National Academies Press: OpenBook

Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping (2009)

Chapter: Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics

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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
×
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Suggested Citation:"Chapter 3. Measured Aircraft Taxiing Source Noise Characteristics ." National Academies of Sciences, Engineering, and Medicine. 2009. Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping. Washington, DC: The National Academies Press. doi: 10.17226/22992.
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28 CHAPTER 3. MEASURED AIRCRAFT TAXIING SOURCE NOISE CHARACTERISTICS This Chapter summarizes the available measured aircraft taxi and static noise data. A blend of acoustic measurements of varying fidelity, complexity and breadth will be examined and pertinent acoustic signature data will be summarized. This measurement data is vital for guiding development of a realistic taxi noise modeling techniques and for determining viable low thrust NPD data for use within the framework of INM7. Where possible, data from the multitude of sources have been compared to one another. The spectral data presented here are normalized to 70dB at 1000 Hz, as are the spectral classes in INM. 3.1. Taxi / Idle Condition Measurement Data Sources 3.1.1. T. F. Green State Airport (PVD) Taxi Measurement Data Aircraft taxiway noise was measured at T. F. Green State Airport (PVD) on March 24, 2008. Taxiway noise, SEL (dBA) for 7 aircraft operations, ranging from a small propeller aircraft to 737-700s was measured as the aircraft passed by the microphone setup (Table 7). Details of the T. F. Green measurements may be found in Appendix B. Run up Measurements made at T.F. Green previously are also cited below. TABLE 7 Summary of TF Green Noise Taxi Events Aircraft Carrier Plane Type Speed (knots) Time Start Time Stop dt SEL 1 - small prop - 11:01:39 11:02:31 52 68.6 2 Continental Express Regional Jet - 11:05:58 11:06:47 49 83.6 3 Southwest 737-700 22.79 12:23:16 12:24:37 81 91.8 4 US Airways Express Embraer 170 17.82 12:35:56 12:37:14 78 93.4 5 Southwest 737-700 19.37 13:10:53 13:12:07 74 88 6 US Airways Express Embraer 170 11.79 13:34:41 13:35:50 69 90.6 7 Continental Express Embraer ERJ-145 12.17 13:38:27 13:40:02 95 88.9 Sound level meter placed 71.57 meters from centerline of taxiway Ambient levels around 45 dB: noise from other planes at gates notes did not have camera did not have camera, either a ERJ-145, ERJ-135 or CRJ200LR 3.1.2. Milwaukee Airport (MKE) Static Run up Noise Measurement Data A limited amount of run up measurement data was obtained at Milwaukee Airport (23) from static run up measurements of a DC-9 at idle conditions, made along two radials situated approximately 40o and 135o off the nose of the aircraft (Table 8). Details of the Milwaukee Measurement Program may be found in reference (23). TABLE 8 DC-9 Run up Measurements at T. F. Green Airport D(ft) Angle(deg) Free Field (dBA) Lmax - Avg 3161 40 61.3 53 2525 40 64.25 55.5 925 40 78.1 72 500 40 86.3 86 2846 135 73.6 76.5 1775 135 79.05 88.5 886 135 87 94.5 500 135 93.7 98.5

