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3 CHAPTER 2. AIRPORT TAXIING OPERATIONS Taxiing is the controlled movement of the aircraft under its own power on the ground. Noise levels in the community generated by taxiing aircraft will clearly depend on the operation of the aircraft. Taxiing operations encompass the movement of departing aircraft from the gate- push-back position to the assigned runway, and of arriving aircraft from the runway to their assigned gate. Cumulative noise levels in the community will depend not only on the total time for the taxiing aircraft to reach its assigned destination, but also on the duration of each operating mode â stationary operations, moving taxiing operations at ground idle settings, and brief periods of acceleration which for some aircraft types is preceded by an increase in thrust. Chapter 2 will explore in Section 2.1, the various rules and regulations which impact taxi ground operations. Holding queues will be discussed in Section 2.2, along with several existing tools, TAAMÂ® and EDMS, often employed in studies for the prediction of airport delays. Section 2.3 will focus on the details of the aircraft taxiing motion, present some generalizations about aircraft behavior during taxi operation, drawn from analysis of a large flight data recorder database. Typical aircraft ground speed, engine use during taxi and nominal thrust settings during motion and holds will be presented. Section 2.4 provides a characterization of aircraft accelerations into "Gentle" and "Burst behavior and describes a technique for evaluating Breakaway thrust from FDR data. The final Section 2.5 provides a summary of engine operating state information as was compiled for CAEP as an Alternative Emissions Methodology. 2.1. FAA ATC Rules and Regulations Affecting Taxi Operations Unlike flight operations, the taxiing process is not rigidly influenced by the FAA, so that many of the decisions on taxiing procedures are left to the airline, and ultimately to the Captain. There are some general discussions of airport operations to be found in current literature (9). In order to gain a better understanding of taxi procedures, and specifically to find out how both airline and FAA rules and regulations impact taxi operations, interviews were held with pilots across a variety of airlines (4, 5, 6, 7, 8). This section will discuss the interplay between the FAA guidance, specific airline protocols and the like. The FAA states that a pilot should taxi at a âsafe taxi speedâ, so as to maintain positive control and have the ability to stop or turn where and when desired. The FAA does not designate a specific speed or power setting for aircraft taxiing operations, but local Tower personnel do prescribe the taxi pathway. As a result, aircraft and engine operation during taxi varies from one airline to another and from one pilot to another. For instance, upon being given a slot time, a departing aircraft will be pushed back from the gate; where upon the pilot will start the No 1 engine. Depending on aircraft size and type, some airline Standard Operating Procedures (SOP) may advise pilots to taxi on one engine in order to conserve fuel and reduce emissions, but this is generally only possible for the lighter aircraft. Many pilots prefer not to use one engine because of the non- symmetrical engine wear, and because it requires a two- to four-minute run up of the second engine prior to take-off. Normally, for the first flight of the day two engines are used. The aircraft will then briefly accelerate to taxiing speed and proceed to the end of the assigned runway. En route, taxiing may be interrupted to await ATC clearances, allow for the movement of other aircraft, or enter a departure queue due to congestion. The taxiing process
4 will generally involve constant speed in the range 15 to 20 knots with the engine(s) at ground idle thrust, with brief accelerations for time-sensitive movements, such as crossing active runways. The actual thrust settings are not fixed, but for ground idle are in the range of 5 to 15 percent for regular commercial aircraft, and can be up to 35 percent for regional jets, Thrusts of up to 35 percent are sometimes needed for short periods of time for accelerating from a hold- short position. On some of the lighter aircraft, such as the A320, the pilot may only have to release the brakes to taxi. FAA rules state that engine power is not permitted to exceed 45 percent on the ramp. Several airlines interviewed reported a self-imposed limit of 35 to 45 percent thrust for all taxi operations. Actual thrust settings are aircraft-dependent. Taxiing operations at commercial airports are strongly influenced by aircraft separation standards for departing and arriving aircraft. These standards are dependent on many parameters, including the equipment available (Radar vs. Non-Radar), distances between runways, runway configurations, phase of flight, altitude, and wake turbulence separation Departure separation must be assured by air traffic controllers. The departing aircraft can not begin their takeoff roll until the preceding aircraft has departed and crossed the runway end, or turned to avoid conflict. The above rule only applies if controllers cannot reference landmarks to determine departure distance. If controllers are able to determine departure distances, as related to landmarks on the airfield, if the two aircrafts are Category I aircraft, then a minimum distance of 3,000 feet between them must exist. If a Category II aircraft departs in front of a Category I aircraft, then 3,000 feet between them exist. If the succeeding or both aircraft are Category II aircraft, 4,500 feet must exist between the aircraft. If either aircraft, departing or preceding, is a Category III aircraft then 6,000 feet must exist between the aircraft. (Category I aircraft is a small aircraft weighing 12,500 lbs or less, with a single propeller driven engine, and includes all helicopters, Category II is a small aircraft weighting 12,500 lbs or less with a propeller driven twin-engines, and a Category III includes all other aircraft.) (10) Air traffic controllers are not to issue clearances to small aircraft to taxi into position and hold on the same runway behind a departing heavy jet aircraft, due to the wake turbulence. There is a 2 minute wake turbulence hold when an IFR/VFR aircraft is departing behind a heavy jet or B757 on the same runway or a parallel runway that is separated by less than 2,500 feet. A small aircraft landing behind a large aircraft on the same runway must be separated by 4 miles; a small aircraft landing behind a B757 must be separated by 5 miles, a small aircraft landing behind a heavy aircraft must be separated by 6 miles, a large or a heavy aircraft landing behind a B757 must be separated by 4 miles, and a large behind a heavy must be separated by 5 miles, and a heavy aircraft landing behind another heavy aircraft must be separated by 4 miles. Runway capacity plays a role in aircraft wait times as do Visual Meteorological Conditions (VMC) and Instrument Meteorological Conditions (IMC). VMC and IMC refer to the conditions under which Visual Flight Rules (VFR) and Instrument Flight Rules (IFR) are used, respectively. A single runway configuration, during VMC, can accommodate up to 99 operations per hour for smaller aircraft and approximately 60 operations per hour for larger commercial service aircraft. The capacity of a single runway is reduced during IMC to approximately 42 and 53 operations per hour. If parallel runways exist, depending on the distance between the two runways, capacity is increased. Capacity for parallel runways that have at least 4,300 feet between the two centerlines of the runways, are double the capacity of a single
