PNLTM –10 dB Tone corrected perceived noise level (PNdB) PNLTM Time kLkMkF Time kLkMkF PNLTM –10 dB PNLTM Tone corrected perceived noise level (PNdB) Figure 9. Illustrated example of identification of first and last 10 dB-down records 401(e) Reporting of Data to the FAA §A36.5 [ETM 4.1.5] 401(e)(1) Compliance records GM §A36.5.2 [ETM GM A2 5.1]
(1) Compliance records For compliance with A36.5 of part 36, all data measured during noise certification testing, including time-histories of physical measurements, noise recordings, instrument calibrations, etc., are to be recorded in permanent form and made available to the FAA for review, inspection and approval. A common procedure is for the applicant to submit representative samples of test data for each noise measurement point and adjustments to measured data to permit the FAA to determine compliance with part 36. The applicant may either submit the complete test records along with the required data adjustments, or when approved by the FAA, the applicant may instead submit samples of test data along with the required data adjustments.
401(e)(2) EPNL average values GM §A36.5.4.2 [ETM GM A2 5.4]
(1) Average EPNLr levels when using the NPD equivalent procedure Page 133
For airplanes the average value of EPNL r from an NPD database (see 402( a)(1)(ii)(A)) is the noise level determined along the regression line through the adjusted data set at the appropriate thrust (power) and distance values, including any other additional adjustments necessary (e.g., adjustment to the aircraft reference speed).
(2) Single test values When more than one noise measurement system is used at any one noise measurement point, the resulting noise level is to be the average of the measured noise levels for each noise measurement po int. This requirement does not apply to noise levels measured by microphones not required for acquisition of noise certification data.
(3) Valid conditions All valid noise measurements are to be included in the confidence interval calculations even when they produce results that are outside the 90 percent confidence limit of ±1.5 dB. The cause of erratic or possibly invalid noise data may include testing under different temperature and humidity extremes, anomalous winds aloft, changes in noise measurement system components, changes in aircraft hardware, background noise, shift in instrument calibrations, or not testing in accordance with the approved test plan, etc. The FAA is to make a determination, during the course of noise certification testing, as to the validity of all noise measurements. A noise measurement may not be excluded from the confidence interval calculations at a later date without approval by the FAA. Noise measurements determined in the field to be invalid for any reason may need to be repeated in order to achieve the required minimum number of valid test runs.
401(e)(3) Calculation of 90 percent confidence intervals AMC §A36.5.4.2 [ETM AMC A2 5.4]
(1) Methods for calculating 90 percent confidence intervals Section 305, provides confidence interval calculation methods for clustered measurements, regression mean line, static-test-derived NPD curves and analytically derived NPD curves, along with wo rked examples. Calculation methods for determining 90 percent confidence interval values for clustered and pooled data sets are presented in 305(f)(2) and 305(f)(5) of this AC.
(2) Retest requirements The FAA may require an applicant to retest or provide additional test data for any of the three noise measurement points when the reported results indicate:
a) a required measurement is reported to be invalid;
b) an insufficient number of measurements were conducted by the applicant to determine a suitable data sample;
c) data scatter indicates that the data are not from a normal population or trend (e.g., a discontinuity due to low power SBV operation);
d) the 90 percent confidence interv al for a noise measuring condition exceeds the allowable ±1.5 dB; or
e) the test was not conducted in accordance with an approved noise certification compliance demonstration plan.
401(f) Nomenclature: Symbols and Units §A36.6 [ETM 4.1.6] (Reserved) Page 134
401(g) Sound Attenuation in Air §A36.7 [ETM 4.1.7] (Reserved) 401(h) Adjustment of Helicopter Flight Test Results §H36.205 [ETM 4.1.8] The objective of a noise certification test is to acquire da ta for establishing an accurate and reliable definition of a helicopter’s noise characteristics. Appendix H establishes a range of test conditions and procedures for adjusting measured data to reference conditions.
