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
highest level indicated by the moving needle in the meter display during each event. Other, more complex instruments can be set to hold the maximum noise level reached during each event and show this level on a digital display. Some currently available digital units are capable of storing entire time-histories of noise levels for multiple runs. These histories can be recalled to the instrument’s display, transmitted to a printer or downloaded to a computer.
501(d)(2) Recording systems AMC §G36.105(a) [ETM AMC A6 4.3.1]
(1) Audio recorders One method is to record each noise even t using an audio recorder. This reco rded data can be played back and analyzed as much as necessary to verify that consistent results have been obtained.
(2) Other methods Other methods include the following:
a) reading graphic level recorder charts;
b) reading a sound level meter in the field as the event occurs and keeping a handwritten log in ink; and
c) printing, or transferring to a personal computer, th e entire time history after the test has been completed.
Appropriate measures should be taken to ensure the validity of the data, and their use is subject to approval by the FAA.
501(d)(3) Noise characteristics GM §G36.105(e) [ETM GM A6 4.3.4]
(1) Filtered noise level and meter response speed The noise level from each flyover test should be measured in terms of the maximum A-weighted sound level, in decibel (dB(A)) units, using an A-weighting filter with dynamic characteristics (meter response characteristics) designated as “S” (for “slow”) as defined in Reference T3 and specified in G36.105(e) of part 36. The slow response results in an effective two- second averaging period (i.e., one-second time constant), which should be used in Appendix G noise tests.
(2) Maximum sound level The measured or indicated A-weighted sound level will increase as the airplane approaches the measurement site and will decrease after the airplane passes over the site. The highest value of the A-weighted sound level that occurs during the flyover is called the maximum A-weighted sound level. This is the value that should be measured during each test.
Note.— This maximum value may not occur at the exact moment when the airplane is directly over the microphone. It usually occurs slightly before or after the airplane reaches the overhead position due to the directivity characteristics of propeller, engine and exhaust noise emissions.
501(d)(4) Measurement system sensitivity GM §G36.107(b) [ETM GM A6 4.3.5]
(1) Noise level variability Page 221
There can be variability in the noise levels indicated by the test equipment, primarily due to environmental factors and the internal warm-up that is required by most types of equipment. Occasionally, there may be other changes due to cable problems or even equipment damage. Proper use of acoustic calibration devices can help identify such occurrences.
501(d)(5) Calibration process AMC §G36.107(b) [ETM AMC A6 4.3.5]
(1) Equipment calibration A suitable sound calibrator should be used to provide a reference sound level. This is usually accomplished by placing the calibrator on the microphone and adjusting the gain of the measuring system so that the reading corresponds to the known sound level of the calibrator. Initial, final and periodic calibrations should be used to verify that any changes in sensitivity are identified. It is important that the manufacturer’s recommended system warm-up time be observed in the field prior to equipment calibration. Calibration equipment should be identified in the test plan and is to be approved by the FAA.
501(d)(6) Microphone configuration GM §G36.107(a) [ETM GM A6 4.4.1]
(1) Ground plane microphone The specified ground plane microphone configuration gr eatly minimizes the interfer ence effects of reflected sound waves inherent in pole-mounted microphone installations. For a 4 ft (1.2 m) microphone, such effects typically occur in the frequency region that is most significant for propeller-driven aircraft noise.
(2) Microphone sensitivity The specified ground plane configuration places the microphone diaphragm into an effective sound pressure field for the frequency range of interest. Microphones designed for uniform pressure response are appropriate for use in such installations.
501(d)(7) Microphone configuration AMC §G36.107(a) [ETM AMC No. 1 A6 4.4.1]
(1) Inverted microphone The inverted microphone setup shown in Figure 47 is an example of the design and construction of the microphone holder and the ground plate. The legs of the microphone holder should be firmly attached to the plate so that the microphone holder does not vibrate during the test. The plate should be painted white to reflect the sun’s rays, as such reflection will reduce the thermal effects on the microphone-sensing elem ent. A metal spacer is a practical tool to use in setting the space between the microphone diaphragm and the ground plate. The spacer thickness should be 7 mm minus the space between the microphone protective grid and the microphone diaphragm.
(2) Microphone placement The spacing of the microphone diaphragm relative to the pl ate is critical, since it should be inserted completely within the effective sound pressure field, and the depth of this field varies with frequency and sensor size. For frequencies of interest, 7 mm spacing has been determined to provide the best compromise of associated technical considerations.
