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Ergonomic Issues in Air Traffic Management
August 1997

Karol D. Kerns, MITRE/CAASD Philip J. Smith, The Ohio State University C. Elaine McCoy, Ohio University Judith Orasanu, NASA Ames Research Center

MITRE Center for Advanced Aviation System Development McLean, Virginia

This paper was prepared for publication in The Industrial Ergonomics Handbook, W. Karwoski and W. Marras (Eds.), Boca Raton, FL: CRC Press, Inc.

Introduction | The air traffic management (ATM) system poses a broad set of human factors challenges. These range from the design of tools to support work by individual controllers, to the development of procedures and tools to support cooperative work by flight crews, controllers, traffic managers and dispatchers, to the allocation of tasks within an overall system architecture such that the cognitive demands of the work performed by individuals are at acceptable levels while still achieving a high level of system performance [Wickens, Mavor, and McGee, 1997].

This paper describes a system-wide view of the human factors in the current and future ATM system. The term, air traffic management, is a relatively new one, that denotes both of the systemÕs primary functions, air traffic control (ATC) and traffic flow management (TFM). This term also recognizes the ongoing changes in the organizational culture and operating philosophy of the system, greater emphasis on service to and collaboration with system users; less on control. With this in mind, this paper provides an overview of the main human factors topics in ATM and an explanation of the operational context in which they occur. A major focus of the paper is on the current and future operating philosophies, processes, and technologies, and their implications for human performance, human factors research, and practice.

This paper first briefly outlines the components of the system as it exists in the United States (which is the context that will be used for discussing human factors issues). Then, the air traffic control system is discussed, giving a broad survey of the current and emerging issues and an assessment of the role and status of human factors. Following that, the traffic flow management system is discussed, emphasizing the importance of the impact of the overall system architecture on the demands placed on individuals within the system. Finally, there is a discussion of the human factors implications of proposed future designs for the air traffic management system in the United States.

System Overview | The design of the ATM system reflects important concepts about information management and social organization. Figure 1 illustrates the basic hierarchical nature of this design and of the management domains that make up the air traffic environment. Information regarding air traffic can be partitioned into a hierarchy of domains based on its quality and granularity. As is shown in the figure, long-run strategic planning based on aggregate traffic demand data must be done to make decisions about resource allocations and about rules and procedures, along with daily strategic traffic flow management decisions, while tactical activities, based on flight-specific data, must be done to assure separation of individual aircraft. The goal of these strategic planning activities is to ensure that controllers, pilots, and dispatchers can safely coordinate the activities of specific aircraft in order to assure safe, effective utilization of the airspace and airport facilities [Klein, 1992].

The ATM system is also organized hierarchically to take advantage of the informational structure (see Figure 2). At an organizational level, the ATM system has a hierarchical structure in which the tactical and strategic air traffic management functions are allocated among various ATC and TFM organizational units. Although it is useful to discuss these functions as if they were associated with discrete organizational units, in practice the elements of the functions are intertwined with overlapping and redundant responsibilities to ensure system reliability.

Figure 1. The Hierarchical Nature of Air Traffic Management [From Klein, 1992]

Figure 2. Representation of Air Traffic Management Organizational Structure

At the lowest level of organization, the ATM system is organized to assure aircraft separation throughout all phases of flight from takeoff to landing. To accomplish this, the ATC component of the system is composed of controllers and specialists working in several different types of facilities. These include:
•  Towers at airports, with air traffic controllers responsible for directing arrivals and departures;
•  Terminal radar-approach control facilities (TRACONs), with controllers guiding flights for roughly the first and last 40 miles of flight;
•   Air route traffic control centers (ARTCCs), with controllers in charge of flights while en route;
•   Flight service stations (FSSs) with flight service specialists providing services such as flight plan filing preflight and inflight weather briefings to general aviation pilots.

Pilots file a flight plan and obtain a clearance in order to fly through airspace controlled by the system. Once the initial clearance has been obtained, the pilot maintains radio communication with the controllers at these facilities to receive ATC services. The controllers monitor specific flights within the airspace (sectors) that they are in charge of and issue instructions to pilots in order to ensure safe separation and efficient use of the airspace.

At an intermediate level of organization, the ATM system includes local TFM units. Each ARTCC and TRACON also has a traffic management unit and there are a few towers where traffic management coordinator positions have been established. These organizations are responsible for helping to plan and adjust the flow of traffic within their airspace. At the national level, a centralized facility, the Air Traffic Control Systems Command Center (ATCSCC) coordinates the activities of the local units [Garland, Hopkin, and Muller, 1996]. Finally, the dispatchers within airline operations centers (AOCs) have an increasingly significant impact on traffic flows, and thus must be considered in any discussion of the system. Details on the tactical and strategic operations within this structure are discussed in the following sections.

Human Factors in Air Traffic Control | The function of the ATC system is to provide safe, orderly, and expeditious flow of air traffic to its users. In accomplishing this function, the ATC system operates on two fundamental principles. First, the system exists to serve the users. The users are the pilots, aircrews, and passengers on board the aircraft being controlled by the system, plus the owners and operators of the aircraft. Control of aircraft by the ATC system also benefits operations of aircraft not being controlled directly, because ATC operations provide a systematic environment in which such "non-controlled" aircraft may be flown safely and efficiently.

Second, the system's direct service is provided by the controllers. This service consists of outputs which include the following:
•  Clearances and control instructions;
•  Advisories on traffic or other flight conditions;
•  Manual flight data handling, including keyboard entries, handling of entries on Flight Progress Strips or notepads or other flight data displays;
•  Controller-to-controller voice coordination and controller-pilot voice communications;
•  Plans for and selection of control techniques and actions.

In addition, first-line supervisors in ATC facilities support and coordinate the operational outputs of the controller positions, including coordination of requests for traffic volume control and metering services [Kinney, Bell, and Ditmore, 1983].

As discussed earlier, the ATM system has evolved a specialized, hierarchical structure in which the TFM component works to simplify and expedite traffic flows, ensuring that the operational environment is compatible with the capabilities of the controllers and pilots responsible for conducting tactical flight operations. Safety or separation assurance is the first aim of the ATC component of the ATM system. Efficiency is second, implying orderliness more than expedience.

Controllers collaborate with pilots to provide ATC services to participating (commercial, general aviation, and military) aircraft during takeoff, while en route, and upon landing. The nature of the controller's mission, the ATC organization, the technological systems, and the operating environment vary across the different ATC facilities that support aircraft during each of these flight phases.

The following section describes the organizational, technological, and operational environments in which controllers are embedded. It also discusses factors such as mission requirements and operational constraints that affect the performance and workload of controllers. For each environment, examples of human factors and ergonomic issues are also discussed.

Figure 3. The role of terminal and en route facilities in ATC. (From Mundra, A.D. 1989. A Description of Air Traffic Control in the Current Terminal Environment. MTR88W167, The MITRE Corporation, McLean, VA. With permission.)

Missions and Tasks Within the ATC System | Air traffic control specialists or controllers may work in one of three areas: terminal, en route, or flight service. The terminal area includes controllers working in the Airport Traffic Control Towers (henceforth called simply towers) and the approach control environment. The en route area includes controllers working in ARTCCs or en route centers that manage the domestic and the oceanic airspace environments. The flight service area includes specialists working in FSSs which provide a variety of information and support services to general aviation pilots throughout the system. Because the flight service specialists do not provide services to active flights operating under the control of the ATC system, they are not discussed further in this section.

