ACCEPTABLE (DNL not exceeding 65 decibels)—The noise exposure may be of some concern but common building constructions will make the indoor environment acceptable and the outdoor environment will be reasonably pleasant for recreation and play.

AIP—See AIRPORT IMPROVEMENT PROGRAM.

AIR CARRIER, CERTIFICATED ROUTE—An air carrier holding a Certificate of Public Convenience and Necessity, issued by the Federal Aviation Administration (FAA), to conduct sched-uled services over specified routes and a limited amount of nonscheduled operations.

AIR CARRIER, COMMUTER—An air taxi operator who (1) performs at least five round trips per week between two or more points and publishes flight schedules that specify the times, days of the week, and places between which such flights are performed; or (2) transports mail by air pursuant to a contract with the U.S. Postal Service.

AIRCRAFT DELAY—The additional travel time, caused by aircraft congestion, taken by an aircraft to move from point A to point B.

AIRCRAFT OPERATION—An aircraft arrival (landing) or an aircraft departure (takeoff) represents one aircraft operation. A low approach below traffic pattern altitudes or a touch-and-go operation is counted as both a landing and a takeoff, that is, as two operations. Aircraft operations are recorded by the FAA in four categories: air carrier, air taxi, general aviation, and military.

AIR CARRIER—Operations performed in revenue service by certificated route air carriers.


AIR TAXI/COMMUTER—Operations performed by operators of aircraft holding an air taxi certificate under Part 298 of the FAA regulations. This category includes commuter airline operations (excluding certificated commuter airlines), mail carriers under contract with the U.S. Postal Service, and operators of nonscheduled air taxi service.


GENERAL AVIATION—All civil aircraft operations not classi-fied as air carrier or air taxi operations.


MILITARY—Operations performed by military groups, such as the Air National Guard, the U.S. Air Force, or the U.S. Marine Corps.
Aircraft operations may also be described as local or itinerant:


LOCAL—Local operations are performed by aircraft that (1) operate in the local traffic pattern or within sight of the airport, (2) are known to be departing for, or arriving from, flight in local practice areas within a 20-mile radius of the airport, and (3) execute simulated instrument approaches or low passes at the airport.


ITINERANT—All aircraft operations other than local operations.

AIRCRAFT PARKING APRON—See APRON.

AIRCRAFT PARKING POSITION—The area on the ramp where aircraft park for servicing and preparation for flight.

AIRFIELD CAPACITY (HOURLY)—The maximum number of aircraft operations (landings or takeoffs) that can take place on an air-field in one hour under specific conditions.

AIR NAVIGATION FACILITY (NAVAID)—A facility designed for use as an aid to air navigation, including landing areas, lights, any apparatus or equipment for disseminating weather infor-mation, for signaling, for radio direction-finding, or for radio or other electronic communication, and any other structure or mechanism having a similar purpose for guiding and controlling flight in the air or the landing or takeoff of aircraft.

AIRPORT ACCESS AND PARKING PLAN—A plan that indicates the proposed routing of airport access facilities to central business districts and to points of connection with existing or planned arteries and based on airport access studies that take into account traffic demands, existing and poten-tial access problems, highway and rapid rail facilities, and in-town terminal facilities. The plan also incorporates on- and off-airport parking facilities for passengers, employees, and visitors and is a required element of an airport master plan.

AIRPORT APPROACH AND RUNWAY PROTECTION ZONE LAYOUT PLAN—A plan map showing the imaginary surfaces that specify the maximum height of structures, trees, and other phenomena around an airport and which is prepared in accordance with FAR Part 77, "Objects Affecting Navigable Airspace." The plan is required as part of an airport master plan.

AIRPORT ELEVATION—The highest point of an airport's usable runways measured in feet above mean sea level (msl).

AIRPORT ENVIRONS—The area surrounding an airport that is con-sidered to be directly affected by the presence and operation of the airport.

AIRPORT IMAGINARY SURFACES—Imaginary surfaces established at an airport for obstruction determination purposes, and consisting of primary, approach-departure, horizontal, vertical, conical, and transition surfaces.

AIRPORT IMPROVEMENT PROGRAM (AIP)—A program administered by the Federal Aviation Administration to provide financial grants-in-aid for airport planning, airport development projects, and noise compatibility programs. The program was estab-lished through the Airport and Airway Improvement Act of 1982, which was incorporated as Title V of the Tax Equity and Fiscal Responsibility Act of 1982 (P.L. 97-248).

