The Federal Aviation Administration (FAA) uses established policies to assess potential noise effects of implementing performance-based navigation (PBN) at airports. FAA has been implementing PBN to allow aircraft to fly more precise flight paths intended to reduce flying time, fuel use, and emissions, and PBN may reduce aircraft noise for some communities. FAA uses the Day-Night Average Sound Level (DNL) metric to meet legal requirements in assessing how these more precise flight paths—which can concentrate noise over a smaller area—might affect noise levels at various locations surrounding airports. DNL accounts for the noise intensity, duration, frequency, and time of occurrence for flights above a particular location over an average day.

GAO's analysis showed that because DNL combines the effects of several components of noise into a single metric, it does not provide a clear picture of the flight activity or associated noise levels at a given location. For example, 100 flights per day can yield the same DNL as one flight per day at a higher decibel level, due to the averaging effect of FAA's metric. GAO's analysis and other research demonstrate the limitations of FAA relying solely on DNL to identify potential noise problems. Also, community concerns about increased noise after PBN implementation, among other factors, have led to legal challenges and delays, reducing the realized benefits of PBN. Since no single metric can convey different noise effects, using additional metrics—such as changes in number of flights overhead—in designing proposed flight paths could help FAA identify and address potential noise concerns.

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