dynamic range is further decreased. Significant improv ement of amplitude linearity can be obtained via the implementation of techniques such as oversampling and dithering. Therefore, testing must be performed to determine the actual limits for each digital recording system. Note that assumptions based on experience with analog systems do not always apply.
401(c)(6) Pre-emphasis AMC §A36.3.6.2(a) [ETM AMC A2 3.6.2]
(1) Pre-emphasis systems Use of pre-emphasis systems will be allowed only if the system also employs complementary de-emphasis.
Attempts to compensate for the effects of a pre-emphasis filter by applying one-third octave band de-emphasis adjustments, either numerically to analyzed data via a pink noise adjustment, or on a band-by-band basis using separate gain stages for each one-third octave band filter, are not allowed. In addition, use of a pre-emphasis/deemphasis system will require testing and documentation of all filters and gain stages involved to ensure that any errors are quantified and minimized and that the system performs predictably and reliably.
401(c)(7) Attenuator specifications AMC §A36.3.6.9 [ETM AMC A2 3.6.9]
(1) Attenuator specifications Attenuator specifications allow for the use of switchable voltage input range settings, now commonplace on DAT recorders, as controllable attenuation steps for gain-settin g purposes. In all cases, attenuators should have fixed repeatable steps. Any devices in the measurement system that use vernier or continuously-adjustable gain controls should also have some demonstrable means of being fixed, or locked at a specific setting, to eliminate non-traceable gain errors.
401(c)(8) Linear integrating analyzer specifications GM §A36.3.7.2(c) [ETM GM A2 3.7.2]
(1) Externally controlled linear-integrating analysers In cases where a computer or other external device is used to control and/or communicate with an analyser performing linear integration, extra care should be taken to ensure that the integration period requirements are met. Some analysers from major manufacturers have required a factory modification in order to provide an integration time within 5 ms of the specified 500-ms integration period.
401(c)(9) Analyzer perf ormance specifications GM §A36.3.7.4 [ETM GM A2 3.7.3]
(1) Analyzer specifications Reference T2 specifies the electrical performance requi rements of one-third octave band filters, including tolerances for the attenuation in the transition bands (i.e ., “skirts”) adjacent to the one-third octave passbands.
Most digital one-third octave band analysis systems o ffer only hardwired filtering algorithms that emulate the response of a traditional third-order analysis filter havi ng a maximally flat passband. However, some analysis systems allow the selection of other filtering algorithms which might not provide equivalent performance.
Applicants should demonstrate the effects that alternate filter design response characteristics might have on noise certification EPNL values.
(2) Determination of bandwidth error adjustments The manufacturer can establish the geometric center frequen cies of one-third octave band filters using either Page 127
Base 2 or Base 10 systems. While the use of either method results in frequencies close to the nominal center frequencies referred to in Table A36-3, it is important to note which system is used so that the bandwidth error adjustment can be properly determined. Use of test frequencies calculated by a different base-number system than that for which the analyzer was designed can result in erroneous values for these adjustments.
401(c)(10) Microphone incidence adjustments AMC §A36.3.9.3 [ETM AMC A2 3.9.3]
(1) Applications of adjustments for incidence When using microphones whose frequency response is nearly flat at grazing incidence, and when the angles of incidence of sound emitted from the aircraft are within ±30 of grazing incidence, a single set of data adjustments for free-field response and windscreen insertion loss, based on grazing incidence, is considered sufficient to account for incidence effects. When it is impractical to orient the micr ophone properly to maintain grazing incidence, provided that a continuous record of TSPI is available, free-field and windscreen insertion-loss incidence data adjustments can be applied to the noise da ta on a spectral-record by spectral-record basis. These adjustments are obtained by calculating the angle of in cidence for each record, usin g the point of time which characterizes the 2-second averaging period (see A36.3.7.6 of part 36) and determining the aircraft’s sound emission coordinates and angle of incidence for the sound measured at that time.
