Acoustic Terms

Decibel (dB)

The unit of sound pressure used to measure noise. Decibel means 1/10 of a Bell, after Alexander Graham Bell (thus the capital B). Because decibels are such a small measure, they are computed logarithmically and cannot be added arithmetically. An increase of ten dB is perceived by human ears as a doubling of noise.

dBA

A-weighted decibels adjust the sound pressure to conform with the frequency responses of the human ear. Airport noise is almost always measured in dBA.

DNL

Day/night noise level, previously known as Ldn. This is a 24-hour average hourly noise level with a 10 decibel (dB) penalty for nighttime noise events between 10 PM and 7 AM.

DNL Contour

The "map" of noise exposure around an airport. It is computed through an FAA model called the Integrated Noise Model (INM) which calculates the annual noise exposure from an input consisting of the actual fleet operated at the airport, the runway use, number of operations, and time of day. FAA defines significant noise exposure as any area within the 65dB DNL contour; that is the area within an annual average noise exposure of 65 decibels or higher.

Single Event Noise

The total noise emitted by one overflight or aircraft. Each single event will have a total noise level over the duration of the event called SEL (sound exposure level) and a peak, which is the highest noise level reached by that event. It is important to distinguish single event noise levels from cumulative noise levels such as DNL. Single event noise level numbers are often higher than DNL numbers, because DNL represents an average noise level over a period of time, generally a whole year.

Time Above

An expression of the amount of time noise exceeds a threshold level. The threshold can be set at any point, for instance, 65 or 75 dBA. Generally time above is expressed in minutes per day that the threshold is exceeded.

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Air Traffic Control and Airport Planning Terms

Air Taxi

Non-scheduled passenger aircraft with 50 or fewer seats

ALP

Airport Layout Plan

ATA

Airport Traffic Area

ATC

Air Traffic Control

Center

Air-traffic controller who guides the flight from originating TRACON to destination TRACON. Responsible for "in route" portion of flight.

Commuter Aircraft

Scheduled passenger aircraft with fewer than 50 seats.

FAA

The Federal Aviation Administration

FAR

Federal Aviation Regulations are the rules and regulations, which govern the operation of aircraft, airways, and airmen.

FAR Part 36

A Federal Aviation Regulation defining maximum noise emissions for the manufacture of aircraft. Under this regulation aircraft fall into three categories: Stages 1, 2 and 3 where Stage 1 is the noisiest and Stage 3 the quietest. In the U.S. all large commercial aircraft are Stage 3, but aircraft weighing less than 75,000 pounds can still be Stage 1, 2 or 3. Hushkitted Stage 3 aircraft are previously Stage 2 aircraft that have been adapted to meet Stage 3 requirements.

FAR Part 91

A Federal Aviation Regulation that prescribes rules governing the operation of aircraft within the United States, including the waters within 3 nautical miles of the U.S. coast.

FAR Part 150

A Federal Aviation Regulation governing noise and land use compatibility studies and programs.

FAR Part 161

A Federal Aviation Regulation regarding the imposition of any airport noise regulations that are considered to be an access restriction. Under Part 161, an airport is required to conduct an extensive cost/benefit analysis, and depending on the type of restriction contemplated the regulation may be subject to FAA approval.

FMS

Flight Management System refers to a computer installed aboard an aircraft to aid in navigation. Installation is not mandatory but at the airlines' discretion.

General Aviation

Non-commercial airline aviation—primarily business aircraft and individuals traveling in private aircraft, including those making connections to commercial flights.

GIS

Geographic Information Systems

GPS

Global Positioning System

Ground Run-up Enclosure

A walled structure, usually without a roof, in which aircraft perform run-ups.

Instrument Flight Rules (IFR)

Rules governing flight procedures during limited visibility or other operational constraints. Under IFR, pilots must file a flight plan and fly under the guidance of radar.

Instrument Landing System (ILS)

A precise landing aid consisting of several components giving the pilot vertical and horizontal electronic guidance. Elements usually include: 1. an outer marker, a radio beam 4 to 6 miles from the touchdown point where the electronic signal begins; 2. an approach lighting system at the runway end; 3. a localizer radio beam which provides the horizontal guide; and 4. a glide slope which provides vertical guidance on the angle of descent for landing.

Run-ups

After routine maintenance, aircraft engines must be tested or run-up at partial or full power to ensure that they operate properly during flight.

TRACON

The radar air traffic control facility where controllers give directions to departing and arriving aircraft at an airport. After leaving the jurisdiction of a TRACON, aircraft are handled by a Center.

Visual Flight Rules (VFR)

Air traffic rules allowing pilots to land by sight without relying solely on instruments. VFR conditions require good weather and visibility.