29 3.1.3. Ronald Reagan Washington National (DCA) Taxi Measurement Data Aircraft taxiway noise was measured at Ronald Reagan National Airport (DCA) on June 30, and July 3, 2008. Sound level meters and a video system were deployed in order to obtain noise levels, aircraft speed, and positional information. Accessible measurement locations were selected in conjunction with airport operations personnel based on the prevailing weather conditions and active runway configuration. On June 30 and July 3, 2008, Wyle measured the taxiway noise of 47 and 35 aircraft operations, respectively, ranging from regional jets to 737-700s. A total of 47 taxiway pass-by events were measured on 30 June 2008. From that dataset 20 were found to be free of extraneous noise such as arriving or departing operations. All events were arrivals taxiing from the runway to the terminal. From these measurements we were able to obtain directivity information. Subsequent measurements were made on 3 July 2008 to measure taxiway and holding block idle and acceleration noise. . From that dataset 16 were found to be free of extraneous noise such as arriving or departing operations. Unfortunately due to the amount of traffic during this measurement period, we were unable to obtain data from a single aircraft accelerating from a stop. There were only three brief times when only one aircraft was in the holding block. This measurement dataset will be useful for validation of a future taxiway model, however at present due to a commingling of noise events with various aircraft in the holding block at the same time, the data could not be used directly to extract a single noise event. Measurements of MD-88s were made covering a speed range from 8 to 23 knots and normalized to 150 ft. Radius. The data indicates no apparent speed sensitivity in the taxiing noise (Figure 21). It was noted that B737s and A319s all had CFM56 engines and resulted in similar directivity curves (Figure 22). The general trend for all engines, including the regional jets (Figure 23) indicates a significant high frequency (3 kHz) content at 30o directivity (measured from the nose of the aircraft), and higher noise levels from the inlet (30o) than from the exhaust (120o).

30 MD88 Taxiway Longitudinal Directivity 70 75 80 85 90 95 100 20 40 60 80 100 120 140 160 Directivity Angle (deg) O A SL (d B ) MD88 8kts Evt7 MD88 12kts Evt10 MD88 23kts Evt18 Figure 21. MD88 longitudinal directivity for 8 – 23 knot taxiing speeds. 737 & A319 Taxiway Longitudinal Directivity 70 75 80 85 90 95 100 20 40 60 80 100 120 140 160 Directivity Angle (deg) O A SL (d B ) B737 13kts Evt2 B737 12kts Evt6 B737 14kts Evt15 B737 8kts Evt19 A319 17kts Evt14 Figure 22. B737 and A319 directivity normalized to 150 ft radius.

31 CRJ Taxiway Longitudinal Directivity 70 75 80 85 90 95 100 20 40 60 80 100 120 140 160 Directivity Angle (deg) O A SL (d B ) CRJ 16kts Evt1 CRJ 10kts Evt3 CRJ 12kts Evt5 CRJ 15kts Evt8 CRJ 14kts Evt13 Figure 23. CRJ directivity normalized to 150 ft radius. 3.1.4. Static Engine Idle Data Pratt and Whitney provided spectral directivity data for three engines covering a range of nominal thrusts: 20,000 lbs to 100,000 lbs. Engine 1 is near the low end, engine 2 is about the middle, and engine 3 is near the top end of the thrust range. The data is sea level static, free field, acoustic standard day conditions at 150 ft radius on a polar arc from 5 degrees off the inlet to 160o off the inlet. Data was obtained from a static test stand with an inlet control device in place provides data with ‘clean’ flow entering into the engine inlet. The inlet control device is designed to minimize the distortion of the air stream and avoid entrainment of ground plane vortices, atmospheric turbulence, and vorticity caused by flow over the test stand structure. These devices have been optimized to provide a simulated in-flight environment. There are some tones present in the dataset provided by Pratt and Whitney. It is the opinion of the engine manufacturer that these tones are quite high in amplitude relative to other engine sources and "unstable." That is, they are generally not repeatable if the points are acquired again later on. Also it is possible to encounter non-rotor related tones in the spectra at these low power conditions. SAE has provided an Aerospace Information Report documenting methods for controlling distortion of inlet airflow during static testing (24). The effectiveness of ICD conditioning on entrained airflow has been demonstrated to eliminate the majority of blade pressure fluctuations on inboard compressor stations. Towards the tips of the blades there are still differences between in flight and static test + ICD data. These variations are on the order of 3-7 dB and span the entire frequency range with slightly more variation at the lower frequencies (below 3000 Hz). Given the boundary layer along the inner edge of the inlet, one would expect that a certain amount of radiated noise from fluctuating blade pressures to be present under all operating conditions, on the ground, with or without an ICD, or while in flight.