AIRPORT LAND USE PLAN—A generalized plan depicting proposed land uses within the airport boundary. The land use plan is a required element of an airport master plan.

AIRPORT LAYOUT PLAN (ALP)—A plan for an airport showing boundaries and proposed additions to all areas owned or controlled by the sponsor for airport purposes, the location and nature of existing and proposed airport facilities and structures, and the location on the airport of existing and proposed nonaviation areas and improvements thereon. The ALP is a required element of an airport master plan.

AIRPORT MASTER PLAN—An assembly of appropriate documents and drawings covering the development of a specific airport from a physical, economic, social, and political jurisdictional perspective. The airport master plan includes an airport land use plan, airport layout plan, airport approach and runway protection zone layout plan, terminal area plan, airport access and parking plan, staging plan, and financial plan.

AIRPORT NOISE AND CAPACITY ACT OF 1990—Public Law 101-508, enacted November 5, 1990. Two important provisions of the law were the establishment of a national aviation noise policy (Sections 9308 and 9309) and the creation of a passenger facility charge (Sections 9110 and 9111), which enables airport sponsors to impose fees on the tickets issued to enplaning passengers. An amendment to FAR Part 91, "Transition to an All Stage 3 Fleet Operating in the 48 Contiguous United States and the District of Columbia," and new FAR Part 161, "Notice and Approval of Airport Noise and Access Restrictions," implement the national noise policy. New FAR Part 158, "Passenger Facility Charges," implements that portion of the Act authorizing the imposition of such charges.

AIRPORT SPONSOR—A public agency or tax-supported organization, such as an airport authority, that is authorized to own and operate an airport, to obtain property interests, to obtain funds, and to be legally, financially, and otherwise able to meet all applicable requirements of current laws and regulations.

AIRPORT SURVEILLANCE RADAR (ASR)—Radar providing position of aircraft by azimuth and range data. It does not provide elevation data. ASR is designed for range coverage up to 60 nautical miles and is used by terminal area air traffic control.

AIRPORT TRAFFIC CONTROL TOWER (ATCT)—A central operations facility in the terminal air traffic control system, con-sisting of a tower cab structure, including an associated instrument flight rule (IFR) room if radar equipped, using air/ground communications and/or radar, visual signaling and other devices, to provide safe and expeditious movement of terminal air traffic.

AIRSPACE—Space in the air above the surface of the earth or a particular portion of such space, usually defined by the boundaries of an area on the surface projected upward.

AIR TRAFFIC CONTROL (ATC)—A service operated by appropriate authority (the FAA) to promote the safe, orderly, and expeditious flow of air traffic.

AMBIENT NOISE—The total of all noise in a system or situation, independent of the presence of the specific sound to be measured. In acoustical measurements, strictly speaking, ambient noise means electrical noise in the measurement system. However, in popular usage, ambient noise is also used to mean "background noise" or "residual noise."

APRON—A paved area that provides the connection between the terminal buildings and the airfield. The apron includes aircraft parking areas, called ramps, and aircraft circula-tion and taxiing areas for access to these ramps. On the ramp, aircraft park in locations typically designated as gate positions or gates.

ATC—See AIR TRAFFIC CONTROL.

AUTOMATED RADAR TERMINAL SYSTEM (ARTS)—Computer-aided radar display subsystems capable of associating alphanumeric data with radar returns.

AVERAGE DAILY TRAFFIC (ADT)—The average traffic flow on a spe-cific street, road, or highway segment. ADT can be either total average flow or the average traffic in each direction.

AVIATION SAFETY AND NOISE ABATEMENT ACT OF 1979—Public Law 96-193, enacted February 18, 1980. The purpose of the Act is to provide assistance to airports in preparing and carrying out noise compatibility programs and in assuring continued safety for aviation. The Act also contains provi-sions that extend until January 1, 1988, the requirement for certain types of aircraft to comply with Part 36 of the Federal Aviation Regulations (see also FAR Part 36).

BACKGROUND NOISE—See AMBIENT NOISE.

BENEFICIAL OCCUPANCY—See DATE OF BENEFICIAL OCCUPANCY.

