AC 36-4D

(10) No acoustical change (NAC) documentation Aircraft type design changes that are categorized as NAC require FAA approval under the provisions of part 21 rather than part 36. Applicants’ proposals for approval of NAC must be submitted to the FAA in writing. The written submittal must define the planned changes in aircraft type design, and must include information, data and analyses that will substantiate that the specified type design change will not result in an “acoustical change” (see Subpart A, §36.1(c)(2) of part 36).

NAC substantiation documentation must include:

a) introduction (concepts and requirements);

b) description of aircraft baseline and proposed type design changes;

c) methodology for substantiating an NAC;

d) description of tests and/or analyses performed; and

e) test and/or analyses results relating to complia nce with FAA NAC criteria (s ee Subpart A, §36.1(c)(3) of part 36).

Note 1.— Section 21.93(b) of part 21 states that, “…any voluntary change in the type design of an aircraft that may increase the noise levels of that aircraft is an ‘acoustical change’.” Therefore NAC is determined on an aircraft by aircraft basis rather than just on the basis of the type certificate data sheet (TCDS) information for the aircraft model.

Note 2.— Section 183.29(i) of part 183 specifies that Acoustical DERs may not determine that a type design change is an NAC. As such, Acoustical DERs may not make recommendations for approval of NACs under Form 8110-3.

(11) Other noise-related documentation FAA develops noise-related documentation other than that required of applicants under Subpart O, §1501 of part 36 as follows:

a) Findings required by public law 103-272 (Reference G2):

Compliance with Public Law 103-272 requires that FAA, before issuing an original TC for any aircraft of any category, regardless of whether part 36 applies to the aircraft, must make a finding to determine whether:

Substantial noise abatement cannot be achieved for that aircraft by prescr ibing standards and regulations consistent with the limitations of section 44715(b), or Substantial noise abatement may be so achieved in which case the regulatory process must be used to determine the extent of noise reduction to be required before an original TC may be issued.

Note.— The specific process required for FAA to complete Noise Finding documentation is presented in paragraph 7-3 of FAA Order 8110.4C (Reference G4).

b) Environmental Assessments:

Compliance with the National Environmental Policy Act of 1969 may require that the FAA conduct an Page 49

Environmental Assessment (EA) as specified in Ch apter 4 of FAA Order 1050.1F (Reference G1 xx).

An EA is conducted when type certificate actions (New, Amended, or Supplemental) are initiated for aircraft types for which part 36 requirements do not exist (see Note 1). An Environmental Impact Statement (EIS) or Finding of No Significant Impact (FONSI) must be prepared by the FAA (see Note

2). The FAA may grant an interim aircraft noise certification upon completion of an EA while appropriate rulemaking is being completed.

Note 1.— Applicants may be requested to provide appropriate noise data to FAA to support the EA and/or rulemaking processes Note 2.— Procedures for generation of an EA, EIS or FONSI are also found in FAA Order 1050.1E (Reference G1).

(12) Procedures

a) Applicant’s responsibility : Applicants must generate and submit to FAA the following noise certification documentation:

 Noise certification plans;

 Noise compliance demonstration plans (including proposed equivalent procedures);

 Noise certification reports;

 NAC substantiation.

b) DER’s responsibility : Acoustical DERs responsibilities are as defined in FAA Order 8110.37E (Reference G6).

201(d)(2) Non-flight test data §36.1501(b)

(1) Airplane families Chapter 4 of this AC, 402(a)(3) for jets, and 402(b)(3) for heavy props, present guidance on static engine noise tests and projection of static engine noise data to flight conditions. Comparisons of projections of static engine noise data from the engines of an originally certified airplane with those of modified configuration engines could enable certification of modified configurations without further expensive and time consuming flight tests (see §36.1501(b) of part 36). An originally certified airplane and its derived versions are known as an “Airplane Family”.

(2) Tests for supplemental data:

An applicant should attempt to obtain static engine or flig ht test noise data at engine thrust (power) settings and ranges, and airplane flight configurations and conditions that may be needed in the foreseeable future. This may necessitate over-boosting of the engine thrust (power) (with the permission of the engine manufacturer), resetting of the engine idle thrust (power) to lower limits, testing to higher or lower range of airspeed and angles of attack, testing at higher or lower altitudes, etc.

