Advisory U.S. Department of Transportation Circular Federal Aviation Administration AC 36-4B
NOISE CERTIFICATION
HANDBOOK March 23, 1988 Initiated by: AEE-3
Advisory U.S. Department Circular of Transportation Federal Aviation Administration Subject: Date: 3/23/88 AC No: 36-4B Initiated by: AEE-3 Change:
NOISE CERTIFICATION HANDBOOK
FOREWORD The aim of this technical manual 1s to promote uniformity of implementation of the noise certification requirements of Part 36 of the Federal Aviation Regulations (FAR) by presenting test, analysis, and documentation procedures for subsonic turbojet airplanes that have been determined by the FAA to be technically acceptable for demonstrating compliance with that regulation.
where appropriate, FAA policy governing such certifications is reviewed.
Naryan I luna Norman H. Plummer Director of Environment and Energy
CONTENTS Page CHAPTER 1. GENERAL.
1. Purpose..
2. Cancellation.
3. Background.
4. Objective.
5. Noise Demonstrations. 2 Flight Tests. 2
b. Supplemental Noise Demonstration Methods 2 C. Changes to Noise Certification Levels for Derived Versions 2
6. Witnessing of Tests. 2
7. [Reserved].. 3 (and 4) CHAPTER 2. NOISE DEMONSTRATION FLIGHT TEST UNDER PART 36 APPENDICES A AND B. 5
8. Test Conditions. 5
a. Test Site Characteristics 5
b. Atmospheric Conditions 5
9. Flight Procedures.
a. Selection of a Flight Test Noise Demonstration Method.
b. Nominal Conditions for Development of a Reference Noise-Power-Distance Data Base (NPD).....
c. Measurements at Non-reference Points.
d. Test Day Flight Procedures
e. Target Test Conditions. 11
10. Measurements11
a. Sound Measurements.. 12
b. Airplane Position and Performance. 13 I1. Sound Measurement System Specifications 14
12. Analysis System. 14 111
Page
13. Sound Measurement Equipment Calibrations
and System Checks15
a • Frequency Response.. 15
b. Amplitude15
14. Computation of Effective Perceived Noise
Level (EPNL)15
Data Processing.. 16
b. Test Day EPNL17
Normalized EPNL17
d. Duration Correction for Simplified Adjustment Method... 17
e. Reference EPNL Evaluation 18 CHAPTER 3. SUPPLEMENTAL NOISE DEMONSTRATION METHODS UNDER PART 36 APPENDICES A AND B. 21
15. Alternative Methods21
16. Technical Considerations 22 8• Examples of Demonstration Applications. 22
b. Noise Source Characteristics. 22
c. Noise Increment Derivation Using Common Demonstration Method. 22
d. Supplemental Test Data Compatibility. 24
e. Component Noise Sources 24
f. Noise Increments Caused by Nacelle Acoustic Treatment Changes... 24
17. Static Engine Tests24
Limits on the Projection of Static to Flight Data....... • 25 Test Site Requirements. 25
Test Hardware25
Inflow Control Devices Acoustic Shadowing... ICD Calibration...... 25 26 26 Microphone Locations. 28 Engine Power Test Conditions. 28 Data Analysis System Compatibility. 28 Data Acquisition, Analysis and Normalization. 28 1v
Page
18. Projection of Static Engine Data to Airplane
Flight Conditions29
a. Engine Installation Effects. 29
b. Normalization to Reference Conditions.. 32
c. Engine Component Noise Source Separation. 32
d. Noise Source Position Effects 33
e. Engine Flight Conditions... 34
f. Noise Source Motion Effects. 34 Airframe Noise Component.. 35
h. Extrapolation to Airplane Flight Path. 36
1. Total Noise Spectra. 36 j• EPNL Computations... 37
k. Noise Increment Computations 37
19. Supplemental Flight Tests 37
20. Supplemental Analyses... 37
a. Noise Increments Derived from an Approved NPD. 38
b. Analytical Modeling of Noise Components 38
