I had an interesting exchange yesterday.
London Control cleared me to leave CAS by descent.
I then descended through FL075 which was the CAS base. But later, at -500fpm, I would have infringed the 5500ft CAS stepdown, so I turned west to continue descent outside that.
I then asked whether had I continued, it would be a CAS bust. During a previous such conversation, I got a vague reply saying it would probably be ok. This time, the controller told me that once “cleared to leave CAS by descent” you can re-enter it during the descent.
The interesting thing is that one guy got MORd and possibly busted by the CAA for this, in the MOR reports I “get”. Of course I got yesterday’s exchange recorded to mp3. This discussion has been squashed on the UK sites (they are dominated by NATS/CAA people in disguise) so LTCC must be reading EuroGA 
Peter wrote:
I then asked whether had I continued, it would be a CAS bust. During a previous such conversation, I got a vague reply saying it would probably be ok. This time, the controller told me that once “cleared to leave CAS by descent” you can re-enter it during the descent.
Did you still have your original squawk at that time or had you changed to 2000? (Or 7000…)
I retained the London Control squawk (6474, curiously, issued by LC on the handover to them) all the way to tarmac.
That is prob99 the key to not getting busted because the CAIT software cannot know what clearance you have; it must just compare your txp code against the airspace and a list of txp codes which are valid for that airspace. There could be further integration, with ATC typing in a clearance limit, but I cannot see anybody spending the money on that, just to catch a very small number of GA pilots who fly IFR in CAS; maybe 1% of GA.
If London Control asked me to set 7000 (or “conspicuity” as it is now called) upon leaving the FL075 CAS, and I clipped the 5500ft CAS, that would have triggered a CAIT report and then some “internal procedures” would have been carried out on whether to MOR it or not; ATC is not allowed discretion on whether to ignore that.
Nothing happens if you stay on airways squawk as far as CAIT is concerned, but ATC could still flag it as bust, especially if it happens on other sector with no handover, although they likely won’t notice
Even on 7000/2000 you have 15 seconds to exit CAS that is 200ft margin 
Do you have a reference for that?
No reference but when CAIT was introduced it used to filter out some codes (7010 was one that my old club used for tugging gliders in CTR without Class D clearance but has changed later on to some code in BGA/NATS letter of agreement), the 10s-15s, I read that in some technical paper but I am lazy to fetch it up…
While I am not sure on the legalities, it happens over here all the time – on every descent from above FL95 one ends up clipping the edges of the TMA reverse wedding cake, unless one opts for a steep descent (within or outside of controlled airspace). I’ll ask how it works from the controller’s POV next time I run into one.
Does it also work for nipping polish military airspace on descent
Any reasons why one can’t keep his airways code on descent? I keep mine until circuit or engine is shutdown, unless when I get to talk to Southend or Thames, I don’t recall anyone have complained about this?
I gave up on calling LARS or FIS for the last 20nm under LTMA, it’s just too distracting with zero operational benefit
Any reasons why one can’t keep his airways code on descent?
If instructed to set 7000 then No, but if you “forget” to set 7000 they can’t do very much since you will have changed frequency Actually I have genuinely forgotten to set 7000 quite a few times…
Yes that is probably a good tactic.
You can set 7000 on CAS boundary, not before but later on one can flip from 7000 they were instructed to use to 2000/5050 without asking anyone? why putting back your FPL sqwaks for IFR OCAS is problematic?
I really think no one give a cent or mind and it’s unlikely to cause any harm as long as you remain outside controlled airspace, anyway, one will always be busy flying single pilot IFR in IMC after getting dumped OCAS trying to avoid airspace, terrain, traffic…surely setting 7000 in the box is very down the list of priorities, one can always do it when they have runway in sight while on final or bellow radar altitude…
