requirements, and in H36.3(a) of Appendix H of part 36 for helicopters, as a homogenous atmosphere with temperature of 77°F (25°C), and relative humidity of 70%;
d) the series of sound propagation distances (in metres) between the aircraft and the microphone at the time of emission of each noise data record, SR(k), as determined in 403(a)(2);
e) the series of reference sound pr opagation distances (in meters) betw een the aircraft and the reference microphone at the time of emission of each reference-condition noise data record, SRR(k), as determined in 403(a)(2); and
f) the series of effective durations, δtR(k) for the series of reference-condition noise data records, k, as determined in 403(a)(2).
403(a)(4)(B) Adjustmen t of sound pressure levels to reference conditions [ETM 4.3.1.4.2] Adjust each measured test-day one-third octave band SPL in the spectral time history for the effect of spherical spreading (also known as “inverse square law”) and the effect of the change in sound attenuation due to atmospheric absorption for differences between the test and reference flight paths, using the equation provided in A36.9.3.2.1 of part 36:
For all of the one-third octave bands, i, in each of the measured test-day acoustic data records, k:
SPLR(i,k) = SPL(i,k) + 0.01 [α(i) – α(i)0] SR(k) + 0.01 α(i)0 (SR(k) – SRR(k)) + 20 log (SR(k) / SRR(k)) Note.— Sufficient spectra in the measured test-day spectral time history should be adjusted to encompass the integrated reference-condition EPNL noise duration, which may exceed the test-day EPNL noise duration due to adjustments to SPLs that may result in different first and last 10 dB-down points being selected.
403(a)(4)(C) Determi nation of reference-condition tone-corrected perceived noise level time history [ETM 4.3.1.4.3] Calculate the reference-condition tone-corrected perceived noise level, PNLTR(k) for each adjusted spectrum, k, in the spectral time history using the procedures specified in A36.4.6 of part 36, including calculation of perceived noise level, PNLR(k) and tone-correction factor, CR(k).
403(a)(4)(D) Identification of integrated reference-condition maximum tone-corrected perceived noise level [ETM 4.3.1.4.4] Using the reference-condition time history of PNLT R(k) obtained in 403(a)(4)(C), identify the record at which the maximum level occurs, PNLT R(kM), and determine and apply the integrated reference-condition bandsharing adjustment, ΔBR, as described in A36.4.4.2 of part 36, to obtain the bandsharing-adjusted reference PNLTMR:
PNLTMR = PNLTR(kM) + ΔBR Note that due to adjustments for differences between test-day and reference conditions, it is possible that the acoustic data record at which PNLTM R occurs will be different from the record at which the measured test-day PNLTM occurred. It is also possible that the bandsharing adjustment determined from the integrated reference-condition data set. ΔBR, will be different from the bandsharing adjustment determined from the measured test-day data set, ΔB.
403(a)(4)(E) Identification of integrat ed reference-condition EPNL Noise Duration [ETM 4.3.1.4.5] Using the reference-condition time history of PNLT R(k) obtained in 403(a)(4)(C), but including the bandsharing - adjusted value for PNLTM R obtained in 403(a)(4)(D), identify the limits of the reference-condition EPNL noise duration in accordance with A36.9.4.3.1 of part 36.
Note that due to adjustments for diffe rences between test-day an d reference conditions, the first and last referencecondition 10 dB-down points, kFR and kLR will quite likely be different from the first and last 10 dB-down points for the measured test-day EPNL noise duration, kF and kL.
403(a)(4)(F) Summation of integrated reference-condition EPNL [ETM 4.3.1.4.6] Compute the integrated reference-conditi on effective perceived noise level, EPNL R, by summing the PNLT R(k) “energy” between the limits, kFR and k LR, of the integrated reference-conditio n EPNL noise duration, obtained in
4.3.1.4.5, as follows:
ܮܰܲܧோ ൌ 10logሾሺ ଵ ሻ ∑ಽೃಷೃሺ10.ଵே்ೃሺሻሻሺݐߜோሺ݇ሻሻ ሿ where:்బ — the series of PNLT R(k) values is obtained in 4.3.1.4.3 (substitu ting as necessary the bandsharing-adjusted value of PNLTMR in place of the value for PNLTR(kM)); and — the effective durations, δtR(k), is obtained in 4.3.1.2.
