DFC
I was aware that slots don't provide separation but my most recent involvement in an ATC's loss of separation was on an overloaded sector so there is a link between flow management and the ability of ATC to provide the services they they are required to provide in various airspaces.
There is a link but slots are wide (15 minutes) and there might be a wide variance on actual times to get to the sector from those planned due to high winds or reroutes so it's not a guarantee that a sector won't be overloaded. It's not impoossible to have all your hours worth of traffic in the first half of the hour bunched together. Overload is also something which is not necessarily linked to a numerical value. A controller might be overloaded when working at less than 50% capacity if a whole bunch of things go wrong. At the end of the day there is a definition of overload which controllers can use to assess whether they think they were overloaded or not. The flow figures, slot busting, etc will be used as supporting evidence in any incident investigation but might not necessarily have been the prime cause.
You said: The priority will be to ensure the safe and efficient flow of IFR traffic in that area and nats said By staying VFR, priority will be given to IFR flights
Asuming the flight had filed a VFR flight plan before departure i.e. was operating normally in accordance with a filed flight plan........where does it say that IFR flights have priority in class D TMAs?
No one wants to get in anyone's way. But not operating on the long established (and sill used by the CFMU) rule of first come first served is operating outwith the rules I believe.
The IFR traffic is already in CAS, is already working the sector controller who will have to give the ATC clearance to the VFR joiner, and is already forming part of the ATCOs existing capacity. Ergo, they are first come first served and take priority
Of course the VFR flight (with a flight plan) could choose to hold awaiting safe clearance. How long would you hold the VFR flight for before you end up with the VFR flight stating PAN, divert me, I am low on fuel?
If it's the peak time of the day, then it could be a couple of hours ... if it's just a holding peak being dealt with then maybe 30 minutes or so. Much easier for the VFR pilot to use airmanship in that case, descend to remain clear of the Class D and enter the Class E where no clearance or communication is required
Don't forget that there is no requirement for a VFR flight to carry diversion fuel - simply destination fuel plus final reserve. It would be a good idea to carry a bit extra to allow for delay in gaining access to the TMA (20 minutes?) but that is it really.
If the pilot has an emergency due to low fuel then all he has to do is shout and the waves will part. The CAA can deal with any fuel planning issues after the event over a cosy chat with the pilot.
One good reason for filing VFR and compying with the more onerous weather requirements is the fact that the fuel load can be reduced!
As entry to CAS is never guaranteed without a delay, then that's a calculated judgement on the part of the pilot. Mostly it will work out well but if it doesn't then they should already have briefed themselves on what Plan B is.
Don't forget the TMA is established for traffic into and out of the TMA airfields and by NATs' own definition, Cumbernauld is a STMA airfield.
At the moment Class E caters well for the VFR traffic inbound to TMA airfields (or overflying) which doesn't want any delay. When it changes to Class D on November 24th (?) then entry for VFR traffic at levels below 6000' should still be achievable for the vast majority of the time, or the new raised bases can be used to fly under CAS VFR with no need for a clearance or communication. Entry above 6000' will still be possible as well, it's just a case of avoiding the peak periods when the airspace is saturated. In the case of airspace around TLA the rough peaks are 0700-0930 local and 1600-1900 local. Of course, bad weather, European and other UK slots, etc, can cause knock on delays to airline rotations and move these peaks around or extend them. But as a rough guide I'd try and avoid these times if you want the best chance of a clearance.
Spoke to someone today that told me the reason why ATC (at an ACC) will try to separate VFR flights from IFR flights eg in the STMA and suchlike places is that the separation is monitored by a computer that does not make any allowance for VFR/IFR. Thus if ATC place you 2nm to one side of another flight at the same level, the system reports a loss of separation even if the two are VFR. This means an investigation even if it is a short one. So ATCOs do not want the situation. Perhaps ATCOs can confirm this separation monitor?
There is a Separation Monitoring Function (SMF) in use at NATS units, however there is capacity for completing the short reporting form to signify that the encounter involved VFR traffic and is thus a non event. The investigation involves the Supervisor viewing the encounter on a replay and asking the ATCO involved for their justification. Takes a couple of minutes at most (unless it's an actual event of course !!). I don't think the 'snitch' would prevent anyone from getting traffic closer than IFR separation just because they run the risk of a tap on the shoulder. As long as any required separation is provided there's no need to fear SMF. And for VFRs the required separation is usually at the pilots discretion