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12 Airfield Capacity Concepts Airfield capacity is a function of the airportâs physical facilities or components; its layout or geometry; its operating environment, including the airspace allocated to the airport and spe- cific air traffic control (ATC) and flight procedures; the mix of aircraft using the airport; and weather conditions (i.e., ceiling, visibility, and winds). Understanding these factors, where to obtain information about them, and the required inputs and assumptions related to each factor, are all important prerequisites for analyzing airfield capacity. Airport Components An airport encompasses many features that allow aircraft to take off and land and that allow pilots and passengers to access facilities on the ground. Typically, an airportâs facilities are divided into three components: airside, terminal, and landside (Figure 2-1). These three components have specific functions and capacities, and their capacities must be in reason- able balance. Airside facilities generally include those that support the transition of aircraft from air to ground or the movement of aircraft from parking or storage areas to departure and flight. The airfield itself is one component of the airside facilities, the dominant feature of an airport, and typically encompasses the largest land area. In general, the airfield includes the airportâs runway and taxiway system; along with various aircraft hold pads or holding bays. Airside support facili- ties include airfield maintenance, marking and lighting, navigational aids, weather reporting stations, and ATC facilities. Airside facilities generally are common to all sizes of airports (every airport needs at least some type of runway), with varying degrees of complexity depending on the type and level of activity at the airport. Terminal facilities are used to transfer passengers and aircraft crews from the landside to the door of the parked aircraft. Terminal facilities are provided at both commercial service and general aviation airports. The terminal itself is typically a passenger-processing building for ticketing and baggage claim, along with concourses and gates. Terminal facilities at commer- cial service airports usually are larger and have greater security, access, and general footprint requirements. Terminal building functions at general aviation airports are typically provided in a fixed base operator (FBO) building that houses a pilotâs lounge, access to weather data, restrooms, and so forth. Landside facilities provide the link between air and ground transportation. Landside facilities include airport access roadways, terminal area access and circulation roadways, terminal curb- sides, automobile parking facilities, intermodal access, and commercial ground transportation staging facilities. C h a p t e r 2
airfield Capacity Concepts 13 Airside Components Including Airfield Geometry All of an airportâs componentsâairside, landside, and terminalâwork in conjunction with one another in operation of the airport. The airside components, including airfield geometry, are generally the limiting factor governing the ultimate capacity of the airport. As previously noted, airside facilities are designed to accommodate the movement of aircraft between final approach and aircraft parking for arrivals and from aircraft parking to initial climb-out for departures. At commercial service airports, these facilities usually are not accessible by the general public, but they are generally accessible to the public at smaller airports that do not have the same security and access requirements. Airside components include the following: ⢠Runways ⢠Taxiways ⢠Holding bays ⢠Aprons ⢠Gates Runways A runway is a strip of hard or paved level ground on which aircraft take off and land. A runwayâs surface typically is hard and can be made of concrete or asphalt as well as grass/turf, dirt, or gravel. At airports used mostly by commercial and larger general aviation aircraft, a concrete or asphalt runway is standard. Airports can have a single runway or multiple runways that may or may not be operated simultaneously. Figure 2-2 shows typical runway numbering and marking. The number of runways and the way the runways are operated can have a substantial effect on airfield capacity. Runway length, width, pavement strength, and orientation determine whether a runway is usable by a particular aircraft type for landing or takeoff. The runway must be long enough for an aircraft to accelerate to takeoff speed or slow down sufficiently to exit the runway. The runway must be wide enough to accommodate the width of the landing gear and provide Source: Landrum & Brown. Figure 2-1. Major airport components: airside, terminal, and landside facilities.
14 evaluating airfield Capacity wingtip clearance to adjacent buildings and aircraft. Runways are generally oriented in the direc- tion of the prevailing winds; aircraft operate best with a headwind, and most aircraft have limited ability to fly with strong cross winds or tailwinds. Pavement must be strong enough to support the design aircraft (i.e., the largest aircraft regularly operating at the airport) without structural damage to the aircraft or pavement. Taxiways A taxiway is also a hard or paved strip of level ground along which aircraft taxi from the run- way to a parking position (and vice versa) or from one part of the airport to another. Taxiways can be used to temporarily hold aircraft waiting to take off or waiting for a gate, but it is best to hold aircraft on an apron. There are three major types of taxiways: 1. Parallel (full or partial) taxiways, which generally provide a route for aircraft to reach the runway end or to use after exiting the runway (see Figure 2-3) 2. Entrance/exit taxiways, which connect runways to parallel taxiways or some other type of taxiway, and which provide a path for aircraft to enter the runway for departure or exit the runway after landing 3. Access (or circulation) taxiways, which provide paths for aircraft to move between the vari- ous airside components of the airport (and which include bypass and crossover or transverse taxiways, including those that cross active runways, and apron-edge taxiways) The number, location, and layout of taxiways can significantly affect airfield capacity. Taxi- ways provide space for the temporary staging and sequencing of aircraft prior to takeoff Source: Landrum & Brown. Figure 2-2. Runway numbering and marking. Source: Landrum & Brown. Figure 2-3. Taxiway types and locations.