32 To use the test stand data for estimating taxiing noise levels it is necessary apply a correction from the SAE AIR to account for the acoustic difference between clean (ICD) and distorted (no ICD) airflow. One would therefore expect that engine measurements for taxi operations to be about 3 to 7 dB higher than ICD measurements, with a 3 dB difference above 6 kHz and a 7 dB difference below 3 kHz. The spectra of ground effect taxi operations due to ground vortex ingestion can be quite different from the flight idle spectra or engine test stand spectra obtained when using an inlet control device. Unfortunately data comparing back to back measurements with and without an ICD were not available. 3.1.5. Madrid Taxi Measurement Data A comprehensive measurement program at Madrid-Barajas Airport, Spain (25, 26), was conducted by the Universidad Politenica de Madrid obtaining a significant amount of measurement engine taxi noise data. This published dataset is based on in-situ measurements of 240 taxi events at nominal taxi speed, representing a wide variety of aircraft, listed in Table 9. TABLE 9 Aircraft Measured at Madrid-Barajas Airport under Taxiing Conditions A310-300 B717 MD-82 ATR-72-500 A-319 B737A (-300, -400, -500) MD-83 CFJ A-320 B737B (-600, -700, -800) MD-87 DHC8Q3 A-321 B747 MD-88 Fokker 50 A-340-300 B757-200 B767 Measurements were obtained in the form of 1 second time histories at five locations along taxiway. From these measurements sound power was determined according to ISO 3740 and ISO 9613. Data obtained includes a nominal spectra and directivity for each engine class in units of sound power level based on ISO 3740 based method. Key findings in this report are: • Overall sound power level by aircraft (AC) family (type); • Spectral directivity of various jet AC families; • Low (<200 Hz), mid (200-1250 Hz), and high (>1250 Hz) frequency directivity for propeller AC; • Propeller AC demonstrate similar directivity irrespective of aircraft type; • Jet AC demonstrate different directivity depending on aircraft type (engine config., size, etc.); • Determined line source sound power level spectra (63-8k Hz) for studied taxiway; • Most AC moved at constant speed of about 8-12 m/s (18-27 mph, 26-39 ft/s, 15-23 knots); and • Average speed was 10.2 m/s (23 mph, 33 ft/s, 19.8 knots).

33 3.1.6. IAD Directivity and Breakaway Thrust Measurement Data A characterization of breakaway thrust from taxiing aircraft noise was successfully made based on measurements conducted at Washington Dulles International Airport (IAD). This type of event occurs when certain aircraft, often very large jets, increase engine power in order to overcome static friction and begin to roll. Breakaway thrust has been determined to be apparent and distinguishable from normal taxi noise through measurement and analysis. For an Airbus A320-232 and a Boeing B757-222 breakaway thrust noise has been quantified from a stop-and- go taxi operation for each vehicle as they stopped to wait for a plane in front of them to take off. The microphone used for analysis was located at a directivity angle of 50° as measured off the nose of the aircraft. The A320-232 was found to exhibit an increase of un-weighted overall sound pressure level of 4 dB over the course of 10 seconds. The static aircraft spooled up its engines from idle power to begin roll, maintained an increased thrust level for 10 seconds, then spooled-down its engines while rolling to the runway. The B757-222 had an A-weighted increase of 7 dB, and maneuvered slightly differently from the A320 in that the increased thrust was only maintained for 1 second. However, the spool-down behavior of these particular engines may lend itself to require less time of maintained increased thrust. This is evident in the B757 spectrogram by the gentle spectral slow down. In summary, measurements were conducted to acquire noise source levels of taxiing aircraft. The report reviewed other taxi source noise databases and identified a considerable variation between measured values. Static engine idle data was evaluated and the use of an ICD to simulate in-flight conditions precludes the adoption of such low thrust acoustic data for taxiing aircraft. Insufficient information about the particular taxi operation (specifically lack of engine operating state data) prevented the identification and quantification of plausible explanations for these discrepancies between datasets. It also precluded an empirical based determination of acoustic – thrust sensitivity. This is one of the key findings of this analysis and leads directly to the recommendation that a more comprehensive taxi noise measurement program which captures concurrent acoustic and FDR data. 3.2. Source Directivity 3.2.1. Lateral Directivity The aircraft lateral directivity utilized in INM may be applied in the creation of 3D noise spheres as are used in AAM for detailed simulation noise modeling. Figure 24 from the INM manual (1) shows the lateral directivity corrections for wing and fuselage mounted engine configurations in both polar and Cartesian plot formats.

