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Evaluating Airfield Capacity (2012)

Chapter: Chapter 5 - How to Select the Appropriate Airfield Capacity Model

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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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Suggested Citation:"Chapter 5 - How to Select the Appropriate Airfield Capacity Model." National Academies of Sciences, Engineering, and Medicine. 2012. Evaluating Airfield Capacity. Washington, DC: The National Academies Press. doi: 10.17226/22674.
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71 How to Select the Appropriate Airfield Capacity Model Selecting the appropriate level of model sophistication is not as easy as a one-size-fits-all approach. Many factors contribute to the type of model that is best suited to analyze a particular capacity issue. This chapter is intended to guide a capacity analyst to the appropriate level of modeling sophistication by presenting a decision-support tool and describing additional con- siderations in selecting a level of modeling sophistication. The guidance presented in this chapter is not intended to provide a definitive, unique answer to the question, “Which model should I use in a given situation?” Rather, it is intended to guide the user through the factors to be considered in making a reasonable choice for a given set of circumstances. Rarely is there only one answer as to which model should be used, and many factors can affect a decision that cannot be captured in a decision hierarchy. Nevertheless, the guidance contained in this chapter should help the reader narrow down the choices regarding which levels of modeling sophistication are appropriate to a reasonable set of options, any of which would be satisfactory. Decision Factors Selecting an appropriate level of modeling sophistication depends primarily on the purposes of the capacity analysis and the characteristics of the specific airport. Purposes of Capacity Analysis: What’s the Question? What types of capacity changes need to be analyzed, and why are the changes being analyzed? The answers to these “what and why” questions will largely determine the specific capacity fac- tors and issues that must be addressed. In this chapter, the following types of capacity changes will be considered in relation to the ways they affect data requirements and the choice of model sophistication level: • New runways and runway extensions • New taxiways, aprons, or gates (holding bays, parallel taxiways, runway exits, etc.) • Changes in flight procedures and navigational aids • Noise abatement procedures (e.g., multiple versus single headings, aircraft type restrictions, etc.) • Aircraft fleet mix and stage-length mix changes • Runway crossings • In-trail terminal airspace restrictions • Changes in navigational aid critical areas • Multiple instrument approach procedures and staggered instrument approach procedures • Effects of airport traffic control towers (ATCTs) C h a p t e r 5

72 evaluating airfield Capacity • Changes in air traffic control (ATC) rules and procedures • ATC workload and human factors (such as pilot proficiency) • Next Generation Air Transportation System (NextGen) benefits of reduced aircraft separa- tions and new airspace procedures New Runways and Runway Extensions The choice of the level of modeling sophistication to analyze new runways and runway exten- sions depends on (a) the complexity of the airfield, (b) the location and function of the new runway or runway extension, and (c) the purpose and anticipated benefits of the improvement. New runways present the most extensive capacity change that can occur at an airport. The capacity effect of new runways depends most on (a) orientation and dependence in relation to other runways (i.e., parallel, converging, intersecting), and (b) expected runway use (i.e., arriv- als, departures, or mixed mode). A runway extension generally only affects airfield capacity if it would result in a significant change in (a) how the runway is used, (b) what aircraft could use it, or (c) whether it results in or eliminates an intersection with another runway or some other capacity constraint. For example, if the extension transforms the runway from a general aviation runway to an air carrier runway, then capacity could increase significantly. However, if the extension simply allows a few larger aircraft to operate more safely or several long-haul international flights to use the runway in off-peak periods, then its effect on airfield capacity would probably be negligible. If the runway extension results in an intersection with another runway, capacity could be reduced if both run- ways are used in a particular wind or weather condition. Below are some additional considerations for deciding on a level of modeling sophistication for evaluating the capacity changes associated with new runways and runway extensions: • Airfield Complexity. For simple or complex airfields, the model sophistication level required to analyze the change in capacity primarily depends on the additional capabili- ties provided by the improvement. Ironically, the greater the change in capabilities, the less sophisticated the model needed to measure it. Analytical models typically are adequate for analyzing major changes, such as increasing the number of runways from one to two. On the other hand, adding a fourth or fifth runway to an already complex airfield would almost certainly require an airfield simulation analysis to determine the expected marginal increase in capacity. • Degree of Change. In most capacity analyses, the change in capacity is the most critical factor; therefore, it often requires a more sophisticated model or a higher level of fidelity and refine- ment to appropriately measure small changes in capacity. Less sophisticated modeling tools are usually sufficient to measure large changes in capacity. • Location and Function of Runway Improvement. The location and function of a proposed runway are the primary determinants of the additional capabilities that would be provided. If the proposed runway would be independent of the other runways, then its benefits might easily be estimated using a Level 3 analytical model. However, if the proposed runway would be dependent on the other runways, a more sophisticated level (e.g., Level 4) model may be required. Moreover, if the proposed runway would create additional runway crossings or a complex runway intersection, then a more sophisticated level model may be appropriate, particularly because most Level 3 analytical models do not explicitly account for the effects of taxiways and runway crossings. In cases where a new runway would be located far away from the existing runways and terminal building, a Level 5 model is typically required to measure the trade-off between increased capacity and increased aircraft taxiing times. • Purpose and Anticipated Benefits of New Runway or Runway Extension. The modeling sophistication level required to evaluate the capacity change with a new runway or runway