In practice the adjusted EPNL value takes into account the contributions of sound energy associated with each individual time increment (record), and is calculated from a summation from the first 10 dB-down point, kFR, to the last 10 dB-down point, kLR. Note that due to the effects of the adjustment for differences between test and reference conditions the effective duration, δtR(k), associated with each record, is not likely to be uniform. Table 14 provides an example of how this calculation may be performed.
403(a)(4)(G) Worked example calculat ions illustrating the integrated method [ETM 4.3.1.4.7] Table 14. Example calculation of adjusted EPNL value when using the integrated method of adjustment.
Labels PNLT R(k) δtR(k) s “Energy”= ሺ10.ଵே்ೃሺሻሻሺݐߜோሺ݇ሻ
- 84.62 0.3950 -
- 85.84 0.3950 -
- 85.37 0.3951 - kFR 88.57 0.3951 284254291.2
- 88.82 0.3952 301173624.8
- 88.03 0.3953 251146317.4
- 88.76 0.3954 297191692.3
- 87.06 0.3956 201027875.5
- 86.92 0.3957 194700044.3
- 90.39 0.3960 433206721.0 Page 210
“Energy”= Labels PNLT R(k) δtR(k) s ሺ10.ଵே்ೃሺሻሻሺݐߜோሺ݇ሻ
- 89.89 0.3963 386388393.4
- 91.00 0.3967 499415710.9
- 90.08 0.3973 404686358.5
- 89.71 0.3981 372384998.9
- 89.61 0.3992 364914006.0
- 90.21 0.4009 420761559.6
- 91.14 0.4033 524358390.8
- 92.10 0.4066 659427985.6
- 93.68 0.4108 958584572.0
- 94.89 0.4153 1280447955.7
- 95.87 0.4196 1621195835.7
- 97.06 0.4231 2150022601.5 PNLTM 97.40 0.4256 2338845959.1
- 96.23 0.4273 1793630138.6
- 94.73 0.4285 1273358894.6
- 92.30 0.4294 729225824.4
- 88.75 0.4299 322379520.6 kLR 86.96 0.4304 213733335.2
- 85.41 0.4307 -
- 83.88 0.4309 -
- 83.01 0.4311 -
- - Total energy 18276462607.5
- EPNL = 10 log (total energy) – 10 92.61892 403(b) Jet Airplanes [ETM 4.3.2] 403(b)(1) Control of nois e certification computer program software and documentation related to static-to-flight projection processes [ETM 4.3.2.1] 403(b)(1)(A) General [ETM 4.3.2.1.1] Procedures for computer program software control must be developed, approved by the FAA, and maintained and adhered to by each applicant utilizing static-to-flight equivalencies (SFEs).
The procedures must consist of four key elements which, when implemented by the noise certification applicant, will result in documentation which properly describes and valid ates the applicable SFE noise certification computer program and data output. Throughout the development of a given airplane type, adherence to these procedures will enable the tracking of critical computer programs in orde r to verify that the initial software design has not been changed without substantiation.
The four key elements of configuration index, software control plan, design description, and verification process are described in 403(b)(1)(ii).
403(b)(1)(B) Software control procedures — four key elements [ETM 4.3.2.1.2] 403(b)(1)(B)(i) Configuration index [ETM 4.3.2.1.2.1] A configuration index must be established for each unique SFE software system. It will include all applicable elements of the software system and provide historic tracking of documents and software under control. Where appropriate, the index may be maintained in a general database.
403(b)(1)(B)(ii) Software control plan [ETM 4.3.2.1.2.2] A procedure for SFE software change management must be established that includes the baseline design identification, a software change control system and a method of reviewing and auditing software changes and maintaining a status accounting of changes.
Control of software changes must be maintained by establishing baselines within the verification process described below and by documenting modifications to the baseline case that result from program coding changes. Review and auditing procedures will be established within the verificat ion process to allow the validity of the program coding changes for the “modified” configuration to be assessed relative to the “baseline” configuration.
The configuration index must be updated to reflect, historically, the changes made to the software system.
