AC 36-4D

AdvisoryU.S. Department of Transportation Federal Aviation Administration Circular Subject: NOISE STANDARDS: AIRCRAFT Date: 10/12/2017 AC No: 36-4D

TYPE AND AIRWORTHINESS

CERTIFICATION Initiated by: AEE-100 Change:

1. PURPOSE This advisory circular (AC) is intended to promote uniformity of implementation of part 36 of the noise certification requirements of the United States Code of Federal Regulations (CFR) Title 14. It provides (1) guidance to certificating authorities and applicants regarding the intended meaning and stringency of the Standards in part 36 as amended through October 4, 2017 (Amendment 36-31), and (2) the specific pr ocedures that are deemed acceptable in demonstrating compliance with those Standards.

Note.— All references in this AC to parts such as part 36, part 21, etc. refer to parts of Title 14 of the United States Code of Federal Regulations.

a. Contents: This AC provides FAA policies for implementation of the regulatory language in the form of guidance material and technical procedures relating to the requirements of Appendices A, B, G, J, H and K of part 36 as appropriate. Those appendices describe the noise evaluation methods for compliance with the corresponding chapters of part 36 for jet airplanes, propeller-driven he avy and light airplanes, helicopters, and tiltrotor aircraft.

Guidelines are also provided on flight test windows and adjustment of land-use planning data for rotorcraft.

The document also includes general information (including administrative information), general and technical references, definitions and bibliographies related to part 36. Information is also provided on equivalent procedures and demonstrating “no acoustical change” for jet airplanes, propeller-driven heavy and light airplanes, and helicopters.

b. Scope: This AC provides information for interested parties (such as regulatory authorities, aircraft manufacturers, acoustic consultants, air line and airport operators and others) on compliance with the part 36 noise certification regulations, including FAA policies and interpretations of regulatory language and other technical guidance applicable to noise certification processes for airplane and rotorcraft classes of aircraft. It is applicable for noise certification of normal, utility, acrobatic, commuter and transport category airplanes and normal and transport category rotorcraft as defined in part 21. Information is also provided on flight test windows and adjustment of landuse planning for rotorcraft as developed by the International Civil Aviation Organization (ICAO).

This AC does not change regulatory requirements and does not authorize changes in, or deviations from, regulatory requirements. This document contains some mandatory language, where it is appropriate, as well as language that is permissive and advisory in nature. Mandatory language (e.g., the term “must”) is used in the AC for “Explanations of regulatory language”. Mandatory language is also used for certain “Guidance material” and “Technical procedures” to reflect FAA policies regarding compliance with the intent of regulatory language. Permissive language (e.g., the term “should”) is used in this AC, to identify conditions and/or methods and procedures that may be acceptable to the FAA for purposes of aircraft noise certification based upon experience to date, but where an applicant is not constrained from considering or validating and proposing alternatives for compliance with part 36.

c. Document structure: The basic framework of this AC is structured to provide various forms of noise certification guidance material for these aircraft. To a major extent this docum ent follows the structure of the ICAO Environmental Technical Manual (ETM) document 9501 (Reference G5). Chapter 1 provides information on noise certification requirements in part 21. Chapter 2 provides information on FAA regulatory policies. Chapter 3 provides guidance applicable for all aircraft types and subsequent chapters provide guidance unique to different aircraft types.

The general format of Chapters 1 through 8 includes one or more of three types of information: explanatory information, equivalent procedures and technical procedures which are described as follows:

Explanatory information Explanatory information has the following purpose:

a) explains the language of part 36 noise Standards;

b) states current policies of the FAA regarding compliance with part 36; and

c) provides information on critical issues concerning approval of applicants’ compliance methodology proposals.

Explanatory information may take the form of either:

a) guidance material (GM) which helps to illustrate the meaning of a specification or requirement; or

b) acceptable means of compliance (AMC) which illustra tes a means, but not the only means, by which a requirement specified in part 36, can be met. It may contain reference to an equivalent procedure described in this AC.