airfield Capacity Concepts 15 and after landing. Such taxiway use frees the runways to be used efficiently for takeoffs and landings. The combination of runways and taxiways is generally referred to as the movement area. Holding Bays Located near the ends of runways or near the terminal building, holding bays (or hold pads) are intended to provide an area off the taxiway system for aircraft that must wait until ready to take off or until a gate is ready. Typical holding bay locations and geometries are shown on Figure 2-4. Hold- ing bays can affect capacity if sufficient space does not exist for aircraft to wait without occupying a needed gate or blocking a taxiway. Hold pads are provided primarily at busy commercial airports to stage and store aircraft awaiting departure so air traffic controllers can properly sequence them. Deicing pads are a spe- cial category of hold pads that are especially needed and important to airport operations during snow and ice conditions. Holding bays are useful, and in many situations necessary, near depar- ture runway ends to allow aircraft to be bypassed for takeoff. Without this bypass capability, an aircraft holding for ATC clearance or because of inclement weather at its destination could unduly delay other departures attempting to use the same runway. Holding bays also are necessary when, for a variety of reasons, a gate may not be immediately available for arriving aircraft. This type of holding bay should be located near the terminal com- plex to allow easy access to the gate area when a gate becomes available. Aprons Aprons typically are defined areas of land intended to accommodate parked aircraft for pur- poses of fueling, maintenance, or loading or unloading passengers, mail, or cargo (Figure 2-5 and Figure 2-6). Aprons typically surround buildings, such as terminals and hangars, but also can be designed specifically to store aircraft out in the open using tiedowns. Source: Landrum & Brown. Figure 2-4. Holding bays or hold pads at or near runway ends.
16 evaluating airfield Capacity Large aprons can include taxilanes, which are areas identified to provide access between taxi- ways and aircraft parking positions. The combination of the apron and taxilanes is generally referred to as the non-movement area. At certain busy air carrier airports, spots are established as points on the apron where aircraft leaving a non-movement area are expected to contact ground control for taxi clearance. Sometimes also referred to as the ramp, aprons at small general aviation airports are widely accessible, and both aircraft and automobiles may park on its surface. Gates An airportâs gates are the access points between the aircraft and the terminal at which pas- sengers typically embark or disembark the aircraft (Figure 2 -7). An airport can have one or more gates, and these gates may be at ground level or on an upper level, for which a loading bridge is provided to connect the aircraft to the door of the terminal building. At general aviation airports, Figure 2-5. Commercial aircraft parking apron. Source: Landrum & Brown. Source: Landrum & Brown. Figure 2-6. General aviation parking apron (ramp).
airfield Capacity Concepts 17 terminal entrance points are not typically called gates; this term is typically used at commercial service airports. Gates are designed to accommodate different types of aircraft and can affect capacity if the number of gates provided is inadequate to accommodate arriving or departing aircraft, or if the layout of gates impedes taxiing aircraft from reaching the runway ends in a safe and efficient manner. Airport and Airspace Operating Environment In addition to the ground-level physical layout and facilities, an airportâs operating environ- ment also includes the surrounding airspace. The structure and design of the airspace determine the number of routes that can be followed by aircraft flying into and out of the airport. Airspace constraints, such as high terrain, tall structures, special-use airspace, and aircraft operations at another nearby airport, may limit the number of such routes, thereby adversely affecting airfield capacity. Therefore, airspace is a very important consideration in evaluating airfield capacity. Airspace is defined as the portion of the atmosphere above a certain land area. This land area can be defined in terms of the political subdivision that it overlays (e.g., the country or state), or it can be defined based on proximity to the airport. In the United States, FAA maintains and reg- ulates civilian airspace to provide for a safe and efficient movement of air traffic. Aircraft flying in U.S. airspace are subject to a system of controls designed to serve one primary purposeâthe safe separation of aircraft from one another and from other hazards. Such aircraft are subject to varying degrees of control depending on the specific airspace and meteorological conditions in which they operate. FAA is also responsible for the air traffic control system in the United States. Two basic types of flight rules (or flights) are recognized in the U.S. air traffic control system: 1. Visual flight rules (VFR). Aircraft operating under VFR (VFR flights) depend primarily on the see-and-be-seen principle for separation. VFR flights are conducted primarily by smaller aircraft. FAA does not require pilots of VFR flights to file flight plans andâexcept for those services provided by FAA Flight Service Stations and local air traffic control towers (ATCTs)â such flights are not provided service (such as separation assurance and flight following) by the ATC system. 2. Instrument flight rules (IFR). Aircraft operating under IFR (IFR flights) are provided mini- mum radar separations by air traffic controllers. Pilots of IFR flights must file IFR flight plans to receive radar separation assurance and operate in certain controlled airspace. Pilots of large commercial flights nearly always file IFR flight plans and use instruments to navigate from point to point so they can fly in certain adverse weather conditions. Source: Landrum & Brown. Figure 2-7. Terminal gates.
