3/23/88 AC 36-4B approval of data that have been adjusted from the actual measurements to represent data that would have been measured at the reference points in reference conditions. (See paragraph 8(a).) Reasons for such a request may be:
(1) to allow the use of a measurement location that is closer to the airplane flight path so as to improve data quality by obtaining a greater ratio of signal to background noise;
(2) to enable the use of an existing, approved certification data base for an airplane type design in the certification of a derivative of that type when the derivative is to be certificated under reference conditions that differ from the original type certification reference conditions;
(3) to avoid obstructions near the noise measurement station(s) which could influence sound measurements. By using a flight path intercept nay tee located as technique, takeoff and approach noise measurement stations may be located as necessary to avoid undesirable obstructions. Sideline measurement stations 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 paths that occur during flight testing. Approval has been granted to applicants for the use of data from non-reference noise measurement points provided that measured data are adjusted to reference conditions in accordance with the requirements and limitations of Part 36, Appendices A and B.
d. Test Day Flight Procedures. Airplane weight and flight trajectory can be properly simulated by controlling those parameters most directly related to the airplane/engine noise source strength and directivity. Test day flight procedures should be chosen to provide an adequate definition of the airplane noise source characteristics. The selection of test day flight procedures will depend upon the methodology chosen to derive certification noise levels,
i.e., either by repeating the reference conditions or from an NPD.
(1) Test Day Procedures. Selection of test day flight procedures should be compatible with the methodology chosen to derive certification noise levels. If an applicant chooses to derive certification noise levels by averaging at least six conditions flown at or near reference power and distance, then corrections should account for differences in airplane source noise between test and reference airplane power settings. To qualify these corrections, testing at other operating conditions may be required to define in sufficient detail the airplane source noise levels. This testing may be in addition to the number of measurements at or near reference power necessary to achieve a 90 percent confidence interval not exceeding + 1.5 EPNdB. The applicant may, however, choose to derive the certification noise level directly from a generalized NPD. The NPD is developed by conducting a sufficient number of sound measurements over a range of airplane operating conditions so that the 90 percent confidence interval about a mean line does not exceed + 1.5 EPNdB. To describe a power line an applicant should conduct a minimum of six flights, each at a different power setting, at a common target altitude. To attain the required confidence interval, more than six flights may be necessary. Typically, the power-related parameter which governs noise generation is corrected low pressure rotor speed, N1Vot2' corrected net thrust, Fn/O amb› or EPR, where:
Chap 2 10 Par 9
N1 is the actual low pressure rotor speed, is the ratio of absolute total temperature of the air at the height of the airplane to the absolute temperature of the air for an international standard atmosphere (ISA) at mean sea level (i.e., 288.15°K) EPR is engine pressure ratio.
is the actual engine net thrust per engine, and ambient air at the height of the airplane to ISA air pressure at mean sea is the ratio of absolute static pressure of the level (i.e., 29.92 in HgA or 101.325 kPa).
Simulated Takeoffs and Approaches. Where shown by the applicant to be equivalent to the procedures specified in Part 36, simulated takeoffs and approaches, consisting of flight path intercepts may be used in lieu of starting and ending each flight from a full stop. For takeoff, flight path intercepts consist of intercepting and following the desired climb profile beginning at a target distance above ground level. Approach intercepts maintain the approach path over the measurement location without actually landing. In either case, the airplane is stabilized and following the target trajectory throughout the sound measurement period. (The sound measurement period includes levels within 10 PNdB below the maxímum.) Test airplane weights, speeds, and aerodynamic configurations should be selected to allow achieving target test conditions for those performance parameters having the major effect on noise generation. Approved inflight operable Auxiliary Power Units (APU's) must be on during all approach test conditions.
