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

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C-1 APPENDIX C: Washington Dul les Internat ional Airport Breakaway Thrust Noise Measurements C.1 . Measurement Descr ip t ion 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. Many smaller aircraft simply release the brakes, but an idle power setting does not provide sufficient forward thrust for some large aircraft to begin moving. The amount of increase in thrust for the start of taxi roll has not been well defined and may conceivably change from pilot to pilot. The primary goal of this measurement was to determine the general characteristics of breakaway thrust noise and provide substantiation for a recommendation that more comprehensive and detailed breakaway thrust measurements be performed. A small array of ground microphones and sound level meters were deployed in order to record the noise emanating from aircraft as they wait to proceed to the runway for take-off. Due to construction activity at the airport, only the two westernmost runways were operational. The measurement location is signified by the blue box south of the hold block near Runway 30 (Figure C-1). Two of the microphones in this measurement were placed directly on the concrete with half-sphere windscreens. Microphone #1 is 30 feet from the hold line and the two microphones are spaced 30 feet apart from each other as in Figure C-2. The microphone used for analysis was Microphone # 1, located at an angle from the centerline 50° from the nose of the aircraft (theta = 50°) on a 75 foot line perpendicular to the vehicle centerline intersecting the vehicle near the nose (See Figure C-2). The distance from the microphone to the nearest engine was approximately 70-75 feet. Due to lack of aircraft tracking data these dimensions are approximate.

C-2 Figure C-1: Map of Washington Dulles International Airport (IAD) (Image credit: airnav.com)

C-3 Figure C-2: IAD Taxi Noise 2 Microphone Setup (Background image Copyright 2008 Commonwealth of Virginia) Table C-1 lists all aircraft passing the microphone array throughout the measurement period. The “x” column signifies possible breakaway thrust events. This appendix will showcase the two events highlighted in Table C-1, which are assumed to be representative of typical breakaway thrust noise events. Special attention was placed on two large aircraft – the Boeing 757 and the Airbus A320. Engineers on site were able to clearly discern the spooling up of the engines as is anticipated during application of breakaway thrust. These particular recordings were chosen because they were particularly free of unwanted noise from sources such as wind and other aircraft. In Table C-1 the Event column signifies possible breakaway thrust events. The two highlighted events for Tail Numbers 469UA and 561UA, are showcased in this appendix. Table C-1: Aircraft passing the microphone array during the measurement period. Event Tail Number Time (24-hr) Operator Aircraft Engine Mfgr Engine Type 265QS 10:40 GA - NetJets FALCON 2000 C F E CO CFE 738-1-1B 849UA 10:44 United A319-131 IAE V2500 SERIES 825MJ 10:48 Delta EMB-145LR ALLISON AE3007C SER 17175 10:49 Canadair CANADAIR CL-600-2B19 GE CF-34-1A 518FX 10:51 GA Bombardier BD-100-1A10 HONEYWELL AS907-1-1A x 216WR 10:52 Southwest 737-7H4 CFM INTL CFM56 SERIES 258JB 10:56 JetBlue EMBRAER ERJ 190-100 IGW GE CF34-10E6 953AT 10:57 AirTran Boeing 717-200 BMW ROLLS BR 700 SERIES 659UA 10:58 United Boeing 767-322 P&W PW4000 SER 819JR 11:02 GA Autoflight HAWKER 900 HONEYWELL TFE731-50R 955DL 11:07 Delta MD-88 P & W JT8D SERIES x 379PH 11:07 Continental Connection DHC-8-202 (Dash 8) P&W Canada PW123 183JB 11:20 JetBlue EMBRAER ERJ 190-100 UNKNOWN UNKNOWN ENG Hold Line Take-off Ground Mics # 1 # 2 150 feet Taxi Traffic