BUILDING CODE—A legal document that sets forth requirements to protect the public health, safety, and general welfare as they relate to the construction and occupancy of buildings and structures. The code establishes the minimum acceptable conditions for matters found to be in need of regulation. Topics generally covered are exits, fire protection, struc-tural design, sanitary facilities, light, and ventilation. Sound insulation may also be included.

BUILDING PERMIT—A permit issued by a local political jurisdic-tion (village, town, city, or county) to erect or modify a structure.

BUILDING RESTRICTION LINE (BRL)—The BRL should be located on an airport layout plan to identify suitable locations for building areas on airports. It is recommended that the BRL encompass the runway protection zones, the runway visibility zone, areas required for airport traffic control tower clear line of sight, and all airport areas with less than 35-foot clearance under the FAR Part 77 surfaces.

CAPITAL IMPROVEMENT PROGRAM (CIP)—A multiyear (sometimes a single year) schedule of capital expenditures for construction or equipment at an airport.

CEQ (COUNCIL ON ENVIRONMENTAL QUALITY) REGULATIONS—CEQ Regulations implementing the National Environmental Policy Act of 1969 (NEPA) were published in the Federal Register on November 29, 1978. References to the 4 Regulations in FAA Order 5050.4A (Airport Environmental Handbook) identify a given section, e.g., CEQ 1500 or CEQ 1508.8. (See also IMPACT.)

CONTOUR—See NOISE CONTOUR.

CONTRAFLOW- The FAA approved a procedure called "contraflow" as part of our current Noise Compatibility Program. Weather permitting, this procedure calls for all operations between the hours of 10:00 p.m. and 7:00 a.m. to be conducted south of the airport, where extensive mitigation programs have been implemented with Federal, state, and local resources. Specifically, this procedure sets aside the first few overnight hours to accommodate arrivals from the south, and the last few for departures to the south, with some room for transition before and after each block.

DATE OF BENEFICIAL OCCUPANCY (DBO)—The date on which the replacement terminal facilities are as substantially complete that they are usable by Airport tenants and the public without hazard or undue inconvenience.

DAY-NIGHT AVERAGE SOUND LEVEL (DNL)—A method for predicting, by a single number rating, cumulative aircraft noise that affects communities in airport environs. The DNL value represents decibels of noise as measured by an A-weighted sound-level meter (see also). In the DNL procedure, the noise exposure from each aircraft takeoff or landing at ground level around an airport is calculated, and these noise exposures are accumulated for a typical 24-hour period. (The 24-hour period often used is the average day of the peak month for aircraft operations during the year being analyzed.) Daytime and nighttime noise exposures are considered separately. A weighting factor equivalent to a penalty of 10 decibels is applied to operations between 10 p.m. and 7 a.m. to account for the increased sensitivity of people to nighttime noise. The DNL values can be expressed graphically on maps using either contours or grid cells. DNL may also be used for measuring other noise sources, such as automobile traffic, to determine combined noise effects.

dBA—See A-WEIGHTED SOUND LEVEL.

DECIBEL (dB)—A unit for measuring the volume of a sound, equal to the logarithm of the ratio of the intensity of the sound to the intensity of an arbitrarily chosen standard sound.

DEPLANED PASSENGERS—The volume of passengers inbound to an airport. The annual passenger volume of an airport is the total of deplaned and enplaned passengers (see also).

DEREGULATION ACT—Airline regulatory reform act of 1978. Designed, among other things, to encourage competition among domestic airlines, the Act allows an airline greater freedom to enter and leave any given market.

DEVELOPMENT PLAN—A detailed land use plan for all or specific areas on an airport. The plan usually includes a plot map depicting parcel size and configuration, access, land use categories, utilities, and performance standards for each parcel and use category.

DISPLACED THRESHOLD—A runway threshold that is located at a point other than the designated beginning of the runway.

DNL—See DAY-NIGHT AVERAGE SOUND LEVEL.

EFFECTS—See IMPACT.

ENGINE RUNUP AREA—An area on an airport where aircraft engines are serviced or tested. The noise from such servicing or testing can affect neighborhoods adjacent to the airport.

ENPLANED PASSENGERS—The volume of passengers outbound from an airport. The annual passenger volume of an airport is the total of enplaned and deplaned passengers (see also).

