[Updated February 21, 2014]

Editor's note: This glossary is intended to be a comprehensive explanation of technical terms that are important in understanding the jargon used by airport managers, noise abatement managers, and Federal Aviation Administration bureaucrats to discuss noise issues. If a term does not appear here and you believe it's important to know, use the button at the end of the glossary to communicate with the editor. See also: Pilot/Controller Glossary

The A-weighting characteristic modifies the frequency response of a sound measuring instrument to account approximately for the frequency characteristics of the human ear. The A-weighted decibel is a measure of sound pressure level modified by attenuating the low frequencies. (See Noise level and Noise exposure level.)

Air carriers
Airlines holding a certificate of public convenience and necessity issued under 49 U.S.C. 1371 (Federal Aviation Act of 1958) that operate aircraft designed to have a maximum seating capacity of more than 60 seats or a maximum payload capacity of more than 18,000 pounds or conduct international operations. There are four different types of air carriers: majors (annual operating revenues greater than $1 billion), nationals ($100 million to $1 billion), large regionals ($20 million to $100 million), and medium regionals (0 to $20 million).

Air quality standards (See National Ambient Air Quality Standards)

Air taxis
Planes that either (1) fly at least five round trips per week between two or more points according to flight schedules that specify the times, days of the week, and places between which such flights are performed, or (2) transport mail pursuant to a current contract with the U.S. Postal Service.

Airport noise compatibility program
The program developed and documented in accordance with Appendix B of 14 CFR 150, including the measures proposed or taken by the airport operator to reduce existing noncompatible land uses and to prevent the introduction of additional noncompatible land uses within the area.

Aviation, commercial
The sum total of air carrier and air taxi flights.

Aviation, general
All aviation that is not commercial or military.

Avigation easement
An easement that stipulates that a person's property is exposed to aircraft noise; it usually provides legal protection to the airport against noise lawsuits.

Community noise equivalent level (CNEL)
Same as day-night average sound level, except that it includes a 5 decibel penalty during the hours of 7:00 p.m. to 10:00 p.m. in addition to the 10 decibel nighttime penalty. The annual average CNEL is used by the State of California and the Federal Aviation Administration to describe the noise impacts of airports. The symbol for noise equivalent level is Leq.

Compatible land use
Land use identified under 14 CFR Part 150 as normally compatible with the outdoor noise at that location (or an adequately attenuated noise level for any indoor activities involved) because the annual average day-night average sound level is at or below the level identified for that use (or similar uses) in Appendix A (Table 1) of 14 CFR Part 150.

Day-night average sound level (DNL)
A level of noise expressed (in decibels) as a 24-hour average. Nighttime noise, between the hours of 10:00 p.m. and 7:00 a.m. is weighted; that is, given an additional 10 decibels to compensate for sleep interference and other disruptions caused by loud nighttime noise. An annual average of DNLs is used by the Federal Aviation Administration to describe airport noise exposure. Areas with noise impacts less than 65 dB DNL are considered "compatible" with residential use; areas at or above 65 dB DNL are designated "incompatible" with residential use. The symbol for DNL is Ldn. (14 CFR Part 150) (Cf. Community noise equivalent level.)

Decibel (dB)
A unit of sound (sound pressure level) measurement, expressing a logarithmic ratio between a measured sound pressure and a reference level, which is normally 0.0002 dynes per square centimeter (about the threshold of hearing). A sound doubles in loudness for every increase of 10 decibels. See Comparison of Noise Sources in Decibels.

Pollutants discharged into the atmosphere from smokestacks, other vents, and surface areas of commercial or industrial facilities; from residual chimneys; and from motor vehicle, locomotive, or aircraft exhausts. The U.S. Environmental Protection Agency sets minimum air quality standards for a variety of pollutants, but individual states are permitted to set tougher standards (as does California).

Environmental Impact Report (EIR)
A document required by the California Environmental Quality Act. It describes the positive and negative effects of the undertaking and lists alternative actions. In principle, it is a basis for deciding whether to permit the project. (Cf. Environmental Impact Statement)

Environmental Impact Statement (EIS)
A document required of federal agencies by the National Environmental Policy Act for major projects or legislative proposals that "significantly" affect the environment. It describes the positive and negative effects of the undertaking and lists alternative actions. In principle, it is a basis for deciding whether to permit the project. (Cf. Environmental Impact Report)

Equivalent noise level (Leq)
The average (on an energy basis) noise level (usually A-weighted sound level) integrated over some period of time. (See Community noise equivalent level.)

Freedom of Information Act (FOIA)
The Freedom of Information Act (5 U.S.C. §522) requires that a federal agency deliver a copy of any public record on demand. The Act permits certain exceptions. Each federal agency has its own procedures for complying with FOIA. See U.S. Department of Transportation FOIA Procedures.

Instrument approach procedure
A series of predetermined maneuvers for the orderly transfer of an aircraft under instrument flight conditions from the beginning of the initial approach to a landing or to a point from which a landing may be made visually. It is prescribed and approved for a specific airport by competent authority. U.S. civil standard instrument approach procedures are approved by the Federal Aviation Administration as prescribed under 14 Code of Federal Regulations Part 97, and are available for public use.

