(i) Substantial noise abatement cannot be achieved for that aircraft by prescribing standards and regulations consistent with the limitations of section 44715(b); or
(ii) 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 FAA Order 8110.4 B (reference 7 of this AC), paragraph 6-3(c)
b. ENVIRONMENTAL ASSESSMENTS: Compliance with the National Environmental Policy Act of 1969 may require that the FAA conduct an Environmental Assessment (EA) as specified in Appendix 4, section 3(a) of FAA Order 1050.1D (reference 1 of this AC). 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 A). An Environmental Impact Statement (EIS) or Finding of No Significant Impact (FONSI) must be prepared by the FAA (See Note B). The FAA may grant an interim aircraft noise certification upon completion of an EA while appropriate rulemaking is being completed.
Notes:
A. Applicants may be requested to provide appropriate noise data to FAA to support the EA and/or rulemaking processes
B. Procedures for generation of an EA, EIS or FONSI are also found in FAA Order 1050.1D
c. Procedures
(1) Applicant’s Responsibility: Applicants are responsible to generate and submit to FAA the following noise certification documentation:
i. Noise Certification Plans;
ii. Noise Compliance Demonstration Plans (Including Proposed Equivalent Procedures);
iii. Noise Certification Reports;
iv. NAC Substantiation.
(2) DER”S Responsibility: Acoustic DERs responsibilities are as defined in FAA Order 8110.37C (reference 10 of this AC)
78. Section 36.1501(b)
Where supplemental test data are approved for modification or extension of an existing flight data base, such as acoustic data from engine static tests used in the certification of acoustical changes, the test procedures, physical configuration, and other information and procedures that are employed for obtaining the supplemental data must be developed and approved.
a. Explanation
The FAA permits use of non-flight test data to supplement flight databases and to allow wider flexibility in the use of that data for aircraft modifications and extension of flight test data. This includes static engine noise test data used to modify or expand NPD databases for noise certification of derivative airplanes.
b. Supplemental Information
(1) Airplane Families: Section 2.3 of the appended ICAO TM presents 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 section 36.1501(a), Supplemental Information Items (3) and (8)). An originally certified airplane and its derived versions are known as an “Airplane Family”.
c. Procedures
(1) Tests For Supplemental Data: An applicant should attempt to obtain static engine or flight 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.
79. Section 36.1581 Manuals, markings, and placards
80. Section 36.1581(a)
If an Airplane Flight Manual or Rotorcraft Flight Manual is approved, the approved portion of the Airplane Flight Manual or Rotorcraft Flight Manual must contain the following information, in addition to that specified under § 36.1583 of this part. If an Airplane Flight Manual or Rotorcraft Flight Manual is not approved, the procedures and information must be furnished in any combination of approved manual material, markings, and placards.
(1) For transport category large airplanes and jet airplanes, the noise level information must be one value for each flyover, lateral, and approach as defined and required by Appendix B of this part, along with the maximum takeoff weight, maximum landing weight, and configuration.
(2) For propeller driven small airplanes the noise level information must be one value for flyover as defined and required by Appendix F (or Appendix G) of this part, along with the maximum takeoff weight and configuration.
(3) For rotorcraft the noise level information must be one value for flyover, lateral and approach as defined and required by Appendix H (or flyover for Appendix J) along with the maximum takeoff weight, maximum landing weight and configuration.
a. Explanation
This section specifies information that must be contained in airplane and rotorcraft flight manuals.
b. Supplemental Information
(1) An approved Airplane Flight Manual (AFM) or Rotorcraft Flight Manual (RFM) is required for certification of each aircraft type in compliance with section 1581 of the Airworthiness Standards for parts 21.24 (Primary Category), 23, 25, 27, and 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 contained in an FAA approved section of an AFM or RFM other than the limitations section. For example, the performance section is an appropriate place to include the certificated noise level compliance information. The certificated noise levels are to be reported to 0.1 dB in the AFM or RFM. Further, an AFM or RFM must specify the type certificate limitations if any, that are established as a result of part 36 compliance, combined with the airworthiness limitations.
(2) Airplane Flight Manual Limitation Section:
(i) 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.
