Aviation Environmental Impacts and Airport-Level Mit- Mit-igations
6.2 Aircraft Noise Background
Noise is any undesirable or unwanted sound. During the early decades of aviation, there were few aircraft movements and hence limited aviation noise concerns. The first-genera-tion jet aircraft in the 1950s led to a rapid expansion in commercial aviafirst-genera-tion and their en-gines created significant noise. The resulting severe disruption of living patterns in nearby communities prompted the establishment of formal and informal groups opposing airport expansion, drawing considerable media attention and, ultimately, government intervention.
To allay public concerns in the 1960s, authorities put in place airport-specific noise limits as traffic grew at major airports such as London/Heathrow and New York/Kennedy. In the 1970s, the U.S. Federal Aviation Administration (FAA) introduced the first noise certifica-tion standards and the Internacertifica-tional Civil Aviacertifica-tion Organizacertifica-tion (ICAO) promoted similar standards globally (Smith, 1989).Chapter 2of ICAO’s “Environmental Protection/Annex 16 to the Convention on International Civil Aviation” (ICAO, 2008a) defined noise stand-ards for aircraft certified before October 6, 1977 (with some exemptions); Chapter 3 for aircraft certified between then and December 31, 2005; and Chapter 4 for aircraft certi-fied thereafter. ICAO member states adopt these standards into national legislation, for ex-ample U.S. Federal Aviation Regulation (FAR) Part 36 Stages 2/3/4 correspond to ICAO Chapters 2/3/4. The standards outline noise limits at approach, sideline and flyover certi-fication points (Fig. 6.3) and cumulative across all three points. All new aircraft must meet these certification standards in order to gain approval to operate. Sound level is measured in decibels (dB), and each new ICAO chapter imposes increasingly stringent noise lim-its, resulting in a 10-to 20-dB cumulative reduction in allowable noise. These standards have significantly driven down the noise impacts of individual aircraft of a given size over time. For example, the first-generation Boeing 747-100/200 was introduced in 1970 under Chapter 2rules, the Boeing 747-400 in 1989 underChapter 3rules, and the Boeing 747-8 under Chapter 4rules. Each successive generation of the aircraft has been required to be significantly quieter than its predecessors.
FIGURE6.3 Aircraft noise certification points (Chapters 3and4).
These increasingly stringent certification standards (coupled with the other mitigations to be discussed) have dramatically decreased the number of people exposed to significant noise levels from airport operations in the last several decades. For example, from 1975 to 2005 there was a 95 percent reduction in the number of people in the United States living inside 55-dB DNL contours around airports (a noise metric discussed in detail later) (NRC, 2002). However, as technology enhancements experience diminishing returns and demand for aviation continues to grow, ICAO projects that the number of people exposed to 55-dB DNL noise will increase globally, from approximately 20 million in 2005 to 25 to 35 mil-lion in 2035 (ICAO, 2010a). As a result, airports will need to continue to address noise impact concerns.
Aircraft Noise Sources
There are two general sources of noise from aircraft: the engines and the airframe, as shown inFig. 6.4.
FIGURE6.4 Primary aircraft noise sources.
Aircraft generate noise whenever there is speed or turbulent airflow and/or high-speed mechanical movement and rotation. Turbofan engine noise [and noise from auxiliary power units (APUs) used to provide power when aircraft are on the ground] comes from the flow of air through and rapid rotations of the various components of the engine fan and core elements, as well as the high-speed gases in the engine exhaust being expelled into the outside air. Turboprop (propeller) engine noise also includes the turbulent air shed from each blade and the interactions between the blades. Airframe noise is caused by the flow of air over the surfaces of the aircraft and the turbulent flows created by the structure and
cav-ities introduced by the deployment of high-lift devices and landing gear. See Smith (1989) for more detailed discussion of aircraft noise sources.
Engine noise tends to dominate on the ground, especially during takeoff when the en-gines are at very high thrust level, on landing when using thrust reversers and when taxiing at low speed. By contrast, airframe and engine noises are about equally important during approach and landing operations when aircraft are at low altitudes in “dirty” aerodynamic configuration with high-lift devices and landing gear extended and engines at lower thrust levels than at takeoff. Another source of aircraft noise is the sonic boom created by air-craft flying at supersonic speeds which can be very disruptive to activities on the ground.