403(b)(1)(B)(iii) Design description [ETM 4.3.2.1.2.3] A technical description of the methods used to accomplish the SFE certification must be provided, including an overview and a description of the soft ware system design to accomplish the t echnical requirements. The software design description should include the program structure, usage of subroutines, program flow control and data flow.
403(b)(1)(B)(iv) Verification process [ETM 4.3.2.1.2.4] The validation process for the SFE software system, or modifications to it, must include a procedure to verify that the calculations described in the documentation are being performed properly by the software. The process may include manual calculations compared to computer output, stepwise graphical displays, software audits, diagnostic subroutines that generate output of all relevant variables associated with the modifications, or other methods to establish confidence in the integrity of the software. The process results must be monitored and tracked relative to software calculation changes.
403(b)(1)(C) Applicability [ETM 4.3.2.1.3] Although the software control plan is applicable to all SFE-specific computer program software and documentation established through the specific procedur es and processes of each applicant, it may not be necessary to review and audit ancillary software such as, but not limited to, subroutines dealing with the sound attenuation coefficients for the effects of atmospheric absorption, noy calculations and tone corrections for each main program source code change.
403(b)(2) Identification of spectral irregularities [ETM 4.3.2.2] Page 212
403(b)(2)(A) Introduction [ETM4.3.2.2.1] Spectral irregularities that are not produced by aircraft noise sources may cause tone corrections to be generated when the procedures of A36.4.3 of part 36 are used. These spectral irregularities may be caused by:
a) the reflected sound energy from th e ground plane beneath the microphone mounted at 4 ft (1.2 m) above it, interfering with the direct sound ener gy from the aircraft (The reinforcing and destructive effects of this interference are strongest at lower fre quencies, typically 100 Hz to 200 Hz and diminish with increasing frequency. The local peaks in the one-third octave spect ra of such signals are termed pseudo-tones; above 800 Hz this interference effect is usually insufficient to generate a tone correction when the part 36 tone correction procedure is used.);
b) small perturbations in the propagation of aircraft noise when analyzed with one-third octave bandwidth filters; or
c) the data processing adjustments such as the background noise adjustment method and the adjustment for sound attenuation due to atmospheric absorption. (In the case of the latter, the sound attenuation coefficients, a, given in Reference T16 ascribe values at 4 kHz to the center frequency of the one-third octave band whereas at 5 kHz the value of “a” is ascribed to the lower pass frequency of the one-third octave. This difference is sufficient in some cases to generate a tone correction).
The inclusion of a tone correction factor in the computa tion of EPNL accounts for the subjective response to the presence of pronounced spectral irre gularities. Tones generated by aircraft noise sources are those for which the application of tone correction factors is appropriate. Tone correction factors that result from spectral irregularities (i.e., false tones produced by any of the causes cited above) may be disregarded. This section describes methods which have been approved for detecting and removing the effects of such spectral irregularities. Approval of the use of any of these methods however remains with the FAA.
403(b)(2)(B) Methods for identifying false tones [ETM 4.3.2.2.2] 403(b)(2)(B)(i) Frequency tracking [ETM 4.3.2.2.2.1] Frequency tracking of flyover noise data is useful for th e frequency tracking of spectral irregularities. The observed frequency of airplane noise sources decreases continuousl y during the flyover due to Doppler frequency shift, fDOPP, where:
݂ை ൌ ଵିெ௦ఒ, where:
— f is the frequency of the noise at source;
— M is the Mach number of the airplane; and — λ is the angle between the flight path in the direc tion of flight and a line connecting the source and observer at the time of sound emission.
Reflection-related effects in the spectra (i.e., pseudo-tones) decrease in frequency prior to, and increase in frequency after, passing overhead or abeam the microphone. Spectral irregularities caused by perturbations during the Page 213
propagation of the noise from the airplane to the microphone tend to be random in nature , in contrast to the Doppler effect. These differing characteristics can be used to separate source tones from false tones.
403(b)(2)(B)(ii) Narrow-band analysis [ETM 4.3.2.2.2.2] Narrow-band analysis with filter bandwidths narrower than those of one-third octave is useful for identifying false tones. For example, when the analysis is produced such that the spectral noise levels at an instance are presented in terms of image intensity on a line, the overall flyover analysis clearly indicates the Doppler-shifted airplane tones and those due to reflection as described above.
