When must the approach clearance for Vicksburg be issued?

The approach clearance for Vicksburg must be issued prior to the DORTS center estimate because this timing is critical for ensuring that the aircraft is properly sequenced into the approach pattern. The DORTS center estimate refers to the point where the center air traffic control provides its estimated time for an aircraft's arrival at a particular fix or waypoint. By issuing the approach clearance before this estimate, controllers ensure that pilots have sufficient time to prepare for their descent and approach, thereby enhancing safety and efficiency in managing traffic.

Issuing the clearance at this juncture allows the pilot to make necessary adjustments and comply with air traffic protocols without the pressure of imminent arrival, which is essential in busy airspace. This procedure aligns with standard aviation practices where efficient communication between pilots and air traffic controllers is paramount for maintaining safe and orderly flight operations.