Sensitivity analysis of NA "Number Above" noise indicator

DGAC/DTA asked STAC to make this study to assess the potential use of noise index NA “number above” in France.

Three cases were studied : a virtual airport, Bâle-Mulhouse airport (noise insulation map 2008) and Orly airport (noise insulation map 2004).

The impact of different parameters on NA noise contours was studied to better-understand NA behaviour. This sensitivity analysis concerns :

• parameters linked with traffic assumptions : total number of movements, relative impact of landings and take-offs, relative impact of heavy aircraft / medium aircraft / regional aircraft / general aviation, and impact of fleet characteristics;
• modelling parameters : flight profiles, INM database, track dispersion, topography, grid refinement;
• parameters linked with the noise index construction: NA-SEL, NA-night The NA noise contours (NA70, NA65, NA62) were calculated for many scenarios.

The analysis of surface and graphical comparisons allowed to better-understand NA behaviour and to compare with Lden behaviour.

This study identifies not only the well-known advantages of NA (simple definition easily understandable by the public, arithmetic behaviour, frequency effect taken into account in NA definition), but also the principal technical drawbacks of this index:

• Difficulty to define common and relevant limit values for every airfield;
• Important threshold and border effects;
• Noise index more unstable and unforeseeable than Lden index;
• Limited and, in some cases, non existing effects of fleet modernization on NA contours.

The matter of correlation between NA index and noise annoyance expressed by the neighbours around airfields is outside the scope of this study and shall be studied in serious epidemiologic survey which could, for example, conclude on the relevance to use NA-night index to evaluate sleep disturbance and long term health effect.