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2Introduction The modernization and complexity of the National Air- space System (NAS) have increased considerably in the last four decades, and its components require continuous improve- ments to increase capacity and safety. Airports are one of the main components of this complex and dynamic system. To meet higher demands for flights, airlines are operating larger aircraft with greater seating capacity. However, the air- field configurations at many airports were established years ago, and many existing airports were designed and constructed in the 1960s and 1970s to accommodate the smaller aircraft in use at that time. As a consequence, some of these airports have air- field separations that are not compatible with current Federal Aviation Administration (FAA) design requirements and rec- ommendations. Moreover, existing airports need to increase their capacity to meet demand. Many of these airports are con- strained by urban development and physical and environmen- tal restrictions, or they do not have the necessary real estate to accommodate the applicable separation standards. With this potential traffic growth, many airports will need to bring larger aircraft and use instrumented runways to increase their capacity; however, existing separations may not be appro- priate for larger aircraft or for upgraded approach categories. When existing separations do not meet FAA standards, a request for modification of standards (MOS) may be required, and authorization for the operation may be granted if the MOS fulfills the criteria described in FAAâs Advisory Circular (AC) 150/5300-13, Airport Design (FAA, 1989). In the context of this study, MOS means any change to FAA dimensional standards, other than those for runway safety areas (RSAs), applicable to an airport design or construction project (new, reconstruction, expansion, or upgrade). So far, requests dealing with airfield separations have been analyzed according to non-risk-based methodologies, and the only available guidance is described in AC 150/5300-13 (FAA, 1989). This process can affect airport efficiency and capacity and, potentially, prevent certain aircraft from using an airfield. On the other hand, some of the factors that lead to aircraft accidents/incidents are considered under subjective criteria. This research addresses a more consistent risk-based decision- making process to analyze the separation standards at those constrained airports. Project Goals The main objective of this research was to develop a method- ology for assessing the risk of aircraft collisions associated with non-standard airfield separations. The methodology is intended to provide a quantitative basis to support MOS requests by airport operators for airfield separations that do not meet FAA standards. This study is not aimed at modify- ing existing standards but rather at allowing an airport oper- ator to assess the level of safety when those standards cannot be met. The methodology was developed to be simple and practical, allowing airport operators to estimate whether the level of risk is acceptable and to compare it to the level of risk achieved when the standard separations are met. Major Challenges Associated with Airfield Separations In the coming years, it is expected that air transportation will experience greater growth than has been observed during the 2008 to 2011 recession period. Anticipating the higher demand for flights, airlines are operating larger aircraft with greater seat- ing capacity. However, it is sometimes impracticable to meet the separation standards for larger aircraft due to a number of physical, social, economical, and environmental limitations. There is no acceptable method available to estimate the level of protection provided by existing airport layouts when evalu- ating the operation of large aircraft in terms of the probability of collision with another aircraft, vehicle, or object. Current separation standards have provided an excellent level of safety, as evidenced by the small number of accidents associated with C H A P T E R 1 Background