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).
(4) Flight path/height determination The flight path is required to be “straight and level”. Since there is no requirement for the terrain over which the helicopter is flying to be perfectly level, the height of the helicopter above the ground may vary slightly over the distance corresponding to the 10 dB-down period. If a ground-based system such as a differential GPS base station or a three-camera system is used, then the flight path /height determination will n eed to account for the actual ground elevations at which the system components are placed.
(5) Flight track deviations The allowable off-track deviation from the vertical above the reference track is limited by ±10 or ±65 ft (±20 m), whichever is the greater. The height above the flight track noise measurement point must be within ±30 ft (±9 m) of the reference height of 492 ft (150 m). The allowed off-track deviation is ±81.5 ft (±24.9 m) at the lower height limit of 462 ft (141 m) and is ±92 ft (±28 m) at the higher height limit of 522 ft (159 m). Thus the helicopter must pass through a “test window” located above the reference flight track, as illustrated in Figure 16, throughout the 10 dB-down period.
Test window 9 m 20 m 9 m 20 m Reference flight track Reference height 150 m 10° 10° Figure 16. Flight boundaries for flyover test condition
(6) Number of test runs At least six flyover test runs are required, with equal numbers with headwind and tailwind. Since the data will be adjusted, there are no requirements for these to be flown in pairs immediately one after the other. Conducting the test runs in pairs, however, would alleviate the need to take the wind direction into account. The applicant will therefore typically find it expedient to conduct tests in such a manner and include additional pairs of test runs in case any of the test runs are proved invalid on subs equent analysis. In addition to the simultaneous noise measurement at the three measurement points, the applicant should note that synchronized noise and flight path measurements are required throughout the 10 dB-down pe riod. If additional test runs are conducted and more than six valid noise measurements are simultaneously obta ined at all three measurement points, then the results of such test runs are also required to be included in the averaging process for calculating EPNL. The results of test runs without simultaneous noise measurements at all three measurement points are not included in the calculation process.
Note.— If the absolute wind speed component in the direction of flight, as measured at a height of 10 m (33 ft) above ground, is less than 5 kt (2.6 m/s), then the effect of wind direction can be considered to be negligible. In this case the measured flyover can be considered to be either a headwind or tailwind test run.
(7) Test height Since most test sites will not be completely flat, the height (distance) between th e helicopter and ground track will vary during the flyover The flight path and the relative position between the helicopter and the reference profile can be determined using a number of different systems (see 302 of this AC).
(8) Test airspeed The flyover test airspeed will be either the reference airspeed if a source noise adjustment is applied, or the adjusted reference airspeed if the equivalent Mach number method is used. The applicant should also note that the airspeed defined in part 36 is the true airspeed (TAS). Since most airspeed instruments do not indicate the TAS value, airspeed calibration curves and test-day meteorological conditions should be used to determine the IAS for use by the pilot.
(9) Source noise adjustment Source noise adjustments have to be developed for noise data measured at the center, left sideline and right sideline microphones. Test runs are conducted in two directions. “Left sideline” and “right sideline” are defined relative to the direction of flight for each test run. It follows that if a microphone is “left sideline” for a test run in one direction then it is “right sideline” for a test run in the other direction. The applicant should take care to ensure the measured noise is correctly designated.
Two methods have been adopted by various applicants to establish the source noise adjustment. The first involves testing, relative to the reference flight speed, at a number of fixed airspeeds such as Vr – 10 kt (9 km/h), Vr – 20 kt (37 km/h) and Vr + 10 kt (19 km/h). To retain the same accuracy as associated with the reference condition, six test runs (three in each direction) at each of the additional flight airspeeds are typically needed. A sensitivity curve is then developed from this data as indicated in Figure 17. Other applicants have tested over a range of airspeeds from, for example, Vr – 20 kt (19 km/h) to Vr + 10 kt (9 km/h) and developed a sensitivity curve in this manner.
In this case at least six valid test runs are of course still required at the reference airspeed. A statistically acceptable curve using this method is illustrated in Figure 18. Th e number of test runs required for either method for developing source noise sensitivity curves is subject to approval by the FAA.