21. Supplemental Component Tests. 39 (and 40)
22. Methods for Demonstrating No Acoustical Change. 39 (and 40) CHAPTER 4. DOCUMENTATION REQUIREMENTS 41
23. General.. 41
24. Noise Compliance Demonstration Plan 41
a. Introduction. 41
b. Test Item41
c. Type of Noise Demonstration Method. 41
d. Test Description41
e • Data Acquisition Systems. 41
f. Data Analyses and Normalization Procedures. 41
g. Equivalencies41
25. Noise Certification Compliance Report 42
a. Introduction42
b. Airplane, Engine, and Nacelle Descriptions. 42
c. Airplane Performance at Reference
Conditions42
d• Certification Noise Levels. 42
e. Noise Certification Test Results 42 V
Page
26. DER Witness Informal Report 43
27. Noise Control Act Findings. 43
28. Airplane Flight Manuals. 44 a • General Requirements for Limitation. 44
b. AFM Maximum Cert1ficated Weight Limits. 45
c. Multiple Noise Certifications. 45 Landing Flap Restriction. 45
e. Optional Engine Thrust Ratings (Derate and Reduced Thrust).. 46
f. Noise Level Information. 46
29. ICAO Cert1ficat1ons 46 APPENDIX 1. NOISE DATA CORRECTIONS FOR TESTS AT HIGH ALTITUDE TEST SITES.
Introduction.
2. Jet Noise Source Correction APPENDIX 2. GUIDANCE MATERIAL ON METHODS TO ACCOUNT
FOR THE EFFECTS OF BACKGROUND NOISE ON AIRPLANE RECORDED NOISE DATA 1
1. Introduction.. 1
2. Background Noise. 1
3. Correction Procedures 3 APPENDIX 3. GUIDANCE MATERIAL ON METHODS TO IDENTIFY
PSEUDOTONES/FICTITIOUS TONES IN AIRPLANE RECORDED NOISE DATA 1
Introduction1
Spectral Irregularities. 1 Methods for Identification of Pseudotones/Fictitious Tones. 2 APPENDIX 4. COMPUTATION OF EFFECTIVE PERCEIVED NOISE LEVEL (EPNL) ... .... 1 (and 2) APPENDIX 5. GUIDANCE MATERIAL ON METHODS TO CALCULATE CONFIDENCE INTERVALS. 1
1. Introduction..
2. Confidence Interval for Clustered Data.
Confidence Interval for Regression Curve.
Confidence Interval for Pooled Data Sets v1
CHAPTER 1. GENERAL PURPOSE. The aim of this advisory circular (AC) 1s to promote uniformity of implementation of the noise certification requirements of Part 36 of the Federal Aviation Regulations (FAR) by presenting technically acceptable test, analysis and documentation procedures for subsonic turbojet airplanes that may be used by applicants for demonstrating compliance with Part 36.
appropriate, FAA policy governing such certifications is reviewed.
2. CANCELLATION. Advisory Circular 36-4A, Noise Certification Test and Analysis Procedures, dated January 21, 1986, 1s cancelled.
3. BACKGROUND. Part 36 contains noise certification standards for turbojets, large transport category airplanes, and propeller-driven small airplanes.
These regulations require that standard procedures be used unless an equivalent procedure 1s approved by the FAA. However, airplane noise measurement and analysis technology has developed considerably since Part 36 was adopted in 1969. Through aircraft noise certification, a body of technically sophisticated measurement and analysis methods has been developed by certificate applicants. As a result of this experience, alternative compliance methods have evolved and have been approved by the FAA. This AC describes those procedures. Applicants should always obtain prior approval from the responsible FAA certificating official before initiating any noise certification action. It should also be noted that the methods presented in this AC are not the only ways to demonstrate compliance with Part 36 nor the only ones that may be approved in the future.
