CHAPTER 3 CHAPTER 3 Airport Noise Control
3.4 Noise-Control Strategies
There are many ways in which operations on and in the vicinity of airports can be modified to control noise and to decrease its impact on airport neighbors (ICAO 2007). As early as 1986, 37 categories of noise-control actions had been identified as being in use at over 400 U.S. airports (Cline 1986). Several of these are discussed briefly.
Quieter Aircraft
Although considerable noise is generated by aerodynamic flow over the aircraft frame, most of the noise from modern transport aircraft has the engine as its source. The two principal component of engine noise are high-velocity exhaust-gas flows and air flows in the compressor fan system. The early turbojet engine was extremely noisy owing to the high velocity of the compressor tip speeds and the jet exhaust. Subsequent generations of high-bypass-ratio engines have included a number of achieved and proposed improvements, including
• Low-noise fans with swept stators
• Quieter intake liners, bypass and core stream liners
• Improved nozzle-jet noise suppressors
• Active noise-control fans
• Reduction in airframe noise
• Low-noise inlets
• Low-noise flaps, slats, and gear
Both the National Aeronautics and Space Administration (NASA) and the European Commission (EC) had research and development programs that sought to reduce aircraft noise through improved low-noise design resulting in a 6-to 7-dB reduction from the levels of aircraft in the mid-1990s.
Noise-Preferential Runways
Modern transport aircraft are not particularly sensitive to the crosswind component on landing and takeoff. Consequently, these operations can be conveniently carried out on a less than optimally oriented runway if that facility will reduce the noise nuisance to the community at large. Schiphol Amsterdam is an example of an airport that might well have abandoned the use of a particular runway were it not for the fact that this runway is well suited to direct noise nuisance away from the heavily populated suburbs of Amsterdam. At Los Angeles, an over-ocean operational procedure provides some relief from arriving aircraft noise to the close-in communities to the east of the airport between the hours of 0000 and 0630. During this period, aircraft approach the airport from over the ocean toward the east and depart over the ocean toward the west unless air traffic control (ATC) determines that the weather conditions are unsafe for such operations. Very much related to the noise-preferential runway concept is that of minimum-noise routings (MNRs) or preferred-noise routings (PNRs), which are designed to direct departing aircraft to follow routes over areas of predominantly low population density. Although the size of the noise footprint is not altered significantly, the impact in terms of disturbed population is much decreased. The use of MNRs and PNRs has been hotly contested by those adversely affected in terms of the social justice of the few bearing high noise exposure levels in order to protect the many. In the United Kingdom, the Noise Advisory Council has examined the practice of using MNRs and has recommended its continuation as being the best course of action for the community as a whole.
Operational Noise-Abatement Procedures
Several operating techniques are available that can bring about significant and worthwhile reduction in aircraft noise in the takeoff and approach phases in the vicinity of an airport as well as during operations while on the ground (ICAO 2010b).
Takeoff
To reduce noise over a community under the takeoff flight path, power can be cut back once the aircraft has attained a safe operating altitude. Flight continues at reduced power
cutback, noise levels can be reduced. Of course, there will be reduced benefits further down route, where noise levels are likely to be higher than those produced by a full-power climb. This is called noise displacement. However, by carefully planning the noise- abatement procedure (NAP) on takeoff, the level of noise exposure on the total community can be reduced.
Figure 3.4 shows the easily calculated theoretical effect of a power-cutback procedure on maximum flyover noise levels for points directly below the flight path. Staged-climb NAPs are common at many airports around the world. Figure 3.5 shows the results of tests carried out on a B747-100 by the Civil Aviation Authority (CAA) in Britain, in which noise improvements of 2 dB were widespread in the areas of high noise exposure in the initial climb paths, but the displacement of noise to downstream areas was negligible and considered not discernible. Noise experts point out that a more significant factor in noise of departing aircraft is the operator’s practice of minimizing takeoff thrust to minimize fuel burn. Many aircraft thus are substantially lower when they pass the 6.5 km from rollout point than they would be with full-thrust takeoffs (Ollerhead et al. 1989). Later work is documented by ICAO (2008b).
FIGURE 3.4 Effect of power cutback on ground noise levels.
FIGURE 3.5 (a) Comparison of on-track noise levels for a Boeing 747 with and without power cutback. (b) Differences in on-track noise levels for a Boeing 747 with and without power cutback. (Source: Ollerhead et al., 1989.)
Approach
Noise on approach can be reduced by adopting operational NAPs that keep aircraft at increased heights above the ground. Some of the following have been used at various airports:
• Interception of glide slope at higher altitudes when interception is from below the slope. Manchester Airport prohibits descent below 2000 feet (610 m) until the glide
the visual approach slope indicator system (VASIS) to avoid unnecessary low flying.
• Performing the final descent at a steeper than normal angle. Descents of 4 degrees have been used, but 3 degrees is a more normal angle.
• Two-segment approaches with the initial descent at 5 or 6 degrees of flaring to 3 degrees for the final approach and touchdown.
• Low-grade approaches with reduced flap settings and lower engine power settings demonstrate some reduction of noise. Reducing flap settings on the B737 from the normal 30 degrees reduces the noise by 2 EPNdB.
• Use of continuous-descent approaches employing secondary surveillance radar for height information. This prevents the use of power in a stepped descent and consequently reduces noise under some parts of the descent path. A combination of low-power and low-drag approach procedures has been used in the past with
considerable success at Frankfurt Airport, which has severe environmental noise problems owing to its position within an urban area.
Runway Operations
The most significant improvement in noise impact that can be achieved when aircraft are on runways is control of the use of thrust reversal. Although thrust reversal is usually about 10-dB below takeoff noise, it is an abrupt noise that occurs with little warning. Aircraft operations should be restrained from the use of thrust reversal on noise-nuisance grounds, except in cases where no other adequate means of necessary deceleration is available or where the airport setting does not require noise-control strategies.
Insulation and Land Purchase
Some relief to noise nuisance can be attained by the use of sound insulation. In some countries, those adversely affected by defined levels of noise nuisance are eligible for governmental or airport authority grants that must be used for double glazing or other sound-insulation procedures. Schemes of this nature operate, for example, in the noise- impacted areas around London Heathrow and Schiphol Amsterdam.
A more direct, although more expensive, method of reducing noise nuisance was adopted at LAX, where many homes and businesses in the immediate vicinity of the airport were purchased by the airport through mandatory purchasing procedures (eminent domain). In some cases, this type of action is the only recourse open to an airport when continued operation means intolerable living and working conditions for the neighboring population.