Figure 3 shows the role of the terminal and en route areas by depicting the ATC phases through which an aircraft passes from its point of origin to its destination [Mundra, 1989]. Towers control the surface movement of aircraft, their landings, and takeoffs. Once an aircraft takes off and before it lands the aircraft is under the control of the TRACON. TRACON airspace generally extends about 40 nautical miles (nmi) from the airport. Outside of this region an en route center has responsibility for control of the flight. The cruise phase of an aircraft may be conducted within one or more centers. For international flights, the aircraft may pass into oceanic airspace and be handed over to an international control facility. The oceanic airspace is divided into flight information regions operated by non-US civil aviation authorities.

Terminal Environment - Tower Operations | The Federal Aviation Administration (FAA) operates over 400 towers to control traffic on airport runways, taxiways, and in the immediate vicinity of the airport. ATC in the tower is based on the visual confirmation, pilot reports of aircraft locations, and face-to face-interaction between controllers. Tower equipment types vary depending on each facility's operational needs, and the equipment configuration is often customized to a given tower's structure. As in all ATC environments, flight strips are used to record, maintain, and coordinate flight plan and clearance data. Many towers also have radar displays to aid visual acquisition of arriving aircraft, surface surveillance equipment for displaying aircraft locations on the airport surface, and a data link communications system for transmitting selected messages to pilots.

The tower may be staffed with a local, ground, clearance delivery/flight data, and supervisor position. Each position has different responsibilities depending on an aircraft's phase of flight. Only the local and ground controllers have traffic movement responsibility.

Figure 3. The Role of Terminal and En Route Facilities in ATC [from Mundra, 1989] | For departing aircraft, the flow of information and responsibility goes from flight data/clearance delivery to ground control and then to local control. The flight data/clearance delivery position processes flight plan data and manages the predeparture clearance (PDC) process. This process ensures that the aircraft has received current airport information (the Automatic Terminal Information Service (ATIS)) and an approved flight plan. At most major airports, controllers prepare digital messages and pilots request the ATIS and PDC by using a data link. The ground controller is responsible for issuing the taxi clearance and directing the aircraft through the system of taxiways that lead to the runway. The local controller has responsibility for the active runway and clears the aircraft for takeoff.

For arriving aircraft, the flow goes from local to ground control. The local controller clears the aircraft to land and issues instructions regarding where the aircraft should exit the runway. The local controller then directs the movement of the aircraft toward a gate.

Research on the tower operations highlights some characteristics of the environment that have important effects on controller performance and workload. Tower controllers spend a considerable amount of their task time, approximately one-third, visually tracking traffic movements outside the tower [Bruce, 1996]. The task of visually tracking aircraft and knowing how to communicate with them is not a trivial one [Wickens, Mavor, and McGee, 1997]. It entails extensive cross referencing of display aids, controller movement around the tower to view aircraft movements, and face-to-face interactions with the other controllers. Although tower controllers accomplish many tasks concurrently such as moving flight strips and cueing the radio microphone while looking out the tower cab window, they do not conduct keystroke entries or read display information.

For several years, the FAA and the National Transportation Safety Board have been concerned with reducing runway incursions and related surface incidents. These incidents may result from a variety of causes, including errors made by controllers as a result of reduced visibility. Display-based aids for detection of unauthorized movement of aircraft on runways are now being deployed by the FAA. However, because tower controllers have a major responsibility to continuously scan the terminal airspace such aids must be carefully integrated with the controller's visual and auditory scanning tasks [Bales, Gilligan, and King, 1989].

Another important characteristic of the tower environment is that it is communications-intensive. Tower communications are generally time-sensitive and often time-critical. During busy periods, a tower controller accomplishes time-critical communications at a rate only marginally ahead of air traffic movements around the airport. Moreover, the only way tower controllers can manage their workload in heavy traffic situations is to delay aircraft movements [Bruce, 1996]. Analyses of tower communications indicate that a variety of factors are adversely affecting the pilot's ability to correctly understand and remember controller instructions [Adam, Kelley, and Steinbacher, 1994; Burki-Cohen, 1995]. In order to minimize time on the radio frequency at busy times, there is a tendency for controllers to speak more rapidly, making it difficult for pilots to understand messages. Furthermore, many of the messages transmitted by tower controllers, such as taxi instructions, are complex and lengthy, making them difficult for pilots to remember. Finally, frequency congestion, a pervasive problem at busy towers, often prevents pilots from reading back their instructions to ATC, eliminating the verification that the pilot has heard and understood the instructions.

Terminal Environment - Approach Control | Approach control facilities provide ATC services to arrival and departure aircraft transitioning between the tower and the en route airspace. In most busy terminals, the TRACON room is housed in the same building as the tower. ATC in the TRACON, as in the en route domestic airspace, is primarily radar-based. However, the equipment, traffic environment, and control procedures used in the TRACON are quite different from those in the en route environment. The automated radar terminal system equipment which provides a radar display of air traffic is fairly standard across facilities but system capabilities may vary somewhat depending on the level of traffic handled by the facility. Flight strips are also used in the TRACON.

A typical TRACON control room includes two arrival feeder, two final, and two departure positions. Each position is assigned responsibility for a sector of airspace with arrival and departure traffic segregated into dedicated airspace corridors. The arriving traffic in the terminal area funnels in from higher altitudes and speeds for landing. The arrival controllers sequence and space the aircraft. A pair of arrival or feeder controllers each work the arriving traffic for one side of the airspace, e.g., north or south arrivals, establishing the aircraft on their initial approach to the airport. Two final controllers, one for each side, are responsible for the final approach phase.

The departing traffic fans out to higher altitudes and speeds for the cruise phase. Departure traffic is also divided between two departure controllers who each work one side of the airspace, establishing the aircraft on headings to the planned route of flight.

The physical layout of the TRACON reflects the degree of coordination required between operational positions. The four arrival positions work as a team and are situated next to each other. The final controllers are next to each other, as they typically need to coordinate frequently. The arrival feeder controllers are each situated next to the final controller they feed. Departure positions are separated from the arrival positions, mirroring the segregation of arrival and departure traffic.

Mundra [1989] identified several characteristics of the TRACON operating environment that affect the performance and workload of the controllers. There is high frequency of aircraft maneuvering in altitude, speed, heading, and a rapid convergence of many traffic streams. Once within the TRACON airspace aircraft are neither expected to nor allowed to follow a predefined route, instead control is exercised through headings (i.e., vectoring), altitude, and speed instructions. As a result, system performance is sensitive to controller skill level in planning and achieving throughput and aircraft efficiency (fuel-efficient speeds, altitudes or paths).

The TRACON lacks planning data and a route structure on which to base flight planning for individual aircraft. Instead, it uses a vectoring plan that only the controller knows. Today, the path flown is a specific response to a tactical situation, and must be visualized in the controller's mind. The controller must also keep track of the situation and required actions under high workload conditions.

Because of the high frequency of controller-initiated maneuvering, voice communication is a significant portion of the controller's workload. This implies many of the same communication problems discussed in the preceding section.

The uncertainty and rapid pacing of activity in the TRACON environment requires flexibility and teamwork. The limited advance information on controlled traffic and the lack of any advance information on uncontrolled traffic means that controllers must respond quickly to adapt their plans, fit in new traffic, and share or redistribute tasks.

En Route Environment - En Route Domestic | The FAA operates 22 en route centers to separate aircraft traveling between airports. The host computer system, display equipment, and facility designs in the en route centers are standardized. ATC in the en route centers is radar-based and each operational position is equipped with a radar display of traffic and flight strips. However, this common equipment configuration accommodates a wider range of staffing plans, missions, and tasks than is found in other ATC environments. En route centers provide an ATC link with other centers, with TRACONs, and with towers. As in the TRACON, the physical layout of the sectors in the en route facility reflects the degree of coordination required between controller positions.