C-4 IGW 227AA 11:32 American DC-9-82(MD-82) P & W JT8D SERIES 375P 11:37 GA - Private PIPER PA-28-161 LYCOMING 0-320 SERIES 139SK 11:40 GA - Private LEARJET 55 GARRETT TFE 731 SER 803TA 11:54 GA - Flight Options HAWKER 800XP ALLIEDSIGN TFE731-5BR 691WN 12:15 Southwest Boeing 737-3G7 CFM INTL CFM56 SERIES x 75996 12:18 United Express / Mesa CANADAIR CL-600-2B19 GE CF34 SERIES 654UA 12:24 United Airlines Boeing 767-322 Pratt & Whitney PW4000 SER 351UA 12:25 United Airlines Boeing 737-322 CFM INTL CFM56 SERIES 27172 12:27 United Express CANADAIR CL-600-2B19 GE CF34 SERIES 17156 12:28 United Express CANADAIR CL-600-2B19 GE CF34 SERIES 508QS 12:28 GA - Private Gulfstream GV BMW ROLLS BR 700 SERI 470UA 12:30 United Airlines Airbus A320-232 IAE V2500 SERIES 242CJ 12:37 United Express SAAB 340B GE CT7-Series 854UA 12:37 United Airlines Airbus A319-131 IAE V2500 SERIES x 512MJ 12:40 United Express/Mesa CL-600-2C10 GE CF34 SERIES UNK 12:40 United Express UNK UNK UNK x 196CJ 12:42 United Express / Colgan Air SAAB 340B GE CT7-9B x 469UA 12:43 United Airlines Airbus A320-232 IAE V2500 SERIES 933UA 12:45 United Airlines Boeing 737-522 CFM INTL CFM56 SERIES 290SK 12:46 United Express Embraer EMB-145LR Rolls Royce /Allison AE 3007A1P x 809HK 12:49 Trans States Airlines Embraer EMB-145EP Rolls Royce /Allison AE 3007A 856RW 12:49 United Express Embraer ERJ 170-100 SE GE CF34 SERIES 210CJ 12:53 United Express/Colgan Air SAAB 340B GE CT7-Series 292SK 12:54 United Express Embraer EMB-145LR Rolls Royce /Allison AE 3007A1P 220MJ 12:55 United Express / Colgan Air SAAB 340B GE CT7-SER 37208 12:56 United Express CL-600-2B19 GE CF34 SERIES 311CE 12:57 United Express SAAB 340B GE CT7-SER x 561UA 12:58 United Airlines Boeing 757-222 Pratt & Whitney PW2040 341CJ 12:59 United Express / Colgan Air SAAB 340B GE CT7-SER 679DA 13:03 Delta Boeing 757-232 Pratt & Whitney PW2037 845HK 13:05 United Express / Trans States EMB-145LR Rolls Royce /Allison AE3007 SER 835HK 13:07 United Express EMB-145LR Rolls Royce /Allison AE3007 SER 977RP 13:09 United Express EMB-145 Rolls Royce /Allison AE 3007A1P x 47202 13:14 United Express CL-600-2B19 GE CF34 SERIES 849HK 13:15 United Express EMB-145LR Rolls Royce /Allison AE3007 SER 982NA 13:16 GA Private Cessna 550 P&W Canada JT15D-4 x 555AN 13:17 American Airlines DC-9-82(MD-82) Pratt & Whitney JT8D SERIES 346SW 13:23 Southwest Boeing 737-3H4 CFM INTL CFM56 SERIES 840HK 13:25 United Express EMB-145LR Rolls Royce /Allison AE3007 SER

