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.
Overhead Beginning of test Allowable lateral deviation Extended runway Point M Point K Point C Point F Microphone location PNLTM – 10 dB Overhead test window End of test Flight path Allowable vertical deviation test target altitude PNLTM – 10 dB Figure 24. Takeoff flight path tolerances 401(i)(3) Takeoff test procedure AMC §A36.9.2.1(c) [ETM AMC A2 8.1.1.2]
(1) Target test conditions Target test conditions are established for each noise me asurement. These target conditions specify the flight procedure, aerodynamic configuration to be selected, airplane weight, engine thrust (power), airspeed and, at the closest point of approach to the noise measurement point, airplane height. Regarding choice of target airspeeds and variation in test weights, the possible combinations of these test elements may affect the airplane angle-ofattack or airplane height and, therefore, possibly the airplane noise generation or propagation geometry (see 402(a)(1)(ii)(A) for guidance on the choice of target airspeeds and variation in test weights).
(2) Flight test procedures Before the start of noise testing the FAA should approve flight path tolerances (see A36.9.2.1 of part 36). Except when takeoffs with thrust (power) reduction are being demonstrated, the engine thrust (power), airplane flight path and aerodynamic configuration should be kept constant between points C and F (see Figure 24) during each approved certification flight test.
(3) Invalid test data Noise measurements obtained when the airplane flies outside the approved flight path envelope between points C and F (see Figure 24) during a noise certification test are c onsidered invalid, and the noise measurement is to be repeated.
401(i)(4) Approach test configuration GM §A36.9.2.2/B36.7(c) [ETM GM A2 8.1.1.3]
(1) Approach tests Figure 25 depicts the reference approach flight test configuration for noise certification testing of airplanes. The approach angle (steady glide angle) for this condition is 3 0.5, and the target airplane height vertically above the noise measurement point is 394 ft (120 m). Maximum PNLT may occur before or after the approach noise Page 157
measurement point.
6 562 ft 10 dB-down period PNLTM –10 dB Touchdown Distance Point P Point N Point O Point J Point I Point H End of test Flight path Stabilized conditions PNLTM – 10 dB Point G Glide slope = 3° ± 0.5° Microphone Altitude Figure 25. Approach with full landing
(2) Flight path deviations Approved height and centerline deviations along the extended runway approach flight path (see Figure 26) define an approved flight path envelope within which the flight crew should fly between points G and I. In cases where the flight crew has a clear view of the airport runway during the approach, it is common for the crew to consistently fly within the approved flight path envelope. Therefore, the approved centerline and height deviations for approach conditions may be smaller than during takeoff conditions.
Allowable lateral deviation PNLTM – 10 dB PNLTM – 10 dB 10 dB-down period Microphone Point P Point N Point H Flight path Point I Point G Beginning of test Allowable altitude deviation Test target altitude = 394 ft End of test Point O Extended runway Figure 26. Approach flight path tolerances 401(i)(5) Approach test procedure AMC §B36.7(c) [ETM AMC A2 8.1.1.3]
(1) Target test conditions Target test conditions should be established for each noise measurement. These specify the selected aerodynamic configuration, system operation, airplane weight, flight procedure (such as complete landings or flight path intercepts) height, thrust (power) and airspeed during each noise measurement. The applicant is required to select the approved airworthiness configuration for the approach noise certification that produces the highest noise level (i.e., the most critical from the standpoint of noise). The airspeed requirement for subsonic jet airplanes is VREF + 10 kt (VREF +19 km/h)). This airspeed is kept constant, within 3.0 percent, throughout the 10 dB-down period (i.e., between points G and I in Figure 26). The airplane configuration (e.g., flap setting, air conditioning and/or APU system operation) is to remain constant during the noise measurement period. Airspeed variations are measured in terms of IAS as determined by the pilot’s airspeed indicator.