This issue severely limited the market for supersonic commercial aircraft introduced in the 1970s. Only the Aérospatiale-BAC Concorde found a niche market serving transatlantic routes (overland flights were banned due to the sonic boom concerns) until its retirement in 2003 on economic grounds.
The increasingly stringent noise certification standards have spurred the development of low-noise technologies for new aircraft. These have significantly reduced noise impacts, as illustrated inFig. 6.5. Most reductions in aircraft noise have been achieved through im-provements in engine technology, especially the transition from turbojets to high bypass ra-tio turbofan engines. The bypass rara-tio is the rara-tio between the amount of air drawn in by the fan that bypasses the engine core relative to that passing through the core. Large modern turbofan engines have a bypass ratio of around 10:1; that is, ten times more of the air that is ingested by the fan goes around the engine core than goes through it. This configuration achieves a given thrust level with minimum size of core and the slower moving bypass air mixes with the high-speed core air, resulting in a significantly lower exhaust velocity that in turn reduces exhaust noise. Although bypass ratios have generally increased over time for modern turbofan engines, a limit is being reached which manifests as the plateauing in the noise reduction curve in Fig. 6.5. Higher bypass ratios require larger fan diameters that increase the weight and drag of the engine and thus increase fuel burn. This implies a tradeoff between environmental impacts of noise and climate change from fuel burn emis-sions discussed later in this chapter.
FIGURE6.5 Aircraft source noise reduction.
Meeting future noise targets [such as the European Commission’s goal for a 65 percent reduction in perceived aircraft noise level relative to 2000 levels by 2050 (EC, 2011) and NASA’s long-term goal for a cumulative 62 dB reduction below Chapter 4 standards (NSTC, 2010)] will require new noise reduction technologies. Some candidate technolo-gies are illustrated on the right side ofFig. 6.5. Near-term incremental technology enhance-ments include engine core and nacelle chevrons (which increase the mixing of the core and bypass air, reducing engine exhaust noise), and streamlined landing gear fairings (but these also increase weight and hence have fuel burn impacts). In the medium-term (possibly by 2020), geared turbofans and ultrahigh bypass ratio (UHBR, also called unducted fan) en-gines are being promoted for significant fuel savings, but their impact on noise needs to be carefully monitored. Longer-term (unlikely to be available commercially until at least 2025), more integrated airframe/engine designs afforded by blended-wing body configura-tions are being explored. These absorb or heavily shield engine noise, leading to signific-antly lower noise impacts on the ground. Their operational usability is an area of ongoing research, for example regarding airport infrastructure. It is unlikely that new-generation su-personic aircraft will reenter the commercial airline fleet in the foreseeable future, although new airframe technologies (such as low boom shaping) are being developed, which may enable smaller supersonic business jets to become a reality in the near future.
Measuring Aircraft Noise and Its Impacts
Aircraft noise propagates in the form of sound waves that travel through the atmosphere.
When these waves reach the human ear, they create pressure fluctuations that are processed mentally. The wide range of pressures to which the human ear responds and the nonlinear
response to pressure levels have led to the use of a logarithmic scale for quantifying sound levels. As previously introduced, the unit of measurement used internationally is the decibel (dB): a tenth of a bel, a unit named after the Scottish innovator Alexander Graham Bell. A sound level of intensity, I measured in dB (LdB) is defined as
(6.1) where Irefis the sound intensity at the threshold of hearing for the healthy human ear, which by convention equates to 0 dB. The range of sound levels perceptible to the human lies in a range of roughly 0 to 120 dB: those just above 0 dB are barely perceptible by the most sens-itive ears in a perfectly quiet environment, whereas those above 120 dB lie at the threshold of causing pain and physical injury to the ear.
Common everyday events mapped to the decibel scale are illustrated inTable 6.1, along with their intensity ratios and approximate perceived loudness ratios. The formulas for de-termining perceived loudness are complex and vary significantly with sound characterist-ics such as frequency (Smith, 1989). As a rough rule of thumb, the human ear perceives an increase of 10 dB in sound level as approximately twice as loud. Noise events within 2 miles of a major airport when under the flight paths from aircraft taking off and landing typically fall within the 70-to 110-dB range, depending on aircraft type, exact location, and atmospheric conditions.