In the Explanatory sections of Chapters 4 through 8, a heading is provided for each AC section, including a subject or title, and the part 36 appendix and section number to which it refers. For example, GM §A36.2.1.1 is guidance material concerning §2.1.1 of Appendix A. Adjacent to this heading is a bracketed reference to the corresponding section of the ICAO ETM. For this example, “[ETM GM A2 2.1]” is guidance material concerning §2.1 of Appendix 2 of ICAO Annex 16, Volume I (Reference G7). Similarly, AMC §A36.2.1 .1 presents an acceptable means of compliance concerning §2.1.1 of Appendix A and “[ETM AMC A2 2.1]” is an acceptable means of compliance concerning §2.1.1 of Appendix 2 of Annex 16, Volume 1.

Equivalent procedures The FAA Office of Environment and Energy (AEE) (located in Washington DC) has approved a substantial number of equivalent procedures proposed by applicants. An equivalent procedure is by definition a test or analysis procedure which, while differing from one specified in part 36, in the technical judgment of the FAA, yields effectively the same noise levels as the specified procedure. Therefore, equivalent procedures provide some flexibility for the applicant in conducting noise certification in accordance with part 36, and may be ap proved for the convenience of an applicant in conducting measurements that are not strictly in accordance with the part 36 procedures, or when a departure from the specifics of part 36 is necessitated by field conditions.

Many of these equivalent procedures are referred to in this document because, through their extensive usage by applicants, they have become acceptable practices for aircraft noise certification applications. The acceptability of an equivalent procedure for an aircraft noise certification application is subject to FAA review and approval on a project by project basis. Equivalent procedures such as flight intercept need FAA approval as all other equivalencies, but may not require AEE involvement.

Equivalent procedures fall into two broad categories:

a) those which are generally applicable; and

b) those which are applicable to a particular aircra ft type; for example, some equivalencies dealing with measurement equipment may be used for all types of aircraft, but a given test procedure may be appropriate only for jet airplanes and not for turboprop airplanes.

Typical applications of equivalent procedures requested by applicants are to:

a) use previously acquired certification test data for the aircraft type;

b) permit and encourage more reliable demonstration of small level differences among derived versions of aircraft; and

c) minimize the cost of demonstrating compliance with the requirements of part 36 by keeping aircraft test time, airfield usage, and equipment and personnel costs to a minimum.

Technical procedures A technical procedure is a test or analysis procedure not defined in detail in part 36 but which the FAA have approved as being acceptable for compliance with the general provisions of part 36.

The procedures described in part 36 must be used unless an equivalent procedure or alternative technical procedure is approved by the FAA. Procedures should not be considered as limited only to those described herein, as this AC will be expanded as new procedures are developed. Also, their presentation does not infer limitation of their application or commitment by the FAA to their further use.

Chapter 9 provides guide lines for recertification of airplanes to Stage 4 or St age 5 based upon assessment of those with approved existing noise levels or those specially “modified” to achieve compliance with Stage 4 or Stage 5.

Conversion of units Conversions of some non-critical numerical values between U.S. Customary (English) and SI units are shown in the context of acceptable approximations.

2. CANCELLATION Advisory Circular 36-4C, Noise Standards: Aircraft Type and Airworthiness Certification, dated July 15, 2003, is cancelled.

3. REFERENCES General references References that identify FAA Orders, Advisory Circulars, and Reports that provide direction and/or guidance on acceptable processes and procedures for aircraft noise certification are listed in Appendix 1. GENERAL REFERENCES of this AC. These references also include current ICAO noise standards and are noted as Reference G1, Reference G2, etc. in the text. As these documents are referenced in this AC, information is also provided as to the extent that their provisions apply to a given type of aircraft noise certification process. Applicants are encouraged to contact FAA AEE to determine if mo re recent editions are available and app licable to noise certification actions that are being considered for FAA approval.