34 SAE AIR 5662 Lateral Source Directivity -5 -4 -3 -2 -1 0 1 -90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90 Angle (Deg) En gi ne In st al la tio n Ef fe ct (d B ) Fuselage mounted Engines Wing Mounted Engines Starboard Port Figure 24. Aircraft lateral directivity applied to 3D noise spheres. A series of steps were taken to create 3D Noise Spheres for use in Wyle’s Advanced Acoustical Model (AAM) (27), a simulation model. The INM NPD curve for a B737-700 at Approach power using 3000 lbs thrust was matched by building an omni-directional 3D sphere containing the INM NPD Spectral class simulating an infinite flight and linearly offsetting it by a fixed amount in each band in order to best fit the NPD data at all distances. The inability to match the slope at the larger distances is due to different absorption models. INM’s absorption method is based on SAE ARP 866A (28) while AAM’s absorption based on ANSI standards (29). Since this project is assessing taxi noise at regions closer to the airfield the NPD data was readjusted in order to better match the INM at distances of 5000 ft or less. INM NPD comparison with AAM Omni Spectral Spheres 40.0 50.0 60.0 70.0 80.0 90.0 100.0 0 5000 10000 15000 20000 25000 Distance (Ft) SE L- dB A AAM-SEL-dBA-004*Unif-160kt AAM-SEL-dBA-005*Unif-160kt-To4k INM-NPD-SEL INM NPD comparison with AAM Omni Spectral Spheres 70.0 75.0 80.0 85.0 90.0 95.0 100.0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Distance (Ft) SE L- dB A AAM-SEL-dBA-004*Unif-160kt AAM-SEL-dBA-005*Unif-160kt-To4k INM-NPD-SEL Figure 25. B737-700 AAM omni directional sphere a) best match all distances and b) best match under 5000 ft.

35 X Y Z B737-700 Based on matched INM NPD SEL Data 3000 Lbs Approach Thrust Omnidirectional Spectral Sphere with AIR5662 Lateral Directivity Applied Figure 26. B737-700 AAM 3D source - omni directional sphere with SAE lateral directivity applied. One of the manifestations of applying only lateral directivity to a sphere, due to the geometric topology, is an increase in noise in the front and rear of the spheres. While INM does not need to consider this implication since it uses an integrated approach to modeling (an entire segment of SEL data applied to a ground mesh at one time), it shows the importance in simulation modeling of coupling the lateral directivity with the longitudinal directivity. Figure 27 compares the ground noise predictions made using AAM of a sphere that is omni directional with a sphere containing only lateral directivity. One of the obvious implications of the lateral-only directivity on the sphere is the fore and aft spikes in the contour predictions. These are due to purely geometric considerations and show up when a line connecting source and receiver intersects the higher amplitude areas on the sphere near the nose and tail. The second implication can be seen in the size of the contours at large distances from the source location. Lateral directivity has a maximum reduction in the base omni directional sphere in the plane of the wings. This can be seen as a reduced contour area for very low propagation angles. Figure 28 contains a view in closer to the operation, spanning an area of +/- 10,000 ft. Because of the formulation of the lateral directivity adjustment, a uniform integrated 3D energy level is not maintained at the sphere. This is a primary contributor to the difference in the size of the contour levels (application of lateral directivity reduces the contour areas) both at the long and short propagation distances. For flight operations it is important to preserve Lmax and SEL directly overhead; for taxi and ground operations levels directly under the aircraft are not of concern. Nose Tail