Target Test Conditions. Target test conditions can be established for These target test conditions will define the selected aerodynamic configuration, airplane weight, flight procedure (such as takeoff flight path intercept), altitude, power, and airspeed at the closest point of approach to the measurement location. For example, a takeoff aerodynamic configuration may be defined by the climb gradient and flap setting. NOIE:
The airplane angle of attack will remain approximately constant for all weights if the tests are conducted at a constant flap setting and the airplane sound measurement period. Speed corrections, however, are generally permitted up to 15 kts. Alternatively, for many airplanes the airplane attitude remains approximately constant for all weights if the tests are conducted at the V, + 10 kts for the maximum takeoff weight. In the execution of the flight for purposes of obtaining sound measurements, the pilot should "set up" the airplane with the selected configuration in order to pass by the measurement location at the target altitude, power, and airspeed.
10. MEASUREMENTS. This section provides guidelines on how to obtain sound measurements and relevant airplane position and performance data necessary for normalizing the measured sound data to conditions differing from the actual test day conditions. All of these are synchronized.
Chap 2 Par 9 11
a. Sound Measurements.
(1) All sound measurements are made with equipment meeting the specification in Part 36. The measurements are made such that, when processed, they will provide one-third octave band sound pressure levels as a function of time for the calculation of EPNL.
The location of each noise measuring station must be surrounded by relatively flat terrain having no excessive sound absorption characteristics, such as tall (in excess of 3 inches) grass, shrubs, or wooded For measurement sites located on the extended runway centerline, a closely mowed grass circle 50 feet in diameter is acceptable. For sideline locations, the grass may be mowed in a 50-foot semicircle facing the line of flight. Any earthen or sandy surface should be reasonably tamped down.
Plowed furrows, silt, or soft powdered surfaces are unacceptable. In the winter time, a 50-foot radius microphone area that is clear of snow may be considered acceptable by the onsite FAA witness when other weather Microphones should be located at the certification reference height of 1.2 m (4 ft) above the ground surface. In some cases, such as for pseudotone identification, it may be desirable to obtain additional data exhibiting free-field spectral shapes by selecting a microphone location to minimize the interference effects of ground reflections on the measured sound spectra (see Appendix 3 of this AC). Free field spectral shapes can be obtained by:
mounting the microphone high above the ground so that the first interference frequency falls below the range of interest. A height of 10 m (33 ft) is generally adequate.
(b) mounting the microphone diaphragm in the plane of, or very close to, a large, acoustically hard (reflective) ground-plane surface so that the direct and reflected waves arrive in phase causing a doubling of the sound pressure. Care must be taken to ensure that the hard surface does not have air pockets beneath the surface or edges that could influence the acoustic measurements.
(4) Orientation of the microphone will depend on the microphone type. Most commonly used are grazing incidence microphones which are designed so that nearly a flat frequency response is maintained at grazing incidence to the microphone; the microphone is oriented to maintain near grazing incidence throughout the airplane flyover. Perpendicular incidence microphones are designed so that maximum sensitivity occurs when they are oriented so that the sound impinges upon the diaphragm in a perpendicular direction when the airplane is at the closest point of approach. Data obtained using perpendicular incidence microphone are acceptable provided that the data are corrected for directivity and frequency response effects and the maximum Noy value for the corrected data is located in the one-third octave band centered at 5000 Hz or less.
Chap 2 12 Par 10
(5) If the airplane sound pressure levels within the sound measurement period do not exceed the background sound pressure levels by at least 10 dB in each one-third octave band, they may be adjusted to eliminate the additive influence of the background sound. When the airplane sound is less than 10 dB, but more than 3 dB above the background, the corrected airplane sound levels are obtained by logarithmically subtracting the background sound levels. For those situations where the airplane levels are less than 5 dB above the background, this influence may be avoided by obtaining measurements at distances closer to the airplane and then adjusting the measurement to the desired distance or by adjusting the measured sound pressure levels using technically supported analytical shaping methods (see Appendix 2 of this AC). NOTE: If the measurements are not adjusted to delete background noise, the resultant EPNL values will be higher than if measured in the absence of background noise.
(6) Maximum sideline noise may be determined by positioning several measurement stations parallel to the airplane flight track as prescribed in Part 36. An alternative to this method may be to use two measurement stations, symmetrically positioned about the nominal flight track, and fly the airplane at several altitudes and thus simulate the data that would have been acquired during a "climb-out" using a series of measurement stations.