(2) Engine idle trim For engines where the idle trim may affect the inter-c ompressor bleed valve schedule during the approach condition, the engine in-flight idle trim should be adjusted to the highest engine speed setting permitted by the engine manufacturer and consistent with airworthiness requirements. The engine may also provide ground idle trim adjustment, but the trim that needs adjustment is that which is operable during flight. In-flight idle trim may be adjusted to improve engine acceleration characteristic s to satisfy airworthiness compliance. The higher idle trim will cause the highest engine speed, and hence idle thrust (power), which results in a greater airplane angleof-attack and will result in the loudest approach noise requi red for certification. The applicant is to make those adjustments necessary to satisfy the airworthiness regulations. This idle trim adjustment may affect the performance or evaluation of approach NPD testing.
(3) Internal compressor bleed adjustment The internal compressor bleed operation, sometimes referred to as the surge bleed valve (SBV) operation, should be adjusted within the engine manufacturer’s specification to represent reference conditions as closely as possible. Most Page 159
turbojet engines are equipped with internal compressor bleed systems. The internal compressor bleed operates to reduce the possibility of internal engine surges during rapid throttle movements. Some jet engines have overboard bleed systems that generate high noise levels. These systems normally operate above in-flight idle and do not present a problem unless the applicant chooses to prepare an NPD database and the thrust (power) settings higher than inflight idle. The applicant is responsible for substantiating that either the internal compressor bleed operation does not affect the reference EPNL values during noise certification reference conditions, or the data contains the effects of the internal compressor bleed operation.
(4) Invalid test data Noise measurements obtained when the aircraft flies outside the approved flight path envelope between points G and I are invalid, and the noise measurement must be repeated.
401(i)(6) Lateral noise measurement GM §B36.4(b) [ETM GM A2 8.1.1.1]
(1) Measured lateral noise levels Measured lateral noise levels may not be the same at symmetrical noise measurement points even when the data are adjusted for airplane position for flight directly over the extended runway centerline. This non-symmetrical nature of measured sideline noise is primarily attributable to the direction of engine or propeller rotation. Because of inlet shielding, jet-powered airplanes may exhibit 1 to 2 dB di fferences in lateral noise levels. Turbo-propeller-powered airplanes can exhibit differences in lateral noise levels in excess of 6 dB. Due to their inherent lateral noise asymmetry, B36.4(b) of part 36 specifies that, fo r propeller-driven airplane s, simultaneous measurements be made at each and every test noise measurement point at its symmetrical position on the opposite side of the flight track.
401(i)(7) Integrated procedure adjustment GM §A36.9.4 [GM A2 8.4]
(1) Integrated procedure adjustments Section A36.9.4 of part 36 provides details of an approved integrated adjustment method when the airplane is operated at stabilized flight path and thrust (power) conditions dur ing the noise measurement period. Measured and reference flight paths are illustrated in Figures A36-11 (a) and A36-11 (b) of part 36.
401(i)(8) Acoustic emission angles GM §A36.9.4.2 [GM A2 8.3.1.2]
(1) Acoustic e mission angles For the integrated method, each one-half-second noise data record will define a separate acoustic emission angle. This angle will then define the location of each data record along the reference flight path. The distance between consecutive data records along the reference flight path, divided by the reference path speed, provides the time interval between reference data records. The reference duration of each of these data records can be determined by obtaining the average of the two intervals between the adjacent data records. This may be different than 0.5 seconds. Section 403(a)(1)(iv) provides methods for time interval computations using the integrated method.
EQUIVALENT PROCEDURES INFORMATION [ETM 4.2] 402(a) Subsonic Jet Airplanes [ETM 4.2.1] Page 160 402
402(a)(1) Flight test procedures [ETM 4.2.1.1] The following methods have been used to provide equivalent results to the procedures for jet airplanes described in part 36.
402(a)(1)(A) Flig ht path intercepts [ETM 4.2.1.1.1] Flight path intercept procedures, in lieu of full takeoff and/or landing profiles described in A36.9.2.1 and A36.9.2.2 of part 36, have been used to meet the demonstration re quirements for noise certification. The intercept procedures have also been used in the implementation of the generalized flight test procedures described in 402(a)(1)(B). The use of intercepts eliminates the need for actual takeoffs and landings, with significant cost and operational advantages at high gross weight, and substantially reduces the test time required. Site selection problems are reduced, and the shorter test period provides a higher probability of stable meteor ological conditions during testing. Airplane wear and fuel consumption are reduced, while greater consistency and quality of noise data are obtained.
