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ARTICLES Table of Contents   
Year : 1999  |  Volume : 1  |  Issue : 3  |  Page : 27-44
Non-acoustical factors in noise management at heathrow airport

1 Institute of Sound and Vibration Research, University of Southampton, United Kingdom
2 Airside Environment Manager, BAA Heathrow, United Kingdom

Click here for correspondence address and email

Non-acoustic factors in environmental noise can be broadly defined as all those factors other than noise level alone which contribute to noise annoyance and similar effects. Noise levels such as LAeq provide a good indication of the amount of physical noise present, and changes in physical noise level can be expected to correlate with changes in resident's perception, at least to some degree. On the other hand, a flexible approach to noise management based on wide consultation and communication can be extremely important. At Heathrow Airport over the last 20 to 30 years, overall aircraft noise levels have reduced because of the phasing out of the older noisier Chapter 2 aircraft types, but there are also other strategies in place which are clearly regarded as being effective both by airport management and by local community representatives and which are not as easily quantified in terms of noise level alone. This paper describes the background to noise management at Heathrow in some detail and draws a general conclusion that taking non-acoustic factors into account in addition to physical noise levels alone has been of considerable benefit over the years.

How to cite this article:
Flindell IH, Witter IJ. Non-acoustical factors in noise management at heathrow airport. Noise Health 1999;1:27-44

How to cite this URL:
Flindell IH, Witter IJ. Non-acoustical factors in noise management at heathrow airport. Noise Health [serial online] 1999 [cited 2023 Nov 28];1:27-44. Available from: https://www.noiseandhealth.org/text.asp?1999/1/3/27/31715

  Introduction Top

Heathrow is the world's busiest international airport, and because it is also relatively close to densely populated parts of West London, at least some aircraft noise problems are inevitable. Between 1st April 1997 and 31st March 1998 Heathrow handled 431,904 air transport movements, 58 million passengers and over 1 million tonnes of cargo excluding airmail (BAA Heathrow, 1998). The most recently published official air noise contours (for summer 1996) show around 299,000 residents within the 164.7 sq. km area enclosed by 57 LAeq,16hr day contour (DETR, 1998). For guidance, this aircraft noise level has been defined by the UK government as representing 'the onset of low annoyance'. In 1980, the number of residents included within the 57 LAeq,16hr aircraft noise contour (equivalent to 35 NNI as used at that time) was estimated to be around 944,000, and in 1972 it was estimated to be around 2,092,000 (Neil, 1997) indicating a considerable improvement in the aircraft noise climate overall. However, aircraft noise is only one of many contributors to the ambient noise environment in the residential areas around Heathrow and there has been and continues to be a considerable amount of road traffic noise in many areas (Flindell, 1995). There is no evidence that the amount of road traffic noise has similarly reduced. The relative contribution made by aircraft noise over the area as a whole has almost certainly diminished.

The airport has been well-established for many years and over 66,800 people are directly employed there. The total wages bill exceeds £3 billion and a further 125,000 jobs across the UK economy as a whole are estimated to be dependent on the airport (BAA Heathrow, 1998).

And of course, 58 million passengers use the airport every year. Because of this, all noise management issues have to be finely balanced against the economic and social benefits provided by the airport.


Air noise management at Heathrow is principally a government responsibility under the terms of the Civil Aviation Act 1982 and various international agreements, but BAA Heathrow (the privatised company which now owns and operates Heathrow Airport) has made an increasing contribution to a number of noise management issues in recent years. Much of the detail for this section can be found in various proofs of evidence submitted to the Heathrow Terminal 5 public inquiry (see Neil, 1997)


The airport has two main parallel runways (north and south) which are oriented approximately east-west. There is a shorter cross-wind runway which is used relatively infrequently. A system of westerly preference has been operated at Heathrow since 1962 which together with the prevailing south westerly wind direction determines that around three-quarters of all arrivals approach the airport from the east, thereby flying over some of the most densely populated parts of West London. Around three­quarters of all departures climb away from the airport over mostly less densely populated areas to the west of the airport. Around a quarter of all arrivals approach over parts of Windsor and Slough to the west of the airport, while around a quarter of all departures climb away along various routes over West London.

Departures noise limits

Noise limits for departing aircraft were first defined at Heathrow in 1959 by the government of the day (DETR, 1997). The limits were set at 110 PNdB (0700 to 2300 hrs day) and 102 PNdB (2300 to 0700 hrs night), A number of different noise monitoring systems have been deployed over the years and when the current Noise and Track Keeping (NTK) system was installed by BAA Heathrow in 1992-93 these limits were formally redefined in terms of their LAmax equivalents at 97 dBA (day) and 89 dBA (night). At the same time, the existing departures noise monitors were moved to a standard distance of 6.5 km from the start of roll position, or as close to that distance as could be achieved given the various geographical constraints on the ground. There are currently eight noise monitors positioned at various sites around the airport. Positional adjustments are applied where the actual distances are either slightly less or slightly more than the desired 6.5 km. More recently, the government proposed reducing the departures noise limits to 94 dBA (day) and 87 dBA (night) (DETR, 1997), but no decision has been taken as of the time of writing.

Noise infringements

The Noise and Track Keeping system is operated by BAA Heathrow which levies financial penalties on operators exceeding these noise limits. Each infringement of 3 dBA or less is charged at £500 and each infringement exceeding 3 dBA is charged at £1000. There are a small number of exemptions including Concorde, and additional allowances are made where aircraft are required to take-off under tail wind conditions. In the 1997/98 year there were 242 daytime infringements out of the total of 217,759 daytime departures (0.11%) and 188 night-time infringements out of the total of 3,206 night-time departures (5.86%) (BAA Heathrow, 1998).

Night restrictions

Additional restrictions have been applied by the government to limit the amount of aircraft noise at night. The recent government second stage consultation paper on its proposals for future night restrictions provides a good background to this subject (DETR, 1998), and there is also a comprehensive historical record available (Terminal 5 public inquiry Position Statement PS/7.2, 1997). The first night restrictions imposed in 1962 simply limited the total numbers of movements between 2300 and 0700 hrs. In 1965, both the number of movements and the times within which the restrictions applied were reduced. Various minor changes were made up until 1978 when separate night-time movement limits for noisier and quieter aircraft were introduced.