Experience suggests that the maximum noise level will occur at an elevation angle near 34 degrees (approximately 1000 ft altitude for airplanes with 1,476 feet [450 meter] sideline measuring stations) and the levels will be rather insensitive within several degrees of this value. When the two-station method is used, the sideline noise for each run is the arithmetic average of the corrected EPNL from each of the two lateral measurement points. The confidence level should be calculated using the method in Appendix 5 (paragraph 2 of this AC).
b. Airplane Position and Performance.
(1) The airplane height and lateral position relative to a fixed location (the nominal flight track along the extended centerline of the runway) should be determined by a method independent of normal flight deck instrumentation. Methods found to be acceptable include radar tracking, theodolite triangulation, laser trajectography, microwave positioning, and photographic scaling techniques (when the simplified method is used). Care must be taken when the photographic scaling technique is used to ensure that the airplane position can be reliably related to the sound measurement positions. In the past, inaccuracies in space positioning have been introduced by using hand-held cameras to take single photographs of aircraft supposedly overhead. The aircraft are often off-center from the reference flight path. Firm mounting on a fixed tripod can eliminate some of the sources of possible error The simplified method for correcting EPNL should be used when continuous tracking is not available.
(2) The airplane position along the flight path can be related to the sound pressure levels recorded at the measurement locations by using synchronized signals. Airplane position should be recorded during the sound measurement period.
Chap 2 Par 10 13
(3) Measurements of airplane/engine performance parameters for calculation of test day performance should be recorded throughout the sound measurement period. These measurements should be time-related to the airplane position along the flight path. Examples of parameters for measurement are airplane attitude, engine power setting, climb angle, and airplane speed.
Flap position and gear position should be fixed and noted for each test condition.
SOUND MEASUREMENT SYSTEM SPECIFICATIONS. The noise certification test should use a quality sound measurement system. Output should be measured in the form of one-third octave band sound pressure levels covering the frequency range from 45 Hz to 11,200 Hz for each one-half second over the sound measurement period. Included in the measurement system should be components that perform the following functions: transform sound pressure waves to electrical signals (microphones), condition the signals, store signals for later analysis, and retrieve and analyze the signals in terms of one-third octave band levels as a function of time.
a. Corrections to compensate for instrumentation non-uniform frequency response characteristics should be determined and applied to the sound pressure level data. These should include response characteristics of
(a) microphone free field and directivity, (b) windscreen insertion loss and directivity, (c) preemphasis, (d) signal conditioning equipment, (e) data storage and retrieval system, and (f) analysis system. All corrections should be identified, documented, and FAA approved.
b. When selecting sound measurement system components, compliance with requirements may be determined from manufacturers' specifications, user calibrations, or a combination of both. When multiple systems of the same type are used, demonstration of compliance of at least one system is satisfactory. However, this does not preclude the need to perform the system calibrations and checks described in Part 36 and paragraph 13 of this AC.
Analysis systems that meet the specifications defined in the following paragraphs have been approved.
12. ANALYSIS SYSTEM. The analysis system may be either analog or digital and provide a root mean square (RMS) sound pressure level in one-half second increments in each of the 24 one-third octave bands having geometric mean frequencies from 50 Hz to 10 kHz. The averaging properties of the integrator
a. For instruments with exponential averaging characteristics, the requirements for Type 1 sound level meter with "slow" response as defined in International Electrotechnical Commission (IEC) Publication No. 651.
b. For true integrating instruments compliance with the "slow" response as defined in IEC 651 may be demonstrated by combining successive readouts from the data processor, such as -3, 0, +1 half-second samples or -2, 0, +2 half-second samples. This combination of readouts is required since the effective "slow" response averaging time is greater than the sampling interval. When a true integration system is used, an acceptable alternative Chap 2 14 Par 10
to combining "successive" readouts is to set the integration time equal to the sampling interval. The use of this type of data reduction system has required the use of the integrated EPNL method.
