Each center tracks flight plans for aircraft which will pass through or near its geographic area. If a flight passes through multiple facilities, its flight plan must also be passed. Usually, this is also accomplished automatically via computer. The receiving center’s computer must then assign a radar beacon code to the flight. The transferring computer passes the beacon code used by the flight to the receiving computer with the flight plan. The receiving computer is programmed to keep the flight on the same beacon code, if possible. However, if that beacon code is already in use at the receiving facility, the computer will be forced to assign the flight a new beacon code. This is why a controller might occasionally change your transponder code enroute.
Each center also has its own traffic management (TMU), commonly referred to as flow control. TMU controllers manage the flow of air traffic so that a single controller or facility does not become overloaded. They also deal extensively with the staff meteorologist in order to move traffic around areas of active or potential severe weather.
To accomplish these tasks, the center may initiate a ground stop or delay program for a particular airport. TMU will take these actions whenever the number of arrivals for a particular time frame at an airport is expected to be above the capacity of the air traffic control facility in charge of that airport. A ground stop, just as it sounds, is a stop of all traffic that has not yet departed for that airport. In a ground delay program, the TMU delays departure traffic so that arrivals are dispersed from arriving at a concentrated time frame.
At large airports, traffic jams often occur during busy times. By delaying traffic on the ground, in flight holding for the arrival airport can be reduced or eliminated. Another common reason for a ground stop or ground delay program is weather. During times of poor weather, approach controllers are forced to use more restrictive separation requirements. This lowers their capacity to receive airplanes. The TMU and center controllers work together to ensure an orderly, even flow of traffic into each approach control and tower facility.
TMU controllers manage departures busy airports by requiring the controller in charge of the flight to call the TMU prior to releasing the flight for departure. When the pilot requests to depart for a busy airport, the approach, tower, or center controller handling the flight calls the TMU to request a release time. The TMU controller then looks at all the flights inbound for the busy airport and finds a space for the departure. The controller in charge of the flight is then given a release and void time for the flight. This controller does not have a picture of the flow and simply departs the flight between the release and void times. Often times, the local controller handling the flight will be in communication with TMU requesting updated information.
In addition to managing traffic for particular airports, TMU controllers also manage traffic through each sector of airspace. Most of the time, a particular sector of airspace becomes busy due to weather closing down airspace. If a particular sector becomes closed or busy, TMU may move traffic vertically or laterally.
TMU controller’s have several options in addition to delaying traffic on the ground. On commonly used option is to issue a spacing requirement for an airport. In this case, traffic for an airport is required to be spaced by a certain mileage or minutes in trail. Controllers meet these requirements by issuing vectors and speed assignments.