The present night noise quota scheme was established in 1993 and is currently under review. Each aircraft type is assigned an arrivals and a departure quota count classification on the basis of the certificated approach, departure and sideline noise levels. The government has set overall summer and winter night quotas which apply between 2330 and 0600 hrs and which are intended to encourage the use of quieter aircraft within those periods. In parallel with this there are also separate overall movement limits for the summer and winter seasons. Additional restrictions are applied to the noisiest aircraft types within the overall night period between 2300 and 0700 hrs.

Differential landing charges

In addition, BAA Heathrow has for many years differentiated between noisier and quieter aircraft types in respect of landing charges. The noisier aircraft types certificated to ICAO Chapter 2 pay a 30% surcharge. (Modern, quieter aircraft types are certificated to ICAO Chapter 3). The very small numbers of aircraft without even a Chapter 2 noise certificate pay a further 30% on top. BAA also discourages operators from re-introducing non-Chapter 3 aircraft types to Heathrow through its Conditions of Use. To reward airlines investing in the quietest aircraft types (defined as having a quota count classification of 1 or less) there is a discount of 10% on the standard charges.

Noise insulation schemes

Four separate Noise Insulation Grant Schemes (NIGS) were set up by the government of the day under various Civil Aviation Acts around Heathrow between 1966 and 1980, with a subsequent amendment in 1989 (Terminal 5 public inquiry Position Statement, PS/7.4, 1997). Each of these historic schemes was fully funded by the then British Airports Authority who managed the airport before privatisation. Each of these schemes provided internal secondary glazing, acoustic ventilators and secondary works such as additional ceiling insulation, blocking up chimneys, etc. within set cost limits and were administered by the local authorities concerned. Eligibility was defined in terms of the property being within specified aircraft noise contour areas, using first the NNI (noise and number index) and then in combination with a composite 95 PNdB footprint representative of aircraft operations at night. The total expenditure between 1980 and 1986 on the 1980 scheme was £22,893,000.

The 1995 Noise Insulation Scheme (NIS) is slightly different from the various historic NIGS schemes in that it was set up and administered on an entirely voluntary basis by BAA Heathrow. All residential properties within a defined 69 LAeq,18hrs aircraft noise contour were eligible whether or not they had been insulated under any previous scheme. The 69 LAeq,18hrs contour was constructed using predicted 1994 0500 to 2300 hrs (18 hours) daytime traffic with all movements between 0500 and 0700 counted twice (weighted by 3 dB) to reflect local concerns about early morning traffic at Heathrow. When the air noise contours based on actual 1994 summer traffic were published in 1997, they were found to extend beyond the air noise contours based on predicted 1994 traffic and the scheme was extended to take this into account.

Residents were offered traditional internal secondary glazing at no cost or a 50% contribution towards the cost of replacement windows with either standard or high performance sealed unit double glazing. Loft insulation and acoustic ventilators were also provided at no cost. Field trials at Heathrow showed that replacement windows fitted with sealed unit double glazing can outperform the standard secondary glazing systems specified in earlier schemes (Davis, 1993), but it is of course, more expensive. Take-up during the 1996 pilot scheme was 62%. It was estimated that the total cost of the 1995 scheme to BAA Heathrow would be around £10,000,000 on the assumption that the overall take-up would be between 60% and 70% of the 7,385 properties then determined to be eligible.

Complaints investigations

Statistics on aircraft noise complaints have been collected by government departments, the British Airports Authority, and by local authorities around Heathrow for many years (Terminal 5 public inquiry Position Statement PS/7.3, 1997). In September 1994, BAA Heathrow took over responsibility for handling aircraft noise complaints from the then Department of Transport. For the last three years the new BAA Heathrow Community Information Office at the purpose built Heathrow Airport Visitor Centre has recorded all complaints using a Geographic Information System database linked to the airport Noise and Track Monitoring system. Anyone wishing to lodge a complaint, or indeed to make any kind of enquiry, is encouraged to use a freephone telephone Noise Line. All enquiries are investigated by the staff at the Community Information Office. Where appropriate the precise details of all aircraft movements taking place near to the caller's address at the time of the enquiry are sent on to the caller. Residents are frequently invited in to the open access Visitor Centre where any particular concerns can be addressed.

After an initial surge when the freephone telephone NoiseLine was first opened, the total numbers of complaints has steadied out. Between 1st April 1995 and 31st March 1996 there were 6,796 separate enquiries recorded from 3,676 callers referring to a total of 10,158 incidents or events. Two years later, between 1st April 1997 and 31st March 1998 there were 7,070 separate enquiries recorded from 3,332 callers referring to a total of 13,061 incidents or events. The number of callers has gone down, while the numbers of enquiries and incidents or events reported by those callers has gone up. An average caller might only call in once or twice per year and an increasing proportion of complaints are made by only a small minority of callers. The types of enquiries made cover a complete spectrum from a small minority who obviously have very great difficulty in living adjacent to a major international airport to other residents who merely wish to report aircraft apparently straying from their customary route, or apparently flying too low, or some such similar enquiry. The overall number of callers at 3,332 per annum is quite substantial, yet this represents just over one percent of the around 300,000 residents included within the 1996 57 LAeq,16hrs air noise contour area.

It is obviously impossible to compare the situation with what might have happened had there been no Community Information Office, but there is a general consensus that the work of the office has been extremely beneficial in terms of general attitudes towards the airport as a whole. Complaints investigations have no direct effect on noise levels as such, although discussions held with operators as a result of enquiries have contributed to improved track keeping over recent years. In addition, the patterns and trends of complaints are monitored by BAA Heathrow and this has resulted in better communications with communities affected by noise.

Formal consultation

The main consultative forum is the Heathrow Airport Consultative Committee (HACC). The HACC includes representatives from local authorities and councils, environmental, consumer and tourism groups, groups representing passengers, the business community, airlines, and the government as an ex-officio member. The HACC receives regular reports on noise complaints, runway alternation, noise infringements and night noise quota use and is a forum which allows members to raise any issues concerning noise. The work described below under the headings of the westerly preference, the Cranford Agreement and night alternation was carried out under the supervision of a special Noise Management Initiatives sub­group of the HACC.

