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73 CHAPTER 5. SUGGESTIONS Considering the wide range of measured taxi noise and the spread of the source noise data presented thus far in this report, it would appear that the chief burden of improvements to current state of the art techniques for taxi noise modeling falls upon the creation of an adequate taxi noise source database for use in INM and AEDT. The wide range of geometric operational modeling fidelity, a strong driver of the noise contours, can already be accommodated in INM, albeit some with significant user burdens. The finer details of the source noise modeling (spectra, lateral and longitudinal directivity and lateral attenuation) are present in INM, though not currently implemented in the most applicable manner for taxi noise. The remainder of this chapter covers the principal elements involved in taxi modeling, gathering together the key points and suggestions of the sensitivity portion of the study. 5.1. Source and Propagation Modeling 5.1.1. INM NPD Data A new taxi-NPD class should be created in addition to the existing approach-NPD and departure-NPD datasets. An interim extrapolation procedure for INM7 could be based on a simple dB / lbs Thrust correction as a short term patch to extrapolate existing NPD data down to taxi idle thrust conditions. This approach would require analysis of measurement noise data across a range of low thrust settings and engine types. 5.1.2. Thrust Settings for Taxi Operations The thrust label in the NPD data is a label that points at the appropriate source SEL value. Taxi thrust is not a performance factor as it is in flight profile modeling. CAEP data (18) does not indicate a strong dependence of taxi thrust or breakaway thrust with aircraft takeoff gross weight, therefore one does not expect the noise from taxiing events to exhibit a strong dependence with aircraft takeoff gross weight either. Breakaway thrust can be considered a secondary taxi engine and noise state. There is precious little flight data recorder data documenting breakaway thrust and detailed engine operating states, and even less capturing the noise from a breakaway thrust operation. We were not able to locate any data for which contains a synchronized time trace of the vehicle motion and engine operating state and acoustic measurement data. Limited measurements suggest that noise at the breakaway thrust condition for larger commercial aircraft is on the order of 3 - 7dB louder than idle thrust taxi settings. Simulation modeling shows some potentially important effects of breakaway thrust noise; however limited historical data on the frequency and duration of breakaway thrust use, combined with the limited acoustic data suggests further investigation is warranted. 5.1.3. Lateral and Longitudinal Directivity Within INM the distinction between source modeling and propagation modeling is not completely distinct. Section 4.3.7 âSource Height Modeling and INM Directivityâ considered the current INM process for applying lateral and longitudinal directivity to the taxi noise computations. Section 4.1 âSingle Event Analysisâ considered detailed simulation techniques with AAM using 3D spectral noise sources to assess the sensitivity of various source features on
74 noise contours. AAM treats the lateral and longitudinal directivity as a source modeling input, not as propagation corrections as is the case within INM. Expansion of the taxi modeling capability within INM and in the future, AEDT will need to ensure consistency in propagation and source modeling effects between static and moving operations. 5.2. Trajectory/Airspace Modeling Under some circumstances one may have access to detailed operational information documenting exact taxi paths, times spent holding at various locations and specific details of exact paths taken between terminals and runways, and an ambitious noise modeler might choose to undertake inclusion of such details in taxi noise predictions on the local community. Under these circumstances, (which admittedly will become more frequent when AEDT and EDMS become available), it is important that the future model for taxi noise feasibly permits the user to representing such aircraft motions by stringing together combinations of aircraft run up and over flight operations at altitudes near the ground. The effects of combining operations and their relative spatial distribution, as was examined in detail in Chapter 4, is important to preserve the multitude of potential modeling scenarios already implemented within the current INM7 architecture. 5.2.1. Terminal & Gate Modeling Chapter 4 described changes in taxi noise contours from a series of operational and geometric modeling scenarios with decreasing complexity. These results indicate that the spatial coverage of the aircraft movements has the greatest impact on the contours. It is reasonable to simplify taxi tracks from a terminal area instead of individual gates if the tracks are selected appropriately. 