36 X-Ft Y -F t -50000 0 50000 -40000 -20000 0 20000 40000 SEL-unwt 90 85 80 75 70 65 60 55 50 45 40 B737-700 Omnidirectional sphere Matched NPD under 5k ft X-Ft Y -F t -50000 0 50000 -40000 -20000 0 20000 40000 SEL-unwt 90 85 80 75 70 65 60 55 50 45 40 B737-700 Omnidirectional sphere Matched NPD under 5k ft Lateral Directivity Applied Figure 27. B737-700 AAM ground contour predictions with omni and lateral directivity spheres (+/- 50 k ft). X-Ft Y -F t -10000 -5000 0 5000 10000 -10000 -5000 0 5000 10000 SEL-unwt 100 98 96 94 92 90 88 86 84 82 80 78 76 74 72 70 B737-700 Omnidirectional sphere Matched NPD under 5k ft X-Ft Y -F t -10000 -5000 0 5000 10000 -10000 -5000 0 5000 10000 SEL-unwt 100 98 96 94 92 90 88 86 84 82 80 78 76 74 72 70 B737-700 Omnidirectional sphere Matched NPD under 5k ft Lateral Directivity Applied Figure 28. B737-700 AAM ground contour predictions with omni and lateral directivity spheres (+/- 10k ft).

37 3.2.2. Longitudinal Directivity 3D spheres were created that incorporate longitudinal directivity as obtained from measurements in addition to the lateral directivity discussed above. An assessment of longitudinal directivity was obtained by comparing several data sources: measurements performed at Washington National Airport, directivity data obtained from static engine testing provided by a major engine manufacturer and datasets obtained from published reports. Continuing with the B737-700 assessment and including data for an Airbus A319, due to the common engines types, one can obtain a nominal longitudinal directivity (Figure 29). The selected normalized directivity is also shown in Figure 30 as implemented in the sphere. During the measurements and flight recorder data was not available so the thrust of the engines is unknown, and could be contributing to the differences between the various directivity curves. Details of the measurement data analysis process is provided in Appendices A, B and C. 737 Taxiway Longitudinal Directivity 60 65 70 75 80 85 90 95 100 20 40 60 80 100 120 140 160 Directivity Angle (deg) O A SL (d B ) Ground Idle for 20-40k T Engine B737 13kts Evt2 B737 12kts Evt6 B737 14kts Evt15 B737 8kts Evt19 A319 17kts Evt14 Kbos-Meas-B737-300 Fit Figure 29. B737-700 measured longitudinal directivity – all available data.

38 Normalized Taxi Directivity based on available data, B737-700 -10 -8 -6 -4 -2 0 2 4 6 8 0 15 30 45 60 75 90 105 120 135 150 165 180 Angle from Nose (deg) N or m al iz ed L ev el re : 9 0- de g Figure 30. B737-700 normalized longitudinal directivity. For comparison, the measured longitudinal directivity for a Bombardier CRJ aircraft is given in Figure 31. Details of the measurements and process of correcting the taxi data to free field standard day conditions directivity at 150 ft are provided in Appendix A. One of the primary differences between the 737 and the CRJ directivity is the marked increase of noise in both the front and rear of the CRJ. The measured data was depropagated using the ART technique (27) which takes advantage of a known geometrical and temporal relationship between source and receiver. Since vehicle tracking information could not be independently obtained, we relied on video instrumentation and scaling based on the vehicle length in order to determine a nominal taxi speed. This constant speed was then applied to obtain the temporal relationship between source and receiver. For the smaller CRJ aircraft one would expect its speed to inherently have more variability than the larger B737. It is likely that discrepancies between actual and presumed source location can cause greater uncertainty in the forward and aft portions of the longitudinal directivity evaluation so directivity data at the extremes, near the nose and tail were not included.