401(c)(11) Pink noise specification GM §A36.3.9.4 [ETM GM A2 3.9.4]
(1) Pink noise Pink noise contains equal energy in each octave band or fractional octave band (e.g., the octave from 100 Hz to 200 Hz contains the same amount of energy as the octave from 1 kHz to 2 kHz, although for the lower-frequency octave, it is distributed over a frequency range 10 times narrower).
(2) Pink noise usage Because of the dynamic nature of the pink noise signal, longer samples produce statistically better measurements.
A minimum duration of 30 seconds of pink noise should be recorded.
401(c)(12) Measurement sy stem field calibration AMC §A36.3.9.5 [ETM AMC A2 3.9.5]
(1) Measurement system field calibration (all components of the measurement system, except microphones) All components of the measur ement system, except microphones, should be tested while deployed in the field, using pink noise at a level within 5 dB of the calibration level (see A36.3.9.5 of part 36). The signal should be recorded for a duration of at least 30 seconds so that one-third octave band system frequency response adjustments can be determined and applied during analysis. The pink noise generator should be calibrated within 6 months of the measurement and is acceptable for certification use only if its output in each one-t hird octave band does not change by more than 0.2 dB between calibrations.
401(c)(13) Field acoustical calibrations AMC §A36.3.9.8 [ETM AMC A2 3.9.8]
(1) Field acoustical calibrations All components of the system, excluding the windscreen, should be in place at this time, including cables, attenuators, gain and signal-conditioning amplifiers, filters (including pre-emphasis) and power supplies. During Page 128
calibration, attenuators and gain stages should be set to prevent overload and to maintain the calibration signal level on the reference level range within the limits specified in A36.3.6.6 of part 36. If any switchable filters that could affect the calibration signal are utilized during measurements, then calibrations should be performed both with and without these filters enabled. Components of the electrical system should not be added, removed or replaced without re-calibrating the entire system immediately before and after each change.
401(c)(14) Windscreen loss adjustment AMC §A36.3.9.10 [ETM AMC A2 3.9.10]
(1) Determination of windscreen data adjustments The physical condition of a windscreen can significantly affect its performance, and manufacturer-provided windscreen data adjustments for insertion loss are valid only for new, or clean, dry windscreens. For these adjustments, a single set of values based upon windscreen insertion loss tests at grazing incidence may be used when the angles of incidence of sound emitted from an aircraft are within +30° of grazing incidence. For other cases, the windscreen insertion loss adjustments should be determined and applied on the basis of intervals between angles tested not exceeding 30°.
When the windscreen data adjustments provided by the manufacturer are presented in the form of curves, care should be taken to include the insertion loss throughout each one-third octave band, rather than just at the nominal midband frequency. Windscreen insertion loss can vary substantially within the frequency range of a single band and must be averaged or faired to more accurately correct one-third octa ve band data for the presence of the windscreen. Windscreen data adjustments may also be obtained by free-field calibration in an anechoic chamber.
401(c)(15) Measurement sy stem background noise AMC §A36.3.10.1 [ETM AMC A2 3.10.1]
(1) Measurement system noise Since measurement system noise can add energy to m easured aircraft noise levels, the background noise measurement described in A36.3.10.1 of part 36 should be made with all gain stages and attenuators set as they would be used during the aircraft noise certification measurements. If it is expected that multiple settings will be required during the measurements, background noise data should be collected at each of these settings. Care should be taken to ensure that the background noise is truly representative of that present during the aircraft noise certification tests.
(2) Mean background noise assessments At least 30 seconds of background noise data must be time-averaged to determine the mean level for each onethird octave band. The PNL value for this averaged spectrum should then be calculated using the procedures defined in A36.4.1.3 a) of part 36. The aircraft noise level data should also be analyzed and PNL values calculated for each spectral record. The maximum aircraft PNL value should be at least 20 dB above the PNL of the averaged background noise spectrum for the data to be considered acceptable.
