Tasks in Air Traffic Control6
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 safely 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.
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 TRACON's 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 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). 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 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-ofthe-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 humanautomation 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 1 970s-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, 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.
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 facilityspecific 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.
Prepared by National Academies Press (NAP)
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