Some further thoughts re why this airspace model.
At BRM, ERSA says:
FLIGHT PROCEDURES
1. During published HRS CAGRS is provided to aircraft operating within the following designated broadcast area:
a. Horizontal limits: 30NM radius centred on YBRM/DME
b. Vertical Limits: ground level to any height above the AD at which an aircraft may conflict with another aircraft that is arriving, departing or carrying out local operations at the AD.
2. Pilots of arriving aircraft shall broadcast on the CTAF at 30NM
3. Pilots of outbound aircraft shall monitor the CTAF until 30NM
4. Refer to CASA Instrument 490/05 for broadcast requirements at AD when a CAGRS is in operation.
In short, currently all traffic within 30NM is known, because radio carriage and use is mandatary for all aircraft within 30NM.
So putting in a Class D tower and "standard" Class E overhead would actually be a degradation from what is the case now. All traffic within 30NM would no longer be known, and in particular "standard" Class E would mean no communication required with ATC for VFR - only at best monitoring - and as previously outlined even no-radio types could be in the E.
Either CASA OAR recognised this degradation in service or it was pointed out to them, and to get around the problem they'll revise the broadcast area Instrument to make the E 2500-4500 Class E+.
However this upper limit of the broadcast area being 4500 is interesting, given ERSA item 1b above