TABLE6.1 Sound Levels and Typical Noise Events
Although the logarithmic scale for measuring the loudness of sound is technically con-venient, it causes immense confusion in informing the public about aircraft noise. When told that measurements at some location show that the noise generated on takeoff by the average aircraft has been reduced from a typical value of 100 to 90 dB, most people will (not surprisingly) interpret this statement to mean that aircraft noise has been reduced by 10 percent, when in fact the intensity of the sound has dropped by 90 percent (i.e., a factor of 10) whereas the human perceives the relative loudness has dropped by approximately 50 percent.
In addition, the decibel measurement of the sound generated by an aircraft movement does not fully characterize its impact on humans: the frequency or pitch is also important.
People may perceive the loudness of two sounds with equal decibel level but different fre-quencies as significantly different. Although the healthy human ear can hear sounds in the
general frequency range of 16 to 16,000 hertz (Hz), it is most sensitive to sounds in the range of 2000 to 4000 Hz. Measurements of the loudness of sound thus typically undergo a further calibration, resulting in an “A-weighted adjustment”, to better reflect the human response to noise in the different frequencies. In practical terms, this adjustment adds ap-proximately 2 to 3 dB to sounds in the high-sensitivity frequency range of 2000 to 4000 Hz and subtracts a few decibels from sounds outside this range. Noise measurement devices installed around airports are designed to report A-weighted sound levels automatically. To indicate explicitly that the decibel scale has been adjusted to account for the sensitivity characteristics of the human ear, the A-weighted decibel units are denoted as dB(A) or dBA.
The most commonly used measures of airport noise can be subdivided into single-event metrics (associated with a single aircraft movement) and cumulative metrics (measuring noise from many movements over a specified time period). Audible noise generated by a single aircraft movement lasts for an amount of time T that varies from about 10 seconds to a few minutes, depending on the location of the listener relative to the aircraft and on the type of movement (approach, departure, overflight, surface movement, etc.). Analysts and regulators typically use three measures to describe single event noise:
• Lmax (maximum sound level) measures the peak sound level reached during T. It is simply the highest reading, in dBA, recorded by a noise sensor during T.
• SEL (sound exposure level) is a measure of the total noise impact of an event by integrating the noise impacts over time T which is then normalized to a 1-second duration.
• EPNL (effective perceived noise level) is similar to SEL but accounts for the dur-ation and tone of an event (e.g., by assigning additional weight to certain discrete frequency tones that are particularly irritating to the ear). It is the measurement used for certification purposes, and its units in this case are termed EPNdB. Because of the complexity of its definition, the generation of EPNL estimates requires sophist-icated computation. As a result, airport environmental studies typically utilize SEL to measure single-event noise, not EPNL.
Cumulative measures of noise estimate the total noise effect over multiple aircraft move-ments over a specified time near a particular location. They are thus more appropriate for representing the general noise environment around an airport. Their definitions attempt to capture the combined impact of the A-weighted loudness of the total individual noise events. Two cumulative measures are particularly important:
• Leq (equivalent sound level) is a time-averaged cumulative equivalent sound level whose specific parameters can be adapted to a given situation. It measures noise exposure by computing the average dBA of noise per unit of time during the spe-cified period. For example, to compute Leqfor a 2-hour period, the SEL of all the aircraft-generated noise events occurring during that period would be added on a logarithmic scale and the resulting total would be averaged (i.e., spread equally) over 7200 seconds.
• Ldnor DNL (day–night average sound level) is a special case of Leqfor an entire day (86,400 seconds) with a 10-dB increase for nighttime (10:00 pm–7:00 am) noise to account for its greater impact at these times. Importantly, it is the standard metric of the FAA for determining the noise impacts of aggregate operations around airports in the United States.
Because cumulative measures represent average noise exposure over time, they may not be able to distinguish between quite different situations. For example, one noise event gen-erating a painfully loud noise for a short period of time might have the same average noise over a longer period as many events each generating moderate noise. The Leqvalue may be similar for both cases, but most people would distinguish between them. Public hearings on airport noise often bring up this deficiency of cumulative measures of noise.
