Ground Noise Predictions
London Gatwick Airport
Operational noise from airports is considered either as air noise (aircraft taking off, in flight or landing) or ground noise (anything which does not come under the heading of air noise). Ground noise includes taxiing to and from either end of the runway but whilst an aircraft is on the main part of the runway this is classified as part of the procedure for either take-off or landing. Air noise is generally much louder than ground noise and has a greater impact on most residential receiver locations. However, ground noise affects locations off to either side of the airport which are not under flight tracks but close to aprons and taxiways. The more continuous nature of ground noise contributes to the local noise environment, particularly when the airport is otherwise quiet, i.e. between take-offs and landings.
Following the formation of the Airports Commission towards the end of 2012, Gatwick Airport Ltd began looking at a number of different scenarios for a second runway. Hayes McKenzie were commissioned to carry out predictions for assessing potential ground noise effects associated with a number of different scenarios for a second runway at Gatwick. Ground noise predictions and associated assessments were summarised in a report prepared as part of Gatwick’s submission to the Airports Commission in 2013.
The submission included five different operational scenarios for a second runway and a base-case model which predicted future levels of ground noise assuming no second runway development. For each of these 6 scenarios, ground noise was modelled separately for easterly and westerly runway operations. Since the level of air-traffic around the airport varies significantly throughout the day, noise was also separately modelled for day, evening, night and 24 hour periods. This resulted in a total of 48 separate ground noise models covering all of the potential operational scenarios presented to the Airports Commission.
Although ground noise encompasses a large variety of operations at the airport, the noise generated by an aircraft’s main engines during taxiing and auxiliary power unit (APU) whilst parked at a stand are generally much louder than any other ground based operations. Since other sources make very little contribution to the overall level of ground noise, predictions are simplified by limiting the sources to taxiing (main engines) and stands (APUs).
The noise models were made up of large numbers (around 1000 depending upon the proposed layout) of point sources, each with its own directivity characteristics, sound power and ‘on-time’ that had been calculated based on traffic data, assumed taxi routes and average turnaround times. Measured sound power and directivity data were available for two different aircraft types that were used to represent ‘small’ and ‘large’ aircraft. Airport traffic forecast data was filtered to separate numbers into either ‘large’ or ‘small’ aircraft arriving and departing. This information was then used to calculate the percentages of ‘small’ compared to ‘large’ aircraft that would be present on a given taxiway or stand (over a given period) and this in turn defines an average sound power and directivity for the point sources (based on the percentage of each aircraft type). On-time for each point source was calculated in one of two ways: for stands it was calculated from an average turnaround time multiplied by an average (fractional) number of aircraft (based on traffic data and an even distribution); for taxi routes it was calculated based on the length of taxi route that was represented, an assumed speed and the volume of traffic going through.
An assessment was carried out by comparing different scenarios to the base-case such that the relative impact of each option could be determined at various representative noise-sensitive locations surrounding the airport.
The results of the assessment provided our client (Gatwick Airport Ltd) with comprehensive information about the overall impact and the relative differences between the different options for a second runway. The information fed into an overall appraisal of the various scenarios that was submitted to the Airports Commission