403(b)(2)(B)(iii) Microphone mounting height [ETM 4.3.2.2.2.3] Comparison of one-third octave spectra of measurements taken using the 4 ft (1.2 m) high microphone and corresponding data obtained from a neighboring microphone mounted flush on a hard reflecting surface (a configuration similar to that described in A36.4.4 of part 36 or at a height substantially greater than 4 ft (1.2 m), such as 33 ft (10 m), may be used to identify false tones. Changes to the microphone height alter the interference spectra irregularities from the frequency range of data from the 4 ft (1.2 m) high microphone, and when a comparison is made between the two data sets collected at the same time, noise source tones can be separated from any false tones which may be present.
403(b)(2)(B)(iv) Inspection of noise time-histories [ETM 4.3.2.2.2.4] Spectral irregularities which arise following data adjustment as described in this section will occur in the frequency range of between 1 kHz to 10 kHz, and the resulting false t one corrections will normally vary in magnitude between
0.2 dB to 0.6 dB. Time-histories of PNLs and PNLTs, which exhibit constant level differences, are often indicative of the presence of false tone corrections. Supplementary narrow-band analysis is useful in demonstrating that such tone corrections are not due to airplane -generated noise.
403(b)(2)(C) Treatment of false tones [ETM 4.3.2.2.3] When spectral irregularities give rise to false tones that ar e identified by, for example, the methods described in this section their values, when computed according to Step 9 of the tone correction calculation described in A36.4.3 of part 36, may be set to zero.
403(b)(3) Noise data adjustments for tests at high altitude test sites [ETM 4.3.2.3] 403(b)(3)(A) Introduction [ETM 4.3.2.3.1] Jet noise generation is somewhat suppressed at higher altitudes due to the difference in the engine jet velocity and jet velocity shear effects resulting from the change in air density. The use of a high altitude test site for the noise test of an airplane model that is primarily jet-noise dominated should include making the following adjustments. These jet source noise adjustments are in addition to the standard pistonphone barometric pressure adjustment of about 0.3 dB/1000 ft
(0.1 dB/100 m) which is normally used for test sites not approximately at sea level. The jet source noise adjustments are applicable to tests conducted at sites at or above 1200 ft (366 m) mean sea level (MSL).
403(b)(3)(B) Jet source noise adjustment [ETM 4.3.2.3.2] Flight test site locations at or above 1200 ft (366 m) MSL, but not above 4000 ft (1219 m) MSL, may be approved Page 214
provided certain criteria are met (see Figure 46) and the source noise adjustments are applied.
Alternative criteria or adjustments require the approval by the FAA.
403(b)(3)(B)(i) Criteria [ETM 4.3.2.3.2.1] Jet source noise altitude adjustments are required for each one-half-second spectrum when using the integrated procedure and, for the PNLTM spectrum, when using the simplified procedure (see A36.9.3 and A36.9.4 of part 36) and are to be applied in accordance with the criteria described in Figure 46.