In most cases, the main product of noise analyses is a set of noise contours. These are lines on a map defining the areas around an airport that are estimated to be subjected to spe-cific levels of noise after completion of the proposed project: an example is shown inFig.
6.6. It can be seen how noise exposure areas are impacted by arrival and departure flight patterns. Airports often publish aggregate contours annually to show their noise perform-ance over time. Airports often use noise monitoring systems with sensors at strategic loca-tions to assess their actual noise performance. Airports may also employ web applicaloca-tions to allow the public to have timely access to aircraft flight track and noise impact informa-tion, which can facilitate communication between airports and community stakeholders.
FIGURE6.6 Sample noise contours. (Source: NATS.)
Noise contours are typically drawn for DNL or Leq values in the range 50 to 80 dBA in appropriate increments. They can be generated by computer models, such as the U.S.
FAA’s Integrated Noise Model (INM)1 and Noise Integrated Routing System (NIRS), the U.K. Department for Transport’s Aircraft Noise Contour version 2 (ANCON-2) model, and EUROCONTROL’s SysTem for AirPort noise Exposure Studies (STAPES). For INM, tra-jectory information is required in the form of aircraft type, ground track, and altitude pro-files over a given time period, as well as airport characteristics such as length of runways and proportion of time in each runway configuration. The model then uses noise-power-distance (NPD) characteristics for different aircraft to calculate the noise contours on the ground. Users need to spend considerable effort preparing good-quality, location-specif-ic inputs for INM and carefully calibrate the results with appropriate field measurements.
Population distributions (e.g., from census data) can then be overlaid on the noise con-tours to determine how many people and properties are subject to noise of different levels.
This can then be used to determine which properties qualify for noise mitigation funds for sound-proofing or relocation, as well as help land planners determine appropriate usage of certain areas regarding new development. For example, approval of noise-sensitive activ-ities (e.g., schools, hospitals, religious institutions, and residences) would generally not be
recommended in high-noise areas, but they might be acceptable for industrial and commer-cial purposes (“employment zones”) instead.
Past airport environmental assessments in the U.S. have concentrated on the number of people living in areas that experience noise above a certain level (e.g., DNL values of 65 dBA or higher), based on the premise that this group suffers the most and reacts most strongly to noise. This was consistent with research conducted in 1970s relating transport-ation noise exposure to annoyance level (Schultz, 1978) which became the U.S. govern-ment’s preferred noise impact metric based on the recommendations of the U.S. Federal In-teragency Committee on Noise (FICON). However, more recent research (e.g., Fidell and Silvati, 2004) suggests that the annoyance curve is shifting such that people are becoming highly annoyed at lower DNL levels (seeFig. 6.7). For example, at the 65 dBA DNL level, the fraction of people expected to be highly annoyed has moved from 15 percent using the older data to around 25 percent using more recent studies. Although most of these studies have been conducted in the United States and Europe, and scatter in the data is relatively high, the general trends are likely to be similar in other world regions.
FIGURE 6.7 Annoyance level as a function of noise level. [Source: based on data from (Schultz, 1978; Fidell and Silvati, 2004; Mahashabde et al., 2011).]
There are extensive studies into the behavioral and physiological impacts from short-and long-term exposure to aircraft noise. Potential impacts include sleep disturbance;
stress-related health effects such as hypertension, hormone changes, and mental health ef-fects (Mahashabde et al., 2011); deteriorations in work performance; and child learning dis-ruption. Attributing impacts to specific aircraft operational and performance parameters is
challenging due to the many confounding variables such as income and dietary habits, but research is identifying some well-defined exposure-response relationships at much lower levels than 65 dBA DNL. The World Health Organization has recommended that a limit of an Leqvalue of approximately 50 dBA (16-hour time base) in exterior sound levels is ne-cessary to avoid serious annoyance (WHO, 1999). In addition to suspected human health effects, aircraft noise also leads to monetary impacts in terms of reducing property values.
This effect is commonly captured using noise depreciation indices (NDIs) that relate the percent loss in housing stock value for each dB of aircraft noise. Typical NDI values of 0.5 to 1.0 percent per dB of noise have been reported (Nelson, 2004), that is, a 0.5 to 1.0 percent loss of housing value for each dB of noise. However, significant variations exist between regions and countries and careful consideration is required in any specific analys-is.