Instrument flight rules (IFR)
Rules governing the procedures for conducting instrument flight. Also a term used by pilots and controllers to indicate type of flight plan.

Instrument landing system
A precision instrument approach system that normally consists of the following electronic components and visual aids: localizer, glideslope, outer marker, middle marker, and approach lights. (For a more detailed description see instrument landing system; see also Visual approach.)

Integrated Noise Model (INM) See Noise models

Landing and takeoff (LTO) cycle
The basis of calculating ground-level aircraft exhaust emissions. The components of an LTO are approach, taxi/idle-in, taxi/idle-out, take-off, and climbout. The LTO cycle thus includes only those emissions created within 3,000 feet of the earth's surface, all of which affect ground-level air quality.

National Ambient Air Quality Standards (NAAQS)
Standards established by the U.S. Environmental Protection Agency for the levels of a variety of pollutants in the ambient (outside) air. At prresent there are NAAQs for ozone, carbon monoxide, particulates, sulfur dioxide, lead, and nitrogen dioxide. Each state is required to submit to the EPA for approval a State Implementation Plan (SIP). Individual states are permitted to set tougher standards (as California does). The standards are enforced by a complex system of regulations for specific industries. Regulations require permission of the state to build new facilities or enlarge existing facilities that emit pollutants for which standards have been set. The process of setting and amending regulations on air quality is highly contentious, as air quality standards impose the cost of pollution abatement on industry.

Noise exposure level (NEL)
The noise exposure level is the level of noise accumulated during a given event, with reference to a duration of one second. More specifically, NEL (in decibels) is the level of the time-integrated A-weighted squared sound pressure for a stated time interval or event, based on the reference pressure of 20 micronewtons per square meter and reference duration of one second. (21 Calif. Code Regs. 5001) (See Single event noise exposure level; Community noise equivalent level; Day-night average sound level; and Noise level.)

Noise exposure map
A scaled, geographic depiction of an airport, its noise contours, and surrounding area developed in accordance with section A150.101 of Appendix A of 14 CFR 150, including the accompanying documentation setting forth the required descriptions of forecast aircraft operations at that airport during the fifth calendar year beginning after submission of the map, together with the ways, if any, those operations will affect the map (including noise contours and the forecast land uses). (14 CFR Part 150)

Noise level
Noise level (NL) is the measure (in decibels) of an A-weighted sound pressure level as measured using the slow dynamic characteristic for sound level meters specified in American National Standard Specification for Sound Level Meters (ANSI S1.4-1983, as revised by ANSI S1.4A-1985). The reference pressure is 20 micronewtons per square meter (2 x 10-4 microbar).

Noise models
"Noise models" are computer models used to predict the impacts of aircraft noise over a geographic area. Such models are used to develop the noise exposure contours and noise exposure maps submitted to the Federal Aviation Administration and other government agencies (state and local). Models use aviation operations data collected by the airport operator. Models must be approved by the FAA. Presently the FAA has approved the the Integrated Noise Model (INM) for airports and the Heliport Noise Model (HNM) for heliports. In considering approval of a methodology or computer program, key factors include the demonstrated capability to produce the required output and the public availability of the program or methodology to provide interested parties the opportunity to substantiate the results. (14 CFR Part 150 sec. A150.103) See Aircraft Noise Measurement and Modeling.

Preferential runway use
Taking off or landing on specified runways during certain hours to avoid residential areas.

An aircraft maintenance procedure; a "gunning" of the engine.

Single event noise level (SENL)
The single event noise level (in decibels) is the noise exposure level of a single event, such as an aircraft flyby, measured over the interval between the initial and final times in which the noise event exceeds a threshold noise level. (21 Calif. Code Regs. 5001)

Sound exposure level
The level, in decibels, of the time integral of squared A-weighted sound pressure during a specified period or event, with reference to the square of the standard reference sound pressure of 20 micropascals and a duration of one second. (14 CFR Part 150) (See Noise level and Noise exposure level.)

Stage 2 and stage 3 aircraft
Commercial jet engines currently meet either stage 2 or stage 3 noise standards. Stage 2 engines are older and noisier than stage 3 engines. By the year 2000 most jet engines used in the United States must meet stage 3 noise standards (Airport Noise and Capacity Act of 1990). Stage 3 aircraft incorporate the latest technology for suppressing jet-engine noise and, in general, are 10 dB quieter than stage 2 aircraft. This represents a halving of perceived noise; however, actual noise reduction varies by aircraft.

State Implementation Plan (SIP) (See National Ambient Air Quality Standards)

Visual approach
An approach wherein an aircraft on an IFR (instrument flight rules) flight plan, operating in VFR (visual flight rules) conditions under the control of an air traffic control facility and having an air traffic control authorization, may proceed to the airport of destination in VFR conditions. (Compare Instrument landing system.)

Visual flight rules (VFR)
Rules that govern the procedures for conducting flight under visual conditions. The term "VFR" is also used in the United States to indicate weather conditions that are equal to or greater than minimum VFR requirements. In addition, it is used by pilots and controllers to indicate type of flight plan. See FAA Advisory Circular 91-36D, "Visual Flight Rules (VFR) Flight Near Noise-Sensitive Areas" (2004).