Figure 47. Configuration for one-half-inch inverted microphone 501(d)(8) Microphone installation AMC §G36.107(a) [ETM AMC No. 2 A6 4.4.1]
(1) Plate installation in local ground surface Care should be taken during installation to ensure that the ground surface beneath the plate is level and contains no voids or gaps. One way to achieve this is by pressing the plate into the ground surface at the desired location, applying slight pressure, then removing the plate to de termine if any areas under the plate are recessed. These recesses can then be filled in with loose material, such as sand or soil, to obtain a level, uniform underlying surface. Care should also be taken to ensure that the edges of the plate are flush with the surrounding ground surface. This is especially important for plates that are thicker than the specified minimum of 2.5 mm.
In some cases it may be appropriate to moisten the soil with water immediately before installation to allow the surface to mould itself around the plate. In such cases, acoustical measurem ents should not be performed until the ground has dried.
(2) Design and construction of microphone support The support should be designed so that it minimizes any potential interference with sound waves from the aircraft arriving in the vicinity of the microphone. If a spider-like structure such as that in Figure 47 is used, the number of legs should be limited to three or four. As specified in the figure, the legs should be no larger than 2 mm diameter. Ideally the support collar should be as small as possible, an d it should also implement some sort of tightening device, such as a set screw, to facilitate ad justment of the microphone diaphragm height above the plate. The support should be stable and should orient the microphone in such a way that the diaphragm is parallel to the plate.
(3) Cable support In some cases, it may be desirable to provide additional support to the microphone ca ble as it leads away from the plate. A metal rod or similar sort of support may be used for this purpose. Any such support should be as small as possible and located as far away from the plate as is practical. The microphone cable should lead directly away from the plate without crossing above any more of the plate’s surface than is necessary.
(4) Windscreens Consideration should be given to using windscreens when wind speed exceeds 5 kt (2.6 m/s) (see 502(b) of this AC).
501(d)(9) Background noise alleviation AMC §G36.107(c) [ETM AMC A6 4.4.4]
(1) Increased airplane noise If a site with lower noise levels cannot be used, it may be necessary to fly the airplane so that the target height over the microphone is less than it would be at the reference microphone station (8202 ft (2500 m) from the start of the takeoff roll). In this case, the airplane height at the microphone location is likely to be outside the ±20 percent tolerance specified in G36.111(a) of part 36. Adjustment of data to reference conditions should be performed in an approved manner.
501(e) Adjustment to Test Results [ETM 5.1.5] 501(e)(1) Atmospheric absorption adjustment GM §G36.201(b) [ETM GM A6 5.2.1 a)]
(1) Atmospheric absorption The temperature and relative humidity of the air affect the sound propagation. This correction accounts for the difference in atmospheric absorption along the sound pr opagation path that occurs between temperature and relative humidity under noise certification test conditions and temperature and relative humidity under reference conditions 59 F (15ºC) and 70 percent relative humidity (see G36.201 of part 36 for additional atmospheric absorption correction information).
501(e)(2) Noise path adjustment GM §G36.201(d) [ETM GM A6 5.2.1 b)]
(1) Noise path length The airplane test limitations are that the height over the microphone must be within 20 percent of the reference height and that the lateral position must be within 10 of the vertical. The noise path length correction adjusts the measured noise levels for the difference in noise path length between actual noise test conditions and reference conditions (see G36.201 of part 36 for additional path length correction information).
501(e)(3) Noise source adjustment GM §G36.201(d)(3) [ETM GM A6 5.2.1 c)]
(1) Helical tip Mach number Page 224
The noise generated by a propeller-driven airplane depend s on the rotational speed of the tip of the propeller, more specifically the helical tip Mach number. Data corrections are based on the relationship between the helical tip Mach numbers determined for test and reference conditions (see 502(d)).
Note.— The reference helical tip Mach number, MR, is the one corresponding to the reference conditions above the measurement point.
501(e)(4) Noise source adjustment GM §G36.201(d)(4) [ETM GM A6 5.2.1 d)]
(1) Engine power Corrections are required to account for non-reference engine power settings that are used during noise certification tests. The procedures for determining the engine power to be used in the calculations depend on the design characteristics of the engine-propeller combination. In most cases, this power is not published and does not have to be determined for airworthiness purposes. It is therefore necessary to determine the power for noise certification purposes (see 502(d)).