BAA Heathrow also chairs a Noise and Track Keeping (NTK) Working Group which includes representatives from the HACC. This group was initially set up as a forum for discussing the use of the noise and track keeping system and assisting the HACC with interpretation of the results. Subjects covered include ground noise, arrivals noise and departures noise. Reports are presented on trials underway as well as on completed work.

The government publishes consultation papers regularly on all aspects of the noise management regime within its spheres of interest. All written responses to consultation papers are made available for inspection and summarised in subsequent documents. The Aircraft Noise Monitoring Advisory Committee (ANMAC) was set up by the government to advise the Secretary of State and includes representatives from the Heathrow, Gatwick and Stansted Airport Consultative Committees, the airlines, the individual airport operating companies and the National Air Traffic Services (NATS).

In addition to formal consultation, Heathrow Airport managers regularly attend public meetings where noise issues are often discussed. The various consultation mechanisms are kept under review, and there have been changes from time to time.

Runway operations at Heathrow

The existing runway operating regime at Heathrow has evolved over many years in response to previously expressed public concerns over noise impacts. It seems reasonably clear that the existing runway operating regime is a major non-acoustic factor influencing people's attitudes to aircraft noise in the Heathrow area. It is generally assumed that the community would react unfavourably to any major changes to the existing runway operating regime unless the benefits of any such changes could be clearly demonstrated and unless agreement with community representatives had been achieved.

The three main components of the existing runway operating regime are as follows;

  • Westerly preference
  • Cranford agreement
  • Daytime runway alternation

It is important to note that conventional noise indicators, such as LAeq,16hrs, can be relatively insensitive to small changes in the runway operating regime which anecdotal evidence suggests could be of major significance as far as resident's perceptions are concerned. Daytime runway alternation compared to non-alternation really has no effect at all on the 16 hour average noise level.

Westerly preference

The westerly preference increases the proportion of westerly operations over and above that which would occur naturally. The main purpose of the westerly preference is to keep to a minimum the number of easterly departures over the most heavily populated areas close to the eastern end of the airport. Normally of course, aircraft take­off and land into the wind, and under the prevailing wind conditions at Heathrow, this would give a natural long term easterly/westerly split of around 36% and 64% respectively on a 24 hour basis (Flindell, 1996). Under the westerly preference, the runways continue to be operated in westerly mode even in moderate easterly winds providing that the surface tail wind component does not exceed 5 knots, that the runway surface is dry and the associated cross-wind component does not exceed 12 knots. When deciding on a change of runway direction, the air traffic controllers have also to take the upper winds and the amount of traffic flying on different routes into account. There is always a certain amount of hysterisis in the system that prevents too frequent changes of runway direction.

The available wind speed and direction statistics suggest that the proportion of easterly operations with a 5 knot westerly preference could be as low as 13%, although in practice it is closer to 25%. The difference is attributable to the effect of wet runways, the effect of the upper winds, the natural hysterisis in the system and the fact that the meteorological records do not take into account the effect of short term gusting which might otherwise preclude tail wind operations even where the hourly average tail wind component is below 5 knots. Assuming that these same factors would apply in reverse if there was a 5 knot easterly preference, then the proportion of easterly operations could be increased to 61%.

The Noise Management Initiatives (NMI) sub­group was set up by the Heathrow Airport Consultative Committee and supervised a study of the environmental costs and benefits of operating the westerly preference at Heathrow during the daytime. Using separate mode aircraft noise contours for arrivals and departures on each runway, it was possible to calculate the numbers of residents exposed under each alternative operating scenario. The combined population counts for easterly arrivals and departures for different air noise contour values were in each case between 15% and 65% greater than the combined population counts for westerly arrivals and departures. This provided strong support for continuing with the 5 knot westerly preference. On the other hand, the proportion of the overall time that residents are exposed to aircraft noise under different scenarios is also relevant to any future decisions on this topic.

Further analyses taking into account the population counts for each runway mode multiplied by the relative proportion of time that each runway mode might be expected to be in operation under different tail wind preference options gave a less clear-cut result, whereby certain areas around the airport would clearly benefit from a change to the westerly preference at the expense of other areas that would lose out. The NMI sub-group did not come to any clear conclusions as to how best to deal with the problem of different winners and losers in different areas around the airport, but the point was made that the westerly preference should be kept under review as the relative balance between arrivals and departures noise might change in the future.

As originally adopted, the system of westerly preference applies on a 24 hour basis. The complaints record suggests that early morning arrivals over the heavily populated areas around the eastern end of the airport are a major source of public dissatisfaction (in 1997/98 28.2% of all recorded complaints at the Heathrow Community Information Office were attributed to early morning flights, which are mostly easterly arrivals). The government has now suggested that 'it makes little sense from a noise amelioration standpoint artificially to increase the number of occasions when early morning arrivals overfly the densely populated parts of London by maintaining the westerly preference at times when no take-offs are scheduled' and has invited comments on the possibility of changing the preference arrangements during the early morning period (DETR, November 1998). A decision is expected during late Spring/early Summer 1999.

The Cranford agreement

The Cranford residential area is located very close to the eastern end of the north runway. The Cranford agreement was negotiated at a public meeting many years ago at which an undertaking was given to avoid overflying the Cranford area as far as possible (Munro, 1995). No written record of the original agreement has survived. The Cranford agreement is now generally understood through custom and practice to mean the avoidance of easterly departures from the north runway over Cranford. Without the Cranford agreement, and with the current westerly preference still in place, around 12% of all departures would climb away over the Cranford area. A very small number of easterly departures do take place off the north runway from time to time, but only where there is an over-riding need. In 1994, the proportion of all departures from the north runway to the east over Cranford was around 0.3% (Flindell, 1996). The Cranford agreement is not currently understood as applying to arrivals, and around 37% of all arrivals do in fact approach over the Cranford area.