ENVIRONMENTAL IMPACT STATEMENT (EIS)—A statement prepared under the requirements of the National Environmental Policy Act of 1969 (NEPA), Section 102(2)(c). The EIS represents a federal agency's evaluation of the effects of a proposed action on the environment. Regulations relating to the preparation of an EIS are published in FAA Order 5050.4A.

FAA—See FEDERAL AVIATION ADMINISTRATION.

FAA ADVISORY CIRCULAR 150/5300-13—This document, titled "Airport Design," contains airport design standards, includ-ing descriptions of various subdivisions of FAR Part 77 (see also) such as obstacle free zones (OFZs), object free areas (OFAs), and runway protec-tion zones (RPZs)—formerly referred to as "clear zones"—on airports. According to Paragraph 211, "Safe and efficient operations at an airport require that certain areas on and near the airport be clear of objects or restricted to objects with a certain function, composition, and/or height." To achieve this requirement, object clearing criteria contained in the handbook describe the type of objects tolerated within various subdivisions of FAR Part 77. Aircraft are controlled by aircraft operating rules and not by these criteria. However, objects not in conformance with these criteria may result in aircraft operating restrictions.

FAA HANDBOOK 7400.2—This document, titled "Procedures for Handling Airspace Matters," contains procedures and guide-lines for conducting aeronautical studies and determining effects of existing or proposed objects that exceed FAR Part 77 (see also) standards. Objects that exceed FAR Part 77 standards are subject to an aeronautical study and are presumed to be hazards to air navigation unless an aeronautical study determines otherwise. However, once an aeronautical study has been initiated, Part 77 standards are not the basis for determining whether or not an object would be a hazard. Other standards, including operational, proce-dural, and electronic requirements, are used to determine if the object being studied would actually be a hazard to air navigation. The outcome of an FAA aeronautical study is either a "Determination of No Hazard" or "Determination of Hazard to Air Navigation."

FAA HANDBOOK 8260.3B—This document, titled "TERPS," contains obstruction clearance criteria for instrument procedures. Imaginary surfaces for each particular type of instrument procedure are described. If an object would penetrate the imaginary surfaces for a particular procedure and could not be relocated or sufficiently reduced in height, one of the following actions would be necessary: (1) alteration of the procedure, to minimize or eliminate effects; (2) increase in the minimum cloud ceiling and/or visibility requirements for conducting the procedure; (3) some combination of (1) and (2); or (4) preclusion of the affected procedure.

FAA ORDER 5050.4A—This document, entitled "Airport Environ-mental Handbook," was published by the FAA on October 8, 1985. It contains all of the essential informa-tion an airport sponsor needs to meet both procedural and substantive environmental requirements.

FAR PART 36—Federal Aviation Regulations Part 36, "Noise Standards: Aircraft Type and Airworthiness Certification." Establishes noise standards for the civil aviation fleet. Some extensions for compliance are included in the Aviation Safety and Noise Abatement Act of 1979 (see also).

FAR PART 77—Federal Aviation Regulations Part 77, "Objects Affecting Navigable Airspace." Establishes standards for determining obstructions and conducting aeronautical studies to determine the potential effects of obstructions on aircraft operations. Objects are considered to be obstructions to air navigation according to FAR Part 77 if they would exceed certain heights or penetrate certain imaginary sur-faces established in relation to airports. Objects classi-fied as obstructions are subject to an aeronautical study by FAA to determine their potential effects on aircraft operations.

FAR PART 91—Federal Aviation Regulations Part 91, "General Operating and Flight Rules." On September 25, 1991, the FAA issued an amendment to FAR Part 91 (14 CFR Part 91) in con-formance with requirements of the Airport Noise and Capacity Act of 1990 (see also). The amendment to the aircraft oper-ating rules requires a phased transition to an all Stage 3 fleet operating in the 48 contiguous United States and the District of Columbia by December 31, 1999. The amendment places a cap on the number of Stage 2 aircraft allowed to operate in the United States and provides for a continuing reduction in the population exposed to noise from Stage 2 aircraft.