(ii) The AFM may address a single configuration (hardware 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 configuration. Furthermore, if not all configurations are approved for the airplanes listed in the AFM, the AFM must clearly identify by serial number, the proper operating limitations for each airplane.
(iii) 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.
(3) Multiple Airplane Noise Certifications
(i) Multiple noise-limited gross weight pairs (takeoff and landing) for one airplane configuration are not permitted by Subpart A, section 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.
(ii) An airplane configuration (hardware build) must be certificated to only a single “Stage” as appropriate under part 36 (Stage 1, Stage 2, or Stage 3). The limitations section of the AFM for an airplane configuration must not contain operating limitations that would result in noise levels exceeding part 36 noise limits.
However, “Stage, 2” airplanes may be recertificated as “Stage 3”, provided AFMs are revised and approved for configurations appropriate for compliance with “Stage 3”, and the “Stage 2” approval is deleted.
(4) Landing Flap Restriction
(i) 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.
(ii) For some Stage 3 airplanes, a “softguard” (such as a crushable cover plate) or frangible device must be installed on the flap selection control. 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 considered 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 by a maintenance action accomplished in accordance with part 43 prior to the next flight.
c. Procedures
(1) 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.
(2) Implementation of Landing Flap Restrictions: Long standing FAA policy has resulted in a choice of two options for an applicant to implement landing flap restrictions, where their approval is consistent with applicable airworthiness requirements.
(i) 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
(ii) If flap settings not considered in demonstrating compliance with part 36 are to remain selectable then, to prevent their use in normal operations:
A. Restrictions against their use, except under emergency (declared and undeclared) operations, must be incorporated into the AFM Limitations Section.
B. Operational information must be relocated to the Emergency Procedures Section.
C. For Stage 3 airplanes, install a “softguard” or frangible device over the flap selection control.
D. Placards may be installed to provide appropriate information to the flight crew.
(3) ICAO Certifications: The FAA is not authorized to approve aircraft compliance with Annex 16, Volume 1 Noise Standards. (reference 11 of this AC). This is the responsibility of the foreign airworthiness authority involved. FAA participation is limited to witnessing tests and reviewing data. 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 14 CFR 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-(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, 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.”
81. Section 36.1581(b)
If supplemental operational noise level information is included in the approved portion of the Airplane Flight Manual, it must be segregated, identified as information in addition to the certificated noise levels, and clearly distinguished from the information required under § 36.1581(a).
a. Explanation
This section specifies requirements for including supplemental noise level information in an approved AFM.
b. Supplemental Information
(1) The FAA permits publication of supplemental noise information in an approved AFM or RFM for configurations other 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.
c. Procedures
(1) Applicant's Responsibility: Applicant’s 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.
82. Section 36.1581(c)
The following statement must be furnished near the listed noise levels:
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.
a. Explanation
This section specifies that the FAA does not appraise aircraft in terms of their index of acceptability as related to airport operations.
b. Supplemental Information
(1) Certification vs. Operational Rule: Part 36 is a certification rule and not an operational rule.
The certificated noise levels may or may not meet the operational noise level requirements for any particular airport. The aircraft noise certification provisions of part 36 do not place any operational permissibility or limitations on the operation of certificated aircraft at any airport.
c. Procedures
(1) Applicant’s Responsibility: Applicants are responsible to develop an AFM or RFM and obtain FAA approval prior to the aircraft entering service. The approved flight manual must list the certificated noise levels shown to comply with part 36 maximum noise levels (noise limits) and contain a disclaimer as specified in section 36.1581(c).