OBJECTIVE. The objective of noise certification is the demonstration of compliance with Part 36 for one or more specific airplane versions. The demonstration method used to determine noise levels will depend upon a number of factors. For example, Part 36 requires an applicant for a new type design to conduct a noise demonstration flight test. However, depending upon the extent of the data already in existence for the type design, the applicant for a change to an existing type design may submit for FAA approval a proposal either to conduct a flight test or to conduct supplementary tests and/or analyses. When supplementary data are used, they serve as the basis to adjust flight noise levels already approved for an earlier version. Dialogue between the applicant and the certificating authority at the beginning of an airplane certification program is necessary to identify anticipated changes in airplane type design that can impact noise certification. A noise compliance demonstration plan outlining the planned tests and/or analyses should be submitted as soon as possible to the FAA by the applicant. This plan should contain the method by which the applicant proposes to show compliance with the noise certification requirements. Because of the unique nature of each noise demonstration, the test plan is reviewed with the certificating authority before each test and, if necessary, modified to reflect any unique aspects or current developments related to the immediate noise demonstration test. With this flexibility, a single test plan, submitted to encompass more than the immediate test, is often advantageous. Guidance on preparation of noise compliance demonstration plans and certification compliance data reports 1s provided in Chapter 4 of this AC.
Chap 1 Par 1 1
5. NOISE DEMONSTRATIONS.
a. Flight Tests.
data from a sufficient number of flights of a particular airplane to derive the three required Effective Perceived Noise Level (EPNL) levels. Guidelines are also provided on acquisition and normalization methods such that a generalized noise data base (defined by noise-power-distance relationships) may be developed for an airplane type. Suggested procedures for determining the certificated Part 36 noise levels derived from this generalized noise data base are also provided. This generalized data base is directly applicable to changes in type design which retain the same basic noise source characteristics.
b. Supplemental Noise Demonstration Methods. For those applicants seeking approval for changes in type design, guidance is also provided on suggested procedures for determining noise level changes relative to a previously approved version of the type. These procedures rely on supplementary testing and/or analysis techniques in conjunction with approved noise data already available for the type. The use of such methods for demonstrating noise compliance is optional, subject to FAA approval, and does not preclude an applicant from using a flight test for each noise Changes To Noise Certification Levels For Derivative Versions. Many of the equivalent procedures given in this AC relate to derivative versions, where the procedure used yields the information needed to obtain the noise certification levels of the derivative version by adjustment of the noise levels of the "flight datum" aircraft (1.e., the most appropriate aircraft for which the noise levels were measured during an approved Part 36 flight test demonstration). The physical differences between the "flight datum" aircraft and the derivative version can take many forms, for example, an increased takeoff weight, an increased engine thrust, changed powerplant or propeller types, etc. Some of these changes will alter the distance between the aircraft and the noise certification reference points, others the noise source characteristics. Procedures used in the determination of the noise certification levels of the derived versions will, therefore, depend upon the change to the aircraft being considered. However, where several similar changes are being made, for example, introduction of engines from different manufacturers, the procedures used to obtain the noise certification levels of each derivative aircraft should be followed in identical fashion.
WITNESSING OF TESTS. All flight and other tests conducted in support of Designated Engineering Representative (DER) appointed under Order 8110.37. Under that Order, Acoustic DER's may: noise certification need to be witnessed by FAA personnel or by an Acoustic 1C 025 ay: (DER).
- Witness and approve noise certification tests conducted in accordance by the FAA. with an approved test program, when specifically authorized to do so 2 Chap 1 Par 5
- Approve noise analysis techniques and computer programs and certify the noise values reduced by these programs that were measured and evaluated as prescribed in Subparts B and/or F of Part 36 or by an equivalent procedure previously approved for that noise test series by the Office of Environment and Energy.
Prior FAA approval is required for the re-delegation by Acoustic DER's of the authority to witness tests. In addition, Acoustic DER's may not determine whether a type design change is an acoustic change under Part 21. Acoustical DER's also may not approve:
- Test plans or equivalent procedures,
- Operating limitations or other Aircraft Flight Manual information, or
- Certificated aircraft noise levels.