5 runway capacity. If lateral separation between parallel runways is less than 4,300 feet and IFR conditions exist, capacity is reduced significantly. Parallel runways that are separated by less than 2,500 feet must operate, and are treated, as a single runway configuration for IFR operations. The FAA does not designate a minimum distance that aircraft on the ground must be from each other. A safe distance should be maintained at all times. An interview with an air traffic controller (8) who worked in the tower at Teterboro Airport in New Jersey, indicated that distance between aircraft stopped on taxiways depends on the type of aircraft is in front of the line. If preceding aircraft is one where the engines are located on the wings such as a B737, aircraft following might be as close as 50 feet behind. A preceding aircraft whose engines are located on the fuselage or tail, such as a MD80, might have 100-150 foot of separation in between the tail of one aircraft and the nose of the following aircraft. A simple model of aircraft in a holding queue is depicted in Figure 1. 9 27 Figure 1. Single taxiway, multiple aircraft holding queue scenario. In summary, when considering ATC Rules and Regulations affecting taxi operations and based on the wide variability seen in practice of operational procedures, both in terms of pilot discretionary decisions and regulatory/safety requirements, it was not possible to draw any general taxi operational guidelines such as minimum separation distances, speed limits or the like. 2.2. Holding Queues Queuing time refers to the amount of time aircraft executing departures wait to depart from the runway or aircraft executing arrivals wait to cross active runways as they travel from the runway to a terminal. As mentioned in Chapter 1, a myriad of things may impact the travel of an aircraft between a gate and a runway. â¢ FAA guidance and rules; â¢ Weather; â¢ Airport Congestion; â¢ Runway Capacity; and â¢ Crossing over active runways. Often runway queues consist of parallel taxiways with multiple aircraft holding. As one can imagine the prediction of the interaction between the runways, terminal and taxi operations can be quite complex, especially for larger airports. Simplified time in mode modeling may rely on data sources for total taxi times, as is commonly employed for emissions assessments, and may include FAA maintained databases. For example Boston Logan International Airportâs Annual
6 Environmental assessment (11, 12) bases Aircraft taxi-times on data obtained from the FAA Aviation System Performance Metrics (ASPM) database (13) for 2006. For Boston, aircraft taxi- times for 2006 averaged 25.32 minutes, an increase of less than 1 percent from 2005. Another source of time information is the ICAO guidelines (14) for emissions calculations. These specify 26 minutes total taxi time (taxi-in + taxi-out). These 26 minutes are broken down into 9 minutes taxi-in and 17 minutes taxi-out. This reference time is applied also within the Emissions and Dispersion Modeling System (EDMS) (15). The alternative to the aforementioned two taxi time data sources is the prediction of taxi times based on simulation which take into account the aforementioned issues. Such analysis can a sophisticated model such as the Total Airspace and Airport Modeler (TAAMÂ®) (16) or the queuing component of EDMS (15). TAAMÂ® is a very high fidelity tool capable of simulating complex scenarios and is currently being used for airport modeling. We utilized TAAMÂ® because of the opportunity to leverage a concurrent Wyle noise study at a major US International airport thereby utilizing realistic aircraft taxiing movements including aircraft congestion queuing for this study. EDMS, an emissions model for assessing air quality around airports, is being incorporated into AEDT. The queuing engine within EDMS consists of an Airside Delay Model (WWLMINET) in conjunction with a sequencing model. The primary modes of operation for EDMS, inventory and dispersion, require significantly different input data and model considerably different level of detail for aircraft operations. Further information on EDMS can be found in Appendix D. In the remainder of this chapter the aforementioned tools and the potentially symbiotic analyses which can be performed in support of emissions and taxi noise prediction will be addressed in subsequent sections. 2.2.1. Total Airspace and Airport Modeler (TAAMÂ®) TAAMÂ®, a time-based simulation and modeling tool (16) is an application for the simulation of airspace and airport operations. It is a gate-to-gate system that models the entire airside and airspace environment in detail, including pushback, runways, terminals, en-route and oceanic airspace. TAAMÂ® is often employed to evaluate the efficiency, capacity, and safety of airspace and airport operations. TAAMÂ® simulates aircraft movements in detail in fast-time, facilitating quicker results and cost efficiencies. TAAMÂ® is not a noise model, but its results can provide some sample test cases for use in activities developing and assessing various fidelity taxi operation modeling scenarios. In subsequent sections of this report, we will be leveraging the results of a TAAMÂ® analysis performed for a major International US Airport. This TAAMÂ® analysis utilized input associated with air traffic procedures, weather and operational constraints and predicted a set of complex taxi movements for the average day of the peak month of existing operations. The intent of the analysis from which this dataset is being leveraged, was to calculate estimated capacity metrics, add proposed noise abatement alternatives, and assess their effect on capacity of the a major US International airport. The TAAMÂ® model provides aircraft type, taxi track (segments of each route), numbers of operations and queue times. A TAAMÂ® simulation can also provide modeled taxi routes and unimpeded taxi times and delays (i.e., additional taxi/queue time) separately for arrivals and departures, by route
7 segment. Unimpeded aircraft ground times are measured as the un-interrupted time from gate to lift-off for departures, and un-interrupted time from touch-down to gate for arrivals. This unimpeded time is the amount of time it would take the aircraft to traverse its taxiing route if it were the only aircraft in the simulation. The unimpeded taxi movements were not considered in this study; however this TAAMÂ® capability is noted here because it could be utilized for those airports surrounded by communities where taxi noise and impeded taxi/queuing time can dominate and be of significant concern. 2.2.2. EDMS and the WWLMINET Queuing Module EDMS predicts air quality inventory and also models emission dispersion around airports. In the future users of AEDT will be able to simultaneously model noise and emissions impacts. The two different types of EDMS computations along with the input data and outputs has some applicability to taxi noise modeling in that both emissions and noise modeling requires knowledge of the aircraft operations, specific airplane and engine performance as well as detailed motions the aircraft employ in the airfield. Within EDMS, for computation of an annual emissions inventory, the user-specified taxi time option may be used. This option, which requires summary data about the airport operations (emission sources and annual activity for each source) may not be used for dispersion analysis. The methodology employed for prediction of annual emissions inventory does not consider any kind of geometric distribution of the emissions sources. Hence, for emissions inventory computations within EDMS, the user is not required to provide an airport layout. The question naturally arises as to whether such a summary assessment of emission sources (aircraft spent in various modes of operation, taxiing and holding) could be useful for aircraft taxi noise modeling. This subject is addressed later in Chapter 4. Dispersion modeling requires knowledge of detailed aircraft movements. Within EDMS this higher fidelity input option may be used to calculate an annual emissions inventory in addition to providing data critical for dispersion modeling using AERMOD (17) thus, in addition to the aircraft schedule, the future AEDT user will be required to provide the following: â¢ Detailed airport layout (gates, taxiways, runways, etc.); â¢ A set of taxipaths connecting gates to runways and runway exits to gates; â¢ Airport configurations; â¢ Hourly weather; and â¢ Location of receptors. Each operation (departure or arrival) is characterized, among the others, with its assigned gate and its expected (scheduled) operation time. Based on the weather input file, EDMS identifies the most appropriate airport configuration for each hour, which further identifies: 1) the airport runway capacity and 2) the aircraft runway distribution (based on the aircraft weight class). EDMS allows users to identify one (1) taxi path for each gate-runway pair (departures) and runway-exit-gate (arrivals). Therefore, each departure gets assigned a unique taxi path when the appropriate gate-runway pair is identified. Also, each arrival operation gets assigned a unique taxi path when the appropriate runway-exit-gate pair is identified. EDMS then models the movements of individual aircraft along the taxi paths.
