A-Weighted Sound (dBA) -- A measurement representing a sound generally as the human ear hears it by filtering out as much as 20 to 40 decibels of sound below 100 hertz (Hz). Used for aircraft noise evaluations.

Air Route Traffic Control Center (ARTCC) -- An FAA facility established to provide air traffic control service to aircraft operating on an IFR flight plan within controlled airspace during the en route portion of a flight.

Air Traffic Control (ATC) -- A service operated by appropriate authority to promote the safe orderly, and expeditious flow of air traffic.

Airman's Information Manual -- A publication containing basic flight information and ATC procedures designed primarily as a pilot's information and instructional manual for use in the National Airspace System.

Airport Elevations -- The highest point on an airport's usable runways expressed in feet above mean sea level (MSL).

Airport Improvement Plan (AIP) -- A Federal funding program for airport improvements. Funds are derived from sources such as airline tickets, aviation fuel, etc.

Airport Layout Plan (ALP) -- An airport layout plan is a scaled drawing of existing and proposed land and facilities necessary for the operation and development of the airport. Any airport will benefit from a carefully developed plan that reflects current FAA design standards and planning criteria. The ALP shows 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 non-aviation areas and improvements thereon.

Airport Operations -- The total number of movements in landings (arrivals) plus takeoffs (departures) from an airport. 

Airport Surveillance Radar (ASR) -- A radar system which allows air traffic controllers to identify an arriving or departing aircraft's distance and direction from an airport.

Airport Traffic Control Tower (ATCT) -- The air traffic control facility located on an airport and responsible for traffic separation within the immediate vicinity of an airport as well as on the surface of the airport.

Airway -- A corridor of controlled airspace whose centerline is established by radio navaids. Low altitude airways (between 3,000 and 18,000 feet AGL) are identified by the letter V. High altitude airways (above 18,000 feet MSL) are known as Jet airways and are identified with the letter J.

Ambient Noise -- The totality of noise in a given place and time - usually a composite of sounds from varying sources at varying distances from the observer.

Annual Service Volume (ASV) -- A planning term which describes the number of annual aircraft operations which is possible at an airport with an acceptable amount of delay. The measure is specific to individual airports because it is derived from their own particular capacity characteristics.

Attenuation -- Acoustical phenomenon whereby a reduction of sound energy is experienced between the noise source and the receiver. This energy loss can be attributed to atmospheric conditions, terrain, vegetation, man-made features (e.g., sound insulation) and natural features.

Automated Radar Terminal System (ARTS) -- Computer-aided radar display subsystems capable of associating alphanumeric data such as aircraft identification, altitude, and airspeed with aircraft radar returns.

Azimuth -- An arc of the horizon measured between a fixed point (as true north) and the vertical circle passing through the center of an object.

Baseline Condition -- The existing conditions or conditions prior to future development, which serve as a foundation for analysis.

Building Restriction Line (BRL) -- A line which identifies suitable building area locations on airports. The BRL encompasses the runway protection zones, the runway visibility zone areas required for airport traffic control tower clean line of sight and all airport areas with less than 35 foot (10.5) clearance under the FAR Part 77 surfaces.

Capacity -- The number of aircraft that can land or depart from an airport under specific conditions during a particular time. Capacity is determined by a number of complex factors including the length of runways in use, air traffic rules, the mix of airplanes using the airport, the current weather and visibility, the number of available gates, and other limiting factors such as getting to and from the airport. 

Class A Airspace -- Generally, that airspace from 18,000 feet MSL up to and including FL600, including the airspace overlying the waters within 12 nautical miles of the coast of the 48 contiguous States and Alaska; and designated international airspace beyond 12 nautical miles of the coast of the 48 contiguous States and Alaska within areas of domestic radio navigational signal or ATC radar coverage, and within which domestic procedures are applied.

Class B Airspace -- That airspace from the surface to 10,000 feet MSL surrounding the nation's busiest airports in terms of IFR operations or passenger enplanements. The configuration of each Class B Airspace area is individually tailored and consists of a surface area and two or more layers (some Class B airspace areas resemble upside-down wedding cakes), and is designed to contain all published instrument procedures once an aircraft enters the airspace. An ATC clearance is required for all aircraft to operate in the area, and all aircraft that are so cleared receive separation services within the airspace. The cloud clearance requirement for VFR operations is "clear of clouds." 