Technical references References that identify technical documents other than part 36 are listed in Appendix 2. TECHNICAL REFERENCES of this AC. These documents include inte rnational instrumentation standards incorporated by reference in part 36 and are noted as Reference T1, Reference T2, etc. in the text.

Bibliography A suggested bibliography relating to certain texts of this AC is provided in Appendix 3. BIBLIOGRAPHY.

FAA/DOT addresses Appendix 4. DOT/FAA ADDRESSES lists current addresses of U.S. Department of Transportation and FAA organizations (and noise certification specialists) that might provide assistance to an applicant.

4. DEFINITIONS This section contains two categories of definitions that are used within the regulatory language, policies or other guidance information of this Advisory Circular. The first category defines general terms or organizational designations that depict FAA and applicant roles in the part 36 nois e certification process. The second category defines specific aircraft types that must comply with part 36 regulations and other items related to those aircraft. References to part 1 are to the definitions of 14 CFR Part 1.

General definitions:

Administrator: The Federal Aviation Administrator or any person to whom he has delegated his authority in the matter concerned. (Ref: part 1) Aircraft certification office (ACO): The (FAA) office that administers the type certificate and production approval of products in the area where the manufacturer is located.

Applicant: Section 21.13 of part 21, states: “Any interested person may apply for a type certificate”. The person that applies to the FAA for an aircraft type certificate, whether a new (original), amended, supplemental or provisional type certificate, is identified as the “applicant” and is the entity that must comply with the procedural, certification and production requirements of part 21.

Noise certification specialist (NCS): The Directorate (Transport, Small Airplane, or Helicopter) Noise Certification Specialist.

Person: An individual, firm, partnership, corporation, company, association, joint-stock association, or government entity. It includes a trustee, receiver, assignee, or similar representative of any of them. (Ref: part 1).

Type inspection authorization (TIA): The TIA is the official FAA docu ment that provides direction and communication between the local ACO engineering group and the FAA flight test crew. The TIA usually mandates the provisions of an applicant’s approved demonstration compliance plan and any additional restrictions, inspections, tests, limitations, procedures, etc., that are required by the ACO to be observed during the conduct of the flight or ground testing.

Definitions related to aircraft:

Acrobatic category airplane: The acrobatic category is limited to airp lanes that have a seating configuration, excluding pilot seats, of nine or less, a maximum certificated takeoff weight of 12,500 pounds or less, and intended for use without restrictions, other than those shown to be necessary as a result of required flight tests. Acrobatic category airplanes are required to show compliance with part 23.

Aircraft: A device that is used or intended to be used for flight in the air. (Ref: part 1) Aircraft engine: An engine that is used, or intended to be used, for propelling aircraft. It includes turbosuperchargers, appurtenances, and accessories necessary for its functioning, but does not include propellers. (Ref: part 1) Airplane: An engine-driven fixed-wing aircraft heavier than air that is supported in flight by the dynamic reaction of the air against its wings. (Ref: part 1) Airport: An area of land or water that is used or intended to be used for the landing and takeoff of aircraft, and includes its buildings and facilities, if any. (Ref: part 1) Civil aircraft: Aircraft other than a public aircraft. (Ref: part 1) Commuter category airplanes: The commuter category is limited to propeller-driven, multiengine airplanes that have a seating configuration, excluding pilot seats, of 19 or less, and a maximum certificated takeoff weight of 19,000 pounds or less, intended for non-acrobatic operations. Commuter category airplanes are required to show compliance with part 23.

Helicopter: A rotorcraft that, for its horizontal motion, depends principally on its engine-driven rotors. (Ref: part 1) Large aircraft: Large aircraft means aircraft of more than 12,500 pounds, maximum certificated takeoff weight. (Ref:

part 1) Normal category airplane: The normal category is limited to airplanes that have a seating configuration, excluding pilot seats, of nine or less, a maximum certificated ta keoff weight of 12,500 pounds or less, and intended for nonacrobatic operation. Normal category airplanes are required to show compliance with part 23.