39 CRJ Taxiway Longitudinal Directivity 60 65 70 75 80 85 90 95 100 20 40 60 80 100 120 140 160 Directivity Angle (deg) O A SL (d B ) 737 Fit CRJ 10kts Evt3 CRJ 12kts Evt5 CRJ 14kts Evt13 CRJ 15kts Evt8 CRJ 16kts Evt1 Figure 31. B737-700 normalized longitudinal directivity. In summary, source directivity was identified in a range of measurement datasets. Although the levels of the directivity vary (likely due to differences in engine operating states) the shape of the directivity is similar suggesting that perhaps a nominal fleet-based directivity curve (as is currently implemented in INM for behind the start of takeoff roll) is plausible. Additionally, a collection of empirical taxi directivity data was presented for a wide variety of aircraft. There is a considerable amount of scatter in the normalized aircraft directivity assessments; however a wide variety of measurement techniques and atmospheric conditions were used for measurements. The most comprehensive and consistent measurement dataset for nominal taxi noise directivity and spectra is that obtained in Madrid. The documentation presents one longitudinal spectral directivity for each aircraft type in the form of sound power and does not address breakaway thrust. 3.3. Source Spectra 3.3.1. Taxiing Noise Levels and Spectra A set of spectra, normalized to 70 dB at 1000 Hz, are compared in Figures 32, 33 and 34 for radials forward, abeam, and aft of the engine inlet. Measurement data obtained from airfield measurements conducted as a part of this study are labeled Event. Static engine test stand data for a similar thrust rated engine (Test + ICD) are also displayed along with the INM spectral classes for both approach (INM Arr) and departure (INM Dep). Additional data from Reference (30, 31) is indicated (HMMH) as well as data from the Madrid measurements (Madrid) documented in Reference (25, 26).

40 B737 Average Ground Idle Spectra 30-50 degrees 40 45 50 55 60 65 70 75 80 85 10 100 1000 10000 Frequency [Hz] Le ve l N or m al iz ed to 7 0 dB a t 1 k H z EventsAVG Event2 Event6 Event15 Test+ICD INM_Dep INM_Arr HMMH B737-300 Madrid 737 A Madrid 737 B Madrid A319 Figure 32. Normalized spectra – forward quadrant. B737 Average Ground Idle Spectra 80-100 degrees 40 45 50 55 60 65 70 75 80 85 10 100 1000 10000 Frequency [Hz] Le ve l N or m al iz ed to 7 0 dB a t 1 k H z DCA Avg DCA Event 2 DCA Event 6 DCA Event 15 Eng1 INM_Dep INM_Arr HMMH - B737-300 Madrid 737 A Madrid 737 B Madrid A319 Figure 33. Normalized spectra – abeam.

41 B737 Average Ground Idle Spectra 120-140 degrees 40 45 50 55 60 65 70 75 80 85 10 100 1000 10000 Frequency [Hz] Le ve l N or m al iz ed to 7 0 dB a t 1 k H z DCA Events Average DCA Event 2 DCA Event 6 DCA Event 15 Eng1 INM_Arr INM_Dep HMMH - B737-300 Madrid 737 A Madrid 737 B Madrid A319 Figure 34. Normalized spectra – aft quadrant. One important consideration of taxi way noise modeling is that of the aircraft spectra, and in particular the change in spectra with directivity angle. Given the geometric orientations of taxiways to community receptors, it is possible for certain regions to be repeatedly exposed to taxi operations at a narrow range of directivity angles relative to the source. For measurements at the limits of the front and back directivity angle, the aircraft is at a considerably greater distance than point of nearest approach. When accounting for propagation (spherical spreading, absorption and ground effect) one can see that as an aircraft is approaching there is considerably more high frequency content. This is likely due the turbine whine and blade passage frequency emanating from the front of the engine. As the aircraft passes-by and moves away there is more low frequency content. This is likely due to the thunder and turbulent wake emitted from the aft of the engine. At all frequencies, there is a spike in the 63 and 160 Hz one- third octave band. One can also conclude from the wide range of measurement data with spreads of over 10 dB at some frequencies as shown in Figures 29, 30 and 31, that there is considerable variability in noise emanating from an engine at taxi conditions. 3.3.2. The Impact of Spectral Class Selection The effect of changing the spectral class of a departure noise ID to an approach spectral class can not be studied in INM directly, because there is no flexibility to change the spectral class assigned to an NPD curve, nor is there a feature to create user defined spectral classes. The run up duration analysis presented here therefore relies on a surrogate setup: utilizing the approach NPD versus the departure NPD both with different spectral classes. The primary