401(d) Calculation of EPNL from Measured Data §A36.4 [ETM 4.1.4] 401(d)(1) Instantaneous sound pressure levels GM §A36.4.1.2 [ETM GM A2 4.2]
(1) Instantaneous sound pressure levels For the purposes of this procedure, “instantaneous” sound pressure levels are considered to be one-third octave band sound pressure levels for each one-half second record obtained using a continuous exponential averaging Page 129
process, as described in A36.3.7.5 of part 36 or its equivalent.
401(d)(2) Tone correction calculation AMC §A36.4.3.1 [ETM AMC A2 4.3.1]
(1) Data precision for tone correction computation Prior to Step 1, it is recommended that all one-third octave band sound pressure levels be temporarily rounded to
0.1 dB resolution. The tone correcti on procedure presented here includes several steps that utilize decibel level criteria to characterize the significance of tonal content. These criteria can become artificially sensitive to small variations in level if a resolution finer than 0.1 dB is used in the computations.
401(d)(3) Adjustments relati ng to background noise AMC §A36.4.3.1(d) & (e) [ETM AMC A2 4.3.1 (Steps 4, 5)]
(1) Data adjustments for background noise When the technical procedure presente d in 306(c)(2) of this AC is used for adjustment for the effects of background noise, Steps 4 and 5 of this tone correction procedure should be modified as follows:
— Step 4. The LGB should be used in place of the highest frequency band (i = 24); and — Step 5. A new slope, s’(25,k), should be calculated for the band beyond LGB as described for an imaginary 25th band. This slope should be used in place of the slope derived from the actual level of the band beyond LGB.
401(d)(4) Data resolution afte r tone correction calculation AMC §A36.4.3.1(j) [ETM AMC A2 4.3.1 (Step 10)]
(1) Data precision (after calculation of tone correction factor) At this point, the original sound pressure level resolu tion of 0.01 dB should be restored. Although the required precision of reported EPNL is 0.1 dB, all other intermedia te calculations external to the tone correction process should maintain a precision of at least 0.01 dB.
(2) Identification of pseudo-tones Section 403(b)(2) presents guidance material on methods for identifying pseudo-tones. Note that the use of ground plane or 33 ft (10 m) microphones is supplemental to the required 4 ft (1.2 m) microphones and is allowed only for identification of frequency bands within which pseudo-tones might occur and not for the determination of aircraft certification noise levels.
(3) Tone correction factor adjustment When tone correction f actors result from false or fictitious tones, recalculation is allowed using revised sound pressure level values, based on narrow-band analysis, of the smoothed spectral levels obtained in Step 7. Once the levels have been revised, the tone correction factor should be recomputed for the revised one-third octave band spectrum. This recomputed maximum tone correction factor should be applied, even if it occurs at or near the band associated with an artific ial tone, and approval by the FAA should be obtained for the methodology used.
401(d)(5) Band sharing adjustment GM §A36.4.4.2 [ETM GM A2 4.4.2] Page 130
(1) Band-sharing adjustment concept The one-third octave band filtering process specified for analysis of aircraft noise certification data in A36.4.3.2 of part 36 may allow the tone correction procedure to und er-predict a tone correction factor when the frequency of a tone is located at or near the edge of one or more one-third octave bands. To account for this phenomenon, a band-sharing adjustment is computed that takes advantage of the fact that, as a result of the Doppler effect, a tone that is suppressed at PNLTM will probably appear normally in the spectra that occur before or after PNLTM. By averaging the tone correction factors calculated for the spectra within a 2-second period around PNLTM, the tone correction factor that would have occurred at PNLTM if it were not suppressed can be reasonably estimated.