FAR PART 150—Federal Aviation Regulations Part 150, "Airport Noise Compatibility Planning." An FAR Part 150 Program is an FAA-assisted study designed to increase the compatibility of land and facilities in the areas surrounding an airport that are most directly affected by the operation of the air-port. The specific purpose is to reduce the adverse effects of noise as much as possible by implementing both on-airport noise abatement measures and off-airport noise mitigation programs. The basic products of an FAR Part 150 program typically include (1) noise exposure maps for the existing condition and for five years in the future; (2) workable on-airport noise abatement measures, such as prefer-ential run-way use programs, new or preferential flight tracks, curfews; (3) off-airport noise mitigation measures (land use control programs and regulations), such as land acquisi-tion, soundproofing, or special zoning; (4) an analysis of the costs and the financial feasibility of the recommended measures; and (5) policies and procedures related to the implementation of on- and off-airport programs. A community involvement program is carried on throughout all phases of development of the program.

FAR PART 158—Federal Aviation Regulations Part 158, "Passenger Facility Charges." Adopts new regulations to establish a passenger facility charge (PFC) program. The rule imple-ments Sections 9110 and 9111 of the Airport Noise and Capacity Act of 1990 (see also), which requires the Department of Transportation to issue regula-tions under which a public agency may be authorized to impose a PFC of $1, $2, or $3 per enplaned passen-ger at a commercial service airport it controls. The proceeds from such PFCs are to be used to finance eligible airport-related projects that pre-serve or enhance safety, capacity, or security of the national air transportation system, reduce noise from an airport that is part of such system, or furnish opportuni-ties for enhanced competition between or among air carriers. The rule sets forth proce-dures for public agency applica-tions for authority to impose PFCs, for FAA processing of such applications; for collec-tion, handling, and remittance of PFCs by air carriers; for recordkeeping and auditing by air carriers and public agencies; for terminating PFC authority; and for reducing federal grant funds apportioned to large and medium hub airports imposing a PFC.

FAR PART 161—Federal Aviation Regulations Part 161, "Notice and Approval of Airport Noise and Access Restrictions." Establishes a program for reviewing airport noise and access restrictions on the operations of Stage 2 and Stage 3 aircraft. This rule is in response to specific provisions in the Airport Noise and Capacity Act of 1990 (see also) and is a major element of the national aviation noise policy required by that statute.

FEDERAL AVIATION ADMINISTRATION (FAA)—The FAA is the agency of the U.S. Department of Transportation that is charged with (1) regulating air commerce to promote its safety and devel-opment; (2) achieving the efficient use of navigable airspace of the United States; (3) promoting, encouraging, and developing civil aviation; (4) developing and operating a common system of air traffic control and air navigation for both civilian and military aircraft; and (5) promoting the development of a national system of airports.

FINDING OF NO SIGNIFICANT IMPACT (FONSI)—A finding by the FAA that a proposed action by an airport sponsor will have no significant impact (on the environment). Specific guide-lines for the preparation of a FONSI report are included in FAA Order 5050.4A.

FLIGHT TRACK—The average flight path flown by aircraft within specific corridors. Deviation from these tracks occurs because of weather, pilot technique, air traffic control, and aircraft weight. Individual flight tracks within a corridor are "averaged" for purposes of modeling noise exposure using the Integrated Noise Model (see also).

FONSI—See FINDING OF NO SIGNIFICANT IMPACT.

GATE—The designated location in a terminal building that con-tains an airline podium area where ticketed passengers check in for a specific flight. (See also APRON.)

GENERAL AVIATION (GA)—All civil aviation except that classified as air carrier or air taxi. The types of aircraft typically used in GA activities vary from multiengine jet aircraft to single-engine piston aircraft.

GENERAL PLAN (sometimes referred to as a comprehensive plan or community plan)—An overall plan of a political jurisdiction setting forth the goals and objectives of the jurisdiction, policies for development and redevelopment, and maps showing the spatial arrangement of land uses, circulation routes, and community facilities.

GPS (Global Positioning System)- A satellite navigation system designed to provide instantaneous position, velocity and time information almost anywhere on the globe at any time, and in any weather.

IFR—See INSTRUMENT FLIGHT RULES.

IFR AIRPORT—An airport with an authorized instrument approach procedure.

IFR CONDITIONS—Weather conditions that require aircraft to be operated in accordance with instrument flight rules.

IFR MINIMUMS AND DEPARTURE PROCEDURES (FAR PART 91)—Prescribed takeoff rules. For some airports, obstructions or other factors require the establishment of nonstandard takeoff minimums or departure procedures, or both, to assist pilots in avoiding obstacles during climb to the minimum en route altitude.

ILS—See INSTRUMENT LANDING SYSTEM.