how to Select the appropriate airfield Capacity Model 73 extension also may depend on the purpose and anticipated benefits of the improvement, which, in turn, may determine the evidence the model must provide to convince stakehold- ers that the benefits of the improvement are at least as great as its cost. For example, there may be a need for convincing evidence that the benefit of the improvement is justified because it would require significant capital investment or it may have adverse environmental effects. In either case, a more sophisticated level of modeling may be required to provide that convincing evidence at a level of detail appropriate for a benefit-cost analysis (BCA). New Taxiways, Holding Bays, and Runway Exits Detailed airfield simulation modeling is the only method now available to measure the effects of many types of aprons and gates on airfield capacity. Current analytical models do not include gates and have only limited capability to evaluate holding bays. Most analytical models strictly address airfield capacity and do not take into account other airfield elements except through the assumptions of runway occupancy times and, possibly, aircraft separations. New aprons at smaller general aviation airports do not necessarily require modeling. Regional airport system plans would not likely require measurements of the effects of aprons, only of runways. Analytical models provide the means to evaluate some of the effects of taxiways on runway capacity, especially the effects of runway entrances and exits and parallel taxiways. More discus- sion of the modeling of these elements is provided in Chapter 4 of this guidebook. Changes in Flight Procedures and Navigational Aids Changes in flight procedures can be evaluated using either analytical or simulation models. Such changes will affect the percent of time that particular procedures can be conducted, aircraft separation requirements, and the number of simultaneous movements that can be conducted. The determining factor in choosing an analytical model versus a simulation model may depend primarily upon the effect of the flight procedures on the number of simultaneous move- ments, which, in turn, depends on the complexity of the airfield. Changes in Noise Abatement Procedures The effects of noise abatement procedures can be estimated using analytical models if the effects of such procedures can be defined in terms of increased or decreased in-trail separations, or significant changes in runway use. Otherwise, airfield simulation modeling may be needed. For example, a noise abatement procedure may require turbojet aircraft to fly a long common path but allow turboprop and propeller aircraft to make immediate turns. The net effect of such a noise abatement procedure is complex and dynamic and would depend on how well control- lers can sequence departures such that they avoid sending two successive turbojets to the same departure fix. Analyzing such effects may require a higher level of modeling sophistication than most analytical models can provide. Aircraft Fleet Mix and Stage-Length Mix Changes Analytical models should be sufficient to estimate the effects of a change in aircraft fleet mix on airfield capacity if that change does not significantly affect how the runways are used or what aircraft can use which runway. Such a change would be reflected primarily in how frequently certain aircraft-pair combinations occur and, therefore, the average time interval between operations. A change in stage-length mix may affect runway use because the pilots of longer stage-length flights may request use of a longer runway. Unless the change in stage-length mix is significant, however, its effect on capacity is probably negligible and may not justify a higher level of model- ing sophistication.