Considered objectively, the Cranford agreement benefits the residents of Cranford at the cost of some disadvantages to other residents around the airport. The three main negative consequences of the Cranford agreement are that nearly all easterly departures necessarily take-off from the southern runway over similarly densely populated areas; that most easterly arrivals consequently approach onto the north runway; and that there can be no runway alternation (see below) during easterly operations. The total number of residents overflown by easterly arrivals onto the north runway, while much smaller than the total numbers of residents overflown by westerly arrivals, is much greater than the total numbers of residents overflown by easterly arrivals onto the south runway. Easterly landings onto and westerly departures off the south runway overfly the smallest numbers of residents around the airport.

The relative costs and benefits of the Cranford agreement in terms of noise exposure can be seen much more clearly by inspecting separate mode noise contours than by looking at overall average LAeq,16hrs contours, although it is still difficult to quantify the actual benefits that might follow from the introduction of runway alternation on easterly operation which might then be adopted. Of course, while abandonment of the Cranford agreement could lead to overall benefits considered in aggregate, it is unlikely that the residents of Cranford would willingly relinquish a practice that they probably consider to be only a small concession on behalf of the airport, given the numbers of landings that take place directly overhead even with the agreement in place.

Daytime runway alternation

Runway alternation refers to the operating regime adopted under daytime westerly operations by which the use of one main runway for departures and the other main runway for arrivals for most of the day (segregated mode) is alternated according to a pre-arranged pattern requiring a changeover at 1500 hrs. Under westerly operations the north runway is used for arrivals between 0700 hrs and 1500 hrs and for departures between 1500 hrs and 2300 hrs for one week and then it would be used for departures in the mornings and arrivals in the afternoons for the following week

To understand the perceived benefits of runway alternation it is necessary to consider the theoretical alternatives. Assuming that arrivals and departures are kept separated onto different runways, then one theoretical alternative to alternation would be permanent segregated mode. This is effectively what happens on easterlies because of the Cranford Agreement. Comparing runway alternation to permanent segregated mode, this has significant effects on the distribution of aggregate aircraft noise exposure around the airport and would therefore change both the shape and area of the resulting aircraft noise contours to a considerable degree. Alternation gives a wider distribution of noise than permanent segregated mode (without alternation), and reduces overall noise exposure for those most heavily exposed while at the same time increasing overall noise exposure for other areas around the airport that would not otherwise have been overflown.

Alternatively, if arrivals and departures could be allowed to take place in sequence on either runway, then this could also be described as a type of non-alternation. At present, there is no alternation at night but the government is consulting on the possibility of runway alternation at night (DETR, 1998). Comparing alternation with this type of non-alternation shows no effect on the average aircraft noise contours because it merely changes the distribution of traffic over time without affecting the aggregate distribution of routes over the surrounding residential areas.

It is widely accepted (although not on the basis of any quantitative community research) that the daytime runway alternation scheme operated at Heathrow provides significant benefits for the overflown community. This is generally understood to be because the alternation scheme provides periods of relief from what could otherwise be more continuous noise throughout the day and because it is also seen as providing a more equitable sharing of the overall noise burden around the airport. On the other hand, and because runway alternation is effectively a method of redistributing aggregate noise exposure over time, there will be some areas and times of the day when the hourly rate of aircraft noise events is higher than it would otherwise have been under any scheme of non-alternation.

Runway alternation at night

As a result of the widely accepted view that daytime runway alternation is generally beneficial at Heathrow, the suggestion emerged through public consultation that runway alternation should be extended to the night period as well. This topic was first considered in a government consultation paper published in 1977. Responses at that time were evenly divided and the suggestion did not proceed. However, since this was an obvious topic for research, the Heathrow Airport Consultative Committee decided to commission an independent study (MVA 1997 and 1998) into the perceived advantages and disadvantages of any new night alternation scheme.

Two separate night alternation trials were carried out, the first between 28th October 1996 and 16 March 1997 (winter trial), and the second between 15 June 1997 and 25 October 1997 (summer trial). The comparison of the Wave 1 and Wave 2 interviews (see below) before and after the winter trial showed some support for the continuation of the trial and so a second trial (with a further set of Wave 3 interviews) was carried out over the following summer chiefly to investigate the possibility that seasonal factors might have had some influence on the results (people in England tend to sleep with their windows closed at night more often in winter than in summer and there was a possibility that this could have explained part of the differences in results).

Since most operations at night are arrivals, and to avoid any conflict with the Cranford Agreement, which relates to departures only, the two night alternation trials were applied only in respect of landings. In each case night alternation followed a pre-arranged weekly schedule as for daytime alternation which continued throughout.

The three separate waves of interviews were carried out as follows;

Wave 1: 1415 residents and 351

NoiseLine users - before winter trial

Wave 2: 1415 residents and 349

NoiseLine users - end of winter trial

Wave 3: 2063 residents and 319

NoiseLine users - end of summer trial

Each residents survey was carried out using a structured questionnaire administered in-home with representative samples selected from each census enumeration district in four distinct areas around the airport as follows;

Eastern area north (under westerly approach to north runway)

Eastern area south (under westerly approach to south runway)

Inner area (immediately surrounding airport and potentially subject to ground noise) Western area (under easterly approach to both runways)

The western area was not divided into north and south runway approach areas because the resident population underneath the westerly approach to the south runway is much smaller than under the other three approaches, and this would have caused statistical sampling problems, had it been defined as a separate area. The interviews amongst NoiseLine users were carried out by telephone, but using substantially the same structured questionnaire format as for the residents surveys. The questionnaire items used were adapted from those used in previous noise surveys carried out at Heathrow using the same scale wording wherever practicable. The detailed design of the survey was also based on a limited number of pilot interviews carried out in each of the study areas to test that the questionnaire items used could be understood by the public.

Results of night alternation trials research

The night alternation trials were publicised in the local media and full information about them was made freely available to all enquirers. Nevertheless, only a minority of respondents (17% in Wave 2 and 18% in Wave 3) reported that they were aware of the trials at the time of being interviewed, although the proportion of NoiseLine users who were aware of the trials was much higher (38% in Wave 2 and 43% in Wave 3), suggesting that NoiseLine users probably take more interest in the workings of the airport than do the general public as a whole. Of those who were aware of the trial in Wave 2, 44% of residents and 31% of NoiseLine users felt it had made no difference and in Wave 3, 47% of residents and 34% of NoiseLine users felt it had made no difference. Of those who aware of the trial in Wave 2, 35% of residents felt night time aircraft noise was better for them personally and 15% felt it was worse. The corresponding figures for NoiseLine users were 50% and 13% respectively. Of those who aware of the trial in Wave 3, 34% of residents felt night time aircraft noise was better for them personally and 10% felt it was worse. The corresponding figures for NoiseLine users were 39% and 12% respectively.