IMPACT—In environmental studies, the word "impact" is used to express the extent or severity of an environmental problem, e.g., the number of persons exposed to a given noise envi-ronment. As indicated in CEQ 1500 (Section 1508.8), impacts and effects are considered to be synonymous. Effects or impacts may be ecological, aesthetic, historic, cultural, economic, social, or health related, and they may be direct, indirect, or cumulative.

INM—See INTEGRATED NOISE MODEL.

INSTRUMENT APPROACH—An approach to an airport, with intent to land, by an aircraft flying in accordance with an IFR flight plan, when the visibility is less than 3 miles and/or when the ceiling is at or below the minimum initial altitude.

INSTRUMENT APPROACH RUNWAY—A runway served by an electronic aid providing at least directional guidance adequate for a straight-in approach.

INSTRUMENT FLIGHT RULES (IFR)—Rules specified by the FAA for flight under weather conditions in which visual reference cannot be made to the ground and the pilot must rely on instruments to fly and navigate.

INSTRUMENT LANDING SYSTEM (ILS)—A system that provides in the aircraft the lateral, longitudinal, and vertical electronic guidance necessary for an instrument landing.

INSTRUMENT OPERATION—An aircraft operation in accordance with an IFR flight plan or an operation where IFR separation between aircraft is provided by a terminal control facility or air route traffic control center.

INSTRUMENT RUNWAY—A runway equipped with electronic and visual navigation aids and for which a straight-in (precision or nonprecision) approach procedure has been approved or is planned.

INTEGRATED NOISE MODEL (INM)—A computer model developed by the FAA and required by the FAA for use in environmental assess-ments, environmental impact statements, and FAR Part 150 studies for developing existing and future aircraft noise exposure maps.

LAND USE COMPATIBILITY—The compatibility of land uses surrounding an airport with airport activities and particularly with the noise from aircraft operations.

LAND USE COMPATIBILITY ASSURANCE—Documentation provided by an airport sponsor to the FAA. The documentation is related to an application for an airport development grant. Its pur-pose is to assure that a reasonably appropriate action, including the adoption of zoning laws, has been taken or will be taken to restrict the use of land adjacent to the airport or in the immediate vicinity of the airport. Such uses are limited to activities and purposes compatible with normal airport operations, including the landing and takeoff of aircraft.

LAND USE CONTROLS--Controls established by local or state governments to carry out land use planning. The controls include zoning, subdivision regulations, land acquisition (in fee simple, lease-back, or easements), building codes, building permits, and capital improvement programs (to pro-vide sewer, water, utilities, or other service facilities).

LAND USE PLANNING—Comprehensive planning carried out by units of local government, for all areas under their jurisdic-tion, to identify the optimum uses of land and to serve as a basis for the adoption of zoning or other land use controls.

LOUDNESS—The judgment of the intensity of a sound by a person. Loudness depends primarily on the sound pressure of the stimulus. Over much of the loudness range, it takes about a threefold increase in sound pressure (approximately 10 decibels) to produce a doubling of loudness.

MITIGATION MEASURE—An action that can be planned or taken to alleviate (mitigate) an adverse environmental impact. Mitigation includes:

(1) Avoiding the impact altogether by not taking a certain action or parts of an action.
(2) Minimizing the impact by limiting the degree or magni-tude of the action and its implementation.
(3) Rectifying the impact by repairing, rehabilitating, or restoring the affected environment.
(4) Reducing or eliminating the impact over time by preser-vation and maintenance operations during the life of the action.
(5) Compensating for the impact by replacing or providing substitute resources or environments.
A proposed airport development project, or alternatives to that project, may constitute a mitigation measure.

NAVAID—See AIR NAVIGATION FACILITY.

NOISE—Any sound that is considered to be undesirable because it interferes with speech and hearing, or is intense enough to damage hearing, or is otherwise annoying.

NOISE ABATEMENT PROCEDURES—Changes in runway use, flight approach and departure routes and procedures, and other air traffic procedures that are made to shift adverse aviation effects away from noise-sensitive areas (such as residential neighborhoods).

NOISE ATTENUATION OF BUILDINGS—The use of building materials to reduce noise through absorption, transmission loss, and reflection of sound energy.