Class C Airspace -- That airspace from the surface to 4,000 feet above the airport elevation (charted in MSL) surrounding those airports that have an operational control tower, are serviced by a radar approach control, and that have a certain number of IFR operations or passenger enplanements. Although the configuration of each Class C airspace area is individually tailored, the airspace usually consists of a 5 NM radius core surface area that extends from the surface up to 4,000 feet above the airport elevation, and a 10 NM radius shelf area that extends from 1,200 feet to 4,000 feet above the airport elevation. 

Class D Airspace -- That airspace from the surface to 2,500 feet above the airport elevation (charted in MSL) surrounding those airports that have an operational control tower. The configuration of each Class D airspace area is individually tailored and when instrument procedures are published, the airspace will normally be designed to contain the procedures.

Commuter Aircraft -- Commuters are those operators that provide regularly scheduled passenger or cargo service with aircraft seating 72 passengers or less. A typical commuter flight operates over a trip distance of less than 300 miles.

Connecting Passenger -- An airline passenger who transfers from an arriving aircraft to a departing aircraft in order to reach their ultimate destination. 

Constructive Use -- Refers to the possible indirect impacts to DOT Section 303(c) properties such as parks. Constructive use is considered to occur when a transportation project does not incorporate land from a Section 303(c) resource but the projects proximity impacts are so severe that the protected activities feature or attributes that qualify a resource for protection under section 303(c) are substantially impaired. Substantial impairment occurs only when the protected activities, features or attributes of the resource are substantially diminished. For example, a substantial increase in noise levels at a park due to transportation project may represent a constructive use, even though the park is not directly affected through acquisition or development.

Controlled Airspace -- An airspace of defined dimensions within which air traffic control service is provided to IFR flights and to VFR flights in accordance with the airspace classification. Controlled airspace is designated as Class A, Class B, Class C, Class D, or Class E airspace. Aircraft operators are subject to certain pilot qualifications, operating rules, and equipment requirements as specified in FAR Part 91, depending upon the class airspace in which they are operating.

Day-Night Average Sound Level (DNL) -- A noise measure used to describe the average aircraft noise levels over a 24-hour period, typically an average day over the course of a year. DNL considers aircraft operations that occur between the hours of 10 p.m. and 7 p.m. to be 10 decibels louder than they actually are to account for increased annoyance when ambient noise levels are lower and people are trying to sleep. DNL may be determined for individual locations or expressed in noise contours. DNL is currently the accepted measure for aircraft noise analysis.

Decibel (dB) -- Sound is measured by its pressure or energy in terms of decibels. The decibel scale is logarithmic; when the scale goes up by ten, the perceived sound is two times as loud.

Delay -- The difference, in minutes, between the scheduled time and actual time of an aircraft arrival or departure. For airport planning purposes, it is often expressed as an annual average delay per aircraft operation (in minutes).

Displaced Threshold -- A threshold that is located at a point on the runway other than the designated beginning of the runway. The portion of pavement behind a displaced threshold may be available for takeoffs in either direction and landings from the opposite direction.

Distance Measuring Equipment (DME) -- A flight instrument which measures the distance from a navigational radio station in nautical miles.

Enplanements -- The total number of departure passengers boarding an aircraft.

En route System -- That part of the National Airspace System where aircraft are operating between departure and destination airports.

Equivalent Sound Level (Leq) -- The steady A-weighted sound level over any specified time period. It is used to identify the average sound level over a period of time.

Federal Aviation Administration (FAA) -- The FAA is responsible for insuring the safe and efficient use of the Nation's airspace, for fostering civil aeronautics and air commerce, and for supporting the requirements of national defense. The activities required to carry out these responsibilities include: safety regulations; airspace management and the establishment, operation, and maintenance of a system of air traffic control and navigation facilities; research and development in support of the fostering of a national system of airports, promulgation of standards and specifications for civil airports, and administration of Federal grants-in-aid for developing public airports; various joint and cooperative activities with the Department of Defense; and technical assistance (under State Department auspices) to other countries.

Federal Aviation Regulations (FAR) -- The body of Federal regulations relating to aviation. Published as Title 14 of the Code of Federal Regulations.

