airplane is V2 + 15 kt, then by setting the target airspeed at the V 2 + 15 kt appropriate for each test weight, while the actual airspeed will vary according to each test weight, the airplane test angle-of-attack will remain approximately constant. Alternatively, for many airplanes the airplane attitude remains approximately at the attitude associated with the takeoff reference airspeed corresponding to the maximum takeoff weight. Review of these potential airplane sensitivities may dictate the choice of target airspeeds and/or test weights in the test plan in order to limit excessive changes in angle-of-attack or airplane attitude that could significantly change measured noise data. In the execution of each condition, the pilo t should “set up” the airplane in the approp riate condition in order to pass by the noise measurement location within the target height window, while maintaining target power and airspeed, within agreed tolerances, throughout the 10 dB-down period.
A sufficient number of noise measurements are made in order to establish noise-power curves at a given distance for both lateral and flyover cases These curves are extended, either by calculation or by the use of additional flight test data to cover a range of distances, to form the generalized noise database for use in the noise certification of the “flight datum” and derived versions of the airplane type and are often referred to as NPD plots (see Figure 28). If over any portion of the range for the NPD plot, the criteria for calculating the EPNL given in A36.9.1.2 of part 36 requires the use of the integrated procedure, then this procedure must be used for the whole NPD plot. The 90 percent confidence intervals about the mean lines are constructed through the data (see 305 of this AC).
Note.— The same techniques may be used to develop NPD plots that are appropriate for deriving approach noise levels by flying over an under-flight-path microphone for a range of approach powers, using the speed and airplane configuration given in B36.7(c) of part 36 or, more typically, the flight test procedures described in 402(a)(1)(i)(B).
EPNL at a given speed Minimum distance (m) OR FN amb F1 t2 Engine noise performance parameter ( ) Figure 28. Form of noise-power-distance (NPD) plot for jet-powered airplanes The availability of flight test data for use in data adjustment (e.g., speed and height) should be considered in test planning, Page 163
as such availability may limit the extent to which a derived version may be certificated without further flight testing, especially where the effects of airspeed on source noise levels become significant. The effects of high altitude test-site location on jet source noise levels should also be considered in test planning. High altitude test-site locations have been approved under conditions specified in 403(b)(3), provided that jet source noise adjustments are applied to the noise data.
The correction method described in 403(b)(3) has been approved for this purpose.
Flyover, lateral and approach noise measurements should be corrected to the reference speed and atmospheric conditions over a range of distances in accordance with the procedures described in Appendix A of part 36. NPD plots can then be constructed from the adju sted EPNL, power and distances. These plots present the EPNL values for a range of distance and engine noise performance parameters.
The parameters are usually the corrected low pressure rotor speed ሺ ேభ ሻ or the corrected net thrust ሺ ிಿ ఋೌ್ ሻ (see Figure √ఏమ
28), where:
— N1 is the actual low pressure rotor speed;
— ߠ௧ଶ is the ratio of the absolute static temperature of the air at the height of the airplane to the absolute temperature of the air for an ISA at mean sea level (i.e., 288.15 K);
— FN is the actual engine net thrust (power) per engine; and — δamb is the ratio of the absolute static pressure of the ambient air at the height of the airplane to ISA air pressure at mean sea level (i.e., 101.325 kPa).
Generalized NPD data may be used in the certification of the flight-tested ai rplane and derivative versions of the airplane type. For derived versions, these data may be used in conjunction with analytical procedures, static testing of the engine and nacelle, or additional limited flight tests to demonstrate compliance.
402(a)(1)(B)(ii) For flight test procedures for determination of changes in airplane certification noise levels [ETM 4.2.1.1.2.2] 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 reference to the noise levels of the “flight datum” airplane. These noise level changes are added to or subtracted from the noise levels obtained from individual flights of the “flight datum” airplane. Confidence interval s of new data are statistically combined with the “flight datum” data to develop overall confidence intervals (see 305 of this AC).
402(a)(1)(C) De termination of the lateral certification noise levels [ETM 4.2.1.1.3] The lateral full-power reference noise m easurement point for jet-powered airplanes is defined as the point on a line parallel to and 1476 ft (450 m) from the runway center line, where the noise level is a maximum during takeoff.
