(3) Engine Idle Trim: For engines where the idle trim may affect the inter-compressor bleed valve schedule during the approach condition, the engine in-flight idle trim must be adjusted to the highest engine rpm 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 characteristics to satisfy airworthiness compliance (e.g., with Part 25.119). The higher idle trim will cause the highest engine rpm (idle thrust (power)), which results in a greater airplane angle of attack, and will result in the loudest approach noise required 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.
(4) Internal Compressor Bleed Adjustment: The internal compressor bleed operation (sometimes referred to as the surge bleed valve (SBV) operation) must be adjusted within the engine manufacturer’s specification to represent reference conditions as closely as possible. Most 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 in-flight idle may be affected by the internal compressor bleed operation. The applicant is responsible for substantiating that either (1) the internal compressor bleed operation does not affect the reference EPNL values during noise certification reference conditions, or (2) the data contains the effects of the internal compressor bleed operation.
(5) Flight Path Deviations: The approved approach flight path altitude and centerline deviations identify the tolerances of the flight path that are agreed on by the FAA and the applicant before the start of noise certification flight testing. Between Points G and I (see Figure 8), the engine thrust (power), system operational configuration, and aerodynamic configuration should be kept constant throughout the noise measurement (10 dB- down points).
(6) Approach Test Airspeed: The airspeed requirement for subsonic airplanes is VREF + 10 knots.
This airspeed is kept constant (within ±3.0 percent) between the approach measuring points (Points G and I in Figure 8). All test conditions should be conducted within 15 knots of the reference approach airspeed.
(7) 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.
244. Section A36.9.3 Simplified method of adjustment.
245. Section A36.9.3.1 General
As described below, the simplified adjustment method consists of applying adjustments (to the EPNL, which is calculated from the measured data) for the differences between measured and reference conditions at the moment of PNLTM.
a. Explanation
This section specifies a methodology, known as the simplified method, for adjusting noise data from test to reference conditions. Use of this method for adjustment of take-off and approach noise data is limited by the criteria given in sections A36.9.1.2(a) and (b).
b. Supplemental Information
None
c. Procedures
None
246. Section A36.9.3.2 Adjustments to PNL and PNLT.
(a) The portions of the test flight path and the reference flight path described below, and illustrated in Figure A36-6, include the noise time history that is relevant to the calculation of flyover and approach EPNL. In figure A36-6:
(1) XY represents the portion of the measured flight path that includes the noise time history relevant to the calculation of flyover and approach EPNL; XrYr represents the corresponding portion of the reference flight path .
(2) Q represents the airplane’s position on the measured flight path at which the noise was emitted and observed as PNLTM at the noise measuring station K. Qr is the corresponding position on the reference flight path, and Kr the reference measuring station. QK and QrKr are, respectively, the measured and reference noise propagation paths, Qr being determined from the assumption that QK and QrKr form the same angle q with their respective flight paths .
(b)The portions of the test flight path and the reference flight path described below, and illustrated in Figure A36-7(a) and (b), include the noise time history that is relevant to the calculation of lateral EPNL.
(1) In figure A36-7(a), XY represents the portion of the measured flight path that includes the noise time history that is relevant to the calculation of lateral EPNL; in figure A36-7(b), XrYr represents the corresponding portion of the reference flight path.
(2) Q represents the airplane position on the measured flight path at which the noise was emitted and observed as PNLTM at the noise measuring station K. Qr is the corresponding position on the reference flight path, and Kr the reference measuring station. QK and QrKr are, respectively, the measured and reference noise propagation paths. In this case Kr is only specified as being on a particular Lateral line; Kr and Qr are therefore determined from the assumptions that QK and QrKr:
(i) Form the same angle q with their respective flight paths ; and
Figure A36-6. Profile characteristics influencing sound level
(ii) Form the same angle y with the ground.