En route ATC operations are divided into various types of airspace sectors. Each operational position is assigned responsibility for a sector and each sector may be staffed with one to three controllers, depending on the volume of traffic. When multiple controllers work a sector, task assignments vary among teams. Typically, a radar (R) controller is in charge of the sector operation. This controller uses the radar situation display and voice communications to apply radar separation procedures and separate the aircraft from all others within the sector. A radar associate or data (D) controller is responsible for separation planning activities. The D controller uses of flight strips which provide advance information on the aircraft's planned route and interphone communications with other controllers to identify potential problems and coordinate preventative control actions. When the sector is too busy for the R and D controllers to handle, a radar coordinator (tracker) or handoff (H) controller may share the R controller's load, serving as a redundant "set of eyes and ears" to support situation monitoring and as a second pair of hands to perform data entry and intersector coordination tasks.

En route sectors can be classified into types according to various sector and duty characteristics. High altitude sectors, generally above 24,000 feet, handle departure traffic climbing to cruise altitude, overflying en route traffic flying level, and traffic descending to a lower level in preparation for arrival. Low altitude sectors normally have a mixture of aircraft which fly at lower altitudes, slower speed arrival and departure traffic, and higher performance aircraft transitioning to the high altitude sectors. Within these two broad divisions, finer breakdowns can be made based on the homogeneity of the traffic flows and the sector's mission. For example, en route arrival/departure sectors coordinate traffic flow in and out of approach or airport control, transition high and transition low sectors have a majority of their traffic climbing or descending to reach cruise altitude, and en route high and en route low sectors have most of their traffic flying level.

Controller tasking and taskload in different sectors results from a combination of static and dynamic factors. Static factors, such as the type of traffic flows and the service that must be provided each aircraft, imply routine tasks that are known in advance and performed for every aircraft or for a specific subset of the traffic such as a particular flow. For example, en route controllers currently have a major responsibility to implement routine ATC procedures, such as routing and altitude constraints specified in interfacility directives, and flow management procedures to establish an orderly flow of traffic within the airspace. Dynamic factors are associated with the particular airspace delegated to the sector (e.g., size of the airspace for radar vectoring, time an aircraft remains in the sector), the number and characteristics of aircraft within the airspace, and the nature and frequency of requests by users to alter their flight plans. One dynamic factor thought to be an important determinant of complexity involves the number and pattern of potential conflicts that can occur within the sector airspace for a given period of time. The decision-making process associated with the detection and resolution of conflicts has a great impact on determining controller workload and sector capacity.

In the current environment, a mix of procedural solutions and automated capabilities are used to anticipate and manage complexity and controller workload. On a scheduled basis, controller positions are closed and opened and responsibility for sector airspace is combined and decombined under a position. Staffing arrangements also vary with sector load. During busy rush periods throughout the day, en route sectors are routinely staffed with two and sometimes three controllers. When traffic volume is expected to exceed the sector's capacity for an extended period of time, traffic flow management is alerted by an automated monitor alert function and procedures are activated to limit the volume of traffic handled by the sector.

The controller's ability to manage traffic in the current environment is also affected by equipment. The three-person sector team poses problems in performance because the sector equipment is set-up and laid out for a two-person operation and is awkward for three-controller staffing [Kinney, 1977]. For example, when the H controller sits down the R controller moves farther away from the flight strips and has difficulty reading them. The three-controller operation also requires coordination between the R and H controllers, a task that does not exist with the two-person sector. In general, the fact that equivalent tools and technological capabilities are not available on each side at the en route position limits the ability of the assistant controllers to contribute substantially to team performance [Shingledecker and Darby, 1995]. Finally, the outdated message composition, editing, and list management capabilities available on the workstation may be contributing to deficiencies in controller performance with manual inputs. Early research on manual data entry tasks indicated that these tasks which make up a large part of the controller's workload are demanding and time-consuming and that data entry errors appeared as contributing causes in many system error case histories [Kinney, 1977].

En Route Environment - En Route Oceanic | The FAA also provides ATC service within a large area of international airspace, including the western half of the Atlantic Ocean, the Gulf of Mexico, and a significant portion of the Pacific Ocean [Nolan, 1990]. In contrast to the domestic en route airspace, oceanic airspace has no radar coverage over most of the area. For this reason, monitoring and control of the oceanic traffic is based on flight plan data, position estimates, and relayed voice position reports from the pilots. Route structures are used to impose order and separate the traffic flows. In addition, controllers apply a diverse set of non-radar separation criteria to provide ATC services in the ocean. Paper flight strips constitute the primary information display for aircraft separation; these are updated by controllers with the current flight plans and latest position reports for each aircraft. The oceanic computer system also provides a plan view display of traffic positions based on the pilot's reports. Unlike the other ATC environments, air-ground communications over the ocean are indirect. Controllers use a telecommunications processor to send messages to a service provider who relays them to pilots. The telecommunications processor is also capable of sending data link messages directly to aircraft via a satellite. In part of the airspace, controllers have begun to use the data link to communicate directly with pilots.

An oceanic controller is assigned responsibility for a sector of airspace. However, oceanic sectors are significantly larger than those in the domestic airspace environment (e.g., an oceanic sector may be several thousand miles in length) and a single controller may have responsibility for many more aircraft than in the en route environment. Procedural ATC requires large lateral and longitudinal separation between aircraft that takes into account the lack of timely, independent surveillance, uncertainties, and delays associated with the communications. Oceanic controllers plan for separation by mentally calculating fix crossing time differences between aircraft based on flight strip data. If aircraft are not traveling on structured routes, controllers may evaluate separation visually on the plan view display or estimate distance spacing by plotting position data on charts [Couluris, 1985]. To ensure that the planned separations are maintained, controllers monitor flight progress based on pilot reports submitted as they cross compulsory reporting fixes (hourly or every 10 degrees longitude/latitude). With each progress report, the controller reexamines the traffic situation and searches for potential conflicts.

Hamrick and Reierson [1993] discuss some characteristics of the oceanic environment that affect the controller's performance and workload. Separation planning is complex and must allow for uncertainty and delay. It often requires that controllers plan a sequence of clearances involving several aircraft in order enable aircraft at different altitudes on the same route to climb or to merge traffic onto a single route.

Separation assessment entails significant mental calculations of fix crossing time differences and often requires cross referencing and integration of information from multiple sources to form a single "picture" of the situation. A variety of techniques and separation standards are currently applied in the oceanic airspace [FAA, 1992]. Determining the applicable separation criterion is not a trivial task. It requires consideration of multiple factors, such as aircraft type, method of navigation, altitude, geographic region, speed, route of flight, flight origin and destination. In some regions, many standards apply and the controller will select the standard that provides the best operational advantage in a given situation.

Today's oceanic controllers have few tools and automation is limited to use for traffic visualization not separation. Use of flight strips as the primary means of separation means that controllers have a considerable taskload bookkeeping pilot progress reports and monitoring for overdue reports. At times, oceanic sectors have so many flight strips that the controllers must stand and walk back and forth to examine and update the flight data. At the same time, the large sector sizes that must be accommodated on the plan view display cause aircraft positions to appear to be in close proximity, making it difficult to determine if aircraft are separated just by monitoring a situation display. Because the communications system is slow and cumbersome, controllers must take extra precautions in planning separation strategies that minimize the need for communication.