5.2.2. Static Operations, Holding Queues and Breakaway Thrust Modeling It is important that there is consistency between the NPD data used for moving operations and the Lmax data used for static operations. This consistency will permit taxi predictions from time in mode operational parameters if the geometric area of coverage is preserved. The impact of holding queues on the nearby community can be important for distances under one mile. An initial assessment of the impact of breakaway thrust on contours from noise simulation using nominal taxi parameters was inconclusive due to the lack of noise sensitivity data at low thrust settings. 5.2.3. Moving Aircraft along Constant Speed Segments One item which must be considered when making suggestions for taxi noise modeling is the scope of effort and the availability of the data required to perform the analysis. The high fidelity taxi noise assessment highlighted in Section 4 involves a significant amount of resources, from considering the overall annual operations, determining the average busy day, performing detailed queuing and delay assessments under a range of environmental conditions, modeling every individual taxi way at the airport, considering the distribution of airlines, gate assignments, equipment usage and determining individual aircraft taxi paths from gate to runway. Obviously the higher fidelity the available data, the more precise the taxi noise assessment will be. However in many cases such a study is unwarranted. The remaining sections in Chapter 5 will pull together the key findings of the sensitivity assessments presented in Chapters 1-4, and provide some guidelines and considerations for selecting the appropriate level of modeling. 5.2.4. Simplified Time in Mode Modeling
75 When computing taxi noise contours, the application of simplified emissions parameters for time in mode modeling is not recommended. The acoustic impact will depend on the difference between the actual and modeled times in mode. FAA databases are available to obtain taxi times from historical operations. As seen in the noise contours presented in Chapter 4 combining various techniques for moving and stationary operations and considering the duration computation, the taxi noise contour sizes are related to taxi operational duration. Of course for those airports where flight operations dominate the overall noise contours, the impact of the taxi portions can be significantly diluted. 5.3. Taxi Noise Modeling Implications on INM Data Requirements 5.3.1. NPD Data The thrust settings used in taxiing are well below the high thrust settings used for takeoff, but can be comparable to the thrust settings used for approach and landing. Taxi and idle operating state noise data is not included in the standard Noise-Power-Distance (NPD) curves employed in INM7. The noise levels defined for approach are associated with much higher airspeeds, and include the contribution from airframe noiseâa noise source not associated with taxiing operations. It is questionable that the lower thrust approach condition NPD data present in INM is applicable to taxi noise modeling. Comparisons with measurements indicate an over prediction of noise by INM when using the standard NPD curves. Extrapolating the curves down to these low thrust levels appears to be a simple solution, but the current extrapolation procedures employed in INM do not yield results that agree with measurement data. This report presented a multitude of acoustic measurement acoustic test data for low- thrust taxi conditions obtained from various sources, supplemented with noise data measured specifically for this study. A wide variety of aircraft types have been measured internationally using several different measurement techniques and analytical methodologies for assessing engine source characteristics. Differences in the measured SEL values for taxiing aircraft varied considerably. 5.3.2. Spectral Classes Measured data was examined and assess differences between the published INM spectral classes and spectral directivity for low-thrust engine operations. Chapter 3 documented one process by which limited taxi measurement data could be applied to 3D spectral noise spheres and then simulated trajectories could be used to analytically build 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 nearby communities. 5.3.3. Directivity Considerations A collection of empirical taxi directivity data has been 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, however the documentation presents one longitudinal spectral directivity for each aircraft type in the form of sound power and does not addresses breakaway thrust.
76 5.3.4. Aircraft Performance/Operational Thrust Data Data assessing the operating state of aircraft engines during taxi operations was presented as was acoustic data for idle taxi and breakaway thrust noise events. It is suggested that in the long term, additional measurements be made for taxi operations where synchronized noise and engine operating parameters can be obtained. This data will be required to determine the noise sensitivity at low thrust settings and allow a realistic evaluation of breakaway thrust impact.