Note: For the lateral noise measurement, sound propagation is affected not only by inverse square and atmospheric sound attenuation, but also by ground absorption and reflection effects which depend mainly on the angle y.
a. Explanation
This section specifies the portions of flyover, lateral, and approach measured and reference flight paths that are relevant to the calculation of EPNL, and shows the symbols and geometry of the respective airplane positions, resulting noise angles, and sound propagation paths.
b. Supplemental Information
None
c. Procedures
Figure A36-7. Lateral measurement - determination of reference station
None
247. Section A36.9.3.2.1
The one-third octave band levels SPL(i) comprising PNL (the PNL at the moment of PNLTM observed at K) must be adjusted to reference levels SPL(i)r as follows:
A36.9.3.2.1(a) For calculations using the English System of Units:
( ) ( ) ( ) ( )[ ] ( ) ( ) ( )rr rr r i iiii KQQKlog20 + KQQK001.0 + QK001.0SPLSPL 0 0 − −+= a aa
In this expression,
(1) The term ( ) ( )[ ]QK001.0 0ii aa − is the adjustment for the effect of the change in atmospheric sound attenuation coefficient, and a(i) and a(i)0 are the coefficients for the test and reference atmospheric conditions respectively, determined under section A36.7 of this appendix;
(2) The term ( ) ( )rri KQQK001.0 0 −a is the adjustment for the effect of the change in the noise path length on the sound attenuation;
(3) The term ( )20 log QK Q Kr r is the adjustment for the effect of the change in the noise path length due to the "inverse square" law;
(4) QK and QrKr are measured in feet and a(i) and a(i)o are expressed in dB/1000 ft.
A36.9.3.2.1(b) For calculations using the International System of Units:
( ) ( ) ( ) ( )[ ] ( ) ( ) ( )
SPL SPL QK + QK Q K + QK Q K i i i i i r r r r r = + − − 0 01 0 01 20 0 0 .
.
log a a a
In this expression,
(1) The term ( ) ( )[ ]0 01 0. a ai i− QK is the adjustment for the effect of the change in atmospheric sound attenuation coefficient, and a(i) and a(i)0 are the coefficients for the test and reference atmospheric conditions respectively, determined under section A36.7 of this appendix;
(2) The term ( ) ( )0 01 0. a i r rQK Q K− is the adjustment for the effect of the change in the noise path length on the atmospheric sound attenuation;
(3) The term ( )20 log QK Q Kr r is the adjustment for the effect of the change in the noise path length due to the inverse square law;
(4) QK and QrKr are measured in meters and a(i) and a(i)o are expressed in dB/100 m.
a. Explanation
This section specifies the required adjustments to flyover, lateral, and approach one-third-octaveband sound pressure levels to account for the difference between test and reference conditions.
b. Supplemental Information
(1) Test and Reference Conditions: Most noise certification testing is accomplished at other than reference conditions (see section B36.7). When testing is conducted at conditions other than reference conditions, appropriate and approved adjustments are to be made to the measured noise data.
(2) Noise Emission Angles: The noise emission angle between the noise propagation path and the airplane flight paths must be equal for both test and reference evaluations.
(i) For the simplified method, the acoustical record at which PNLTM occurs during the noise measurement defines the test noise emission angle. This angle will then define the location along the reference flight path where PNLTr at time of PNLTM occurs.
c. Procedures:
(1) Calculation of Average Atmospheric Sound Attenuation Coefficients: When using a “layered” atmosphere, the average atmospheric sound attenuation coefficient, α(i), for each one-third octave band must be calculated by apportioning the coefficient values of the individual layers. The goal is to account for each layer’s contribution to the atmospheric attenuation of sound propagated through the layered atmosphere. Since propagation is assumed to occur in a straight line between the aircraft and the microphone, and since the proportion of the propagation distance through each layer is constant to the vertical distance through the layer, the average atmospheric sound attenuation coefficient through the layers can be determined using the aircraft altitude.
To determine the proper proportion of each layer’s coefficient in the average value, first determine the aircraft altitude above the microphone. (In this case, at the time of PNLTM.) Then, determine the effective depth of each layer; that is, the vertical distance that the sound propagates through the layer. Next, for each layer, multiply the layer’s atmospheric sound attenuation coefficient by the ratio of the layer’s effective depth to the aircraft altitude above the microphone, and sum the resulting values.