18 evaluating airfield Capacity The National Airspace System (NAS) includes more than 19,000 airports, about 5,200 of which are open to the public; 400 ATCTs; 197 Terminal Radar Approach Control (TRACON) facilities; and 22 Air Route Traffic Control Centers (ARTCCs). Traffic management of the NAS is directed by the Air Traffic Control System Command Center (ATCSCC) located in Herndon, Virginia. Control of air traffic within specific sub-areas of the NAS is delegated to one of the 22 ARTCCs that have specific jurisdiction. Each ARTCC is responsible for providing ATC services to a large segment of the NAS, which routinely involves dozens of airports and often encompasses all or part of a multistate area (Figure 2-8). The ARTCC further delegates responsibility for air traffic management for smaller geographic areas within its boundary to TRACON facilities and ATCTs. The area of control exercised by the TRACON and ATCT is limited to a maximum defined altitude as well as a specified geographic area. FAA Flight Service Stations provide pilot briefings and VFR search and rescue services, assist lost aircraft and aircraft in emergency situations, originate Notices to Airmen, broadcast aviation weather and NAS information, and receive and process IFR flight plans, among other services. As an aircraft travels through a given airspace sector, it is monitored by one or more air traf- fic controllers responsible for that sector. As the aircraft leaves that airspace sector and enters Source: Federal Aviation Administration. Note: Not pictured: ZAN â Anchorage (Alaska), and ZHN â Honolulu (Hawaii). Figure 2-8. FAA air route traffic control centers (ARTCCs).
airfield Capacity Concepts 19 another, the air traffic controller passes the aircraft off to controllers responsible for the other airspace sector. Much of the current ATC system relies on ground-based navigational aids and radar. Radarâthe acronym stands for radio detection and rangingâdepends on line of sight for detecting targets. During instrument weather conditions, referred to as instrument meteorological conditions (IMC), approaching aircraft must be provided full radar separations by air traffic controllers until the pilots confirm that they can see their runway and the aircraft they are following, and certain movements may not be possible because of increased dependencies between runways (e.g., the operations on one runway may be dependent on operations on an adjacent runway in IMC when visual separation cannot be applied). IMC can result in significant reductions of airfield capacity. During visual or clear weather conditions, referred to as visual meteorological conditions (VMC), approaching aircraft may be issued a visual approach by controllers, under which the pilot is responsible for aircraft separation (including wake turbulence separation) and can visually follow the aircraft in front to the runway. Moreover, in VMC, there is less dependence between runways and air traffic controllers in the ATCT can apply visual separations between aircraft, which can result in a significant increase in airfield capacity. Marginal VMC (MVMC) conditions are defined as ceiling and visibility conditions below visual approach minimums, but better than instrument conditions. Under MVMC, conducting visual approaches is either not possible or requires additional controller workload; however, the full radar separations do not need to be enforced once the pilot has the airport or the preceding aircraft in sight. FAA establishes airspace classes to enhance the safety of aircraft operations by protecting arriving and departing IFR aircraft using ATC services from uncontrolled VFR aircraft. Depending on the class of airspace, VFR operations are subject to certain operational restric- tions and, when operating in certain restricted classes of airspace, must remain under controller/ radar communications and surveillance at all times. These airspace-class restrictions most often affect general aviation users because general aviation aircraft flights account for the majority of VFR traffic. Training activity at an airport is of particular note for airspace and general airport operations as it relates to determining airfield capacity. Training operations, also called touch-and-goes, are defined by how the aircraft perform. In a touch-and-go operation, aircraft make a landing fol- lowed by an immediate takeoff without coming to a full stop or exiting the runway. Each aircraft that conducts a touch-and-go accounts for two operations (a landing and a takeoff) even though the operations are conducted in rapid succession. Airports that have a high level of touch-and-go activity can accommodate a high number of operations. If the pilot brings the aircraft to a full stop before taking off again, it is called as a stop-and-go operation. Stop-and-go operations can have a significantly longer runway occupancy time and therefore can adversely affect the number of operations that an airport can accommodate. Training operations also affect an airportâs traffic pattern, which is the standard path followed by aircraft when taking off or landing while maintaining visual contact with the airfield (Figure 2-9). A traffic pattern is established to be used by aircraft that remain close to the airport, including training aircraft, and is more commonly used by general aviation aircraft and at smaller airports. Traffic patterns also are defined for all sizes of airports for purposes of aborted or missed approaches, but these patterns may not follow the same path as that used for training or local operations at an airport. Traffic patterns are defined as left-hand or right-hand according to the