A short explanation of the trial was given to all respondents who reported that they had not previously been aware of it. In Wave 3, after everyone had been made aware of the trial, 49% of all residents and 36% of all NoiseLine users felt that night time alternation had made no difference as compared to the previous summer (there was no night alternation the previous summer) and 35% of all residents and 22% of all NoiseLine users described themselves as being not sure. 10% of all residents and 21% of all NoiseLine users felt that night alternation had made aircraft noise better. 7% of all residents and 21% of all NoiseLine users felt that night alternation had made aircraft noise worse.

Considered overall, the results suggested that for those respondents who were aware of the trial or who noticed any difference in aircraft noise at night, around twice as many thought that night alternation had made aircraft noise better for them personally, rather than worse. On the other hand, the majority of respondents were not aware of the trial despite the publicity given to it and either had not noticed any difference in noise or could not say whether aircraft noise was better or worse. While night alternation appears to offer some net benefits, it also appears not to have been noticed by the majority of residents.

Given the strong support generally found for the continuation of day-time runway alternation during the day, it is perhaps surprising that so few people noticed any real difference when it was tried at night. On the other hand, all the available evidence suggests that most people would be asleep at the time. The recent field study of aircraft noise and sleep disturbance which was carried out partly at Heathrow suggested that objectively measured sleep disturbance is not greatly influenced by the amount of aircraft noise present (DOT, 1992, Horne et al, 1994). There is no conflict between this finding and the results of public consultation exercises which repeatedly show that many residents consider noise at night to be one of the most sensitive environmental issues around airports.

As a result of the trials, the government is now conducting a wider consultation exercise to determine if night alternation should now become the norm at Heathrow (DETR, 1998). Obviously, there may be other matters which were not investigated in the HACC research, and since the net benefits of night alternation were found to be relatively small, there might be other considerations which could be considered as over-riding these benefits.

Flight operations at Heathrow

In general, there are three areas in which flight operations can have an effect on noise levels on the ground. The actual routes flown by arriving and departing aircraft do not affect the maximum noise levels on the ground but they can influence the areas over which those noise levels are distributed. The precise ways in which aircraft are flown during departures can have an effect on noise levels on the ground. Even the precise ways in which aircraft are flown on approach might have some effect on noise levels on the ground.

Noise preferential routes

The standard noise preferential routes (NPRs) for departing aircraft have been in existence at Heathrow for over 30 years. Similarly, all arrivals are expected to descend along a defined glide slope at 3 degrees elevation in line with the landing runway. Small modifications to the departure routes have been made from time to time for largely navigational reasons, but the original principle adopted was to avoid overflying the most densely populated areas as far as was practicable at the time. Each route is defined by navigational instructions, which if followed precisely, can be represented as a combination of straight line and curved segments on a map. In practice, there is always a certain amount of variation in flight track to either side of the nominal route and the expected limits of deviation are shown on the published NPR maps as permitted swathes up to 1.5 km to either side of the nominal route centre-lines. The NPRs only apply up to an altitude of 4,000 ft after which aircraft can be vectored off in different directions as required.

Until recently, there was no practical method of auditing conformity with the standard noise preferential route system on a routine basis. This situation changed with the purchase and bringing into use of a comprehensive Noise and Track Keeping (NTK) system at Heathrow after 1992­93. Individual flight tracks are recorded and can be plotted out on printed maps showing deviations from the defined routes in relation to centres of population. This system has shown a number of areas where aircraft have difficulty in keeping to the published flight tracks for many different reasons. Where such problems have been identified, these are brought to the attention of the operators concerned and considerable improvements in the percentage of departing aircraft remaining within the permitted swathes have been achieved.

The policy adopted by the government of concentrating all arriving and departing aircraft over a limited number of routes to and from the airport benefits residents who do not live underneath those routes, but it also increases the amount of noise for those that do. The alternative of dispersing the routes of individual aircraft as widely as possible would have the effect of sharing the overall noise burden more widely. This alternative would not be practical for arrivals because of the need to descend in line with the landing runway on a shallow 3 degree glide slope using the instrument landing systems, but it might be a possibility for departures. However, any changes from the current routing system would interfere with the policy of maintaining stability wherever possible. People moving house into or around the areas around Heathrow are able to consult the published NPR maps and, if they have a choice, they are able to take this information into account when deciding where they want to live. It seems likely that people who have specifically selected an area which is not under an NPR and who then find that an NPR is moved to directly overhead might then feel considerable aggrieved. In addition, maintaining stability over a long period of time provides at least a theoretical opportunity for people who are especially sensitive to aircraft noise to move away while others who are less sensitive might then take their place.

Heathrow departures noise study

Noise on the ground at different distances away from the airport depends both on the noise at source and also on the height reached when the aircraft passes over the receiver point. Noise at source can be reduced by cutting back from take­off power to a reduced climb power setting at some appropriate safe height. This will give corresponding small reductions in noise level directly beneath the point of cut-back. However, the reduced power setting will also reduce the rate of climb such that at some point further out the effect of the reduced noise at source could be cancelled out by the effects of the aircraft climbing at a slower rate and noise levels might then exceed what they would otherwise have been without the thrust cut-back. The problem is further compounded by the need to divert some of the energy put in to climbing the aircraft into accelerating it to a speed where the high lift devices (flaps) used during take-off can be withdrawn and thus reduce drag. With drag reduced, more of the available energy can go into climbing and accelerating the aircraft.

The first objective during departure operations is to achieve the optimum cruising speed and height as safely and efficiently as possible. Without compromising safety, there will normally be some flexibility to trade energy consumption for minimum noise, but the noise considerations are further complicated by the different effects of different procedures at different distances from the airport. Airframe manufacturers are able to carry out theoretical calculations of the likely costs and benefits of different departures procedures but real-life data collected under actual field conditions is much harder to find. Accordingly, BAA Heathrow and British Airways decided to carry out an extended field study of alternative departures procedures at Heathrow between June and September 1997 using normal commercial B747 operations (Flindell et al, 1998 and 1999). The complete database included 2214 departures, of which 524 were confirmed as flying over the noise instrumented route and matching noise data was obtained for 487 of these flights.