401(h)(1) Adjustments to reference conditions GM §H36.205(a) [ETM GM No. 1 A2 8.3]
(1) Adjustments to reference conditions Most noise certification tests are conducted during cond itions other than the reference conditions. This includes differences in height, lateral positio n, airspeed, rotor speed, temperature and relative humidity. Therefore, measured noise data should be adjusted to reference conditions to determine whether compliance with the noise certification limits of Appendix H may be achieved. Both positive and negative adjustments must be applied for the differences between the test and reference conditions. Adjustment procedures and analysis methods should be reviewed and approved by the FAA. The FAA should ensure that data adjustment and analysis methods that are proposed by applicants satisfy the requirements of part 36 and approved procedures. Any changes, including software revisions, firmware upgrades or instrumentation changes, are subject to the review of the FAA before they can be used for noise certification evaluations. Program validation should be planned and the required information submitted to the FAA early in the certification cycle, since th e time required for evaluation and approval may vary depending upon the issues encountered.
(2) Non-positive SPLs Whenever non-positive one-third octave band aircraft noise levels are obtained, whether as part of the original one-third octave band analysis or as a result of adjustments for background noise, or other approved procedures, their values should be included in all relevant calculations. The practice of “band-dropping”, where masked levels are methodically set equal to zero, is not considered to be an acceptable substitute for reconstruction of masked levels as per the background noise adjustment guidance provided in 306. For any aircraft noise spectrum subject to adjustment to reference conditions, all one-third octave bands, including those containing masked levels or reconstructed levels, including values less than or equal to zero dB, should be adjusted for differences between test and reference conditions.
(3) Direction of flight considerations Since flyovers are made in two directions with headwind and tailwind components, the lateral (sideline) microphones will be either “left sideline” or “right sideline” depending on the direction of flight. Hence sideline flyover data need to be sorted by left microphone and right microphone for data adjustments and reporting. Note that sorting by left and right sideline microphone is also appropriate for takeoff and approach if more than one direction of flight is used.
It should also be noted that an equal number of flyover test runs with headwind and tailwind components are required. If after analysis the applicant finds that there is at least the required minimum of three measured values in each flight direction, but there are more in one direction than in the other, the applicant then will need approval by the FAA as to which are to be used in the determination of the final EPNL value for flyover.
401(h)(2) Reference data sources GM §H36.111(c) [ETM GM No. 2 A2 8.2.1]
(1) Manufacturer’s data Adjustment of noise values from test to reference cond itions should be based on approved manufacturer’s data.
Manufacturer’s data should include:
a) reference flight profiles;
b) takeoff and flyover engine power settings at reference conditions; and
c) reference airspeeds.
401(h)(3) Adjustments to measured noise data GM §H36.205 [ETM GM A2 8.3.1.2]
(1) Reference flight path noise propagation angle In calculating the position of the PNLTM on the reference flight path, the acoustic emission (i.e., noise propagation) angle () relative to the flight test path must be kept the same as for the test flight path. The elevation angle () relative to the ground plane is not constrained, and determination and reporting of this angle is required.
(2) Maximum adjustments To prevent excessive adjustments to the measured data , the summation of all the adjustments for differences between the test flight path and the reference flight path for flyover and approach is limited to 2 EPNdB. For takeoff the summation of the adjustments is limited to 4 EPNdB of which the sum of 1 and the –7.5 log term from 2 must not exceed 2 EPNdB. The additional allowance for takeoff acknowledges th at larger differences between the test flight path and reference flight path can occur for this condition as a result of the influence of wind speed on the test flight path. It is recommended, however, that the applicant note that methods discussed in H36.3(c)(a)(c)(2) can be used to minimize this difference for takeoff.
401(h)(4) Takeoff profile GM §H36.205(b) [ETM GM A2 8.1.2.1]
(1) Reference takeoff profile Figure 10 illustrates the reference takeoff profile and an idealized test or measured takeoff profile under zero wind conditions.
20 m (65 ft) 500 m (1640 ft)
10/12/2017 AC 36-4D L + A F C Sstarboard Cr Lr r Fr Ir Reference take-off path projection Reference take-off path Measured take-off path Sport 20 m (65 ft) 500 m (1 640 ft) 150 m (492 ft) 150 m (492 ft) Figure 10. Comparison of mea sured and reference takeoff profiles The reference takeoff profile is a straight line segment. It starts from a defined point C r that is 1640 ft (500 m) from the center microphone location A an d at a height of 65 ft (20 m) above the ground. The reference climb angle ( β) of the straight line path will depend on the certificated best rate of climb and V y at the reference conditions. The reference profile ends at a point I r which will encompass the 10 dB-down period of the noise measurements.