Missions and Tasks Within the ATC System - Summary | In all areas of ATC, controllers operate as part of a system of interacting components. These components include: the airspace and airport surface layouts, communications links, information display and management tools, and procedures, as well as pilots, other controllers, and traffic managers. Human factors and ergonomics issues in ATC arise as a result of controller interaction with each of these components. The present system is characterized by standard operating procedures that establish an orderly flow of traffic within the airspace and controller's rely on this traffic organization to anticipate problems and manage separation. Yet, against this basic traffic organization, the process of separation planning and selection of control techniques is highly dependent on the controller and varies across environments with respect to the temporal demandsand information processing and response resources involved. While the basic information required by controllers is fairly constant across environments, the quality and format of information available varies across the domains. Overall, the best information is available in the en route environment but there is also a controller cost associated with maintaining the data. Throughout the system, air-ground communications is a demanding controller task and capacity limitations in the current communications links contribute to information transfer problems. For the most part, controllers are working with aging equipment. Equipment-related issues vary across environments but they include lack of information integration and inefficient display formats, workstations and equipment layouts that impede team coordination, and outdated data handling capabilities that induce errors and reentry of data. Controller and pilot workload tends to be concentrated in busy environments such as the tower and the approach control. In these busy environments, where there is a concomitant need for controllers and pilot to coordinate their activities, it is particularly difficult to time the information exchanges so that they do not interfere with higher priority duties.

Ergonomic Issues in ATC | There is a sizable body of research literature on ergonomic issues in ATC [see, for example, Wickens, Mavor, and McGee, 1997; Hopkin and Wise, 1996; Hopkin, 1988]. This section focuses on the relevance and application of this research in the context of new and emerging ATC capabilities. Some of the main ergonomic issues in ATC relate to the mental demands and uncertainties imposed by the ATC process and the environments in which the controller is embedded. Complexity management and control strategies are key issues in this area. Another source of issues relates to the cooperative nature of the activity [Leroux, 1995]. Information transfer and communications are key issues in this area.

Complexity Management | Current air traffic control systems are functioning near, at, or even beyond their planned maximum traffic handling capacity, and all current projections foresee continuing and cumulative increases in air traffic for a long time [McAlindon and Gupta, 1993]. Consequently, there has been a long-standing interest in understanding and quantifying the controller's traffic handling capacity and in predicting when this capacity breaks down. However, despite a lengthy history of research on controller workload and sector complexity, the operational relationship between the two concepts remains elusive and practical application of complexity measures in the current system is limited. Today, the most widely used complexity measure is based exclusively on aircraft counts. In the field, operational decisions on complexity management continue to be made by instinct and local judgment.

Several factors make the analysis of controller workload in the current system a complex matter [Wickens, Mavor, and McGee, 1997]. Controller taskload has been analyzed extensively to derive traffic and airspace characteristics that predict workload, however this approach seems to be most applicable to measurements of the observable, motor and manual components of workload. Research on workload has also shown that successful controllers use various adaptive strategies to manage their performance in the face of increasing complexity [Sperandio, 1971]. These adaptive strategies allow the controller to handle more aircraft without error or excessive workload. Furthermore, research relating complexity and workload to operational errors tends to confirm that this relationship is probably mediated by other factors, such as control strategies. Studies of operational error reports show that errors are not uniquely associated with high complexity and workload but also occur under low to moderate complexity and workload [Canadian Aviation Safety Board, 1990; Rodgers, 1993; Kinney, 1997]. However, one of the chief difficulties encountered in interpreting the error reports is that there are no normative data. The relative frequencies of the levels of traffic volume and complexity are unknown, which precludes determining whether or not the reported frequencies of errors in these levels are disproportionately related to chance.

On one hand, these results emphasize the great need for better baseline and normative data on human and system performance as standards for evaluating causal relationships [FAA, 1995; Benel, 1995; Galushka et al., 1995]. On the other hand, they may suggest a practical problem with the research questions [Sarter, 1996]. From the standpoint of complexity management, it may be more useful to focus on prediction of complexity with respect to a specific operational context and decision. At a tactical level, controllers need detailed information on current and predicted sector complexity to plan control actions. Supervisors need information on predicted complexity for multiple sectors in order to plan for staffing. At a more strategic level, traffic managers need summary information predicted over a longer period of time and for a larger area to plan for flow management procedures and programs [Klein, 1992].

Controller needs for complexity information are being addressed by development programs in the US and in Europe [Schultheis and Tucker, 1996; Makins and Drew, 1995]. Both programs are developing controller tools for conflict prediction and resolution. With these tools, an indicator of sector complexity is provided in the form of display information on the number of predicted conflicts and their temporal distribution. Currently, both the US and Europe rely on a second controller to plan traffic and ensure that the primary controller is not overloaded. Advance information on conflict resolution workload should allow the second controller to better schedule and manage conflict resolution tasks. Such tasks account for a significant and increasing component of the primary controller's work. Particularly in the US, where the ATC environment is gradually evolving away from structured routings toward user-preferred routings, the relative contribution of the conflict resolution to overall sector workload is likely to increase, making it a better predictor of complexity [Carlson, Rhodes, and Cullen, 1996]. The ultimate goal for future ATC is a control-by-exception paradigm in which controller interventions are limited in extent and duration to correct identified problems [RTCA, 1995].

Control Strategies and Efficiency | The growing demand for aviation services and the constraints on budget that both the industry and the FAA face have stimulated interest in improving the efficiency of ATC. As mentioned in preceding section, controllers cope with increasing traffic demand and complexity by employing adaptive strategies. Research on controller strategies has identified how specific adaptations tend to lower the efficiency of individual flights and the overall traffic flow when demand is high. New ATC capabilities are helping reduce the need for these specific adaptations and preserve flight efficiency.

In general, strategies that are economical for the controller are those that preserve the primary objective of safety but take less account of secondary objectives such as flight efficiency, user-preferred paths, and fuel economy path [Bruce, 1996; Bellorini and Decortis, 1995]. Early on, Sperandio [1971] showed that controllers handled an unexpected increase in traffic load by adaptively decreasing the amount of time they spend on each aircraft. For example, the controller may structure the traffic so that all of the flights are in-trail and traveling at a uniform speed, thereby reducing the difficulty of monitoring for conflicts.

Another way controllers adapt their strategies is to focus on the immediate tactical situation and abandon planned strategies. To plan and execute control strategies that are more flight efficient, controllers must coordinate with each other. Under high workload, a shift from cooperative toward individual work has been observed in both the TRACON and en route environments. Bellorini and Decortis [1995] found that in the TRACON environment controllers shed coordination tasks under high demand, resulting in less efficient sequencing of arrival aircraft. Sperandio [1978] observed that in the en route environment, both sector controllers focus attention on the current tactical situation, reacting quickly and employing less efficient tactical control techniques. He further notes that an increase in the workload in the sector tends to make the support tasks more and more dependent on the central task and tends to overload the principal operator even more, so that the assistant becomes less and less efficient at a time when he is more and more necessary.

Controller-pilot coordination may also increase as controllers abandon planned strategies and react to the tactical situation. In many traffic situations, reactive strategies are communications-intensive, with the controller assuming greater responsibility for flight paths and making continuous tactical adjustments. Increased communications tax the controller's perceptual, cognitive, and speech motor capacities and the pilot's ability to understand and respond to instructions, resulting in more frequent requests for repetition and clarification [Shingledecker and Darby, 1995; Adam and Kelley, 1996].