83. Section 36.1581(d)
For transport category large airplanes and jet powered airplanes, for which the weight used in meeting the takeoff or landing noise requirements of this part is less than the maximum weight established under the applicable airworthiness requirements, those lesser weights must be furnished, as operating limitations in the operating limitations section of the Airplane Flight Manual. Further, the maximum takeoff weight must not exceed the takeoff weight that is most critical from a takeoff noise standpoint.
a. Explanation
None
b. Supplemental Information
(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 appropriate 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-flight APU operation) that are more restrictive than the airworthiness limited configurations. These noise limited configurations are to 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 part 21 section 21.93(b) 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 section B36.7(b) of Appendix B.
c. Procedures
None
84. Section 36.1581(e)
For propeller driven small airplanes and for propeller-driven commuter category airplanes for which the weight used in meeting the flyover noise requirements of this part is less than the maximum weight by an amount exceeding the amount of fuel needed to conduct the test, that lesser weight must be furnished, as an operating limitation, in the operating limitations section of an approved Airplane Flight Manual, in approved manual material, or on an approved placard.
85. Section 36.1581(f)
For primary, normal, transport and restricted category helicopters, if the weight used in meeting the flyover, lateral, or approach noise requirements of Appendix H of this part, or the weight used in meeting the flyover noise requirement of Appendix J of this part, is less than the certificated maximum takeoff weight established under either § 27.25(a) or § 29.25(a) of this chapter, that lesser weight must be furnished as an operating limitation in the operating limitations section of the Rotorcraft Flight Manual, in FAA-approved manual material, or on an FAA approved placard.
86. Section 36.1581(g)
Except as provided in paragraphs (d), (e), and (f) of this section, no operating limitations are furnished under this part.
87. Section 36.1583 Noncomplying agricultural and fire fighting airplanes
88. Section 36.1583(a)
This section applies to propeller-driven, small airplanes that--
(1) Are designed for "agricultural aircraft operations" (as defined in § 137.3 of this chapter, effective on January 1, 1966) or for dispensing fire fighting materials; and
(2) Have not been shown to comply with the noise levels prescribed under Appendix F of this part -- iI) For which application is made for the original issue of a standard airworthiness certificate and that do not have any flight time before January 1, 1980, or
(ii) For which application is made for an acoustical change approval, for airplanes which have a standard airworthiness certificate after the change in the type design, and that do not have any flight time in the change configuration before January 1, 1980.
a. Explanation
This section specifies the noise certification limitations and exemptions for propeller-driven, small airplanes that are specifically designed for agricultural aircraft operations or for dispensing fire fighting materials.
b. Supplemental Information
(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 part 137.3, means the operation of an aircraft for the purpose of:
(i) dispensing any economic poison, or
(ii) dispensing any other substance intended for plant nourishment, soil treatment, propagation of plant life, or pest control, or
(iii) 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, means:
(i) 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 shall declare to be a pest, and
(ii) any substance or mixture of substances intended for use as a plant regulator, defoliant or desiccant.
c. Procedures
None
89. Section 36.1583(b)
For airplanes covered by this section an operating limitation reading as follows must be furnished in the manner prescribed in § 36.1581:
Noise abatement: This airplane has not been shown to comply with the noise limits in part 36 and must be operated in accordance with the noise operating limitation prescribed under FAR §
91.815.
a. Explanation
This section specifies an operating limitation that is to be included in an AFM for propeller-driven, small airplanes that are designed for agricultural operations and for dispensing fire fighting materials.
b. Supplemental Information
(1) Limitation on Operations: No person may operate, in accordance with the restrictions of part 91, section 91.815, an airplane that complies with the noise certification exemptions of this section, except:
(i) to accomplish the work activity directly associated with the purpose for which it is designed;
(ii) to provide flight crewmember training in the special purpose operation for which the airplane is designed; and
(iii) to conduct “nondispensing aerial work operations” related to agriculture, horticulture, or forest preservations.
c. Procedures
None
90. - 99 [RESERVED]
100. Appendix A to Part 36-Aircraft Noise Measurement and Evaluation Under § 36.101
Sec.
A36.1 Introduction.
A36.2 Noise certification test and measurement conditions.
A36.3 Measurement of aircraft noise received on the ground.
A36.4 Calculations of effective perceived noise level from measured data.
A36.5 Reporting of data to the FAA.
A36.6 Nomenclature: symbols and units.
A36.7 Sound attenuation in air.