13. SOUND MEASUREMENT EQUIPMENT CALIBRATIONS AND SYSTEM CHECKS. Part 36 requires calibration of the sound measurement system. This section describes approved procedures to calibrate and check the complete measurement system.
These procedures provide guidance which ensures proper equipment performance over the frequency range from 45 Hz to 11.2 kHz.
a. Frequency Response.
(1) The free-field frequency response characteristics of each microphone are determined by a pressure response calibration (which may be obtained from an electrostatic calibrator) in combination with manufacturer provided corrections or by a free-field calibration in an anechoic facility.
directivity response characteristics of the microphone need to be defined and cover a sufficient angular range to encompass the sound measurement period.
(2) Within 5 days of test the electrical response characteristics of each measurement system, excluding the microphone and windscreen, should be determined at a level within 10 dB of the full-scale reading used during the test, using one of the following input signals: sine waves at the center frequency of each one-third octave band, or pink noise. Sufficient determinations are made to ensure that the overall calibration of the system is known for each test. The calibration signal generator should have been checked within 6 months of the test series with instruments which meet standards set by the National Bureau of Standards.
b. Amplitude An acoustic calibration of the system, including the microphone, should be performed in the field at least once every 5 days. The acoustic calibration should contain a reference signal of known amplitude and frequency in order to provide correlation with the calibrated sound pressure level.
pistonphone operating at a nominal 124 dB and 250 Hz is generally used. In addition, immediately before and after each day's testing, a recorded calibration of the system can be made in the field by injecting a 250 Hz sine wave signal into each microphone preamplifier.
(2) Each magnetic tape reel should contain a reference signal of known amplitude and frequency in order to provide correlation with the calibrated sound pressure level. The reference signal (which represents a known sound pressure level) is generated when the acoustic calibrator is in the proper position over the microphone or an electrical signal of known amplitude is inserted into the tape recorder.
14. COMPUTATION OF EFFECTIVE PERCEIVED NOISE LEVEL (EPNL). SPL data used in compliance demonstrations should be corrected to the airplane and environmental reference conditions prescribed in Part 36. Methods to determine a corrected EPNL at each measurement location for evaluating compliance with Part 36 noise level requirements are provided in this section.
Chap 2 Par 12 15
a. Data Processing. Before computing an EPNL, the measured SPL, meteorological, and airplane position and performance data acquired as described in paragraph 10 should be reviewed to ensure there are no anomalous data. However, it should be noted that Part 36 requires that no data may be omitted without specific FAA approval. Measured data should be processed in accordance with the guidelines provided below.
(1) Sound Pressure Level Data (SPL). Sound pressure levels for each of the one-third octave bands covering the frequency range from 45 Hz to 11200 Hz should be provided for each one-half second over the sound measurement period by analyzing measured signals with equipment that is consistent with the performance specifications defined in paragraph Il.
Corrections for instrument responses, as described in paragraph 13 (such as frequency response), should be taken into account during the data processing.
Measured SPL data may be adjusted to account for the presence of the following undesirable data contamination. (NOTE: If the measurements are not adjusted to delete the presence of contamination the resultant levels will be higher than if measured in the absence of such contamination.)
Extraneous Sound Sources. Contamination of the measured airplane-generated signal may occur occasionally due to the presence of extraneous sound sources. Examples of such sound sources include birds chirping in the vicinity of the microphone or tape anomalies (tape anomalies can be caused by an infrequent, temporary loss of the FM carrier signal).
measured SPLs may be adjusted to reflect the SPL which would have been measured in the absence of the extraneous source. Where appropriate, spurious one-third octave band spectrum levels may be replaced by linear interpolation of adjacent, uncontaminated spectral levels.
Background Noise Corrections. The resultant airplane sound pressure level values, after adjustment, should correspond to the sound pressure levels which would have been measured under test conditions in the absence of any background noise. As discussed in paragraph 10(a) (5) of this AC, sound pressure levels recorded during the airplane flyover and which do not exceed the mean background noise sound pressure levels by at least 3 dB, or the analyzer floor by 1 dB, may be adjusted using a technically supported analytical shaping method. Acceptable methods are provided in Appendix 2.