402(a)(1)(A)(i) For takeoff [ETM 4.2.1.1.1.1] Part a) of Figure 27 illustrates a typical takeoff profile. The airplane is initially stabilized in level flight at point A and continues to point B where takeoff power is selected an d a steady climb is initiated. The steady climb condition is achieved at point C, intercepting the reference takeoff flight path and continuing to the end of the noise certification takeoff flight path. Point D is the theoretical takeoff rotati on point used in establishing the reference flight path. If thrust (power) reduction is employed, point E is the point of application of thrust (power) reduction, and point F is the end of the noise certification takeoff flight path. The distance TN is the distance over which the position of the airplane is measured and synchronized with the noise measurement at point K.
402(a)(1)(A)(ii) For approach [ETM 4.2.1.1.1.2] The airplane usually follows the planned flight trajectory while maintaining a constant configuration and power until there is no influence on the noise levels within 10 dB of the PNLTM. The airplane then carries out a go-around rather than continuing the landing (see part b of Figure 27).
For the development of the NPD data for the approach case, the speed and approach angle constraints imposed by B36.3(c) and B36.7(c) of part 36 cannot be satisfied over the typical ranges of thrust (power) needed. For the approach case, a steady speed of VREF + 10 kt (VREF + 19 km/h) should be maintained to within ±5 kt (±9 km/h), and the height over the microphone should be 394 ± 100 ft (120 ± 30 m). Within these constraints the test approach angle at the test thrust (power) should be that resulting from the test aircra ft conditions (i.e., weight, configuration, speed and thrust (power)).
The flight profiles should be consistent with the test requirements of part 36 over a distance that corresponds at least to noise levels that are 10 dB below the PNLTM (i.e., throughout the 10 dB-down period) obtained at the measurement points during the demonstration.
a) Take-off intercept
b) Approach (break-off) Time Time Intercept flight path Normal Full power take-off C C E TD K N BA F F I 10 dB 10 dB 120 m Stabilized approach flight path Noise measuring point Normal touchdown Break-off path Break-off point beyond last 10 dB-down point PNLT PNLT Intercept point prior to10 dB-down point Figure 27. Flight path intercept procedures 402(a)(1)(B) Generalized flight test procedures [ETM 4.2.1.1.2] The following equivalent flight test procedures have been used for noise certification compliance demonstrations.
402(a)(1)(B)(i) For derivation of noise-power-distance (NPD) data [ETM 4.2.1.1.2.1] For a range of thrust (power) covering full takeoff and reduced thrust (power), the airplane is flown past lateral and under-flight-path microphones according to either the takeoff procedures defined in B36.7(b) of part 36 or, more typically, the equivalent flight path intercept procedures described above in 402(a)(1)(i) . Target test conditions are established for each sound meas urement. These target test conditions defi ne the flight procedure, the aerodynamic configuration to be selected, airplane weight, power, airspeed and the height at the closest point of approach to the measurement location. Regarding choice of target airspeeds and variation in test weights, the possible combinations of these test elements may affect the airplane angle-of-attack or airplane attitude and, therefore, possibly the airplane sound generation or propagation geometry.
The airplane angle-of-attack will remain approximately constant for all test weights if the tests are conducted at takeoff reference airspeed appropriate for each test weight. For example, if the appropriate takeoff reference airspeed for the Page 162
airplane is V2 + 15 kt, then by setting the target airspeed at the V 2 + 15 kt appropriate for each test weight, while the actual airspeed will vary according to each test weight, the airplane test angle-of-attack will remain approximately constant. Alternatively, for many airplanes the airplane attitude remains approximately at the attitude associated with the takeoff reference airspeed corresponding to the maximum takeoff weight. Review of these potential airplane sensitivities may dictate the choice of target airspeeds and/or test weights in the test plan in order to limit excessive changes in angle-of-attack or airplane attitude that could significantly change measured noise data. In the execution of each condition, the pilo t should “set up” the airplane in the approp riate condition in order to pass by the noise measurement location within the target height window, while maintaining target power and airspeed, within agreed tolerances, throughout the 10 dB-down period.