A36.8 [Reserved] A36.9 Adjustment of airplane flight test results.
a. Explanation
Appendix A addresses measurement and evaluation of noise generated by subsonic transport category large airplanes and jet airplanes. Helicopter noise measurement regulations are covered in Appendix H, unless otherwise noted within Appendix H.
b. Supplemental Information
None
c. Procedures
None
101. Section A36.1 Introduction
102. Section A36.1.1
This appendix prescribes the conditions under which airplane noise certification tests must be conducted and states the measurement procedures that must be used to measure airplane noise. The procedures that must be used to determine the noise evaluation quantity designated as effective perceived noise level, EPNL, under §§ 36.101 and 36.803 are also stated.
103. Section A36.1.2
The instructions and procedures given are intended to ensure uniformity during compliance tests and to permit comparison between tests of various types of airplanes conducted in various geographical locations.
104. Section A36.1.3
A complete list of symbols and units, the mathematical formulation of perceived noisiness, a procedure for determining atmospheric attenuation of sound, and detailed procedures for correcting noise levels from non-reference to reference conditions are included in this appendix.
105. Section A36.2. Noise certification test and measurement conditions
106. Section A36.2.1 General
XIII. APPENDIX A TO Part 36-AIRCRAFT NOISE MEASUREMENT AND EVALUATION UNDER § 36.101
107. Section A36.2.1.1
This section prescribes the conditions under which noise certification must be conducted and the measurement procedures that must be used.
Note: Many noise certifications involve only minor changes to the airplane type design. The resulting changes in noise can often be established reliably without resorting to a complete test as outlined in this appendix. For this reason, the FAA permits the use of approved "equivalent procedures". There are also equivalent procedures that may be used in full certification tests, in the interest of reducing costs and providing reliable results. Guidance material on the use of equivalent procedures in the noise certification of subsonic jet and propeller-driven large airplanes is provided in the current advisory circular for this part.
a. Explanation
b. Supplemental Information
(1) Test Conditions: Appendix A of part 36 specifies test site requirements, noise measurement procedures and data evaluation procedures for airplane noise certification testing of subsonic transport category large airplanes and jet airplanes. The applicant may use equivalent procedures, subject to FAA review and approval.
(2) Flight Path Intercept: The “flight path intercept” test procedure has been approved as an equivalent procedure wherein the airplane does not start from a static position on the airport runway for each flyover and lateral noise measurement (or land after each approach noise measurement), but remains in flight throughout the test series. See sections 2.1.1 and 3.1.1 of the appended ICAO TM. Stabilized airplane configuration and thrust (power) are maintained until it is determined that the measured noise level falls below PNLTM -10 dB. An A-weighted noise level is often used as a metric for determining this; noise measurement for a duration delimited by levels 15 dB (A) below the maximum A-weighted noise level is usually sufficient. The flight path intercept test procedure may be applied to all noise testing, including: flyover, lateral and approach noise measurements. This procedure saves valuable time by eliminating the takeoff, landing and turn-around for each measurement. This procedure also provides wider flexibility in the choice of test sites. The target airplane attitude, engine thrust (power), altitude over the microphones, constant configuration, etc., should be the same using the flight path intercept test procedure as it would be if the test were conducted from a standing start on the runway. See sections 2.1.1 and 3.1.1 of the appended ICAO TM for guidance on the use of the flight path intercept procedure.
(3) Equivalent Procedures: Equivalent procedures are discussed in sections 2 and 3 of the appended ICAO Technical Manual (TM). The procedure for obtaining FAA approval of an equivalent procedure is discussed under section 36.101 of this AC. Equivalent procedures are also discussed in sections 36.1501, A36.5.3.1, A36.5.4.3, A36.9.1.2, B36.4 and B36.7(a) of this AC. Information on equivalent procedures is also provided in paragraph 6-3(e) of reference 7.
c. Procedures
(1) Applicant’s Responsibility: An applicant must prepare a noise compliance demonstration plan that specifies a proposed certification process, including equivalencies. This plan is to be submitted to the appropriate ACO noise specialist allowing sufficient calendar time (usually 2 months prior to the scheduled test) to permit adequate FAA review and possible revisions prior to the start of any noise certification testing.
