1.2.1. Introduction
In this section of the guide various aspects of the noise problems associated with take-off and landing operations of present day civil aircraft are discussed. Aircraft interior noise, APU noise on the airport ramp as well as noise generated by SSTs is outside the scope of this section. Some basic background information as well as factual information on aircraft noise is given.
Noise can be defined as undesirable sound. Aircraft noise is a topic that has gained considerable attention all over the world since the introduction of jets. Civil aviation is usually quite pleased with interest shown in it by the public and the press, however, this topic is undoubtedly undesirable publicity. The conflicting interests of airlines, airport authorities and residents in the neighbourhood of airports have led in many occasions to emotional headlines in the newspapers.
The purpose of this section is to give a better understanding of the physics of the aircraft noise phenomenon in order to achieve – within the boundaries of the current directives – optimum noise abatement results. Specific procedures are not given, these are covered in the AOM.
1.2.2. Historical review of the development of the jet noise situation
In the following survey a few developments of the past 20 years are mentioned which have had an important effect on the present (1973) situation.
In the period when the first jet transports were developed, the airlines were assured by the aircraft manufacturers that the new jets would make no more noise than the propeller-driven aircraft of that time; they would be much quieter than military jets.
The Port of New York Authority assigned the consulting firm Bolt, Beranek and Newman to measure the noise of propeller aircraft out of Idlewild (Kennedy) airport. On the basis of these measurements the Port Authority established in 1958 jet noise limits which were intended to ensure that jets would not cause more annoyance than propeller-driven aircraft. These noise rules proved to be far more restrictive than anticipated, due to the fact that – although the overall sound level of the jets was not higher than that of the propeller-driven aircraft – the sound frequency spectrum of the jets was much more annoying than the spectrum of propeller-driven aircraft. The jet spectrum contains more annoying high frequency tones.
One of the results of the noise studies was the establishment of a unit for aircraft noise, the so-called Perceived Noise decibel (PNdB). This unit is still the most widely used unit for measuring and expressing aircraft noise.
The severe restrictions of the Port of New York Authority have led amongst other things to the development of noise suppressors as installed on the company’s DC-8-63 aircraft.
In 1963 the final report of the UK Wilson-committee was published. The committee had been established in 1960 to examine and advise on general noise problems. An important part of the report refers to aircraft noise. The committee has carried out extensive social surveys and has established criteria for the effect of the number of aircraft operations on the noise annoyance. These studies led to the establishment of noise critical areas near Heathrow and to subsidizing noise insulation of houses.
In 1966 the London Noise Conference was held. This conference covered all aspects of the aircraft noise problem and may be considered as a starting point of the more recent developments such as noise certification.
In 1969 the US FAA issued noise certification rules (FAR 36) for transport aircraft. All DC-10 aircraft and B-747 aircraft delivered after 1st December, 1971 are noise certified accordingly.
During the 1969 ICAO Noise Conference, ICAO Annex 16 on Aircraft Noise was developed. This Annex covers in addition to noise certification rules compatible with the FAR 36 rules, noise monitoring, noise unit for land use planning and guidance material on noise abatement procedures.
One of the main current topics is the discussion on “noise retrofit” of the large fleets of first and second generation jet aircraft.
1.2.3. Measuring units and basic concepts
01. The decibel (dB)
The range of sound pressures accepted by the human ear is very large. The lowest sound pressure detectable by the healthy young human ear under very quiet conditions is 0.0002 microbar. This figure is used as a basic reference. It is however not suitable as a measuring unit. For this purpose the decibel (dB), expressing the sound pressure level, is used. The decibel scale is a logarithmic one, with the following relationship between sound pressure in microbar and sound pressure level in decibel:
| Sound pressure (microbar) | Sound pressure level (dB) | Typical sounds |
|---|---|---|
| 0.0002 | 0 | Hearing threshold for good ears |
| 0.002 | 20 | Whispering |
| 0.02 | 40 | |
| 0.2 | 60 | Conversation |
| 2 | 80 | Shouting |
| 20 | 100 | |
| 200 | 120 | |
| 2000 | 140 | Pain threshold |
A 10 dB increase/decrease of the sound pressure level is generally experienced by the human ear as doubling/halving of the strength of the sound. This means for example, that shouting is considered approx. four times as loud as normal conversation.
