FAA Selected the Day-Night Sound Level Method for Measuring Noise Exposure

The Aviation Safety and Noise Abatement Act of 1979 required the Department of Transportation -- after consultation with the Environmental Protection Agency -- to establish, by regulation, a single system for measuring noise from airports and surrounding areas. The act also required the Secretary to establish a single method for measuring the exposure of individuals to noise resulting from airport operations; that method had to consider noise intensity, duration, frequency, and the time of occurrence. According to a Senate committee report, the act was intended to establish a uniform approach for measuring airport-related noise in order to facilitate the administration of a federal noise abatement program that could, in turn, lead to a uniform approach for dealing with noise problems in general. Pursuant to that directive, in 1981, FAA selected the A-weighted decibel and the Day-Night Sound Level method for measuring airport-related noise. [FN 16]

[FN 16] 4 C.F.R. 150.9.
In 1992, the Federal Interagency Committee on Noise noted that the Day-Night Sound Level method was practical and widely accepted. [FN 17] After a comprehensive review of measurement approaches, the interagency committee determined that this method best met the statutory requirements. The committee concluded that there were no other measurement methods of sufficient scientific standing to replace this method as the primary cumulative noise exposure measurement method and that the method correlates well with analyses of community annoyance at various noise exposure levels. [FN 18] The committee also noted that there were no new data to justify a change in the use of extra weighting for nighttime operations. These conclusions are still valid, according to the chairman of the Federal Interagency Committee on Aviation Noise (the successor to the Federal Interagency Committee on Noise), which focuses on aviation research related to noise.
[FN 17] The Federal Interagency Committee on Noise, and its successor -- the Federal Interagency Committee on Aviation Noise -- include the Departments of Transportation, Defense, Justice, Veterans Affairs, and Housing and Urban Development; the Environmental Protection Agency; and the Council on Environmental Quality.

[FN 18] Federal Interagency Review of Selected Airport Noise Analysis Issues (Federal Interagency Committee on Noise; Aug. 1992).

A frequent criticism levied against the Day-Night Sound Level method is that it does not effectively convey to people what they can actually expect to hear in any given area, primarily because it does not identify the noise levels generated by single aircraft takeoffs or landings. The noise level produced by the Day-Night Sound Level method is not the noise level that people actually hear on an event by event basis—it is an average of the cumulative sound levels over time.

To address this concern, the 1992 interagency committee report noted that supplemental information -- particularly information on noise generated by individual takeoffs and landings -- has been, and could continue to be, useful, especially in characterizing specific events and in conveying a clearer understanding of the potential effects of noise on people living and working in the area. The interagency committee recommended that federal agencies continue to be allowed to use supplemental information at their discretion when dealing with environmental impact analyses and requirements. An official of the interagency committee noted, however, that while single event information is useful as a supplement, there is no methodology for aggregating the effects of a single event into cumulative impact analysis, as is the case with the Day-Night Sound Level method.

Because the interagency committee reiterated the usefulness of the Day-Night Sound Level method, all federal agencies have adopted it for analyzing airport-related noise in their environmental assessments and impact statements. Some agencies, however, such as the Department of Defense, use supplemental noise information, such as single event noise measures, to provide a fuller picture of noise conditions and their potential effects. A proposed revision to FAA’s requirements for environmental analyses states that FAA will also use supplemental information where warranted. [FN 19] The revision adds new guidance on the kinds of supplemental information available and their use.

[FN 19] FAA Order 1050.1E. The public comment period closed on Jan. 11, 2000. Prior to this revision, the order stated only that FAA would consider the use of supplemental information.



Aircraft Noise Standards Vary By Aircraft Design and
Do Not Apply to Some Lighter Aircraft

FAA establishes the standards limiting the noise that civil subsonic turbojet aircraft are permitted to generate. [FN 1] Those standards are generally based on an aircraft’s weight and the number of engines and generally allow heavier aircraft to generate more noise than lighter aircraft. The statutory deadline of December 31,1999, for compliance with "stage 3" standards did not apply to aircraft weighing 75,000 pounds or less that were already in operation. As of October 1, 1999, more than 2,750 aircraft were not subject to the stage 3 compliance deadline.

[FN 1] These regulations appear in 14 C.F.R. part 36. Sideline noise is measured at points equidistant from both sides of an aircraft when the aircraft reaches a certain altitude during takeoff -- the altitude where sideline noise is at a maximum. Aircraft subject to regulation as civil subsonic turbojets include such aircraft as Boeing 737 and 747, MD-80, and Gulfstream IV.


