Extended arrival delays resulting from high demand can cause problems for some operators.  Weather and mechanical problems can cause emergencies to arise at any time.  The AACV can automatically accommodate many of these situations by the use of a priority word in the on-condition ADS-B message.  We could use 3 bits to describe four levels of urgency, granting clearance according to priority first, then position/time precedence:
0) Normal Operations.  Sequence generated as described in Section 2.2.2.

1) Re-request.  Set only by system, not user selectable; priority supercedes initial transition and departure requests.

2) Pilot-initiated Urgency request.  e.g. Low fuel state; requesting aircraft assigned next priority status; “priority granted on basis of urgency request” message transmitted to ATC.

3) Emergency.  e.g. Declaration of an in-flight emergency; priority status of current “priority aircraft” rescinded if practical (for instance, priority aircraft has not yet entered the ACV) and earliest possible clearance of emergency aircraft for the approach; “declaration of emergency” message transmitted to ATC for investigation.

2.4 Extensions Beyond the One-In/One-Out Concept 

The next step in the development of automated approach and departure procedures, allowing more than one aircraft into the ACV at a time, depends on tools which enable pilots not only to self-separate, but also to order and merge themselves on or near the approach.  Self-Spacing concepts under development at NASA Langley may provide such aids11.  These tools may help pilots adjust their flight using speed and path guidance generated from interval and target-referenced data rather than ground-referenced instrumentation.  This would allow a pilot to effectively fly a similar guidance cue to today’s course deviation indicator, but by maintaining both course and speed guidance, they would also be assuring separation from the reference target as confirmed by the cue-generating automation.  
2.5 Regulatory considerations

As this procedure's main purpose is to provide separation assurance, all of the airspace within an ACV would necessarily be defined as controlled airspace in order to exclude non-participatory traffic.  Mixing AACV operations with standard IFR operations cannot occur without full surveillance of all traffic targets, or a means to share IFR airport usage/cancellations data between ATC and the AACV.  Preferably, the AACV will have surveillance data for all traffic targets in the area. Assuming an ADS-B-based system, mandatory use of ADS-B transmitters in the vicinity of an active ACV would be one solution. Alternatively, a combination of ground-based passive (or active Mode-C based) technology and airborne reception equipment could supplement those targets not ADS-B equipped.  In fact, these types of systems could be the primary means of surveillance, though they would most likely be cost prohibitive for many municipal facilities.
　
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