The Airport Control Volume is similar in concept to a class E surface area, though the ACV would also include the Initial Approach Fixes (IAF’s) associated with approaches at the airport and would be restricted to typical initial approach altitudes (see Figure 1).  In order to accommodate the FAA’s basic RNAV approach, the ACV is defined nominally as 12 NM from the airport, from the surface to 2500’ AGL. As with class E airspace, there is no reason for the ACV to be effective when the airport weather is above VFR minima, although some of the associated equipment may prove  useful as an aid to visual separation during operations in VMC when the exclusionary rules do not apply.

 Figure 1: Airport Control Volume (ACV)
2.2.2 Access protocols

Arriving aircraft must request a clearance to enter the ACV and they must remain clear of the ACV until clearance to enter has been granted.  Either the pilot or the automation will initiate the clearance request based on a specific triggering event such as distance or time to destination.  Similarly, departing aircraft must also request a departure clearance, and shall remain on the ground, clear of active runways until a departure clearance has been granted.  Aircraft can also request to transition through the ACV enroute to another destination.
Only one aircraft (referred to as the “priority aircraft”) at a time will be granted access to the ACV by the automated system.  Aircraft will be granted access to the ACV by the automated system in order of their priority as established in accordance with a mutual exclusion protocol that locks all aircraft out of the approach/departure airspace while it is in use by the priority aircraft.   Once the priority aircraft is clear of the ACV, sequence is reestablished among all known requests, and the new priority aircraft will receive an automated clearance message granting access to the ACV.
Approaching 
aircraft must request a clearance prior to entering  the “approach airspace”.  Aircraft may proceed to IAF only after clearance is granted.

AC

ApproachAirspacelocked. Onlyone operationat a time ispermitted

Although the specifics are a matter for further study, the protocol design will need to accommodate special circumstances arising from events such as aborted takeoffs and missed approaches.  There are a number of ways these events can be accommodated, but for the present study it suffices to recognize that the ACV will have to remain locked until the situation is resolved.
The priority list is generated by a sequencing protocol in the local automation system based on requests for approach, departure or transition operations.  The ranking algorithm will take into account a number of factors in creating the ranking. Even with the simple  “one-in/one-out” operational model, it is possible to order requests to make better use of the airport resource.  A simple study of arrival sequencing scenarios showed as much as a 46% decrease in the summative delay taken by all participating aircraft by assigning priorities to all requests received within a particular period on the basis of airport usage rather than in the order the requests were received.  The type of operation, the position of arriving aircraft in relation to the IAF’s, aircraft ground speed and the available approach profiles must all be considered in determining the ranking which make most efficient use of the airspace because each has an effect on the duration of the requested operation.  For example, it may be more efficient to assign a departure or a transition request (which are relatively quick operations) a higher priority than an approach request that was received earlier.
　
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