Noise Standards Generally Permit Heavier Aircraft
to Generate More Noise Than Lighter Aircraft

FAA regulations establish the maximum noise levels that civil subsonic turbojet aircraft are allowed to generate for takeoff, landing, and "sideline" measurements. The standards for each of these kinds of measurements are different, but, in general, these standards vary with the weight of the aircraft. The standards allow heavier aircraft to be noisier than lighter aircraft because, according to FAA, the noise generated by an aircraft is generally determined by the thrust powering the aircraft; the amount of thrust an aircraft needs is proportional to the weight of the plane -- that is, the heavier the aircraft, the more thrust it needs. [FN 2] According to an aircraft noise expert, lower noise standards for lighter aircraft is one of the reasons that a stage 2 aircraft weighing 75,000 pounds or less may make less noise than a heavier aircraft that meets the more stringent stage 3 standards. For takeoff, stage 3 noise standards also vary on the basis of the number of engines; generally, the more engines an aircraft design has, the higher the permitted takeoff noise levels. Stage 3 standards for takeoff, sideline, and approach are shown in appendix VII.

[FN 2] According to representatives of the Air Transport Association of America, Inc., there is a tradeoff between reductions in noise and increases in air emissions because modifications to aircraft to reduce noise often add to the weight of an aircraft, thereby causing it to burn more fuel.
The United States is a member of the International Civil Aviation Organization -- the international authority on civil aviation standards-- and as such participates in that organization’s activities regarding aircraft noise standards. Members of the organization are considering more stringent noise standards. The organization’s Committee on Aviation Environmental Protection is reviewing several options, identified by its Noise Scenarios Group in a November 1999 report, including: (1) taking no action on more stringent standards, (2) adopting a standard only for new aircraft designs, or (3) adopting more stringent standards with various schedules for the phaseout of noisier aircraft. Guidance governing the Committee’s work directs it to consider such factors as technical feasibility, economic reasonableness, and the environmental benefit to be achieved. The organization is expected to adopt a resolution when it meets in September 2001 on a more stringent standard and the phaseout of stage 3 aircraft. Implementation of the new standard, and phaseout of the noisier aircraft, would be up to the member nations. [FN 3]
[FN 3] According to an FAA official, many of the organization’s member nations, particularly developing nations, have not yet imposed stage 3 standards on aircraft.
The European Union has banned, after May 1, 2000, stage 2 aircraft that were modified to meet stage 3 noise standards, [FN 4] unless the aircraft were already operating or registered in a member country before that date. The European Union also adopted restrictions on operating modified aircraft after April 1, 2002. The United States filed a formal complaint with the International Civil Aviation Organization on March 14, 2000, alleging that the European Union’s ban discriminates against U.S. aircraft in violation of the agreement establishing the organization.
[FN 4] Modifications can include new engines or other modifications such as hushkits. Hushkits reduce aircraft engine fan and compression noise by modifying various engine components and by adding acoustic treatment and noise suppression technology.


Existing Aircraft Weighing 75,000 Pounds or Less
Have Been Exempt from Operating Restrictions

Both stage 1 and stage 2 aircraft that did not meet more stringent noise standards by specified dates have been prohibited from operating after those deadlines, but that prohibition does not apply to aircraft in service that weigh 75,000 pounds or less. FAA did not require the retirement of the lighter stage 1 aircraft that did not meet stage 2 standards because FAA concluded it was not technologically practicable or economically reasonable to modify these aircraft. The statute prohibiting the operation of stage 2 aircraft that did not meet stage 3 standards by a certain date does not apply to aircraft weighing 75,000 pounds or less.


Exemption From the Stage 1 Operating Deadline Was for Technological and Economic Reasons

When FAA amends regulations controlling aircraft noise, it must consider several factors, including whether the proposed regulations are technologically practicable, economically reasonable, and appropriate for the types of aircraft, aircraft engines, or aircraft certifications that the regulations apply to. [FN 5] FAA must also consider the extent to which any proposed amendments protect the public health and welfare.

[FN 5] Amendments to 14 C.F.R. part 36 and part 91 are governed by provisions of section 611 of the Federal Aviation Act of 1958, as amended.
In 1976, FAA considered amending its regulations to require stage 1 aircraft already in service to meet stage 2 noise standards or be prohibited from operating at U.S. airports. At that time, the Environmental Protection Agency recommended that the deadline for compliance be applied to all civil subsonic turbojet aircraft regardless of weight. That agency contended that all of those aircraft were capable of meeting stage 2 standards by using various engine modifications or replacement options. It determined that because all newly produced aircraft weighing 75,000 pounds or less had to comply with stage 2 noise standards after January 1, 1975, there seemed to be no valid justification for permitting stage 1 aircraft to operate indefinitely. While some who commented on FAA’s proposed amendment supported the Environmental Protection Agency’s conclusion, others challenged it, contending, for example, that (1) the technology was not available to enable lighter aircraft to meet the stage 2 noise standards or (2) other sources, such as heavier aircraft or traffic from regularly scheduled passenger service flights, were the primary causes of the noise problems.