The results of the study indicated that small but statistically significant noise level reductions (~ 1.5 dBA) at the ICAO noise certification point distance of 6.5 km (actually 6.7 km in this study) could be achieved by introducing an early power cut-back at 1000 ft above the runway instead of waiting until between 1500 ft and 2000 ft to cut­back power under the previous standard departures procedure. Further out from the airport, the reduced rate of climb largely cancelled out the small noise level reductions at source leading to no significant change in noise levels. A third procedure based on maximising the rate of climb out to 3000 ft was found to be best at the 10.5 km and 12.3 km noise monitoring positions used in the study. The differences in average noise levels between the three procedures, even where statistically significant, were in all cases small compared to the effectively random variation in noise levels associated with changes in other parameters which would normally be outside the control of the operator. Both alternative procedures were found to consume a significant amount of additional fuel during the climb as a result of extra drag.

When the results of the study were put to the Heathrow Airport Consultative Committee, the community representatives confirmed that they would welcome a change to the early cut-back procedure because of the small reductions in noise level achieved. This was not withstanding the fact that the reductions are small, and might not be particularly noticeable on the ground. As a result of the study and the way in which the results were received, British Airways have now adopted the early cut-back procedure as standard throughout its B747-400 fleet world-wide except where precluded by local conditions such as altitude and air temperature.

Heathrow arrivals noise studies

With the continued replacement of older ICAO Chapter 2 aircraft by generally quieter ICAO Chapter 3 aircraft types, noise levels generated at source on departures have been steadily reduced. The main technical reason for these noise level at source reductions has been the adoption of increasingly higher by-pass ratios in modern turbofan engines which has both reduced the amount of jet noise generated at the engine tail pipe and also allowed for the introduction of increasing amounts of physical noise absorptive treatments in the fan cowlings. A secondary factor is the increasing climb performance of modern aircraft types which can often climb to considerable heights before they cross the end of the runway. Neither of these noise benefits can be applied to aircraft on approach which must be properly stabilised on a standard 3 degree glide slope at some appropriate low engine power setting with landing flaps and gear deployed well before crossing the runway threshold.

An additional factor is that at a busy airport like Heathrow with only a single landing runway in use at any one time there will often be an almost continuous stream of arriving aircraft following each other in succession down the approach track in use at the time. Due to flight track dispersion along different routes, the same situation does not apply on departure. For these and other reasons, approach noise could be much more important in terms of community perceptions than it used to be 10 or 20 years ago. Most in-flight noise management efforts have historically concentrated on departures noise, but this situation is beginning to change.

Over the last few years, BAA Heathrow have increasingly been investigating approach noise using semi-portable noise monitors deployed under one of the main runway approach glide slopes (Flindell & McKenzie, 1999). The phase 1 arrivals noise study carried out in 1994 to 1995 (Flindell & McKenzie, 1995) compared instantaneous noise levels on the ground against recorded aircraft engine parameters using a large sample (57) of British Airways B757 commercial flights to investigate the extent to which instantaneous noise levels on the ground could be correlated with instantaneous changes in engine power settings or other flight parameters. No consistent relationship between engine power setting and maximum noise levels could be found, although there was some evidence of instantaneous noise level changes associated with power setting changes. Over a much larger sample of 2292 arrivals, a simple model based on height over the noise monitor and equivalent sound power level at source gave good agreement with noise level data on the ground, although there was considerable residual variability in the data.

For the phase 2 study, a much larger sample of all arrivals flying over an existing departure noise monitor for the year April 1995 to March 1996 was investigated in some detail. A similar degree of residual variability was found, but because the existing departure noise monitor was not optimally positioned for arrivals noise monitoring directly underneath an approach track, the data was found to be additionally confounded by the effect of small variations in approach track off to either side of the nominal approach track centreline.

An extended phase 3 study was carried out between November 1996 and March 1997 with three noise monitors deployed underneath the westerly approach to the south runway in order to obtain a large enough sample size to be able to statistically distinguish between the relative noise performance of different operators when the effects of other noise level determining factors had been taken into account. The general intention here was to contribute to the development of best practice over the longer term on the assumption that if operators have any flexibility to influence arrivals noise levels on average, then it would be worthwhile to obtain actual data and then use it constructively in future discussions. The results indicated that the main determinants of noise level continued to be the aircraft type/engine fit combination and the actual height over the noise monitor with around 90% of individual events within plus or minus 2 to 4 dB above and below the mean for each aircraft type/engine fit combination. A number of meteorological variables were found to have statistically significant effects but the major part of the observed residual variability still could not be explained by any combination of measured variables. A rank ordering of different operators while controlling for baseline noise capabilities of each aircraft type/engine fit combination as indicated by the US FAA certificated approach noise levels database suggested that there are small but important differences between some operators which might potentially be explained by differences in standard operating procedures. This is now the subject of an extensive phase 4 study which includes the collection of a considerable amount of basic operational data by using a short pro-forma questionnaire which is completed by flight crews immediately after arrival.

Presentations on the work to the various consultative committees has confirmed that clearly focused research of this type is generally very much welcomed by the local community. This is not withstanding the fact that no significant progress has yet been made in identifying methods of reducing arrivals noise for the future. The results to date indicate that aircraft commanders probably have very little flexibility to choose quieter operational procedures once they are firmly established on the glide slope. For example, if an aircraft should momentarily descend below the glide slope, it may be necessary to increase thrust to return it to the proper position. Any such increase in thrust could add to any noise problems caused by the aircraft being nearer to the ground than usual. In any such situation, operational safety considerations have precedence over noise considerations, for obvious reasons.

