Public Notice: Clarifying the Definition of “Substantial Restoration of Natural Quiet” at Grand Canyon National Park, AZ

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On April 9, 2008, the National Park Service (NPS) published a Public Notice of agency policy in the Federal Register with the above title (73 FR 19246-19248), clarifying the NPS definition of substantial restoration of natural quiet at Grand Canyon National Park (GCNP) to distinguish between aircraft noise generated above and below 17,999 feet mean sea level (MSL), and requesting comments on the proposed clarification. Specifically, the NPS proposed the following clarification: (a) Substantial restoration of natural quiet at GCNP will be achieved when the reduction of noise from aircraft operations at or below 17,999 feet MSL results in 50% or more of the park achieving restoration of the natural quiet (i.e., no aircraft audible) for 75% to 100% of the day, each and every day; and (b) The NPS defines the substantial restoration of natural quiet from all aircraft above 17,999 feet MSL to mean that there will be an overall reduction in aviation noise generated above 17,999 feet MSL above the park over time through implementation of measures in accordance with Federal Aviation Administration (FAA) commitments. The NPS also clarified that 50% of the park is a minimum in the restoration