B17 B23 B27 100% 100% 0% 0% Applied correction Applied correction 60 90 120 150 Acoustic angle — degrees Integrated method Simplified method One-third octave frequency band number Figure 46. Criteria for jet source noise correction 403(b)(3)(B)(ii) Adjustment procedures [ETM 4.3.2.3.2.2] An acceptable jet source noise adjustment is as follows:
a) adjust each one-half-second spect rum or PNLTM one-half-second spectru m, as appropriate, in accordance with the criteria of 403(b)(3)(ii)(A) by using the following equation:
ΔSPL = [10 log (dR/dT) + 50 log (cT/cR) + 10k log (uR/uT)] [F1] [F2] w h e r e :
— subscript T denotes conditions at the actual airplane test height above MSL under standard atmospheric conditions (i.e., ISA + 18°F (ISA + 10ºC) and 70 percent relative humidity);
— subscript R denotes conditions at the airplane refe rence height above MSL (i.e., airplane test height above MSL minus the test-site altitude) under standard atmospheric conditions (i.e., ISA + 18°F (ISA + 10ºC) and 70 percent relative humidity);
— SPL denotes sound pressure level;
— dR is the density for standard atmosphere at the airplane reference height in lb/ft3 (kg/m3) ;
— dT is the density for standard atmosphere at the airplane test height in lb/ft3 (kg/m3);
— cR is the speed of sound corresponding to the absolu te temperature for a standard atmosphere at the airplane reference height in ft/s (m/s) — cT is the speed of sound corresponding to the absolu te temperature for a standard atmosphere at the airplane test height in ft/s (m/s) Page 215
— k = 8, unless an otherwise empirically derived value is substantiated;
— u = (ve – va) is the equivalent relative jet velocity in ft/s (m/s);
w h e r e :
— ve is the equivalent jet velocity as defined in Appendix C of Reference T12 and obtained from the engine cycle deck in ft/s (m/s);
— va is the aircraft velocity in ft/s (m/s);
— uR is the equivalent relative jet velocity in ft/s (m/s) where ve is determined at N1C TEST for standard atmosphere at the airplane reference height;
— uT is the equivalent relative jet velocity in ft/s (m/s) where v e is determined at N1C TEST for standard atmosphere at the airplane test height;
— N1C is the corrected engine rpm ܰଵ/√ߠ௧ଶ;
— F1 is a factor corresponding to the percentage of applied adjustment related to acoustic angle in Figure 46 (values range from 0.00 to 1.00); and — F2 is a factor corresponding to the percentage of applied adjustment related to the one-third octave band in Figure 46 (values range from 0.00 to 1.00);
b) for each one-third octave band SPL, arithmetically add the height jet noise adjustment in 403(b)(3)(ii)(B)
a)to the measured SPLs to obtain the altitude jet source noise adjusted SPLs for the derivation of PNL described in A36.4.1.3 a) of part 36; and
c) the height adjustment is to be applied to all measured test data including approach conditions unless it can be substantiated that the jet noise during approach does not contribute significantly to the total aircraft noise.
Chapter 5
GUIDELINES FOR PROPELLER-DRIVEN SMALL AIRPLANES AND PROPELLER-DRIVE COMMUTER CATEGORY AIRPLANES EVALUATED UNDER APPENDIX G OF 14 CFR PART 36
CHAPTER TABLE OF CONTENTS
501 EXPLANATORY INFORMATION218
501(a) Noise Measurements218
501(b) Noise Certification Test and Measurement Conditions218
501(b)(1) Meteorological conditions218
501(b)(2) Atmospheric measurements219
501(b)(3) Airplane flight path220
501(c) Noise Unit Definition220
501(c)(1) A-weighting220
501(d) Measurement of Airplane Noise Received on the Ground220
501(d)(1) Recording systems220
501(d)(2) Recording systems221
501(d)(3) Noise characteristics221
501(d)(4) Measurement system sensitivity221
501(d)(5) Calibration process222
501(d)(6) Microphone configuration222
501(d)(7) Microphone configuration222
501(d)(8) Microphone installation223
501(d)(9) Background noise alleviation224
501(e) Adjustment to Test Results224
501(e)(1) Atmospheric absorption adjustment224
501(e)(2) Noise path adjustment224
501(e)(3) Noise source adjustment224
501(e)(4) Noise source adjustment225
501(f) Reporting of Data to the FAA and Validity of Results225
501(f)(1) Reporting of meteorological data225
501(f)(2) Reporting of airplane information225
501(f)(3) Reference noise levels/confidence intervals225
501(f)(4) Confidence limit compliance226
502 EQUIVALENT PROCEDURES INFORMATION226
502(a) Installation of Add-on Silencers (Mufflers)226
502(b) Guidance on Use of a Windscreen226
502(c) Takeoff Test and Reference Procedures227
502(d) Source Noise Adjustments227
502(d)(1) Fixed pitch propellers228
502(d)(2) Variable pitch propellers228
502(e) No-Acoustical Change Guidance for Derived Versions of Propeller-Driven Airplanes229
502(e)(1) No-acoustical change guidance for airplanes fitted with fixed pitch propellers230
502(e)(2) No-acoustical change guidance for airplanes fitted with variable pitch propellers230
502(e)(2)(A) Engine change without power change230
502(e)(2)(B) Power increase without changing the propeller rpm230
502(e)(2)(C) Change in weight230
502(e)(2)(D) Drag change231
502(e)(2)(E) Different blade count propeller231
502(e)(2)(F) Different blade tip shape231
503 TECHNICAL PROCEDURES INFORMATION232
503(a) Worked Example of Calculation of Reference Flyover Height and Reference Conditions232
503(a)(1) Introduction232
503(a)(2) Takeoff reference procedure232
503(a)(3) Expression for reference height232
503(a)(4) Reference conditions for source noise adjustments233
503(a)(5) Worked example for the calculation of reference flyover height234
503(a)(5)(A) For reference flyover height calculation234
503(a)(5)(B) For calculation of reference atmospheric conditions235
EXPLANATORY INFORMATION [ETM 5.1] See 301 of this AC for technical procedures generally app licable for noise certification tests of all aircraft types including those evaluated under the provisions of Appendix G of part 36. Procedures specific to Appendix G are presented in the following section.
