Dla wykopowych znafcuf:
In certain special cases, controllers may reduce separation below the usually required minima.
In the vicinity of an aerodrome
Aerodrome or "Tower" controllers work in tall towers with large windows allowing them, in good weather, to see the aircraft flying in the vicinity of the aerodrome, unless the aircraft is not in sight from the tower (e.g. a helicopter departing from a ramp area). Also, aircraft in the vicinity of an aerodrome tend to be flying at lower speeds. Therefore, if the aerodrome controller can see both aircraft, or both aircraft report that they can see each other, or a following aircraft reports that it can see the preceding one, controllers may reduce the standard separation to whatever is adequate to prevent a collision.
Under its SESAR project, Eurocontrol is optimizing the slack of the separation, reducing it from 3 to 2.5 nmi (5.6 to 4.6 km): a simulation increased throughput by up to 14% at Zurich’s Runway 14, which cannot have any high-speed exit, inducing a longer runway occupancy. "