Effect of flight path dispersion on airport noise

The social impact of aircraft noise is usually assessed by way of estimates of the “energy sum” of noise delivered to points on the ground. Though in such estimates one commonly supposes that the noise generating aircraft fly along specific trajectories, it has been shown that statistical dispersion about the mean flight path can be significant. The literature suggests that the effect is dominated by lateral excursions due to fluctuations in heading during departure.In this paper an additive term is presented to correct “energy sum” based noise indices for lateral flight path dispersion. Gaussian distribution of flight paths and logarithmic decay of noise level are assumed in the mathematical model. Under these conditions, the correction term depends only on the horizontal and vertical distances between the observer and the closest point on the mean flight path, both distances being normalized by the standard deviation of the lateral fluctuations.The limited data that have been reported indicate that the standard deviation tends to grow linearly with distance after liftoff. If so, the correction for a given aircraft category will tend to be constant along lines radiating outward from the lift-off point. Lateral dispersion always reduces the noise index along the ground track. This reduction decreases to either side, vanishing at some 3 to 6 degrees azimuth. For larger azimuth angles, dispersion increases noise energy-the maximum occurring at some 10 degrees. In typical airport situations this maximum increase may be as great as four decibels. Accordingly, as these results represent a lower bound for them, dispersive effects may significantly influence airport exposure.A numerical example illustrates how lateral flight path dispersion broadens and foreshortens contours of equal noise exposure.