Note.— For clarity the location of the test and reference PNLTM points, L and Lr, are illustrated at the same position for both the centerline noise measurement point A and the starboard lateral (sideline) noise measurement point S. Normally however L, and hence Lr, will be a different position on the test and reference flight paths for each noise measurement point.
(2) Reference climb angle The reference climb angle, β, is based on the best rate of climb and V y airspeed determined from approved manufacturer’s data for the takeoff performance of the helicopter at the reference conditions. Since airspeed is defined as being in the direction of the flight path, the climb angle β is the arcsine of the ratio of best rate of climb to Vy. On a helicopter that is engine-power-limited at the reference conditions, the best rate of climb has to be calculated from the minimum specification engine(s) performance. On many helicopters the takeoff characteristics will be dependent on gearbox torque limit, and this will be typically less than the torque associated with minimum specification engine(s) at the reference conditions. Since all procedures have to be consistent with the airworthiness regulations, the gearbox takeoff torque limit should be used to calculate the applicable best rate of climb at the maximum noise certification weight fo r those helicopters which are performance-limited by the gearbox characteristics at the reference conditions.
401(h)(5) Takeoff test conditions AMC §H36.101(d) [ETM AMC No. 1 A2 8.1.2.1]
(1) Takeoff requirements The takeoff profile is commenced from a level flight at a height of 65 ft (20 m). After reaching position C, takeoff power has to be applied to initiate the climb. The takeoff power will either be dependent on the gearbox torque Page 137
limit for takeoff or minimum installed engine(s) takeoff power torque at the reference conditions at sea level and 77ºF (25C).
(2) Test airspeed The best rate of climb airspeed V y to be used is that determined from the takeoff performance at sea level and 77F (25C) during airworthiness certification. This is to be maintained during the complete takeoff procedure.
To account for test-to-test variation and slight variations during each test run, a tolerance of ±5 kt (±9 km/h) is allowed.
(3) Rotor speed The mean value of the rotor speed during the 10 dB-down period is to be within ±1 percent of the maximum normal operating rotor speed value at the reference takeoff condition.
(4) Flight path deviations To minimize lateral flight path deviations, and hence the difference in noise levels due to off-track position at the PNLTM emission point, the helicopter must fly over the reference flight track during the 10 dB-down period within ±10 or ±65 ft (±20 m) from the vertical, whichever is the greater. This is illustrated in Figure 11. There is no direct height limitation, but the adjustments that take into account differences between the reference and test sound propagation distances at PNLTM are limited to 2 EPNdB as discussed in H36.205(f) and H36.111(c) & (d) of part 36.
Lateral cross-sectional view of take-off corridor 20 m 20 m Reference flight track 10° 10° Ground surface Figure 11. Lateral deviation tolerances for takeoff
(5) Helicopter test weight Page 138
The weight of the helicopter during the noise certificatio n demonstration (see H36.101(b) of part 36) must lie within the range of 90 percent to 105 percent of the maximum takeoff weight for the takeoff demonstration. No adjustment of the noise data to maximum takeoff weight is required. At least one takeoff test run must be completed at or above this maximum certificated takeoff weight. If the value of the maximum takeoff weight selected for noise certification is less than that used fo r airworthiness certification, then the lower weight may become the operating limitation defined in the appropriate section of the RFM 401(h)(6) Takeoff flig ht test procedures AMC §H36.103 [ETM AMC No. 2 A2 8.1.2.1]
(1) Test takeoff profile The test takeoff profile requires stabilized flight conditions only over the 10 dB-down period in the climb portion of the procedure.
(2) Number of test runs At least six test runs are required with simultaneous noise measurements at each of the noise measurement points.
It should also be remembered that synchronized noise and flight path data are required. Since it cannot be determined until the analysis is partly completed if each test run meets all the requirements of Appendix H, the applicant will find considerable merit in conducting additional takeoff test runs. Experience suggests that 8 to 10 test runs would normally provide an adequate safeguard against some test runs being determined invalid during subsequent analysis. If additional test runs are conduc ted and more than six valid noise measurements are simultaneously obtained at all three measurement points, then the results of such test runs are also required to be included in the averaging process fo r calculating EPNL. The results of test runs without simultaneous noise measurements at all three measurement points are not included in the calculation process.