74 evaluating airfield Capacity Changes in Runway Crossings Airfield simulation modeling generally is required to reflect the effects of runway crossings on airfield capacity primarily because of the complex interactions between the aircraft arriving and/or departing on the runway and the aircraft trying to cross that runway. Changes in the fre- quency of runway crossings can be caused by addition or removal of runway crossing points, use of land and hold short operations (LAHSO), provision of end-around taxiways, and changes in runway use. Some analytical models are designed to evaluate simple runway-crossing scenarios, as described in Chapter 4, but they may not be adequate to evaluate crossings involving multiple runways. In-trail Terminal Airspace Restrictions The effects of in-trail terminal airspace restrictions can be estimated either implicitly using analytical models or explicitly using airfield simulation models. Only Level 5 simulation models enable the user to explicitly measure the controller’s ability to sequence departures to minimize the effects of having to send two successive aircraft to the same restricted departure fix. There- fore, aircraft delay simulation models (Level 5) are preferred for this purpose if sufficient time and budget are available. Such models provide a more fine-grained analysis of the effects of such restrictions on airfield capacity and proposed improvements to mitigate those effects. Changes in Navigational Aid Critical Areas Navigational aid critical areas can affect capacity by restricting access to the runway. For exam- ple, an instrument landing system (ILS) glide slope critical area may encroach on the taxiway leading to the end of a departure runway. If the aircraft is held short of the critical area, it will take longer to taxi to the departure end of the runway to line up and wait or be cleared to take off, which, in turn, will increase the separation that controllers must provide between arrivals to release the departure between arrivals. Analytical models can be used to measure such an effect by either (a) increasing the arrival- arrival separation required to release a departure between arrivals, or, equivalently, (b) increasing the required arrival-departure separation (i.e., the distance out from the threshold that an arrival must be in order to release a departure). Unless this distance is increased, the ability of controllers to clear departures between arrivals in an arrival-priority operating mode could be restricted. Simulation models may be required if the effect of the navigational aid critical area is more complex or to explicitly measure the effect of the navigational aid critical area through assump- tions regarding the additional aircraft taxiing distances and clearance times that the critical area would impose on departures. For example, to avoid having to hold departures short of an ILS glide slope critical area, controllers could, if advantageous, taxi aircraft across the runway to a parallel taxiway on the other side of the runway so that they can be more easily cleared onto the runway for departure. The effect of using such a runway crossing versus having to wait at the ILS hold line would have to be measured using a Level 5 simulation model. Another possible effect of eliminating the need to hold departures short of a navigational aid critical area may be a change in runway use. For example, depending on the complexity of the airfield, controllers could make greater use of a runway for departures if the need to hold departures short of a navigational aid critical area were eliminated. In this case, Level 4 or Level 5 airfield simulation modeling may be required to estimate the capacity benefit of such an improvement. Multiple Dependent and Independent Instrument Approach Procedures A higher level of modeling sophistication, such as Level 4 and Level 5 airfield simulation modeling, would be preferable for estimating the effects of introducing a new multiple-approach

how to Select the appropriate airfield Capacity Model 75 procedure, such as a simultaneous offset instrument approach procedure or a dependent con- verging instrument approach procedure using a converging runway display aid. Such improvements increase the complexity of an existing airfield operation to the extent that analyzing their benefits using analytical models becomes very difficult. In particular, modeling staggered instrument approach procedures can be very complex if the procedures require that standard wake turbulence separations be provided behind a heavy jet or a B-757. Effects of Airport Traffic Control Towers The effects of installing or removing an ATCT can be measured most easily through before- and-after analyses using an analytical model that would include appropriate assumptions about the changes in aircraft separations made possible with the ATCT. Without the ATCT, aircraft would have to follow uncontrolled airport approach and departure procedures, relying on visual separation from the cockpit and pilots announcing their intentions over a universal communications (UNICOM) system. Although such uncontrolled airport pro- cedures can be efficient, pilots operating at a busy uncontrolled airport tend to be very conserva- tive in separating themselves from other aircraft. With the ATCT, controllers can apply visual separation and provide clearances to land and take off, which generally results in shorter intervals between operations and greater runway use, thereby increasing airfield capacity. Except in rare cases, simpler techniques for evaluating airfield capacity, such as table lookup or simple analytical models, would be most appropriate. The use of airfield simulation models would typically not be justified unless the airport has a complex runway layout. Changes in Air Traffic Control Rules and Procedures Most changes in ATC rules and procedures can be analyzed using either analytical models or simulation models, depending on the complexity of the airfield being analyzed and other factors, such as those addressed in previous sections of this guidebook. Such changes usually affect the magnitude of aircraft separations or the degree of dependence between runway operations. In the past, such changes have included (a) the ability to operate 2.5-nautical-mile spacing on final approach for runways on which average arrival occupancy times of 50 seconds or less can be demonstrated, (b) dependent ILS approaches with 1.5-mile staggered separations, (c) addition of divergent runway headings to runway ends which currently do not have them, and (d) restric- tions on the ability of larger aircraft to overtake smaller aircraft on final approach to closely spaced parallel runways to avoid hazardous wake turbulence interactions. The first three of these changes could be analyzed easily using simpler analytical models, such as the FAA’s ACM. The fourth change—restrictions on larger aircraft overtaking smaller aircraft on final approach—has been more difficult to analyze using the ACM because of the complex modeling required to measure the effects of the aircraft fleet mix on such a restriction. Therefore, the choice of model for analyzing such changes in ATC rules and procedures would depend on the complexity of the change under consideration and the complexity of the airfield layout. Air Traffic Control Workload and Human Factors Restrictions (Including Pilot Proficiency) Human factors are not very well represented in available analytical models or simulation mod- els for evaluating airfield capacity. Sometimes, for an airport-specific procedure or restriction, an analyst can receive controller input on an average aircraft separation or acceptance rate to adjust the model separation minimums or associated buffers to reflect high controller workload. For example, conducting visual approaches to closely spaced parallel runways, where extensive

