(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.
Reference flight track noise measurement station Reference flight path Reference touchdown point +0.5° –0.5° Test window 20 m 20 m Reference flight track Helicopter height 150 m 10° 10° 120 m (394 ft) 1 140 m (3 748 ft) 6° 6° 6° + 0.5° 6° – 0.5° Figure 20. Flight boundaries for approach test condition
(3) Flight path guidance The helicopter has to fly within the 6° ± 0.5 approach angle range and within ±10 or ±65 ft (±20 m), whichever is greater, of the vertical above the reference flight track throughout the 10 dB-down period. Thus the helicopter has to fly within a “rectangular funnel” as illustrated in Figure 20. To ensure flight within these limits, positive guidance to the pilot will most likely be required. This guidance can take many different forms, varying from onboard instrumentation providing, for example, a box in which the pilot flies the aircraft, cross hairs where the pilot flies the helicopters at the center, or an external light guidance system such as a visual approach slope indicator (VASI) or pulsating light approach slope indicator (PLASI) located at or near the imaginary touchdown point where the 6 angle reaches the ground. The system chosen by the applicant should be approved by the FAA prior to testing.
(4) Flight path intercept H36.3(f) of part 36 specifies that each approach test run be continued to a normal touchdown. The noise data are taken during stabilized flight conditions within the 10 dB-down period and thus may not be impacted by the flare Page 151
or the final touchdown. Also, for flight safety reasons, it may not be desirable to continue the test run on a 6 profile to the ground. As a result an equivalent procedure may be used, subject to approval by the FAA, where the helicopter can break off from the descent after the second 10 dB-down point is reached. This can be completed without the need to actually land the helicopter, offering considerable savings in flight time, providing the other requirements are met.
(5) Wind direction Although part 36 does not specifically require that the test runs be conducted into the wind, this is advisable since it will provide a safer and more stable flight environment.
(6) Rotor speed guidance The comments on rotor speed guidance discussed in H36.3(c) and H36.101(b) of part 36 are equally applicable for approach noise testing.
(7) Other test requirements The comments on the height, flight airspeed variation and rotor speed measurements discussed in H36.3(d) & (e), H36.105(b), and H36.105(c) & (d) of part 36 are equally applicable for approach.
401(h)(12) Adjustments of PNL and PNLT GM §H36.205(f) [ETM GM A2 8.3.2]
(1) Units For calculations in SI units, the distances are measured in meters and (i) and (i)0, used in determining 1, are expressed in dB/100 m. In this case a constant factor of 0.01 is used for the first and second terms of the 1 adjustment. If the U.S. Customary (English) system of uni ts is used the distances are measured in feet and (i) and (i)0 are expressed in dB/1000 ft. In this case a constant factor of 0.001 is used for the first and second adjustment terms.
(2) Zero adjustment test window If the test conditions fall within the “zero attenuation adju stment window” shown in Fi gure 37of 402(c)(1), the sound attenuation adjustment for the effects of atmospheric absorption of the test data may be taken as zero subject to prior approval by the FAA (see 402(c)(1) for details).
401(h)(13) Duration adjustment to EPNL AMC §H36.205(g) [ETM AMC A2 8.3.4]
(1) Adjustment for flight path The distances associated with the PN LTM position used to calculate the ad justments under H36.205 of part 36 are used in the calculation of the first term of the 2 duration adjustment to EPNL.
Note.— If the test conditions fall within the window shown in Figure 37of 402(c)(1), the ratios of the reference and test slant distances for the propagation path adjustments in the first term of the adjustments to the duration correction may be replaced by the ratios of the reference and test distances to the helicopter when it is overhead the flight track noise measurement point (see 402(c)(1) for details).
(2) Adjustment for ground speed differences Page 152
The ground speed must not be confused with the actual airspeed used during the tests and will be a function of both the flight test airspeed and wind speed. The reference ground speed, VGr (based on the assumption of a zero wind condition) is, for takeoff an d approach, the horizont al component of the reference airspeed, V y (in true airspeed) defined in H36.103(b) and H36.107(b) of part 36 and, for flyover, the reference airspeed defined in H36.105(c) of part 36. For takeoff the reference ground speed V Gr is the reference airspeed V y times the cosine of the reference climb angle .
(3) Microphone height To make the necessary adjustments, the microphone height above the ground, 4 ft (1.2 m), is to be taken into account when calculating the sound propagation path fr om the position at which the PNLTM occurs, to the microphone.
Note.— For each noise measurement point during each test run, the PNLTM will normally occur at a different position on both the test and reference flight paths.