501(a) Noise Measurements [ETM 5.1.1] 501(b) Noise Certification Test and Measurement Conditions [ETM 5.1.2] 501(b)(1) Meteorological conditions GM §G36.101(b) [ETM GM A6 2.2.2]
(1) Atmospheric conditions Atmospheric conditions can affect the generation and pr opagation of sound for non-r eference helical tip Mach numbers (see G36.201 of part 36). Propellers generate higher noise levels at higher propeller helical tip Mach numbers. Usually the actual tip velocity is close to refere nce propeller-tip velocity, but the speed of sound is a function of air temperature which is often different than the reference value. Off-reference tip Mach numbers can occur because of off-reference air temp erature. Part 36 specifies the need for correction for non-reference tip Mach numbers under most circumstances. However, limiting the permissible test temperature range reduces the potential magnitude of this correction. Corrections are also required to account for non-reference atmospheric absorption of sound. The magnitude of this correction is also limited by restricting the range of permissible temperature and relative humidity.
(2) Non-uniform atmosphere The atmosphere between the source (i.e., airplane, propeller and/or exhaust) and the microphone is not uniform.
There can be strong temperature gradients, positive and negative, variations in relative humidity and variations in wind. Turbulence is also associated with strong winds, which can cause irregular sound propagation. Corrections are not required to account for wind. The wind limits provide only a mean s of determining the acceptability of the data.
(3) Weather monitoring Based on the above considerations, weather conditions sh ould be monitored. Procedures used in the noise certification process for transport category airplanes and turbojet-powered airplanes call for measurement of the weather conditions between the ground and the height at which the airplane is flying (see A36.2.2.2(b) of part
36). The absorption of sound in air can then be computed based on these measurements. This process requires an appreciable investment of time and resources. For light propeller-driven airplanes, the magnitude of the adjustment for atmospheric absorption is less than that for jet airplanes. An adjustment procedure based on measurements of the weather near the surface is therefore considered sufficient and more appropriate for airplanes covered by this section.
(4) Temperature inversions The effects of inversions and anomalous wind conditions are difficult to quantify. When temperature inversions are present (i.e., when the air temperature increases with height over any po rtion of the atmosphere between the ground and the airplane) flight conditions may be unstab le, which hampers the ability of the pilot to set up a consistent, stabilized climb within the permitted operational tolerances. Also, under these conditions, it is possible to have a situation in which the surf ace temperature and relative humidity m eet the permissible test criteria but the conditions aloft are much drier, with consequent hi gh sound absorption characteristics and the possibility of underestimating the noise level. The noise spectrum of propeller-driven airplanes contains relatively less high frequency noise than that of jet airplanes, so the effect s may not be very significant unless there is a severe inversion.
501(b)(2) Atmospheric measurements AMC §G36.101(b) [ETM AMC A6 2.2.2]
(1) General weather measurements The applicant should measure weather conditions near the surface and in the vicinity of the noise measuring point.
The acceptability of noise data is contingent on the conditions being within the specified limits of G36.101(b) of part 36. These measurements are to be made at a height between 4 ft (1.2 m) and 33 ft (10 m) above ground level.