76 evaluating airfield Capacity voice communication is required on the part of the pilots and controllers, often is characterized as being driven by how fast controllers and pilots can talk. Pilot proficiency is rarely modeled in terms of how it affects airfield capacity except in cases where pilot proficiency with the English language limits the ability of controllers at busy inter- national airports to conduct visual approaches. Pilots who are not proficient in English generally will not accept a visual approach clearance. In the United States, visual approaches are almost uni- versally accepted, except at large international airports where many pilots are flying foreign-flag aircraft. At most other international airports around the world, where ATC communications are conducted in multiple languages, visual approach clearances are not accepted or conducted at all. NextGen Benefits of Reduced Aircraft Separations and New Airspace Procedures The capacity benefits of anticipated NextGen operational improvements have been estimated using a variety of analytical and simulation models. Consistent with previous recommenda- tions, the choice of model for evaluating these benefits depends primarily on the complexity of the airport operation and the complexity of changes associated with the NextGen operational improvement under consideration. For example, Boeing recently used an analytical model to estimate the capacity increases at 35 airports associated with a set of operational improvements anticipated with NextGen. At the other end of the modeling sophistication scale, researchers at NASA Ames Research Center have used the Airspace Concept Evaluation System (ACES) model to evaluate the effects of NextGen on the national airspace system (NAS). ACES is an agent-based, NAS-wide, non- real-time simulation model. MITRE Corporation has been estimating the effects of NextGen technologies on airfield capac- ity using its runwaySimulator, which is a Level 4 airfield capacity simulation model designed to estimate throughput using input parameters similar to those used in the ACM. Members of the research team that developed this guidebook have estimated the effects on airfield capacity of various NextGen technologies using the ACM. In this modeling exercise, the primary parameter assumed to change as a result of NextGen technologies was the statisti- cal spacing buffer applied to the minimum aircraft separation requirements. NextGen also was assumed to have an effect on runway occupancy times and certain minimum wake turbulence aircraft separation requirements. These same changes in parameters were used in modeling NextGen operational improvements for FAA and the Joint Planning and Development Office (JPDO), a cross-agency entity created in 2003 to manage the public-private partnerships that were designed to implement NextGen. Airport Characteristics The level of modeling sophistication appropriate for a particular application depends to a certain extent on the characteristics of the airport being analyzed. The following airport charac- teristics can affect the choice of modeling sophistication level: • Level of capital investment and complexity of airport operations • Types of activity at the airport • Capacity issues to be addressed • Airport size Level of Capital Investment and Complexity of Airport Operations Evaluations of airport improvements requiring significant capital investment or at airports with highly complex operations or airfield configurations typically justify using higher levels of