8 In summary, the timing of aircraft in holding queues is affected by a multitude of factors including FAA rules, weather, congestion and capacity. Empirical or modeling data from EDMS and TAAMÂ® and ultimately AEDT are suitable sources for such queuing information. Based on the complexity of taxi operations, especially at larger airports we found that we could not provide a generally applicable recommendation for queuing time determination nor a particular level of fidelity with which taxiing queuing should be modeled. 2.3. Typical Taxi Behavior: Ground Speed, Engine Use and Thrust During an aircraftâs travel between gates and runways, a series of different operating states might be encountered. These include stationary operations, moving taxiing operations at ground idle settings, and brief periods of acceleration which for some aircraft types is preceded by an increase in thrust. An examination of flight data recorder (FDR) information from a major European airline was used to generate some statistical generalities about vehicle taxi operating behavior, including aircraft taxiing speeds, engine use and operating states / thrust levels. The FDR data include 1 year of operational data from a major European carrier (flagship plus their affiliate regional carriers) and include all operations, from gate to runway to air to runway to gate, across a multitude of international airport pairs. Since itâs a European carrier there are more European airports represented than US airports, but the data can be considered generally applicable to US domestic operations as well. It is important to note that this comprehensive dataset covers only the operational half of the situation â the noise half is not covered since concurrent taxi acoustic data was not gathered during any of these operations. Subsequent sections will delve into the subject of taxiing aircraft acceleration after a stop (excluding on-runway and takeoff acceleration) and breakaway thrust. Simple queries were employed in the prediction of the parameters presented below. Appendix E contains expanded information about the analysis algorithms. To develop the summary data, the flight record was split into operational segments from gate to gate: departure, enroute flight and arrival. This included segments such as parked at the gate, pushback, taxi to the runway (including any holding queues encountered), and departure operation on the runway. On the ground FDR data is spaced 5 seconds apart while enroute data incorporates a logarithmic time spacing algorithm. The departure segment was further examined and aircraft with ârolling departuresâ were separated from those who âheldâ at the end of the runway before departing. The next segment was the runway takeoff, followed by the enroute flight segment. At the destination region, the records were split up to include approach up to the touchdown point along with the runway deceleration period. Segments where the aircraft had left the runway and was on a taxiway (regardless of speed) are included as part of the taxi for an arrival. The aircraft at the gate was considered part of the taxi segment up until the time when the fuel flow was reported as zero for all engines. Operations at the gate while engines were spooling down (and thrust / operating state parameters were reported as non-zero) were not included in the taxi segment. Subsequent examination of the average operational parameters for stationary portions (or holds) included this stationary gate portion of the taxi operation. The departure taxi segment and the arrival taxi segments were assessed separately. An assessment of the use of engines expressed as a function of total taxi time was performed in order to determine whether single or multiple engine operations could be a factor in taxi analysis. Stationary segments were defined as those with reported ground speed less than 1 knot and
9 moving segments those with speeds at or above 1 knot. During these stationary and moving segments average ground speed and thrust parameters were obtained from the FDR data. Table 1 itemizes the average and standard deviation of ground speed during departure and arrival taxi operations. One would expect that taxi speeds immediately after leaving the runway on arrivals to be greater than those for departing aircraft as is indicated in the average and standard deviation of ground speed (Table 1). Table 2 summarizes the percentage of time each engine is operating during the various taxi operations. For this analysis, an operation is defined as those records when the fuel flow for at least one engine is greater than zero. The engine operating state parameters as reported in the FDR data is presented in Table 3. The definition and units of the engine operating state parameters are: â¢ N1avg: N1, average (all engines, percent of maximum) at start of event; â¢ %Thrust: percent of maximum thrust at start of event; â¢ EMS Thrust: EMS thrust per engine, averaged over all engines at start of event, lbs; and â¢ EMS enhanced: EMS enhanced thrust per engine, averaged over all engines at start of event, lbs. In summary, an examination of a comprehensive flight data recorder dataset yielded some statistical information about historical commercial aircraft ground speeds, engine use and thrust settings. Such data could be applied by noise modelers to their specific analyses. Engine operating parameters were computed for a range of aircraft types and categorized into arriving and departing operations as well as during moving as well as stationary periods. This section provides some of the numerical basis from which subsequent sensitivity studies will draw. TABLE 1 Ground Speeds for Taxiing Operations Aircraft Average Ground Speed (knots) Standard Deviation GS (knots) A319 9.26 3.34 A320 9.10 2.92 A321 9.39 3.31 A330 10.05 3.32 A340 9.26 2.98 B757 8.87 2.28 B767 11.13 3.13 B777 8.97 3.18 RJ100 9.14 3.57 RJ85 8.23 3.08 Ground Speed - Moving Aircraft, Departures Aircraft Average Ground Speed (knots) Standard Deviation GS (knots) A319 11.72 3.27 A320 11.08 3.27 A321 11.28 4.67 A330 13.07 3.21 A340 9.88 2.92 B757 13.23 2.68 B767 12.65 2.60 B777 11.45 2.26 RJ100 14.10 4.44 RJ85 14.67 4.77 Ground Speed - Moving Aircraft, Arrivals
