Airport slots

Slot allocation isn't an issue at many airports, as there is enough capacity for airlines to fly at whatever time they'd like. However, major delays would occur at airports, such as Heathrow, where capacity is constrained if airlines wanted to operate too many flights at peak times.

So a system of slot allocation has been developed. A ‘slot’ is a licence to plan the operation of a flight at a particular time - either an arriving or departing flight - and only a limited number of licences (‘slots’) are available for each hour.

In the UK, the slot allocation process is run by an independent slot coordinator, Airport Coordination Limited (ACL), in line with European legislation and long-standing industry practices. There are two scheduling seasons: the summer season which runs from the end of March to the end of October, and the winter season which is the remainder of the year. ACL allocates slots in advance of each season. 

On the day of the flight itself, however, an airline files its flight plan with the air navigation service provider - in the UK this is NATS. The departure time request will take account of any operational problems being faced on the day, for example, foggy weather, and may therefore be different to the airport slot time which that flight has been given.