how to Select the appropriate airfield Capacity Model 77 modeling sophistication. In most cases, these conditions occur at larger airports rather than at smaller airports. However, this generalization does not always apply, such as when a complex issue needs to be addressed at a small airport that requires a high level of fidelity in the airfield capacity modeling to be able to distinguish the effects of small differences among alternatives. Types of Activity at the Airport Another airport characteristic that affects the choice of modeling sophistication level relates to the types of activity at the airport. In particular, the presence of high levels of training activity or wide ranges of aircraft types operating on the airfield affect the selection of an appropriate level of modeling sophistication. Instrument approach procedures in effect at the airport also influence the choice of an appropriate capacity evaluation technique. Capacity Issues to Be Addressed One of the most important factors affecting the selection of a level of modeling sophistica- tion is the capacity issues to be addressed (i.e., the reasons for the analysis). For example, all master plans require some form of airfield demand-capacity analysis to examine the need for additional capacity at the airport. However, if airfield capacity is not known to be an issue at the airport, the level of modeling sophistication necessary to meet this master planning requirement could be minimal. In such cases, a simple methodology that uses the Level 1 lookup tables in AC 150/5060-5 (the AC) or the Airport Design Computer Program model may be sufficient to address this requirement. Similarly, state system plans typically do not address airfield capac- ity other than through a high level review of annual service volume (ASV) and comparison of annual operations to reveal potential capacity needs. However, some regional airport system plans are being developed primarily to mitigate capacity constraints caused by conflicts between airports in the region, and such plans could require a sophisticated Level 5 model to address specific capacity issues. Airport Size The size of the airport typically has a significant effect on the appropriate level of modeling sophistication. However, making a determination based on simple descriptive categories, such as large, small, commercial service, or general aviation, is not always appropriate. To determine the appropriate level of model sophistication, the size of the airport must be determined in terms of the types of facilities and amount and/or type of activity at the airport. At airports that may typically be referred to as small, the following factors have been identified as being relevant in determining the necessary level of modeling sophistication: • Presence or Absence of an ATCT. An airport with an ATCT can likely accommodate a higher number of operations (hourly, daily, and annually) because the ATCT can provide pilots with information to maintain a safe and efficient flow of traffic. Without an ATCT, pilots rely on seeing each other and announcing their intentions using radio communication (UNICOM) to determine where other aircraft are in relation to the airport and the ability of the pilot to safely land the aircraft. Typically, airports without an ATCT are less busy from an operational perspective and the issue of capacity is one of perspective, as reliable data on actual opera- tional activity are not available. The absence of an ATCT may be related to the limited opera- tional activity at the airport, as less busy airports are not likely to be able to meet the FAA’s criteria for establishing an ATCT. • Presence or Absence of Commercial (Air Carrier) Passenger Service. Commercial passenger service can affect airfield capacity depending on the types of aircraft that provide service and the number of operations conducted by the commercial passenger airline(s). If passenger ser- vice is provided using a mix of small and larger aircraft, the capacity calculations are affected more than if the sizes of the aircraft are more homogeneous. A significant level of passen- ger service would also require taking into account airline schedules, parking areas, gates, and

78 evaluating airfield Capacity other factors that may affect airfield capacity. Airports without commercial passenger service, referred to as general aviation airports, can still be served by a diverse aircraft fleet and may still require a Level 4 or Level 5 airfield capacity evaluation based on this fleet and the number of operations at the airport, particularly if airport taxiways and aircraft parking areas are an issue. • Presence or Absence of an Instrument Approach Procedure. The lack of an instrument approach procedure limits airfield capacity, as activities are not considered that cannot be accommodated during conditions below visual approach minimums or basic visual flight rules conditions (i.e., when the ceiling is less than 1,000 feet or visibility is less than 3 miles). The AC lookup tables are based on specific assumptions regarding the presence of specific facilities and approach procedures. These include a precision approach (an instrument land- ing system is noted specifically), a full-length parallel taxiway, and a runway configuration. The lack of an instrument approach procedure may be related to limited operational activity at the airport, as less busy airports are not likely to be able to meet the FAA’s criteria for estab- lishing an instrument approach procedure. • Number of Annual Aircraft Operations. The current process for evaluating airfield capacity for any specific time period requires comparing the number of operations to the calculated capacity for that period. Airports with a low number of aircraft operations compared to the calculated capacity for that period would not be identified as having a capacity issue and, therefore, would not require detailed analysis of airfield capacity. Even if the types of activ- ity accommodated include a diverse range of aircraft types, a low number of annual aircraft operations likely indicates that airfield capacity is not an issue and that a low level of analytical modeling sophistication would be sufficient. • Number of Training or Touch-and-Go Operations. Training or touch-and-go operations typically increase the operational capacity of an airport because the pilots conducting these operations have aircraft in the pattern continually ready to take off and land, and each touch- and-go counts as two operations: one landing and one takeoff. The AC Level 3 methodology specifically includes a factor related to the percent of training operations in total operational activity in calculating an hourly capacity and an ASV for the airport. Additional Considerations Certain additional considerations may override the general logic and hierarchy discussed in previous sections of this chapter. These additional considerations include the availability and adaptability of legacy models, time and budget constraints, availability of data, levels of stake- holder involvement in the capacity analysis, magnitude of investment, and the level of accuracy required to discern benefits versus costs. Availability of Legacy Models and Risks of Updating Previous Modeling Efforts If various models have been used for capacity analysis during previous planning projects, continuing to use such legacy models can reduce the time and cost required to complete a new analysis. Using a legacy model means it is not necessary to develop a new baseline model. Using a legacy model also avoids the need to calibrate the model, assuming that no significant changes at the airport would dictate a recalibration. It also maintains consistency in measures and methods for comparison with prior analyses. Using previously developed models, at least as a starting point, typically is most beneficial for simulation modeling that has a long start-up and calibration time. Before deciding to use a legacy model, however, it is important to review the model to ensure that it is not erroneous and that any assumptions are still current. It should be verified that the