Alternative procedures using two microph one stations located symmetrically on either side of the takeoff reference track have proven to be effective in terms of time and cost savings. Such an arrangement avoids many of the difficulties encountered when using multiple microphone arrays along the lateral lines. The procedure consists of flying the test airplane at full takeoff thrust (power) at several different specified heights above a track at right angles to and midway along the line joining the two microphone stations. When this procedure is used, matching data from both lateral microphones for each fly-past should be used for the lateral noise determination. Fly-pasts where data from only one microphone are available must be omitted from the determination. The following paragraphs describe this equivalent procedure for determining the lateral noise level for jet airplanes.
a) For airplanes being certificated under Append ix B, two microphone locations are typically used, Page 164
symmetrically placed on either side of, and 1476 ft (450 m) from, the airplane reference flight track.
b) Fly-pasts are performed at constant full takeoff power, configuration and airspeed as described in B36.7(b)(3) and B36.7(b)(4) of part 36.
c) The airplane should be flown along a track that intersects, at right angles, the line joining the two microphones. A number of flights should be performed such that the height of the airplane as it crosses this line typically covers a range between 200 ft and 2000 ft (approximately between 60 m and 600 m).
d) Adjustment of measured noise levels should be made to the acoustical reference day conditions and to reference airplane operating conditions as specified in A36.9 of part 36.
e) If the adjusted noise levels show a reasonable degree of symmetry between the left and right sides, as will generally be the case for jet airpla nes, the arithmetic average of the EPNL r values for the lateral microphone pair should be plotted against either the height of the airplane opposite the microphones or the average of the sound emission heights for PNLTM. A regression curve, which is typically second order, is plotted through the data points. The reported lateral EPNL at the reference condition needed for the purpose of demonstrating compliance with the applicable noise limit is the maximum value of the curve.
f) For airplanes for which the adjusted noise levels exhibit a marked degree of asymmetry, the EPNL r values for the left and right side should be plotted against either the height of the airplane opposite the microphone location or the height at the time of emission of PNLTM. Separate regression curves, which are typically second order, are plotted through the data points for the left and right sides. The reported lateral EPNL at the reference condition (EPNL r) needed for the purpose of demonstrating compliance with the applicable noise limit is the maximum value of the curve midway be tween the left and right curves.
g) It should also be established that the confidence interval associated with the reported lateral EPNL (i.e., the maximum “regression” value of EPNLr) is within the ±1.5 dB 90 percent confidence interval specified in A36.5.4 of part 36 (see 305(b)(2) of this AC).
Note.— Exceptionally, and in order to obtain a curve fr om which a maximum value can be clearly determined, either a third order regression curve or the removal from the analysis of some outlying data points might be permitted.
Applicants will be required to provide technical justification for the use of such exceptional procedures, which will be subject to the approval by FAA Certification lateral noise levels have also been determined by using multiple pairs of laterally opposed microphones rather than only one pair. In this case the microphones must be sufficiently spaced along the lateral line to ensure that the noise levels measured at each microphone are statistically independent. A sufficient number of data points, resulting from a minimum of six runs, must be obtained in order to adequately define the maximum lateral EPNL r value and provide an acceptable 90 percent confidence interval.
Lateral noise measurements for a range of conventionally configured airplanes with under-wing and/or rear-fuselage mounted engines having a bypass ratio of more than two have shown that the maximum lateral noise at full power normally occurs when the airplane is close to 984 ft (300 m) in height during the takeoff. Based on this finding, and subject to the approval by the FAA, the airplane may be flown on a minimum of six acceptable occasions such that it passes the microphone stations at a target height of 984 ft (300 m) while staying within +328 ft, –164 ft (+100 m, –50 m) of this target height.
402(a)(1)(D) Takeoff flyover noise levels with thrust (power) reduction [ETM 4.2.1.1.4] Page 165
Flyover noise levels with thrust (power) reduction may also be established without making measurements during takeoff with full thrust (power) followed by thrust (power) reduction (see 402(a)(2)(a) for details).