Fix -- A geographical position determined by reference to one or more radio NAVAIDS; celestial plotting; or by some other means such as satellite navigation. Examples for Dayton International Airport include BRUNY, DANEI, KEKEE, and BUCKE.

Flight Level -- A level of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury. Each Flight level is expressed in three digits representing hundreds of feet. For example FL 250 represents a barometric altitude of 25,000 feet. Aircraft operating at altitudes greater than 18,000 feet MSL use Flight levels as their altitude reference.

Flight Track Utilization -- The use of established routes for arrival and departure by aircraft to and from the existing runways at the airport.

Glide Slope (GS) -- Provides vertical guidance for aircraft during approach and landing. The glide scope consists of the following:

Electronic components emitting signals which provide vertical guidance by reference to airborne instruments during instrument approaches such as ILS, or 
Visual ground aids, such as VASI, which provide vertical guidance for VFR approach or for the visual portion of an instrument approach and landing. 

Global Positioning System (GPS) -- A system of 24 satellites used as reference points to enable navigators equipped with GPS receivers to determine their latitude, longitude, and altitude. The accuracy of the system can be further refined by using a ground receiver at a known location to calculate the error in the satellite range data. This is known as Differential GPS (DGPS).

Grid Analysis -- A type of aircraft noise analysis which evaluates the noise levels at individual points rather than through generation of noise contours.

Ground Effect -- Noise attenuation attributed to absorption or reflection of noise by man-made or natural features on the ground surface.

Hub -- An airport which service airlines that have hubbing operations.

Hubbing -- A method of airline scheduling that times the arrival and departure of several aircraft in close period of time in order to allow the transfer of passengers between different flights of the same airline in order to reach their ultimate destination. Several airlines may conduct hubbing operations at an airport.

Hubbing Complex -- The period of time in which an airline times the arrival and departure of several aircraft to accomplish hubbing. An airline may operate several complexes at an airport each day.

Instrument Approach -- 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.

Instrument Flight Rules (IFR) -- That portion of the Federal Air Regulations (14 CFR 91) specifying the procedures to be used by aircraft during flight in IMC. These procedures may also be used under visual conditions and provide for positive control by ATC. (See also VFR).

Instrument Landing System (ILS) -- An electronic system installed at some airports which helps to guide pilots to runways for landing during periods of limited visibility or adverse weather. 

Instrument Meteorological Conditions (IMC) -- Weather conditions expressed in terms of visibility, distance from clouds, and cloud ceilings during which all aircraft are required to operate using instrument flight rules (IFR).

Integrated Noise Model (INM) -- A computer model developed and maintained by the FAA to predict the noise impacts generated by aircraft operations.

Knots -- Airspeed measured as the distance in nautical miles covered in 1 hour.

Land Use Compatibility -- The ability of land uses surrounding the airport to coexist with airport-related activities with minimum conflict.

Landing and Takeoff (LTO) Cycle -- The time that an aircraft is in operation at an airport. An LTO cycle begins when an aircraft starts its final approach (arrival) and ends after the aircraft has made its climb-out (departure).

Ldn -- (See DNL). Ldn used in place for DNL in mathematical equations only.

Leq -- Equivalent Sound Level. The steady A-weighted sound level over any specified period of time (examples: 24 hours, 8 hours). This metric is used to identify the average sound level over a specified period of time.

Local Passenger -- A passenger who either enters or exits a metropolitan area on flights serviced by the area's airport. The opposite of a connecting passenger.

Localizer -- The component of an ILS which provides lateral course guidance to the runway

Location Impact Analysis -- An analysis conducted to determine if noise level increases associated with projected development would approach the FAA threshold of a 1.5 DNL increase within the 65 DNL or greater noise contours over any noise-sensitive land use.

Loudness -- The subjective intensity of sound.

Master Plan Update -- An update to the long-range airport development requirements. These plans are typically updated every 5-7 years.

Mean Sea Level (MSL) -- The average height of the surface of the sea for all stages of the tide, used as a reference for elevations. Also called sea level datum.

Missed Approach -- A prescribed procedure to be followed by aircraft that cannot complete an attempted landing at an airport.

Mitigation -- The avoidance or minimization of an adverse impact.

Mitigation Measure -- An action take to alleviate adverse impacts.