20 evaluating airfield Capacity direction in which they are flown. Left-hand patterns are standard, because most small aircraft are piloted from the left seat; but right-hand patterns are used frequently for other reasons (e.g., to accommodate parallel runways, for noise abatement, and to manage terrain issues). Traffic patterns are usually rectangular in shape, with the runway serving as one of the long sides of the rectangle. When training operations begin to affect the ability of the airport to efficiently accommodate aircraft traffic due to too many aircraft in the traffic pattern, air traffic controllers may ask pilots to conduct full-stop landings instead of touch-and-goes or stop-and-goes. With full-stop land- ings, the training aircraft land and exit the runway to stop and wait on an approved taxiway or apron until cleared to re-enter the runway for another takeoff. Switching to full-stop landings thus reduces the number of aircraft active in the traffic pattern. Next Generation Air Transportation System (NextGen) NextGen is the umbrella term used in the industry to describe the ongoing, wide-ranging transformation of the NAS. The transformation is focused on changing the legacy, radar-based ATC system and the legacy, ground-based navigation system to satellite-based systems. As pro- posed, the satellite-based technologies are expected to significantly improve the safety, capacity, and efficiency of runways in the NAS while providing environmentally friendly air traffic proce- dures and technologies that reduce fuel burn, carbon emissions, and noise. NextGen is a collaborative effort among FAA and partners from the airlines, the Aerospace Industries Association, federal agencies, airports, and state and local governments. NextGen is part of a worldwide effort to modernize ATC systems, and FAA is collaborating with other air traffic service providers to ensure that future communications, navigation, and surveillance technologies and procedures are harmonized and interoperable internationally. One of the most important technologies behind NextGen is the global positioning system (GPS), an application that has been used in aircraft approach procedures to airports of all sizes. Through the use of new navigation procedures, such as Area Navigation (RNAV) and Required Navigational Performance (RNP)âillustrated in Figure 2-10âaircraft will be capable of flying more direct and narrowly defined routes, even during inclement weather conditions, allowing the possibility for the airport to be operated with reduced separation standards, thereby increas- ing airfield capacity. Source: Federal Avia�on Administra�on. Figure 2-9. Traffic pattern operating segments and procedures.
airfield Capacity Concepts 21 While the primary emphasis of NextGen is on safety and efficiency, the potential improve- ments in airfield capacity are significant. Airfield capacity improvements are anticipated as more precise surveillance, navigation, and controller automation tools reduce effective separation between aircraft. Separation buffers built into todayâs operations will be reduced so that aircraft can achieve average separations closer to published minimum standards. For airports with parallel runway systems, the required separation between the runways is expected to be reduced, which would allow greater flexibility for designing additional runways and adding capacity to existing parallel runways that meet the reduced standards. Collaborative deci- sion making in airport surface management improves departure sequencing and taxiing efficiency. Other innovations are ongoing in areas such as weather forecasting, data networking, and dig- ital communications. In addition to technological changes, new airport infrastructure, new and renovated aircraft fleets (including advanced engines and airframes), and new aircraft approach and departure procedures will be part of NextGen implementation. NextGen has been discussed for many years, and some of the technologies, standards, and procedures have already been implemented; the gradual evolution of the NAS is under way and will continue for many years. Aircraft Fleet Mix and Performance Measures Aircraft fleet mix refers to the size, engine power (i.e., piston, turboprop, or jet), wake tur- bulence category, and performance (e.g., approach speeds and runway occupancy times) of all aircraft types operating at an airport. The fleet mix is a core parameter that affects every capacity analysis with respect to the following considerations: 1. Aircraft separation criteria. Separation requirements between arrivals and departures are enforced through ATC rules and procedures, which are typically based on an aircraftâs maxi- mum gross takeoff weight capability. Aircraft wings generate lift, a byproduct of which is wake turbulence. Separation requirements vary depending on the difference in size between the leading aircraft and the trailing aircraft, with larger separations required behind heavier aircraft to protect for wake turbulence. Figure 2-11 illustrates the wake vortices that come off Source: Federal Aviation Administration. JFK Visual/VOR vs. RNP Approaches JFK Descent Profile View Figure 2-10. Potential RNP approach versus existing VOR approach: plan and profile.