To preserve flight efficiency, a more proactive approach to ATC is desirable. Decision support tools for tactical planning and selection of control strategies are being tested in the en route and TRACON environments. As mentioned in the preceding section, the FAA is currently testing a conflict probe capability to provide early detection of conflicts and tools for conflict resolution in the domestic en route environment [Schultheis and Tucker, 1996]. A similar capability is also under development for the oceanic environment [Hamrick and Reierson, 1993]. This capability will reduce the mental calculations and extrapolations involved in separation monitoring and afford a longer lead time to plan resolution maneuvers that are less disruptive to the user's flight intent. Team performance will be aided by providing the assistant controller with more powerful tools for visualizing the future traffic situation and evaluating proposed maneuvers. In addition, an automated coordination aid will also allow controllers to share and approve plans.

In the TRACON, controller aids for merging flows and sequencing aircraft for approach are already in use or under test at field facilities [Mundra, 1990; Lee and Davis, 1995]. These tools provide advance information on the predicted sequence of arrival aircraft. One tool, the converging runway display aid (CRDA), has been deployed to assist the controller in conducting staggered approaches. Staggered approaches require specific separations between aircraft landing on adjacent runways as well as between in-trail aircraft. Staggered approaches have been characterized as more complicated than simultaneous approaches which have only in-trail spacing requirements [FAA, 1991]. The CRDA reduces the complex mental calculations and extrapolations involved in staggered approaches by projecting false targets or ghosts for aircraft arriving in one of two converging streams onto the other stream, thus allowing the controller to visualize and manage simple in-trail spacing on a single approach.

Another tool, the final approach spacing tool (FAST) is one element of a Center TRACON Automation System (CTAS). CTAS comprises a set of tools to assist controllers handling aircraft arrivals starting at about 200 nmi. from the airport and continuing to the final approach fix. FAST provides the controller with landing sequence numbers and runway assignments to achieve an accurately spaced flow of traffic onto the final approach course [Davis et al., 1991]. Based on the displayed sequence, the controller formulates appropriate instructions for merging and spacing the arrivals. Research indicates that FAST advisories improve the runway delivery precision and reduce controller workload by reducing the number of vectors issued to each aircraft and reducing the need for (verbal) coordination between controllers [Credeur et al., 1993; Lee et al., 1995].

Communications | There is ample evidence in the research literature that controller-pilot communications are a common and persistent problem in today's operations [Kerns, 1994]. Analyses of incident and error reports and of recordings of routine controller-pilot communications offer a cogent explanation of how often and why problems occur. Field experience and simulation studies on data link offer a useful perspective on how this technology can be used in the operational environment to improve communications.

Some of the primary factors which contribute to communications problems arise from the use of spoken language to transfer information. ATC communications have been designed to ensure that spoken dialogues can be conducted efficiently and with minimum possibility of error or misunderstanding [Hopkin, 1988]. Controllers and pilots have adopted a standardized phraseology, language conventions, and procedures which define the process for conducting the dialogue, including the cues that tell a listener when a transaction has been completed and whether a readback is required. However, despite years of refining the language and procedures, research confirms the intractable nature of many of the problems inherent in the exclusive use of spoken language for controller-pilot communications. Grayson and Billings [1981] analyzed aspects of human speech processing and conversational behavior that mediate communication performance. Their analysis found that a tendency to fill-in information, the expectancy factor, and timing problems were implicated in many types of controller-pilot communication problems. The expectation factor contributes to misinterpretations and inaccuracies because controllers and pilots sometimes hear what they expect to hear. This generates what have been called "readback and hearback" errors in which, respectively, a pilot perceives what he expected to hear in the instruction transmitted by the controller and a controller perceives what he expected to hear in the readback transmitted by the pilot.

Congestion on the voice radio frequency has also been implicated in communications problems. During busy periods, controllers issue longer, more complex messages in an attempt to minimize use of the radio frequency. However, as more transmissions are crowded onto the frequencies, the procedural steps (callsign identifications, readbacks) that assure communication are being dropped [Adam, Kelley, and Steinbacher , 1994; Cardosi, 1993]. Analyses of routine communications [Morrow et al., 1993; Cardosi, 1993; Cardosi, Brett, and Han, 1996] have highlighted the contribution of message complexity and the resulting memory burden to miscommunications. In both the terminal and en route environments, errors and procedural deviations increased as clearances increased in complexity.

The frequency congestion problem is most pronounced in the tower environment where controller transmission rates have been observed at 3.9 and 8 transmissions per minute for the local and ground controllers, respectively, as compared to 1.8 transmissions per minute in the en route environment [Burki-Cohen, 1995]. A concentration of controller and pilot workload in the tower environment also accounts for failures to transmit information and untimely transmissions in busy environments. Because of high workload, controllers may fail to initiate lower priority traffic advisory messages, precisely when the pilot's need for this information is greatest. Conversely, pilots may be preoccupied with external vigilance and flight control tasks. They may not wish to receive messages and may fail to respond.

One of the ways the FAA is responding to communications problems is by developing alternative means of information transfer. Data link communications have already been introduced in environments where communications problems and limitations are the most severe: the tower and the oceanic en route. The PDC and the ATIS are being transferred via data link at many major airports. Direct controller-pilot communications via a satellite data link are being conducted in the oceanic airspace. The selection and design of these services have benefited directly from research on human interaction with voice and data link communication systems [Kerns, 1991;1994].

Research indicates that data link offers several advantages for transfer of lengthy, repetitive messages such as PDC and ATIS. Data link capabilities for message storage and retrieval reduce the controller's burden when preparing messages and the pilot's memory burden when receiving them. Data link also allows controllers and pilots to pace the transmission and processing of the information, thus avoiding conflicts with higher priority tasks.

In the oceanic environment, the tempo and highly structured format of the controller-pilot communication is well suited to data link transmission. The design of the controller and pilot data link communication protocols and their message handling capabilities have also been guided by data link simulation research [Kerns, 1994]. Based on the study results, operational communication protocols that minimize switching between voice and data link media have been implemented. Moreover, the procedural steps required to conduct communications within each medium are consistent. In terms of the level of automation, message handling capabilities have been designed so that controllers and pilots have the final authority to approve the transfer of information to each other and to their automation systems. At the same time, computer aiding of message composition has been implemented using menu style user interfaces to minimize input errors and relieve the human operators of these functions.

Although the experience and results to date indicate that these initial applications of data link offer important operational benefits at little or no cost to the human operators, future applications of data link must be selected carefully, addressing key human engineering challenges. Visual display and manual control of transmitted information may not be appropriate in environments where the controller or pilot visual and manual resources are already reaching an overload state. Delay factors associated with message composition may limit the utility of data link in rapidly changing condition while transmission delays may limit its utility for time-critical transmissions. Finally, new procedures will be needed to maintain team performance when the communication medium is silent and may be less readily observable by multiple operators.

Human Factors in Traffic Flow Management | The function of the Traffic Flow Management (TFM) system [Nolan, 1990; Odoni, 1987] is to provide strategic planning and control when necessary to try to avoid situations where potentially unsafe or inefficient operations are likely to arise. As discussed earlier, within the United States (which is the ATM system that will be used to provide examples in this Chapter), the FAA has organized this system into a hierarchical structure including ATCSCC (which supervises and coordinates planning at a national level), traffic management units at en route Centers and TRACONs, as well as FAA staff at towers and airport facilities. In addition, although not a formal part of the FAA's TFM system, AOCs [Airline Dispatchers Federation, 1995] play an increasingly important role in influencing traffic patterns.

Sample Control Methods Within the TFM System

To make the concept of TFM clearer, several examples are provided below.

Predeparture Interventions by ATCSCC | There are a variety of situations where the normal flow of traffic into an airport or some portion of the airspace needs to be restricted, and where information is available early enough to change plans before the affected flights depart. Such situations include forecasts of bad weather (thunderstorms, low visibility at an airport, etc.), runway restrictions or closures and forecasts of heavy traffic congestion.

Under such circumstances, traffic managers at ATCSCC (in consultation with the affected traffic management units) may choose to employ a variety of procedural tools to reduce traffic. They may, for example, initiate a ground delay or ground stop for flights departing from airports within one or more Centers. Alternatively, they may prevent AOCs from filing flights along a particular jet route or may reduce the flow of traffic along that route by requiring additional spacing between aircraft (for instance, requiring aircraft to fly 25 miles-in-trail). They may also give an airline several options to choose from in filing a particular flight.

Predeparture Interventions by AOCs | When capacity-limiting situations arise, the airlines may also choose to change their plans without any intervention by ATCSCC. They may, for example, cancel a number of their flights to a particular airport that is expected to have a reduced arrival rate due to bad weather, because a failure to do so could result in expensive diversions of some of their flights. Similarly, they may choose to file their flights along an alternative route because they are forecasting poor weather along the normally preferred routing.

At the other extreme, they may choose to file a few additional flights over and above the forecast arrival rate for an airport expected to have bad weather in order to provide a "reservoir" of flights to take advantage of the situation if the forecast bad weather doesn't develop. (In this latter situation, they run the risk of having to divert these additional flights if the forecast weather does develop, and must fuel the aircraft to handle the resultant diversion to an alternate airport.)

Interventions While En Route | Situations also arise that require interventions while flights are airborne. As examples, traffic managers at en route Centers may ask controllers to reroute certain flights to avoid predicted traffic congestion somewhere further along their routes, or may impose metering over an arrival fix, limiting the number of flights arriving at that fix within some period of time. Airline dispatchers may similarly request their flight crews to request rerouting if they foresee some problem further along the route.

Sample Actions - Summary | The point of these examples is first, that strategic planning decisions are made to try to avoid situations that could affect safety or efficiency. A second point is that there are a variety of alternative tools to influence traffic flow. A third is that, in the current TFM system, some of the decisions are made by FAA traffic managers, while others are made by individual AOCs (and that these decisions clearly interact with each other to determine the ultimate impact on traffic flow).

Traffic Flow Management - Conceptual Framework

To better understand the interactions of these organizational units that make up the TFM system, and to understand the performances of the individuals within them, it is useful to describe it as a distributed cognitive system, where the primary task is planning in the face of uncertain events. It is distributed in many senses, both within these organizational units and across them [Layton, Smith, and McCoy, 1994].

Distributed Problem-Solving | There are many senses in which TFM can be viewed as a distributed problem-solving task [Davis and Smith, 1983]. First, the organizations (ATCSCC, ARTCCs, TRACONs, Towers and AOCs) are geographically separated, so that direct face to face communication cannot occur. Second, in many cases information access is distributed among these different organizations, so that a traffic manager at an en route Center may not have the same information as a dispatcher at an AOC or a specialist at ATCSCC. Third, different types of knowledge or expertise are distributed among these organizations. Fourth, different types of decision-support tools are available at the different organizations.

Tasks and responsibilities are not only distributed across organizations, they are also distributed within organizations. There are specialists at the Command Center to deal with weather forecasting, to design severe weather avoidance programs, etc. Similarly, dispatchers at AOCs have responsibility for developing flight plans for flights in different parts of the world, and interact with airline meteorologists and specialists with expertise in such things as aircraft maintenance, crew scheduling and aircraft scheduling.

Competing and Complementary Goals | Another important characteristic of this cognitive system is that different organizations and individuals have different goals and priorities. FAA traffic managers have as their primary responsibility ensuring traffic flows that allow the safe separation of traffic from all sources (commercial airlines, general aviation and Department of Defense flights). They are also concerned with the efficient use of airspace capacity. Some of the decisions they make further require consideration of equity among the different airlines.

AOC staff likewise have the safety of their flights as the most important consideration. However, within that constraint, their goal is the efficient and effective operation of their own airline's schedule, as they are in competition with the other airlines.

Thus, even from a broad systems perspective, disagreements between FAA traffic managers and AOC staff must be expected. When constraints such as workload and information access further intervene, or when there are differences of opinion about weather or traffic forecasts, such disagreements are even more likely to occur.

Decision-Making Under Uncertainty | A third major factor that must be considered to understand human performance within the TFM system is the high degree of uncertainty associated with decisions. Weather is a major issue [Andrews, 1993], whether it concerns a forecast regarding the development of a line of thunderstorms, a snowstorm closing East Coast airports, strong crosswinds impacting a particular runway at an airport, or low visibility due to fog. There is also considerable uncertainty about traffic patterns. Flight departures are frequently delayed, for instance, which can influence traffic congestion at some further point along their routes, and there are numerous reasons why runway use at an airport must be restricted. Finally, controller staffing and workload limitations can introduce unexpected capacity constraints.

Conceptual Framework - Summary | In short, to cope with the complexities of managing traffic in the face of numerous sources of uncertainty, the TFM system has evolved into a very complex network of organizations and individuals where subtasks are allocated to specific individuals or organizations in order to reduce the cognitive demands on any one individual. This task decomposition includes considerable redundancy to catch errors and inefficiencies. This task decomposition also introduces all of the classic concerns associated with finding localized (suboptimal) solutions to problems, as well as the classic concerns regarding the strengths and weaknesses of group decision-making processes.

Ergonomics in TFM

The conceptual framework provided above begins to suggest some of the main ergonomic issues that arise in TFM [Wiener and Nagel, 1988]. These concerns stem from the fact that this is a very complex group decision-making process under conditions with a high degree of uncertainty, and are highlighted in the subsections below. It is also worth noting that these same characteristics (the complexity resulting from a large number of factors interacting to influence performance, and the fact that it is the interaction of a network of organizations and individuals) have had a significant influence on the type of research conducted thus far. With a few exceptions [Layton, Smith, McCoy, 1994], the research has been limited to descriptive observational studies of performance in field settings.

Goal Allocation as a Strategy to Reduce Complexity | To deal with complexity, the TFM system has been designed to distribute responsibility between FAA TFM facilities and the airlines by distributing responsibility for different goals. Traffic managers within the FAA have as their primary responsibility assuring safe and efficient use of overall system capacity. Thus, if an airport is predicted to have a reduced arrival rate due to a forecast of low visibility, ATCSCC, in consultation with the affected ATC facilities, will select a control strategy, such as an "all Centers" ground stop for all flights to that airport for a certain period of time. This strategy is then simply imposed on the airlines, as ATCSCC has the ultimate authority in making such a decision. Under such a "control by directive" paradigm, the airlines' business concerns are considered only in the limited sense that safe, efficient overall use of the system's capacity is desirable from an airline perspective as well.

Such a paradigm reduces the complexity of the decision-making task faced by traffic managers, as they don't have to consider as many factors in making decisions. However, it also clearly can lead to sub-optimal decisions from an industry perspective (as there can be several alternatives that are equivalent from a TFM perspective in terms of safety and overall use of system capacity, but which are quite different in terms of their economic impacts on individual airlines).