02. Perceived noise level and “effective” perceived noise level
The Perceived Noise dB (PNdB) was developed as a measure to express community annoyance. It is a calculated unit which gives more weight to the more annoying high frequencies of the noise spectrum. The PNdB cannot be measured with a simple noise meter. Usually the aircraft noise is recorded on tape and each of the frequency bands is rated in NOYs, a unit which expresses linearly the overall annoyance due to the combination of sound pressure level and frequency. The frequency band with the highest NOY value has a great influence in the subsequent PNdB calculation.
Shortcomings in the original PNdB concept are that the PNdB does not reflect the discomforting effects of long duration and pure tones of the noise of overflying aircraft in a realistic way. This is allowed for by a refined calculation method and by applying certain corrections for these effects. This revised unit for measuring and describing noise is now referred to as the “effective PNdB” (EPNdB). This unit was internationally accepted as the official unit for describing aircraft noise.
03. dBA, dBC and dBD
Sound pressures may be measured with a noise meter with a so-called “A-scale”, “C-scale” or “D-scale”. The associated readings are expressed in “dBA”, “dBC” or “dBD”.
With the A-scale the sound pressure contributions of the lower frequencies are suppressed to a considerable extent so that the reading is mainly determined by the higher frequencies.
The dBC reading can be considered equivalent to the Overall Sound Pressure Level because with the C-scale only very light frequency filtering is applied.
With the dBD scale the PNdB weighting of the sound pressure contributions of the noise spectrum is approximated.
Although there is no fixed relationship between the dBA reading and the Perceived Noise Level it may be assumed that for the present jets, the PNdB value is approx. 13 dB higher than the dBA reading. When using the dBD scale the reading must be increased by 7 dB to find the approx. PNdB value.
04. Basic methods to decrease noise
Noise can be abated by:
- reducing noise generation at the source;
- increasing the distance from the source to the noise sensitive area;
- applying sound insulation at the noise sensitive object.
All three methods are applied in cases where noise annoyance is experienced.
Method (1) refers to engine noise which may be reduced by thrust reduction, engine and nacelle design. Fig. 1 illustrates how the noise from the two main sources of noise as well as the total noise of a straight jet engine varies with thrust.
The exhaust noise decreases significantly with decrease of exhaust velocity and as a consequence with decrease of thrust. Fig. 1 shows that, since exhaust noise is dominating in the higher thrust brackets, thrust reduction is quite effective.
Bypass engines have good possibilities for exhaust noise reduction. By mixing the primary exhaust flow with fan air a reduction of jet velocity is achieved which effectively reduces the primary exhaust noise. Fig. 2 gives a picture how the total noise and this noise from various sources of a typical low bypass engine (bypass ratio approx. 1.3:1) vary with thrust.
It appears that at take-off thrust the total noise level is significantly lower than for straight jet engines, that the high pitch fan whine (caused by interference between fan blades and guide vanes) is dominating and that thrust reduction is ineffective to reduce the PNdB values.
For the high bypass engines that have been developed for the B-747 and DC-10, noise reduction is achieved in the following areas:
- high bypass ratio’s (approx. 5:1). Although in the past fuel economy was the main reason for high bypass designs, noise has become an increasingly important factor for high bypass ratio’s;
- since for low bypass engines the fan is the dominating noise source, full attention was paid to reduce the fan noise of new engines by deletion of in-take guide vanes and by ample spacing between fan blades and exit guide vanes;
- a further effective measure is to provide noise insulation for various parts of the engine nacelle.
With regard to method (2) it should be noted that the sound waves expand spherically from the source. Their intensity decreases inversely proportional to the area of the sphere (i.e. inversely proportional to the square of the distance).
This means that the sound pressure decreases inversely proportional to the distance from the source. For instance, by doubling the distance from the source the measured sound pressure will decrease to 50% and the associated reduction in sound pressure level amounts to approx. 6 dB.
Method (3) is used for houses etc. near airports.
05. Noise patterns on the ground produced by aircraft take-off and landing
As indicated in para 04, the sound pressure decreases inversely proportional to the distance from the source of the sound. The PNdB values decrease with distance at a somewhat higher rate because the high frequencies are rapidly damped out at greater distances. Disregarding possible atmospheric and directional effects of the sound produced by an aircraft, this means that during a straight flight aircraft produce concentric “iso sound pressure levels” or “iso PNdB” cylinders. (See Fig. 3.)