22 evaluating airfield Capacity the tips of the aircraft wing and trail behind the aircraft, creating a danger for smaller aircraft that might be caught in the wake. Behind large aircraft, these wake vortices can be very strong, and controllers must build in extra separation to protect against a dangerous wake encounter. Two other determinants of aircraft separation criteria are airport surveillance radar and aircraft navigation precision. How precisely an aircraftâs position is known to ATC, and how precisely an aircraft is able to follow a path through airspace both drive the need for separa- tion requirements, particularly in instrument weather conditions. 2. Runway use restrictions. The use of a runway may be restricted depending on the operating requirements of an aircraft type or runway use preferences at an airport. For example, certain runways may be designated for smaller aircraft or non-jet aircraft for noise abatement pur- poses. In addition, runway length requirements may preclude certain runways from being used by larger aircraft or may dictate a preferred mode of operation for certain classes of aircraft. 3. Final approach speeds. An aircraftâs size, weight, and engine power determine its typical speed on final approach. Final approach speed affects airfield capacity because higher approach speeds allow higher throughput rates; however, airfield capacity can be adversely affected when there are significant differences in final approach speeds of aircraft in the fleet for which controllers must provide protection from loss of separation. The typical aircraft classification system depicted in Table 2-1 is included in this guidebook as it relates to evaluating airfield capacity. The specific aircraft mix categories appropriate for an airfield capacity analysis may differ from airport to airport. Refer to Appendix A and Appendix B of FAA Order JO 7110.65, Air Traffic Control, for current aircraft classifications. All B-757 aircraft models were reclassified in 2010 as large aircraft, but the B-757 still requires special wake turbulence separation criteria that place it in its own aircraft class. Other Factors That Affect Airfield Capacity The airport, airspace, and aircraft considerations discussed in this chapter all affect airfield capacity. In each of these categories, specific characteristics have a greater or lesser effect and, therefore, require proportionate consideration in the calculation of airfield capacity. The follow- ing four important factors can have a significant effect on airfield capacity: Source: Federal Aviation Administration. Figure 2-11. Extra separation is required to protect for wake turbulence.
airfield Capacity Concepts 23 1. Airfield geometry 2. Aircraft mix, activity type, and scheduling 3. Weather, runway use, and ATC procedures 4. Airspace Airfield Geometry Factors Runway Exit Design Runway exit taxiways are most important during periods of mixed mode operations on a single runway, allowing arriving aircraft to exit the runway as quickly as possible and increas- ing the likelihood that a departure will have time to take off before the next arrival occupies the runway, thereby increasing runway capacity. However, even on a runway used for arrivals only or departures only, runway occupancy times can limit airfield capacity. Runway exits that have a shallow angle to the runway centerline allow an aircraft to exit the runway at a higher speed than those with larger angles to the runway centerline. The standard geometry for a high-speed exit is shown in Figure 2-12. In cases where the centerline separa- tion between a runway and parallel taxiway is insufficient to allow full high-speed exits, acute- angle exits can be used. Acute-angle exits are not true high-speed exits but allow aircraft to Aircra� Class Descrip�on Maximum Gross Takeoï¬ Weight Sample Aircra� Small-S Single engine Less than 12,500 pounds Cessna 172, Piper Warrior Small-T Twin engine Less than 12,500 pounds Beach 35, Piper Seneca, Turbo Commander Small + Mixed engines Between 12,500 pounds and 41,000 pounds Lear 35, Hawker 400, Cita�on 10 Large Mul�ple engines Between 41,000 pounds and 300,000 pounds B-737, A319, Global Express, CRJ-200 B-757 Boeing 757 300,000 pounds B-757 Heavy Mul�ple engines More than 300,000 pounds B-747, B-767, B-777, A330 Super Heavy A380 1,200,000 pounds A380 Source: FAA Order 7110.65, Air Traï¬c Control. Table 2-1. Aircraft classifications. Source: Federal Avia�on Administra�on. Figure 2-12. FAA standard high-speed runway exit geometry.
