 Additionally, the aircraft must be placed in a flow consistent with the aircraft's route of flight.This effort is complicated by crossing traffic, severe weather, special missions that require large airspace allocations, and traffic density.When the aircraft approaches its destination, the center is responsible for issuing instructions to pilots so that they will meet altitude restrictions by specific points, as well as providing many destination airports with a traffic flow, which prohibits all of the arrivals being "bunched together".