A sufficient number of noise measurements are made in order to establish noise-power curves at a given distance for both lateral and flyover cases These curves are extended, either by calculation or by the use of additional flight test data to cover a range of distances, to form the generalized noise database for use in the noise certification of the “flight datum” and derived versions of the airplane type and are often referred to as NPD plots (see Figure 28). If over any portion of the range for the NPD plot, the criteria for calculating the EPNL given in A36.9.1.2 of part 36 requires the use of the integrated procedure, then this procedure must be used for the whole NPD plot. The 90 percent confidence intervals about the mean lines are constructed through the data (see 305 of this AC).
Note.— The same techniques may be used to develop NPD plots that are appropriate for deriving approach noise levels by flying over an under-flight-path microphone for a range of approach powers, using the speed and airplane configuration given in B36.7(c) of part 36 or, more typically, the flight test procedures described in 402(a)(1)(i)(B).
EPNL at a given speed Minimum distance (m) OR FN amb F1 t2 Engine noise performance parameter ( ) Figure 28. Form of noise-power-distance (NPD) plot for jet-powered airplanes The availability of flight test data for use in data adjustment (e.g., speed and height) should be considered in test planning, Page 163
as such availability may limit the extent to which a derived version may be certificated without further flight testing, especially where the effects of airspeed on source noise levels become significant. The effects of high altitude test-site location on jet source noise levels should also be considered in test planning. High altitude test-site locations have been approved under conditions specified in 403(b)(3), provided that jet source noise adjustments are applied to the noise data.
The correction method described in 403(b)(3) has been approved for this purpose.
Flyover, lateral and approach noise measurements should be corrected to the reference speed and atmospheric conditions over a range of distances in accordance with the procedures described in Appendix A of part 36. NPD plots can then be constructed from the adju sted EPNL, power and distances. These plots present the EPNL values for a range of distance and engine noise performance parameters.
The parameters are usually the corrected low pressure rotor speed ሺ ேభ ሻ or the corrected net thrust ሺ ிಿ ఋೌ್ ሻ (see Figure √ఏమ
28), where:
— N1 is the actual low pressure rotor speed;
— ߠ௧ଶ is the ratio of the absolute static temperature of the air at the height of the airplane to the absolute temperature of the air for an ISA at mean sea level (i.e., 288.15 K);
— FN is the actual engine net thrust (power) per engine; and — δamb is the ratio of the absolute static pressure of the ambient air at the height of the airplane to ISA air pressure at mean sea level (i.e., 101.325 kPa).
Generalized NPD data may be used in the certification of the flight-tested ai rplane and derivative versions of the airplane type. For derived versions, these data may be used in conjunction with analytical procedures, static testing of the engine and nacelle, or additional limited flight tests to demonstrate compliance.
402(a)(1)(B)(ii) For flight test procedures for determination of changes in airplane certification noise levels [ETM 4.2.1.1.2.2] Noise level changes determined by comparison of flight test data for different configurations of an airplane type have been used to establish certification noise levels of newly derived versions by reference to the noise levels of the “flight datum” airplane. These noise level changes are added to or subtracted from the noise levels obtained from individual flights of the “flight datum” airplane. Confidence interval s of new data are statistically combined with the “flight datum” data to develop overall confidence intervals (see 305 of this AC).
402(a)(1)(C) De termination of the lateral certification noise levels [ETM 4.2.1.1.3] The lateral full-power reference noise m easurement point for jet-powered airplanes is defined as the point on a line parallel to and 1476 ft (450 m) from the runway center line, where the noise level is a maximum during takeoff.
Alternative procedures using two microph one stations located symmetrically on either side of the takeoff reference track have proven to be effective in terms of time and cost savings. Such an arrangement avoids many of the difficulties encountered when using multiple microphone arrays along the lateral lines. The procedure consists of flying the test airplane at full takeoff thrust (power) at several different specified heights above a track at right angles to and midway along the line joining the two microphone stations. When this procedure is used, matching data from both lateral microphones for each fly-past should be used for the lateral noise determination. Fly-pasts where data from only one microphone are available must be omitted from the determination. The following paragraphs describe this equivalent procedure for determining the lateral noise level for jet airplanes.
a) For airplanes being certificated under Append ix B, two microphone locations are typically used, Page 164