how to Select the appropriate airfield Capacity Model 79 model was calibrated properly. New data should be collected to refresh any needed inputs related to forecasts and demand patterns. A particular model or level of sophistication should not be selected solely because a legacy model is available. More important considerations are the capac- ity issue being analyzed, the time and budget available, and the factors affecting airfield capacity that need to be incorporated into the analysis. Time and Budget Constraints The available time and budget may limit the choice of a modeling sophistication level for the airfield capacity analysis. If time and budget resources are limited, analytical models or spread- sheets may be the most appropriate choice. In many cases, an airport sponsor has only a short window for decision making, whether for policy decisions or funding decisions (e.g., to proceed further with a capital improvement). Such time constraints may dictate the use of less sophisticated models, even though other consider- ations would suggest a different choice. In such cases, it is not unusual to follow such analyses with more sophisticated modeling in support of further funding decisions, planning or design requirements, proof of concept, environmental analyses, or phasing plans. Notwithstanding the guidance provided in this chapter for choosing an appropriate level of modeling sophistication, exceptions will always exist related to time and budget constraints. Availability of Data The data available to the capacity analyst can be a deciding factor in the level of modeling sophistication selected. Data may not be available for many reasons, including the following: • The data may not exist or be recorded. • The data may not be publicly available. • The data may be too costly to acquire. • The data may be too time-consuming to process. For example, even if a non-ATCT airport is facing a complex capacity issue related to taxiways, it may not be feasible to construct a simulation model because of the lack of data regarding flight schedules and characteristics of operations. Or, at an airport with no scheduled passenger ser- vice, it may be difficult to calculate fleet mix, which eliminates many capacity analysis methods from consideration. However, default values often can be used in lieu of particular inputs in cases where data are unavailable or infeasible to obtain. Figure 5-1 summarizes the data that are typically required for the various levels of modeling sophistication. Figure 5-1. Choosing the level of modeling sophistication based on data availability.

80 evaluating airfield Capacity See Chapter 3 for a discussion of the data requirements for each level of sophistication. A guide to data sources is provided in Appendix B of this guidebook. Level of Stakeholder Involvement in the Capacity Analysis Process For controversial or high-profile projects it may be necessary to work with agencies that are unfamiliar with the nomenclature or metrics used in airfield capacity evaluations (e.g., when conducting extensive public workshops or when providing briefings to elected officials). In such cases, graphic output ranging from simple diagrams and charts to animations of airfield/ airspace options can aid in explaining (1) the issue that has led to a proposed improvement, (2) the purpose of the proposed improvement, and (3) the benefits to be derived from the pro- posed improvement. In these cases, the sophistication of the model tends to be less important than the ability to communicate the critical issues to the non-aviation community. However, it is important to be consistent with metrics previously reported for an airport (i.e., throughput capacity, annual capacity, or aircraft delay). Magnitude of Investment The magnitude of investment does not always correlate to the level of modeling sophis- tication needed. This correlation applies in some, but not all, situations. For example, some smaller changes in capacity may not be able to be distinguished and evaluated using anything except simulation modeling (e.g., the expected change in runway throughput with proposed taxiways or holding bays that enable improved departure staging and sequencing). At the same time, smaller investments may not warrant the use of expensive, high level simulation tools. For large-scale projects, especially those that are subject to FAA BCA requirements for Airport Improvement Program grant consideration, simulation modeling may be needed to provide the appropriate level of fidelity even if the magnitude of investment is relatively modest. The more important determinant of required modeling sophistication is the expected change in capacity. For example, the changes in capacity expected from the addition of a new runway can easily be estimated with lower level models, tables, or spreadsheets. For an investment as large as a new runway, however, a higher fidelity level of modeling that includes simulation usually is recommended. Level of Accuracy Required to Discern Benefits versus Costs The level of accuracy is directly correlated to the level of modeling sophistication: as the level of modeling sophistication increases (and the more airport-specific data inputs are required), it is expected that the level of accuracy of the results would increase. The level of accuracy needed to discern benefits is project-specific. Often, smaller projects have a small margin of benefits, which requires a higher level of accuracy to be able to measure the benefits. On the other hand, large projects that provide a large change in capacity can be captured using analytical or other lower-level modeling techniques. Notice that a high level of accuracy may not be required to discern the benefits of a project (e.g., the capacity increase associated with a new runway). A high level of modeling sophistica- tion may be needed, however, because of the magnitude of the investment or because of other factors. Alternatively, the change in capacity of a small project (e.g., a departure sequencing hold pad) may be best measured using simulation, but the time and budget available for the analysis may preclude the use of the more time-consuming and costly simulations.