Terminal 5 development proposals

Most experts agree that unconstrained demand at Heathrow will soon rise to 80 million passengers per year and beyond. The existing four passenger terminals have been continually adapted and improved to keep up with this increasing demand. However, this process cannot go on indefinitely. Around 10 years ago BAA decided to bring forward plans for a fifth passenger terminal to accommodate the expected rise in passenger numbers. The Terminal 5 development proposals have been examined in a very comprehensive public inquiry convened at Heathrow. The government appointed Inspector will eventually prepare a report which the Secretary of State will then take into account before announcing whether the development can proceed or not, or under what conditions. At the time of writing the Terminal 5 public inquiry had been going on for longer than any previous inquiry held in the UK. This caused a considerable delay to BAA's anticipated construction programme and has cost a great deal of money for all parties involved in the inquiry. On the other hand, a large number of issues were very thoroughly examined both in evidence and in subsequent cross-examination. The examination of noise issues alone occupied many months of inquiry time, and of course, all the evidence was and is available for public scrutiny.

Because the two main runways are already operated at capacity for much of the day, the BAA predictions of future airport capacity with Terminal 5 in place assumed only very small increases in the total numbers of air traffic movements. The increased numbers of passengers would be accommodated within almost the same number of aircraft by increasing the proportion of larger aircraft types in the overall mix. If all other things are equal, larger aircraft tend to be a bit noisier than smaller aircraft. Mainly for this reason, predicted air noise levels were higher with a fully developed Terminal 5 than for a future Heathrow constrained to the existing four passenger terminals, but none of the differences exceeded 2 dB (Neil, 1997). In addition, the published data showed reductions in air noise contours as compared to the present day irrespective of whether Terminal 5 is built or not.

The new Terminal 5 would be positioned between the existing north and south runways on a new site to the west of the current central terminal area, mostly at some considerable distances from the nearest residential areas outside the airport boundary. BAA predictions showed increases in airport ground noise around the western end of the airport associated with the new focus of ground operations in and around the new terminal areas, but in most cases these increases would be above a low base and are mostly of limited significance when considered in the context of the existing ambient noise environment in these areas (Flindell, 1997). BAA proposed a number of measures to minimise ground noise, to include the provision of fixed ground power and pre-conditioned air and some restrictions on the use of outer taxiways and aircraft parking stands at night. A new ground running pen would be provided by British Airways designed to attenuate engine test noise to the lowest practicable levels. Finally, BAA proposed extensions to the current noise insulation scheme to deal with any residual problems attributable to ground noise associated with Terminal 5.

Issues - absolute vs. relative impacts

Heathrow Airport is and will remain a busy airport, whether or not Terminal 5 is built. Secondly, the BAA case assumed that no changes would need to be made to the existing patterns of operational use currently in place to accommodate the changes in the proportion of larger aircraft associated with the development. BAA recognised that 'many issues arise in connection with air noise, but by and large the development of Terminal 5 is not material to them, and where it is material it has a small effect' (Neil, 1997). On the other hand, a number of objectors at the public inquiry emphasised their concerns about the absolute level of noise impact of the existing airport, and there was some evidence submitted on possible non­auditory health effects of noise and on the possible effects of aircraft noise on schools. These objectors were debating the absolute impact of the airport, whereas the BAA case was essentially that the relative impact would be small. Since the absolute impact would be much the same whether Terminal 5 is built or not, it is difficult to see how this debate would assist the Inspector in reaching any recommendation as to whether the proposals should be permitted or not.

It seems reasonably clear that the level of absolute impact as judged by reported annoyance in the noise exposed communities is likely to be significantly affected by many non-acoustic factors as discussed above. On the other hand, where there is no reason to believe that there would be any changes in any of these factors other than average noise level expressed as LAeq,16hrs, then there is no reason to doubt the validity of this indicator as showing relative impact at least as well as any other noise indicator. It is usually very important that the objectives of any noise assessment should be very clearly defined as to whether they are concerned with either absolute or relative impacts as the processes and indicators used and the type of conclusions which may be drawn will often be very different.

Assurances to the public

To provide some assurance to the public, BAA stated that it was 'prepared to be committed to ensuring that the area of the 57 LAeq,16hrs contour at Heathrow with Terminal 5 will be no bigger than it was in 1994 when calculated on the same basis' (Neil, 1997). In effect, this assurance requires that the replacement of the remaining older noisier aircraft types with ICAO Chapter 2 noise certificates by newer and quieter aircraft types with ICAO Chapter 3 noise certificates will continue; that no future aircraft types will be significantly noisier than existing aircraft types with ICAO Chapter 3 noise certificates; and that there will be no significant increases in the numbers of air traffic movements. The precise definition and application of any such cap would be a matter for the government, but this does not affect the principle of the assurance. It is hoped that this assurance will provide some confidence to the public that the airport management are committed to maintaining the growth of the airport over the next few years without generating any additional noise as compared to the present day. As a potentially significant non­acoustic factor, it is obviously very important that this type of assurance should be widely perceived as being completely binding.

General discussion

The continuing development of an effective noise management strategy at any major international airport which is located in relatively close proximity to major population centres is a complex process involving many inter-related factors. Working within an overall statutory framework as defined by the government, the airport management at Heathrow have to strike a fine balance between minimising noise impact in the surrounding residential areas while at the same time not compromising the major economic and social contributions made by the airport. Maintaining open and wide-ranging communications with the local communities through both formal and informal consultation is seen as a vital part of this process. Of course, merely listening to public concerns is not enough on its own, the airport also has an obligation to take whatever action is found on investigation to be economically affordable, does not compromise safe and efficient operation, is feasible and practical, and is likely to be effective. This means that where there are a number of noise management options available, all of them should receive appropriate consideration. Given that not all options will be feasible anyway, it is probably important that there should still be a portfolio of noise management strategies in place, and that the reasons behind the non­adoption of any remaining strategies should also be explicit.

It is important to note that not all non-acoustic factors involved in effective noise management can be investigated by quantitative research of the type conventionally used to establish noise dose-effect relationships. To take but one academic example, if it was desired to carry out any sort of quantitative investigation of the benefits of daytime runway alternation, which as described above is widely perceived as providing considerable benefit, it is difficult to see how any type of control condition could be devised which would enable a fully objective comparison between alternated (test) and non-alternated (control) options to be carried out. The problem here is that, since daytime alternation has been in place for such a long time, the overall aircraft noise climate has changed to a significant extent anyway over the same time period. If non­alternation was being considered as a serious policy option, then a comparative trial could be carried out (in theory of course, there may be serious practical considerations which preclude this possibility), but on the other hand, there would have to be a very good reason for carrying out the trial in the first place and since consultations suggest that alternation is almost universally preferred, it would be difficult to justify this.