24 evaluating airfield Capacity exit at nearly the same speeds. Usually an aircraft will need to slow down on a runway to use a 90-degree or reverse-angle exit, thereby taking more time on the runway and reducing airfield capacity. A landing aircraft exiting the runway at a higher speed reduces runway occupancy time, allow- ing the runway to be used by other aircraft more efficiently. Runway Entrance Design/Departure Staging and Sequencing Taxiways Having more than one entrance taxiway to a runway or holding bay may increase runway capacity. A secondary entrance taxiway may enable an intersection departure, allowing a departing aircraft to use only the portion of a runway that does not intersect with another runway rather than the full length of the runway. Additionally, multiple runway entry points or a holding bay at the departure end of a runway could allow flexibility for air traffic con- trollers to optimize the sequence of departing aircraft or remove aircraft from the queue before they reach the runway end if they have a mechanical problem or are awaiting takeoff clearance. Parallel Taxiway A parallel taxiway allows departing aircraft to reach a runway entrance without taxiing on the runway and can keep arriving aircraft from having to back-taxi on the runway to access the ramp if they roll past the last runway exit. The length of the parallel taxiway (full or partial) can affect airfield capacity. The efficiency of a partial parallel taxiway depends on its length and location in conjunction with the direction of departing or landing aircraft. The lack of a full-length parallel taxiway may greatly increase runway occupancy time for arrivals and departures, thereby reduc- ing airfield capacity. Runway Crossings Aircraft taxiing across an active runway impede the runwayâs primary purpose of providing a space for landing and departing aircraft. Crossing aircraft also divert ATC resources from the task of controlling landing and departing aircraft. Large gaps may sometimes occur in the opera- tions on a particular runway (e.g., due to a large wake turbulence separation behind a heavy jet aircraft). Such large gaps may permit one or two aircraft to cross the runway; however, gaps that are not large enough to permit such crossings will reduce the time available for the runway to be used for landings and takeoffs. Providing multiple runway crossing points may mitigate the adverse effect of runway crossings. Number of Runways and Relative Location Airports provide varying numbers of runways to accommodate aircraft operations. These runways can have a variety of configurations depending on conditions at the airport, when the runways were constructed, and other factors. A single-runway airport can accommodate a high level of aircraft operations depending on the availability of other infrastructure, such as instru- mentation and taxiways, as well as operating conditions. Two or more runways can provide increased capacity compared with a single runway, but it depends on the runway configuration and layout (i.e., runway lengths, separation between runway centerlines, and whether the run- ways are parallel, intersecting, converging, or otherwise dependent). Aircraft Mix, Activity Type, and Scheduling Factors Aircraft Fleet Mix and Approach Speeds As previously discussed, aircraft fleet mix includes the size, engine power, performance, and wake turbulence of the aircraft types serving the airport. A significant mixture of different air-
airfield Capacity Concepts 25 craft types operating on the same runway can significantly affect the capacity of an airfield: the greater the differences in aircraft performance and wake turbulence categories, the greater the adverse effect on capacity. The size, engine power, and weight of an aircraft also affect its approach speed for landing. Approach speed varies and is expressed by FAA using the letters A through E. Table 2-2 lists aircraft approach speed categories. Activity Type Commercial passenger service can affect airfield capacity depending on the types of aircraft that provide the service and the number of operations conducted by the commercial passenger airline(s). If the passenger service is provided with larger aircraft, the capacity calculations are affected to a greater degree than if the size of the aircraft is more homogeneous. The presence of touch-and-go training activity is also an important consideration. Touch-and-go operations typically increase the runway operational throughput that can occur in an hour because aircraft land, followed by an immediate takeoff without coming to a full stop or exiting the runway, and therefore have shorter runway occupancy times. Airports that have a high proportion of touch- and-go activity can accommodate a high number of landings and takeoffs for a given capac- ity and set of assumptions regarding aircraft separations, approach speeds, runway occupancy times, and fleet mix. By contrast, a stop-and-go operation, where a pilot comes to a full stop on the runway and then takes off, can significantly increase runway occupancy time and reduce actual throughput. Daily Distribution of Aircraft Activity/Detailed Flight Schedules Aircraft activity scheduling patterns also can influence airfield capacity, depending on how capacity is defined. When capacity is defined as a function of average aircraft delay, it is affected by the demand peaking patterns at the airport. When capacity is defined in terms of annual service volume, then it is also a function of the demand peaking distribution and the seasonality of traffic at the airport. At small commercial and general aviation airports, traffic tends to peak in the morning and at night, sometimes influenced by weather or air traffic. The presence of pilot training, typically at smaller commercial or general aviation airports, can also result in scheduling that affects capac- ity. At large airports, airlines may group arriving or departing aircraft to promote passenger connections. This practice is often called hubbing. Hubbing creates unique stresses on airfield capacity because a comparatively large number of aircraft are attempting the same activity nearly simultaneously (see Figure 2-13). Aircra� Category Approach Speed Example A < 91 knots Cessna 172 B 91 to < 121 knots King Air 200 C 121 to < 141 knots B-737 D 141 to < 166 knots B-767 E 166 knots or more SR-71 Source: FAA Advisory Circular AC 150/5300-13, Airport Design. Table 2-2. Aircraft approach speed categories.
26 evaluating airfield Capacity Long distance flights also affect airfield capacity as airlines schedule flights when pas- sengers demand them. Most passengers generally do not want flights that arrive or depart in the middle of the night. International flights in particular need to meet airline hubbing windows for airline connection efficiency, making the windows for their arrival or departure particularly narrow. For commercial service airports with high levels of aircraft activity, detailed flight schedules can be used in the capacity calculation. These schedules, which can be purchased through com- mercial vendors, provide aircraft type, airline, time of day, and other relevant information criti- cal to capacity evaluations. Weather, Runway Use, and ATC Procedural Factors Weather Weather conditions affect runway use, runway orientation, and aircraft separation require- ments. Wind speed and direction determine aircraft speed over the ground during flight (i.e., the aircraftâs ground speed). Wind speed variability (gusts) may cause a pilot to increase speed. Higher approach or departure speeds generally increase runway length requirements. In addi- tion, strong winds can limit the runway orientations that can be used at any given time, which may limit an airport with multiple runway orientations to a single orientation and a reduced airfield capacity. Cloud ceiling and visibility at the airport, which define whether aircraft are operating in VMC, MVMC, or IMC, also affect airfield capacity. These weather categories are typically defined by FAA as follows: ⢠VMC: Ceiling and visibility allow for visual approaches. Visual approach minima, expressed as a combination of ceiling and visibility, are specific to each airport. ⢠MVMC: Ceiling and visibility are below visual approach minima but better than instrument conditions. ⢠IMC: Ceiling less than 1,000 feet or visibility less than 3 statute miles. Under these conditions, instrument flight rules apply and radar separation between aircraft is required. Source: Oï¬cial Airline Guide for Lambert St. Louis Interna�onal Airport. Figure 2-13. Demand pattern showing peak arrival and departure banks at major connecting hub.