FAA chose not to apply the operating deadline for stage 1 aircraft to aircraft weighing 75,000 pounds or less. FAA concluded that it could not impose operating noise limits on the lighter aircraft at that time in a manner that was fully consistent with its obligations under the law for two reasons. First, FAA determined that the cost-effectiveness of implementing the kinds of modifications needed to retrofit an existing aircraft was questionable and, therefore, not technologically practicable. It concluded that noise reduction modifications to the lighter aircraft could be applied during the original design and manufacture of an aircraft, but such modifications involved substantial redesign efforts that, while reasonable when spread over the production process, were of doubtful cost-effectiveness if accomplished by retrofitting. FAA considered only retrofitting options -- engine modification or replacement -- as acceptable for meeting noise standards; flight operation noise abatement procedures were not an acceptable means for complying with the noise standards. [FN 6]

[FN 6] FAA did consider flight operations, however, to be an appropriate operational noise abatement procedure to further reduce noise where circumstances warranted.
Second, FAA determined that available information was not sufficient to assess the economic impact on owners of an across-the-board requirement to retrofit the lighter aircraft. Available information was limited because the aircraft were so varied in their use and mission and were frequently the only -- or one of a few -- aircraft owned by the owner. In addition, FAA determined that the availability of supplies for small engine manufacturers needed further study before FAA could assess the overall economic impact of specific compliance dates on aircraft owners.

In December 1997, however, the National Business Aviation Association, a membership organization of companies that operate aircraft, passed a resolution calling for the group’s 5,200 members to refrain from adding new stage 1 aircraft to their fleets beginning in January 2000 and to end the operation of stage 1 aircraft by 2005.


Statutory Deadline for Stage 2 Retirement Did Not Apply to the Lighter Aircraft

The Airport Noise and Capacity Act of 1990 established December 31, 1999, as the deadline for phasing out stage 2 aircraft that were not modified to meet stage 3 noise standards. The statute, however, specifically applied the phaseout only to aircraft weighing more than 75,000 pounds. The legislative history of the act provides no discussion on why the statutory phaseout was not applied to the lighter aircraft. [FN 7] As of October 1, 1999, just over 9,000 civil subsonic turbojet aircraft that weighed 75,000 pounds or less were certified by FAA as airworthy. [FN 8] About 31 percent of those, or just over 2,770, are stage 1 or stage 2 aircraft that may still operate at U.S. airports after December 31, 1999.

[FN 7] According to the General Aviation Manufacturers Association, this exception is consistent with guidance adopted by the International Civil Aviation Organization.

[FN 8] As of Oct. 1, 1999, FAA had certified nearly 300,000 fixed-wing aircraft as airworthy. About 5.3 percent, or just over 15,500, of those were civil subsonic turbojet aircraft.

The 1990 act, however, also established federal review requirements when an airport wants to control noise by imposing more stringent limitations on aircrafts’ use of the airport than federal regulations provide. The act directed the Secretary of Transportation to establish a national program for reviewing airport restrictions on the operation of stage 2 and stage 3 aircraft. It also required the Secretary to study whether federal review should be applied to restrictions on stage 2 aircraft weighing less than 75,000 pounds. The study recommended that the same procedures should apply to all stage 2 aircraft, regardless of weight. [FN 9] FAA adopted that recommendation. Thus, an airport may impose a noise or access restriction on stage 2 aircraft, whatever its weight, if the airport operator publishes the proposed restriction and prepares and makes certain analyses available for public comment at least 180 days before the effective date of the restriction. [FN 10] Unlike noise or access restrictions proposed for stage 3 aircraft, FAA approval is not required.
[FN 9] Study of the Application of Notice and Analysis Requirements to Operating Noise/Access Restrictions on Subsonic Jets Under 75,000 Pounds, Federal Aviation Administration, Office of Environment and Energy, June 1991.

[FN 10] The statute requires analyses of such things as the costs and benefits of the proposed restriction and a description of alternatives. The airport operator also must describe alternative measures considered that do not involve aircraft restrictions and a comparison of the costs and benefits of these measures with those that do involve aircraft restrictions. (49 U.S.C. 47524).