Note that the full depth of the lowest (and in some cases, the highest) layer is not utilized. The depth of the lowest layer must have the microphone height subtracted. (i.e., for a nominal 100-foot layer, only 96 feet of vertical depth would affect the sound propagation to the four foot high microphone.) Also, unless the aircraft altitude coincides with the upper boundary of one of the layers, the layer containing the aircraft altitude will have only a partial effect on sound propagation.
(2) Example: Using the same layered atmospheric data presented in Procedure (1) under section A36.2.2.3 of this AC, and assuming an altitude above the ground at the microphone station (In this case, at the time of PNLTM,) of 394 feet, the average atmospheric sound attenuation coefficient for the 3150 Hz band would be 16. 79 dB per 1000 feet.
Layer Height, ft. α (3150), dB/1000 ft.
Effective Layer Depth, ft.
Ground to 100 17.55 96 (100 minus 4) 100 to 200 17.15 100 200 to 300 16.63 100 300 to 400 15.81 94 (394 minus 300)
Table 4: Example - Effective Layer Depth
The effective depth of the lowest layer is 96 feet, since the layer is 100 feet high and the microphone extends four feet into the layer. The effective depths of the 2nd and 3rd layers are 100 feet each, since the sound propagates through the full layer depth. The effective depth of the 4th layer is only 94 feet, since the aircraft altitude at the time of PNLTM is 394 feet, which is 94 feet above the lower boundary of the 4th layer.
(17.55 * 96 / 390) + 1st layer minus microphone height
(17.15 * 100 / 390) + Entire 2nd layer depth
(16.63 * 100 / 390) + Entire 3rd layer depth
(15.81 * 94 / 390) + Portion of 4th layer affecting propagation = 16. 79 dB / 1000 feet Average atmospheric sound attenuation coefficient, 3150 Hz
This calculation is repeated to obtain the average atmospheric sound attenuation coefficient, for each one-third octave band. These values are then used in the calculation presented in section A36.9.3.2.1.
248. Section A36.9.3.2.1.1 PNLT Correction.
Convert the corrected values, SPL(i)r, to PNLTr;
(b) Calculate the correction term D1 using the following equation:
∆1 = −PNLT PNLTMr
a. Explanation
None
b. Supplemental Information
None
c. Procedures
(1) Application of BandSharing Adjustment for the “Simplified” Method: See the Supplemental Information and Procedures provided under section A36.4.4.2 of this AC.
(2) Application of High-Altitude Site Jet Noise Adjustment for “Simplified” Method: When the Equivalent Procedure for use of high-altitude test sites (Elevation > 1200 feet MSL) is employed, and when an adjustment has not been applied to measured one-third-octave-band sound pressure levels, SPL(i), the adjustment must be applied to the one-third-octave-band sound pressure levels that have been adjusted to reference conditions, SPL(i)r, before calculation of PNLTr at time of PNLTM and ∆1. See Appendix 6 of the appended ICAO TM, for information on the equivalent procedure for testing at high-altitude sites.
249. Section A36.9.3.2.1.2
Add D1 arithmetically to the EPNL calculated from the measured data.
250. Section A36.9.3.2.2
If, during a test flight, several peak values of PNLT that are within 2 dB of PNLTM are observed, the procedure defined in section A36.9.3.2.1 must be applied at each peak, and the adjustment term, calculated according to section A36.9.3.2.1, must be added to each peak to give corresponding adjusted peak values of PNLT. If these peak values exceed the value at the moment of PNLTM, the
maximum value of such exceedance must be added as a further adjustment to the EPNL calculated from the measured data.
a. Explanation
None
b. Supplemental Information
(1) Identification of Multiple Peak PNLT Values: Peak PNLT values within 2 dB of PNLTM must be identified for use in the adjustment of test to reference EPNL values. This adjustment is calculated and applied as follows:
(i) For each identified peak value of PNLT, adjust the associated test condition one-third octave band spectrum to reference conditions, per the procedure described in section A36.9.3.2.1.