This allows the use of hand-held equipment but does not preclude the use of more complex equipment of the type identified in Appendix A of part 36 if the applicant so chooses. The weather data may be recorded on a chart, or a record of the observations, witnessed by the FAA, may be kept.
(2) Wind Consistent with the less complex requirements for small propeller-driven airplanes, wind measurements may be made using a hand-held device if its specifications are similar to instrumentation that the FAA would approve for use in testing under Appendix A of part 36. If the device used does not provide enough information to compute the crosswind, then the wind in any direction should be limited to the crosswind limit of 5 kt (2.6 m/s).
The wind limits are based on a 30-second average.
(3) Temperature and relative humidity limits Noise data are acceptable only if the air temperature is in the range of 36°F (2°C) to 95°F (35°C), and the relative humidity is in the range of 20 to 95 percent. Temp erature and relative humidity may be measured with a psychrometer, a device that measures wet and dry bulb temperatures of the air. Relative humidity is then computed from these temperatures. Sufficient m easurements should be made to dete rmine all adjustments specified by Appendix G of part 36. Persons responsible for performing the test should be alert to changes in the conditions.
At a minimum, measurements should be made immediately before the first run in a series and immediately after the last run. This interval should not exceed more than one hour because of the requirement for adjustment of the airplane test weight due to fuel loss. In marginal or changing conditions, shorter intervals would be more appropriate.
(4) Anomalous winds The presence of anomalous wind conditions may be assessed by noting the airspeed variation as the airplane climbs. If the wind is uniform or changes speed or di rection slowly with altitude, there is no difficulty in maintaining a constant climb speed. If there are strong variations in the wind (i.e., wind shear) or rising and descending air, there will be variations in airspeed that are not easily controllable. Variations of ±5 kt (±2.6 m/s) during the flyover relative to the reference velocity (Vy) are permitted by Appendix G of part 36, and this criterion may be used to evaluate the presence of anomalous wind conditions.
(5) Air temperature measurements versus altitude Page 219
At the beginning of the test and, if considered necessary, at intervals during the test, an observer on the test airplane may consider monitoring the air temperature during a climb. This climb may be a noise data–recording climb or may be dedicated to temperature measurement. The information must be assessed if a judgment is to be made about the acceptability of the conditions for noise measuremen ts. The presence of anomalous wind conditions can be assessed during the data acquisition.
501(b)(3) Airplane flight path GM §G36.111 [ETM GM A6 2.3.5]
(1) Airplane position Appendix G of part 36 specifies determination of the noise level at a single location relative to the start of takeoff roll. Limits on the permissible deviation from the reference flight path are specified for the flight tests. These limits are based on the ability to obtain consistent, representative results, without placing excessive restrictions on the flight test. The initial takeoff weight should be equal to the maximum approved takeoff weight, and after an hour of flight time, the weight is to be increased back to maximum to account for fuel burn. This procedure ensures that the flight parameters, primarily angle-of-a ttack, do not vary significantly from the reference. The airplane position is to be approved by the FAA for each test flyover.
501(c) Noise Unit Definition [ETM 5.1.3] 501(c)(1) A-weighting GM §G36.105(e) [ETM GM A6 3.0]
(1) Basis of measurement The A-weighting correction curve has been precisely defined by national and international standards for the measurement of sound, such as environmental noise, and is a standard feature in sound level meters and other sound analysis equipment used for noise assessments.
501(d) Measurement of Airplane Noise Received on the Ground [ETM 5.1.4] 501(d)(1) Recording systems GM §G36.105 [ETM GM A6 4.3.1]
(1) Audio recorders An audio recorder can be used to preserve a complete acoustical record of the events. If there are questions about the data observed during the tests, the recorded data can be replayed, multiple times if necessary, to verify the results. A more detailed analysis of the airplane noise signal may also be useful to the applicant for research and development purposes.
(2) Graphic level recorders A graphic level recorder can be used to provide a pe rmanent record of the noise levels, but no replay or reproduction of the acoustical signal is possible.
(3) Sound level meters The record that results from the use of a sound level meter depends on the design features of the instrument. The least complex instrument uses an el ectromechanical metering mechanism, requiring the operator to observe the Page 220