201(d)(3) Manuals, markings and placards §36.1581(a)

(1) Aircraft flight manual Page 50

An approved Airplane Flight Manual (AFM) or Rotorcra ft Flight Manual (RFM) is required for certification of each aircraft type in compliance with §36.1581 of the Airworthiness Standards for parts 21.24 (Primary Category), 23, 25, 27, or 29. Section 36.1581 of part 36 requires that certificated noise level compliance information be included in an AFM or RFM. These levels must be cont ained in an FAA approved section of an AFM or RFM other than the limitations section. Fo r example, the performance section is an appropriate place to include the certificated noise level compliance inform ation. The certificated noise levels ar e to be reported to 0.1 dB in the AFM or RFM. Further, an AFM or RFM must specify the ty pe certificate limitations if any, that are established as a result of part 36 compliance, combined with the airworthiness limitations.

(2) Rotorcraft flight manual In addition to the requirements of §36.1581, §H36.3(e) and §J36.3(c)(1) of part 36 require that the values of V H and VNE defined for noise certification must be listed in the RFM. The value of V NE is located in the limitations section of the RFM. The value of VH should be listed in the section of the RFM containing the certificated noise level compliance information.

(3) Airplane flight manual limitation section

a) An AFM may be issued for a single airplane or a group of similar airplanes (including more than one series of a specific model). The AFM must clearly identify (by airplane serial number) the operating limitations, including the maximum weight limits that apply to an airplane.

b) The AFM may address a single configuration (har dware build) or multiple configurations. If an AFM includes information for more than one configuration (hardware build), the appropriate airplane operating limitations must be clearly identified for each config uration. Furthermore, if not all configurations are approved for the airplanes listed in the AFM, the AFM must clearly identify by se rial number, the proper operating limitations for each airplane.

c) The operating limitations contained in the Limitations Section (including any noise-limited weights) must be expressed in mandatory language, not permissive language. The terminology used in the AFM must be consistent with the relevant regulatory language.

(4) Multiple airplane noise certifications

a) Multiple noise-limited gross weight pairs (takeoff and landing) for one airplane configuration are not permitted by Subpart A, §36.1(g) for compliance with part 36. Only one set of gross weight limits that pertain to a particular configuration (hardware build) may be established under part 36 for a particular airplane.

b) An airplane configuration (hardware build) may be certificated only to a single Stage as it qualifies under part 36 (Stage 1, Stage 2, Stage 3, Stage 4, or Stage 5). The limitations section of the AFM for an airplane configuration may not contain operating limitations that would result in noise levels exceeding part 36 noise limits. However, a Stage, 2 airplane may be recertif icated as Stage 3, provid ed that it meets the noise limits for Stage 3 and the AFMs are revised and approved for configurations appropriate for compliance with Stage 3, and the Stage 2 approval is deleted.

(5) Landing flap restriction

a) An operating limitation preventing the use of an approved landing flap setting cannot be imposed under part 36 and must be established under the airworthiness requirements or through a voluntary type design change.

If such a restriction is requested by an applicant in order to comply with part 36 certification requirements that flap setting limit must be incorporated into the design and operation of the airplane.

b) For some Stage 3 airplanes, a “softguard” (such as a crushable cover plate) or frangible device must be Page 51

installed on the flap selection control to safeguard its use in instances other than emergency. This device makes obvious the flap settings that are not to be used for normal operation. Fracture or deformation of the softguard would indicate use of landing flap settings that have not been approved in demonstrating compliance with part 36. For airworthiness purposes, the design of these devices allows the flight crew use of airworthiness approved restricted flaps in emergencies (declared and undeclared). The device may not be resettable by the flight crew; a damaged softguard or frangible device must be repaired as a maintenance action accomplished in accordance with part 43 prior to the next flight.

(6) Procedures

a) Applicant responsibility:

An applicant must identify limiting configurations that may be required to satisfy airworthiness and/or noise regulatory requirements. The limiting configurations, including approved aircraft gross weights, are identified in the “Limitation Sections” of the approved aircraft AFM or RFM.

(7) Implementation of landing flap restrictions FAA policy allows a choice of two options for an applicant to implement landing flap restrictions, where their approval is consistent with applicable airworthiness requirements:

a) where permitted and possible, remove the cockpit flap selection control and performance information from the Performance Section of the original AFM that is relevant to flap settings not considered in demonstrating compliance with part 36; and

b) if flap settings not considered in demonstrating comp liance with part 36 are to remain selectable then, to prevent their use in normal operations:

— Restrictions against their use, except under emer gency (declared and undeclared) operations, must be incorporated into the AFM Limitations Section.

— Operational information must be relocated to the Emergency Procedures Section.

— For Stage 3 airplanes, must install a “softguard” or frangible device over the flap selection control.