402(a)(1)(E) Me asurements at non-reference points [ETM 4.2.1.1.5] In some instances test measurement points may differ from the reference measurement points specified in B36.4 of part 36.Under these circumstances an ap plicant may request approval of data that have been adjusted from actual measurements in order to represent data that would have been measured at the reference noise measurement points at reference conditions. Reasons for requesting approval of such adjusted data may be:
a) to allow the use of a measurement lo cation that is closer to the airplane flight path so as to improve data quality by obtaining a greater ratio of signal to b ackground noise; whereas 306(e) of this AC describes a procedure for removing the effects of background noise, the use of data collected closer to the airplane avoids the interpolations and extrapolations inherent in the method;
b) to enable the use of an existing, approved noise certification database for an airplane type design in the certification of a derivative of that type, when the deri vative is to be certificated under reference conditions that differ from the original type certification reference conditions; and
c) to avoid obstructions near the noise measurement points, which could influence sound measurements; when a flight path intercept technique is being used, flyover and approach noise measurement points may be relocated as necessary to avoid undesirable obstructions; lateral noise measurement points 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 re quirements of A36.9 of part 36 and the magnitudes of the adjustments do not exceed the limitations cited in B36.8(d) of part 36.
402(a)(1)(F) Atmospheric test conditions [ETM 4.2.1.1.6] The FAA has found it acceptable to exceed the sound attenuation coefficient limits of A36.2.2.2(c) of part 36 in cases when:
a) the dew point and dry bulb temper ature are measured with a device which is accurate to ±0.5°C and are used to obtain relative humidity, and when “layered ” sections of the atmosphere are used to compute sound attenuation coefficients in each one-third octave band in co mpliance with the provisions of A36.2.2.3 of part 36, or
b) the peak noy values at the time of PNLT, after ad justment to reference conditions, occur at frequencies of less than or equal to 400 Hz.
402(a)(1)(G) Layering equivalency [ETM 4.2.1.1.7] A36.2.2.3 of part 36 defines the procedure for layering the atmosphere and determining the sound attenuation coefficients to be used in the adjustment of aircraft noise levels. The procedure requires that the atmosphere from the ground to at least the height of the airplane must be divided into layers of 100 ft (30 m) depth. Subject to the approval by FAA, the applicant may use layers of more or less, and not necessarily equal, depth. The applicant should demonstrate that the layering procedure being proposed is equivalent to the procedure defined in part 36.
402(a)(2) Analytical procedures [ETM 4.2.1.2] Analytical equivalent procedures rely upon available noise and performance data obtained from flight tests for the airplane type. Generalized relationships between noise, power and distance (see 402(a)(1)(B)(i) for derivation of NPD plots) and adjustment procedures for speed changes in accordance with the methods of Appendix A of part 36 are combined with certificated airplane aerodynamic performance data to determine noise level changes resulting from type design changes. These noise level increments are then applied to noise levels in accordance with 402(a)(1)(ii).
402(a)(2)(A) Flyover noise levels with thrust (power) reduction [ETM 4.2.1.2.1] Flyover noise levels with thrust (power) reduction may be established from the merging of PNLT versus time measurements obtained during constant power operations. As illustrated in Figure 29, the 10 dB-down PNLT noise time history recorded at the flyover point may contain portions of both full thrust (power) and reduced thrust (power) noise time-histories. As long as these noise time-hist ories, the average engine spool-down thrust (power) characteristics, and the airplane flight path during this period (see Figure 30), which includes the transition from full to reduced thrust (power), are known, the flyover noise level may be computed.
Point of power cutback Spool-down Flyover noise time-history with power cutback 10 dB B alone Fullpower A alone Time PNLT Noise time-history Figure 29. Computation of cutback takeoff noise level from constant power tests Page 167
Noise measuring point Engine spool-down period Cutback power flight path Point of power cutback Full take-off power flight path Flight path over flyover measuring point with power cutback Figure 30. Computation of reduced thrust ta keoff noise level from constant thrust tests Where the full thrust (power) portion of the noise time history does not intrude upon the 10 dB-down time history of the reduced thrust (power), the flyover noise levels may be computed from a knowledge of the NPD characteristics and the effect of the average spool-down thrust (power) characteristics on the airplane flight path.
