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122 Aircraft Noise: A Toolkit for Managing Community Expectations The subsequent paragraphs provide a short definition of the purpose of each measure, the lim- itations to its implementation, the parties responsible for its implementation and management, the general public reaction to its use and the sources of additional information about its appli- cation. Examples of how the action may be displayed to explain its effects to a public audience is provided for the most effective of these techniques. Flight Management Techniques To abate the noise of aircraft in flight, the following techniques have been used at airports across the United States in efforts to seek relief for those affected. Continuous Descent Approach (CDA) Purpose: An automated arrival procedure designed to reduce noise impacts on communities located under the approach path to a runway. The procedure is designed for aircraft to utilize a continuous descent of aircraft on a gradual slope while idling without the deployment of flaps and landing gear to reduce airframe noise. Procedure provides benefit to communities located 7-15 miles from airport and reduces fuel burn where approaching aircraft will typically fly level segments of their arrivals to sequence into the air traffic flows. These level flights may result in higher noise levels as aircraft use more thrust to remain level and have gear and/or flaps extended to maintain slow speeds. The CDA technology is new and has been implemented only at Louisville, KY, and London Heathrow Airports. CDAs are in development at several other air- ports, including Atlanta Hartsfield, Mather Field, and Los Angeles International. Limitations: CDA requires extensive research and coordination in the development of the descent profile. Procedures are designed for one or more runway ends, but usually both. It has not yet been shown to work well in environments exposed to conflicts between arriving and departing traffic, nor during peak operating periods. Procedure works very well in single operation, off-peak periods when properly designed. Implemented by: Airlines/operators with guidance from the FAA Air Traffic and Flight Standards. Airports may seek their approval if believed to be a desirable noise abatement action. Public reaction: Public experiences lesser noise levels along the full course of the approach through the absence of thrust adjustments required to maintain episodes of periodic level flight. Reaction to date on this relatively new procedure has been generally positive. The investigation of CDAs is one of the PARTNER projects undertaken by the FAA/NASA/ Transort Canada sponsored Center of Excellence. Additional information about CDAs may be found at: http://web.mit.edu/aeroastro/partner/projects/project4.html (164). Preferential Runway Use Program Purpose: This is an airport-initiated program that delineates selection procedures for the use of each runway at an airport. The runway use program can be either informal (voluntary) or for- mal (mandatory). These programs attempt to manage the number of aircraft that fly over areas along the approach and departure routes leading to or from each runway end. By doing so, the programs manage the overall noise energy present, over time, in those areas. To be effective for noise abatement, they should always be indexed to the population overflown or incorporate techniques to provide relief and respite to those persons under the preferential routes of flight. Limitations: Preferential runway use programs are driven by wind availability. When wind conditions are favorable (up to a five knot tail wind component) and runway length is adequate