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.
Read more about this topic: Air Traffic Flow Management
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