NOISE CONTOURS—Lines drawn on a map that connect points of equivalent Ldn or CNEL values. They are usually drawn in 5-Ldn intervals, such as connections of Ldn 75 values, Ldn 70 values, Ldn 65 values, and so forth.

NOISE CONTROL PLANS—Documentation by an airport sponsor of actions to be taken by the sponsor to reduce the effect of aviation noise. These actions are to be taken by the spon-sor either alone or in cooperation with the FAA, airport users, and affected units of local government, with appro-priate comments from affected citizens. Alternative actions should be considered, particularly where proprietary use restrictions (see also) on aircraft operations are involved.

NOISE LEVEL REDUCTION (NLR)—The noise reduction between two areas or rooms is the numerical difference, in decibels, of the average sound pressure levels in those areas or rooms. A measurement of noise reduction combines the effect of the transmission loss performance of structures separating the two areas or rooms and the effect of acoustic absorption present in the receiving room.

NOISE-SENSITIVE LAND USE—Land uses that can be adversely affected by high levels of aircraft noise. Residences, schools, hospitals, religious facilities, libraries, and other similar uses are often considered to be sensitive to noise.

NORMALLY UNACCEPTABLE (DNL above 65 but not exceeding 75 decibels)—The noise exposure is significantly more severe; barriers may be necessary between the site and prominent noise sources to make the outdoor environment acceptable; special building constructions may be necessary to ensure that people indoors are sufficiently protected from outdoor noise.

OBSTACLE FREE ZONE (OFZ)—The OFZ is a three-dimensional volume of airspace that supports the transition of ground-to-airborne-aircraft operations (and vice versa). The OFZ clearing standard precludes taxiing and parked airplanes and object penetrations, except for frangible navaids whose location is fixed by function. The runway OFZ and, when applicable, the inner-approach OFZ, and the inner-transitional OFZ compose the obstacle free zone.

OBSTRUCTION—An object that exceeds a limiting height or pene-trates an imaginary surface described by current Federal Aviation Regulations (Part 77).

PATTERN—The configuration or form of a flight path flown by an aircraft, or prescribed to be flown, as in making an approach to a landing.

PRECISION APPROACH PROCEDURE—A standard instrument procedure for an aircraft to approach an airport in which an elec-tronic glide scope is provided—for example, an instrument landing system and precision approach radar.

PREFERENTIAL RUNWAY USE (PROGRAM)—A noise abatement action whereby the FAA Air Traffic Division, in conjunction with the FAA Airports Division, assists the airport sponsor in developing a program that gives preference to the use of a specific runway(s) to reduce overflights of noise-sensitive areas.

PRIORITY ACTION PROGRAM—See STAGING PLAN.

PROPRIETARY USE RESTRICTIONS—Restrictions by an airport sponsor on the number, type, class, manner, or time of aircraft operations at the airport.

RAMP—See APRON.

RELEASE POINT - A point on approach where aircraft are free to start their maneuver to centerline as needed.

RETROFIT—The retroactive modification of existing jet aircraft engines for noise abatement purposes.

RUNWAY OBJECT FREE AREA—The runway object free area (OFA) is a two-dimensional ground area surrounding the runway. The runway OFA clearing standard precludes parked airplanes and objects, except objects whose location is fixed by function.

RUNWAY PROTECTION ZONE (RPZ)—The RPZ (formerly the runway clear zone) is trapezoidal in shape and centered about the extended runway centerline. It begins 200 feet beyond the end of the area usable for takeoff or landing. Displacing the threshold does not change the beginning point of the RPZ. The RPZ dimensions are functions of the design aircraft, type of operation, and visibility minimums.

RUNWAY THRESHOLD—The beginning of that portion of a runway usable for landing.

RUNWAY USE PROGRAM—See PREFERENTIAL RUNWAY USE PROGRAM.

SEVERE NOISE EXPOSURE—Exposure to aircraft noise that is likely to interfere with human activity in noise-sensitive areas; repeated vigorous complaints can be expected and group action is probable. This exposure may be specified by a cumulative noise descriptor as a level of noise exposure, such as DNL 75. (See also SIGNIFICANT NOISE EXPOSURE.)

SIGNIFICANT EFFECT ON THE ENVIRONMENT—A substantial, or potentially substantial, adverse change in any of the physical conditions within the area affected by the project, including land, air, water, minerals, flora, fauna, ambient noise, and objects of historic or aesthetic significance. An economic or social change by itself is not considered a significant effect on the environment. A social or economic change related to a physical change may be considered in determining whether the physical change is significant.