(c) Pseudotone/Fictitious Tone Identification. Spectral irregularities resulting from ground plane reflections, known as pseudotones, or any spectral irregularities other than those resulting from airplane sound, known as fictitious tones, may be excluded from the tone correction evaluation in the EPNL calculation. Acceptable methods for identifying pseudotones/ fictitious tones are provided in Appendix 3 of this AC.
(2) Airplane Position and Performance Data. Representative airplane position and performance data for the sound measurement period should be computed for each measurement location.
(3) Meteorological Data. The ambient air temperature and relative absorption of sound, are those which are representative of the atmospheric Chap 2 16 Par 14
conditions over the sound propagation path at the time of each sound measurement. An absorption value determined from the meteorological data acquired at 10 m (33 feet) provides an adequate representation of the atmospheric conditions, if the absorption coefficient in the 3150 Hz one-third octave band does not vary more than † 0.5 dB/100 m over the sound propagation path corresponding to the maximum PNLT. When the atmospheric absorption coefficient in the 3150 Hz one-third octave band, over the sound propagation path relative to the PNLTM varies more than + 0.5 dB/100 m, then 'layered" sections of the atmosphere should be used to compute equivalent weighted sound attenuations in each one-third octave band. The arithmetic average of values of temperature and humidity at the top and bottom of each layer are used to represent the sound path atmospheric absorption rate for each layer. (Also see paragraph 8(b)(1) of this AC.)
Test Day EPNL. Using the SPL data, corrected for instrumentation and background noise and processed in a manner described in paragraph 14(a), an EPNL corresponding to test day conditions should be calculated per the method provided in Part 36, Appendix B. To reduce the influence of random spectral irregularities, one or more of the correction methods provided in Appendix 3 of this AC may be used to meet the requirements of Part 36, Appendix B.
Normalized EPNL. Measured SPL data are normalized to the reference ambient conditions specified in Part 36 and to nominal airplane position and engine performance conditions. Nominal airplane position and engine performance data should be derived by using approved aerodynamic performance data at the reference ambient conditions. The measured SPL data should be corrected for the effects caused by differences between test day and nominal conditions. The corrections account for the effects of atmospheric absorption, sound propagation distance, airplane speed, and airplane source noise. A normalized EPNL should be calculated using either the "simplified" or 'integrated" adjustment methods as described in Part 36, Appendix A (section A36.11(f)) for takeoff. The integrated method has been extended to sideline and approach noise measurements for some applications. However, the "simplified" method should generally be used for adjusting sideline noise measurements. Some NPD procedures have required the use of the integrated adjustment method. When an array of sideline microphones located on one side of the airplane flight path is used, the maximum sideline noise level and the associated airplane altitude should be determined from a least squares polynomial regression of sideline noise as a function of airplane altitude.
The regression curve, and thus the maximum sideline noise, is adjusted by an amount equal to one-half the difference between noise levels observed at the symmetric sideline locations. When the airplane is flown past two symmetrical microphones at several altitudes, the arithmetic average of the two microphone levels is used as the datum for each flight.
d. Duration Correction for Simplified Adjustment Method.
calculating a normalized EPNL using the 'simplified" method of adjustment, a duration correction may be applied as follows!
A 2 = -7.5 10g (DM/DR) Chap 2 Par 14 17
where: DM is the minimum distance to the measured flight path for takeoff and approach. For sideline, DM is the measured sound propagation distance associated with maximum PNLT.
DR is the minimum distance to the reference flight path for takeoff and approach. For sideline, DR is the reference sound propagation distance for maximum PNLT.
e. Reference EPNL Evaluation. Reference EPNL values for each of the three certification reference measurement locations and their 90 percent confidence intervals can be produced from the test results and reported.
