From memory part of the justification to roll back NAS 2B – specifically E over D - was an Air Services (AsA) Airspace modelling report based upon Albury and the D/E airspace design under the 2B changes. The report used the AsA software program, which calculated the various collision pairs, level of safety etc. This was based on various inputs including aircraft numbers, flight category, mistakes rates of pilots, controllers etc. I’m sure someone reading this knows more about the actual system than I, please feel free to expand on this description. The report demonstrated that the risk level in E over D was marginally higher that the accepted ALARP level.
The problem was AsA intentionally manipulated the inputs to achieve the required outcome, specifically the VFR pilot mistake levels were artificially inflated to make them appear to pose a greater risk than they actually were. Several industry groups wrote to AsA formally complaining about this assumed incompetence of VFR pilots. And the controller/IFR pilot mistake levels were artificially lowered so that IFR/IFR conflicts would never enter the equation, and that controllers were able to administer the large area of airspace at Albury. I'm an IFR pilot with 20+years of experience,. The rate of mistakes AsA allowed for IFR pilots almost made me fall off my chair when I saw how incredibly brilliant AsA considered me to be!!
Problem was the VFR rate couldn’t be inflated too high because then the risk in the Class D airspace would also become intolerable.. So it had to relate only to VFR aircraft overflying Albury and remaining in Class E. But the problem was there wasn’t enough VFR only flights overflying Albury and remaining in Class E. The number of aircraft initially used was based on factual data, historical figures ie facts.
No problem here, AsA decided off their own back that VFR overflight of a Class D was a new procedure that must be taught, therefore the numbers of aircraft in that airspace must surely increase. Note that this training requirement was never identified by the NAS team, or CASA, it was never published in any syllabus of training for any licence. AsA decided to increase the numbers off their own back, until – guess what – the ALARP level was exceeded. Oh dear, we will have to reverse the airspace.
Oh by the way, during all of this 2B rollback, did you know that AsA was awarded a contract by the FAA to run several class D towers in the USA? Utilising the same airspace model they claimed was unsafe in Australia? And no, class D towers in the US don’t have radar, their tower controllers aren’t required to be radar rated, they administer the airspace within about 5NM of the aerodrome. And yes, they are a damn sight busier than Australian Class D, with 121/135 and 91 operations.
So in all reality the rollback was never based on safety, it was purely industrial.