how to Select the appropriate airfield Capacity Model 81 High-Level Matrix for Preliminary Screening of Levels of Modeling Sophistication Figure 5-2 presents a high-level matrix of the major decision factors to be considered in choos- ing a level of modeling sophistication for a particular set of circumstances and conditions. This matrix is intended for preliminary screening purposes, and can be used in conjunction with the more detailed decision hierarchy described in the text. Decision Hierarchy The decision hierarchy developed as part of ACRP Project 03-17 supports an airport oper- ator’s decision-making process in selecting an appropriate method of capacity analysis. It is important to note that this decision hierarchy is not meant to provide a definitive answer, but instead should be used as a decision-support tool. For any capacity issue being analyzed, different levels of modeling sophistication could be used. Moreover, the decision-support tool provides only the recommended level of sophistication “in a perfect world.” Exceptions and special cir- cumstances will always exist that could dictate a different choice or that do not follow the logic presented in the decision hierarchy. The decision hierarchy is intended to distinguish between the factors for which each level of sophistication can account, allowing a capacity analyst to identify the specific attributes of the capac- ity issue that would drive the use of a certain level of modeling sophistication. The distinguishing factors between one level and the next are presented in a hierarchy for a capacity analyst to consider in selecting the appropriate level of sophistication. The questions and characterizations of each level are meant to answer the question, “In a perfect world, which level of modeling sophistication should you use?” In many situations, however, multiple levels of modeling sophistication can be used. The questions in the decision hierarchy are presented in Figure 5-3 and explained in more detail in Table 5-1. Examples of Level of Modeling Sophistication Used in Airfield Capacity Case Studies ACRP Project 03-17 included the preparation of 27 case studies of applications of airfield capacity analysis. These case studies were selected to include a wide-ranging sample of applica- tions of capacity analyses and levels of modeling sophistication. A brief overview of the case studies is provided in this section of ACRP Report 79. The ACRP Project 03-17 final report, including detailed descriptions of the case studies, has been posted to the ACRP Project 03-17 web page at http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=2579. Each case study is categorized into one of the following categories: • Airport master plans • Airport system plans • Airport capacity studies • Airport environmental studies • FAA airfield capacity studies • Academic and research studies Each case study was examined to determine which of the following series of characteristics were included in the analysis: • Applications: Capacity benefits, aircraft delay, future technologies, environmental constraints, system planning

82 Evaluating Airfield Capacity Figure 5-2. High-level matrix for choosing a level of modeling sophistication.

How to Select the Appropriate Airfield Capacity Model 83 Figure 5-3. Decision hierarchy for selecting level of modeling sophistication.

84 Evaluating Airfield Capacity Table 5-1. Explanatory comments on questions in decision hierarchy.

How to Select the Appropriate Airfield Capacity Model 85 Source: ACRP 03-17 Research Team. Table 5-1. (Continued).

