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FAQs - Frequently Asked Questions

What is a Part 161 Study? 

´Part 161´ refers to Federal Aviation Regulation ("FAR") Part 161, Notice and Approval of Airport Noise and Access Restrictions, which is codified under 14 C.F.R. (Code of Federal Regulations) Part 161. This regulation sets forth a process for airport sponsors to follow if they wish to impose certain restrictions on the use of their airport. A Part 161 Study is a technical and legal analysis of a proposed noise and access restriction at an airport. The analyses will result in an application to the Federal Aviation Administration requesting authoization to establish the restriction. It is described in more detail on the FAA's Airport Noise website by clicking here.

What is Part 150 and How Does It Relate to Part 161? 

Part 150 (14 C.F.R Part 150), "Airport Noise Compatibility Planning", sets forth standards for airport operators to use in documenting noise exposure in the airport environs and obtaining FAA approval of programs to reduce or eliminate incompatibilities between aircraft noise and surrounding land uses. A formal submission to the FAA under Part 150 includes two principal elements: (1) Noise Exposure Maps (NEMs) and (2) a Noise Compatibility Program (NCP).

Part 150 prescribes specific standards and systems for:

  • measuring noise
  • estimating cumulative noise exposure
  • describing noise exposure (including instantaneous, single event and cumulative levels)
  • coordinating NCP development with local land use officials and other interested parties
  • documenting the analytical process and development of the compatibility program
  • submitting documentation to the FAA
  • FAA and public review processes

FAA reviews NEM submissions to determine whether the airport has prepared them in compliance with the applicable requirements of Part 150. FAA then reviews the NCP submission and prepares a "Record of Approval" (ROA) that addresses each proposed measure. The FAA reviews the measures on the basis of their effectiveness and potential conflict with federal policy and prerogatives, including safe and efficient use of the nation´s airspace, undue burden on interstate commerce, unjust discrimination, and interference with federal regulations. FAA approval of actions that the airport proprietor recommends be taken by the FAA does not constitute a decision to implement the actions; those decisions are subject to subsequent FAA consideration under applicable environmental or other review requirements. The full Part 150 regulation may be reviewed here.

The FAA found the Noise Exposure Maps (NEMs) in compliance on April 20, 2009 and issued a Record of Approval (ROA) for the NCP on October 16, 2009.

What is Part 36? 

The FAA has established limits on allowable levels of aircraft noise emissions, under 14 C.F.R. Part 36, "Noise Standards: Aircraft Type and Airworthiness Certification," that sets noise limits airplanes must meet to receive new or revised "type" or "airworthiness" certificates, to operate in the U.S. In general, permissible noise levels increase with maximum gross takeoff weight. The limits are in terms of a noise metric called Effective Perceived Noise decibels (EPNdB), which takes into account the noise level, duration, and presence of any potentially annoying "pure tones" during the aircraft fly by.

The measurement locations and procedures, and noise limits vary with aircraft "design" criteria, including factors such as subsonic versus supersonic speed capabilities, type of propulsion (e.g. jet- or propeller-driven), weight categories, helicopter versus fixed-wing aircraft, operating category (e.g., agricultural, transport, and commuter), date of initial flight, and, in limited cases, specific engine manufacturer, model, or characteristics.

The full Part 36 regulation may be reviewed here. 

What are Part 36 "Stages?" 

Part 36 groups aircraft into different categories, usually called "stages," as follow:

  •  Turbojet aircraft and transport category "large" aircraft (with maximum takeoff weights of 12,500 pounds or more) are categorized in four stages: "Stage 4" aircraft are the quietest; they meet the most stringent standards, adopted in 2005. Stage 3 aircraft meet intermediate standards set in 1977; Stage 2 aircraft meet initial 1977 standards. Stage 1 aircraft are "uncertified"; they have not been shown to comply with Stage 2, 3, or 4 limits. All jet and transport-category airplanes with maximum gross takeoff weights of 12,500 pounds or more for which application of a new type design is submitted on or after January 1, 2006, will have to meet new noise certification levels.
  •  Helicopters are categorized into two stages: Stage 2 helicopters meet standards adopted in 1988. Stage 1 helicopters have not been shown to meet those standards. Stage 2 has a different meaning for helicopters than it does for jets; a Stage 2 helicopter meets the highest standard, whereas a Stage 2 jet meets outdated standards and is considered relatively noisy.
  • "Propeller-driven small aircraft" (with maximum takeoff weights less than 12,500 pounds) are termed either "certificated" or "uncertificated," with no stage references.

The following figure compares the "noise dose" associated with combined arrival and departure operations of a range of corporate jets from a single manufacturer — Gulfstream Aviation. This comparison shows the significant differences between Stage 2 and 3 models. The comparison is in terms of a noise measure called Sound Exposure Level (SEL), which takes into account the total noise energy associated with an aircraft flyby.

SEL - Sound Exposure Level 

Why is Part 36 Relevant to Part 161? 

Part 36 is highly relevant to Part 161, because it is the potential effect on fixed-wing or helicopter aircraft classified under Part 36 as Stage 2 or 3 that determines whether a Part 161 study is required.

What is FAA Advisory Circular (AC) 36-3? 

This FAA publication, titled "Estimated Airplane Noise Levels in A-Weighted Decibels," provides listings of estimated maximum airplane noise levels in dBA, ranked in descending order and also by manufacturer, at maximum takeoff and landing gross weights at the Part 36, certification measurement locations (6,500 meters from start of takeoff roll, and 2,000 meters from the runway threshold for approach). Many airport operators, including LAWA, use this advisory circular as a basis for formal or informal (voluntary) noise control measures. Click to view the most current version of AC 36-3.

What is CNEL? 

Community Noise Equivalent Level (CNEL) is a measure of cumulative noise exposure over a 24-hour period, with adjustments to the noise during the evening and nighttime to reflect the added intrusiveness of noise during certain times of the day. CNEL is described in greater detail here. 

What is SEL? 

Sound Exposure Level (SEL) is the most commonly used measure of cumulative noise exposure associated with an individual aircraft operation (or other noise source). The Integrated Noise Model (INM) permits the calculation of SEL contours for individual flights. SEL is essentially identical to the Single Event Noise Exposure Level (SENEL). It is discussed in greater detail here.