Note 1.— The selection of the height of an airplane within the reference flight path for initiation of thrust (power) reduction should take into account both the “average engine” spool-down time and a 1.0-second delay for flight crew recognition and response prior to movement of the throttles to the reduced thrust (power) position.
Note 2.— To ensure that the full thrust (power) portion of the noise-time history does not intrude upon the 10 dB- ܶܮܰܲܯܶܮܰܲ down noise levels, ܾ݇ܿܽݐݑܿݎ݁ݐ݂ܽ െ ݁ݎ݂ܾ݁ ܾ݇ܿܽ݁ݐݑܿ 10.5ܤ݀ .
402(a)(2)(B) Equivalent procedures based upon analytical methods [ETM 4.2.1.2.2] Noise certification approval has been given for applications based on type design changes that result in predictable noise level differences, including the following:
a) changes to the originally certificat ed takeoff or landing weight, which in turn lead to changes in the distance between the airplane and the microphone and/or reduced thrust (power) for the takeoff case, and changes to the approach power; in this case the NPD data may be used to determine the certification noise level of the derived version;
b) noise changes due to engine power changes; however, care should be taken to ensure that when NPD plots are extrapolated, the relative contri bution of the component noise sources to the EPNL remains essentially unchanged and a simple extrapolation of the NPD curves can be made; among the items which should be considered in extending the NPD are:
1) the 90 percent confid ence interval at the extended thrust (power);
2) airplane /engine source no ise characteristics and behavior;
3) engine cycle changes; and Page 168
4) quality of data to be extrapolated;
c) airplane engine and nacelle configuration and acoustical treatment changes, usually leading to changes in the values of EPNLr of less than 1 dB;
Note 1.— It should however be ensured that new noise sources are not introduced by modifications made to the airplane, engine or nacelles. A validated analytical noise model approved by the FAA may be used to derive predictions of noise increments. The analysis may consist of modelling each airplane component noise source and projecting the sources to flight conditions in a manner similar to the static test procedure described in 402(a)(3)(D). A model of detailed spectral and directivity characteristics for each airplane noise component may be developed by theoretical and/or empirical anal ysis. Each component shoul d be correlated to the parameters which relate to the physical behavior of source mechanisms. The source mechanisms, and subsequently the correlating parameters, should be identified through use of other supplemental tests such as engine or component tests. As described in 402(a)(3)(D), an EPNLr value representative of flight conditions should be computed by adjusting airplane component noise sources for forward speed effects and for the number of engines and shielding, reconstructing the total noise spectra, and projecting the total noise spectra to flight conditions by accounting for propagation effects. The effect of changes in acoustic treatment, such as nacelle lining, may be modeled and applied to the appropriate component noise sources. The computation of the total noise increments, the development of the changed version NPD, and the evaluation of the changed version EPNLr values should be made by using the procedures described in 402(a)(3)(D). Guidance material on confidence interval computations is provided in 305.
Note 2.— Some engine or aircraft design changes may change the nature of the noise signature of the engine or aircraft and prevent an ordered extension of noise data. In this case, static engine noise tests may not provide adequate proof that an extension of the noise database is valid. Changes that may cause transition of the fan tip velocity to supersonic, interaction of the mid-span fan shrouds or stator vanes, choking of the fan exit guide vanes or primary compressor entrance, operation of the surge bleed valve, an increase in the inlet bypass airflow or a change in aircraft configuration interaction are examples of such changes.
d) airframe design changes (e.g., changes in fuselage le ngth, flap configuration and engine installation) that could indirectly affect noise levels because of an effect on airplane performance (e.g., increased drag).
Note 1.— Changes in airplane performance characteristics derived from aerodynamic analysis or testing have been used to demonstrate how these ch anges affect the airplane flight path and hence the demonstrated noise levels of the airplane.
Note 2.— In these cases care should be exercised to ensure that the airframe design changes do not introduce significant new noise sources or modify existing source generation or radiation characteristics. In such instances the magnitude of such effects may have to be established by test.