501(f) Reporting of Data to the FAA and Validity of Results [ETM 5.1.6] 501(f)(1) Reporting of meteorological data GM §G36.109(d) [ETM GM A6 6.1.3]
(1) Interpretation of “each test” For clarification, this refers to each test series (i.e., test) and each test flyover (i.e., run). The meteorological measurements should be made at the time of each test run, since each noise measurement will be corrected by use of the meteorological data.
(2) Wind measurement The provisions of G36.101(b)(4) of part 36 set the limits on testing, based on a 30-second average wind speed, not to exceed 10 kt (5.1 m/s), with a 5 kt (2.6 m/s) crosswind limitation. There are no additional limitations based on the surface wind.
501(f)(2) Reporting of airplane information AMC §G36.109(g)(5) [ETM AMC A6 6.1.5]
(1) Equipment calibrations All equipment utilized to determine the required parameters should be calibrated, and the calibrations are to be applied before being reported to the FAA in the test report and before being used to make reference airplane corrections. The temperature at the airplane height should be acquired for tip Mach number correction.
(2) Mechanical tachometers Separate validation of the in-flight reading should be made if a mechanical tachometer is used because mechanical tachometers are subject to potential indicating errors as a result of the cable drive system.
501(f)(3) Referen ce noise levels/confidence intervals AMC §G36.203 [ETM AMC A6 6.2.1]
(1) Average noise level calculations Page 225
502 Calculation of average noise and associated confidence intervals should be accomplished as described in 305.
When the 90 percent confidence limit calculated using data from six or more test flights is within 1.5 dB(A), then the average corrected noise level (L Amax)avg resulting from the validated data can be used to determine conformity with the maximum noise levels specified in G36.301 of part 36.
501(f)(4) Confiden ce limit compliance GM §G36.203(b) [ETM GM A6 6.2.2]
(1) Confidence limit exceedance If the 90 percent confidence limit does not satisfy the 1.5 dB(A) standard, additional test data points should be obtained, increasing the number of events until the confidence limit is reduced to 1.5 dB(A). The variability of data obtained under controlled conditions should be substantially less than 1.5 dB(A). If the 90 percent confidence interval is near or above the permitted limit, the approved test procedures and/or correction procedures should be carefully reviewed.
EQUIVALENT PROCEDURES INFORMATION [ETM 5.2] The procedures described in this chapter have been used as equivalent in stringency for propeller-driven airplanes with maximum certificated takeoff weight not exceeding 19,000 lb (8618 kg), as provided in Appendices F and G of part 36.
502(a) Installation of Ad d-on Silencers (Mufflers) [ETM 5.2.1] Installation of an add-on silencer (muf fler) may be an effective method for reducing the noise levels of a propellerdriven airplane powered by a reciprocating engine. However, an add-on silencer (muffler) may also degrade the performance of the airplane and therefore adversely affect the aircraft’s noise characteristics.
The airplane performance characteristics must be re-evaluated after the installation of the add-on silencer (muffler).
The type design change represented by the silencer (muffler) installation can be accepted as an NAC for compliance with Appendix F or G of part 36 if the following conditions are verified to the satisfaction of the FAA:
a) for aircraft certificated according to Appendix F of part 36, the airplane’s takeoff and climb performance, as determined by the performance correction defined in F36.201 of part 36, is not adversely affected; or
b) for aircraft certificated according to Appendix G of part 36, the airplane’s takeoff and climb performance, as determined by the reference height calculated in accordance with G36.111 of part 36, is not adversely affected.
In either case, the add-on silencer (m uffler) has no significant effect on th e engine performance (i.e., power and rotational speed).
502(b) Guidance on Use of a Windscreen [ETM 5.2.2] For noise certification tests conducted acco rding to Appendix G of part 36, the microphone must be installed in accordance with G36.107(a) of part 36, which describes how the microphone is mounted in an inverted position so that the microphone diaphragm is 0.25 in (7 mm) above and parallel to a circular metal plate. With this configuration, the FAA has approved the use of a windscreen in order to minimize wind- and turbulence-induced pseudo-sound levels and Page 226
to protect the microphone during the test.
A windscreen prepared and used in the following manner will cause no significant effect on the test result. The windscreen must be made from a commercially available s pherical foam windscreen cut into a hemispherical shape in order to accommodate the microphone over the plate. In preparing the hemispherical windscreen, the following points must be ensured:
a) the cut surface of the windscreen must not be damaged by the cutting process; and
b) with the microphone properly inserted into the hemispherical windscreen and mounted over the ground plate, the microphone diaphragm must be at the specified distance from the plate’s surface.
