Air traffic flow management
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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, and to ensure that available capacity is used efficiently.
Reason for use
Because only one aircraft can land or depart from a runway at the same time, and because aircraft must be separated by a certain 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. Especially the weather can cause large variations in capacity because strong winds may limit the number of runways available, and poor visibility may necessitate increases in separation between aircraft. Air traffic control can also be limiting, there are only so many aircraft an air traffic control unit can safely handle. Staff shortages, radar maintenance or equipment faults can lower the capacity of a unit. This can affect both airport air traffic control as well as en-route air traffic control centers.
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. Because aircraft flying in circles is an inefficient and costly way of delaying aircraft, 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. Obviously, careful calculation of en route time for each flight (and the effect of current wind upon it) and traffic flow as a whole is needed, which is highly dependent on computers.
Operation in Europe
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.
Slot and calculated take-off time
The CFMU issues delays by means of a CTOT (calculated take-off time), also known as slot time or simply slot. The slot is actually a period of time within which take-off has to take place; in Europe (Eurocontrol) it is defined between −5 and + 10 minutes from CTOT. The aircraft is required to be at the runway, ready for departure at its CTOT, the leeway is for air traffic control to integrate the aircraft into the other traffic.
If a slot is missed (or if it is already certain in advance that it will be missed), CFMU assigns a new one. A different aircraft which has a slot because of the same regulation may be issued an improvement on its slot to make use of the newly available capacity. The slot and any revisions are communicated to the aircraft operator as well as the air traffic control unit at the departure airport via a special network called AFTN.
It is perhaps surprising to some, that, for example, a delay in Istanbul may be incurred because inclement weather is expected at the destination in London, 3 hours later, even though the weather in Istanbul is good and there is no congestion. Blaming a delay on the departure airport or the airline is often not correct. Capacity limitations of the airspace between the two aerodromes, in the en-route segment, can also be a reason for delays.
- "CFMU Objectives". Retrieved 2008-05-22.