In level flight the iso PNdB cylinders are intersecting the ground by straight lines parallel to the flight path. If the aircraft is climbing or descending the cylinders intersect the ground by ellipses (Fig. 4).
Typical examples of ground sound patterns produced during take-off and during approach are shown in Fig. 5 and Fig. 6. The differences in shape of the patterns are due to the fact that the take-off flight is flown steeper than the approach path, which results in a more pointed shape of the approach ellipses.
1.2.4. Noise properties of present aircraft
In the Figures 7 to 10 incl. the noise levels (PNdBs) are given for the various types of aircraft of our fleet for three power settings against altitude.
Figure 7 shows that the DC-8-33 take-off noise at the lower altitudes is quite high as compared with for instance the 112 PNdB New York noise limit. Take-off thrust reduction reduces the noise level by some 8 PNdBs which means that this procedure is quite effective.
Figure 8 shows the noise properties for DC-8-53/55 and DC-8-63, which are essentially the same. The striking difference with the DC-8-33 is, that thrust reduction on take-off has hardly any effect on the PNdB values. This may be explained by the fact that for the JT3D engines the fan is mainly responsible for the peak noise levels. (See also 1.2.3, 03.)
However, contrary to the PNdB, the Overall Sound Pressure Level (dBC) of the JT3D decreases significantly upon thrust reduction. There is, therefore, a case for thrust reduction at locations such as Montreal where in fact the dBC is monitored. The lower take-off noise level as compared with DC-8-33 makes fan-equipped aircraft more acceptable from noise point of view. Because of the better nacelle the DC-8-63 noise levels are slightly lower than the DC-8-53/55 at comparable power settings.
In Figure 9 the DC-9 noise with JT8-D7 engines is shown. The D9 engine, delivering approximately 500 lb more take-off thrust, is producing some 1 – 1.1 more PNdBs at take-off power. Figure 9 shows that the noise levels of this two-engined aircraft are lower than the levels of the JT3 equipped DC-8s at comparable heights. It is also noteworthy that for the DC-9 thrust reduction is quite effective.
Figure 10 illustrates the noise of the B-747 equipped with JT9-D7 engines and fixed lip nacelles. With the original flow-in-door nacelles the noise levels are approx. 4 PNdB higher. Fixed lip nacelles are required for noise certification. The DC-10 noise levels are shown in Figure 11.
Fig. 10 and 11 clearly illustrate the advantages of high-by-pass engine technology: although the weights at which the B-747 and DC-10 operate are multiples of the DC-9 weights, the noise levels of the B-747 and DC-10 are less than for the DC-9.
1.2.5. Operational measures for noise abatement
01. Turns away from the noise sensitive areas
A procedure which increases the distance to the noise sensitive area, is a turn after take-off, away from the measuring point and(or) the noise sensitive area. (See Fig. 12.) Unless the noise sensitive area is situated very close to the end of the runway it is usually a very effective procedure.
02. Thrust reduction
The effect of reducing thrust at some point after take-off is shown in Fig. 13. The width of the noise pattern is reduced considerably upon thrust reduction. However, as a result of the much flatter take-off path, the ground patterns become very long, and may thus affect areas farther away from the airport.
For optimum thrust reduction results, thrust should be reduced just before reaching the noise sensitive area. If thrust is reduced too early the noise sensitive area is overflown at an unnecessarily low altitude resulting in an unnecessarily high noise level (Fig. 13).
03. Weight reduction
Fig. 14 shows that by reducing the take-off weight the flight path becomes steeper and the associated ground noise patterns are reduced in size. It must be pointed out that a small reduction in size requires an unproportional high take-off weight reduction. From airline point of view this is hardly acceptable because of the possible severe weight penalties.
04. Flap setting and climb out speed
Although flap setting and climb out speed have a relatively small effect on the noise reading, it should be understood that one or two PNdB’s reduction is still important.