Guidance on confidence interval computations is provided in Appendix 5 of this AC. No test result should be omitted in the evaluation of the reference EPNL inless specified by the certificating authorities. Applicants should meet the 0 percent confidence interval not to exceed + 1.5 EPNdB as required by section A36.5 (e) (2) of Part 36.
(1) Average of Clustered Flight Conditions. For each measurement location, one method for demonstrating compliance with the 90 percent confidence interval requirements is to conduct a series of sound measurements at prescribed flight conditions. A minimum of six flight conditions must be conducted at or near reference power or at judiciously spaced intervals surrounding the reference power. Measured SPL data for each flight condition should be normalized to reference ambient conditions and to reference airplane position and performance accounting for the effects of atmospheric absorption and sound propagation distance. These normalized values of SPL should be used to compute EPNL. Values of EPNL should be further corrected to account for differences between test and reference airplane speed and source noise.
Source noise level corrections may be derived from approved supplemental data acquired in addition to the test flights clustered around the reference power. The resulting corrected EPNL's for all flight conditions at each single measurement location are arithmetically averaged to derive a single reference EPNL value. (See paragraph 14(e) (2) (b) below when the data are to be used to derive a mean EPNL line for an NPD or for determining sideline noise levels.) The corrections may be derived solely from the test flights provided the test flight conditions were conducted at intervals surrounding the reference power such that the source noise level versus power setting is defined. In this case, the resulting EPNL's at a measurement location are obtained using the NPD methods in the following paragraph.
(2) Noise-Power-Distance Data Base.
An alternate method of demonstrating compliance with the 90 percent confidence interval requirements is achieved by developing an NPD data base. The NPD data base should be developed by conducting a series of flights, each at a different airplane operating condition, while maintaining the same aerodynamic configuration. The measured sound pressure level data for each power condition should be normalized to the reference ambient conditions and specified airplane position and performance. The normalized sound pressure level data are used to compute values of EPNL. For each measurement location, the nominal airplane position and performance will be obtained for each selected aerodynamic configuration. The FPNL at each power Chap 2 18 Par 14
condition is then normalized to an array of minimum distances. All correction techniques in paragraph 14(c) should be applied except the source noise correction which 1s not applicable in the development of the NPD data base.
For each array of corrected EPNL and power values, corresponding to a common airplane to ground minimum distance, a mean line and the 90 percent confidence intervals about the line should be computed and documented. This mean line as shown in Figure 5-1 1n Appendix 5, 1s obtained from a first, second, or third order least-squares polynomial regression analysis. (Note: the choice of the order of curve fit requires FAA approval.) Similarly constructed lines may be used for EPNL versus slant range or altitude.
(c) The reference EPNL value should be derived by entering the NPD data base at the reference location and the aerodynamic performance conditions of power and minimum distance and by applying a duration correction to account for the difference between the nominal airplane speed used in development of the NPD and the reference airplane speed. To minimize errors introduced in the speed correction proçess, hold the variations in test speed around the reference values to less than 15 knots.
(3) Procedures for the Determination of Changes in Noise Noise level changes determined by comparison of flight test data for different configurations of an airplane type have been used to establish certification noise levels of newly derived versions by comparison to the noise levels of the flight datum airplane. These noise changes should be added to or subtracted from the noise levels obtained from individual flights of the "flight datum" airplane.
Chap 2 Par 14 19 (and 20)
CHAPTER 3. SUPPLEMENTAL NOISE DEMONSTRATION METHODS UNDER PART 36 APPENDICES A AND B
15. ALTERNATIVE METHODS. This chapter provides guidance on alternative methods for deriving noise levels in support of Part 36 compliance demonstration for conventional turbofan-powered airplanes that result from a change in type design. The methods addressed are ground static engine testing, supplemental flight testing, analytical adjustment and component testing. Although emphasis is placed on deriving noise increments suitable for adjustment of a Noise-Power-Distance (NPD) data base derived according to the methods outlined in Chapter 2, the guidance provided is also applicable to adjustment of singular noise levels at any of the three Part 36 reference locations. Approval of equivalent procedures for the use of static test information depends critically upon the availability of an adequate approved data base acquired from the flight testing of the flight datum airplane.
each application, the FAA will review the technical validity of the existing certification noise levels/NPD to ensure the data base is sufficient for derivation of certification noise levels for airplanes resulting from a change in type design.