10 TABLE 2 Engine Use for Taxiing Operations Aircraft Average %1eng Standard Dev.%1eng Average %2eng Standard Dev %2eng Average %3eng Standard Dev %3eng Average %4eng Standard Dev %4eng A319 2.40 5.47 97.40 5.68 0.00 0.00 0.00 0.00 A320 3.30 7.62 96.60 7.85 0.00 0.00 0.00 0.00 A321 2.10 3.56 97.80 3.85 0.00 0.00 0.00 0.00 A330 9.60 14.93 90.30 15.12 0.00 0.00 0.00 0.00 A340 1.20 7.25 7.10 15.86 3.20 5.70 88.20 21.51 B757 5.10 5.58 94.70 5.86 0.00 0.00 0.00 0.00 B767 17.10 15.24 82.70 15.38 0.00 0.00 0.00 0.00 B777 7.30 14.01 92.50 14.14 0.00 0.00 0.00 0.00 RJ100 0.00 0.00 0.00 0.00 0.00 0.00 100.00 0.00 RJ85 0.00 0.00 0.00 0.00 0.00 0.00 100.00 0.00 Engine Use - Stationary, Departure Taxi Operations Aircraft Average %1eng Standard Dev.%1eng Average %2eng Standard Dev %2eng Average %3eng Standard Dev %3eng Average %4eng Standard Dev %4eng A319 3.00 15.13 96.90 15.21 0.00 0.00 0.00 0.00 A320 1.10 7.63 98.80 7.77 0.00 0.00 0.00 0.00 A321 0.00 0.00 100.00 0.00 0.00 0.00 0.00 0.00 A330 4.40 19.42 95.50 19.50 0.00 0.00 0.00 0.00 A340 1.00 8.60 1.70 10.36 1.20 10.07 95.90 19.35 B757 1.20 10.87 98.70 10.87 0.00 0.00 0.00 0.00 B767 0.60 5.39 99.30 5.44 0.00 0.00 0.00 0.00 B777 0.00 0.00 100.00 0.00 0.00 0.00 0.00 0.00 RJ100 0.00 0.00 0.00 0.00 0.00 0.00 100.00 0.00 RJ85 0.00 0.00 0.00 0.00 0.00 0.00 100.00 0.00 Engine Use - Stationary, Arrival Taxi Operations Aircraft Average %1eng Standard Dev.%1eng Average %2eng Standard Dev %2eng Average %3eng Standard Dev %3eng Average %4eng Standard Dev %4eng A319 11.20 19.54 88.60 19.41 0.00 0.00 0.00 0.00 A320 9.30 18.39 90.60 18.27 0.00 0.00 0.00 0.00 A321 8.70 16.10 91.10 15.99 0.00 0.00 0.00 0.00 A330 11.30 14.72 88.50 14.75 0.00 0.00 0.00 0.00 A340 1.80 2.98 7.20 8.74 1.40 1.88 89.20 10.83 B757 2.80 4.51 97.10 4.76 0.00 0.00 0.00 0.00 B767 6.60 9.73 93.30 9.90 0.00 0.00 0.00 0.00 B777 10.50 10.87 89.40 10.92 0.00 0.00 0.00 0.00 RJ100 0.00 0.00 0.00 0.00 0.00 0.00 100.00 0.00 RJ85 0.00 0.00 0.00 0.00 0.00 0.00 100.00 0.00 Engine Use - Moving, Departure Taxi Operations Aircraft Average %1eng Standard Dev.%1eng Average %2eng Standard Dev %2eng Average %3eng Standard Dev %3eng Average %4eng Standard Dev %4eng A319 0.40 2.85 99.50 2.99 0.00 0.00 0.00 0.00 A320 1.10 6.28 98.70 6.46 0.00 0.00 0.00 0.00 A321 0.00 0.53 99.90 0.62 0.00 0.00 0.00 0.00 A330 1.70 11.23 98.20 11.36 0.00 0.00 0.00 0.00 A340 0.50 3.05 1.70 10.09 0.10 0.68 97.50 11.04 B757 0.40 3.60 99.50 3.72 0.00 0.00 0.00 0.00 B767 0.50 7.36 99.40 7.36 0.00 0.00 0.00 0.00 B777 0.00 0.18 99.90 0.25 0.00 0.00 0.00 0.00 RJ100 0.00 0.00 0.00 0.00 0.00 0.00 100.00 0.00 RJ85 0.00 0.00 0.00 0.00 0.00 0.00 100.00 0.00 Engine Use - Moving, Arrival Taxi Operations
11 TABLE 3 Engine Operating Parameters for Taxiing Operations Aircraft Average N1average Standard Dev. N1avg Average %Thrust Standard Dev %Thrust Average EMS Thrust Standard Dev EMS Thrust Average EMS Enhanced Standard Dev EMS Enhanced A319 19.42 1.41 8.41 1.18 1975.24 278.42 1975.24 278.42 A320 19.16 1.29 7.45 1.34 2011.77 361.01 2011.77 361.01 A321 20.06 1.55 6.15 1.15 1843.61 345.29 1843.61 345.29 A330 21.16 3.08 3845.13 2484.18 A340 19.50 4.73 2210.70 1027.83 B757 20.47 1.51 2.68 0.67 1077.75 268.58 1077.75 268.58 B767 23.78 2.66 5.74 1.24 3565.83 772.65 0.00 0.00 B777 19.89 2.30 4.86 0.86 5615.41 989.00 0.00 0.00 RJ100 22.67 1.80 21.70 1.99 1518.85 139.30 0.00 0.00 RJ85 22.32 1.59 21.44 1.71 1500.55 120.04 0.00 0.00 Engine Operating Parameters - Stationary, Departure Taxi Operations Aircraft Average N1average Standard Dev. N1avg Average %Thrust Standard Dev %Thrust Average EMS Thrust Standard Dev EMS Thrust Average EMS Enhanced Standard Dev EMS Enhanced A319 17.37 3.98 8.62 2.20 2026.47 516.45 2026.47 516.45 A320 17.51 3.04 7.72 1.89 2083.66 510.60 2083.66 510.60 A321 18.21 3.82 6.78 1.97 2034.77 591.40 2034.77 591.40 A330 21.51 4.25 5.80 4.46 3947.56 3035.05 2815.96 3582.02 A340 19.20 3.93 6.38 2.85 2590.74 839.84 1516.45 1520.39 B757 19.64 2.50 1.39 0.87 560.33 347.74 560.33 347.74 B767 26.79 1.56 6.09 4.47 3781.28 2777.72 0.00 0.00 B777 21.41 0.91 5.46 0.43 6312.84 496.08 0.00 0.00 RJ100 17.51 6.52 16.54 6.41 1157.97 449.01 0.00 0.00 RJ85 18.03 5.69 17.06 5.45 1194.22 381.76 0.00 0.00 Engine Operating Parameters - Stationary, Arrival Taxi Operations Aircraft Average N1average Standard Dev. N1avg Average %Thrust Standard Dev %Thrust Average EMS Thrust Standard Dev EMS Thrust Average EMS Enhanced Standard Dev EMS Enhanced A319 19.56 3.34 9.20 1.92 2162.66 451.69 2162.66 451.69 A320 19.71 3.29 8.22 1.85 2220.48 498.40 2220.48 498.40 A321 20.32 3.13 6.89 1.43 2066.97 429.37 2066.97 429.37 A330 22.28 