2
Tasks in Air Traffic Control

In this chapter, we describe the tasks of people who work in three types of air traffic control facilities: the air traffic control tower, the terminal radar approach control (TRACON), and the air route traffic control center or, as it is often called, the en route center. We also consider the tasks of people at two other locations: the Air Traffic Control System Command Center in Herndon, Virginia, and the flight service stations around the country.

In describing the duties of the controller in the tower, the TRACON, and the en route facilities, we point out their common as well as their distinguishing features. Although the descriptions are fairly generic, it should be emphasized that, within a type of facility, there are vast differences from region to region, dictated by the level of activity (compare the New York TRACON with that at Champaign, Illinois, for example) and by other unique features of the work culture.

AIR TRAFFIC CONTROL ORGANIZATION

The Federal Aviation Administration's current headquarters organizational structure is divided into six ''functional lines of business," each the responsibility of an associate administrator reporting directly to the agency's administrator (FAA Headquarters Intercom, December 13, 1994):

  1. The air traffic control organization, called Air Traffic Services, is responsible for operation of the 20 en route centers, almost 200 TRACON facilities,



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Flight to the Future: Human Factors in Air Traffic Control 2 Tasks in Air Traffic Control In this chapter, we describe the tasks of people who work in three types of air traffic control facilities: the air traffic control tower, the terminal radar approach control (TRACON), and the air route traffic control center or, as it is often called, the en route center. We also consider the tasks of people at two other locations: the Air Traffic Control System Command Center in Herndon, Virginia, and the flight service stations around the country. In describing the duties of the controller in the tower, the TRACON, and the en route facilities, we point out their common as well as their distinguishing features. Although the descriptions are fairly generic, it should be emphasized that, within a type of facility, there are vast differences from region to region, dictated by the level of activity (compare the New York TRACON with that at Champaign, Illinois, for example) and by other unique features of the work culture. AIR TRAFFIC CONTROL ORGANIZATION The Federal Aviation Administration's current headquarters organizational structure is divided into six ''functional lines of business," each the responsibility of an associate administrator reporting directly to the agency's administrator (FAA Headquarters Intercom, December 13, 1994): The air traffic control organization, called Air Traffic Services, is responsible for operation of the 20 en route centers, almost 200 TRACON facilities,

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Flight to the Future: Human Factors in Air Traffic Control hundreds of airport towers, the Air Traffic Control System Command Center, and flight service stations located throughout the United States and Puerto Rico. Air Traffic Services formulates plans and requirements for future air traffic control operations and evaluates and analyzes current operations. Encompassing both air traffic control and airway facilities activities, Air Traffic Services is responsible for requirements; system management; rules and procedures; national airspace system operations, transitions, and implementation; resource management; logistics; flight inspection programs; and airspace capacity planning. The Research and Acquisition organization is responsible for the development of communications, navigation, and surveillance systems; system architecture; aviation research; research and development performed at the FAA Technical Center; and all technology acquisitions. The Regulation and Certification organization is responsible for aircraft certification, flight standards, rule making, aviation medicine, and accident investigation. The Airports organization is responsible for airport planning and airport safety. The Civil Aviation Security organization is responsible for security operations and planning and for civil aviation security intelligence. The Administration organization is responsible for agency human resources, budgeting, and accounting, as well as for the FAA's Aeronautical Center and for administrative functions at the nine FAA regions. Regional administrators at the nine regions are responsible for the administrative functions of the multiple facilities in their regions. Regional administrators report to the Administration organization at FAA headquarters; they do not have line authority over the regional division managers for Air Traffic, Airway Facilities, Airports, or Civil Aviation Security, who report directly to their respective associate administrator or director at headquarters. Thus, the director of Air Traffic Services directly supervises the regional division manager for Air Traffic (FAA order 1100.148B). This chain of responsibility and authority continues through the area level, at which air traffic managers, reporting to their respective regional division managers, are responsible for the day-to-day operations of an assigned group of air traffic control facilities. Air traffic managers are supported when necessary by assistant managers, area managers, and area supervisors, to whom air traffic controllers report (FAA order 1100.126F, April 13, 1990; FAA order 1100.5C, February 6, 1989). Air traffic control services are provided at three types of facilities: Terminals, including tower and TRACON controllers, En route centers, and Flight service stations.

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Flight to the Future: Human Factors in Air Traffic Control Each type of facility provides a particular service to the users of the national airspace system and, in all three, automation is used to assist people in making decisions and in supplying up-to-the-minute information. In 1994, the three types of facilities carried out more than 164 million operations; by the year 2000, they are projected to carry out 207 million operations. THE TOWER The Task Within the terminal area of air traffic control, there are two distinct functions provided by air traffic controllers. The tower controller, located in a glass structure "on top of the tower," controls aircraft on the ground, just after takeoff, and just before landing (Figure 2.1). Tower control tasks are usually divided between the ground controller and the local area controller. The TRACON controller, located in a windowless radar room either below the tower cab or somewhere else in the area, controls aircraft in the wider region of space around the airport. The key responsibilities of tower controllers are to: Issue clearances for the aircraft to push back from the gate and then to leave the ground. These clearances generally involve confirmation of schedules of flight plans that were filed previously through Flight Services and by airline dispatchers. For takeoffs and landings, they involve prior assurance of safe separation from other traffic. FIGURE 2.1 Control tower. Source: Federal Aviation Administration.

