We note in the last posting reference to NAS 2C, that one of the proposed changes in the next airspace change tranche is to amend Class D procedures. Our understanding from examining the NAS documentation is that this will involve a change in VFR entry procedures, so that simple acknowledgement of a call by VFR aircraft at the boundary may be taken as a clearance (implicit rather than explicit) to enter Class D airspace (on-line at the NAS website via Airservices Australia’s web-site) .
We have reviewed your AIP (on-line at Airservices Australia) and note that the current procedures require VFR flights to obtain a positive clearance to enter Class D airspace.
We note that the NAS 2C changes will not amend airspace boundaries for Class D – i.e., VFR flights may enter Class D airspace at a boundary distance of more than 20 nautical miles. At that distance it would be impossible for air traffic control to positively sight and identify the aircraft concerned, and provide a meaningful service between IFR and VFR flights.
We have examined the United States procedures (AIM and ATC Documents (7110.65)) and note that Class D airspace in the United Stated most often extends to a maximum of 7 miles (typically 5 miles) and upper limits in the range of 2000 feet at each of the several hundred Class D towers (refer also FAA web-site at
www.faa.gov). As is the case with aircraft entering your GAAP airspace (ref Australian AIP) it would definitely be possible for a controller to sight, identify, and provide services in respect of that aircraft, even with an implicit clearance.
This begs the question of the potential changes in risk that would be associated not only with the implementation (as has been claimed in respect of other changes) but also with the actual design itself.
We referred again to ICAO DOCUMENT 9689 - MANUAL ON AIRSPACE PLANNING METHODOLOGY FOR THE DETERMINATION OF SEPARATION MINIMA. That document contains an appendix submitted by Australia which outlines an airspace risk model for small terminal areas – CTAF and MBZ.
We understand that in 1996, attempts were made to change the Class C airspace associated with 12 regional non-radar towers to Class D airspace. We have been provided information that indicates that there e was significant community opposition, and significant debate, which forced a review of that decision. A particularly detailed risk study was conducted, a 1997 copy of which was provided to one of our members.
The report of that study indicated that the change from Class C to Class D would increase risk in the order of ten-fold. In some cases, this brought the risk close to (but not above) the limits of tolerability – however Airservices Australia was able to implement the change to Class D in 1997.
The study built on the airspace risk model that had been presented to, and accepted by, ICAO. It is therefore reasonable to say that the modeling carried out on Class C and Class D airspace would fall within a statement that “it was based on ICAO guidance material”.
We have examined the model and find that it contains a number of fault tree decision points, the likelihoods of which were determined from industry panel inputs.
It would therefore be relatively easy to re-assess the likelihood of certain events occurring if VFR aircraft were NOT subject to positive clearance – i.e., were potentially likely to enter controlled airspace by accident. A change in likelihood in just a few decision points can have dramatic effect on risk – to the point where the risk at a small number of the Class D towers would EXCEED the tolerable risk limits.
It should not require a great deal of effort on the part of Airservices Australia to re-model the proposed changes and provide a public risk statement.
We are not in a position to directly intervene here, but we believe that you should challenge your service provider both to re-release the risk analysis already done on Class D airspace – AND to carry out the re-analysis based on the changed procedure.
Summarizing, it is our considered judgment that without a commensurate change in the size of the Class D airspace to replicate the United States situation, the levels of risk to both IFR and VFR flights in Class D airspace would elevate significantly and may exceed already established risk tolerability limits.
Our apologies.
We omitted to add in our previous post that should the Class D airspace limits be amended to those applicable in the United States, a commensurate increase or lowering of Class E airspace would be required (as applies in the United States). This would result in the potential for unannounced VFR flights to transit above Class D towers at 2500 feet.
In short, not only will the risk in residual Class D be raised, but the risk in the volume previously categorised as Class D - i.e,. that airspace that becomes Class E - would increase even further than had it remained Class D with new procedures. The risk in Class E airspace between 2500 feet and 4500 feet can easily and validly be modelled within the model agreed by ICAO and referred to in the previous post (the Class D risk analysis conducted by Airservices Australia).
We re-iterate that it is our considered judgment that the levels of risk at certain Class D locations may rise above previously agreed risk thresholds, and in the risk workshops being conducted by your NAS teams, you must challenge them both to release that study, and commit to a re-calculation of design (and not implementation) risk.