C-5 C.2 . Analys is A certain challenge in analysis of this data was deciphering the noise from departing aircraft from the desired breakaway thrust noise component. Often it seemed the waiting pilot’s cue to engage the engines to initiate a taxi roll was when the departing plane’s pilot throttled the engines to take-off power to begin departure. This means the number of “clean” events was significantly reduced and that the analyzed recordings required some careful listening and close attention to the spectral aspects of the noise. For jets, engine noise is generally broadband, but, as a departing plane gets further away, the high-frequency components diminish due to atmospheric absorption, allowing special attention to be focused on the steady-state noise of the near-by aircraft as it begins its taxi roll. Two other aspects to consider with moving vehicles are engine noise directivity and the noise sources’ proximity to the microphones. It can be assumed that the noise characteristics of an engine may change as the microphone’s position relative to the engine changes. Also, as the vehicle begins to move forward, the engine’s distance to the microphone decreases and the sound pressure will increase due to spherical wave divergence. These two elements’ impact on the data may be minimized by using Microphone # 1. Since it is further away from the engine when the aircraft is stationary, it will be less susceptible to spherical divergence, which behaves exponentially, and thus decreases the chance of an increase in acoustic level being attributed to the wrong cause. The forward position of Microphone # 1 will also make it less susceptible to the changes in noise characteristics due to engine directivity and simple geometry. For these two large, jet-powered vehicles – the B757 and the A320 – the pilot would spool up the engines, increasing the rotational rate of the turbines, and at some point in time the aircraft begins to move forward. From a listener’s perspective, it seemed the thrust was held constant for the taxi operation and often it seemed the pilot would pull the throttle back down towards idle power once the vehicle was able to maintain forward momentum for the short taxi to the start of the runway. Special attention, therefore, is placed on the increase in overall acoustic level which takes place as the tonal aspects of the noise increase in frequency. This segment of time is representative of an engine increasing in rotational rate and therefore increasing thrust. Due to lack of data, it is not possible to know for sure if rolling commenced before the thrust level was stabilized. If time-synched tracking and thrust data were available, then the increase in acoustic level due to spherical divergence would be distinguishable from the increase caused by thrust increase alone. This analysis will assume vehicle rolling commences at the same time the thrust stabilizes, and, if rolling does begin prior to that, a decreasing distance between source and receiver will be at a very slow rate and thus a negligible contributor to acoustic level increase. C.2.1. Airbus A320-232 This particular event was of interest because the waiting pilot did not apply breakaway thrust for a significant amount of time after the aircraft on the runway took off. Figure C-3 shows the overall flat and A- weighted levels of this recording segment. Grey vertical bars at times 36 and 42 seconds enclose a period where the aircraft is stationary and sound level increases are believed to be due solely to thrust increase. The aircraft remains at idle power and at 97 dB overall sound pressure level (OASPL) until the time in the recording reaches 36 seconds. At this point, the engine power is increased, which causes an increase in OASPL that peaks at 105 dB at a time of 46 seconds. As the vehicle rolls past the microphone and begins to turn onto the runway the acoustic level is diminished as the vehicle gets further away and the pilot spools down the throttle for taxi. The main peak in Figure C-3 likely coincides with the closest point of approach of the noise source to the microphone. To extract the acoustic level increase due solely to engine spool up a spectral representation such as Figure C-4 displaying frequency versus time with a monochrome scale for acoustic level is a good tool. Grey vertical bars at time equals 36 and 42 seconds enclose a period where tonal components increase in frequency due to an increase in the engine’s rotational rate. This is when the thrust is being increased by the pilot.

C-6 A look at this time period for the overall sound pressure graph (Figure C-3) correlates to an increase in un- weighted overall acoustic level of 4 dB, from 97 dB to 101 dB. Past 42 seconds the tones remain constant in frequency but the overall level of the noise increases as the vehicle moves closer to the microphone. The acoustic level then decreases not only due to the airplane then moving away from the microphone, but also due to the fact that the pilot has now decreased the thrust to prepare to wait in idle power for permission to take off. The spool-down occurs at 50 seconds. In the far-field, this event would be perceived as an increase in noise of 4 dB over the course of 10 seconds, and can be contributed almost entirely to breakaway thrust. Figure C-5 shows the one-third octave band spectra for the A320 during idle and with the engine throttled to a breakaway thrust level. When the engine is throttled the one-third octave band spectrum shows the main tonal component of the jet engine in band number 29 (800 Hz) 5 dB above the rest of the broadband noise. Figure C-3: OASPL (dB) versus time for Airbus A320 breakaway thrust noise event. Figure C-4. Airbus A320 Taxi Event Spectrogram