3.30 4261.80 2792.32 A340 20.45 4.08 2407.10 1008.95 B757 23.28 2.20 3.73 1.01 1500.41 405.07 1500.41 405.07 B767 26.14 1.71 6.58 1.14 4085.75 708.94 0.00 0.00 B777 20.08 1.94 5.16 0.77 5960.23 884.92 0.00 0.00 RJ100 25.49 2.45 24.61 2.47 1722.56 172.80 0.00 0.00 RJ85 24.59 2.11 23.85 2.38 1669.50 166.76 0.00 0.00 Engine Operating Parameters - Moving, Departure Taxi Operations Aircraft Average N1average Standard Dev. N1avg Average %Thrust Standard Dev %Thrust Average EMS Thrust Standard Dev EMS Thrust Average EMS Enhanced Standard Dev EMS Enhanced A319 19.94 1.05 9.89 0.70 2323.04 164.33 2323.04 164.33 A320 19.62 1.38 8.70 1.32 2350.02 355.09 2350.02 355.09 A321 20.72 1.55 7.62 0.57 2284.95 169.66 2284.95 169.66 A330 23.15 2.23 6.56 4.26 4459.99 2892.61 2886.91 3742.11 A340 20.03 2.55 7.04 2.54 2862.06 555.44 1672.96 1585.15 B757 22.29 1.79 3.34 0.61 1341.46 245.01 1341.46 245.01 B767 27.17 2.13 6.65 0.89 4130.46 549.77 0.00 0.00 B777 21.53 0.45 5.48 0.31 6336.52 359.74 0.00 0.00 RJ100 23.84 6.19 22.85 6.16 1599.68 431.08 0.00 0.00 RJ85 23.44 6.67 22.55 6.63 1578.58 463.89 0.00 0.00 Engine Operating Parameters - Moving, Arrival Taxi Operations Note: Some A330 and A340 Departure values were erroneous in the FDR database and removed.
12 2.4. Accelerating Aircraft and Breakaway Thrust A measure of the aircraft acceleration following a hold was obtained by examining flight data recorder information from a major European Airline. A full description of the analysis process may be found in Appendix E. A hold was defined as any period during which the aircraft speed (as reported by the ground speed indicator in the FDR data) was less than 1 knot. The cause of the hold (wait to cross a runway, queue hold due to traffic, hold after pushback etcâ¦) could not be determined or catalogued. Figure 2 shows the acceleration values for all aircraft types where the acceleration âburstsâ (5 and 10 second duration) display a distinctly higher longitudinal acceleration value. The corresponding Maximum % Thrust parameter for these data records is given in Figure 3. The Maximum Thrust was determined by searching through the time records during the stationary period immediately preceding the acceleration event through the acceleration event itself, and extracting the maximum value of the indicated thrust parameter. It is presumed that these particular acceleration events are due to the application of breakaway thrust and hence a significantly higher, yet shorter duration acceleration region than those other events with very low values of acceleration (less than .05 g) which tend to linger for long times. The resolution of the source FDR files used in this analysis all contained a 5 second time spacing, hence the discrete time intervals in the figures in this section. Acceleration Events following a Hold - All Aircraft Longitudinal Acceleration (g) as a function of Acceleration Time (sec) 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Accel Time (sec) M ax L on g A cc el (g ) A319-Arri A319-Depa A320-Arri A320-Depa A321-Arri A321-Depa A330-Arri A330-Depa A340-Arri A340-Depa B757-Arri B757-Depa B767-Arri B767-Depa B777-Arri B777-Depa Figure 2. Acceleration events following a hold â all aircraft.
13 Acceleration Events following a Hold - All Aircraft Maximum thrust (%) as a function of Acceleration Time (sec) 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Accel Time (sec) M ax T hr us t ( % ) A319-Arri A319-Depa A320-Arri A320-Depa A321-Arri A321-Depa A330-Arri A330-Depa A340-Arri A340-Depa B757-Arri B757-Depa B767-Arri B767-Depa B777-Arri B777-Depa Figure 3. Thrust (% maximum) for the acceleration events following a hold â all aircraft. The increase in thrust required to overcome static friction is referred to as breakaway thrust. During thrust measurements conducted by Wyle (see Appendices A, B and C) the following aircraft behavior was observed: In general the commercial jet aircraft spool up their engines to a higher thrust setting and achieve a "new steady state" (as evidenced by the acoustic spectral trace) after which the pilots release the brakes and the aircraft begins moving. A short period of time later the pilots bring the engines back down to the idle setting. For the purposes of this analysis the aforementioned behavior will be utilized but the applicability of this taxi operation generalization should be verified, possibly by conducting further interviews with pilots, performing additional acoustic measurements of breakaway thrust at other airports for a wide variety of aircraft and airlines, and by examining in more detail flight data recorder information from which engine operating state, aircraft location and speed can be reliably obtained. The data presented in Figure 2 was filtered to capture two distinct operation types: 1) Short Bursts of acceleration (15 seconds or less) where presumably breakaway thrust is applied and 2) âGentleâ longer slower accelerations (below 0.1g) where brakes are released from which it can be inferred that minimal thrust changes are employed. Accelerations below .01g were removed from the analysis. These two groupings are in essence those events clustered near the axes as shown in Figures 4 and 5. The corresponding Thrust values are provided in Figures 6 and 7 for âBurstâ and âGentleâ accelerations respectively. More detail about the processing algorithms employed in this analysis can be found in Appendix E.