— Placards may be installed to provide appropriate information to the flight crew.

(8) ICAO certifications The FAA is not authorized to approve aircraft compliance with Annex 16, Volume 1 Noise Standards. (Reference G7). This is the responsibility of the foreign airworthin ess authority involved. FAA participation is limited to witnessing tests and reviewing data. If the Stage 4 or Stage 5 compliance has been demonstrated refer to §36.105 or §36.106 of part 36. In those cases where Stage 3 compliance has been demonstrated, it is permissible to insert the following statement in the AFM:

“Certification Noise Levels The following noise levels comply with part 36, Appendix B, Stage 3 noise level requirements and were obtained by analysis of approved data from noise tests conducted under the provisions of part 36, Amendment 36-XX (Insert part 36 amendment to which airplane was certificated). The noise measurement and evaluation procedures used to obtain these noise levels are essentially equivalent to those required by the International Civil Aviation Organization (ICAO) in Annex 16, Volume I, Amendment 7, Chapter 3. ICAO Annex 16, Volume I, Chapter 3 approval is applicable only after endorsement by the Civil Aviation Authority of the country of airplane registration.”

201(d)(4) Supplementa l information §36.1581(b)

(1) Publication The FAA allows publication of supplemental noise information in an approved AFM or RFM for configurations other than the maximum takeoff and landing weight configuration or noisiest approach configuration. Such information may be useful to airlines in conjunction with their selection of operational route structures and airport operational requirements.

(2) Applicant’s responsibility Applicants must identify supplemental noise levels as “Supplemental Information” when they are included in an AFM or RFM. Even though noise data used to provide “Supplemental Information” may have been obtained during certification noise testing and witnessed by the FAA, it must still be clearly marked to show that it is not intended to demonstrate compliance with part 36, and may be presented in a non-FAA approved section or page of the AFM or RFM.

201(d)(5) Index of acceptability §36.1581(c)

(1) Certification vs. operational rule Part 36 is a certification rule and not an operational ru le. The certificated noise levels may or may not meet the operational noise requirements for any particular airport or operating rule. The airc raft noise certification provisions of part 36 do not place any operational permissi bility or limitations on th e operation of certificated aircraft at any airport.

(2) Applicant’s responsibility Applicants are responsible for developing an AFM or RFM and obtaining FAA approval prior to the aircraft entering service. The approved flight manual must list the certificated noise levels that have been shown to comply with part 36 maximum noise levels (noise limits) and contain the following statement as specified in §36.1581(c) and reprinted here:

“No determination has been made by the Federal Aviation Administration that the noise levels of this aircraft are or should be acceptable or unacceptable for operation at, into, or out of, any airport.”

201(d)(6) Airplane limitations §36.1581(d)

(1) Airworthiness maximum gross weights Transport category large airplanes and jet airplane gross weights are normally limited for structural, performance or economic reasons and demonstrated to the appropri ate airworthiness standards for the airplane. Those airworthiness limited gross weights may constitute the maximum operational gross weights and are to be identified in the “Limitations Section” of an approved AFM.

(2) Approach noise limiting configurations Applicants may need to limit the landing flaps or the approach gross weight in order to comply with the requirements of part 36. All noise limiting gross weights and configurations are to be included in the “Limitations Section” of an approved AFM.

(3) Takeoff noise limiting configurations An applicant may establish takeoff limiting configurations (e.g., maximum takeoff gross weight, takeoff airspeed schedules, takeoff thrust (power) derate schedule, in-flig ht APU operation) that are more restrictive than the airworthiness limited configurations. These noise limited configurations must be furnished in the Limitations Section of an approved AFM.

(4) Optional engine thrust (power) ratings (derate and reduced thrust) Compliance with part 36 is only required at full rated takeoff thrust (power). However, an airplane may be type certificated at derated/reduced thrust (power) that is less than full rated takeoff thrust (power). This is not an acoustical change as defined in §21.93(b) of part 21 provided that the full rated thrust (power) remains approved for a given airplane configuration. Airplane type certification at derated/reduced thrust (power) does not prohibit an applicant from employing thrust (power) reduction for noise certification as permitted by B36.7(b) of part 36.

201(d)(7) Non-complying agricultural and firefighting airplanes §36.1583(a)

(1) Fire fighting airplanes Those small propeller-driven airplanes that are specifically designed for “dispensing of fire fighting materials" are exempt from the noise certification requirements of part 36.