7. [RESERVED] Chap 1 Par 6 3 (and 4)
CHAPTER 2. NOISE DEMONSTRATION FLIGHT TEST UNDER PART 36 APPENDICES A AND B
8. TEST CONDITIONS. The airplane should be operated such that noise values corresponding to the takeoff, sideline, and approach reference conditions defined in Part 36 can be derived within the limitations of Appendix 1. To meet the test environmental requirements prescribed in Part 36, the following test site and atmospheric conditions have been approved as providing an acceptable test environment.
a. Test Site Characteristics. The noise measurement stations are placed such that the reference locations specified in Part 36 can be adequately simulated. Part 36 prescribes reference noise measurement stations relative to runway brake release or threshold point (e.g., 6500 meters from brake release for takeoff measurements); however, actual test measurement station locations may differ for the following reasons:
(1) To minimize the impact of ambient sound on the measurement of the airplane sound pressure levels. Prior to testing, a comparison of expected airplane sound pressure levels with the expected test-site ambient sound will aid in choosing test microphone locations. It is generally preferable to relocate measurement stations rather than contaminate airplane noise measurements with ambient sound from sources such as nearby road or air
(2) To increase the airplane signal-to-noise ratio to ensure adequate measurement of the airplane's acoustic signature. In such cases, the takeoff measurement station and/or the airplane flight trajectory may be relocated so that the test day sound propagation path length is less than the reference path length to ensure acquisition of acceptable data. In the selection of an adequate signal-to-noise ratio, consideration should be given to limitations on corrections for the influence of ambient sound, as well as limitations on the total correction from test to reference conditions.
(3) When obstructions near the noise measurement station(s) will influence sound measurements. Takeoff and approach measurement stations may be relocated as necessary to avoid undesírable obstructions. Sideline measurement stations (distances) may be relocated by distances which are of the same order of magnitude as the airplane lateral deviations (or offsets) relative to the nominal flight path that occur during flight testing.
NOTE: Flight path intercepts adopted as alternatives to full takeoffs and full stop landings may be adjusted to attain the same performance as would be observed at the reference distances.
b. Atmospheric Conditions.
(1) Temperature and Relative Humidity Limits. When atmospheric conditions of ambient temperature and relative humidity result in atmospheric absorption in the 8 kHz one-third octave band at any point along the sound propagation path exceeding 12 dB/100 m, then instrumentation used to determine relative humidity should consist of an ambient air temperature and dew point Chap 2 Par 8 5
temperature measurement system accurate to within † 0.5°C. When such instrumentation is used, the 8 kHz atmospheric absorption may be extended to 14 dB/100 m and, for approaches only, to 16 dB/100 m. Relative humidity is required to be between 10 and 95 percent.
(2) Wind Limits. Testing may be conducted when: the wind speed does not exceed an average value, over the sound measurement period, of 12 kts and a maximum value of 15 kts; the crosswind component does not exceed an average value over the sound measurement period, of 7 kts and a maximum value of 10 kts. A 30 second moving average is used to define average wind speed.
Part 36 requires the wind measurements to be made 10 meters above the ground and in the vicinity of the measuring station.
9. FLIGHT PROCEDURES.
a. Selection of a Flight Test Noise Demonstration Method. An accurate, reliable definition of the airplane noise characteristics is the primary objective of the demonstration test. To achieve this goal, at least six sound measurements are required by section A36.5(e) and section 8(a) of Part 36 to ensure that a mean noise level for each of the three certification reference conditions can be defined with a 90 percent confidence interval not exceeding † 1.5 EPNdB. This requirement can be achieved by conducting a series of test flights at or near reference power. After adjusting each measured noise level to Part 36 reference conditions, a certification noise value should be computed by averaging the adjusted levels (see paragraph 14). An alternate method is to acquire sufficient measurements (at least six) to allow the generalized noise characteristics of the airplane to be developed (a noise-power-distance data base (NPD)). (See paragraph 14(e) (2)). The three certification levels for any specific configuration are then determined by entering the developed data base at the certification reference distance and power and applying appropriate airplane speed corrections.