C-7 10 15 20 25 30 35 40 60 65 70 75 80 85 90 95 100 ISO Third Octave Band Number Le ve l [d B re 20 m ic ro Pa ] A320 Spectrum During Idle (t = 23 to 33 s) Idle 10 15 20 25 30 35 40 60 65 70 75 80 85 90 95 100 ISO Third Octave Band Number Le ve l [d B re 20 m ic ro Pa ] A320 Spectrum During Taxi (t = 43 to 48 s) Throttled Figure C-5. Airbus A320 third-octave band spectra for an idling and taxiing engine. C.2.2. Boeing B757-222 The second representative noise event for breakaway thrust is portrayed below in Figures C-6 and C-7. A tonal frequency increase occurs from time equals 36 to 44 seconds. This corresponds to a 7 dB increase in A-weighted OASPL from 97 dB to 104 dB. A-weighted levels were considered due to some low- frequency contamination from the entrails of a takeoff noise event leading up to this period. The spool- down period for this vehicle begins just after the throttle ascension ceases at time equals 45 seconds. The vertical grey bars at time equals 36 and 44 seconds encompass the region of breakaway thrust noise. This B757 event would also be perceived as an increase in far-field level on the order of 10 seconds, and the increase of 7 dB far surpasses the 4 dB increase exhibited by the A320. However, the two events differ in that the A320 noise increases, maintains, and spools completely down over the course of 10 seconds, whereas the B757 takes about 8 seconds to spool up and then as it spools down it takes another 4 seconds to reach the original idle-power acoustic noise level. It is the author’s opinion that the two events, while characteristically different, would be perceived as similar levels of annoyance by a listener. Figure C-6. OASPL (dB) versus time for Airbus B757 breakaway thrust noise event.

C-8 Figure C-7. Boeing B757 Taxi Event Spectrogram 10 15 20 25 30 35 40 60 65 70 75 80 85 90 95 100 ISO Third Octave Band Number Le ve l [d B re 20 m ic ro Pa ] B757 Spectrum During Idle (t = 23 to 33 s) Idle 10 15 20 25 30 35 40 60 65 70 75 80 85 90 95 100 ISO Third Octave Band Number Le ve l [d B re 20 m ic ro Pa ] B757 Spectrum During Taxi (t = 41 to 46 s) Throttled Figure C8. Airbus A320 one-third octave band spectra for an idling and taxiing engine C .3 . Conclus ions 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 an angle from the centerline 50° from the nose of the aircraft (theta = 50°) on a 75 foot line perpendicular to the vehicle centerline intersecting the vehicle near the nose (Figure C-2). The distance from the microphone to the nearest engine was 70-75 feet. Due to lack of tracking data these dimensions are approximate and apply to both vehicles. 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

C-9 particular engines may lend itself to require less time of maintained increased thrust. This is evident in the B757 spectrogram (Figure C-7) by the gentle slope down spectrally from time equals 45 to 60 seconds. Two events were analyzed completely for the sake of this appendix and both aircraft were found to exhibit breakaway thrust noise level increases. These two aircraft – the A320 and the B757-222 – are very common to IAD and many other airports. Table C-1 shows when the field personnel perceived a breakaway thrust noise event in the first column. Other aircraft which may potentially capture breakaway thrust noise events include the Dash-8, CL-600, EMB-145, B737, and DC-9. The measurement location was ideal for an operating airport. Meteorological and operational factors, however, preclude a significant portion of the data from analysis. In order to fully quantify noise level increases due to breakaway thrust, a parametric study would need to be performed to understand the relative sensitivities of variables such as aircraft types, pilot operational tendencies, airport operational factors, and engine directivity and type.