402(a)(2)(C) Equivalent procedure for ca lculating the certification noise levels of weight variants of a given airplane type [ETM 4.2.1.2.3] Section 901 of Chapter 9 of this AC speci fies that “In the case of an airplane be ing recertificated to the standard of part 36, Amendment 24, noise certification should be granted on the basis that the evidence used to determine compliance is as satisfactory as the evidence expected of a new type design.” The lateral, flyover and approach noise levels and their 90 percent confidence intervals for the weight variants of a given airplane /engine model and acoustic configuration are typically derived from generalized NPD curves, based on the information in noise certification reports and supporting documentation, and in conjunction w ith certificated aerodynamic performance data for the airplane, as approved by FAA.
Some airplane manufacturers have used the noise level information initially certificated for several weight variants to demonstrate that when the basic airplane performance parameters (e.g., V2 and VREF ) vary in a linear manner over a range of certificated takeoff or approach weight, the resulting noise (EPNL) versus weight relationship can be shown Page 169
to be linear in that range as well. When this situation is demonstrated by the applicant and, subject to the approval by FAA, the applicant may derive the certification noise levels of additional weight variants using linear interpolation between previously certificated points. The confidence interval for the interpolated weight is then to be established in a process that utilizes the polynomial regression models that had been used by the applicant to develop the NPD. For a given airplane type, equivalency of the interpolated noise level values is demonstrated when the noise levels and the associated confidence intervals are calculated and reported in a manner acceptable to the FAA.
402(a)(3) Static engine noise tests and projections to flight noise levels [ETM 4.2.1.3] 402(a)(3)(A) General [ETM 4.2.1.3.1] Static engine noise test data provide valuable definitive information for deriving the noise levels that result from changes to an airplane powerplant or from the installation of a broadly similar powerplant into the airframe following initial noise certification of the “flight datum” airplane. This involves the testing of both the “flight datum” and derivative powerplants using an open-air test facility where the effect on the noise spectra of the engine modifications on airplane noise characteristics may be assessed. It can also extend to the use of component test data to demonstrate that when minor development changes have been made the noise levels remain unchanged (see section 101(d) of this AC).
Approval of equivalent procedures for the use of static engine noise test data depends critically upon the availability of an adequate approved database (NPD plot) acquired from the flight testing of the “flight datum” airplane.
Static engine noise tests can provide sufficient additional data or source noise characteristics to allow for predictions about the effect of changes on the airplane certification noise levels.
Types of static tests accepted for the purposes of demons trating certification compliance in airplane development include engine noise tests. Such tests are useful for assessing the effects on the individual noise sources of mechanical and thermodynamic cycle changes to the engine. Such configuration and/or design changes often occur as engines are developed, subsequent to the initial nois e certification of an aircraft, to eas e production difficulties, reduce cost, improve durability, and for operational reasons.
Static engine noise testing is discussed in detail in subsequent sections. For component tests, the criteria for acceptability are less definable. There are many instan ces, particularly when only small changes in EPNL r are expected, where component testing will provide an adequate demonstration of noise impact. Examples of such changes include:
a) changes in the specification of sound-abs orbing linings within an engine nacelle;
b) changes in the mechanical or aerodynamic design of the fan, compressor or turbine;
c) changes to combustor designs, including material changes;
d) changes to bleed valves; and
e) changes to the exhaust system.
Each proposal by an applicant to use component test data should be considered by the FAA with respect to the significance of the relevant affected source on the values of EPNLr for the airplane that is being certificated.
402(a)(3)(B) Limitation on the projection of static-to-flight data [ETM 4.2.1.3.2] Page 170
Guidance on the acceptability, use and applicability of static engine test data are contained in subsequent sections.
The amount by which the measured noise levels of a derivative engine will differ from those of the reference engine is a function of several factors, including:
a) thermodynamic changes to th e engine cycle, including increases in thrust (power);
b) design changes to major components (e.g., the fan, compressor, turbine, exhaust system); and
c) changes to the nacelle.
Additionally, day-to-day and test-site to test-site variables can influence measured noise levels, and therefore the test, measurement and analysis procedures described in this AC are designed to account for these effects. A limit is needed that can be used uniformly by FAA in order that the degree of change resulting from aspects such as a), b) and c), when extrapolated to flight conditions, is restricted to acceptable amounts before a new flight test is required.
