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 only safely handle 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.
Read more about this topic: Air Traffic Flow Management
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