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
For variable pitch propellers, the source noise sensitivity curves are developed from data taken with the aircraft flying over a range of propeller speeds, typically three, at a fixe d torque or MAP in a manner similar to that described in 502(d)(1) where NMCP or NMAX would in this case be the maximum propeller speed at the maximum permitted torque or MAP. This is repeated for two lower torque or MAP values in order to establish a carpet plot of maximum A- weighted noise levels against propeller speed and torque, MAP or SHP.
A plot of maximum A-weighted noise level (LAmax), helical tip Mach number (MH) and torque or MAP is developed.
This plot is then used to derive the source noise adjustment (LAmax) which is the difference between reference and test conditions at the noise certification power.
Generally the test and reference engine SHP can be derived from the engine manufacturer’s performance curves.
However, where such curves are not available, a correction should be applied to the manufacturer’s published engine SHP, which is normally presented for a range of engine speeds under ISA and sea level conditions, in order to establish the engine power level under the test conditions of ambient temperature and air density. The correction is as follows:
a) for normally aspirated engines:
்ܲ ൌܲ ோ|ሺܶோ/்ܶሻ భ మ|ሾሺߪ െ 0.117ሻ/0.883ሿ ;and ’
b) for turbo-charged engines:
்ܲ ൌܲ ோ|ሺܶோ/்ܶሻ భ మ, where:
— PT and PR are the test and reference engine powers;
— TT and TR are the test and reference ambient temperatures; and — ߪis the air density ratio.
Note.— In this context reference denotes the reference conditions for which the engine SHP is known.
502(e) No-Acoustical Change Guidance for Derived Versions of Propeller-Driven Airplanes Certificated According to Appendix G [ETM 5.2.5] After the certification in their basic configuration, small propeller-driven airplanes are often modified, either by a TC change of the TC holder or by an STC from a supplier. These changes can be of a different nature such as weight increase/decrease, engine change, power change, propeller change, installation of vortex generators, fitting of winglets or external mounted equipment (cargo boxes, floats, etc.). With regard to noise and depending on their nature, some changes might have to demonstrate compliance with the applicable requirements by a new flight test, others by reevaluation of the original noise flights or by demonstrating a no acoustical change.
The propeller and the engine are the main noise sources of small propeller-driven airplanes. Parameters like the diameter, the number of blades, the rpm, the pitch, the blade tip shape or the geometry could have an impact on the propeller noise signature. As for the engine, noise signature could change by modifying the rotor assembly or the exhaust.
Several sections refer to the determination of a new reference height due to a change in performance data, here D50, Vy and RC. New performance data are accepted for the recalculation only if they are established by a method approved by the FAA.
The following sections are intended to provide guidance for applicants concerning an NAC demonstration.
502(e)(1) No-acoustical change guidance for airplanes fitted with fixed pitch propellers [ETM 5.2.5.1] (Reserved) Note.— This guidance is limited to variable pitch propellers only. According to G36.111(c)(2)(iv) of part 36, takeoff rpm must be maintained throughout the noise test runs. During climb at the best rate of climb speed, the fixed pitch propeller can generally not reach its maximum operating rpm value. Therefore the reference propeller speed is defined to be the average propeller rpm calculated from all valid runs. Changing the performance (e.g., due to an engine change or a change in weight) might change the average rpm adversely. Currently the amount of change in the propeller rpm for a fixed pitch propeller cannot be estimated analytically.
502(e)(2) No-acoustical change guidance for airplanes fitted with variable pitch propellers [ETM 5.2.5.2] 502(e)(2)(A) Engine change without power change [ETM 5.2.5.2.1] The engine is one of the main noise sources of the airplane. Changes to the engine can take many forms. They vary from small changes within one engine family, normally addressed by different characters within the engine designation, to a complete re-engine. In the latter case the compliance demonstration can in general be achieved only by new noise flight tests. In the former case it is often obvious that the change has no acoustical impact, and a simple statement should suffice to demonstrate that the change is not acoustically significant.
502(e)(2)(B) Power increase with out changing the propeller rpm [ETM 5.2.5.2.2] Increasing the power output of an engine without changing th e takeoff weight will increase the engine noise level at source but also improve the takeoff performance. A method subject to the approval by the FAA can be applied to evaluate the increase in engine noise source. This increase in engine noise source may be offset by the higher reference height. Takeoff distance is shortened, the climb rate is increased and therefore the flyover height over the microphone is increased. If it can be shown that this increase in reference flyover height offsets the increase in engine source noise, the change in engine power may be considered as an NAC.
An NAC is acceptable if it can be demonstrated that a minimum of six valid flights are within the “new” height window.
The effect of angle-of-attack changes may be included in the analysis by a method approved by the FAA. The method must be robust enough to account for the effects of performance and angle-of-attack changes on noise levels. If the analysis method shows that the noise level does not increase then the noise level of the unmodified airplane can be applied. Otherwise new testing should be required.
Note.— Section G36.111(c)(2)(iv) of part 36 defines that the microphone has to be passed at maximum takeoff power. If source noise sensitivity curves are established in accordance with the procedure laid down in 502(d) of this chapter the noise level can be adjusted up to the highest power covered by the sensitivity curve. In such a case the power correction determined by the sensitivity curve should be used instead of the general adjustment Δ3 = K3 log (PR/PT).
