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In practice, reordering the priority list in the interest of efficiency should, however, be limited to inserting small numbers of departure and transition operations between previously ranked arrival requests and deferring departure requests when predicted holding time for arrivals becomes excessive; the ranking of arrival requests should not be altered in relation to each other.  The reason for this is simple: arriving pilots need some idea when they can expect to receive an approach clearance in order to make potentially critical decisions regarding fuel management and diversion to an alternate destination.  After pilots have been advised of the EFC time, the relative order of arrivals must be preserved in order for this information to remain valid. 
Although the priority list is created on the basis of valid clearance requests by aircraft in the immediate vicinity of the ACV, it may also be advantageous to develop forecasts for airport demand that can be used to generate delay estimates well in advance.  Flight plan data, required enroute updates and data from the FAA’s HOST computer or other ground-based surveillance systems can be used by the AACV automation to periodically compute and update predictions of the sequence of arriving aircraft, periods when the airspace may not be available for departures, or when overall demand on the airport exceeds its capacity.  This information can then be disseminated in the form of pilot advisories.
2.2.3 Automation system and supporting message exchange

As noted above, all automated communication, sequence generation and clearance granting functions are performed by a local AACV data processing system.  All participatory aircraft will be required to be ADS-B equipped, with a 40-mile transmission capability (although preliminary studies suggest that a transmission capability as low as 20 miles may be sufficient). ADS-B will provide the primary means of communication between the aircraft and the AACV system, providing general data transfer and surveillance functions. For our initial implementation, we have attempted to use a minimal message set, assuming substantial cost for all transmitted data.  
For Approaching Aircraft:  A specific on-condition request report would be designated, containing Aircraft ID, intended airport facility, and request code (landing or transition).  This message could be generated by the on-board flight planning and/or navigation avionics at a specified point on the flight path (e.g. 20 NM to destination).  “Priority granted” messages will be transmitted to the aircraft through data link and will consist of airport ID, aircraft ID, time, IAF assignment, and a single priority bit. “Priority denied” messages will consist of airport ID, aircraft ID, EFC time, and a single denial-of-priority bit.  The denied aircraft would re-request at the EFC time to minimize message traffic.  This time is also useful in the event of lost communications.  An “operations complete” message could be used to unlock the airspace. Optionally, ADS-B surveillance data transmitted from the priority aircraft indicating the aircraft’s velocity has dropped below a specified threshold for a specified duration could perform this function. State data as described by RTCA DO-2426 (Time, Lat, Long, altitude, ground-referenced velocity vector, ID, category, navigational data quality) will be sufficient input for the ground-based arbitrator to calculate priority.
　
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