CHAPTER 5 NOISE ABATEMENT PROCEDURES

Contents

          Para      Subject


          5.1       The Need for Noise Abatement
          5.2       Tug Types
          5.3       Technical Improvements
          5.4       Pilot's Knowledge
          5.5       Met Conditions
          5.6       Nature and direction of aircraft noise
          5.7       Flight Paths for Towing
          5.8       Height
          5.9       After Release
          5.10      Descent
          5.11      Priorities
          5.12      Table of Noise-sensitive locations

5.1 THE NEED FOR NOISE ABATEMENT. Lasham, as a matter of policy, wishes to be as good a neighbour as it can to the people living around the airfield. We also wish to minimise the complaints about tug noise that we receive, mainly by telephone, on busy weekends. We do not operate the noisiest aircraft which fly in this area, those are civil and military helicopters and the occasional military fast-jet; but we do operate for long periods of time at a lower noise intensity. Our `good neighbour' policy is to minimise the effects of noise from the aircraft within our control, that is motor gliders, visiting aircraft, and particularly tugs, since, unlike motor gliders and visiting aircraft, they operate almost all the time we are flying. We manage this in various ways, firstly by limiting the first aerotow times (Para 1.7), and secondly by defining areas over which tugs must not fly whilst towing. These areas are shown on the map which follows, and avoid-areas have been defined rather than set flight paths. A rigidly-defined flight path would not be able to take account of the varying winds, the location of thermals, position of the sun, and the differing speeds and rates of climb that are an inevitable result of the many different combinations of tug and glider. Furthermore, our neighbours living under such a rigid flight path would have a legitimate cause for complaint.

5.2 TUG TYPES. Lasham has also chosen tugs that have the best compromise between power for a good climb rate, and noise. In any case, a good climb rate means that you are higher when you have to manoeuvre to avoid the noise-sensitive areas, and the higher you are, the less the decibels on the ground. At one time we thought we might buy more Pawnees but have reversed that decision, at least partly on grounds of noise; this could change with better exhaust silencers and four-bladed props.

5.3 TECHNICAL IMPROVEMENTS. Improvements have been centred on the use of 4 bladed propellers, and new and better silencers, such as the Golmozig. These methods reduce engine power, but this is a price worth paying for better relations with our neighbours. It shows that we are serious about the matter, despite being well within existing CAA and European standards for aircraft noise.

5.4 PILOT's KNOWLEDGE. New tug pilots must realise that Lasham takes the problem of tug noise very seriously. Every pilot has to be concerned about the noise the tug is making and arrange each flight so as it produces the minimum possible disturbance to the people below. Every Tug Pilot must be able to point out the Noise avoid areas, and on every flight Tug Pilots have to plan their route's to avoid these areas by as wide a margin as possible.

5.5 MET CONDITIONS - SLACK WINDS AND INVERSIONS. Tug pilots have to be particularly concerned when there is little wind when sound travels furthest and the tug/glider combination is lower leaving the airfield vicinity. A low inversion makes things worse since sound tends to be reflected off the inversion and that part of the noise which would normally travel upwards can be reflected back towards the ground.

5.6 NATURE AND DIRECTION OF AIRCRAFT NOISE. Aircraft noise is principally created from the engine exhaust and from propellers, mainly the tips (or rotors in the case of helicopters); at the speeds that we fly, airframe noise is small. Fine-pitch props produce more noise than coarse-pitch varieties, and once the prop tips go supersonic the noise increases by several orders, fortunately no Lasham tug has a supersonic prop but if you have heard a Harvard or a 230 hp Rallye Minerva you will know what one sounds like. Prop noise is propagated in line with the prop disc and its frequency is related to prop RPM and the number of blades. The direction, amount, and frequency of exhaust noise depends on the engine rpm, the rate that cylinders fire, and the efficiency of any silencer; thus two-strokes create a higher pitched noise than four-strokes because for a given rpm they fire at twice the rate and the six cylinder engines of some tugs such as Pawnees also produce higher frequency noise than our four cylinder tugs. Measurements made at Lasham on a number of motor gliders and tugs by scientists from Farnborough showed that all of our tugs and motor gliders were well within the legal noise limits for light aircraft (measured in decibels on the so-called `A-weighted' scale, or `dB(A)'), but that most aircraft have specific directions in which their noise is greatest, normally slightly ahead and abeam. Thus from the ground you may also notice that quite small changes in heading, 20 to 30 degrees often brings a noticeable change to the perceived noise. Because of the prop noise propagated in the direction of the wing tips, if you turn so that a noise-sensitive area is at the centre of the turn, it will receive the prop noise for all of the time of the turn. This can be more irritating than a short peak of noise during a straight overflight.

5.7 FLIGHT PATHS FOR TOWING. Towing flight paths as you climb out should not be directly towards or over the more noise sensitive areas, and there should be several small changes of heading but the overall flight path should cover as much ground as possible. Turns should avoid pointing the lower wingtip at a noise-sensitive area. Never `avoid' an area by flying close around it, even up to a mile away. This is no help at all, in fact by subjecting those below to the noise for longer it is probably worse than overflying. Fortunately it is possible to plan routes that minimise the effects of tug noise and this no doubt explains why we are able to do so many tows with few complaints. The map shows the SW sector between Bentworth and Burkham House is the least populated area and should be used whenever the wind direction allows.

5.8 HEIGHT. Noise is propagated in three dimensions away from its source, and its intensity attenuates in proportion to the surface area of a sphere as it expands away from its centre. The volume of noise perceived at a given distance is therefore inversely proportional to the cube of the distance away from its source; distance, in this case the height of the tug aircraft, is a very powerful noise reducer, but also bear in mind that the higher you are, the larger the area over which the noise is spread. Remember that on weekend afternoons, local residents may be wanting a quiet time in their gardens and any noise may annoy them. Nevertheless, when above 1500 ft you have more freedom of manoeuvre, and above 2500 ft on high tows, you can make straight overflights of the noise-sensitive areas (an overflight may be less obtrusive than a turn round an area). To put these heights into perspective, UK air law allows straight overflights by light aircraft at heights down to 500 ft as long as you are within range of a landable field in case of engine failure.

5.9 AFTER RELEASE. Part of the careful engine handling needed after release is that in an effort to cool the engine gradually for the first 10 seconds, you must not over-speed it and allow the rpm to increase. This, combined with the Doppler effect as you increase speed for the descent, can be very irritating to people on the ground.

5.10 DESCENT. On the descent keep the noise down by planning your flight path and height appropriately. As covered in para 3.9, once the CHT is below 200 degrees the rpm can be reduced 2000, and if you need to increase power in the circuit before finals, you are judging things badly.

5.11 PRIORITIES. Flight safety, saving the engine, and noise abatement all take precedence over saving time or fuel or, worst of all, you trying to prove you are the Ace of the Base.

5.12 NOISE SENSITIVE LOCATIONS. The following is a list of places to avoid because of noise. Tug pilots are expected to learn them and have to be shown them as part of their training. Also note their locations on the map on the following page. Distances and bearings listed in the table are in kilometres from the airfield centre, and bearings are in degrees with respect to the grid on the 1:50,000 OS Map (in effect, degrees true).

      NORTH WEST SECTOR


           Place                   Population   Km & Bearing from LAS

Herriard Village (crossroads)         50           2.0  340°

Herriard Grange area                  10           2.7  315°

Ellisfield Village & Manor           250           4.5  305°



     NORTH EAST SECTOR

           Place                   Population   Km & Bearing from LAS

Houses at Powntley Copse              80           3.2  080°

Weston Patrick Village               120           3.6  020°

Upton Grey Village                   320           5.5  025°



     SOUTH WEST SECTOR

           Place                   Population   Km & Bearing from LAS

Lasham Village                        130          1.0  190°

Burkham House & Houses to the NE                   2.3  255°

Bentworth Village NE end                           2.8  200°
Bentworth Village SW end                           4.7  200°

Bradley Village                       140          4.2  245°



     SOUTH EAST SECTOR

           Place                   Population   Km & Bearing from LAS

Shalden Village                       180          2.3  125°

Alton & Holybourne                 16,650 (1991 Census) 7.0  135°

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Last updated by John Leibacher on Friday, September 8, 1995 at 21:21