Normal category rotorcraft: A normal category rotorcraft is a rotorcraft with a maximum weight of 7000 pounds or less and that meets the regulatory requirements of part 27.

Primary category aircraft: A primary category aircraft is an aircraft that (1) is unpowered, or is an airplane powered by a single, naturally aspirated engine with a 61-knot or less stall speed, or is a rotorcraft with a 6-pound per square foot main rotor disc loading limitations, and (2) weighs not more than 2700 pounds, and (3) has a maximum seating capacity of not more than four persons, including the pilot, and (4) has an unpressurized cabin. A primary category aircraft is required to meet the regulatory requirements of §21.24 of part 21.

Propeller: A device for propelling an aircraft that has blades on an engine-driven shaft and that, when rotated, produces by its action on the air, a thrust approximately perpendicular to its plane of rotation. It includes control components normally supplied by its manufacturer, but does not include main and auxiliary rotors or rotating airfoils of engines. (Ref: part 1).

Public aircraft: Aircraft used only in the service of a government, or a political subdivision. It does not include any government-owned aircraft engaged in carrying persons or property for commercial purposes. (Ref: part 1) Restricted aircraft: Used for special purpose operations as defined in §21.25 of part 21.

Rotorcraft: A heavier-than-air aircraft that depends principally for its support in flight on the lift generated by one or more rotors (Ref: part 1).

Small aircraft: Small aircraft means aircraft of 12,500 pounds or less, maximum certificated takeoff weight. (Ref:

part 1) Page 5

Subsonic airplane: A subsonic airplane means an airplane for which the maximum operating limit speed, MMO, does not exceed a Mach number of 1.

Supersonic airplane: A supersonic airplane means an airplane for which the maximum operating limit speed, M MO, exceeds a Mach number of 1.

Transport category aircraft: Those aircraft (large or small) that are demonstrated to meet and are certificated to the regulatory requirements of part 25 for transport category airplanes or part 29 for transport category rotorcraft.

Jet Airplane: Any fixed wing airplane that is powered by a turbojet or turbofan engine regardless of whether it is an airplane certificated to part 23 (small or commuter aircraft) or part 25 (transport category aircraft).

Turbo-propeller airplane: An airplane whose primary source of thrust is from a propeller driven by a gas turbine engine.

Utility category airplane: The utility category is limited to airplanes that have a seating configuration, excluding pilot seats, of nine or less, a maximum certificated takeoff weight of 12,500 pounds or less, and intended for limited acrobatic operation. Utility category airplanes are required to show compliance with part 23.

5. ABBREVIATIONS AND ACRONYMS The following abbreviations and acronyms are used within the text of this AC AC = FAA advisory circular ACO = Local FAA aircra ft certification office AEE = FAA Office of En vironment and Energy AFM = Airplane Flight Manual AMC = Acceptable means of compliance APU = Auxiliary power unit C/A = Coarse/Acquisition CAEP = Committee on Aviation Environmental Protection CDI = Course deviation indicator CG = Center of gravity CFR = Code of Federal Regulations CPA = Closest point of approach DAT = Digital audiotape dB = Decibel DER = Designated Engineering Representative DGPS = Differential global positioning system DMU = Distance measuring unit EPNL = Effective perceived noise level ETM = Environmental Technical Manual (“ICAO Environmental Technical Manual. Volume 1, Procedures for the Noise Certification of Aircraft”) FAA = Federal Aviation Administration GDI = Glide-slope deviation indicator GM = Guidance material GPS = Global positioning system Hz = Oscillatory frequency, cycles per second IAS = Indicated airspeed ICD = Inflow control device INS = Inertial navigation system ICAO = International Civil Aviation Organization ILS = Instrument landing system IOD = Issue of data Page 6