(1) For a range of powers covering full and cutback powers, the airplane is flown past sideline and under-flight-path microphones in accordance with section C36.3 of Part 36. Sufficient noise measurements are made to enable noise-power curves at a given distance for both sideline and cases to be established. These curves are extended either by calculation or by the use of additional flight test data to cover a range of takeoff distances to form the generalized noise NPD data base for use in the noise certification of the "flight datum" and derived versions of the type.
The 90 percent confidence intervals about the mean lines are constructed through the data. This sequence should be repeated for an under-flight-path microphone for a range of approach powers using the appropriate speed and airplane configuration. In compliance with section C36.9 of Part 36, the approach configuration that is most critical from a noise standpoint must be When in doubt, an applicant may be required to test and evaluate more than one configuration.
(2) When approved data are available to define the engine spool down characteristics, it may be appropriate to substantiate noise levels for cutback power without the implementation of a cutback takeoff procedure by making noise measurements during reduced power operations. The average engine Chap 2 6 Par 8
spool down time should be representative of average conditions and should reflect a 1.0 second minimum altitude-recognition lag time to account for pilot response. For determination of the reference cutback noise levels, the cutback initiation should be selected to ensure stabilized cutback power conditions before the initial 10 Tone Corrected Perceived Noise Level (PNLT) down point. Flyover noise levels with power cutback may be established from the merging of PNLT versus time measurements obtained during constant power operations. As seen in Figure 1(a), the 10 dB down PNLT noise time history may contain portions of both the full power and cutback power noise time histories. Provided these noise time histories, the average engine spool-down thrust characteristics and the airplane flight path during this period (Figure 1(b)), which includes the transition from full to cutback power, are known, the flyover noise level may be completed.
(3) An alternate method may be used to derive cutback power noise levels. In some cases, this method results in lower EPNL values, depending upon the characteristics of the forward radiated noise. Actual test runs may be conducted using a power cutback procedure that is initiated close to the overhead point. This technique is different from the procedure described in the previous paragraph in that the engine power is not stabilized during the entire 10 dB down time period. The concept involves delaying power cutback in order to gain extra altitude, but cutting back power before the maximum PNLT (PNLT) exceeds the PNLT that would exist using stabilized cutback power.
Since this is a more difficult flight test procedure, care should be exercised in conducting the test runs to ensure consistency in order to meet the 90 percent confidence interval criteria.
(4) The airplane's source noise characteristics should be measured over the operating range in sufficient detail to define the difference between test and reference day source noise. To aid in obtaining adequate data for these adjustments it may be necessary to identify those airplane/engine parameters, e.g., NI of the airplane's nöise lével and appropriately vary these parameters during for turborans, that are the major controllers the test. For propeller-driven airplanes, the engine parameters are propeller helical tip Mach number and shaft horsepower.
b. Nominal Conditions for Development of a Reference Noise-Power-Distance Data Base (NPD). For the development of a generalized NPD, sound pressure level (SPL) data are normalized to the ambient environmental conditions specified in Part 36 and to nominal takeoff and approach aerodynamic configurations and performance. For example, the selected airplane speed should be representative of the range of speeds for the configurations and weights which are anticipated to be certified. In addition, takeoff and approach configurations may be different; requiring normalizing SPL data to different airplane speeds. Note that the adjustment procedure requires FAA approval when developing an NPD. (See paragraph 14(e) of this AC.) An example of a generalized NPD is shown in Figure 2.
c. Measurements at Non-reference Points. In some instances test measurement points may differ from the reference measurement points in Part 36, Appendix C. Under these circumstances an applicant may request Chap 2 Par 9 7