Appendix B—Derivation of Noise Indices
- B.1 Flight path movements charts
- B.2 Respite charts
- B.3 N70 contours
The jet flight path movements charts shown in Chapter 2 are derived from two sources.
The base flight path ‘zones’ shown on the charts have been derived from examination of flight path maps covering the period represented on the chart. The flight path maps are produced by the airports' noise and flight path monitoring systems. The ‘zones’ are designed to give a good indicative representation of the general flight path patterns, say 90% of jet aircraft movements. They do not represent the total spread of aircraft on the particular flight paths.
The number of aircraft movements assigned to each flight path at any airport is derived from that airport’s Avcharges database. The Avcharges database is established by Airservices Australia to facilitate charging of airport users. The database contains information on runway use, aircraft type, time of day of operations, destination and origin of aircraft, etc.
For some airports the assignation to flight paths is simply derived from runway use (e.g. Coolangatta Airport—Figure 2.2). At airports where the charts show splits on the approach or departure flight paths (e.g. Sydney and Perth Figures 2.1 and 2.3) assignation is based both on runway use and either on interrogating the database on port of origin and/or destination or by estimating the number of aircraft using each flight path through examination of the flight path maps.
The statistical information in the boxes is simply derived from interrogation of the Avcharges database.
These charts use the same flight path base map as the flight path movements charts.
Respite is simply derived from interrogation of the Avcharges database. As indicated in Section 3.1, respite as shown in Figure 3.1 is calculated by counting the number of clock hours when there were no jet movements on a particular flight path and reporting this as a percentage of the total number of hours during the period of interest.
The N70 contours have been derived using the US Federal Aviation Administration's (FAA) Integrated Noise Model (INM).
N70 contours are not on the menu of INM output metrics. The contours have therefore been produced indirectly from the model by, in essence, computing a detailed grid and summing the number of events within the model which register a non zero time above 70 dB(A). The Department has developed a computer program which carries out this task and produces an N70 grid which enables the contours to be drawn using the INM contouring routine or another contouring program.
Figures 4.8 and 4.9, which show N70s for Sydney Airport on the 18 Jan 1999 and 20 Jan 1999, were derived by manipulating the aircraft type and runway use data in the 1998 ANEI files to correspond with the actual activities that took place on those days. The flight paths were not adjusted from those used in the 1998 ANEI—examination of the flight path maps for those days did not show any significant divergence from the flight paths used for modelling the average day. The mix of aircraft stage lengths for each particular aircraft type for the two days was assumed to same as that for the average day.
Figure 4.10 which shows the N70 for the ‘sensitive times’ was similarly derived by interrogating the Avcharges database for 1998, ascertaining the jet movements during the times selected and using this to replace the data for the average day. The flight paths and stage lengths were treated in a similar manner to that for the other partial N70s.
The computations of the PEI/AIE in Chapter 5 involve the number of persons within specified N70 contours. Table 5.1 also shows the total number of persons within the 10 events per day louder than 70 dB(A) contour at specified airports.
These computations have been based on the Australian Bureau of Statistics 1996 census data. Each INM grid point is assigned a population based on the population density within the individual census Collection Districts. This enables the total population exposed to any specified N70 level to be computed.
As indicated above the N70 contours have been derived indirectly using information in the INM model. The N70 is therefore not an FAA verified contour. The Proponent's Statement [ref 20] contains a short discussion on some potential sources of computational error in the contours.
While no formal study has been carried out in Australia to verify the accuracy of the contours, they are checked informally and the work to date indicates that broad confidence can be placed on them as being good indicative contours.
When preparing an N70 an initial informal check of the contours is made which involves ensuring that the N70 values are consistent with the number of operations that took place during the period of interest and with the 70 dB(A) single event contours for the types of aircraft that operated.
An attempt has also been made to verify the INM computed N70 values by comparing them to the measured N70 values recorded at the permanent noise monitoring terminals (NMTs) sited around Sydney Airport. This comparison is shown in Table B.1.
It can be seen that at the noise monitoring terminals the agreement between modelled and measured values is broadly of the order of the mean +/-10%.
The agreement is worst at Annandale but the difference is not biased in one direction—on the Mode 9 day the modelled value is significantly higher than the measured value; this is reversed on the Mode 10 day. It appears that this problem has arisen because Annandale is just on the edge of the 70 dB(A) contour for certain aircraft types. With a threshold type indicator such as the N70, ‘edge effects’ will occur—a reading of 69.9 dB(A) is a ‘no’ while a reading of 70.1 dB(A) is a ‘yes’.
Conceptually it would be expected that, given the number of events involved and the way they have been computed, the N70 for the annual average day would be more accurate than the short term N70s. It would also be expected that, due to the greater variation in the flight paths of departing aircraft, there would be more uncertainty in the N70 values for those areas impacted primarily by noise from departing aircraft (e.g. to the north-west at Sydney—see Figure 2.1) than for those areas impacted by noise from arrivals.
Table B. 1: Comparison of modelled and measured N70s—Sydney Airport
| Noise Monitoring Terminal
|1998 N70||Mode 10 N70||Mode 9 N70|
- INM—N70s modelled using the Integrated Noise Model
- NMT—N70s measured at the Sydney Airport Noise Monitoring Terminals