Narrowbody Aircraft -- A commercial passenger jet having a single aisle and maximum of three seats on each side of the aisle. Narrowbody aircraft include B727, B737, B757, DC9, MD80, MD90 and A320.

National Airspace System (NAS) -- The common network of U.S. airspace; air navigation facilities, equipment, services, airports or landing areas; aeronautical charts, information and services, rules regulations and procedures; technical information, manpower and material which are used in aerial navigation.

National Environmental Policy Act of 1969 (NEPA) -- The original legislation establishing the environmental review process.

Nautical Mile -- A measure of distance equal to 1 minute of arc on the earth's surface (approximately 6,076 feet).

NAVAIDS (Navigation Aids) -- Any facility used by an aircraft for navigation.

Noise -- Unwanted Sound

Noise Abatement -- A measure or action that minimizes the impact of noise on the environs of an airport. Noise abatement measures include aircraft operating procedures and use or disuse of certain runways or flight tracks.

Noise Contour Map -- A map representing average annual noise levels summarized by lines connecting points of equal noise exposure.

Noise Exposure Map (NEM) -- A map of an airport and its environs which identifies the area impacted by various aircraft noise levels. The FAA has specified criteria for presentation of Noise Exposure Maps, which is governed by FAR Part 150.

Noise Impact Routing System (NIRS) -- A computer simulation model that evaluates noise impacts in a defined study area from the ground up to 18,000 feet AGL. This model replaces the FAA's Checklist Analysis 7210.360.

Noise Level Reduction (NLR) -- The amount of noise level reduction achieved through incorporation of noise attenuation (between outdoor and indoor levels) in the design and construction of a facility.

Nondirectional Beacon (NDB) -- A beacon transmitting nondirectional signals whereby the pilot of an aircraft equipped with direction finding equipment can determine his bearing to and from the station. When the radio beacon is installed in conjunction with the ILS marker, it is normally called Compass Locator.

Nonprecision Approach -- A standard instrument approach procedure providing runway alignment but no glide slope or descent information.

Outer Fix -- A general air traffic control term to describe the fixes in the terminal area from which aircraft are normally cleared to the approach fix or final approach course.

Positive Control --The separation of all air traffic within designated airspace.

Precision Approach Procedure/Precision Approach -- A standard instrument approach procedure in which an electronic glideslope/glidepath is provided, e.g.. ILS and PAR.

Primary Commercial Service Airport -- A commercial airport which enplanes 0.01 percent or more of the total annual U.S. enplanements.

Primary Runway -- The runway on which the majority of operations take place. At large, busy airports, there may be two or more parallel primary runways.

Profile -- The physical position of the aircraft during landings or takeoffs in term of altitude in feet above the runway and distance from the runway end.

Public Use Airport -- Any public, airport, any privately owned reliever airport, any privately owned airport which is determined to enplane annually 2,500 or more passengers and receive scheduled passenger service of aircraft, and which is used or to be used for public purposes.

Quadrant -- A quarter part of a circle, centered on a NAVAID oriented clockwise from magnetic north.

Radial -- A magnetic bearing extending from a VOR, VORTAC or TACAN facility.

Reliever Airport -- An airport which, when certain criteria are met, relieves the aeronautical demand on a busier air carrier airport.

Rotational Runway Use -- Variance in use of runways over a specific time period to prevent constant use of one runway.

Run-Up -- A routine procedure for testing an aircraft engine at a high power setting. Engine run-ups are normally conducted by airline maintenance personnel checking an engine following the conduct of maintenance.

Runway -- A defined rectangular area on an airport for the purpose of landing and takeoff of aircraft. Runways are normally numbered in relation to their magnetic direction rounded off to the nearest 10 degrees, e.g., Runway 14, Runway 32.

Runway Protection Zone (RPZ) -- An area (formally the clear zone) trapezoidal in shape and centered about the extended runway centerline, is used to enhance the safety or aircraft operations. It begins 200 feet (60M) beyond the end of the area usable for takeoff or landing. The RPZ dimensions are functions of the design aircraft, type of operation and visibility minimums.

Runway Safety Area (RSA) -- A defined surface surrounding the runway prepared or suitable for reducing the risk or damage to airplanes in the event of an undershoot, overshoot or excursion from the runway.