Given this information, noise levels of 55 dBA DNL and above are now becoming im-portant for aviation impact analyses. This is consistent with maintaining annoyance levels at no more than around 15 percent in Fig. 6.7 to account for the increasing sensitivity to noise in recent years. This is significant because, not only are many more people then included, but also the communities in the 55-to 65-dBA DNL exposure zone are often wealthier (compared to the higher-noise 65 dBA DNL and above regions) and are more ef-fective politically in objecting to airport activities.
Airport-Level Noise Mitigations
ICAO recommends a “balanced approach” to aircraft noise management (ICAO, 2007a, 2010b). This comprises the following:
1. Reductions at source
2. Land use planning and management 3. Noise abatement operational procedures 4. Operating restrictions
Noise charges are a complementary mitigation mechanism and each of these elements is examined in turn.
Reductions at Source
Reductions at source decrease the amount of noise being generated by the aircraft. They comprise the engine and airframe modifications and technology improvements previously described. Airframe and engine manufacturers are developing and implementing these im-provements in response to the certification environment and airline customer requirements.
The main impact for airport planners and operators is to ensure compatibility of airport
in-frastructure to any new airframe and engine configurations introduced in response to source noise reduction efforts. For example, future alternatives such as blended wing bodies would have to overcome significant barriers from an airport operating perspective.
Land Use Planning and Management
Land use planning and management policies should minimize the impact of any noise that is generated. They include appropriate zoning, building codes, and mandated noise disclos-ures in real estate transactions set by the local authorities of residential, municipal, and commercial areas around airports given the noise environment. The airport authority does not directly control these policies but needs to interact with local authorities to ensure ef-fective implementation. In addition, the airport is sometimes required to provide sound in-sulation upgrades to certain properties within the highest noise contours (e.g., as described under FAR Part 150 Airport Noise Compatibility Planning Program in the United States).
Noise Abatement Operational Procedures
Airports can promote runway, taxiway, and airspace designs and associated operational procedures that minimize the noise generated and the number of people impacted by air-craft movements. Some of the best practice operational procedures (regarding noise, air quality, and climate change mitigations) are identified in Fig. 6.8. Note that many of the mitigations can help against multiple environmental impact areas, but, given the low alti-tudes involved, surface, departure, and approach phases are seen to be most important from an airport noise perspective.
FIGURE 6.8 Sample operational mitigation alternatives to address noise (N), air quality (A), and climate change (C) impacts.
Ground operations that minimize noise include taxiing with one engine turned off (single-engine taxiing for two-engine aircraft); extended towing of taxiing aircraft by ground vehicles; minimizing APU usage (using ground-supplied power instead); limits on the use of thrust reversers on landing; and provision of ground run-up facilities with appro-priate noise barriers for engine testing. The amount of time that an aircraft is taxiing and generating noise on the ground can be reduced by appropriate runway and taxi route design and assignment given the terminal and gate that each aircraft is coming from/going to, as well as surface congestion management techniques (to be discussed in detail inSec. 6.4).
Noise abatement departure procedures (NADPs) minimize noise impacts on the ground by modifying the takeoff and initial climb phases of flight. Current ICAO guidance defines two types of NADPs that modify the thrust and speed profiles (ICAO, 2005): “NADP 1”
reduces noise for areas close to the departure end of the runway by delaying the accelera-tion to climb speed until after 3000 ft altitude is reached; “NADP 2” reduces noise to areas more distant from the runway end by accelerating aircraft earlier to get them higher more quickly. Departures off some runways at an airport can have much lower noise impacts than off other runways (e.g., if one runway sends departures over a body of water initially, compared to over residences) and in these cases noise-preferred runway assignments may
be an important aspect in the decision making of ground air traffic controllers. Planners should consider such issues in the very early stages of airport design when they configure runways, taxiways, and terminals. Many airports also have “noise containment corridors”
within which departing and arriving flight tracks should be maintained (e.g., consistent with the “high-noise” corridors used by the land use planning authorities). Defining depar-ture and approach procedures to remain within these regions is another important part of noise management.