(10) Equivalent Mach number test procedure To avoid testing at a large number of airspeeds over a wide airspeed range to develop a PNLTM versus a Mach number sensitivity curve, the applicant may, subject to approval by the FAA, use the equivalent procedure presented in 402(c)(2)(B). With this procedure a single series of test runs is conducted at an adjusted reference airspeed. The minimum number of acceptable test runs is six (three in each direction), and the previous comments on the need for the applicant to consider making additional test runs to cover the case where some test runs may subsequently be found to be invalid is equally applicable . When this procedure is used the airspeed tolerance is reduced from ±5 kt (±9 km/h) to ±3 kt (±5.5 km/h) In addition all the other limits applicable to testing at the reference airspeed also apply.
This equivalent procedure requires measurement of the on- board outside air temperatur e just prior to each test run. Under stable ambient temperature conditions this is relatively straightforward and calculations can be made on the ground prior to each test run. When changes in temperature are occurring during the test period, it may be necessary to take the on-board temperature in flight just prior to reaching the initial 10 dB-down point. These can be used to make the necessary calculations to adjust the flight airspeed appropriately and ensure that the applicable adjusted airspeed and reference advancing blade tip Mach number are used for the test run.
Reference Mach Number
10/12/2017 AC 36-4D Main rotor advancing blade tip Mach number PNLTM (PNdB) r Derive the correlation of noise measurements for each noise measurement station relative to the direction of flight on each test run Test group at reference airspeed 0
0.82 0.83 0.84 0.85 0.86 0.81 –5 +5 Reference Mach number 3 = Mach number correction 3 Figure 17. Example of source noise correlation using pooled (clustered) test data 102 100 98 96 94 92 90
0.85 0.86 0.87 0.88 0.89 0.84 Main rotor advancing blade tip Mach number 7 valid flights at reference airspeed Reference Mach number 3 PNLTM (PNdB) r 3 = Mach number correction Figure 18. Example of source noise correlation using distributed test data When this equivalent procedure is used the test runs are conducted at the reference blade tip Mach number, and hence no additional source noise adjustments are required. The applicant should also note the airspeed defined in part 36 is the true airspeed (TAS), and since most airspeed instruments do not indicate the TAS value, airspeed calibration curves and test-day meteorological conditions should be used to determine the IAS for use by the pilot.
(11) Equivalent Mach number test speed Each flyover noise test must be conducted such that the adjusted refe rence true airspeed (V ar) is the reference airspeed (Vr) specified in H36.105(c) of part 36, adjusted as necessary to produce the same main rotor advancing blade tip Mach number as associated with reference conditions.
Note.— The reference advancing blade tip Mach number (Mr) is defined as the ratio of the arithmetic sum of the main rotor blade tip rotational speed (VT) and the helicopter reference speed (Vr) divided by the speed of sound (cr) at 77°F (25C) 1135.6 ft/s (346.1 m/s) such that:
ܯ ൌ ்ܸ ܸܿ and the adjusted reference airspeed (Var) is calculated from:
ܸ ൌܿ ሺ ்ܸ ܸሻെܸ ்ܿ where c is the speed of sound calculated from the on-board measurement of outside air temperature.
(12) Rotor speed/flight path guidance The comments on rotor speed and flight path guidance discussed in H36.3(c) and H36.103(b) of part 36 are equally applicable for flyover noise testing.
401(h)(9) Approach configuration GM §H36.3(f) [ETM GM No. 1 A2 8.1.2.3]
(1) Reference approach profile The reference approach profile is illustrated in Figure 19 together with an idealized measured test profile. part 36 requires flight tests to be conducted under stable flight conditions within a 6° ± 0.5 approach angle with the noise data adjusted to a 6 reference profile. The reference airspeed is V y, as used for the takeoff test, or the lowest airworthiness-approved speed for approach, whichever is the greater.
Note.— For clarity the location of the test and reference PNLTM points, N and N r, are illustrated at the same position for both the centerline noise measurement point A and the starboard lateral noise measurement point S. Normally however N, and hence Nr, will be a different position on the test and reference flight paths for each noise measurement point.