IRIG = Inter-Range Instrumentation Group IRIG B = Inter-Range Instrumentation Group Standard Serial Time Code Format B ISA = International standard atmosphere LAAS = Local area augmentation system LAmax = Maximum A-weighted sound level log = Logarithmic value to base 10 LUP = Land-use planning MAP = Manifold air pressure MLW = Maximum landing weight MSL = Mean sea level altitude, feet MTOW = Maximum takeoff weight NAC = No acoustical change NCS = Noise Certification Specialist NIST = National Institute of Standards and Technology NMEA = National Marine Electronics Association NPD = Noise-power-distance OAT = Outside air temperature RFM = Rotorcraft Flight Manual RH = Relative humidity, % rms = Root-mean-square грт = revolutions per minute SAE = Society of Automotive Engineers SB V = Surge bleed valve (engine internal operating valves) SR = Slant range STC = Supplemental Type Certificate TIA = Type Inspection Authorization TC = Type Certificate TSPI = Time-space-position information UTC = Coordinated universal time VLA = Very Light Aircraft VNTSC = Volpe National Transportation System Center (DOT) VREF = Reference Landing Speed Curtis Ho sclax Acting Director of Environment and Energy Page 7

Table of Contents Page

1. PURPOSE1

2. CANCELLATION3

3. REFERENCES3

4. DEFINITIONS4

5. ABBREVIATIONS AND ACRONYMS6

6-7. RESERVED7

CHAPTER 1. CERTIFICATION PROCEDURES FOR PRODUCT AND PARTS 14 CFR PART 21

101 EXPLANATORY INFORMATION10

CHAPTER 2. NOISE STANDARDS: AIRCRAFT TYPE AND AIRWORTHINESS

201 EXPLANATORY INFORMATION32

CHAPTER 3. TECHNICAL PROCEDURES AP PLICABLE FOR NOISE CERTIFICATION

O F A L L A I R C R A F T T Y P E S

301 NOISE MEASUREMENT PROCEDURES58

302 FLIGHT PATH MEASUREMENT65

303 ON-BOARD FLIGHT DATA ACQUISITION80

304 TIME SYNCHRONIZATION OF MEASURED DATA82

305 CALCULATION OF CONFIDENCE INTERVALS84

306 ADJUSTMENT OF AIRCRAFT NOISE LEVELS FOR THE EFFECTS97

307 NOISE REDUCTION SYSTEMS105

308 CALCULATION OF THE SPEED OF SOUND107

309 REFERENCE TABLES USED IN THE MANUAL CALCULATION OF107

CHAPTER 4. GUIDELINES FOR TRANSPORT CATEGORY LARGE AIRPLANES,

SUBSONIC JET AIRPLANES, AND HELICOPTERS EVALUATED UNDER APPENDIX A OR H OF 14 CFR PART 36

401 EXPLANATORY INFORMATION116

402 EQUIVALENT PROCEDURES INFORMATION160

403 TECHNICAL PROCEDURES INFORMATION192

CHAPTER 5. GUIDELINES FOR PROP ELLER-DRIVEN SMALL AIRPLANES AND

PROPELLER-DRIVE COMMUTER CATEGORY AIRPLANES EVALUATED UNDER APPENDIX G OF 14 CFR PART 36

501 EXPLANATORY INFORMATION218

502 EQUIVALENT PROCEDURES INFORMATION226

503 TECHNICAL PROCEDURES INFORMATION232

CHAPTER 6. GUIDELINES FOR HELI COPTERS NOT EXCEEDING 7000 LB EVALUATED UNDER APPENDIX J OF 14 CFR PART 36

601 EXPLANATORY INFORMATION237

602 EQUIVALENT PROCEDURES INFORMATION245

CHAPTER 7. GUIDELINES FOR TILTRO TOR AIRCRAFT EVALUATED UNDER APPENDIX K OF 14 CFR PART 3 Page 8

701 EXPLANATORY INFORMATION247

CHAPTER 8. GUIDELINES ON FLIGHT TEST WINDOWS AND ADJUSTMENT OF

LAND-USE PLANNING NOISE DATA MEASURED IN ACCORDANCE WITH ATTACHMENT H TO ANNEX 16, VOLUME I

801 EXPLANATORY INFORMATION253

802 EQUIVALENT PROCEDURES INFORMATION260

803 TECHNICAL PROCEDURES INFORMATION260

CHAPTER 9. GUIDELINES FOR AIRCRAFT RECERTIFICATION

901 INTRODUCTION266

902 ASSESSMENT CRITERIA267

903 RECERTIFICATION GUIDELINES FOR “MODIFIED”AIRPLANES271

Appendix 1. GENERAL REFERENCES273

Appendix 2. TECHNICAL REFERENCES274

Appendix 3. BIBLIOGRAPHY276

Appendix 4. DOT/FAA ADDRESSES277

Chapter 1 CERTIFICATION PROCEDURES FOR PRODUCT AND PARTS 14 CFR PART 21

CHAPTER TABLE OF CONTENTS

101 EXPLANATORY INFORMATION10

101(a) Part 21 General10