401(i) Adjustment of Airplane Flight Test Results §A36.9 [ETM 4.1.9] 401(i)(1) Adjustments to reference conditions GM §A36.9.1 [ETM GM A2 8.2]
(1) Adjustments to reference conditions Most noise certification tests are conducted during conditions other than reference conditions. During these tests the airplane may be at a different height over the microphone, or deviate laterally from the intended flight path.
The engine thrust (power), atmospheric conditions, airplane height and/or gross weight might also differ from reference conditions. Therefore, measured noise data sh ould be adjusted to reference conditions to determine whether compliance with the noise certification limits of Stage 3 or 4 or 5 of part 36 may be achieved. 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 review by the FAA before they can be used for noise certif ication evaluations. Program validation should be planned and the required information submitted to the FAA early in the certification cycle, since the time required for evaluation and approval may vary depending on 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(c). 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) High altitude test sites For test sites at or above 1200 ft (366 m), data must be adjusted to account for jet noise suppression due to the difference in the engine jet velocity and jet velocity sh ear effects resulting from the change in air density. This adjustment is described in 403(b)(3).
401(i)(1)(A) Origin of reference data GM §A36.9.1.1 [GM 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 during takeoff with maximum gross weight;
b) flyover, lateral and approach engine thrust (power) or thrust settings at reference conditions;
c) engine cutback thrust (power) reduction requirements at reference flyover conditions;
d) data defining negative runway gradients (not applicable when an applicant uses flight path intercept techniques); and
e) reference airspeeds during flyover, latera l and approach tests at maximum gross weight.
401(i)(2) Takeoff configuration/profile GM §B36.7(b)/A36.9.2.1 [ETM GM A2 8.1.1.2]
(1) Takeoff tests The reference takeoff configuration selected by the applicant should be within the approved airworthiness certification envelope. Special flight crew procedures or aircraft operating procedures are not permitted.
(2) Takeoffs with thrust (power) reduction Figure 21 and Figure 22 illustrate an example of the effect of thrust (power) reduction on the PNLT time history and the associated flight path. After thrust (power) reduction a slight decr ease in the climb gradient may occur due to the thrust (power) lapse that results from increased height during the 10 dB-down period.
Flyover noise time-history with thrust (power) reduction 10 dB A Alone B Alone Time Spooldown Point of thrust (power) reduction Full power PNLT Figure 21. Takeoff noise time history Engine spool-down period Point of thrust (power) reduction Full take-off thrust (power) flight path Noise measurin g point Reduced thrust (power) flight path Figure 22. Takeoff flight path over flyover measuring point with thrust (power) reduction
(3) Full thrust (power) takeoffs Full thrust (power) takeoffs are also permitted as the reference flyover noise certification procedure and are a requirement for the lateral noise certification procedure. Maximum approved takeoff thrust (power) is to be used from the start-of-roll (see point A in Figure 23). Lift-off from the runway is at point B, after which the landing gear is retracted and flap positions adjusted. At point C, the stabilized climb angle and airspeed are achieved while maintaining full takeoff thrust (power). The airplane continues to climb until sufficiently past point F to ensure that the 10 dB-down time noise value is measured at point K. Between points C and F, the thrust (power), flight path and aircraft configurations are to be kept constant.
Test end PNLTM PNLTM Flight path Point C Point B Lift-off Brake release Point A Test start Microphone Point K Point M PNLTM – 10 dB PNLTM – 10 dB Point F 10 dB Time C F 10 dB-down period 21 325 ft (6 500 m) Altitude PNLT Figure 23. Normal full thrust (power) takeoff
(4) Flight path Figure 24 illustrates the envelope for flight path tolerance within which the flight crew should fly between points C and F. FAA have permitted a ±20 percent tolerance in overhead test height and a ±10° lateral tolerance relative to the extended runway centerline. Th ese tolerances permit the applicant to conduct testing during most wind conditions with minimal risk of retesting being required due to off-target flight paths. In conjunction with the climb gradient and approach angle, these flight path deviation limitations define the takeoff “flight path” through which the aircraft is to fly during and throughout the noise measurements (i.e., throughout the 10 dB-down period).
During flyover and lateral noise measurements, the extended centerline is not visible and it may be more difficult to conduct flight within the approved flight path, especially during conditions with anomalous winds aloft. Several methods have been devised to assist and provide direction to the flight crew in order to stay within the required flight path envelope. Indicators located in the airplane cockpit can provide flight path direction and indicate deviations from the extended runway centerline. Transmissions from the airplane position-indicating system (e.g., microwave position system, precision DMU or DGPS) can also provide useful inputs.