Because operating in UK Class D, a significant element of CAT aircrew, and many GA pilots, have little understanding of the responsibilities pertaining to the provision of ATS. Many CAT crews have an expectation that ATC will ensure that their flight does not encounter conflicting GA traffic, even if pertinant traffic traffic information has been passed to both aircraft, and they can see each other. If a TCAS alert is received, CAT crews have an obligation to respond. This automatically generates a Safety Report from both aircrew and ATS, resulting in a chain of events starting with the pulling of the controller for investigation, and potentially leading to Licence suspension. This has been discussed at great length on another British GA forum.
Some GA pilots operating VFR believe that it is acceptable to fly in close proximity with IFR traffic that they can see and avoid: fine for them, but almost inevitably the CAT crew will file a report. Again, this results in action under the SMS, and, if the number of reports increases, those running the SMS may turn to the ANSP manager and request the adoption of practices that stop the encounters happening.
The guidance being offered recently from the ATS regulatory side is that these conflicts should be prevented, meaing that ATC have to actively segregate or deconflict the aircraft involved.
The upshot of all this is that ANSPs, in order to satisfy the regulators and the local SMS, are likely to become very cautious when granting access to CAS of VFR traffic, and if it is allowed to enter, to proactively manage potential conflicts, using a level of intervention which is far more restrictive that permitting VFR pilots to 'see and avoid'. Access to CAS, and VFR operations within it, have been discussed at length on another British GA forum.
Another consideration that must be be understood is that the ATS system in the UK is not 'national'. ATS operations in many Class D zones, and at those airports hosting CAT which are located outside CAS, are supporting the business of the airport. If the SMS of an airline operating into one of these airports is recording a high number of incidents involving conflict or perceived conflict with GA VFR aircraft, it is likely that the airline's safety department will approach the airport operator for an explanation, or more probably, a resolution that stops their operations being impinged on by the VFR traffic. The business priorities of the Airport, its operator, and its shareholders, will come first. Transiting GA traffic, or the provision of ATSOCAS in adjacent Class G areas, will not generate any income, and will require staffing. If I am managing an airport in the UK, competing with an adjacent airport for high end traffic that pays the highest fees, service provision to aircraft that are not landing at or departing from my airport will not be my priority. I need to keep my paying customer satisfied, and if it means restricting the transit of GA aircraft through our airspace, or minimal service provision to aircraft flying adjacent to it, so be it. I can always use our SMS to claim that I am reducing risk.
There has been much discussion about ATS provision outside CAS, access to CAS, and ATS operations within CAS, on another GA forum. Many of the contributors call for a a level of service provision to the GA community that mirrors that of the USA, or one that offers a level of service that is far in advance of what is available now. What these posters should remember is that ATS outside the airways system in the UK has been privatised, fragmentised, and encouraged to adopt a business-based model. The result is a disconnected service that reflects on the business needs of the airport from which it is provided, and is provided on the basis of the local business requirements and resources. If the GA community believe that this can be replaced by some kind of national joined-up system funded by the taxpayer, they are in clooud cuckoo land.
Assuming the UK complies with the EASA requirements, there will seemingly be an expansion of CAS to accomodate the instrument procedures for those airports presently sitting in Class G. Where the associated controllers are going to come from, and how they will be funded, I cannot say. What I do forsee, however, is an increasing reulctance of ANSPs, for all the reasons I have expanded on above, to permit GA traffic the access to this new CAS on a see-and-avoid basis, and a reduction in ATSOCAS provision to aircraft operating outside CAS, further adding to the level of discontent felt by elements of the GA community.