401(d)(6) Calculation of band sharing adjustment AMC §A36.4.4.2 [ETM AMC A2 4.4.2]
(1) Computation of band-sharing adjustment Although part 36 refers to identification of the frequency bands in which maximum tone corrections occur for the records near PNLTM, the presence or absence of band-sharing cannot be established merely by observing these frequencies. Even though the maximum tone that occurs in a one-third octave band spectrum may not be related to the band of maximum tone correction in the PNLTM spectrum, a related tone may still be present. Therefore, the average of the tone corrections of all spectra within one second (i.e., five, one-half-second data records) of PNLTM should be used regardless of the bands in which maximum tones are found. If the band-sharing adjustment is believed to result from effects other than band-sharing, the applicant should demonstrate its absence for each event.
(2) Adjustment of PNLTM for band-sharing The band-sharing adjustment should be computed before the determination of the 10 dB-down period and should be included in the reported PNLTM and EPNL values for the test condition data.
(3) Application of band-sharing adjustment for simplified procedure When the simplified procedure is used to adjust data to reference conditions, the band-sharing adjustment should be applied to the PNLTr at time of PNLTM before “∆1” and EPNLr are calculated.
(4) Application of band-sharing adjustment for integrated procedure When the integrated procedure is used to adjust data to reference conditions, a new band-sharing adjustment should be calculated as in A36.4.4.2 of part 36. This new band-sharing adjustment uses the average of the tone correction factors of the PNLTMr spectrum and the tw o preceding and two succeeding spectra, after adjusting them to reference conditions, and should be applied to the PNLTMr value prior to identification of the reference condition 10 dB-down points and calculation of EPNLr.
401(d)(7) Noise Duration AMC §A36.4.5 [ETM AMC A2 4.5]
(1) Noise duration (10 dB-down period) This period is the portion of the aircraft flyover in wh ich the measured noise level is within 10 dB of PNLTM
(i.e., the period to be used for the calculation of EPNL). To ensure an adequate duration of recorded noise, recording systems should be activated, and the aircraft maintaining a stable condition, when the noise level at the first microphone location is estimated to be approximately 20 dB(A) below what is expected to be L Amax. Care should be taken during use of the flight path intercept method (see 402(a)(1)(i)) to ensure that noise levels have fallen 20 dB(A) below LAmax before flight path go-around procedures are initiated.
Note.— If recorded data do not encompass the entire 10 dB-down period, an EPNL cannot be calculated from those data, and the event should not be used for aircraft noise certification purposes.
(2) Identification of the first and last records within the noise duration When identifying the records that define the limits of the noise duration, those records having PNLT values closest to the actual value of PNLTM – 10 dB should be used. As a result, the PNLT values for the PNLTM – 10 dB points may not always be greater than or equal to PNLTM – 10 dB.
In order to illustrate the correct identification of the 10 dB-down points, Figure 9 provides examples of PNLT time-histories made up of records calculated from measured one-half-second values of SPL in accordance with the procedures specified in A36.4.2 of part 36. The shaded record k M represents the reco rd associated with PNLTM. Shaded records kF and kL represent respectively the first and last 10 dB-down points.
In the first example the PNLT value associated with kF is greater than PNLTM – 10. The PNLT value associated with kL is less than PNLTM – 10.
In the second example there are two records after kM with a value equal to PNLTM – 10. In this case kL is the last of the two records. The first 10 dB-down point, k F, is the record closest in valu e to PNLTM – 10, ignoring any records that precede it with greater values but which are less in value than PNLTM – 10.
Note.— In all cases in the calculation of EPNL, the contribution of all the records from kF to kL inclusive should be included.
401(d)(8) Equations for co mputing duration correction GM §A36.4.5.4 [ETM GM A2 4.6.2]
(1) Duration correction factor The equation for the duration correction factor, D, in A36.4.5.4 of part 36 is valid only for records of one-half second in length. The constant value 13 is used to normalize the one-half second values to the 10 seconds standard duration
(i.e., 10 seconds duration comprising twenty 0.5-second data records and 10 log (20) = 13.01).
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