As a result, the system has been evolving toward formal procedures for incorporating airline preferences in TFM decisions [Lacher and Klein, 1993; Scardina, Simpson, and Ball, 1996; Wambsganss, 1995]. Initial efforts in this regard focused on the process for approving the flight plans filed by airlines prior to departure. This evolution began in the early 1990s with the implementation of the National Route Program [Federal Aviation Administration, 1992]. This gave the airlines a mechanism for requesting permission to file flights on routes other than FAA preferred routes. Such requests for "non-preferred routes" were submitted to ATCSCC on a daily basis, and were evaluated for approval by Command Center specialists in consultation with traffic managers in the affected Centers.

This "control by permission" paradigm maintained the distribution of goals, as FAA traffic managers did not have to directly consider airline business concerns in approving requests for non-preferred routes. They just had to evaluate the flights plans submitted by the airlines in terms of their impact on safety and system utilization. Similarly, airline dispatchers didn't have to consider the impact of alternative routes for a flight on overall system utilization or on air traffic congestion, they just had to evaluate the routes in terms of safety and cost-effectiveness for their company. Hence, it provided a means for limiting the complexity of the decision-making task for any one individual (a traffic manager or a dispatcher), but provided a means for arriving at closer to optimal solutions.

The available data suggested that such a paradigm shift did indeed produce more cost effective performance without any indication that safety was being compromised [McCoy, Smith, Orasanu et al., 1995; Smith, McCoy, Orasanu et al., 1996a]. There were, however, two concerns expressed regarding the ultimate effectiveness of this approach:

•  Such interactions to get permission to fly non-preferred routes were costly in terms of the staffing requirements for both AOCs and the FAA;

•  Continuing to give FAA traffic managers responsibility for evaluating airline requests provided insufficient impetus to explore the feasibility of new routings (as the traffic managers and controllers involved could achieve the goals for which they were primarily responsible—safe and efficient overall use of system capacity—without significantly changing the status quo).

As a result, a new order was implemented, referred to as the expanded National Route Program (NRP) [Federal Aviation Administration, 1995], which gave the airlines greater autonomy [Denning et al., 1996]. Under the expanded NRP, subject to certain constraints, the airlines were allowed to file flight plans (pre-flight) without seeking permission from ATCSCC [Carlson, Rhodes, and Cullen, 1996]. This new paradigm was an example of "control by exception" [Sheridan, 1987; Sheridan, 1992], in the sense that, under its rules, the ATC system was supposed to intervene only after a flight had been launched, taking corrective actions to deal with traffic bottlenecks tactically, rather than pre-flight. (Under some circumstances, such as the development of a broad area of bad weather, traffic managers could still revert back to "control by directive" by canceling the NRP.) The available data indicates that this further paradigm shift has resulted in additional efficiencies for the airlines [Couluris and Dorsky, 1995; Smith, McCoy, Orasanu et al., 1996b].

Distributing Information and Knowledge to Support Decision-Making | Although such paradigm shifts in the locus of control appear to have produced efficiencies, the available data indicate that, to maximize gains from such paradigm shifts, there is a need for improvements in access to information and knowledge, and for tools to help decision-makers cope with the added complexities that such shifts in control introduce. Indeed, after experience with this "control by exception" paradigm, dispatchers made comments like:

"Under the expanded NRP, it's like shooting ducks in the dark."

"It used to be the weather that was the biggest source of uncertainty. Now it's the air traffic system," [Smith, McCoy, Orasanu et al., 1996b].

Empirical studies indicate that such a lack of information about air traffic bottlenecks can result in fuel losses instead of fuel gains when flights filed under the expanded NRP along fuel efficient routes are rerouted by the ATC system to avoid excessive traffic congestion. As an example of a worst case scenario, Smith, McCoy, Orasanu et al. [1996b] found that flights scheduled to fly NRP routes from Los Angeles to Dallas, and to arrive during the noon rush, on average burned 1.9% more fuel than they would have if filed on the FAA preferred route, instead of achieving the 4.5% fuel savings expected from the NRP route.

The implication of such studies is that, if the locus of control is shifted without a concomitant shift in access to information and knowledge (in this case dealing with ATC responses to air traffic bottlenecks), benefits may be less than expected.

Developing Alternative Models of Cooperative Problem-Solving to Cope with Complexity | The "control by permission" model described earlier represented a model for cooperative problem-solving that reduced complexity for any one individual (traffic manager or dispatcher) by continuing to distribute responsibility for certain goals between AOCs and traffic managers. The "control by exception" paradigm potentially reintroduces this complexity for the dispatcher, as, potentially, he or she is now expected to integrate a much broader set of factors in order to develop "optimal" plans to meet the needs of an airline's schedule. (As discussed earlier, traffic managers still serve as a safety net to identify and prevent situations where traffic flows could impact safety or overall efficiency in the use of system capacity, and do this by reverting back to a "control by directive" paradigm.)

A number of approaches are being explored to deal with this [Adams et al., 1996; Billings, 1997; McCoy et al., 1991; Pujet and Feron, 1996; RTCA, 1995; Smith, McCoy, Orasanu et al., 1996c]. Some of them focus on enhancing data exchange and collaboration between AOCs and traffic managers, including the use of "white-boards" to enhance interactions and provide a framework for developing shared mental models and for sharing knowledge [McCoy, Orasanu et al., 1995; Orasanu, 1991]. Others focus on distributing responsibilities within AOCs, essentially changing the organizational structures to reduce the complexity for any one individual. Still others are exploring the use of decision-support systems to reduce the workload and complexity for individuals. Finally, some proposals focus on reformulating the problem by changing the parameters of control. Under these proposals, instead of having the FAA impose detailed solutions, such as implementing an "all Centers" ground stop, the FAA would instead place a constraint on airline generated solutions, such as "each airline must reduce its arrival rate at a particular airport by 50%."

Ergonomic Issues In TFM - Summary | As outlined above, at a macroscopic level, the critical question regarding the design of the TFM system has been identifying the truly important determinants of performance. In recent years, a number of approaches for revising the design of the system have been discussed. Some of these, such as shifts in underlying paradigm of control from "control by directive" to "control by permission" to "control by exception", have actually been implemented for some aspects of the TFM system. Others, such as changing the parameters for controlling airline performance, have thus far only been proposed. To understand the impact of such factors, a number of observational studies have been conducted. These studies indicate that such changes can in fact have a significant impact on performance, but because of the inherent confoundings in such studies, they do not definitively establish the relative contributions of the different changes that have been made to the system.

Thus, the fundamental human factors question remains:

In an environment where capacity is limited and where there are competing goals (among the airlines), how do we design a system where the cognitive demands on individuals are reasonable, and yet still achieve high levels of performance?

Issues in the Design of the Future ATM System

Recently, the FAA and the aviation industry reached general consensus on a strategic direction for the ATM system in the United States called "Free Flight" [RTCA, 1995]. The goal of free flight is an ATM system based on two fundamental premises:

•  The FAA retains and strengthens its safety mandate for separation of aircraft;

•  The users of the system are given the flexibility to make decisions that allow them to extract the maximum economic benefit from the ATM system.

Among other characteristics, free flight assumes a shift way from the current ground-based, tactical ATC operations toward a more cooperative arrangement in which users have greater flexibility to select and manage their flight paths and to participate routinely in airspace management decisions. As the ATM system evolves toward free flight, consideration of the human element will be critical to the realization of new operating philosophies, design of new functional architectures for integrating ATM system components, and application of advanced technologies to support ATM operations.