SIGNIFICANT NOISE EXPOSURE—Exposure to aircraft noise that is likely to interfere with human activity in noise-sensitive areas; individual complaints may be expected and group action is possible. This exposure may be specified by a cumulative noise description as a level of noise exposure, such as DNL 65. (See also SEVERE NOISE EXPOSURE.)

SOUND INSULATION—(1) The use of structures and materials designed to reduce the transmission of sound from one room or area to another, or from the exterior to the interior of a building. (2) The degree of reduction in sound transmis-sion by means of sound insulating structures and materials.

SOUND LEVEL (NOISE LEVEL)—The weighted sound pressure level obtained by the use of a sound level meter having a standard frequency filter for attenuating part of the sound spectrum.

SOUND LEVEL METER—An instrument, consisting of a microphone, an amplifier, an output meter, and frequency-weighting net-works, that is used to measure noise and sound levels in a specified manner.

STAGE 3 AIRCRAFT - Aircraft Flying in the US and Europe must be certified as Stage 3 compliant after December 31, 1999. Stage 3 is the standard that defines allowable noise emissions and is defined in Part 36 of Title 14 Code of Federal Regulations.

TERPS—Certain airspace needs to be cleared for aircraft operations. This airspace is determined by the application of operating rules and terminal instrument procedures (TERPS). Removing obstructions to air navigation, except those which an FAA aeronautical study determined need not be removed, satisfies these requirements. Subpart C of FAR Part 77 defines obstructions to air navigation. (Also see FAA HANDBOOK 8260.3B.)

TOWER—See AIRPORT TRAFFIC CONTROL TOWER (ATCT).

UNACCEPTABLE (DNL above 75 decibels)—The noise exposure at the site is so severe that the construction cost to make the indoor noise environment acceptable may be prohibitive and the outdoor environment would still be unacceptable.

VFR AIRPORT—An airport without an authorized or planned instru-ment approach procedure.

VFR CONDITIONS—Weather conditions that permit aircraft to be operated in accordance with visual flight rules.

VHF OMNIDIRECTIONAL RANGE (VOR)—A radio transmitter facility in the navigation system radiating a VHF radio wave modulated by two signals, the relative phases of which are compared, resolved, and displayed by a compatible airborne receiver to give the pilot a direct indication of bearing relative to the facility.

VISUAL APPROACH—An approach to an airport wherein an aircraft on an IFR flight plan, operating in VFR conditions under the control of a radar facility and having air traffic control authorization, may deviate from the prescribed instrument approach procedure and proceed to the airport of destina-tion, served by an operational control tower, by visual reference to the surface.

VISUAL APPROACH SLOPE INDICATOR (VASI)—An airport lighting facility in the terminal area navigation system used primar-ily under VFR conditions. It provides vertical visual guid-ance to aircraft during approach and landing by radiating a directional pattern of high-intensity, red- and white-focused light beams, which indicate to the pilot that he is "on path" if he sees red/white, "above path" if white/white, and "below path" if red/red.

VISUAL FLIGHT RULES (VFR)—Rules that govern the procedures for conducting flight under visual conditions (Federal Aviation Regulations Part 91).

VISUAL RUNWAY—A runway intended solely for the operation of aircraft using visual approach procedures, with no straight-in instrument approach procedure and no instrument designa-tion indicated on an FAA-approved airport layout plan, or by any planning document submitted to the FAA by competent authority.

ZONING AND ZONING ORDINANCES—Ordinances that divide a community into zones or districts according to the present and poten-tial use of properties for the purpose of controlling and directing the use and development of those properties. Zoning is concerned primarily with the use of land and buildings, the height and bulk of buildings, the proportion of a lot that buildings may cover, and the density of popu-lation of a given area. As an instrument of plan implemen-tation, zoning deals principally with the use and develop-ment of privately owned land and buildings. The objective of zoning legislation is to establish regulations that provide locations for all essential uses of land and buildings and to ensure that each use is located in the most appropri-ate place. In noise compatibility planning, zoning can be used to achieve two major aims: (1) to reinforce existing compatible land uses and promote the location of future com-patible uses in vacant or underdeveloped land, and (2) to convert existing incompatible uses to compatible uses over time.