Chapter 2—Primary Information—Flight Paths and Movement Numbers
- 2.1 Flight path maps
- 2.2 Runway end movement information
- 2.3 Flight path movements charts
- 2.4 The average day
- 2.5 Sensitive times
- 2.6 Extent of coverage
- 2.7 Discussion
The first information which is sought by a member of the public when looking into aircraft noise for the first time (e.g. when deciding whether to buy a new house in a certain area) is generally the location of the flight paths. If there are no flight paths near the area the person is interested in then aircraft noise is generally no longer an issue for that person. However, if flight path maps indicate that there will be aircraft overflights of the area then the person invariably asks how many aircraft movements will there be on the flight paths.
Historic flight path information is frequently sought by members of the public from the Airservices Australia Noise and Flight Path Monitoring Systems (NFPMS), which are in place at most of the major Australian airports. This information is made freely available and is used as a key indicator in all airport monitoring reports.
The issue of presenting information on future flight paths, for example in an EIS, is more problematic. In the initial phase of developing the Sydney Airport Long Term Operating Plan (LTOP) a large number of detailed flight path maps were provided to the community. These maps proved very useful as they gave persons a long way away from the Airport an indication where the aircraft were likely to be. However, they did have weaknesses.
While the NFPMS maps show the spread of the flight paths the representation of flight paths in the LTOP report by relatively narrow lines gave the impression to some people that aircraft fly on ‘rail lines’ in the sky. There are other shortcomings which apply equally to both the NFPMS maps and the LTOP maps. For example, the maps do not indicate how often the flight paths have been, or would be, used, and how high and loud aircraft were, or would be, at a particular point. They can also give the impression that aircraft will only be audible in areas immediately under, or very close to, the lines.
2.2 Runway end movement information
At Sydney the most basic form of ‘noise’ information is presented simply in terms of the number of movements (and/or percentages of total traffic) over the four geographic quadrants—north, south, east and west. Clearly a statement such as ‘ 25% of the movements during May were to the north of the Airport’ leaves out a large amount of detail (e.g. whether the movements were landings or take-offs, which flight paths were used, etc). Nevertheless, this information does enable a person to gain a broad appreciation of the distribution of noise in areas close to the airport.
Despite its shortcomings, this very simple way of describing ‘noise’ has received very wide usage in Sydney over the past three years.
In order to address some of the key weaknesses in the types of information discussed in the previous Sections flight path movements charts of the type at Figure 2.1 have been developed. These essentially answer the two basic questions: where do the aircraft fly and how many overflights are there. These only include information on jet aircraft, which are the overwhelming source of public complaint, primarily in order to maximise the clarity of the charts.
These charts are simple to derive and are based on airport NFPMS flight path maps and on aircraft movements information contained in an airport's Avcharges database. This is explained in Appendix B. The charts shown in this Chapter report on what has happened. Producing charts to indicate future noise exposure patterns is discussed in Section 6.4.
Examples of flight path movements charts for Coolangatta, Perth and Brisbane are shown in Figures 2.2, 2.3 and 2.4. These reveal some interesting differences between the nature of the noise exposure at different airports. For example, at Sydney almost all of the areas that receive a high average daily number of overflights per year still have a very significant number of days with no movements (see most of the flight paths in Figure 2.1). By comparison at Perth, areas which have a relatively low average number of daily overflights have a much smaller number of days with no overflights (see almost all the flight paths in Figure 2.3). On the other hand a ‘bad’ day at Sydney tends to be much worse than one at Perth.
While the first question on the number of overflights a person may ask is usually couched in average terms—how many flights will there be a day?—this quickly evolves into questions on the hour to hour, day to day and season to season variations. As indicated in the Introduction (Section 1.5) providing ‘average day’ information using any aircraft noise indicator has the potential to be misleading due to the generally wide temporal variations in aircraft overflights at a given location.
In order to address the average day issue the boxes on the charts include, for each flight path, some simple statistical information on the variations in movements over the time period for which the chart applies. Specifically the boxes show the range in the number of daily movements (ie the ‘quietest’ and ‘busiest’ days) for the period and also specify how many days there were when there were no movements. The boxes also give an indication of the proportion of the total jet movements along each of the flight paths to show the extent to which the movements are shared between the flight paths (this is a particularly important issue for Sydney Airport under its ‘noise sharing’ regime).
The information in the boxes has evolved in response to the most frequent questions raised by the Sydney community. Clearly it would be very simple to use alternative statistical descriptors such as the mode or the number of days when the activity is below a specified threshold level if these more closely responded to the questions being raised by communities at other airports.
Alternatively flight path movements charts could be produced daily and the statistics could relate to the hour by hour variations in movements (e.g. the charts could show the quietest and busiest hours and the number of hours with no movements).
Figure 2.5 is another simple statistical representation derived from the Sydney Airport Avcharges database which has been prepared in response to questions on the number of overflights on the average day over a specific area. This shows that under the parallel runway regime (1995) there were typically around 200 jet overflights a day over areas to the north of the Airport. Under the noise sharing regime (1998) there is much greater variability in the number of overflights and on the most common day the activity levels are approximately half of those under the parallel runway regime.