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Flight to the Future: Human Factors in Air Traffic Control Manage ground traffic to and from the gate. This involves lining aircraft up in a sequence for takeoff and coordinating the traffic on the runways so that it does not conflict with other ground traffic (aircraft or vehicles) or with aircraft that are taking off or landing. Hand off the departing aircraft to and accept the arriving aircraft from the TRACON controller residing in the radar room. The ground functions of taxi management are handled by the ground controller, and the takeoffs and landings are handled by a local controller. At smaller facilities, or at times of very low traffic density, the two functions may be carried out by a single individual. Visual Resources The most critical task of the tower controller (both ground and local) is to keep track of who is where. Because all aircraft are nominally within sight of the controllers in the tower, the most important resources at their disposal are their eyes, coupled with a voice communication link. The challenge is always to know how to communicate with an identified aircraft on the ground and in the air. Thus towers are constructed to provide as much full visibility of the entire airport surface as possible, although there are occasional deficiencies caused by airport structures (one such deficiency was identified as a causal factor in the runway incursion accident at the Los Angeles airport in 1991; see number 13 on the list in Chapter 1). The task of knowing whom one is looking at is not a trivial one at a busy airport. Many aircraft look alike; vision is often degraded at night or when ground fog obscures parts of the runway; pilots can add to the tower controller's demands if they become confused and take a wrong turn on a taxiway or ramp; and the visual chaos is often enhanced by the diversity of ground vehicles, traveling this way and that, occasionally without communications to the tower. Some assistance for the local and ground controllers is being provided by airport surface detection equipment (ASDE), a system that provides radar identification of ground vehicles and aircraft at the airport. It is being added to many airports that do not have this equipment and upgraded at airports that do have it; the installations and upgrading are behind schedule, however. Many towers are equipped with radar displays called DBRITE (digital brite, radar indicator tower equipment) to augment visual control of airborne traffic. The DBRITE provides the local controller with (1) a radar presentation of about 15 miles around the airport and (2) alphanumeric information on the aircraft that is received from the

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Flight to the Future: Human Factors in Air Traffic Control FIGURE 2.2 Flight strip. Source: Federal Aviation Administration. automated radar terminal system at a nearby TRACON (discussed at length below). The information from the DBRITE provides the tower controller with a view of arrival aircraft that are not yet under his or her control, so the aircraft that are the controller's responsibility on the ground can be safely and efficiently coordinated with planned arrivals. DBRITE enhances the local controller's ability to control higher volumes of airport traffic by providing key information about the aircraft identity, type, altitude, first fix after departure, and speed while the aircraft is on final approach. Flight Strips The controller's task of maintaining location information is greatly facilitated by paper flight strips (Figure 2.2). These physical representations of each aircraft, which are computer generated at the time the flight plan is filed, represent a visible reminder of an aircraft's status in the sequence of taxi-takeoff (for departure) or landing-taxi (for arrival). As they are physically moved around the controller's workstation, they are a reminder of what each represented aircraft is doing on the terminal surface, thereby generally helping to maintain the big picture of who is where. Along with the visual obstruction cited above, a lost flight strip was also identified as one cause contributing to the 1991 Los Angeles runway incursion. Communications Using standardized phraseology, the controller talks with the pilots on radio. A particular pilot knows that he or she is the recipient of a communication by the header ID ("United two twenty-four: hold short at runway two four left"), and other pilots can also hear the message. Such a "party-line" feature creates added auditory input in all cockpits, allowing all pilots to build a better mental model of what surrounding traffic is doing (Pritchett and Hansman, 1993). Tower controllers communicate with pilots, with each other (ground to local), and with the TRACON controller in the radar room to accept arrivals and hand off departures. This latter communication is handled in three ways: first,