42 impact of changing the spectral class drives the effect of atmospheric absorption over the propagation distances of interest. Appendices A and B in this report use local meteorological conditions in utilizing acoustic measurements for the creation of user defined NPD curves. For such cases it does matter significantly what spectral class is used to model the run up operations. In summary, measured data was examined to assess differences between the published INM spectral classes and spectral directivity for low-thrust engine operations. A process was identified by which 3D spectral noise spheres can be created using limited taxi measurement data in conjunction with a simulation model to create NPD data for INM. Normalized spectral directivity data was shown from a multitude of taxi and static idle engine test stand measurement data. This difference in spectra from the INM spectral classes is primarily an increase in levels at higher frequencies oriented towards the front of the engine which could have an impact on A- weighted noise levels in nearby communities. The difference in the spectral content of existing INM flight operations spectral classes and measured taxi noise spectra is considerable suggesting that an additional taxi spectral class be implemented using similar aircraft spectral class groupings. 3.4. Source Noise Sensitivity at Low Power Settings Within this section, a set of 3D noise spheres will be presented that were created using a combination of experimental data from a nominal taxi conditions and INM data such as Spectral Class and the NPD SEL (dBA). The spheres will be utilized later in a single event analysis in Chapter 4. The absolute level of the noise sphere will be determined by measurement data, but the sensitivity of the noise level with thrust will be based on an extrapolation of the noise sensitivity (dB/ lb Thrust) in the INM NPD curves. A B737-700 taxi operation previously measured (Appendix B) was assigned a thrust of 1285 lbs, 5.25% based on the ICAO / CAEP paper (18) assessing idle taxi thrust settings. Longitudinal directivity was derived from empirical data from Wyle DCA measurements (Appendix A). The baseline sphere spectral class is that provided in INM for the CFM 56 engine. The baseline sphere source level was created based on the INM NPD and then adjusted to match the SEL (dBA) of the T.F. Green taxi measurement of a B737 (Appendix B). The INM NPD was matched by simulating a set of NPD level flight procedures in AAM and minimizing the SEL (dBA) differences for propagation distances under 4000 ft. Since the NPD curves include propagation as well as ground effects (for a nominal 4 ft high receiver) it was necessary to simulate these propagation. The easiest way to perform this calculation was by modeling a level over flight at 15 knots at the various distances in the NPD above a microphone placed 4 ft AGL. This process of applying spectra, level and directivity in both directions completely defines a single noise sphere for the 1285 (5.25%) Thrust condition for a B737-700. The next step in the process is to make a ‘set’ of three dimensional spectral taxi noise spheres – each one representing a different engine thrust setting. Given a lack of noise level data for engines operating at different low thrust settings, the INM NPD curves were utilized (Figure 35) to obtain a simple variation in the SEL level with thrust. The delta-dB values as a function of 1000 lbs Thrust were computed based on the differential for 3000 Lbs Thrust Arrival NPD (the lowest thrust in the Arrival NPD curves) as well as for 6000 lbs Thrust Arrival NPD. For reference, the 10,000 Departure NPD (the lowest thrust in the Departure NPD curves) is also included in Figure 35.