502(c) Takeoff Test and Reference Procedures [ETM 5.2.3] Note.— In planning a test program for noise certification according to the provisions of Appendix G of part 36, it is helpful to note the differences between test-day flight procedures and the standardized takeoff reference profile.
The takeoff reference profile is used to compute the altitude and speed of the aircraft passing over the microphone on a standard day. The requirements for this profile are contained in G36.111(c) of part 36. They require that the first segment be computed by using airworthiness approved data, assuming takeoff power is used from the brake-release point to 50 ft (15 m) above the runway. The second segment is assumed to begin precisely at the end of the first segment with the airplane in a climb configuration, with gear up and climb flaps, an d operating at the certificated speed for best rate of climb (Vy) (see Figure 48).
A worked example of the calculation of reference flyover height and reference conditions for correction of source noise for airplanes certificated according to the Standards of Appendix G of part 36 is presented in 503(a).
The requirements for airplane test procedures are contained in G36.111(a) of part 36. It basically refers only to test tolerances and approval of test plans by the FAA.
Figure 48 illustrates the difference between the test and reference procedures. Note that the actual flight test path need not include a complete takeoff from a standing condition. Rath er, it assumes that a flight path intercept technique is used. As with the turbojet and helicopter standards, the airplane should be flown to intersect the second phase (i.e., segment) climb path at the right speed and angle of climb when going over the microphone within 20 percent of the reference height.
The takeoff reference procedure defined in Appendix G of part 36 requires that the second phase of the procedure be flown at the best rate of climb speed (Vy). The airplane testing procedures described in Appendix G of part 36 require that the flight test be conducted at Vy. The reference height to which the measured noise levels are to be corrected is calculated from the climb rate corresponding to Vy. Recent changes to the airworthiness requirements have eliminated the need to determine Vy for small propeller-driven airplanes. In this case applicants will nevertheless have to determine Vy for the purpose of showing compliance with Appendix G of part 36. If the minimum airworthiness approved climb speed is greater than Vy then this speed must be used and noted in the AFM.
Applicants may alternatively show compliance with Appendi x G of part 36 at the climb speed for which the AFM performance information is calculated provided they demons trate, to the satisfaction of the FAA, that the resulting noise level is not less than would have been obtained using Vy.
502(d) Source Noise Adjustments [ETM 5.2.4] Source noise adjustment data for propeller-driven light airplanes may be obtained by flying the test airplane with a range of propeller speeds for fixed pitch propellers and a range of torque or manifold air pressure (MAP) values for Page 227
variable pitch propellers.
Height (m) 15 0 Test flight path First phase Second phase 2 500 Reference height Reference flight path Distance from brake release (m) Figure 48. Typical test and reference profiles 502(d)(1) Fixed pitch propellers [ETM 5.2.4.1] For airplanes fitted with fixed pitch propellers demonstr ating compliance with Appendix F of part 36, source noise sensitivity curves are developed from data taken by measuring the noise level for the airplane flying at 1000 ft (300 m) (see F36.111(a) of part 36) at the propeller speed for maximum continuous power (NMCP).
Airplanes demonstrating compliance with Appendix G of part 36 should be flown according to G36.111(c) of part 36.
In this way, the aircraft overflies the microphone at the reference height (HREF) defined in G36.111(c) of part 36, the best rate of climb speed (Vy) and at the propeller speed (NMAX) corresponding to that defined in G36.111(c)(iv) of the “second phase” of G36.111(c) of part 36.
For both Appendix F and Appendix G airplanes, noise measurements are repeated at two lower propeller speeds, typically 200 rpm and 400 rpm lower than NMCP or NMAX. For Appendix G airplanes, these should be flown at speed Vy. The maximum A-weighted noise peak noise level (LAmax) is plotted against the propeller helical tip Mach number (MH) in order to obtain the curve from which the source noise correction may be derived.
For fixed pitch propellers, it is generally not possible to separate the two significant noise generating parameters, helical tip Mach number and the power absorbed by the prope ller, by using flight tests. A sensitivity curve of Mach number versus noise level derived from flight tests of a fixed pitch propeller, either level flyovers or fixed-speed climbs, will therefore include, within the curve, the effect s not only of the Mach number but also the power. Under these circumstances, it is not appropriate to apply a separate power correction.
502(d)(2) Variable pitch propellers [ETM 5.2.4.2] Page 228