(2) Agricultural airplane operations Agricultural aircraft operation, as defined in §137.3 of part 137, means the operation of an aircraft for the purpose of:

a) dispensing any economic poison;

b) dispensing any other substance intended for plant nourishment, soil treatment, propagation of plant life, or pest control; or

c) engaging in dispensing activities directly affecting agriculture, horticulture, or forest preservation, but not including the dispensing of live insects.

(3) Economic poisons Economic poisons, as defined in part §137.3 of part 137, means:

a) any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any insects, rodents, nematodes, fungi, weeds, and other forms of plant or animal life or viruses, except viruses on or in living man or other animals, which the Secretary of Agriculture declares to be a pest; or

b) any substance or mixture of substances intended for use as a plant regulator, defoliant or desiccant.

201(d)(8) Operating limitations §36.1583(b)

(1) Limitation on operations No person may operate, in accordance with the restrictions of §91.815 of part 91, an airplane that complies with the noise certification exemptions of this section, except:

a) to accomplish the work activity directly associ ated with the purpose for which it is designed;

b) to provide flight crewmember training in the special purpose op eration for which the airplane is designed; or

c) to conduct “nondispensing aerial work operations” related to agriculture, horticulture, or forest preservations.

Chapter 3

TECHNICAL PROCEDURES APPLICABLE FOR NOISE CERTIFICATION OF ALL AIRCRAFT TYPES

CHAPTER TABLE OF CONTENTS

301 NOISE MEASUREMENT PROCEDURES58

301(a) Test Site Selection58

301(a)(1) Terrain58

301(a)(2) Grass59

301(a)(3) Snow59

301(a)(4) Plowed fields59

301(a)(5) Obstructions59

301(a)(6) Anomalous meteorological conditions60

301(b) Precautions Regarding Effects of Precipitation60

301(b)(1) General precautions60

301(b)(2) Effect of moisture on microphones60

301(b)(3) Microphone internal heaters60

301(c) Calibration of Acoustical Measurement System60

301(c)(1) General calibration process60

301(c)(2) Frequency-dependent system response corrections61

301(c)(3) Single-frequency level calibrations62

301(c)(4) Field sound pressure level calibration considerations62

301(c)(5) Application of calibration corrections63

301(c)(6) Calibration traceability63

301(d) Windscreen Insertion Loss64

301(e) Wind Speed Limitations64

301(f) Measurement of Atmospheric Conditions64

301(g) Meteorological Measurement System64

301(h) Aircraft Position Measurement65

301(i) Measurement System Specifications65

301(i)(1) Validity of measurent system configuration65

301(i)(2) Changes in measurement system configuration65

301(j) Noise Compliance Demonstration Plan65

302 FLIGHT PATH MEASUREMENT65

302(a) Radar or Microwave Tracking System65

302(a)(1) Aircraft equipment66

302(a)(2) Ground equipment67

302(b) Kine-theodolite System67

302(c) Radar/theodolite Triangulation67

302(d) Photographic Scaling67

302(e) DGPS-based Time-Space-Position Information Tracking Systems68

302(e)(1) General68

302(e)(2) System design issues69

302(e)(2)(A) Coordinate frames and waypoint navigation70

302(e)(2)(B) Test site survey70

302(e)(2)(C) Receiver output data71

302(e)(2)(C)(i) Data stored during aircraft noise testing when a real-time data link is used72

302(e)(2)(C)(ii) Data stored during aircraft noise testing when a real-time data link is not used72