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Flight to the Future: Human Factors in Air Traffic Control voice communication is used to coordinate (e.g., accept or decline handoffs). For example, a local controller may refuse to accept a landing aircraft until he or she knows that the runway will be clear for a certain period of time before the approaching aircraft touches down. Second, information on the flight strips is also relayed between tower and radar room, specifically by the FDIO (flight data input/output) computer system. Third, the communication is mediated by the pilot, who, when instructed, changes radio frequency and contacts the next appropriate control facility. Then the handoff is complete. Traffic Management Throughout this process, the tower controller at busy facilities feels continuing pressure from the clients being served: from outbound aircraft, to move each one as soon as possible to the desired "Number 1 for takeoff" position, and from inbound aircraft, to get them off the active runway as soon as possible and, once off, to get them taxied to the gate as soon as possible. When the taxiing aircraft must be cleared to cross an active runway that is accepting a steady stream of departing and arriving aircraft, the scheduling demands can be challenging indeed. THE TOWER The Task The tasks of the terminal radar or TRACON controller, are (1) to manage the safe and expeditious flow of a departing aircraft accepted from the tower to a handoff to the en route controller, a job usually handled by the departure controller, and (2) to manage the arriving aircraft from a handoff from the en route controller to a handoff to the tower controller on a final approach for landing, a job usually handled by the approach controller. A key component of the TRACON controller's job, like that of the tower controller, is sequencing or "lining up" the aircraft in an orderly inbound or outbound flow, at regular spacing. Maintaining the safe separation between aircraft is as important as it is for the tower, but for the TRACON controller it is an even more challenging task because separation is now a three-dimensional problem and aircraft are constantly climbing and descending (in addition to their lateral movement). Thus, the TRACON controller must be ever sensitive to the critical separation criteria for all aircraft operating under instrument flight rules: 1,000 vertical feet and 3, 4, or 5 miles lateral separation, depending on the size of the aircraft. (Large aircraft require more lateral separation because of the wake turbulence they create with their wings.) The pressures for efficiency often dictate separations that are not much greater than this, and the countervailing pressures for safety dictate

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Flight to the Future: Human Factors in Air Traffic Control that these criteria shall never be violated, such that operational errors are recorded. The skills necessary to balance these criteria and maintain the regular, evenly spaced flow are considerable, as we detail in subsequent chapters. Aircraft must be constantly adjusted in their speed, heading, and altitude to meet the dual criteria of safety and efficiency while they follow generally standardized approach and departure routes (a good description of this task is provided by Luffsey, 1990). Empty "slots" in the airspace are sometimes filled by trying to fit in an additional aircraft. The adjustment of flight paths is accomplished through voice communications with the pilot and his or her rapid and accurate compliance. At the same time, the TRACON controller tries to maintains sensitivity to the pilot's needs to avoid excessive and abrupt maneuvering. Each controller will work his or her set of aircraft through a given sector of the airspace. These sectors are sometimes oddly shaped three-dimensional volumes, which include not only the standardized arrival and departure routes, but also features like terrain, structures, special-use (restricted) airspace, and missed approach paths. Sometimes a given controller will work only arrivals, sometimes departures, and sometimes both, an assignment that may vary throughout the day or night, as the TRACON becomes more or less busy. The Information: The ARTS III System The critical information available to the controller is collected by the ARTS (automated radar terminal system) computer system. The most sophisticated version is the ARTS III computer system which supports all high-activity (level 4 and 5) TRACON facilities (Figure 2.3a and 2.3b). The information that is integrated in the computer is provided by primary and secondary radar and by the flight data input/output computer. The primary radar (airport surveillance radar) receives returns from all aircraft in the air. The secondary radar is an active system that receives digital signals from all aircraft equipped with a transponder. The FDIO hosts computer-based flight plans. From these three sources, for each aircraft equipped for instrument flight rules, the ARTS III system has available a data block, which contains information on aircraft call sign, type of aircraft, destination airport, first navigational fix after departure, mode C altitude (if equipped with a mode C transponder), ground speed, and scratchpad information useful to the controller (Figure 2.4). Such information may be accessed at any time via the computer interface. Some of it is contained on the computer-generated flight strips, and some is presented on the radar display. The sophisticated ARTS IIIA system, used at all level 4 and 5 TRACON facilities, contains automated monitoring systems that provide conflict alerts and minimum safe altitude warnings. Level 2 and some level 3 facilities are supported by the less sophisticated ARTS 2 system, which does not have the automated options and is served only by primary radar signals.

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Flight to the Future: Human Factors in Air Traffic Control FIGURE 2.3a ARTS III radar display. Source: Federal Aviation Administration. The Radar Display For the controller, the most critical source of visual information is the radar display, which supports maintenance of the big picture. The primary radar receives a return from anything in the sky and paints this on the scope. The secondary radar receives additional information from aircraft equipped with transponders; this information includes aircraft identity, derived from a code transmitted by the aircraft transponder, and altitude information, if the aircraft is equipped with a mode C transponder. When the ARTS computer has a flight plan

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Flight to the Future: Human Factors in Air Traffic Control FIGURE 2.3b ARTS III radar display detail. Source: Federal Aviation Administration.

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Flight to the Future: Human Factors in Air Traffic Control FIGURE 2.4 Data block. Source: Federal Aviation Administration. associated with the transponder code transmitted by the aircraft, a data tag containing the aircraft's call sign (identity), mode C altitude (if the aircraft is equipped), and ground speed will be displayed on the controller's radar display. The ARTS III computer works to integrate the primary and secondary radar information to provide the most accurate estimate of current aircraft location. The raw blip of the primary radar is always on the screen. In addition, a line at the end of the data tag represents the computer estimation of the position based on both primary and secondary radar. This upgraded system yields more accurate position by time data. Its first priority is to track aircraft using the secondary radar. If for some reason it cannot get a good return on the secondary radar, it will track on the primary radar return. The ARTS tracker is a predict-and-confirm system. Once a target is identified, it establishes a history of direction and speed in the computer memory. Then the computer places the alphanumeric information on a predicted path and waits for the 4-second radar sweep to confirm the position. To the controller, the information always appears to be associated with the actual target. When the system fails to confirm its prediction, it begins a search around the predicted path and alerts the controller with the letters CST (meaning "track lost") displayed in the altitude field of the tag. The controller can communicate with the computer generating the display via a trackball and keyboard system. The trackball can be used to position a cursor on top of a given aircraft symbol, and the keyboard can then be used to enter information into the host ARTS computer pertaining to that aircraft. The particular example of the radar screen, shown in Figure 2.3a and 2.3b, indicates the orderly flow of aircraft across the TRACON area, entering or departing at "gates" in the four corners adjacent to the en route area, and being merged and lined up just short of the two active runways. This is a situation that the skilled controller can handle by an adjustment in the flight path of one or the other. The screen represents a compromise between information and clutter. Naturally the controller would like to have maximum information about each flight, at a location immediately adjacent to the aircraft's accurately depicted position. In the same display the controller also may have information regarding other spatial features, like the location of ground hazards, approach and departure routes, navigational fixes, and even, ideally, severe weather patterns, which are available

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Flight to the Future: Human Factors in Air Traffic Control to most controllers with the new ASR-9 radar. At the same time, this plethora of information, particularly during busy traffic periods, can present a very cluttered display. Data tags may overlap or overlie other display features. Some intelligent features within the ARTS computer will adjust the location of tags to prevent excessive overlap, and the controller does have some options for decluttering the display. Automation also provides conflict alerts for projected and current loss of separation and alerts for loss of separation from the terrain (a minimum safe altitude warning). Data blocks blink and aural alarms are sounded. The controller has some options to declutter the display including removing fixed objects, decreasing sensitivity to weather, and simplifying or removing data blocks. Flight Strips Flight strips are issued by the flight data input/output computer. The existence of the data blocks lessens the controller's degree of dependence on the flight strips, in comparison to the era before the ARTS computer was implemented. But these strips still remain an important augmentation to the controller's memory of what each aircraft is doing or is about to do. Controllers may write on the slips, indicating instructions just issued to aircraft, or they may cock the strips of certain aircraft at odd angles, to remind them of certain unusual circumstances that may need to be addressed in the future, information that will not be known by the ARTS computer (and hence cannot be portrayed in the radar data block). Traffic Management Although controllers strive to preserve an orderly flow of traffic, several forces exist to counteract this goal. The following is a partial list: The sector may be filled by a very heterogeneous mixture of aircraft, including slow-flying but fast-maneuvering general aviation aircraft and fast-flying but slow-maneuvering transport aircraft, particularly the wide-bodied ''heavies" like the 747 and the DC10. Aircraft differ in the extent to which they leave dangerous but invisible wake vortices in trail, which require different separation standards between light and heavy aircraft, both in terms of the vortices they leave and the susceptibility to the vortices they encounter. The sector may be pressured to accept an excessive number of aircraft for approach and departure at heavy traffic periods in the morning and the late afternoon. In a high-workload sector, there may be as many as 10–15 aircraft at one time. Weather can severely disrupt the traffic management plan that a controller is trying to execute. A reported wind shear along one of the arrival strings shown in Figure 2.3a can wreak havoc on the orderly flow. So can a runway that is suddenly closed or a sudden change in wind direction that will force a reversal of the runway direction for arrivals and departures.

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Flight to the Future: Human Factors in Air Traffic Control Controllers may have to deal with aircraft flying under visual flight rules (VFR) that are not communicating with them and that appear to be in a conflicting path with controlled aircraft. Some of these aircraft may not be equipped with a transponder, making it harder for the controllers to see them, since no data tag will be generated. Pilots may not always carry out the instructions issued by the controller or may carry them out differently from what was intended. They may fail to slow or climb through an assigned flight level, thereby not only destroying the controller's careful tactical plan for flow, but also potentially creating a conflict situation with other aircraft, drawing the controller's attention away from other regions of the sector. Communications The problems resulting from pilots' occasional failures to comply reinforce the importance of communications, which, along with the radar display and the flight strips, represent the third critical element of controller interactions with the environment. As with the tower controller, TRACON communications are highly standardized and controllers are trained to deliver these in a clear, coherent fashion, as well as to monitor pilot "readback" of the communications string to ensure that it was correctly heard. However, such readback is not always accurate, and controllers may sometimes fail to detect the inaccuracies (Monan, 1986: Chapter 5). Furthermore, a heavy workload may force the controller to speak more rapidly than is optimal for pilot comprehension, particularly when longer strings are required. Finally, communications back from the pilot to the controller may be considerably less standardized than in the other direction, because there are far more pilots than controllers, and their level of skills and fluency in the English language are far more diverse. As in the tower, communication between controllers in the TRACON is as important as it is between controllers and pilots. Communications and coordination between controllers in adjacent sectors is critical when aircraft cross the sector boundaries. This communications link is just as critical when aircraft make the transition between the TRACON and the adjacent tower or en route airspace. The handoffs from one controller to another are typically accomplished by the automated handoff, in which case a quick sequence of keyboard interaction sends a message to the receiving controller, which gives the latter individual the opportunity to accept, also with a keypress, once the controller feels that the sector is ready to absorb the additional traffic. As in the tower, the handoff process between facilities is also mediated by voice communications with the pilot, as the appropriate frequency to contact the receiving controller is announced.

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Flight to the Future: Human Factors in Air Traffic Control If for some reason an automated handoff cannot be made, then voice communications are used. The Environment The TRACON environment is dark, an appearance dictated by the low contrast of the greenish-yellow blips of the primary radar on the dark screens. Its level of activity (and number of people and amount of chatter) varies radically as a function of the time of day. At busier times, more controllers handle progressively smaller regions, as multiple sectors (which are combined in low-workload periods) are pulled apart as traffic increases. Supervisors within the facility coordinate the staffing in this dynamic fashion and may themselves step in to assist a particularly busy sector as required. It is in part this flexibility of personnel assignment that allows the air traffic control system to respond adaptively to such changing situations as weather and aircraft emergencies. Furthermore, at some facilities, personnel may at times alternate between the TRACON and the tower upstairs. Equipment and Other Failures The TRACON controller's efforts to manage the orderly flow may occasionally be disrupted by equipment failures. Sometimes these disruptions may produce only minor annoyances, as when a data tag for an individual aircraft is temporarily lost. In such cases there is backup information on the flight strips, which can replace the temporarily hidden data tags, and communications can be initiated with the pilot to receive whatever information is necessary. More serious are the more severe breakdowns in the ARTS system, which may result when extremely high traffic density exceeds the computer's capacity. In such cases, the system is designed to fail somewhat gracefully, so that the more powerful automation options (e.g., the predictor algorithms) are lost before the data tags are eliminated. And even in the absence of any information from the ARTS computer, primary radar and flight strips will still allow some representation of aircraft position. Equally serious, if not more so, are the rare losses of power and communications. Although all of these catastrophic failures are rare, they are nevertheless real possibilities that controllers must be prepared to handle. Finally, there are nonequipment failures, such as a crash on a runway or a lost primary radar contact, that have an equally serious need for sudden crisis management, typically by following prepared procedures. The most generic response of the TRACON system to failures of all kinds is to temporarily sacrifice efficiency and preserve safety at all costs—a goal that may well be met by increasing the minimum separation boundaries, if radar resolution or communications ability is degraded, or by diverting aircraft to adjacent sectors or facilities.

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Flight to the Future: Human Factors in Air Traffic Control THE EN ROUTE CENTER At the air route traffic control centers, also called en route centers, the controllers primarily use radar information to provide guidance to aircraft flying across the country. However, there are certain areas for which there is no radar coverage; for these areas, nonradar procedures are used. The implications of this are that the minimum spacing between aircraft must be quite large, sometimes causing a loss of efficiency in traffic flow. Two major nonradar areas are the oceanic areas of the Atlantic and the Pacific oceans, for whose control the New York and the Oakland en route centers, respectively, are responsible. Oceanic controllers rely on aircraft position and intent data provided by pilots. The discussion of en route centers in this chapter focuses on operations for nonoceanic flights. Flights are guided along what is a generally orderly series of linear routes across the sky at different flight levels. The linear paths are defined by navigational aids called VORs, each with a given name. Two crossing radials from a VOR may define an intersection, designated by a pronounceable 5-letter code. Generally eastbound flights travel at odd flight levels, and westbound flights at even levels. In the United States, there are 21 centers that in 1994 handled a total of 39,000,000 operations. Like the TRACON controller, the en route controller must balance concerns of expeditious flow against those of safety. However, the safety separation standards are greater in en route: 5 miles or, depending on the aircraft's altitude, 1,000 or 2,000 feet, rather than 3 miles, 1,000 feet. This increase in separation standards is dictated jointly by the greater difficulty that the more distant radar coverage has in establishing precise location, as well as by the faster speed of travel. The effect of this greater separation on traffic flow is minimal, because the density of traffic is considerably less than in the TRACON area. Each en route center is also divided into a series of irregularly shaped sectors that have both horizontal (lateral) and vertical boundaries. Adjacent controllers may be working aircraft above or below one another. Also, a given sector may overlay a TRACON space beneath. Each sector may be worked by a team of two controllers: a radar position (R-side), whose primary responsibility is to monitor the radar display and ensure separation, and a data position (D-side), whose primary responsibility is to handle data and coordinate. During periods of low traffic, a single controller may handle both responsibilities. When manned by two controllers, however, communication between them is vital. The Information: The En Route HOST Computer Information for the en route controller is gathered from air route surveillance radar and integrated with other information from the FDIO computer by the en route automated system called the HOST. This is a software system, developed

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Flight to the Future: Human Factors in Air Traffic Control in the 1960s and currently "hosted" on hardware introduced in the 1980s. The HOST provides flight data processing and radar data processing. Flight data processing is the system that develops flight plans from information received from automated flight service stations, controller input, and air carriers (dispatchers) that have a direct link to the HOST for filing flight plans. Flight data processing is interfaced with towers, TRACONs, sectors within the en route centers, and with other en route centers so that flight plans can be automatically sent. As for the TRACON controller, the primary tool for the en route controller is the radar display, called the plan view display, or PVD. The PVD is somewhat different from the TRACON display in that it is a fully digitized display of primary and secondary radar targets that are presented to the controller in symbolic format from the IBM HOST computer radar data processor. Hence, what is depicted is an intelligent estimate of the current location of each aircraft, based on computer aggregation of returns from the air route surveillance radars located within the en route center's area. These returns are processed by the HOST computer. The Interface Examining a typical PVD display in Figure 2.5, we see a pattern that bears many similarities to the TRACON display. However, since the information depicted is entirely digital (i.e., no raw radar returns), the operations can be carried out in a more brightly illuminated environment; that is, there is no need to present a light stroke on a dark background. More information is presented and a set of equal-time cross-hatched lines behind the aircraft indicates its part trajectory, providing a good representation of current heading and recent air speed. The levels of automation here are similar to the TRACON, in terms of flight path predictors, minimum safe altitude warnings, and conflict alerts. Here, too, the flight strips remain an important part of the controller's task. However, because of the generally higher levels of automation and the digital information available at all centers regarding flight activity of all aircraft, there are now options to allow many of the manual operations of flight strip updating and manipulation to also be carried out by computer. Communication also is managed in very much the same way between TRACON and en route centers. Traffic Management The primary objective of the en route center is to maintain the expeditious but regular delivery of an aircraft stream to the receiving TRACONs, providing them as rapidly as they can be received (but ideally no faster, since this will produce bottlenecks in the sky). To assist in this process, each center is equipped with a traffic management unit that attempts to coordinate flow across the entire

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Flight to the Future: Human Factors in Air Traffic Control FIGURE 2.5 Plan view display: En route center. Source: Federal Aviation Administration. area and may also coordinate with the air traffic control system command center in Herndon, Virginia (discussed below), to accomplish this. Individual controllers at their workstations monitor the ongoing flights of a multitude of aircraft flying at various speeds and respond to pilot requests and adjust to weather conditions by issuing instructions to alter air speeds, flight levels, and (if necessary) headings, in order to maintain maximum but regular flow. At the same time, the ever-vigilant monitoring for predicted conflicts is ongoing. A well-organized, conflict-free flow through a sector can be suddenly compromised by an aircraft that wishes to climb or descend to seek more favorable tail winds or to avoid turbulence; the winds themselves can have different and unpredictable effects on the speed attainable by different types of aircraft. Some flight path adjustments are therefore second-order ones, issued in response

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Flight to the Future: Human Factors in Air Traffic Control to projected conflicts that may themselves have developed following the granting of a pilot request. TRACON AND EN ROUTE: SIMILARITIES AND DIFFERENCES In addition to the separate areas of airspace for which they are responsible, there are certain other generic differences in the kind of tasks and traffic dealt with by the TRACON and the en route controllers. These differences are best characterized by the more continual changes in aircraft altitude experienced in the TRACON, the greater frequency of flying by visual flight rules by aircraft in the TRACON region (and greater variety of aircraft types), and the greater extent, in the en route centers, to which control can be carried out strategically, through the execution of longer-range plans, than tactically, in terms of resolving more suddenly arising conflicts. At the same time, as noted above, there are far more similarities than differences between the two operations, particularly as these translate into the human factors issues that are discussed in this book. For example, the strategies and tactics for failure management that are described for the TRACON are similar for the two kinds of controllers. In the rest of the book, we often speak generically about controllers, referring to those that occupy both kinds of facilities. One final common feature that characterizes the tasks of controllers at all levels is the nature of changes in work shifts. The impact of the specific shifts (day or night) during which controllers work is addressed in Chapter 6. We note here the critical importance of shift or station changes, when one controller assumes the duties of another at a station. In these circumstances, the traffic situation must be accurately understood by the replacing controller: Who is where in the sky or on the ground? What actions are pending? What conflicts may be forecast in the distant future but are not yet sufficiently imminent to warrant corrective action? Data show that this period, the first minutes following a new time on position, is one in which errors are more likely to occur (Cheaney and Billings, 1981). CENTRAL FLOW CONTROL Metering the number of aircraft in the national airspace on a daily basis is an important task. Flow control is designed to meet user needs to the best ability of the system—that is, to ensure that the national airspace system accepts the maximum number of aircraft yet maintains high levels of safety. Factors that affect flow control are the physical structures of airports, including runway and taxiway availability; the number of arrivals and departures that can be operated safety in a given hour; controller equipment status, including what equipment has failed that reduces their capability to handle workload; the status of the national airspace system equipment; emergency situations; and the main factor, weather.

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Flight to the Future: Human Factors in Air Traffic Control The Air Traffic Control System Command Center, located in Virginia near Dulles Airport, is responsible for the management of traffic throughout the air traffic control system. This facility, in conjunction with traffic management units at each of the en route centers and some designated TRACONs and towers, establishes the daily traffic flows into and out of 28 major airports based on all the factors listed above. En route centers also provide flow control within their airspace to ensure that sectors do not become saturated. Flow control is dynamic, and the flows may change on a minute by minute basis. The primary method for ensuring that the traffic is metered is to hold aircraft on the ground and release them into the system at intervals so that they can be sequenced into the approach without further delay when they reach their destination. Airborne holding is another method and is being used on a limited basis (fuel costs being the limiting factor). Both methods have pros and cons for the commercial airline industry. Holding on the ground saves fuel costs and usually ensures no delay at the destination but has an effect on the customer and airline competition. Airborne holding costs more in fuel, presents greater safety hazards, and creates more workload for the controllers, but it ensures that when an approach slot is available there is an aircraft there to take it, thus making the best use of airspace for meeting demand. Flow controllers at the local facilities, called traffic management coordinators, primarily utilize the HOST system to tell them where the traffic is located and at what time it is expected to affect the airport. The HOST is supplemented by a system called aircraft situation display, which uses a computer to present a picture of all the airborne aircraft in the country at any time. The display gets its information from the HOST by data link through a computer located at Cambridge, Massachusetts. This system is very helpful to the en route centers' traffic management units in determining when a sector will become busy and where to reroute traffic to keep workload efficiencies at a maximum. The working day for flow control starts very early in the morning, when the daily plans of operations are formulated by two organizations. First, dispatchers at the airline operations centers of the major airlines arrange the daily plan of flights to and from the major hubs. The other organization is at the central flow control center at Herndon, Virginia, where each morning a national plan for traffic flow is developed, taking into consideration issues like weather, which may restrict flow to certain regions, and critical events like the Super Bowl, which may create bottlenecks in certain areas. As the national airspace increases its activities and flights begin departing in the East, then the Midwest, and then the West, air traffic control is distributed to the facilities (TRACONs and en route centers). Minor traffic bottlenecks and buildups are addressed by distributed local negotiations between adjacent facilities via their traffic management coordinators. If a TRACON is temporarily saturated, controllers there will coordinate with one or more of the feeding en route centers to slow down the delivery of aircraft. Similar local negotiations

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Flight to the Future: Human Factors in Air Traffic Control FIGURE 2.6 Central flow control aircraft situation display. Source: Federal Aviation Administration. may be carried out between adjacent centers, just as they also take place between adjacent sectors within a facility. Central flow control at the command center continues to actively monitor the biggest picture via the aircraft situation display (ASD) (Figure 2.6). Occasionally it becomes actively involved in implementing ground holds or managing the flow of traffic to and from international destinations, but by and large the philosophy is a fairly hands-off one, to allow local solutions to be achieved within the facilities, unless problems develop that they cannot handle. Such problems may be of two sorts: first, there may be anticipated problems such as the gradual buildup of traffic in a region, a buildup that may need to be addressed by three or more facilities (TRACONs and en route centers), making achievement of a solution difficult with a single phone call. Second, there may be truly abrupt or catastrophic failures in the system, as when severe weather closes an airport or a power outage at some major facilities drastically degrades the ability to monitor traffic position. In these infrequent instances, central flow central must "jump into the loop" as an active participant in control (Huey and Wickens, 1993), suddenly utilizing

