NAS-wide simulation. This delivery allows researchers to use ACES for system-level studies of the complex terminal airspace. As a precursor to use of the tool for research, the software was evaluated against current day arrivals in the Atlanta TRACON using Atlanta's Hartsfield-Jackson International Airport (KATL) arrival schedules, Standard Terminal Arrival Routes (STARs), and traffic flow management (TFM) techniques typical for Atlanta. Results of this validation effort are presented describing data sets, traffic flow assumptions and techniques, and arrival rate comparisons between reported landings at Atlanta versus simulated arrivals using the same traffic sets in ACES equipped with M&S. Since emphasis for intended research is on arrival capacity, this was also the focus of the validation. Before testing began, the simulated system was expected to demonstrate superior capacity over current day Atlanta by managing spacing intervals efficiently and exactly. Initial results, instead, showed the simulation's modeled capacity to be far short of what human controllers currently achieve, despite the efficiently managed spacing. Investigation into the cause of the shortfall revealed aspects of systems-level flow and control techniques that are critical to achieving sustained high capacity in the face of varying traffic loads and type mixes. This new understanding, once applied to the current day validation model, allowed a match of Atlanta's arrival capacity as well as a better understanding of how modern airports are limited by current day route models. Following this validation effort, a sensitivity study was conducted to measure the impact of variations in system parameters on the Atlanta airport arrival capacity.