The requirements for approval of acoustical changes are contained in FAR section 21.93(b), not in Part 36. As a result, once a flight test data base is approved for the baseline airplane, it is seldom necessary to employ full Part 36 noise tests to demonstrate compliance for an acoustical change.
In many instances, approval has been based on compliance demonstration methods not requiring flight tests. In these instances noise level increments between two airplane versions were derived using supplemental test and/or analysis techniques. The results of these analyses were used to demonstrate either no acoustical change, or to establish certification noise levels for the changed airplane version. Certification noise levels for a derived version are determined by applying the appropriate noise increments to noise levels already approved for the airplane prior to the design change provided that the data are within the required confidence interval. This adjustment of previously approved noise levels may be made either at the specific Part 36 reference points or, more generally, to an approved NPD for the type design from which Part 36 noise levels may be derived.
b. Noise increments for certain changes in type design can be established by a specific supplementary demonstration method; for example, noise increments due to airplane performance changes may be assessed using an approved NPD analysis. The available supplementary demonstration methods should be reviewed for the most appropriate method based on the guidelines in this AC. The use of these methods for demonstrating noise compliance is discretionary and does not preclude an applicant from conducting a noise demonstration flight test to show compliance.
c. When it has been determined in advance that more than one aircraft configuration may be certificated from common flight test data bases, it is often useful to the applicant to develop and seek FAA approval of an overall certification plan for that family of aircraft. This is referred to as a "family plan." These plans characteristically involve one or more approved NPD data bases and supplementary demonstrations, such as engine static tests.
Chap 3 Par 15 21
3/23/88 AC 36-4B
16. TECHNICAL CONSIDERATIONS. Selection of a supplementary noise demonstration method for a specific change of type design depends upon a number of general technical considerations. The technical items common to each demonstration method are discussed in this section; technical items associated with a particular type of supplemental demonstration method are provided in the respective sections.
a. Examples of Demonstration Applications. The nature of the change in type design may limit the selection of appropriate supplementary noise demonstration method(s). Examples of the application of supplementary methods are provided in Figure 3. The list is not comprehensive, and, for a specific type change, more than one method may be technically valid.
b. Noise Source Characteristics.
(1) For type changes which retain the same basic noise source characteristics as the approved version that has demonstrated, by flight test, compliance with FAR Part 36, certification noise levels may be derived directly from the approved NPD; for example, noise increments resulting from airplane aerodynamic performance differences can be obtained directly from the NPD by using analytical methods described in paragraph 18 of this AC.
(2) The nature of some type changes may or may not constitute a change in the basic airplane noise source characteristics. In such cases, the provided wen approved NPD may be acceptable for deriving noise increments if supporting data or analyses are provided which demonstrate that the noise source characteristics remain unchanged. For a power rating extension, a certification noise level may be directly derived from an NPD, which has been extended using approved analytical techniques.
(3) For type changes which may result in changes to the basic noise source characteristics, modifications to the approved NPD may be incorporated through use of one of the supplementary methods described in this AC. Noise level increments representative of the differences between the reference and changed version should be algebraically added to the approved NPD values. A new engine type or nacelle are examples of changes in type design which may
(4) For some type changes which may result in altering the basic noise source characteristics, adjustments may be made directly to the approved flight levels. These adjustments reflect the noise difference between two nacelle and/or engine configurations and may be applied to the approved flight levels as one-third octave band adjustments or as EPNL adjustments.
Application of this method follows the same procedures as defined for adjusting an NPD data base, except it 1s applicable only to a discrete airplane flight condition.
c. Noise Increment Derivation Using Common Demonstration Method. To ensure derivation of the correct noise increment between two different airplane configurations, the supplemental test and analysis methods used for both the previously approved and changed version must be of the same type.
Chap 3 22 Par 16