101(b) Airworthiness Certification10

101(c) Changes in Type Design10

101(d) No acoustical changes (NAC)11

101(e) Methods for Demonstrating No Acoustical Change (NAC)14

101(f) Acquisition of In-duct and/or Near-field Data for Demonstration of NAC29

101 EXPLANATORY INFORMATION 101(a) Part 21 General As part of the overall aircraft certification requirements iden tified in part 21, applicants must comply with the noise certification requirements of part 36. Section 21.17 of part 21 specifies that an applicant for a type certificate must show that the aircraft meets the applicable requirements of part 36. In addition, §§21.21 and 21.25 of part 21 specify that an applicant is entitled to a type certificate for an aircraft if the applicant submits the type design, test reports, and computations necessary to show that th e product to be certificated meets the applicable airworthiness and aircraft noise requirements. Similar requirements are imposed for aircraft manufactured in a foreign country for import into the United States (§21.29 of part 21). Refer to the part 21 regulations for specific regulatory language.

101(b) Airworthin ess Certification All aircraft requiring noise certification by the FAA are required to meet the applicable airworthiness requirements of part 21, which identifies the specific airworthiness standards for each aircraft type and product, such as:

a) part 23 for normal, utility, acroba tic, and commuter category airplanes;

b) part 25 for transport category airplanes;

c) part 27 for normal category rotorcraft;

d) part 29 for transport category rotorcraft;

e) part 33 for aircraft engines;

f) part 35 for propellers;

g) primary category aircraft; and

h) JAR-VLA aircraft.

101(c) Changes in Type Design

(1) General Section 21.93(b) of part 21 requires compliance with th e appropriate part 36 “acoustic al change” requirements for a voluntary type design change of an aircraft that ma y increase the noise levels of that aircraft, regardless of whether the change in type design is classified as a minor or major change in §21.93(a) of part 21.

(2) Voluntary changes A general description of FAA handling of various forms of voluntary changes in the type design of an aircraft is addressed in 201(a)(3) of this AC. Procedures for demonstration of compliance with part 21 for specific types of “no acoustical change” (NAC) are presented in 101(d) & (e).

101(d) No acoustical changes (NAC)

(1) NAC definition Aircraft/engine model design changes and airframe/engine performance changes may result in very small changes in aircraft noise certification levels that are not acous tically significant. These cha nges are referred to as no acoustical changes (NACs). For this AC NACs, which do not result in modification of an aircraft’s noise certification levels, are defined as:

a) changes in airplane noise certification levels approved by the FAA which do not exceed 0.10 dB at any noise measurement point and which an applicant does not track;

b) cumulative changes in airplane noise certification le vels approved by the FAA whose sum is greater than

0.10 dB but not more than 0.30 dB at any noise measurement point and for which an applicant has an approved tracking procedure;

c) for helicopters certificated according to the standa rds of Appendix H, changes in any one of the noise certification levels approved by the FAA which do not exceed 0.30 EPNdB; and

d) for helicopters certificated according to the standa rds of Appendix J, changes in the noise certification level approved by the FAA which does not exceed 0.30 dB(A).