14 The key acceleration findings are summarized in Table 4. For these same events, the pertinent thrust parameters were obtained. Occasionally, especially for the A340 the high thrust values are possibly a failure of the flight segment separation logic and indicative of the erroneous inclusion of a rolling takeoff. Due to licensing restrictions we were not able to delve into the FDR time histories for these events and subsequently improve the algorithms, so those events were deleted from the analysis. Also during some of the events, particularly for the A330 and A340, inconsistencies between the various reported FDR thrust parameters was noted. These spurious records, present also in the ârawâ FDR dataset, were also deleted from the analysis presented here. A series of graphics with the acceleration and breakaway thrust events separated by aircraft type may be found in Appendix E. The FDR data included the following data fields which we utilized in this analysis: N1- average-%, defined as âN1: average (all engines, percent of maximum) at start of event, units of %â and thrust-ave-percent defined as âthrust, percent of maximum at start of the event, units of %â. These parameters was averaged over the duration of the identified holding event to obtain the nominal values and then the multiple events were averaged together and are therefore referred to in Table 4 and 5 as the average of the avg-%-thrust. In summary, an examination of a comprehensive flight data recorder dataset yielded some statistical information about historical commercial aircraft application of thrust following holds. Events following a hold were categorized into burst or gentle acceleration periods whereby statistical thrust values were computed.
15 TABLE 4 Engine Operating Parameters for Taxiing Operations Aircraft Type Arri / Depa Operation Type Avg. Accel Time (s) Avg. N1avg Avg. N1Max Avg. avg %Thrust Avg. max %Thrust Avg. Max Long Accel (g) # Events A319 A Burst 5.00 14.88 15.58 8.01 8.50 0.29 2 A320 A Burst 8.33 18.40 20.45 7.21 8.44 0.03 6 A321 A Burst 7.00 18.56 18.71 6.88 6.94 0.03 5 A330 A Burst 7.31 21.89 23.80 5.82 6.27 0.01 13 A340 A Burst 9.50 22.77 25.88 8.07 9.69 0.03 10 B757 A Burst 6.68 18.85 20.50 - - 0.02 95 B767 A Burst 8.00 26.10 27.31 4.61 11.88 0.01 25 B777 A Burst 9.27 21.04 22.10 5.23 5.73 0.01 41 A319 D Burst 6.63 28.24 36.07 15.61 21.24 0.20 92 A320 D Burst 7.07 27.93 35.79 13.74 18.68 0.20 121 A321 D Burst 6.07 28.80 35.45 12.74 16.69 0.18 61 A330 D Burst 8.00 40.07 54.75 10.44 16.58 0.15 95 A340 D Burst 7.50 29.52 41.41 8.57 14.04 0.14 34 B757 D Burst 7.27 35.58 44.34 10.29 14.30 0.15 75 B767 D Burst 8.17 41.16 57.39 14.19 28.82 0.18 71 B777 D Burst 8.17 31.14 40.96 9.45 13.02 0.15 101 Aircraft Type Arri / Depa Operation Type Avg. Accel Time (s) Avg. N1avg Avg. N1Max Avg. avg %Thrust Avg. max %Thrust Avg. Max Long Accel (g) # Events A319 A Gentle 104.41 17.85 26.12 8.71 14.17 0.02 17 A320 A Gentle 115.47 15.32 23.89 6.64 11.30 0.03 43 A321 A Gentle 54.76 22.08 27.72 7.89 11.41 0.03 21 A330 A Gentle 32.20 24.32 29.84 8.76 11.49 0.02 25 A340 A Gentle 29.44 23.28 29.09 8.50 11.40 0.02 18 B757 A Gentle 12.41 19.70 21.94 - - 0.02 106 B767 A Gentle 41.84 21.94 27.73 4.55 16.35 0.02 98 B777 A Gentle 17.50 21.19 22.66 5.24 5.89 0.01 54 A319 D Gentle 71.53 23.07 27.90 10.87 14.49 0.02 334 A320 D Gentle 70.02 22.46 26.77 9.42 12.22 0.03 547 A321 D Gentle 70.63 23.39 28.18 8.22 11.53 0.03 248 A330 D Gentle 59.65 27.27 32.93 6.46 9.18 0.02 103 A340 D Gentle 48.94 23.38 31.10 5.22 8.68 0.03 17 B757 D Gentle 67.99 27.38 33.84 5.14 8.95 0.04 296 B767 D Gentle 74.26 27.64 29.86 6.84 22.62 0.02 291 B777 D Gentle 68.46 22.89 25.80 5.98 7.16 0.02 343 Note: Some B757 %Thrust values were erroneous in the FDR database and removed.
16 "Burst" Acceleration Events following a Hold - All Aircraft Longitudinal Acceleration (g) as a function of Acceleration Time (sec) 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Accel Time (sec) M ax L on g A cc el (g ) A319-Arri A319-Depa A320-Arri A320-Depa A321-Arri A321-Depa A330-Arri A330-Depa A340-Arri A340-Depa B757-Arri B757-Depa B767-Arri B767-Depa B777-Arri B777-Depa Figure 4. âBurstâ acceleration events following a hold â all aircraft. "Gentle" Acceleration Events following a Hold - All Aircraft Longitudinal Acceleration (g) as a function of Acceleration Time (sec) 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Accel Time (sec) M ax L on g A cc el (g ) A319-Arri A319-Depa A320-Arri A320-Depa A321-Arri A321-Depa A330-Arri A330-Depa A340-Arri A340-Depa B757-Arri B757-Depa B767-Arri B767-Depa B777-Arri B777-Depa Figure 5. âGentleâ acceleration events following a hold â all aircraft.