502(e)(2)(C) Change in weight [ETM 5.2.5.2.3] According to G36.111(a) of part 36, the flight tests must be initiated at the maximum takeoff weight. Only increases in Page 230
takeoff weight up to the maximum actually flown during the original flight tests can be accepted without new flight tests.
If it can be demonstrated that a further weight increase and the corresponding loss of performance do not adversely affect the noise level by more than 0.10 dB(A), the certificated noise level may be assigned to this weight without additional flight tests.
A change in the weight of the aircraft will lead to different performance characteristics. A new reference height with the new performance parameters has to be determined to demonstrate the influence on the noise level. Possible impact on the propeller speed should be taken into account.
Similar to the previous section, the effect of angle-of-attack changes may be included in the analysis by a method approved by the FAA.
502(e)(2)(D) Drag change [ETM 5.2.5.2.4] While a change in drag generally has no direct impact on the noise at source, it may have an indirect effect on noise level through a change in performance.
A drag change will in general be introduced by modifications such as the fitting of cargo pods or external fuel tanks, larger tires, floats, etc. In most cases, the change in ae rodynamic noise can be shown to be negligible for small propeller-driven airplanes. However, there may be cases where the aerodynamic noise generated by the modification has to be addressed. The drag change might change the performance characteristics of the aircraft D50, Vy and/or RC leading to a change in reference flyover height. The performance characteristics defined in the AFM are approved by the performance experts of the FAA. In some cases the pe rformance experts agree to ap ply the former performance parameters to the modified aircraft if the applicant can demonstrate that the performance is not worse than the one for the basic aircraft. Three different situations have to be considered:
a) the performance characteristics are bette r than those of the parent aircraft;
b) the performance characteristics are identical to those of the parent aircraft; and
c) the performance characteristics are wors e than those of the parent aircraft.
With regard to noise these three situations should be dealt with as follows:
a) in the case of situation a), independent of whether th e applicant decides to maintain the old performance data or to document the better performance in the AFM, an NAC can be granted and the noise level for the parent version can be applied to the modified aircraft;
b) in the case of situation b), the noise level for the parent version can be applied to the modified aircraft without further investigation; and
c) in the case of situation c), in general a new flight test is required.
502(e)(2)(E) Different blade count propeller [ETM 5.2.5.2.5] The effect of changing the number of the propeller blades on the noise level is difficult to determine by analytical procedures. Typically the applicant is obliged to perform a new flight test. Propeller noise prediction routines are highly sophisticated requiring extensive data sets which can in general be provided only by the propeller manufacturer.
The use of such propeller noise prediction routines has to be acceptable to the FAA to demonstrate an NAC.
502(e)(2)(F) Different blade tip shape [ETM 5.2.5.2.6] Page 231
503 In general rounded tips are quieter than squared ones. The change from squared to rounded blade tips can be accepted as an NAC if the rpm and diameter remain the same.
TECHNICAL PROCEDURES INFORMATION [ETM 5.3] 503(a) Worked Example of Ca lculation of Reference Flyover Height and Reference Conditions for Source Noise Adjustments (Appendix G) [ETM 5.3.1] 503(a)(1) Introduction [ETM 5.3.1.1] The reference flyover height for an airplane certificated to Appendix G of part 36 is defined at a point which is 8202 ft (2500 m) from the start-of-roll beneath a reference flight path determined according to the takeoff reference procedure described in G36.111(c) of part 36. An expression for the reference flyover height in terms of commonly approved performance data and an example of how such an expression may be worked are presented in this section. The relationship between the reference height and the conditions to which source noise corrections are to be made is also explained.
503(a)(2) Takeoff reference procedure [ETM 5.3.1.2] The takeoff reference procedure for an airplane certificated to Appendix G is defined in G36.111(c) of part 36 under sea level, ISA conditions, at maximum takeoff weight for which noise certification is requested. The procedure is described in two phases:
a) the first phase commences at “brakes release” and con tinues to the point where the aircraft reaches a height of 50 ft (15 m) above the runway; the point of interception of a vertical line passing through this point with a horizontal plane 50 ft (15 m) below is often referred to as “reference zero”; and
b) the second phase commences at th e end of the first phase and assume s the airplane is in normal climb configuration with landing gear up and flap setting normal for “second segment” climb.
Note.— The reference “acoustic” flight path ignores the “first segment” part of the flight path, during which the aircraft accelerates to normal climb speed and, where appropriate, landing gear and flaps are retracted.
503(a)(3) Expression for reference height [ETM 5.3.1.3] The reference flyover height is defined according to the takeoff reference flight path at a point 8202 ft (2500 m) from the start-of-roll for an airplane taking off from a paved, level runway under the following conditions:
a) sea level atmospheric pressure of 1013.25 hPa;
b) ambient air temperature of 59°C (15°C) (i.e., ISA);
c) relative humidity of 70 percent; and
d) zero wind.
This height can be defined in terms of the approved takeoff and climb performance figures for the conditions described above as follows: