Too many aircraft in the air at the same time and place can lead to an unsafe situation. One of the tools used by the Network Manager Operations Centre to prevent this from happening is to apply CTOTs (calculated take-off times), which is also known as issuing ATFM slots or simply slot.
The slot is actually a period of time within which take-off has to take place. In Europe, (EUROCONTROL’s area of operations), a slot is defined as the period between 5 minutes before and 10 minutes after the CTOT. The aircraft is required to be at the runway, ready for departure at its CTOT. The leeway is to allow 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), the Network Operations Centre 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. More technical means of communicating slot information are available, such as operational applications and computer-computer interfaces.
Some may be surprised to know, for example, that 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. However, it is often wrong to blame a delay on the departure airport or the airline, as capacity limitations on the airspace between the two aerodromes, in the en-route segment, can also be a reason for delays.