Main Purposes of Airfield Capacity Analysis Elements of Airfield Considered Airfield Capacity Metrics Level of Modeling Sophis�ca�on Case Study (short �tle) Capacity Benefits Aircra� Delay NextGen Technologies Environmental Constraints System Planning Runways Taxiways Gates Terminal Airspace NAS Hourly Throughput Annual Service Volume Level 1 & 2 AC Level 3 Analy�cal Level 5 Simula�on Master Plans 1 Airport Master Plan (ARW) X X X X X X 2 Airport Master Plan (CHD) X X X X X X 3 Master Plan Update (MEM) X X X X X X 4 Master Plan (BWI) X X X X X X X System Plans 5 Airport System Plan Update (New Mexico) X X X X 6 Regional Airport System Demand Study (NY/NJ) X X X X X X X Capacity Studies 7 Update of Airfield Analysis (HOU) X X X X X X 8 Airport Expansion Feasibility Study (PBC) X X X X X X 9 Ul�mate Airfield Capacity Study (OAK) X X X X X 10 Analysis of Airside and Gate Capacity (SFO) X X X X X 11 Airside Capacity Study (JFK) X X X X X X X 12 Delay Reduc�on Study (JFK) X X X X X X X Environmental Studies 13 Part 161 Study (BUR) X X X X X X X 14 Environmental Impact Statement (FLL) X X X X X X X X 15 Part 150 Study (CVG) X X X X X FAA Airfield Capacity Studies 16 AC-150/5060-5 Airport Capacity and Delay X X X X X X X X X 17 Capacity Enhancement Plan (CEP) for MEM X 18 Airport Capacity Benchmark Report X X X X X 19 Capacity Needs in the Na�onal Airspace System X X X X X X X X X Academic and Research Studies 20 Low Visibility Landing and Surface Opera�ons Runway Occupancy Time X X X 21 Op�mal Level of Opera�ons on an Arrivals Only Runway X X 22 Computer Simula�on Model for Airplane Landing Performance X X X 23 Improvements in Simple Models for Es�ma�ng Runway Capacity X X X 24 Valida�on of Runway Capacity Models X X X 25 Delay Impacts of an Airport Enhancement (Detroit) X X X X 26 Scenario-Based Management of Air Traffic Flow X X X X X X X 27 North Airfield Safety Study (Los Angeles) X X X X X X Source: ACRP 03-17 Research Team. Table 5-2. Summary of airport capacity case studies: Conditions and levels of modeling sophistication used—ACRP Project 03-17, “Evaluating Airfield Capacity.”

how to Select the appropriate airfield Capacity Model 87 • Elements of Airfield Considered: Runways, taxiways, gates, terminal airspace, NAS • Airfield Capacity Metrics: Hourly throughput, annual service volume • Level of Modeling Sophistication: Level 1 and 2 table lookup or nomographs (e.g., the AC); Level 3 analytical models (e.g., the ACM); and Level 5 aircraft delay simulation (e.g., SIMMOD and TAAM) Table 5-2 summarizes key features of the case studies. The levels of modeling sophistication used in each study are shown in the three right-hand columns of the table. Notice that in Table 5-2 the most frequently used level of modeling sophistication is Level 3, analytical models. The table also shows that several of the studies involved the use of more than one level of modeling sophistication. Four of the airport planning studies used Level 5 aircraft delay simulation, as did three of the academic and research studies. The smaller airport master planning studies used Level 1, Level 2, and Level 3 capacity models, while the larger airport master planning studies used Level 5 simulation, as one would expect.

Next: Chapter 6 - Subsequent Uses of Airfield Capacity Estimates »
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TRB’s Airport Cooperative Research Program (ACRP) Report 79: Evaluating Airfield Capacity is designed to assist airport planners with airfield and airspace capacity evaluations at a wide range of airports.

The report describes available methods to evaluate existing and future airfield capacity; provides guidance on selecting an appropriate capacity analysis method; offers best practices in assessing airfield capacity and applying modeling techniques; and outlines specifications for new models, tools, and enhancements.

The print version of the report includes a CD-ROM with prototype capacity spreadsheet models designed as a preliminary planning tool (similar to the airfield capacity model but with more flexibility), that allows for changing input assumptions to represent site-specific conditions from the most simple to moderate airfield configurations.

The CD-ROM is also available for download from TRB’s website as an ISO image. Links to the ISO image and instructions for burning a CD-ROM from an ISO image are provided below.

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CD-ROM Disclaimer - This software is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences or the Transportation Research Board (collectively "TRB") be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

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