302(e)(2)(C)(iii) Real-time DGPS messages72

302(e)(2)(C)(iv) Messages for multi-path testing73

302(e)(2)(D) System accuracy and sources of DGPS error73

302(e)(2)(E) Multi-path errors74

302(e)(2)(E)(i) Characteristics74

302(e)(2)(E)(ii) Code-based system ground station74

302(e)(2)(E)(iii) Carrier-based system ground station75

302(e)(2)(E)(iv) Aircraft installation75

302(e)(2)(F) Other sources of DGPS error75

302(e)(2)(F)(i) Correction latency75

302(e)(2)(F)(ii) Tropospheric delay76

302(e)(2)(F)(iii) Mismatched GPS receivers76

302(e)(2)(F)(iv) Mismatched satellite ephemeris/clock data76

302(e)(3) System approval recommendations77

302(e)(3)(A) Design issues77

302(e)(3)(B) Data storage (logging) during noise testing78

302(e)(3)(B)(i) For a system with real-time data link78

302(e)(3)(B)(ii) For a system not using real-time data link78

302(e)(3)(C) Documentation79

302(e)(3)(D) Accuracy verification test79

302(e)(3)(E) Software verification79

302(e)(3)(F) Ground-station multi-path mitigation and verification79

302(e)(3)(F)(i) All systems80

302(e)(3)(F)(ii) Code-based systems80

302(e)(3)(G) Airport survey80

303 ON-BOARD FLIGHT DATA ACQUISITION80

303(a) General80

303(b) Magnetic Tape Recording81

303(c) Automatic Still Photographic Recording81

303(d) Cine Recording81

303(e) Video recording81

304 TIME SYNCHRONIZATION OF MEASURED DATA82

304(a) General82

304(b) TSPI Equipment and Software Approval82

304(c) Continuous Time-Dode Recording82

304(d) Recording of Single Time Marker83

304(e) Measurement of the Interval Between Recorder Start and Overhead83

304(f) Setting of Internal Time-Stamp Clock83

304(g) Additional Time-Synchronization Considerations83

305 CALCULATION OF CONFIDENCE INTERVALS84

305(a) Introduction84

305(b) Confidence Interval for the Mean of Flight Test Data84

305(b)(1) Confidence Interval for the Sample Estimate of the Mean of Clustered Measurements84

305(b)(2) Confidence interval for mean line obtained by regression85

305(c) Confidence Interval for Static-Test-Derived NPD Curves87

305(d) Confidence Interval for Analytically Derived NPD Curves88

305(e) Adequacy of the Model88

305(e)(1) Choice of engine-related parameter88

305(e)(2) Choice of regression model88

305(f) Worked Example of the Determination of 90 percent Confidence Intervals89

305(f)(1) Introduction89

305(f)(2) Confidence interval for a clustered data set90

305(f)(3) Confidence interval for a first order regression curve91

305(f)(4) Confidence interval for a second order regression curve95

305(f)(5) Confidence interval for the pooled data set96

305(g) Student’s t-distribution (for 90 percent Confidence) for Various Degrees of Freedom96

306 ADJUSTMENT OF AIRCRAFT NOISE LEVELS FOR THE EFFECTS97

306(a) Introduction97

306(b) Definitions98

306(c) Background Noise Adjustment Procedure99

306(c)(1) Assumptions99

306(c)(2) Step-by step description100

306(c)(2)(A) Determination of pre-detection noise100

306(c)(2)(B) Determination of post-detection noise100

306(c)(2)(C) Testing of pre-detection noise versus post-detection noise100

306(c)(2)(D) Determination of masking criteria100

306(c)(2)(E) Identification of masked levels101

306(c)(2)(F) Determination of Last Good Band101

306(c)(2)(G) Adjustment of valid levels for background noise101

306(c)(2)(H) Adjustment of valid levels for measurement conditions101

306(c)(2)(I) Reconstruction of low frequency masked bands101

306(c)(2)(J) Reconstruction of levels for masked high frequency bands102

306(c)(2)(J)(i) Frequency extrapolation method102

306(c)(2)(J)(ii) Time extrapolation method103

306(c)(2)(K) Handling of spectra after reconstruction of masked bands103

306(d) General Considerations104

306(d)(1) Limitations and requirements for any background noise adjustment procedure104

306(d)(2) Rejection of spectra due to masking104

305(d)(3) Special tone correction considerations due to masking104

306(d)(4) Handling of masked data in reference conditions data sets105

307 NOISE REDUCTION SYSTEMS105

307(a) Variable Noise Reduction Systems105

307(a)(1) Reference procedures106

307(a)(2) Test conditions and procedures106

307(a)(3) Adjustments to measured noise data106

307(a)(4) Guidance for specific VNRS107

307(b) Selectable Noise Reduction Systems107

308 CALCULATION OF THE SPEED OF SOUND107

309 REFERENCE TABLES USED IN THE MANUAL CALCULATION OF107

301 NOISE MEASUREMENT PROCEDURES [ETM 3.1] The applicant should submit to the FAA for its review an d approval technical procedur es for the measurement of aircraft noise certification levels that the applicant will use. The technical procedures described in the following sections of this chapter should be considered as genera lly appropriate for all aircraft types. See Chapter 4 for procedures specific to Appendices A and H. See Chapter 5 for procedures specific to Appendix G. See Chapter 6 for procedures specific to Appendix J.