(3) Flight airspeed tolerance A ±5 kt (±9 km/h) tolerance about the reference airspeed is specified in H36.103(b) of part 36. This is not intended to allow tests at different speeds but rather to account for variations during the 10 dB-down period which occur during an individual test run as a result of the pilot attempting to maintain the other takeoff requirements and testto-test variations.
The value of Vy is published in the takeoff performance section of the RFM and is typically defined as an IAS.
The applicant should note the reference airspeed is the tr ue airspeed (TAS). Since most airspeed instruments do not indicate the TAS value, airspeed calibration curves and meteorological conditions should be used to convert between TAS and IAS.
(4) Horizontal adjustment of climb initiation Position C in Figure 10 may be varied, subject to approval by the FAA, to minimize the difference between the test and reference heights vertically above the flight track noise measurement point. This difference can result from the effect of wind on the climb angle during testing. Figure 12 illustrates the case of a headwind. Note that even for zero or very low wind, the transition from the horizontal flight to the climb can take a significant time.
This will be the case normally on larger and heavier helicopters. The resulting flight path could be well below the reference profile. In this case there would be merit in moving position C further away from the noise measurement point. This is illustrated in Figure 13.
(5) Vertical adjustment of climb initiation Subject to approval by the FAA, the height of the initial level flight may also be varied in order that the height (distance) adjustment associated with the climb phase can be minimized. This is an equivalent procedure, which Page 139
can be used in the place of adjusting the horizontal location of position C fr om the flight track noise measuring point to achieve the same result.
The applicant should note that under many test conditions no such adjustments are required to comply with the data adjustment procedures defined in H36.205(f).
Fr Cr C F Take-off test path with adjustment to position “C” Take-off test reference profile Position “C ” is adjusted to p r osition “C” to reduce the height difference relative to the reference height over the measurement point Position “C” is moved towards measurement point “A” Wind direction A Figure 12. Adjustment of takeoff profile position “C” for headwind Note.— The above procedures for horizontal or vertical adjustment of climb initiation are based on consideration of the height above the flight track or center noise measuring point, even though the adjustments to the noise measurements are applied for the PNLTM point. However, since the PNLTM point, which is normally within close proximity of the overhead point, cannot be determined until after the noise analysis is conducted, use of height over the noise measuring point to determine the location of position C (or the initial horizontal height) is acceptable.
(6) Practice flight Irrespective of which method is used to control the height over the flight track noise measurement point, an applicant may find it helpful, if not essential, to conduct a number of practice or pre-noise certification test runs to adjust the height/location of position C. With prior ap proval of the FAA, these practice runs can be excluded from the noise compliance evaluation. These runs should also be documented in the noise certification report as practice flights.
(7) Power setting Takeoff power at sea level and 77°F (25 C) has to be applied at position C to initialize the climb. On many helicopters the airworthiness power limit will be set by the takeoff gearbox torque limit. When this is not the case, the takeoff torque will be that torque determined during the airworthiness certification and will be based on the minimum specification engine(s) power.
Cr Fr Ir F A C CA > C Ar At zero or low wind conditions Reference take-off/climb path Resulting take-off path with adjustment at position “C” At zero or low wind conditions, position “C” is adjusted to reduce height difference relative to reference height over noise measurement station “A” Position “C” is moved further away from noise measurement station “A” Figure 13. Adjustment of takeoff prof ile position “C” for zero or low wind In some cases the applicant may find that the takeoff gearbox torque limit to which the helicopter is to be airworthiness certificated has not been approved and hence cannot be used during the noise certification test.
When testing only at a lower torque is possible, the FAA may approve, as an equivalent procedure, the extrapolation of noise data from lower torque settings. Tests conducted at the maximum available, and a minimum of two, lower gearbox torque settings would be required for extrapolation, subject to approval by the FAA, to a higher torque value. Experience suggests that extrapolation of no more than 10 percent is likely to be acceptable.
The applicant will also need to document in detail the extrapolation procedure to be used.
(8) Rotor speed Rotor speed may be manually or automatically varied on some helicopters. On many designs variation in rotor speed can occur due to the limits of the engine/rotor governing system. In order that the noise levels are representative of normal takeoff operation, the rotor speed is to be the maximum normal value associated with the reference takeoff airspeed. Since on most helicopter s small rotor speed changes occur during a stabilized flight, a ±1 percent rpm variation in the rotor speed is allowed.