(ii) Subtract the PNLTr value at the time of PNLTM from each of the identified peak PNLTr values. If any of these differences is positive identify the largest value of exceedance.
(iii) Add this value (defined as Delta Peak) algebraically to the test EPNL value when determining the value of EPNLr.
c. Procedures
None
251. Section A36.9.3.3 Adjustments to duration correction.
252. Section A36.9.3.3.1
Whenever the measured flight paths and/or the ground velocities of the test conditions differ from the reference flight paths and/or the ground velocities of the reference conditions, duration adjustments must be applied to the EPNL values calculated from the measured data. The adjustments must be calculated as described below.
a. Explanation
None
b. Supplemental Information
(1) Duration Adjustments: A duration adjustment must be determined when the simplified data analysis method is used. This adjustment, identified as Delta 2 (?2), accounts for the effect on duration due to:
(1) the altitude difference between a test airplane flight path and a reference airplane flight path, and (2) due to the difference between a test airplane airspeed and a reference airplane airspeed. Delta 2 can be positive or negative.
c. Procedures
None
253. Section A36.9.3.3.2
For the flight path shown in Figure A36-6, the adjustment term is calculated as follows:
( ) ( )rrr VVlog10KQQKlog5.72 +−=∆
(a) Add D2 arithmetically to the EPNL calculated from the measured data.
254. Section A36.9.3.4 Source noise adjustments.
255. Section A36.9.3.4.1
To account for differences between the parameters affecting engine noise as measured in the certification flight tests, and those calculated or specified in the reference conditions, the source noise adjustment must be calculated and applied. The adjustment is determined from the manufacturer’s data approved by the FAA. Typical data used for this adjustment are illustrated in Figure A36-8 that shows a curve of EPNL versus the engine control parameter m, with the EPNL data being corrected to all the other relevant reference conditions (airplane mass, speed and altitude, air temperature) and for the difference in noise between the test engine and the average engine (as defined in section B36.7(b)(7)). A sufficient number of data points over a range of values of mr is required to calculate the source noise adjustments for lateral, flyover and approach noise measurements.
Figure A36-8: Noise Thrust Correction
256. Section A36.9.3.4.2
Calculate adjustment term D3 by subtracting the EPNL value corresponding to the parameter µ from the EPNL value corresponding to the parameter µr. Add D3 arithmetically to the EPNL value calculated from the measured data.
257. Section A36.9.3.5 Symmetry Adjustments.
258. Section A36.9.3.5.1
A symmetry adjustment to each lateral noise value (determined at the section B36.4(b) measurement points), is to be made as follows:
(a) If the symmetrical measurement point is opposite the point where the highest noise level is obtained on the main lateral measurement line, the certification noise level is the arithmetic mean of the noise levels measured at these two points (see Figure A36-9(a));
(b) If the condition described in paragraph (a) of this section is not met, then it is assumed that the variation of noise with the altitude of the airplane is the same on both sides; there is a constant difference between the lines of noise versus altitude on both sides (see Figure A36-9(b)). The certification noise level is the maximum value of the mean between these lines.
Figure A36-9: Symmetry Correction
a. Explanation
This section specifies the methodology for applying a symmetry adjustment to the lateral noise levels determined for each test run.
b. Supplemental Information
(1) Guidance: Section 2.1.3 of the appended ICAO TM provides guidance on determination of lateral noise certification levels.