Single Event -- An occurrence of audible noise usually above a specified minimum noise level caused by an intrusive source such as an aircraft overflight, passing train or ship's horn.

Slant-Range Distance -- The straight-line distance between an aircraft and a point on the ground.

Sound -- Sound is the result of a sound source vibration in the air. The vibration produces alternating bands of relatively dense and sparse particles of air, spreading outward from the source in the same way as ripples do on water after a stone is thrown into it. The result of the movement is fluctuation in the normal atmospheric pressure or sound waves.

Sound Exposure Level (SEL) -- A measure of the physical energy of the noise event which takes into account both intensity and duration. By definition SEL values are referenced to a duration of one second and should not be confused with either the average or maximum noise levels associated with a specific event. People do not hear SEL. SEL is always higher than the average and the maximum as long as the event is longer than one second. Expressed in dB.

Special Use Airspace -- Six types of airspace designated to special uses and defined in the Airman's Information Manual. It includes airspace of defined dimensions identified by an area on the earth's surface wherein activities must be confined because of their nature and/or wherein limitations may be imposed upon aircraft operations which are not part of those activities.

Stage 2 Aircraft -- Aircraft which meet the noise levels prescribed by FAR Part 36 and are less stringent than those established for the quieter designation (State 3). The Airport Noise and Capacity Act requires 
the phase-out of all Stage 2 aircraft by December 31, 1999, with case-by-case exceptions through the year 2003.

Stage 3 Aircraft -- Aircraft that meet the most stringent noise levels set in FAR Part 36.

Standard Instrument Departure Procedure (SID) -- A preplanned IFR air traffic control departure procedure printed for pilot use in graphic and/or textual form. SID's provide transition from the terminal to the en route structure.

Standard Terminal Arrival Routes (STARS) -- A preplanned IFR air traffic control arrival procedure printed for pilot use in graphic and/or textual form. STAR's provide transition from the en route structure to an outer fix or an instrument approach fix in the terminal area.
Statute Mile -- A measure of distance equal to 5,280 feet.

S-Turns -- Turns issued to pilots by air traffic controllers such that aircraft follow the path of the letter "S". These turns are issued when necessary to delay aircraft, or to ensure proper aircraft separation and sequencing .

TACAN -- Tactical Air Navigation. A navigational system used by the military. TACAN provides both azimuth and distance information to a receiver on board an aircraft.

Taxiway -- A defined path established for the taxiing of aircraft from one part of an airport to another.

Terminal Radar Approach Control (TRACON) -- An FAA Air Traffic Control Facility which uses radar and two way communication to provide separation of air traffic within a specified geographic area in the vicinity of one or more airports.

Threshold -- The beginning of that portion of the runway usable for landing.

Through Passenger-- An airline passenger who arrives at an airport and departs without deplaning the aircraft.

Time Above (TA) -- Time above indicates the time in minutes that a given dB(A) level is exceeded during a 24-hour period. The time in minutes at a specific location that a preselected sound level is exceeded due to aircraft operations (e.g., time in minutes that the sound level is above 75 dB(A).

Tower En-Route Control -- The control of IFR traffic en route between two or more adjacent approach control facilities.

Transitional Airspace -- That portion of controlled airspace wherein aircraft change from one phase of flight or flight condition to another.

Vector -- Compass heading instructions issued by ATC to provide navigational guidance by radar.

Very High Frequency Omnidirectional Range Station (VOR) -- A ground-based radio navigation aid transmitting signals in all directions. A VOR provides azimuth guidance to pilots by reception of electronic signals. 

Visual Approach -- An approach conducted on an IFR flight plan which authorizes the pilot to proceed visually and clear of clouds to the airport. 

Visual Flight Rules (VFR) -- Rules and procedures specified in 14 CFR 91 for aircraft operations under visual conditions. Aircraft operation under VFR are not generally under positive control by ATC. 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.

Visual Meteorological Conditions (VMC) -- Weather conditions expressed in terms of visibility, distance from cloud, and cloud ceiling equal to or greater than those specified in 14 CFR 91.155 for aircraft operations under Visual Flight Rules (VFR).

VORTAC -- Very High Frequency Omnidirectional Range with Tactical Air Navigation. A navigational aid providing VOR azimuth and TACAN distance
measuring equipment (DME) at one site.