None of this is relevant to getting busted for busting CAS. You simply need to be below the area shown on the VFR chart – either using QNH or using 1013. All the stuff about TA or TL is immaterial.
Thanks for that @Peter 
Having been busted >1 year ago for what I call a minute infringement, and in view of the passive UK FIS attitude, I’ve had to revise my pro-active philosophy… looking out for birds sure helps 
I agree TA and TL have nothing to do with this question.
I also agree that flying from high pressure to low pressure and low temperatures needs careful calculation of True Altitude.
But if I understand correctly, this also is not the question.
If you fly at FL055 with pressure at 964hp you are flying at altitude 4177ft (country dependent exam question) or 4030ft (mental arithmatic).
So if you are flying above altitude of 4030 ft with 964 set you risk busting a FL 055 CAS without clearance.
If you are flying A-B on the semi circular rule you should actually be flying at either FL035 or FL045 depending on track and of course we have planned for this and used mental arithmatic un flight to avoid a CAS bust.
Or have I totally misunderstood the question.
If OCAS, the semicircular rule is nothing to do with busting CAS.
Sorry, why not? Are you saying the UK doesn’t apply the semi-circular rule OCAS?
I thought we were discussing flying under CAS whose floor is at FL055 and climbing into it because your altimeter is set on QNH.
Peter wrote:
BTW, my vague recollection from many years ago was that when climbing you change to 1013 at the TL, and then descending to change to QNH at the TA. But I don’t recall ATC ever passing the TL figure; only the TA, and that only in a few places (Croatia comes to mind).
IME ATC usually passes the TL. Either directly or in the ATIS. The UK may be different, of course.
Anyway I don’t know anybody who actually flies IFR (at reasonable altitudes like FL150) and who use TA or TL for the altimeter resetting point.
At some point the rules for altimeter settings were changed so that if you are cleared for a climb (descent) through the transition layer and don’t expect it to be interrupted, you can set 1013 (QNH) immediately. When I started IFR flying in the 80’s you were certainly expected to change altimeter setting exactly when passing the TA going up or the TL going down.
gallois wrote:
Sorry, why not? Are you saying the UK doesn’t apply the semi-circular rule OCAS?
I thought we were discussing flying under CAS whose floor is at FL055 and climbing into it because your altimeter is set on QNH.
Semi-circular is not madatory, it should be used. If you are challenged in the UK by an ATC you could say this is due to constraints of adjacent CAS. :)
The UK completely ignores the semicircular rule OCAS as far as enforcement goes. They also ignore it in CAS, which is ~100% under radar control anyway.
The only place where I was “politely reminded” of it was N France, some years ago. The ATCO would not actually say it; he said something like “are you sure you want to be at FL75?” and after I while I worked it out and requested the “proper” one.
That’s the reality.
At some point the rules for altimeter settings were changed so that if you are cleared for a climb (descent) through the transition layer and don’t expect it to be interrupted, you can set 1013 (QNH) immediately.
which also makes perfect practical sense, especially single-pilot. Except I leave altimeter #2 on QNH (if climbing).
bust controlled airspace
Should we not change the title of this thread? Even over here I sometimes hear warnings not to fly into “controlled airspace” without a clearance. This ignores that airspace E is controlled but no clearance required for VFR. What would be the correct term? “VFR clearance required airspace” ?
Class E IIRC doesn’t need a clearance, only when the ceilings, cloud and visibility are above the limits. Otherwise its VFR special which does need a clearance.
gallois wrote:
Class E IIRC doesn’t need a clearance, only when the ceilings, cloud and visibility are above the limits. Otherwise its VFR special which does need a clearance.
Special VFR is only possible in control zones and a control zone can’t have class E.