airfield Capacity Concepts 27 These distinctions are important because, assuming all other factors are equal, fewer aircraft operations can occur when visual approaches are not conducted and aircraft require additional separation from one another. For example, in IMC, controllers can no longer apply visual sepa- ration from the ATCT or direct pilots to conduct visual approaches to the airportâs runways; instead, full radar separations must be applied. Moreover, increased dependencies between run- ways in IMC may limit the number of simultaneous movements that can occur at the airport. In IMC, there are stricter requirements on pilot and aircraft certifications and performance capabilities than in VMC, which may affect the number of pilots that can fly in poor weather conditions. Applicability and Acceptance of VFR and Visual Approaches/ Separation Standards (Arrivals and Departures) Airfield capacity is usually higher in VMC, when pilots accept responsibility for self-separa- tion from other aircraft traffic through see-and-avoid techniques. In IMC, air traffic controllers separate aircraft by applying standard minimum radar separation distances. Because of the vari- ability of weather conditions, controllers will add an additional separation buffer to assure that minimum separation distance standards are not violated. In addition, pilot acceptance of visual approaches depends on the pilotâs level of proficiency with the English language and the use of voice communications to understand the location of nearby aircraft traffic. If visual approaches are being accepted by the majority of pilots operat- ing at an airport, then airfield capacity can be significantly increased. During periods of IMC, controllers are able to handle fewer aircraft operations at an airport because they must provide full radar separations and apply minimum separation and divergence requirements between arriving and departing aircraft. Runway Occupancy Time Runway occupancy time refers to the time interval that an aircraft occupies a runway. This time interval is usually expressed in seconds. For arrivals, runway occupancy time refers to the time an arriving aircraft takes between crossing the runway threshold until it is clear of the runway, meaning it is outside the Runway Safety Area (RSA). For departures, runway occupancy time refers to the time a departing aircraft takes from the moment it occupies an active runway, meaning the time it enters the RSA, until it clears the departure end. Multiple-Approach and Departure Capability Multiple-approach airspace design extends final approach and departure corridors, which tends to magnify the negative effects of aircraft speed variations on individual runway accep- tance rates. (A similar effect occurs on multiple parallel or diverging departure runways.) Alternatively, approach procedures for closely spaced parallel runways (i.e., Simultaneous Offset Instrument Approach, or SOIA) and converging/intersecting runways (i.e., Conver- gence Runway Display Aid, or CRDA) can increase airfield capacity, especially in poor weather conditions. Table 2-3 summarizes the minimum spacing requirements between parallel runways and associated operational capabilities and requirements. For more information regarding these requirements, please refer to FAA Order JO 7110.65, Air Traffic Control. Fleet-Mix-Specific Runway Assignment A runway can be limited by the types of aircraft that are able to use it. Factors such as airplane design group, runway length, or noise restrictions may cause the aircraft fleet mix that can use a
28 evaluating airfield Capacity particular runway to differ from the overall fleet mix serving the airport. Limitations on aircraft types able to use a particular runway can have negative effects on airfield capacity. For example, if only a small percentage of the aircraft fleet mix can use a particular runway, then that runwayâs contribution to the overall airfield capacity is also small. Radar Availability The availability of en route or airport surveillance radar can significantly affect airfield capac- ity, especially for IFR flights in IMC. In the absence of such radar coverage, the air traffic control- lers cannot use radar separations for arriving or departing aircraft. At such airports, procedural separation (e.g., the one-in, one-out rule) is used instead of radar separation, or time-based separation requirements (e.g., 10 minutes between successive arrivals) can be used, which can be many times larger than the minimum radar separation requirements. ATCT Availability The effect on airfield capacity of having an operating ATCT in VMC depends on the nature of the traffic and the makeup of the pilot population at an airport. At some uncontrolled air- ports that are well equipped with a Common Traffic Advisory Frequency (CTAF)âwhich might take the form of a UNICOM or MULTICOM radio frequency over which pilots can transmit and receive advisoriesâthe absence of an ATCT could increase capacity, because there is noth- ing to enforce aircraft separation standards. At other airports that are not equipped for pilots to announce their intentions or to receive airport advisories, an ATCT could increase capacity by providing for a more orderly flow of traffic. Under IMC, the capacity of an uncontrolled airport to accommodate instrument approaches and departures is severely limited. Capacity can be even further reduced if there is a lack of radar coverage. As traffic levels rise and operational complexity