Another and perhaps rather more subtle example is the problem of understanding precisely what quantitative benefits ensue from the long standing policy of wide consultation adopted by the airport management. Clearly, it is not possible to compare what might have happened over the last few years if this policy had not been pursued, and neither would it be particularly scientific to make this comparison against any other airport where there could be many, many other factors contributing to the overall degree of public satisfaction or dissatisfaction with noise. Community research could be carried out to obtain qualitative opinions of the benefits of consultation but it seems unlikely that these could ever be measured in terms of any noise level or decibel equivalent.

Of course it is difficult or impossible to consult everyone, and there will often be such a wide range of individual views that this might not even be helpful anyway. This means that the results of consultation which does take place must be interpreted wisely and with the benefit of experience, and at the end of the day, decisions will have to be taken on what seems to be best on the balance of probability rather than on any basis of absolute certainty. This is really no different from any other business decision of course, and this is what effective management is mostly about. The best solutions might often only be reached by properly understanding the different points of view of the different parties concerned, and different individuals will always have different perceptions of what are the most important aspects of any problem, almost by definition. It will always be necessary to remember that while physical noise levels are important, and in many situations will in fact be the best indicator of the scale of any problems, there are often many other factors involved in noise annoyance and where any such factors have been identified, these should also be dealt with accordingly.[20]

  References Top

1.BAA Heathrow, (1998). Environmental Performance Report 1997/98, BAA Heathrow, Middlesex, UB3 5AP.  Back to cited text no. 1    
2.Davis, R. A., (1993). Sound insulation against aircraft noise - effectiveness of different glazing types. Report R01 4550, ISVR Consultancy Services, University of Southampton.  Back to cited text no. 2    
3.DETR (1997). Consultation paper - Noise limits for aircraft departing from Heathrow, Gatwick and Stansted Airports, November 1997, Department of the Environment, Transport and the Regions.  Back to cited text no. 3    
4.DETR (1998). Night restrictions at Heathrow, Gatwick and Stansted - second stage consultation. November 1998, Department of the Environment, Transport and the Regions.  Back to cited text no. 4    
5.Flindell, I. H., & McKenzie, A. R., (1995). Heathrow Airport arrivals noise study. Report LHRARR13, April 1995 (Report published as Heathrow Terminal 5 inquiry document BAA/715 by BAA plc).  Back to cited text no. 5    
6.Flindell, I. H., & McKenzie, A. R., (1995). Heathrow Terminal 5 - Background noise survey. Report LHR920, September 1995 (Report published as Heathrow Terminal 5 inquiry document BAA/761 by BAA plc).  Back to cited text no. 6    
7.Flindell, I. H., (1996). Heathrow Airport aircraft noise - Westerly preference and easterly alternation. Report LHR86 submitted to Heathrow Airport Consultative Committee, September 1996.  Back to cited text no. 7    
8.Flindell, I. H., (1997). Heathrow Terminal 5 Proof of Evidence - Ground noise assessment. BAA/196, BAA plc.  Back to cited text no. 8    
9.Flindell, I. H., McKenzie, A. R., Knowles, A., & Morris, K. (1998). Heathrow Airport departures noise study. Proceedings of the Institute of Acoustics, Vol 20, Part 1, pp 127-134.  Back to cited text no. 9    
10.Flindell, I. H., & McKenzie, A. R., (1999). Heathrow Airport departures noise study. Report LHRDEP16 submitted to BAA Heathrow, January 1999.  Back to cited text no. 10    
11.Flindell, I. H., & McKenzie, A. R., (1999). Heathrow Airport arrivals noise study - Phase 3. Report LHRARR33 submitted to BAA Heathrow, January 1999.  Back to cited text no. 11    
12.Heathrow Terminal 5 Public Inquiry Air Noise Joint Data Group (1997). Night flying restrictions at Heathrow, 1978 - 1998. Heathrow Terminal 5 Public Inquiry Position Paper PS/7/2.  Back to cited text no. 12    
13.Heathrow Terminal 5 Public Inquiry Air Noise Joint Data Group (1997). Heathrow Airport noise infringement and noise complaint data. Heathrow Terminal 5 Public Inquiry Position Paper PS/7/3 (3 volumes).  Back to cited text no. 13    
14.Heathrow Terminal 5 Public Inquiry Air Noise Joint Data Group (1997). Heathrow Airport Noise Insulation Grant Schemes and voluntary Noise Insulation Scheme 1966 - 1997. Heathrow Terminal 5 Public Inquiry Position Paper PS/7/4.  Back to cited text no. 14    
15.Horne, J. A., Pankhurst, F. L., Reyner, L. A., Hume, K., & Diamond, I. D., (1994). A field study of sleep disturbance: effects of aircraft noise and other factors on 5,742 nights of actimetrically monitored sleep in a large subject sample. Sleep, 17(2), pp 146-159.  Back to cited text no. 15    
16.Munro, A. M., (1995). Heathrow Terminal 5 Proof of Evidence - Supplementary Proof Topic 1. DOT/7, Department of Transport.  Back to cited text no. 16    
17.The MVA Consultancy (1998). Heathrow runway alternation trials - research among local residents - Final Report submitted to Heathrow Airport Consultative Committee March 1998.  Back to cited text no. 17    
18.The MVA Consultancy (1998). Heathrow runway alternation trials - research among local residents - Final Report submitted to Heathrow Airport Consultative Committee. April 1997.  Back to cited text no. 18    
19.Neil, R. I., (1997). Heathrow Terminal 5 Proof of Evidence - Air noise policy. BAA/64 and BAA/65, BAA plc.  Back to cited text no. 19    
20.Ollerhead, J. B., et. al. (1992). Report of a field study of aircraft noise and sleep disturbance. December 1992, Department of Transport.  Back to cited text no. 20    

Correspondence Address:
Ian H Flindell
Institute of Sound and Vibration Research, University of Southampton
United Kingdom
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Source of Support: None, Conflict of Interest: None

PMID: 12689498

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