Chapter 3—Respite—Monitoring and Reporting Breaks from Aircraft Noise
‘…now it is just constant it just never goes away; plane after plane after plane after plane after plane…’
‘…a level of volume which may be tolerable in isolation may become intolerable when it is recurrent—a similar effect to a dripping tap;’
The above quotes are taken from the report of the Senate Select Committee on Aircraft Noise in Sydney and illustrate the respite issue [ref 10]. When the new runway opened at Sydney Airport the noise was concentrated over suburbs to the north of the Airport. The total number of movements to the north of the Airport on a typical day rose from around 160 to around 370. For many people the prime issue became not so much how many movements or how much noise they received in total but whether they were able to get a break from the noise.
The question of providing this break or ‘respite’ from noise therefore became an important part of the discussions during the development of the Long Term Operating Plan (LTOP) for Sydney Airport. Airservices Australia devised a means of providing the respite through a system of rotating the operational runways throughout the day which has been introduced under LTOP. This is of course not possible or suitable for all airports to do. However the monitoring and reporting of respite presented a particular challenge for the noise professionals involved in the process as their training and experience had revolved around the monitoring and reporting of noise not the absence of noise.
Extensive debate took place on how best to define ‘respite’. Is it a period when aircraft noise is below a certain threshold level say 70 dB(A)? Does a period of say 30 minutes without aircraft noise constitute respite? Perhaps it can be defined as say a four hour period when there are no more than four events louder than 70 dB(A)?
A number of different ways of defining and reporting respite were trialed over an approximate two year period. Computational and definitional difficulties led to the exclusion of approaches using a particular dB(A) threshold value. Attention was therefore focussed on methods using the base flight path map from the jet flight path movements charts (see Chapter 2) and analysis of temporal movement patterns using the Avcharges database.
The first promising approach was based on defining respite as any period where the gap between aircraft overflights on a particular flight path exceeded 30 minutes. The total number of minutes in these separate periods was summed to give the total amount of respite say for one day on a particular flight path. For example if there were a jet movement at 10.00am and another at 10.29am on a particular flight path no respite would be recorded. If the movements were at 10.00am and 10.40am then 40 minutes of respite would be registered. Despite its prima facie attractions this approach was abandoned for comprehensibility reasons—when it was trialed the output created confusion and a simpler approach was adopted.
The current approach which is used in the monthly monitoring reports for Sydney Airport is illustrated by the respite chart in Figure 3.1. This is based on computing the number of whole clock hours (e.g. 7am to 8am) when there are no movements on the particular flight paths and reporting these as a percentage of the sum of all the clock hours in the period in question. For example if there were no movements on a particular flight path during 50 clock hours in a 100 clock hour period then it would be reported as ‘Respite Hours 50%’.
Details on the computation of the respite charts is given in Appendix B.
The approach to addressing the average day issue is somewhat different to that adopted in the jet flight path movements charts since the respite charts are focussed on reporting hour to hour variations rather than day to day variations in noise. The respite charts report respite during four discrete periods; three noise sensitive periods (see the next Section) and ‘daytime’ (7am to 8pm) on weekdays.
Similar to the jet flight path movements charts the respite charts are produced on a monthly basis for the Sydney Airport Operational reports and also on an annual basis. The computation of the charts is simple and charts could be produced for shorter periods if more information were required on short term variations in respite.
Information on respite during the sensitive hours is shown on the respite charts. For the purposes of the charts the sensitive times are defined as
Morning Respite 6am to 7am on weekdays Evening Respite 8pm to 11pm on weekdays Weekend Respite 6am to 11pm on Saturdays and Sundays These time periods were selected following an extended period trialing the information with representatives of the Sydney community. There is a strictly enforced curfew at Sydney Airport and therefore reporting on respite during the period 11pm to 6am period has not been a key issue at Sydney. However at other airports which may have a number of movements over suburbs during the night it would very likely be useful to examine and report on respite during this period.
It would of course be theoretically possible to derive some form of weighting for each of the specified sensitive time periods and then produce a figure for the weighted average for each of the flight paths. However similar to the approach spelt out in Section 2.5 the preferred and most robust approach is considered to be one based on reporting the actual respite for each of the designated sensitive periods.
The comments under Section 2.6 equally apply to the respite chart—the charts provide aircraft noise information over a much greater area than the conventional ANEF approach.
The reporting of respite has proved to be a difficult concept and while the respite charts have received some broad acceptance they have not proved as successful as the jet flight path movements charts.
A particular criticism in Sydney has been that computing respite on the individual flight paths does not give an accurate picture as in a number of areas noise is audible (though not necessarily at a high level) from more than one of the flight paths. Therefore it is claimed that the respite on one flight path is disturbed by movements on another flight path. This is a valid comment for certain limited areas which are close to more than one flight path. However no practical suggestions on how this weakness can be overcome have been put forward and for most parts of Sydney the charts give a very good indication of the extent to which respite is being achieved.
Basing respite simply on clock hours is also somewhat a weakness as it only allows the counting of respite in one hour units. This has the tendency to lead to an understatement of the respite. For example if there were one movement at 12.05pm and one movement at 1.55pm no respite would be recorded for that two hour period. This would be despite the fact that for virtually all that period (110 minutes) there were no movements.
With the phase out of the older noisier ICAO Chapter 2 aircraft the current worldwide trend is for aircraft noise problems to increasingly focus on the high numbers of noise events (rather than individual ‘noisy’ aircraft). Given this it is inevitable that community pressures to get a ‘break’ from aircraft noise (and the need to quantify this respite) are not going to disappear. The area of monitoring and reporting of respite is not well established and clearly further development work is required on the concepts trialed at Sydney if mature approaches are to be arrived at.