17 "Burst" Acceleration Events following a Hold - All Aircraft Maximum thrust (%) as a function of Acceleration Time (sec) 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 0.00 5.00 10.00 15.00 20.00 25.00 Accel Time (sec) M ax T hr us t ( % ) A319-Arri A319-Depa A320-Arri A320-Depa A321-Arri A321-Depa A330-Arri A330-Depa A340-Arri A340-Depa B757-Arri B757-Depa B767-Arri B767-Depa B777-Arri B777-Depa Figure 6. âBurstâ acceleration event thrust (% maximum) following a hold â all aircraft. "Gentle" Acceleration Events following a Hold - All Aircraft Maximum thrust (%) as a function of Acceleration Time (sec) 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Accel Time (sec) M ax T hr us t ( % ) A319-Arri A319-Depa A320-Arri A320-Depa A321-Arri A321-Depa A330-Arri A330-Depa A340-Arri A340-Depa B757-Arri B757-Depa B767-Arri B767-Depa B777-Arri B777-Depa Figure 7. âGentleâ acceleration event thrust (% maximum) following a hold â all aircraft.
18 2.5. CAEP Alternative Emissions Methodology For taxi noise assessments, one critical question is that of the engine thrust setting during idle and taxi conditions. Consistency in future analyses using AEDT for both noise and emissions would dictate the use of a common dataset describing the aircraft operations. In Air Quality assessments, 7% of full rated power from the ICAO certification procedures is currently used to model the idle and taxi power setting for aircraft operations. In 2006, the Committee on Aviation Environmental Protection (CAEP) Alternative Emissions Methodology Task Group presented results from a number of surveys of power settings used during normal taxi operations (18). Across a wide range of commercial aircraft, an analysis of fuel flows determined that the actual thrust levels used were approximately 5% to 6% of the maximum rated engine output with some Rolls Royce engines being operated in the 3% to 5% range. The recommendation from the study (18) which is under consideration by Working Group 3 is to modify the current ICAO standard of utilizing a nominal 7% thrust (14) to instead utilize fuel flow for emission assessments, or in situations where actual fuel flows are not available to instead use 5% of the ICAO Databank Rated Output thrust level. Based on the emissions certification specifications %Foo is the ICAO âRated Outputâ at sea level static conditions. Figure 8, from reference (18), summarizes the taxi and idle power data measured during several projects executed at Londonâs Heathrow and Gatwick airports (19, 20, 21). In addition to assessing idle settings, the studies identified the breakaway thrust level used in practice by some aircraft. Figure 8. Recorded idle / taxi settings for a number of aircraft and air carriers (18). Figures 9, 10 and 11, also from reference (18), indicate the sensitivity of breakaway thrust for the B747-400 (RB211), the B777-236 (GE90-76B) and the B777IGW-236 (GE90- 76B) to aircraft mass, expressed as a percentage of maximum takeoff mass. Breakaway thrust values were not reported for other aircraft (18).
19 Figure 9. Recorded idle settings for the B747-400 as a function of % TOGW (18). Figure 10. Recorded idle settings for the B777-236 (GE90-76B) as a function of % TOGW (18). Figure 11. Recorded idle settings for the B777-236IGW (GE90-76B) as a function of % TOGW (18).
20 Additionally, some information was provided about the variation due to taxiing with less than all engines operating (18). This indicated that for most aircraft types an increase in thrust was required to perform the taxi operation (Figure 12) with active engines on twin engine aircraft requiring greater thrust increases than those with four engines. Figure 12. Recorded power settings for the taxi with fewer than all engines operating. In summary, an examination of a recent CAEP working group study (18) provided some additional generalized engine operating state parameters for a range of commercial aircraft types, specifically empirical taxi engine settings, breakaway thrust for a limited number of aircraft and the impact of shutting down engines during taxi operations on the remaining operating engine state. 2.6. Relative Contributions of Ground Operations and Flight Noise The scope of the current study is to propose approaches for modeling taxi noise, not to determine whether or not taxi noise should be modeled within the context of flight operation noise. It is not appropriate to make a blanket conclusion on the importance of taxi modeling relative to flight operation modeling because such judgments depend heavily on the specific situation. However, it was requested that we include a realistic example of the relative contributions between a consistent set of ground and flight operations. We leave it to the readers to draw their own conclusion as to the relative importance. Figure 13 presents the results of a Dicernoâ¢ noise analysis which modeled taxi, APU and flight operation noise based on acoustic simulation predictions for taxi, auxiliary power units, ground power units and ground support equipment along with flight noise. Figure 14 itemizes the number and type of operations modeled in this particular study. The ground operations are consistent and represent a geometric distribution and duration required to support the cited number of flight operations.
21 Figure 13. Ground operations and flight operations compared and combined.
22 Day (7 am - 11 pm) Night (11 pm - 7 am) Total Day (7 am - 11 pm) Night (11 pm - 7 am) Total B737-700 39,642 4,557 44,199 40,098 5,012 45,110 B747-400 1,376 3,734 5,110 4,717 2,752 7,469 B767-300 10,819 6,642 17,461 10,498 4,150 14,648 B767-400 8,885 228 9,113 8,430 1,595 10,025 B777-200 2,506 1,595 4,101 3,190 683 3,873 B777-300 1,367 228 1,595 1,367 228 1,595 A319 25,289 2,734 28,023 26,884 2,734 29,618 A320 38,959 6,835 45,794 38,275 5,240 43,515 A330 10,708 1,595 12,303 10,024 2,050 12,074 A340 12,986 2,962 15,948 13,442 2,278 15,720 EMB145 116,420 19,593 136,013 114,370 18,682 133,052 EMB14L 38,731 4,784 43,515 40,326 6,151 46,477 Grand Total 307,688 55,487 363,175 311,621 51,555 363,176 INM Aircraft Type Departures Arrivals Annual Operations Figure 14. Modeled flight operations. In summary, the relative contribution of taxi noise with flight operation noise can be seen in the DNL contours under typical commercial airport situations. The degree of importance ground operations has on the overall noise environment at an airport is site specific and subject to interpretation. 2.7. Environmental Factors: Terrain, Buildings, Ground Cover Airports and the surrounding communities are often urban in nature and frequently contain high-rise buildings in addition to terminals, hangers and other forms of acoustic shielding on or adjacent to airport property. The geometric proximity of these features, specifically if they block the line of sight between a taxiing aircraft and a receptor, can have a significant impact on the noise contours. Ground cover also impacts sound propagation. Water is considered an acoustically hard surface and sound traveling over bodies of water does not attenuate as rapidly as sound traveling over grassy or forested terrain. Due to the wide variety of site specific conditions it is not be possible to draw a firm conclusion to always or never include building shielding or ground cover in taxi noise analysis. Some examples of propagation effects considering environmental factors such as buildings and ground cover are described in this section. An acoustic simulation study was performed using NMSim for a series of annual commercial flight operations at an International airport taking into account the effect of building shielding on sound propagation. While this study modeled only flight operations, they did include the on-runway portion of the operations. The geometric arrangement of the airport is such that the predominant impact to the contours on either side of the runways is from aircraft directly on the runway or at an altitude below the height of the nearby buildings. The noise modeling shown here utilizes a simple Maekawa shielding (line of sight blockage) model and with buildings modeled as a series of thin screens, as is supported by this theory. Figures 15 and 16 contrast the CNEL noise contours from all operations both without and with the building effects included. All annual operations are included in this comparison and building outlines modeled are drawn in yellow. Figures 17 and 18 contrast the CNEL noise contours from only the top 10 contributors for analyses with and without building effects included. A time sequence of still images from a single arriving flight is shown in Figure 19.
