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1 SUMMARY Risk Assessment Method to Support Modification of Airfield Separation Standards The Federal Aviation Administration (FAA) sets standards for the planning and design of airports to provide safe aircraft operations. These design standards include separation distances between various airfield components such as runways and taxiways, taxiways and taxiways, and taxiways and objects. Many U.S. airports were built according to older FAA design standards and were planned to accommodate smaller aircraft. With the rapid growth of aviation demand since World War II, many airports are facing the need to increase capacity and to accommodate larger and faster aircraft. However, some airports are finding it a challenge to modify existing air- field separations to meet current standards for larger aircraft because they are constrained by physical barriers, environmentally sensitive areas, and encroaching development. When it is not feasible to meet existing separations, airports may submit to the FAA a request for modification of standards (MOS) to demonstrate that there are unique local con- ditions that restrict extending the airfield and to show that the modification will "provide an acceptable level of safety, economy, durability, and workmanship" (FAA, 1989). In requests for an MOS to a separation standard, the main concern is usually the safety of operations. As the FAA and the aviation industry transition to a safety management approach to improve safety, it is important that risks associated with changes be assessed. Authorization for MOS may be granted if the MOS fulfills the criteria described in FAA's Advisory Circular (AC) 150/5300-13, Airport Design, which specifically addresses taxiway and taxilane separations (FAA, 1989). The criteria do not provide an assessment of risk that may be used for specific airfield scenarios, including the separation between runways and other airfield components (taxiways, taxilanes, and objects). Adherence to the FAA requirements can affect airport efficiency and capacity and, potentially, prevent certain aircraft from using an airfield. The objective of this research effort was to develop a simple and practical methodology for assessing the risk of aircraft collisions associated with non-standard airfield separations. The tool developed is intended to support MOS requests for non-standard separations. A practical, risk-based methodology to evaluate airfield separations was developed. The methodology is based on the probability of lateral and vertical deviations from the intended path during landing, takeoff, and taxiing operations. A series of risk plots based on center- line or wingtip separations is provided for each Aircraft Design Group (ADG), and step-by- step procedures are described for each type of separation involved in the analysis, such as runway and taxiway, and taxiway and taxiway. The methodology was validated using actual MOS cases approved by the FAA that cov- ered a spectrum of scenarios, airports, and FAA regions. Relevant information was gathered for each case to characterize the non-standard situation and was analyzed using the method- ology developed in this study. Risk criteria were suggested based on the risk matrix used by the FAA in safety management systems, on the evidence of accident and incident rates, and the consequences gathered in this research effort.