Note.— Noise measurements should be made at the maximum rotor speed during normal operations. Testing at the maximum tolerance rpm is not required.
On some helicopter designs more than one rotor speed ma y be available (see 402(e)(1)(vi)). If multiple rotor speeds can be used for normal operations, then noise certification has to be conducted at the highest value allowed at the reference conditions. If the highest speed is limited to special operations, or if the helicopter is configured such that the highest rotor speed cannot be used at the reference conditions or test height, then, subject to approval by the FAA, testing at a lower rotor speed may be allowed.
On some helicopter designs, rotor speed may be automatically varied within the 10 dB-down period. In such cases, a reference rotor rpm schedule as a function of position along the reference flight path should be defined and tests should be conducted so as to maintain the test rotor rpm within 1% of the reference rpm schedule. If the variation in rotor speed results in changes to the best rate of climb, a non-linear reference flight profile should be defined and used in the calculation of reference dist ances for adjustments of the noise data to reference conditions.
For example, for a helicopter that automatically varies rotor speed (N r) during takeoff, a reference rotor speed Page 141
schedule may be defined as a function of height above ground level along the reference flight path during the 10 dB-down period as illustrated in Figure 14. In the typical case where the helicopter is main gearbox torque limited at the noise certification reference conditions, the rotor speed schedule may also result in a non-constant (curved) reference flight path segment during the N r transition as illustrated in Figure 14. The 1% test requirement for rotor speed would be applied to the reference rotor speed schedule as shown in Figure 14.
86 88 90 92 94 96 98 100 102 104 –600 –500 –400 –300 –200 –100 0 100 200 300 400 500 600 Horizontal distance from flight track measuring station (m) Rotor RPM (% N )r Height AGL (m) 0 40 80 120 160 200 240 280 320 360 400 Rotor rpm (% N )r Height AGL (m) Constant reference flight path segment Non-constant reference flight path segment Constant reference flight path segment Approximate 10-dB down period Reference N schedule r End of N transition r Beginning of N t r a n s i t i o nr –1% Nr –1% Nr +1% Nr +1% Nr Figure 14. Example of a reference takeof f flight path and rotor speed schedule (with ±1% Nr limits) for a variable rotor speed helicopter
(9) Flight path guidance To meet the requirement of being within ±10 of the vertical, the applicant may need to use flight track markings that are clearly visible and/or on-board flight track guidance instrumentation and a real-time position measuring system or some other approved method of checking that this requirement is satisfied. Pilot visibility of the ground when climbing may be somewhat limited and thus markers well ahead of the noise measuring point may be required. Some of the flight path measurement systems outlined in 302, provide flight track data in real time, or in a short period after the test run. In this case the applicant will readily be able to determine if a test run is within the allowable deviation limits. If a simpler system is utilized, such as photographic scaling based on the use of still cameras, the applicant may find it useful, if not essential, to develop a method to enable timely confirmation that the test run is acceptable. The actual height and off-track deviations do not need to be established at the time of the test run. However the applicant needs to ensure that otherwise acceptable test runs are not rejected during analysis for failing to meet the ±10 limit for lateral deviation.
401(h)(7) Flyover configuration GM §H36.105 [ETM GM No. 1 A2 8.1.2.2]
(1) Reference flyover profile Appendix H specifies the reference procedure as a level flyover at 492 ft (150 m) above the ground at the flight track measurement point as illustrated in Figure 15, in which the reference flight profile is indicated as D r to Jr and the test profile as D to J.
G S(sideline) A S Reference overflight path projection Measured overflight path 150 m (492 ft) 150 m (492 ft) 150 m (492 ft) Dr Mr Gr Jr JD r M Reference overflight path Figure 15. Comparison of mea sured and reference flyover profiles The reference airspeed is 0.9 V H, 0.9 V NE, 0.45 V H + 65 kt (0.45 V H + 120 km/h) or 0.45 V NE + 65 kt (0.45 VNE + 120 km/h), whichever is less, throughout the 10 dB-down period. The rotor speed (rpm) is fixed at the maximum normal operating value. Note that if VH is greater than VNE then the reference airspeed will be related to VNE.