For DC-8 a larger flap setting results in a shorter take-off distance and a flatter climb out. Consequently less noise is produced near the end of the runway, but farther out, the picture is reversed and the larger flap setting causes an increase in noise. The same applies basically to the DC-9. However, because of the DC-9 fixed pitch climb out, the cross-over takes place well above 1500 or 2000 ft so that – for all practical purposes – the larger flap setting results in less noise.
Figure 15 illustrates how the ground noise varies with the point of acceleration to climb speed for DC-9. It follows that the acceleration should preferably be delayed until the noise sensitive areas have been passed.
05. Exit and approach routings (SID’s and STAR’s)
Sometimes noise problems arise at populated areas at greater distances from the airport. In these cases well defined exit and approach routings may reduce problems to a minimum.
06. Standard initial climb technique
In 1972 the company adopted for all aircraft types a standard initial climb technique, illustrated in Fig. 16.
This technique is essentially the same as the US/ATA “get ’em high early” procedure and the recently adopted IATA standard climb out procedure. It is considered that this technique provides in general the good compromise between close-by and far-out noise problems.
07. Approach noise
Approach noise is a major problem. Procedures to reduce approach noise (two-segment approach, delayed flap and gear extension, high intercept altitude) are under study.
Until revised approach procedures are published, the pilot can only control approach noise by carefully following the present approach procedure, avoiding to intercept the glide path at too low an altitude, and avoiding power changes and low turns over populated areas.
1.2.6. Noise abatement measures outside the control of the pilot or operator
01. Night restrictions
A very effective measure to decrease annoyance is to impose night restrictions. These restrictions become more and more wide-spread and are obviously very unpopular with the airlines. They make scheduling and high aircraft utilisation very difficult. It is for this reason that the IATA has always opposed night restrictions in the strongest way possible.
02. Noise level restrictions
Since the Port of New York Authority has introduced the 112 PNdB limit for Kennedy Airport, similar and in many cases more stringent noise level restrictions have been imposed at other airports. Such a fixed limit is a quite effective measure for the authorities to control the noise near the monitoring point. However, as explained in 1.2.5. 02., using thrust reduction to meet the limit results in a flatter climb out and consequently there may be more annoyance in the area beyond the monitoring point.
Operators have great problems with fixed noise level limits. In many cases, even when using optimum noise abatement techniques, the limit cannot be met unless the take-off weight is reduced considerably.
03. Preferential runways
A preferential runway system may be very effective in avoiding undue annoyance. At busy airports such as J.F. Kennedy Airport, however, the preferential runway system is impairing airport (ATC) capacity considerably.
04. Zoning and land use control
It goes without saying that there should be no residential areas, schools, etc. in the approach and take-off flight path areas. This ideal situation will probably never be reached. Nevertheless, authorities throughout the world are becoming convinced that noise is one of the most important factors to be considered on land use planning, site selection and runway orientation of new airports.
05. Two segment approach
Recently research into the possibility of a two segment approach i.e. a 6 degrees approach down to approx. 2 NM out and then continuing a 3 degrees approach until the landing, has received quite some attention. This procedure is intended to reduce the approach noise by increasing the distance from noise source and by reducing the required thrust. Such a system may require major changes of both aircraft systems and ground aids.
1.2.7. Noise certification
In the past, noise was hardly considered in the design of aircraft; the well known performance rules on required runway length, take-off climb gradients, etc., were in fact controlling flap settings and maximum take-off weights. Within the framework of those performance rules the aircraft were designed for optimum economy.
The current and future noise certification rules may easily have an overriding influence on the design of aircraft.
The ICAO and FAA noise certification rules, which differ only in detail, may be summarized as follows:
- Three noise measuring points have been specified (see Fig. 17) viz. for the take-off tests a take-off and a side line point and for the landing tests an approach point.
At max. take-off weight resp. max. landing weight and under specified operational conditions the noise levels indicated in Fig. 18a-b-c must not be exceeded. To a certain degree it is permissible to compensate exceedances at one or two points by EPNdB margin(s) available at other points.
- The take-off tests must be carried out with an initial climb speed of at least V2 + 10 kts; cutback is in principle permissible.
- The approach tests must be flown on a 3° glide path, landing flaps, gear down and a speed of not less than 1.3VSO + 10 kts.
In Fig. 18 a, b and c, estimated noise certification levels are given for various types of aircraft. This clearly demonstrates that noise certification will have a significant effect.