increases, an ATCT could become warranted under FAA guidelines in order to foster the safe and efficient flow of traffic. Spacing Between Parallel Runways Enabled Procedures and Requirements Very close (700 feet to 2,500 feet) Independent visual approaches in VMC with wake turbulence avoidance procedures Single stream in IMC Close (2,500 feet to 3,000/3,400/ 4,300 feet) Dependent (staggered--1.5 nau�cal miles) instrument landing system (ILS) approaches Independent departures Independent arrivals and departures Far (greater than 3,000/3,400/ 4,300 feet) Dual simultaneous independent ILS approaches 3,000 feet - 4,300 feet requires Precision Runway Monitor (PRM) 3,000 feet - 3,400 feet requires that one localizer be oï¬set 2.5 degrees Far (greater than 5,000 feet) Triple simultaneous ILS approaches Widely spaced (9,000 feet+) Simultaneous ILS approaches without ï¬nal monitors/No Transgression Zone Source: FAA Order JO 7110.65, Air Traï¬c Control, 2012, and FAA No�ce JO 7210.33, Simultaneous Widely Spaced Parallel Opera�ons, 2012. Table 2-3. Required parallel runway spacing for multiple approaches and departures.
airfield Capacity Concepts 29 Airspace Factors Length of Common Final Approach There are varying types of approaches to an airportâs runways. The length of the common approach is the distance from the runway threshold out to the entry gate of the outer bound- ary, where aircraft operating at different approach speeds may open or close the gap between them during final approach. More specifically, the common final approach path is that link over which controllers can no longer apply speed control or vectoring to adjust the separation between aircraft. Over that final distance to the runway, aircraft are flying at their certificated final approach speeds, which cannot be significantly changed. Therefore, differences in those final approach speeds must be accounted for in separating aircraft. This is particularly important when an overtake situation existsâwhere a faster aircraft is following a slower aircraft on final approachâand controllers must build in extra separation to counter the loss of separation. Such speed differences also have to be accounted for in situations where departures have to follow the same path after takeoff for a significant distance. Buffers Buffers are used to space aircraft in excess of the minimum required separation during various phases of flight, including arrivals and departures, to ensure that separation is not lost, which could result in an operational error. A spacing buffer, typically presented in seconds, is used to manually space arriving aircraft to balance the arrival and departure mix and allow for more departures between arrivals. A departure hold buffer is applied to departure runway occupancy times or the suggested minimum time between departures and is added as an additional time requirement before a departure can be initiated. An arrival hold is similar, but is typically pre- sented in nautical miles. Additionally, minimum separation requirements also have buffers to reflect controllersâ tendencies to add spacing beyond that required between operations to avoid operational errors. These separation buffers have been measured many times at various loca- tions, and are well understood. They are primarily a function of the capability of the controller to sequence and space aircraft on final approach, which is a function of the technologies that the controller has available for such sequencing. Departure Fix Restrictions A departing aircraft flies through a series of waypoints, or fixes, to an arrival runway. These waypoints are generally where flight paths from multiple airports merge, or where an airport departure route merges with an overhead flight route or corridor. If there are too many flights en route to a fix, air traffic controllers may restrict the flow of traffic from an airport by assign- ing a minimum distance or time interval between successive departures. These restrictions are referred to as miles-in-trail or minutes-in-trail restrictions, respectively. The adverse effects of these in-trail departure fix restrictions can be mitigated by providing adequate bypass taxiways and holding bays on the airfield so that controllers can properly stage and sequence departures to minimize the incidents where successive departures are flying to the same fix. Availability of Multiple Divergent Departure Headings On departure, aircraft are assigned initial heading. To allow successive or simultaneous depar- tures from parallel runways, the initial separation between headings must meet certain diver- gence requirements (e.g., a divergence of 15°) described in FAA Order JO 7110.65. The provision of divergent headings is a key factor to determining an airportâs departure capacity. Neighboring Airports A runway needs adjacent airspace to accommodate the approach paths that feed it and the departure paths that flow from it. In some cases, airports are so close together that these
30 evaluating airfield Capacity approach and departure paths cannot be operated simultaneously and must be shared (see Figure 2-14). When this occurs, air traffic controllers must coordinate the runway use configu- rations and air traffic flight paths between the two airports. Such coordination of air traffic at two or more nearby airports usually results in a substantial loss of efficiency and a reduction in airfield capacity. Source: LeighFisher. Figure 2-14. Airspace interactions between JFK and LGA.