The recommended guideline for this limit is that the summation of the magnitudes, neglecting signs, of the noise changes for the three reference certification conditions between the “flight datum” airplane and the derived version at the same thrust (power) and distance for the derived version is no greater than 5 EPNdB, with a maximum of 3 EPNdB at any one of the reference conditions (s ee Figure 31). For differences greater than this, additional flight testing at conditions where noise levels are expected to change is recommended in order to establish a new flight NPD database.
Provided that the detailed prediction procedures used are verified by flight tests for all the types of noise sources (i.e., tones, non-jet broadband and jet noise relevant to the airpla ne under consideration) and th at there are no significant changes in installation effects between the airplane used for the verification of the prediction procedures and the airplane under consideration, the procedure may be employed without the limitations described above.
In addition to the limitations described above, a measure of acceptability regarding methodologies for static-to-flight projection is also needed for uniform application by FAA. This measure can be derived as the residual NPD differences between the flight test data and the projected static-to-flight data for the original airplane version. The guideline for a measure of acceptability is to limit these residual differences to 3 EPNdB at any one of the reference conditions.
In determining the noise levels of the modified or derived version, the same analytical procedures used in the first static-to-flight calculations for the noise certification of the airplane type must be used.
Derivative aeroplane L, T or A Derivative aeroplane µ requirement Derived by extrapolation of datum flight-projected NPD where necessary Flight-projected EPNL from static test Engine noise performance parameter Definitions L = Difference between flight datum and derivative aeroplane EPNL at the power requirement of the derivative aeroplane at the lateral condition.
T = Difference between flight datum and derivative aeroplane EPNL at the power requirement and altitude of the derivative aeroplane at the take-off condition.
A = Difference between flight datum and derivative aeroplane EPNL at the power requirement of the derivative aeroplane at the approach condition.
Limitations
(i) |L| + | T| + | A| shall not exceed 5 EPNdB.
i) L T or A shall not exceed ± 3 EPNdB individually. (i , Figure 31. Limitation on use of static test when no validating flight data exist 402(a)(3)(C) Static engi ne noise test procedures [ETM 4.2.1.3.3] 402(a)(3)(C)(i) General [ETM 4.2.1.3.3.1] Test restrictions defined for flight testing in conformity with part 36 are not necessarily appropriate for static testing.
Reference T13 provides appropriate, detailed guidance for measurement of far-field sound pressure levels during static operation of gas turbine engines installed on an outdoor test stand. The following sections provide guidelines supplemental to sections on test site characteristics, data acquisition and reduction, microphone types, locations and installations provided in Reference T13. These supplemental guidelines are specifically related to static engine noise testing for purposes of airplane noise certification.
Noise data acquired from static tests of engines with similar designs to those that were flight tested may be projected to flight conditions when appropriate. Once approved, noise data acquired from static tests may be used to supplement an approved NPD plot for the purpose of demonstrating compliance with the part 36 provisions in support of a change in type design. The engine designs, as well as the test and analysis techniques to be used, should be presented in the test plan and submitted to the FAA for approval prior to testing.
402(a)(3)(C)(ii) Test site characteristics [ETM 4.2.1.3.3.2]
(1) Inflow control devices (ICD) Static engine noise test data for the noise certification of an airplane with a change of engine to another one of a Page 172
similar design should be acquired by using an approved ICD for high bypass engines (i.e., BPR > 2.0). The ICD should meet the requirements in Reference T13. The speci fic ICD hardware should be inspected by the FAA to ensure that the ICD is free from damage and contaminants that may affect its acoustic performance.
(2) ICD calibration An acceptable ICD calibration method is provided in Reference T13. In some cases large fluctuations in the value of the calibrations across adjacent one -third octave bands and between cl osely spaced angular positions of microphones can occur. These fluctuations can be related to reflection effects caused by the calibration procedure, and care must be taken to ensure that they do not introduce or suppress engine tones. This may be done by comparing EPNL computed with:
a) the ICD calibrations as measured;
b) a mean value of the calibration curves; and
c) the calibration values set to zero.