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Flight to the Future: Human Factors in Air Traffic Control the full situation awareness that has been maintained during the previous routine period to rapidly implement strategic adjustments to traffic plans. (The analogy with situation awareness of the individual operator is apparent.) Such crisis negotiation is accomplished by extensive vocal communications over phone lines to facility traffic management units and to airline dispatchers. Such verbal negotiations may well be supported by the spatial display of national flow available to flow control managers at the facilities as well as at central flow control. The response of the national flow system has in the past worked well. No accidents, for example, have ever resulted from the catastrophic impacts on the flow control system of severe events like power loss or runway closure. The system typically responds adaptively to minimize the consequences of out-of-the-ordinary situations such as power outages, severe weather, and communication failures by increasing the amount of voice communications and separation margins whenever possible. However, it is important to ask: (1) how the response might vary if human-human communications are replaced by human-automation communications and (2) how such a response would be made more vulnerable in an airspace that is far more densely populated than at present, a density that is intended to be the direct result of the increased flow capacity made available by the same automation. FLIGHT SERVICE STATIONS The air traffic controller specialists in the flight service stations provide a myriad of services, primarily to general aviation pilots. The services provided are flight plan filing, preflight and en route weather briefings that include the status of navigational aids, airport conditions reports, search and rescue operations, assistance to lost or disoriented aircraft pilots, provision of instrumental flight rule and special visual flight rule clearances, soliciting pilot reports on flying conditions, and providing special services such as customs and immigration notification. Pilots can receive these services by visiting a flight service station, by telephone, or through air-to-ground communications. In 1994 there were 131 flight service stations, of which 60 are automated. Current congressional plans call for reducing the total number of facilities providing flight services to 61. The automated system, called Model I Full Capacity, is a 1970s-era weather and flight notification distribution system. In the early 1980s it replaced a leased weather display and teletype system. The system interfaces with the national airspace data interchange network communication system and the en route centers' HOST system. It has reduced the workload of flight service station controllers and provides for a much quicker briefing to pilots, but it leaves much to be desired in terms of functionality and basic human factors engineering. The typical automated flight service station contains the following operational positions: preflight weather briefing, inflight, flight data/notice to airmen,

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Flight to the Future: Human Factors in Air Traffic Control weather observer, and area supervisor. At designated stations, specially trained controllers provide en route flight advisory services—that is, timely and pertinent weather data tailored to specific altitudes and routes using the most current available sources of aviation meteorological information. These specialists are in constant communication with the National Weather Service's meteorologists at its field offices and center weather service units. A modernization program began in the late 1970s. Its purpose was to achieve equal or improved service to the user, while reducing personnel and maintenance requirements through the consolidation of 317 manual stations into 61 stations with modern automation tools. The program has been successful to some degree, but it has created many issues at locations where an old facility has been closed or is projected to close. Users have been concerned that they would not receive the same level of service, especially at remote locations—Alaska is a good example. Walk-in services for many pilots were out of the question; some stations were not even accessible from the airport. As a result, business is sometimes done entirely by telephone. The primary concern has been that, with fewer stations, the automated flight service station air traffic controllers would be busy on the telephone and users would be delayed in getting service. To offload some of this unmet demand, the FAA implemented several broadcast programs provided by private contractors to distribute weather information. For example, the DUATS program provided contract awards to two companies to provide free access to on-line computer services for weather information and flight plan filing. The consolidation process has been delayed for over a decade and has been subject to political pressure. The key issue is the downsizing and relocation of controllers from the closing stations to the now-centralized ones. Communities were solicited to bid for station sites, and the selections were driven by their cost to the government. Subsequently, some automated flight service stations were located in areas that are difficult to staff. The National Association of Air Traffic Specialists is the bargaining unit representing the nonmanagement flight service station specialists. They have accepted the new concept but have been very concerned about the relocation of positions and the loss of jobs. SUMMARY Controllers work in three types of air traffic control facility: the tower, the terminal radar approach control (TRACON), and the en route center. The air traffic control organization, called Air Traffic Services, manages all of these facilities. This organization is responsible for formulating plans and requirements for future operations as well as evaluating and analyzing current operations. Division managers at the nine regions manage the air traffic control activities in their region. These regional administrators are supervised by the director of Air Traffic Services.

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Flight to the Future: Human Factors in Air Traffic Control Controllers in all types of air traffic control facility develop strategic plans for traffic flow, monitor these plans with visual inputs to update the big picture of the traffic flow, and communicate with pilots and other controllers to ensure continued safety and efficiency. Controllers in the towers depend heavily on direct visual sightings of traffic at the airport, while those in the TRACON and en route environments are supported by computer-based, partially automated radar displays. The level of automation varies from facility to facility. Controllers depend on paper flight strips to represent the progress and special status of individual aircraft as they pass through the controller's sector of the airspace. All controllers must be prepared to deal with unanticipated events—for example, equipment failure, weather emergency, or pilot noncompliance with instructions—in a flexible manner that preserves safety even if it temporarily disrupts efficiency. Although controllers in any of the three basic positions—tower, TRACON, or en route center—share many competencies, there are important differences among their tasks. Furthermore, there are differences between sites that perform the same functions and even within a site from sector to sector. Anecdotal evidence suggests that each site is likely to have its own culture, composed of shared beliefs among a particular set of operating personnel. Ideally, the introduction of new technology into a large organization would be uniform throughout all its branches. Such uniformity of implementation is particularly difficult in the air traffic control environment because of the facility-specific culture and task environment. Furthermore, it has not been possible to create a common training or job performance evaluation program that covers all air traffic control specialists because of the local variation in job requirements.