Note.— For clarity the location of the test and reference PNLTM points, M and Mr, are illustrated at the same position for both the centerline noise measurement point A and the lateral noise measurement point location S. Normally, however, M, and hence Mr, will be a different position on the test and reference flight path for each noise measurement point.
GM §H36.105 [ETM GM No. 2 A2 8.1.2.2]
(1) Flight path deviations To enable the flyover noise characteristics to be obtained, the flyover test has to be a level flight at a fixed height above the flight track noise measurement point. The test runs also have to be within ±10 or ±65 ft (±20 m), whichever is the greater, from the vertical throughout the 10 dB-down period. The ±65 ft (±20 m) is not relevant in the case of flyover since the off-track deviation allowed, at the test height, is controlled by the ±10 requirement.
(2) Test airspeed tolerance The flight airspeed is defined in H36.3(d) of part 36 and a ±5 kt (±9 km/h) tolerance from the reference airspeed during each flyover is allowed within the 10 dB-down period. The power is to be stabilized, and the mean value of the rotor speed during the 10 dB-down period is to be within ±1 percent of the normal operating rpm value for each flyover.
(3) Source noise adjustment testing The applicant should consider the requirement for source noise adjustments since it is unlikely that the tests can Page 143
be conducted precisely at the reference temperature of 77F (25C), reference rotor speed and reference airspeed.
This dictates that, if the test advanc ing blade tip Mach number is different from the reference Mach value, the development of a PNLTM versus advancing blade tip Mach number sensitivity curve is necessary. This requires testing at different flight speeds around the reference flight speed. The number of additional test runs will to some extent depend on the character of the variation of PNLTM with flight speed, but since this cannot be determined until after the analysis is complete, conservative estimates for the number of additional test runs and the actual airspeeds to be used need to be considered.
Note.— The equivalent Mach number procedure, discussed in section 10 of 401(h)(8), is an acceptable method of compliance that eliminates the need for a source noise adjustment.
(4) Helicopter test weight The weight of the helicopter during the noise certification demonstration (see H36.101(b)(6)(ii) of part 36) must lie within the range of 90 percent to 105 percent of the maximum takeoff weight for the flyover demonstration.
At least one flyover test must be completed at or above this maximum certificated weight. If the value of the maximum takeoff weight selected for noise certification is less than that used for airworthiness certification, then the lower weight may become the operating limitation defined in the appropriate section of the RFM.
401(h)(8) Flyover test procedure AMC §H36.105 [ETM AMC A2 8.1.2.2]
(1) VH VH is defined as the airspeed in level flight at the reference conditions and maximum certificated takeoff weight and is obtained using the minimum specification engine(s) torque at maximum continuous power. V H will need to be determined specifically for the noise certification fl yover tests, since its determination is not required for the airworthiness certification. V H, by itself, is never limited by airworthiness considerations. However the maximum continuous power on which it is based may be limited due to airworthiness issues and that, in effect, could limit the value of VH.
(2) VNE VNE is determined as a part of the airworthiness approval and will therefore be readily available.
(3) Reference airspeed On some helicopters VH may be in excess of the level flight V NE imposed and approved by the FAA. The intent of noise certification is not to relate test airspeeds to reference airspeeds that may be beyond the airworthiness VNE limit of the helicopter. Under H36.3(d) of part 36, V NE would therefore apply in place of VH. Also on some helicopters with high airspeed capabilities, the typi cal cruise airspeed will be less than 0.9 V H (or 0.9 VNE) and thus if 0.9 VH (or 0.9 VNE) were used as the reference, it would no longer be representative of a cruise flight. In this case a lower airspeed of 0.45 VH + 65 kt (0.45 VH + 120 km/h) or 0.45 VNE + 65 kt (0.45 VNE + 120 km/h) is used. This applies when 0.9 VH (or 0.9 VNE) is 130 kt (240.8 km/h) or higher (i.e., when VH (or VNE) is 144.4 kt
(267.5 km/h) or higher). Thus the reference airspeed will be the least of the following four airspeeds:
a) 0.9 V H;
b) 0.45 V H + 65 kt (0.45 VH + 120 km/h);
c) 0.9 V NE; or
d) 0.45 V NE + 65 kt (0.45 VNE + 120 km/h).