108. Section A36.2.2 Test environment
109. Section A36.2.2.1
Locations for measuring noise from an airplane in flight must be surrounded by relatively flat terrain having no excessive sound absorption characteristics such as might be caused by thick, matted, or tall grass, shrubs, or wooded areas. No obstructions that significantly influence the sound field from the airplane must exist within a conical space above the point on the ground vertically below the microphone, the cone being defined by an axis normal to the ground and by a half-angle 80° from this axis.
Note: Those people carrying out the measurements could themselves constitute such obstruction.
a. Explanation
This section specifies the requirements for airplane noise certification test site characteristics to help ensure uniformity in the noise measurement process. It also describes the clear zone above and around the microphone station that must be free from obstructions that significantly influence the sound field from the airplane.
b. Supplemental Information
(1) Test Site Locations: When the flight path intercept test procedure is used, it may not be necessary for the test site to be located at an airport. Proposed noise certification test site locations must be submitted to the FAA for review and approval. Some test site criteria that could support selection of a non-airport test site include: level terrain, reduced air traffic, 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.
(2) Noise Measurement Points: For the flyover, lateral and approach noise measurement points, 25-foot radius circles of mowed grass (not exceeding 3” in height) are acceptable. For the lateral measurement point, the grass may be mowed in a semicircle (25 foot-radius) facing the line of flight.
(3) Snow: Snow in the area surrounding the noise measurement points may provide excessive absorption of airplane sound reflected from the ground. FAA has approved noise measurements during winter conditions when snow within a 50-foot radius of the noise measurement points has been removed. However, snow must not be piled at the borders facing the line of flight.
(4) Plowed Fields: Earthen or sandy surfaces within a 25 foot radius of the noise measurement points must be reasonably tamped down. Plowed furrows, silt, or soft powdered surfaces are unacceptable.
(5) Obstructions: 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.
c. Procedures
(1) Applicant’s Responsibility: An applicant must identify the proposed test site characteristics in a proposed noise certification compliance demonstration plan.
110. Section A36.2.2.2
The tests must be carried out under the following atmospheric conditions.
a. Explanation
This section specifies the atmospheric conditions that are acceptable for airplane noise certification testing.
b. Supplemental Information
(1) Unacceptable Test Data: When testing is conducted outside the limits of atmospheric conditions specified in this section, valid adjustments from test to reference conditions may not be possible, and the data may not be acceptable.
c. Procedures
(1) Applicant’s Responsibility: The applicant is responsible for the following:
(i) Specifying test atmospheric condition limits, meteorological measurement systems, calibration procedures, and data adjustment procedures in a noise compliance demonstration plan. See section A36.2.2.2(c) of this AC for more on atmospheric sound attenuation limits.
(ii) Instrumentation for the conduct of meteorological measurements.
(iii) Obtaining FAA approval for deletion of any noise measurements that were obtained outside the approved limits of atmospheric conditions.
111. Section A36.2.2.2(a)
No precipitation.
a. Explanation
This subsection prohibits noise certification testing during conditions of rain or other precipitation.
Rain and other precipitation (including fog, drizzle, snow) can affect the measurement system and sound propagation during noise measurements.
b. Supplemental Information
(1) Effects of Moisture on Microphones: 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. However, some pre-polarized microphones are less susceptible to electrical arcing during high-moisture conditions (consult the equipment manufacturer's specifications). Special care must be taken to ensure that any windscreens exposed to precipitation be thoroughly dry, inside and out, before use. Foam windscreens can trap water; wet foam windscreens must be avoided.
(2) Microphone Internal Heaters: When internal heaters are provided, microphones are less likely to be affected by moisture in wet, humid, cold, or freezing atmospheric conditions.
c. Procedures
(1) Precautions: Special precautions must be taken by the applicant to protect microphones when a test shutdown is caused by wet, humid, or near freezing atmospheric conditions. Measurement System Components must be thoroughly dry before testing is resumed, to prevent arcing.
112. Section A36.2.2.2(b)
Ambient air temperature not above 95°F (35°C) and not below 14°F (-10°C), and relative humidity not above 95% and not below 20% over the whole noise path between a point 33 ft (10 m) above the ground and the airplane;