K r A150 m (394ft) starboard S150 m (492 ft) 10/12/2017 AC 36-4D Starboard Port E H K S A Er Nr Hr Kr Reference approach path projection Measured approach path Reference approach path 6° = 6° ± 0.5° 150 m (492 ft) 150 m (492 ft) N 120 m (394 ft) r S Figure 19. Comparison of mea sured and reference approach profiles
(2) Reference approach path The touchdown position is located 3740 ft (1140 m) from the intersection of the 6° reference approach path with the ground plane through position A. The flight path reference point, Hr, is located 394 ft (120 m) above position A on the ground.
(3) Helicopter test weight The weight of the helicopter during the noise certification demonstration (see H36.101(b) of part 36) must lie within the range of 90 percent to 105 percent of the maximum landing weight for the approach demonstration. At least one approach test must be completed at or above this maximum certificated weight.
For most helicopters the maximum landing weight will be the same as the maximum takeoff weight and, as a result, the same maximum weight will apply to all three test conditions. If the value of the maximum landing weight selected for noise certification is less than th at used for airworthiness cer tification, then the lower weight may become the operating limitation defined in the appropriate section of the RFM.
401(h)(10) Approach test condition GM §H36.107 [ETM GM No. 2 A2 8.1.2.3]
(1) Test airspeed Since there is no single common or well-defined approach airspeed applicable to helicopters, the tests are conducted at the certificated best rate of climb airspeed, Vy, which approximates to a typical approach speed, or the lowest airworthiness approved speed for approach, whichever is the greater.
(2) Flight path deviations Test runs are to be conducted with a 6 ± 0.5 approach angle using stabilized flight airspeed within ±5 kt (±9 km/h) of the reference V y airspeed, rotor speed within ±1 percent of the normal maximum operating rotor speed, and power. To limit the magnitude of the off-track di stance, the flight path is to be maintained to within ±10 or ±65 ft (±20 m) of the vertical, whichever is the greater, throughout the 10 dB-down period (see Figure
20).
(3) Maximum noise level measurement The intent of the Standard is to obtain noise measurements of the maximum noise levels which are likely to occur in practice during an approach flight condition. Since it is known that the maximum main rotor noise, known as blade vortex interaction (BVI) or blade slap, occurs at around 6º descent angle at a constant speed of Vy, this has been chosen as the reference condition. Only in the case where the lowest airworthiness-approved speed for approaches is greater than Vy can any approach angle exception be allowed. This would require approval by the FAA and, irrespective of approved angle, the height above the ground at the flight-track measurement point would have to be 394 ± 33 ft (120 ± 10 m).
Experience suggests however that normally there is little difficulty in conducting the approach test with a descent angle of 6° at an airspeed of Vy within the allowable limits of ±0.5° and ±5 kt (±9 km/h) respectively.
(4) Blade vortex interaction Since a 6 descent angle at a speed of V y is the approach condition likely to give the highest level of main rotor BVI, the applicant should note that although on some helicopters this will result in a steady noise signature, on other helicopters the BVI noise character can vary even under nominally steady flight conditions. This may be subjectively noticeable but is not a technical problem since the average of six test runs will normally give results well within the maximum acceptable 90 percent confidence interval of ±1.5 EPNdB.
(5) Practice flights Flying a constant 6º approach angle at a constant airspeed may be a somewhat demanding requirement for some helicopters, particularly since, in practice, a decelera ting approach at a varying descent angle is the common method utilized in helicopter operations. The applicant/pilot may find merit, therefore, in the test procedure being practiced prior to any noise certification testing.
401(h)(11) Approach test procedure AMC §H36.3(f), §H36.107 [ETM AMC A2 8.1.2.3]
(1) Reference approach procedure The 6 reference procedure is defined as being under stable flight conditions in terms of torque, rotor speed, airspeed and rate of descent throughout the 10 dB-down period. The reference airspeed is the best rate of climb true airspeed (TAS), Vy, approved by the FAA.
(2) Number of test runs At least six test runs are required with simultaneous noise measurements at each of the noise measurement points.
The applicant should, as in the case of takeoff and flyover, consider additional test runs to ensure that a sufficient number of valid data points are available. Synchronized noise and flight path measurements are required throughout the 10 dB-down period. If additional test runs are conducted 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 th e averaging process for cal culating EPNL. The results of test runs without simultaneous noise measurements at all three measurement points are not included in the calculation process.