43 Noise Change (SEL dBA) per Change in Thrust (kLb) Based on INM NPD CF567B 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 100 1000 10000 Dist N PD C ur ve C ha ng e (S EL -d B A / kL B T hr us t) Dep 10000 Ari 3000 Ari 6000 Figure 35. Noise-power relationship: INM NPD, delta-dB per 1000 lbs Δ-thrust. The arrival NPDs were chosen because they include lower thrust settings. The rationale is that even though the NPDs include airframe / slats / gear / flap noise, the difference in noise between the 3000 and 4000 lbs thrust noise levels (for what is assumed to be roughly the same flight speed) will primarily come from changes in engine operating state. Based on the NPD curves a noise differential with thrust was estimated as 0.70 dB / klb. The average delta-dB / klb was applied to the spheres to scale them up and down to different thrust / noise levels. Six different thrust levels spheres were built and are itemized in Table 10. TABLE 10 Noise Spheres for B737 at Various Thrust Settings Run# Thrust (lbs) Thrust (%) Delta-dB 005 480 2 -0.564 000 1285 5.25 0.0 001 1680 7% 0.277 002 2400 10% 0.781 003 3600 15% 1.621 004 4800 20% 2.461 Unfortunately, subsequent measurements conducted at Dulles International Airport (Appendix C) did not provide a suitable operational situation allowing us to directly measure noise at both taxi idle and at breakaway thrust conditions for a B737-700. However, Breakaway thrust measurements of a B757 and A320 increases of 4 and 7 dB for the breakaway thrust condition respectively. In summary, analytical deductions suggested that changes in thrust could amount to instantaneous changes in taxi noise levels approaching 7dB. Practical application of typical durations of such breakaway thrust noise increases dilute the impact of the noise increases and lend credence to the short term suggestion that a “nominal” taxi thrust setting be implemented

44 while additional research investigates in further detail the sensitivity of taxi and idle engine noise to changes in thrust. 3.5. Combining Spectra and Directivity into a Noise Sphere With the spectra and directivity combined as described in Section 3.4, noise spheres for Run #5, 480 lbs thrust, were created and are shown in Figure 36 using different views and scales. The combination of lateral and longitudinal directivity affects the noise around the edge of the sphere. The noise in this location is what is creating the taxi noise in the community. When determining a suitable implementation for Taxi noise in INM and AEDT this difference in directivity (off the side versus directly under the aircraft) needs to be considered. Theta P hi 0 50 100 150 -50 0 50 dBA 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 B737-700 - G7377005.NC Lateral Directivity from 5662 Longitudinal Directivity from Measurements X Y Z Olvl 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 B737-700 G7377005.NC 3000 Lbs Approach Thrust AIR5662 Lateral Directivity Applied Longitudinal Directivity from Measurements Theta P hi 0 50 100 150 -50 0 50 dBA 93 92.8 92.6 92.4 92.2 92 91.8 91.6 91.4 91.2 91 B737-700 - G7377005.NC Lateral Directivity from 5662 Longitudinal Directivity from Measurements Figure 36. Multiple views of a 3D noise sphere for a B737 at taxi condition.

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Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping Get This Book
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TRB’s Airport Cooperative Research Program (ACRP) Web-Only Document 9: Enhanced Modeling of Aircraft Taxiway Noise, Volume 1: Scoping explores ways to model airport noise from aircraft taxi operations and examines a plan for implementation of a taxi noise prediction capability into the Federal Aviation Administration's integrated noise model in the short term and into its aviation environmental design tool in the longer term.

ACRP Web-Only Document 9: Enhanced Modeling of Aircraft Taxiway Noise, Volume 2: Aircraft Taxi Noise Database and Development Process documents the procedures developed and employed in the creation of a taxi noise database for the U.S. Federal Aviation Administration’s Integrated Noise Model and Aviation Environmental Design Tool (AEDT). The AEDT is currently under development.

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