New Philosophies and Procedures

TFM | Under free flight, the fundamental goal for future TFM is to identify new operating philosophies and procedures that remove restrictions, allowing users to make choices based on business considerations subject to the constraints necessary to ensure safety.

Historically, TFM decision making has been characterized by gaming, in which the users, armed with limited information about the state of the system and the rationale for denying or approving requests, attempt to achieve an advantage over their competitors and "the system" by hiding their true intentions and preferences. This gaming has led to mistrust and suspicion among the participants. In the future, a team perspective is likely to provide important insight into many of the issues in TFM. The team literature addresses the functional requirements for interpersonal collaboration as well as the social dynamics [Fleishman and Zaccaro, 1992; Scerbo, 1996]. With regard to team functions, human factors knowledge and research can contribute to the definition of processes which meet system needs for acquiring and distributing information, coordinating the sequence and timing of participants responses, evaluating team performance, establishing team objectives and means to resolve disputes, and monitoring for compliance with policies. With regard to social dynamics, a progressive research approach is needed to accumulate experience in real world settings that will allow participants to practice team performance and assess the effectiveness and reliability of the collaborative process. Real world experience will also help refine the decision-making process and reduce decision maker biases and mistrust among the participants.

ATC | Currently in ATC, the basic rule of separation is that every controller is responsible for separation of participating aircraft for the duration of time the aircraft is within the controller’s sector of responsibility [Nolan, 1990]. In future ATC, the free flight goal is to evolve toward a more strategic operation that better accommodates user preferences. Under this philosophy, controllers may assume more responsibility for solving each other’s problems as strategic predictions allow for early ATC interventions. In addition, as new technologies provide pilots with information and capabilities commensurate with or surpassing those of the controller, they may also be called upon to participate in solving ATC problems.

As in TFM, a team perspective is likely to be a major focus in addressing human factors issues in future ATC. Considerable research and analysis has addressed the controller’s role (often as a single operator) within an ATC domain. There is much less work that addresses the multi-operator perspective [Benel, 1995]. Mavor, Wickens, and McGee [1997] also note that team training is less formal in ATC than on the flight deck, but teamwork is likely to be a critical component of ATC for the foreseeable future. Drawing on the literature in team performance will help identify functional requirements for collaboration in ATC. In the initial stages of orientation and trust building, laboratory and field evaluations can be used to work out task redesigns, clarify goals and roles, and establish commitment. To sustain team performance, the collaborative philosophy of ATC will need to be clearly communicated to the controller work force and reinforced through training, experience, and feedback programs that address concerns.

Functional Architecture | Already, new ATM technologies such as CTAS cut across traditional divisions of responsibility between en route and TRACON controllers. While strategic planning tools such as conflict probe tend to blur the distinction between ATC and traffic management responsibilities. It is also likely that workload concentrations in the tower environment will motivate further reallocation of tasks across ATC environments and additional capabilities for smoothing workload through greater human control over the tasks, their timescales, their scheduling, and their sequencing [Laios and Giannacourou, 1995]. At the same time, imbalances in air and ground system capabilities in the oceanic environment will motivate exploration of alternative allocations of functions between controllers and pilots which increase the pilot’s role in ATC. These forces, taken together with the new operating philosophies envisioned under free flight, will drive a progression toward new relationships among system participants.

Analyses of human factors issues in free flight identified the design of a new functional architecture as a key issue that cuts across all environments [FAA, 1995]. In TFM, human factors can help analyze the roles and interactions of TFM players, including central and local traffic managers and the AOC personnel. For example, laboratory simulations can resolve issues regarding human cognitive capacities and performance parameters, such as how much and how fast information can be assimilated. These results on the timing and quality of feedback needed by decision makers will help discriminate among alternative allocations of functions and define the interrelationships among groups of decision makers. In ATC, human factors can help evaluate the allocation of responsibility between controllers and pilots in all of the operational environments. And within the ATM organization, human factors can help analyze and evaluate the allocation of responsibility between controller team members and between controllers and traffic managers. For example, laboratory simulations can be used to explore the feasibility and effectiveness of new roles, such as a planner/coordinator controller role that uses automated tools to support multiple sector controllers. Such a role might combine functions currently performed by a D controller, supervisor, and traffic manager.

Advanced Technologies | At present, the technologies needed to support collaborative TFM are still in conceptual stages. Commercial-off-the-shelf technologies and prototypes exist for facilitating meetings between participants at remote locations but their application to real time operational environments such as TFM will take some exploration [Roberts, Zobell, and Blanchard, 1993]. At this stage, human factors can assist in development of advanced technology that is designed with the express purpose of supporting teams. For example, research has shown that process behaviors tend to degrade under conditions of high workload. Automating team processes themselves may allow teams to perform more effectively [Bowers et al., 1996]. In addition, distributed decision making may require multiple modes of information transfer (e.g., text, graphics, and voice) to ensure successful communication [Scerbo, 1996].

In ATC, U.S. research and development on the next step in the evolution of ATC automation is aimed at automated advisories for final approach spacing and conflict resolution. This level of automation will have a profound impact on the way ATC is performed. Not surprisingly, there is considerable concern in the human factors community over the impact of such automation on the controller’s situation awareness and ability to provide back-up for the automation system when necessary. An alternative approach is being pursued in Europe which focuses more on highly interactive problem solving tools, including display formats that enable the controller to plan more efficient strategies [Makins and Drew, 1995]. In the current environments, the en route and TRACON controllers' displays are optimized for vector solutions to separation and spacing problems [Kerns, 1994]. Alternative formats, such as graphical displays of the temporal distribution of problems and predictive displays depicting a vertical view of the traffic situation, may facilitate situation awareness and strategy planning.

It is likely that both the US and European approaches have application in the operational environment. Before questions on human roles can be addressed, more fundamental questions must be answered regarding the efficiency of the alternatives in different task environments and the nature of the back-up mechanisms. Laboratory simulations can help answer questions regarding the efficiency of controller tools under various traffic scenarios. Requirements for maintaining situation awareness and controller skills will vary according to the need for manual back-up in the future environment [Mavor, Wickens, and McGee, 1997]. Human factors analyses can help specify these requirements under various back-up mechanisms.

Conclusion | There are major challenges ahead in analyzing functional requirements and the allocation of functions between human and automated elements of the future ATM system. At this point, there is an opportunity for proactive research in human factors to gain insight and perspective on issues surrounding collaborative ATM. Multiple research approaches, including the review and application of existing data and knowledge, will make it possible to prove the value of new concepts regarding human roles and interrelationships and their impact on the quality and efficiency of the ATM system. As the discussion above indicates, however, this is a system that has historically attempted to match the capabilities of the people with the demands of the system by distributing roles and responsibilities, and by imposing structure. Efforts to enhance efficiency through increased flexibility and shifts in the locus of control need to be based on an understanding of the complexity inherent in this system, and on a realistic assessment of the strengths and limitations of the technologies being considered to support controllers, pilots, traffic managers and dispatchers in their new roles.

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Glossary

AOC Airline Operations Center
ARTCC Air Route Traffic Control Center
ATC Air Traffic Control
ATCSCC Air Traffic Control Systems Command Center
ATIS Automated Terminal Information Service
ATM Air Traffic Management
CRDA Converging Runway Display Aid
CTAS Center Tracon Automation System
D data
FAA Federal Aviation Administration
FAST Final Approach Spacing Tool
FSSs Flight Service Stations
H handoff
nmi nautical miles
NRP National Route Program
PDC Predeparture Clearance
R radar
TFM Traffic Flow Management
TRACON Terminal Radar-Approach Control Facility