(2) Tracking procedures With respect to the tracking procedure referred to in 101(d)(1)(b), noise certification approval has been given based upon the following criteria:

a) ownership by the certification applicant of the noise certification database and tracking process on an aircraft/engine model basis;

b) when the 0.30 dB cumulative change in the airplane noise certif ication level is exceeded, compliance with the part 36 requirements is required; the airc raft noise certification levels may not be based upon summation of NAC noise increments;

c) decreases in noise levels should not be included in the tracking process unless the type design change will be retrofitted to all aircraft in service and included on newly produced aircraft;

d) aircraft/engine design changes resulting in nois e level increases should be included in the tracking process regardless of the extent of retrofit to aircraft in service;

e) tracking of an aircraft/engine model should, in ad dition to engine design changes, include airframe and performance changes;

f) tracked noise increments should be determined on the basis of the most noise-sensitive condition and be applied to all configurations of the aircraft/engine model;

g) the tracking should be revised to account for a tracked design change increment that is no longer Page 11

applicable;

h) changes should be tracked to two decimal places (i.e., 0.01 dB). Round-off should not be considered when judging an NAC (e.g., 0.29 dB = NAC; 0.30 dB = NAC; 0.31 dB = acoustical change); and

i) an applicant should maintain formal documentation of all NACs approved under a tracking process for an airframe/engine model; the tracking list will be reproduced in each noise certification demonstration document.

Due to the applicability dates, some light propeller-driven airplanes are not required to have noise certification levels. However some modifications to these aircraft can be applied which may impact the noise characteristics.

In this case, the NAC criteria application should be treated with a procedure approved by the FAA.

(3) Criteria for helicopter certification Noise certification approval of modified helicopters should be granted according to the following criteria:

a) an NAC approval for a derived version can be made only if the parent “flight datum” helicopter was flight tested to obtain the noise certification levels;

b) noise levels for a helicopter designated as an NAC design cannot be used as the “flight datum” for any subsequent design changes; and

c) for changes exceeding 0.30 dB, compliance with part 36 requirements may be achieved either by testing or, subject to approval of the FAA, by analytical means; if analy tical means are employed, the noise certification levels cannot be used as the “flight datum” for any subsequent design changes.

Note.— Definition for parent “flight datum” aircraft is shown in Table 1.

A flow chart illustrating the criteria for dealing with modified helicopters is presented in Figure 1.

Due to the applicability dates, some helicopters are not required to have noise certification levels. However some modifications to these helicopters can be applied which may impact the noise character istics. In this case, the NAC criteria application should be treated with a procedure approved by the FAA.

(4) Helicopters modifications defined as NACs Changes in helicopter noise level(s) arising from modifi cations associated with the installation or removal of external equipment are defined as NACs and thus need not be determined. For the purposes of this section “external equipment” means any inst rument, mechanism, part, appurtenan ce, or necessary accessory that is attached to, or extends from, the helicopter exterior but is not used, nor is intended to be used, in operating or controlling the helicopter in flight and is not part of an airframe or engine.

The following examples are considered to be no acoustical changes:

a) the addition or removal of external equipment;

b) changes to the airframe made to accommodate the ad dition or removal of external equipment, to provide for an external load attaching means, to facilitate the use of external equipment or external loads, or to facilitate the safe operation of the helicopter with external equipment mounted to, or external loads carried by, the helicopter;

c) reconfiguration of the helicopter by the addition or removal of floats and skis;