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Noise Abatement (Airside) Techniques 123 for the weight of aircraft, a preferential program may result in the preferred runway's accep- tance. Virtually all preferential use programs in the United States are voluntary and require the concurrence of the pilot in charge of the aircraft to be fully implemented. In some cases the pilot may wish to use another runway for safety purposes. When that happens, air traffic controllers will attempt to find gaps in the traffic flow that will allow the off-preference operation to take place, but the pilot may have to delay the operation to use the desired runway. Further, the most preferential runway use pattern for noise abatement may not be the most efficient pattern for operation of the airport. Consequently, preferential runway use programs may be limited to off- peak or nighttime hours to achieve their greatest application. Implemented by: FAA air traffic and airlines/operators. Airports may request the adoption of specific program measures to achieve management goals. Public reaction: When preferential runway use schemes are used to abate aircraft noise, they must take into consideration the number of persons exposed to varying levels of noise under the routes of flight. As in any case, those who are overflown will react negatively while those bene- fited by the program by not being overflown will react positively. Consequently, the correlation of noise exposure to population densities is an important consideration in the design of these programs. Public education as to the conditions and benefits of the program are also critical to their success. Example of Technique for Public Information: Comparison of 2020 No Action and Alternative B1b Noise Exposure Pattern (165) 8-2 Flight Track Modifications to Fly Over Compatible Uses Purpose: To reduce the population within noise-sensitive areas, development of specific arrival/ departure procedures may be utilized to direct aircraft while in flight. On approach or departure, use of specific departure headings, waypoints, and FMS/GPS procedures can be very helpful in keeping aircraft over the designated flight path. Flight track modification has been used to define preferred departure routes and approach courses, to separate large and small aircraft for noise abatement and to increase operational efficiency, and to specify courses for general aviation pro- peller aircraft and helicopters to assure that they do not conflict with faster jet aircraft. Limitations: Requires FAA approval in the United States of procedures, a process which can be time consuming. Some aircraft may not be properly equipped with the correct navigational instruments if the procedure uses FMS/GPS procedures. Aircraft using standard departure head- ings ("Fly heading 090 for 2 NM") may be expected to deviate from the nominal flight path due to wind and weather conditions. When tracks are preferred or advisory (as are some helicopter routes), their use is not required. Implemented by: FAA Air Traffic is responsible for the development and management of air- craft track procedures within a controlled airspace environment. Operators are responsible within uncontrolled airspace. Airports may recommend modifications to improve noise exposure conditions. Public reaction: When flight tracks are moved to overfly compatibly used lands such as farm- land, industrial/commercial areas, transportation corridors or water, the public reaction is gen- erally positive if the traffic has been moved from over residential areas. However, if the traffic is moved from areas of dense residential population to areas of less dense residential population to achieve fewer population impacts, or if new areas of residential use are exposed to overflights beyond the compatibly used property, controversy generally will be an outcome, particularly if those newly impacted persons have not been a part of the creative process to define the courses of flight. This action has been the nexus of most airport noise litigation in the United States during