Approach operations have historically received less attention than departures in community noise model systems. However, recent attention to the study of Continuous Descent Approaches both in the US and Europe have re-focused the research community on the approach noise modeling assumptions in the assessment tools available to the airport planner. These include the source noise definition as well as the general relationship between noise and aircraft performance that is central to modeling systems. Modeling approach noise performance is much more difficult than departure due to the uncertainty in approach trajectory resulting from air traffic control procedures and pilot technique. Existing noise model guidance does not provide a method to make the assessment tractable for the general user. Inaccuracies in the size of aircraft noise contours due to the uncertainties listed above will make the assessment of CDA's error prone for the decision planners who balance CDA's across a broad array of noise mitigation alternatives. In this paper we analyze radar data from two major airports in the US to determine the trajectory variation that is typical in operation, gain insight into the procedural constraints that drive this variation, and develop trajectory clusters that are representative of nominal operations. Results of the analysis indicate that there is a tendency to have average glide slopes slightly higher than the nominal 3 degrees, and that downwind approaches tend to have a higher glide slope than straight-in approaches. It is the intent that the result of this research be made available to the general user through input into guidance given in international noise modeling documents such as SAE-AIR-1845, European Civil Aviation Conference Document 29 and ICAO Circular 205.
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