23 Figure 15. Flight operations, no building shielding. Figure 16. Flight operations, with building shielding. 55.0 dB 85.0 dB 65.0 dB 75.0 dB 60.0 dB 70.0 dB 80.0 dB 55.0 dB 85.0 dB 65.0 dB 75.0 dB 60.0 dB 70.0 dB 80.0 dB
24 Figure 17. Flight operations, no building shielding, top 10 contributors. Figure 18. Flight operations, with building shielding, top 10 contributors. 55.0 dB 85.0 dB 65.0 dB 75.0 dB 60.0 dB 70.0 dB 80.0 dB 55.0 dB 85.0 dB 65.0 dB 75.0 dB 60.0 dB 70.0 dB 80.0 dB
25 Figure 19. Series of acoustic simulation SEL (dB) contours with building shielding for a single approach operation at different times.
26 A measurement project was conducted in 2004 for the US Navy (22) in order to assess the effects of aircraft sound propagation over water. Measurements of 349 commercial aircraft departure operations at Ronald Reagan Washington National Airport & Bolling Air Force Base were obtained for elevation angles from 4o â 6o. PASSEUR Radar data for flight tracks and profiles were obtained for the measurement period. A list of the acoustic events and range of aircraft types is given in Table 5. A DC reference acoustic measurement SEL was used to determine vehicle Thrust setting. Figure 20 shows the geometric layout of the runway, flight track, Potomac River and microphone positions. The primary objective of the study was to experimentally determine suitable ground impedance parameters for representing the surface of the water as an acoustically hard surface using the DoD Integrated model; NOISEMAP 7. NOISEMAP 7 and INM are similar in that they are both integrated noise models, however INM contains source lateral directivity adjustments while NOISEMAP does not. Additionally, NOISEMAP has the capability to propagate over acoustically hard or soft terrain. It is the predicted changes between propagation over ground and water which illustrate a nominal 2 dB effect of ground impedance on sound propagation for sources at low elevation angles (Table 6). In this study, the recorded SEL at the microphone on the airport side of the river was used to determine the source conditions (thrust) of the departing aircraft. The radar data provided the aircraftâs airborne trajectory while a video system recorded the aircraft rotation and liftoff. The results presented in Table 6 illustrate the predicted propagation effects over water. Here the lateral source characteristics for aircraft with wing and tail mounted engines were adjusted from the original study based on the INM lateral directivity difference. Aircraft with fuselage mounted engines are 1.5 dB quieter in the plane of the wing than aircraft with wing mounted engines. One can see in Table 6 that the over water propagation accounts for approximately a 2 dB increase in SEL compared with propagation over acoustically soft ground for the geometric arrangement at DCA and for this particular group of operations. TABLE 5 Measured Aircraft Events ICAO ID (Radar Data) Modeled Aircraft Type Modeled Engine Type Engine Location INM Rotation Airspeed (knts) A319 A319 V2522 Wing 141 A320 A320 CFM56-5A-1 Wing 149 B733 B-737-300 B2 CFM56-3B-2 Wing 152 B734 B-737-400 CFM56-3C-1 Wing 160 B735 B-737-500 CFM56-3B-1 Wing 151 B737 B-737-400 CFM56-3C-1 Wing 146 B738 B-737-400 CFM56-3C-1 Wing 161 B752 B-757-200-PW PW2037 Wing 142 CRJ1 CL-601 TF CF34-3A Tail 164 CRJ2 CL-601 TF CF34-3A Tail 164 DC93 DC-9-30QN9 (Q) JT 8D(AC-LINED) Tail 146 E135 CL-600 TF ALF502L Tail 164 E145 E145 AE3007 Tail 125 F100 F10065 TAY 650-15 Tail 146 MD80 MD-81 JT 8D-209/217 Tail 146
27 Figure 20. Over water propagation measurement configuration. TABLE 6 Predicted Differences in SEL over Ground and Water for all Flight Operations (Predictions modified based on INM source lateral directivity for Wing / Fuselage Engines) Measured SEL â Calculated Over Water SEL (Predictions exactly match Reference SEL) Over Water Pred. SEL â Over Ground Pred. SEL (dB) (Consistent Set of Operations with different ground characteristics) Site S1 Site S2 Site S3 Site S1 Site S2 Site S3 -0.8 Â± 0.3 dB -1.2 Â± 0.3 dB -2.15 Â± 0.3 dB -2.2 Â± 0.3 dB -2.0 Â± 0.3 dB -1.5 Â± 0.3 dB In summary, the impacts of shielding due to buildings and terrain and propagation over hard and soft ground (such as would be encountered at an airport adjacent to a body of water) are apparent in the noise contours. However, due to the wide variety of site specific conditions it is not be possible to draw a firm conclusion to always or never include these effects for taxi noise analysis. Not to scale S1 Primary DC Ref S2 S3 Ronald Reagan Washington National Airport Bolling AFB DC Ref S1, S2, S3