301(a) Test Site Selection [ETM 3.1.1] For airplanes, when the flight path intercept equivalent test procedure is used, and for helicopters, it may not be necessary for the test site to be located at an airport. De tails of the proposed noise certification test-site locations should be submitted to the FAA for review and approval. Some test-site criteria that could support selection of a nonairport test site include level terrain, reduced air traf fic, reduced ambient noise, improved weather conditions (temperature, humidity and wind), improved microphone placement, availability of field surveys, improved locations for aircraft position monitoring and improved pilot sight and handling.

301(a)(1) Terrain [ETM 3.1.1.1] Page 58

Uneven terrain having features such as mounds or furrows can result in reflections that could influence the measured sound levels. Vegetation can reduce the amount of sound that is reflected from the ground surface. In most cases this effect results in a reduced sound level, but under some circumstances the level may be higher. Testing over a smooth hard surface, such as a paved area, will generally result in a higher sound level.

301(a)(2) Grass [ETM 3.1.1.2] For noise measurement points under the flight path 25 ft (7.5 m) radius circles of mowed grass (not exceeding 3 in (8 cm) height) are acceptable. For noise meas urement points located to the side of the flight path, the grass may be mowed in a semicircle of 25 ft (7.5 m) radius facing the line of flight.

301(a)(3) Snow [ETM 3.1.1.3] Snow in the area surrounding the noise measurement points may provide excessive absorption of aircraft sound reflected from the ground. Noise measurement points have been approved when snow within a 50 ft (15 m) radius of the noise measurement points has been removed. However, snow should not be piled at the borders facing the line of flight.

301(a)(4) Plowed fields [ETM 3.1.1.4] Earthen or sandy surfaces within a 25 ft (7.5 m) radius of the noise measurement points must be reasonably tamped down. Plowed furrows, silt or soft, powdered surfaces are unacceptable.

301(a)(5) Obstructions [ETM 3.1.1.5] Obstructions in the vicinity of the noise measurement points such as buildings, walls, trees, vehicles and test personnel, if close enough, may be unacceptable because of reflections that influence measured noise levels. There should be no obstructions that significantly influence the sound field from the aircraft with in a conical space above a point on the ground vertically below the microphone at each noise measur ement point. The cone is defined by an axis normal to the ground and by a half angle of 80º (75º for light propeller-driven airplanes) from the axis as illustrated in Table 2.

Figure 2. Obstruction-free cone defined from the base of the measurements microphone Page 59

301(a)(6) Anomalous meteorological conditions [ETM 3.1.1.6] Certain geographical areas are more susceptible to anomalous meteorological conditions than others (i.e., large variations, or inversions, of temperature or humidity, excessive turbulence, or thermally induced vertical winds). The applicant may conduct certification testing only as approved by the FAA.

301(b) Precautions Regarding Effects of Precipitation [ETM 3.1.2] 301(b)(1) General precautions [ETM 3.1.2.1] Fog, rain, drizzle and snow can have a number of adverse effects. Changes in sound generation and propagation under these conditions are not well documented. Most of the equipment used for measuring noise is not intended for use during conditions of precipitation, and the effects can range from changes in microphone and windscreen sensitivity or frequency response, to arcing of conventional condenser microphones, to possible failure of equipment because of electrical short circuits.

301(b)(2) Effect of moisture on microphones [ETM 3.1.2.2] Most microphones that are used during noise certification testing are susceptible to moisture. Precipitation, including snow, drizzle and fog, or excessive humidity may induce electrical arcing of the microphone sensors, making measured noise data unacceptable. Howeve r, some pre-polarized microphones are le ss susceptible to electrical arcing during high-moisture conditions (consult the equipment manufacturer’s specifications). Special care should be taken to ensure that any windscreens exposed to precipitation be thoroughly dry, inside and out, before use. Foam windscreens can trap water and wet foam windscreens should be avoided.

301(b)(3) Microphone internal heaters [ETM 3.1.2.3] When internal heaters are prov ided, microphones are less likely to be affected by moisture in wet, humid, cold or freezing atmospheric conditions.

301(c) Calibration of Acoustical Measurement System [ETM 3.1.3] 301(c)(1) General calibration process [ETM 3.1.3.1] The process for the calibration of an acoustical measurem ent system (see A36.3.1.1, A36.3.3.1, G36.103, and J36.109(c) of part 36 for definition of the “measurement system”) for purposes of noise certification testing consists of two parts:

a) determination of system response corrections; and

b) sound pressure level calibrations and testing in the field.

Some of these calibrations must be conducted at the aircraft noise test site, with the measurement system configured and deployed for aircraft noise measurement. Note that additional calibrations and checks may be allowed, or required, by the FAA.