Air traffic flow management
Air traffic flow management (usually seen abbreviated as ATFM) is the regulation of air traffic in order to avoid exceeding airport or air traffic control capacity in handling traffic (hence the alternative name of Air Traffic Flow and Capacity Management - ATFCM), and to ensure that available capacity is used efficiently.
Because only one aircraft can land or depart from a runway at a given time, and because aircraft must be separated by a certain distance or time to avoid collisions, every airport has a finite capacity; it can safely handle only so many aircraft per hour. This capacity depends on many factors, such as the number of runways available, layout of taxi tracks, availability of air traffic control, and current or anticipated weather. The weather can cause large variations in capacity; strong winds may limit the number of runways available, and poor visibility may necessitate increases in separation between aircraft.
When an air traffic control unit that will control a flight reaches capacity, arriving aircraft are directed towards holding patterns where they circle until it is their turn to land. However, aircraft flying in circles is an inefficient and costly way of delaying aircraft, so it is preferable to keep them on the ground at their place of departure, called a ground delay program. This way, the delay can be waited out on the ground with engines off, saving considerable amounts of fuel. The careful calculation of en route time for each flight and traffic flow as a whole, which aims to minimize overall delays in the air traffic system, is highly dependent on computers.
Operation in Europe
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All IFR flight plans are tracked by a CFMU (central flow management unit). Each airport and air traffic control sector has a published maximum capacity. When capacity is exceeded, measures are taken to reduce the traffic. This is termed regulation. The aim is to use capacity effectively, keeping the average delay as low as possible, while ensuring capacity is not exceeded.
As a (highly simplified) example, if two flights are scheduled to arrive at an airport at exactly the same time, and the airport can handle one aircraft every 5 minutes, the aircraft may be assigned delays to ensure that the second aircraft arrives 5 minutes after the first. Similarly, the first aircraft will be required to depart on schedule and not allowed to depart late. This way, the second aircraft will not need to wait in the air.
In practice, the process is much more complex and highly computerized. One aircraft may be subject to several regulations at the same time. For example, a flight from Amsterdam to Paris may be regulated both by limited capacity at Paris as well as by limited capacity in Belgian airspace.
In some cases, it may be possible to avoid delay by taking a different route. For instance, if Belgian airspace was the only regulation for the flight in the previous example, changing the route to avoid Belgium and going via Germany instead might allow a flight to depart without delay, although the route might be a bit longer. In many cases, airlines authorize the CFMU to make changes in a flight's route to avoid delay.
Certain flights are exempted from regulation. For instance time-critical flights carrying human organs for organ transplantation. If such flights are scheduled, regular traffic will be delayed instead.
If an airport is completely closed unexpectedly (for instance, because the only runway is blocked), a zero rate may be set for a certain time period (e.g. until the runway is expected to be reopened), which will cause all inbound flights to be issued a delay that will cause them to arrive after the reopening time. Flights already en route would either enter the holding or divert to an alternate airport.
- Traffic Flow Management in the National Airspace System (PDF) (Report). Federal Aviation Administration. October 2009. Retrieved 20 November 2016.
- AC 150/5060-5 - Airport Capacity And Delay (Report). Federal Aviation Administration. 23 September 1983. Retrieved 20 November 2016.
- Klein, Alexander; Kavoussi, Sadegh; Lee, Robert (2009). Weather Forecast Accuracy: Study of Impact on Airport Capacity and Estimation of Avoidable Costs (PDF). Eighth USA/Europe Air Traffic Management Research and Development Seminar. Retrieved 20 November 2016.
- "CFMU Objectives". Archived from the original on 17 November 2008. Retrieved 22 May 2008.