Noise impacts for arriving flights have received greater attention than for departures be-cause arriving aircraft spend more time at lower altitudes. Continuous descent approaches (CDAs) are the most common noise abatement approach technique. They eliminate level segments present in conventional “step-down” approaches, keeping aircraft at higher alti-tude and lower thrust for longer prior to intercepting the final approach glide slope, thereby reducing noise impacts (as well as fuel burn and emissions as discussed later). Figure 6.9 illustrates the basic CDA concept and shows that most noise benefits are achieved within 10 to 40 nautical miles (nm) to touchdown.
FIGURE6.9 Continuous descent approach concept.
Proper design of CDA procedures can reduce noise impacts by several dBA outside the airport perimeter and significantly reduce the number of people within certain noise con-tours. CDAs can also be built into the next generation of approach procedures that take advantage of area navigation/required navigation performance (RNAV/RNP) technologies.
These enable aircraft to fly more precise approach (and departure) paths, so that they can be directed over lower-impact corridors such as rivers and less populated regions (seeFig.
6.10). Noise is therefore more concentrated in these regions compared to the more dis-persed noise impacts with conventional procedures. As an example, approaches to Wash-ington/Reagan airport follow the curves of the Potomac River rather than flying directly over sensitive parts of the city. Careful consideration is required to determine whether pre-defined flight paths are appropriate for any given airport, for example regarding
“concen-trated” versus “dispersed” noise impacts and when the noise procedures result in longer flight tracks with associated higher fuel burn and emissions impacts.
FIGURE6.10 Dispersed versus concentrated approach paths.
Another technique to reduce noise impacts from arriving aircraft involves displacing the landing threshold further down the runway. This keeps aircraft higher outside of the air-port perimeter. However, this has significant infrastructure and procedural implications and can only be contemplated on runways long enough to accommodate displaced landings without adversely impacting safety standards. Frankfurt/International is one airport where a displaced landing threshold has been studied for certain aircraft. Longer-term, steeper ap-proach paths (above the approximate 3° flight path typically used today) are being studied.
These would also achieve greater aircraft altitudes outside the airport boundary and hence reduce noise impacts, but again extensive safety studies are needed and the technology and infrastructure changes required at the airport can be significant.
Operating Restrictions
Operating restrictions reduce or limit access to given airports. They potentially provide re-latively rapid and sizeable noise reductions. However, airports should carefully consider other consequences to the airport, its operators, and local regions (e.g., reduced economic benefits) before implementing such restrictions. ICAO encourages member states only to utilize operating restrictions after first applying the other elements of the balanced
ap-proach and to account for special circumstances of operators from developing countries so as to not unduly disadvantage them or where other modes of transport are unavailable.
Restrictions can take the form of outright bans, curfew limits to certain aircraft at certain times and noise quotas. Chapter 2-certified aircraft were largely banned throughout the world between 2000 and 2010. Curfew limits to certain aircraft at certain times can re-strict the noisiestChapter 3aircraft during the most sensitive nighttime hours. Finally, noise quotas are becoming increasingly common as a way of “capping” aircraft noise impacts at a given airport to an agreed level during a certain time interval. Under these schemes, every movement by an aircraft of a given type during nighttime hours is assigned a noise value (typically based on its noise certification values) which is then subtracted from the total quota. Once the quota is reached for a given period, no further operations are permitted. In this way, airports encourage operators to fly quieter aircraft as much as possible, given that more operations of quieter types would be allowed while still complying with the quota.
The total noise quota also typically decreases over time to further encourage operators to transition to quieter types just to maintain a given number of operations: seeExample 6.2.
Example 6.2 Noise Quotas, Noise and Emissions Charges Noise quotas are used at several airports in Europe, including London/Heathrow, Gatwick, and Stansted, as well as Madrid and Brussels. Each aircraft type is given a quota count (QC) rating for departure and arrival operations based on their certified noise levels. For example, Airbus A380 aircraft are QC2 on departure and QC0.5 on arrival. Limits are then set at each airport on the total QC and number of movements allowed in the 11:30 pm–6:00 am period, totals which decrease year on year to encourage adoption of quieter aircraft (Table 6.2).
TABLE 6.2 London Airport Quota Count Details
At London/Heathrow, landing charges vary by noise certification category and time of day, as shown inTable 6.3[as of August 2011, see IATA (2011)].