(2) 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, turbojet powered airplanes may exhibit 1-2 dB differences 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, for propeller-driven airplanes, section B36.4 requires that simultaneous measurements be made at each and every test noise measurement point at its symmetrical position on the opposite side of the runway.
c. Procedures
None
259. Section A36.9.4 Integrated method of adjustment.
260. Section A36.9.4.1 General.
As described in this section, the integrated adjustment method consists of re-computing under reference conditions points on the PNLT time history corresponding to measured points obtained during the tests, and computing EPNL directly for the new time history obtained in this way. The main principles are described in sections A36.9.4.2 through A36.9.4.4.1.
a. Explanation
This section specifies the methodology of the integrated method used for adjusting measured noise data to reference conditions
b. Supplemental Information
(1) Section 6.6 of the Appended ICAO TM provides details of an approved integrated adjustment method when the airplane is operated at stabilized flight path and thrust (power) conditions during the noise measurement period.
c. Procedures
None
261. Section A36.9.4.2 PNLT computations.
(a) The portions of the test flight path and the reference flight path described in paragraph
(a)(1) and (2), and illustrated in Figure A36-10, include the noise time history that is relevant to the calculation of flyover and approach EPNL. In figure A36-10:
Figure A36-10: Correspondence between measured and reference flight paths for the application of the integrated methods of adjustment
(1) XY represents the portion of the measured flight path that includes the noise time history relevant to the calculation of flyover and approach EPNL; XrYr represents the corresponding reference flight path.
(2) The points Q0, Q1, Qn represent airplane positions on the measured flight path at time t0, t1 and tn respectively. Point Q1 is the point at which the noise was emitted and observed as one-third octave values SPL(i)1 at the noise measuring station K at time t1. Point Qr1 represents the corresponding position on the reference flight path for noise observed as SPL(i)r1 at the reference measuring station Kr at time tr1. Q1K and Qr1Kr are respectively the measured and reference noise propagation paths, which in each case form the angle q1 with their respective flight paths . Qr0 and Qrn are similarly the points on the reference flight path corresponding to Q0 and Qn on the measured flight path . Q0 and Qn are chosen so that between Qr0 and Qrn all values of PNLTr (computed as described in paragraphs A36.9.4.2.2 and A36.9.4.2.3) within 10 dB of the peak value are included.
(b) The portions of the test flight path and the reference flight path described in paragraph
(b)(1) and (2), and illustrated in Figure A36-11(a) and (b), include the noise time history that is relevant to the calculation of lateral EPNL.
(1) In figure A36-11(a) XY represents the portion of the measured flight path that includes the noise time history that is relevant to the calculation of lateral EPNL; in figure A36-11(b), XrYr represents the corresponding portion of the reference flight path .
(2) The points Q0, Q1 and Qn represent airplane positions on the measured flight path at time t0, t1 and tn respectively. Point Q1 is the point at which the noise was emitted and observed as onethird octave values SPL(i)1 at the noise measuring station K at time t1. The point Qr1 represents the corresponding position on the reference flight path for noise observed as SPL(i)r1 at the measuring station Kr at time tr1. Q1K and Qr1Kr are respectively the measured and reference noise propagation paths. Qr0 and Qrn are similarly the points on the reference flight path corresponding to Q0 and Qn on the measured flight path . Q0 and Qn are chosen so that between Qro and Qrn all values of PNLTr (computed as described in paragraphs A36.9.4.2.2 and A36.9.4.2.3) within 10 dB of the peak value are included. In this case Kr is only specified as being on a particular lateral line. The position of Kr and Qr1 are determined from the following requirements:
(i) Q1K and Qr1Kr form the same angle q1 with their respective flight paths ; and
(ii) The differences between the angles y1 and yr1 must be minimized using a method, approved by the FAA. The differences between the angles are minimized since, for geometrical reasons, it is generally not possible to choose Kr so that the condition described in paragraph A36.9.4.2(b)(2)(i) is met while at the same time keeping y1 and yr1 equal.
Note 1: For the lateral noise measurement, sound propagation is affected not only by inverse square and atmospheric attenuation, but also by ground absorption and reflection effects which depend mainly on the angle y.
O MEASURED FIGHT PATH
PROJECTION OF FLIGHT
PATH ON GROUND PLANE An
RUNWAY CENTRE LINE Figure A36-11(a): Measured Flight Path — Y, On REFERENCE FLIGHT PATH X,• Oro
RUNWAY CENTRE LINE
PARALLEL TO THE RUNWAY CENTRE
LINE AT SPECIFIED DISTANCE Figure A36-11(b): Reference Flight Path Page 156