Figure 2.5 Daily number of Jet movements to the North comparison between LTOP and Parallel runway operations
The average day question can also be addressed by producing the charts on a more frequent basis. This approach means that shorter term variations are less likely to be ‘buried’ by a mass of other information. Figure 2.1 shows the chart for a year; similar charts are produced on a monthly basis for Sydney Airport as part of its ongoing environmental reporting. Shorter term charts could be produced if they were considered necessary.
Information on very short term fluctuations (e.g. hour to hour) in the patterns of aircraft movements, which forms part of this issue, has been treated separately and captured under the topic of ‘respite’. This is discussed in detail in Chapter 3.
As indicated in the Introduction (subsection 1.5.2) a key issue is the reporting of, or making allowance for, movements which occur at noise sensitive times particularly evening, night, early morning and weekends. The standard charts at Figures 2.1–2.4 do not include any weighting or explicit information on what happened at the noise sensitive times.
There would appear to be two broad approaches which could be adopted to address this question. The first would be to adopt some form of weighting system, similar to that used in many standard cumulative noise indices (e.g. ANEF, Ldn), where movements at sensitive times are taken to be x times worse than those at other times. If it were possible to agree on a suitable weighting factor or factors it would be simple to produce a weighted chart. However, experience would suggest that any factors chosen would be the subject of debate and criticism.
The second approach is to produce a separate chart specifically for the sensitive times. This latter approach is preferred since it overcomes the problem of gaining agreement on what is the appropriate weighting factor; instead it simply factually reports the movements. It is also preferred as it is a much more transparent approach and does not ‘hide’ (by swamping with other movements) the fact that there may be significant aircraft activity over certain areas at sensitive times. Nevertheless, a balance has to be struck and it is recognised that producing a multiplicity of charts for separate time periods can be confusing.
Figure 2.6 is an example of a ‘sensitive times’ jet flight path movements chart for Sydney Airport (sensitive times have been defined as 6am–7am & 8pm–11pm on weekdays and 6am–11pm on weekends—see Section 3.3). Comparison between this figure and Figure 2.1 shows, for example, that while areas north of the Parramatta River (flight path C) are exposed to more than twice the number of total overflights, the west (flight path J) has a greater average daily exposure to overflights at sensitive times.
As indicated in sub-section 1.5.3 a very major issue has been how to appropriately and accurately present information on aircraft noise for those areas which lie outside the conventional noise contours.
Putting aside the question of comprehensibility, the most obvious response to this issue would have been to produce conventional noise contours to a lower noise exposure level. The standard outer contour on ANEF maps is the 20 ANEF. Many requests have therefore been made to report out to the 15 ANEF (for example see [ref 8]). However, this approach has largely not been successful as an information tool because the 15 ANEF generally does not extend far beyond the 20 ANEF (for example see [ref 9]).
It has been found that one of the key reasons for the acceptance of the flight path movements charts has been that they give information on aircraft movements for areas at some distance outside the 20 ANEF contour. This point is illustrated in Figure 2.7.
The Sydney Airport flight path movements charts have been in use for approximately two years and have received wide acceptance as a simple tool for reporting on the Airport's aircraft noise exposure patterns. They are used in the Airport's monthly operational reports and have appeared in a number of Sydney newspapers.
The charts are now being produced for other airports in response to requests from airports and community representatives. It is too early to gauge how useful they will be at these airports where the flight path and aircraft movement patterns have different characteristics to Sydney. However, the initial response has been very positive across a wide range of interested parties from community representatives to State planning and transport officials.
In broad terms the charts have been very useful in broadening knowledge about the temporal and spatial variations in aircraft movement patterns particularly the extent and frequency of day to day variations. The topic of hour to hour variations in movements is examined in Chapter 3 under the heading of ‘respite’.
Despite the acceptance they have received the charts clearly have their shortcomings.
While the charts considerably expand the areas for which aircraft noise information is available they are not totally comprehensive—aircraft noise complaints still come from residents of areas which lie outside the areas covered by the charts. Further expanding the areas covered by the charts creates computational and presentational difficulties. Nevertheless, the potential for doing this is now being examined.
The charts only represent a surrogate for noise and are not based on any actual noise measurements—a long haul B747 is given the same value as a small business jet, while an aircraft movement a long way from the Airport has the same value as one close to the Airport. Another weakness is that the visual impression can be given that in the areas where the aircraft are dispersed there is more noise than in those where the flight paths are concentrated even when this is clearly not the case (e.g. compare flight paths B & C in Figure 2.1)—there is a natural tendency for the person's attention to be drawn to the graphic image rather than the numerical information.
There is the potential for the latter problem to be addressed by more sophisticated graphics. The issue of the lack of actual noise information underpinning the charts has not proven to be as great a problem as might first be thought. Most individuals have a ‘calibrated ear’ and broadly know the range of noise levels which aircraft produce when operating in the vicinity of their home. For many people therefore the question of interest is not how many dB(A) a particular flight generates but what can be done to reduce the number of overflights, particularly at the noise sensitive times.
The absence of noise information only becomes an important issue when the flight path movement charts are used to compare the noise exposure at two different locations. Understandably any such comparisons based solely on the number of overflights have been challenged as being too simplistic and metrics based on noise levels need to be used (see Chapter 4).