402(a)(3)(C)(iii) Data acquisition and reduction systems [ETM 4.2.1.3.3.3]
(1) Data acquisition, analysis and normalization Data acquisition and analysis systems used for static test and the modus operandi of the test program may well vary according to specific test objectives. In general they should conform to those outlined in Reference T13.
For each engine power setting designated in the test plan , the engine performance, meteorological and sound pressure level data should be acquired and analyzed using measurement systems and test procedures described in Reference T13 or as approved by the FAA.
A range of static engine operating conditions should be selected to correspond to the expected maximum range of in-flight engine operating conditions for the appropriate engine power setting parameter. A sufficient number of stabilized engine power settings over the desired range should be included in the test to ensure that the 90 percent confidence intervals for values of flight-projected EPNL can be established (see 305(c)).
Noise measurements should be normalized to consistent conditions and include 24 one -third octave band sound pressure levels between band center frequencies of 50 Hz to 10 kHz for each measurement microphone. Before projecting the static engine data to flight conditions, the sound pressure level data should be adjusted for:
a) the frequency response characteristic s of the noise measurement system; and
b) contamination by background or electrical system noise (see 306(c) of this AC).
(2) Data system compatibility If more than one data acquisition system and/or data analysis system is used for the acquisition or analysis of static data, compatibility of the airfra me and engine manufacturers’ system s is necessary. Compatibility of the data acquisition systems can be accomplished through appropriate calibration. Compatibility of the data analysis systems can be verified by analyzing the same flight test noise data samples on both systems. The systems can be considered to be compatible if the resulting differences are no greater than 0.5 EPNdB. Evaluation should be conducted at flight conditions representative of those for certification.
The use of measured static engine test noise data or pseudo-random noise signals with spectral shape and tonal content representative of turbofan engines is an acceptable alternative to the use of actual flight test noise data Page 173
samples for determination of analysis system compatibility. The systems can be considered to be compatible if the resulting differences are no greater than 0.5 PNdB for an integration time of 32 seconds.
402(a)(3)(C)(iv) Microphone types, locations and installations [ETM 4.2.1.3.3.4]
(1) Microphone locations Microphones should be located over an angular range sufficient to include the 10 dB-down times after projection of the static noise data to flight conditions. The general guidance in Reference T13 describing microphone locations is sufficient to ensure adequate definition of the engine noise source characteristics.
The choice of microphone location with respect to the test surface depends on the specific test objectives and the methods to be used for data normalization. Certification experience with static engine testing has been primarily limited to microphone installations near the ground or at engine centerline height. In general, because of the difficulties associated with obtaining free-field sound pressure levels that are often desirable for extrapolating to flight conditions, near-ground-plane microphone installati ons or a combination of ground-plane and elevated microphones have been used. Consistent microphone locations, heights, etc., are recommended for noise measurements of both the prior approved and changed version of an engine or nacelle.
(2) Microphone installations Where ground-plane microphones are used, special precautio ns as outlined in Reference T13 are necessary to ensure that consistent measurements (e.g., free from “ac oustic shadowing” refraction effects) will be obtained.
When there is a wind in the opposite direction to the sound wave propagating from the engine, or when there is a substantial thermal gradient in the test arena, refraction can influence near-ground-plane microphone measurements to a larger degree than measurements at greater heights.
402(a)(3)(D) Projection of static engine data to airplane flight conditions [ETM 4.2.1.3.4] 402(a)(3)(D)(i) General [ETM 4.2.1.3.4.1] The static engine sound pressure level data acquired at eac h angular location should be analyzed and normalized to account for the effects identified in the pa ragraphs below. They should then be projected to the same airplane flight conditions used in the development of the approved NPD plot.
As appropriate, the projection procedure includes the:
a) effects of source motion including Doppler effects;
b) number of engines and shielding effects;
c) installation effects;
d) flight geometry;
e) atmospheric propagation, including spherical wave divergence and sound attenuation; and
f) flight propagation effects, including ground reflection and lateral attenuation.
To account for these effects, the meas ured total static noise data should be analyzed to determine contributions from individual noise sources. After projection of the one-third octave band spectral data to flight conditions, Page 174