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Noise Abatement (Airside) Techniques 127 to raise altitude of aircraft as they pass over areas on final approach. When arrival noise is the issue, the full runway length may be used for departures. Arriving aircraft would land at or after passing the displaced landing threshold. Displaced thresholds are more often used for obstacle clearance than noise abatement because their effectiveness in reducing noise is so small. Under a typical approach, the displacement of a runway by 1,000 feet will reduce the altitude of a landing aircraft by only about 50 feet and its accompanying noise levels by less than 1-2 decibels just beyond the clear zone, with decreasing benefits at greater distances from the runway end. On arrival only runways, that effect may reduce overall DNL by a similar amount. Limitations: Different aircraft require different runway lengths for safe operation. If the runway is displaced too far, some larger aircraft may no longer be capable of using it. The measure also may move post-landing ground noise farther to the departure end of the runway. Implemented by: Airports, with FAA approval of any changes to the airport layout plan. Public reaction: The public is rarely aware of the use of a displaced threshold unless the dis- placement is long and the runway is almost exclusively used for landings. Relocated Runway End - Takeoffs Purpose: A relocated runway end for takeoffs is the establishment of a location for the begin- ning of takeoff that is offset from the landing end of the runway. Not used nearly as often as a landing threshold displacement, this technique allows takeoffs to be initiated at greater distance from noise-sensitive uses near the landing end of the runway. The benefit that might be gained by the relocation must be balanced against the decreased safety imposed by shorter takeoff length available and lower altitudes during climb out. With a takeoff end relocated 1000 feet down the runway, a typical aircraft will be about 280 feet lower than without it. Limitations: Different aircraft require different runway lengths for safe operation. If the runway end is relocated too far, some larger aircraft may no longer be capable of using it. The measure may also noticeably increase noise levels near the overflight end of the runway. Implemented by: Airports, with FAA approval of any changes to the airport layout plan. Public Reaction: The public is rarely aware of the use of a displaced threshold unless the dis- placement is long or residences are near the runway ends. High Speed Exit Taxiways Purpose: A high speed exit is a specially designed, angled taxiway that allows arriving aircraft to leave the runway prior to coming to a full stop. Use of high-speed taxiways may diminish the noise generated on arrival at neighborhoods adjacent to the airport by allowing aircraft to exit taxiways earlier and reduce their taxi time to their parking positions at the gate or on the apron. Limitations: May be cost prohibitive for some airports to implement. Implemented by: Airport, subject to ALP approval by the FAA. Public reaction: Generally positive reactions from the public evolve from the removal of taxi- ing aircraft from the runway when the runway is adjacent to populated areas. Conversely, air- craft may apply greater levels of reverse thrust to slow enough to use a high speed exit taxiway. As the aircraft moves away from the populated areas into the core of the airport, the noise asso- ciated with the landing aircraft becomes less obtrusive.