@peter, there is a fresh SI from the CAA
CAP493 SI 2021 04
Page 4:
Note 1:When considering whether an infringement of controlled airspace has occurred, controllers are not to consider aircraft total vertical error (TVE) and errors introduced by ATS surveillance systems in transmitting and processing SSR Mode C/Mode S altitude data
Note 2:Controllers should take into account in their RT exchanges with pilots that the level displayed on the aircraft’s altimeter might not be exactly the same as that displayed to the controller on ATS surveillance systems when the appropriate barometric pressure has been correctly set.
There was some paper from the CAA as well where they said something in the line of: if touch a CAS while on your own – MOR, if you are in contact with an ATCO, then some tolerance is applied. Again, can’t find it.
That’s very interesting, because allowing people to fly at the base of CAS implies making an allowance for TXP error.
Otherwise, 50% of people doing it would get busted 
So now the CAA is finally applying the ICAO principle that if you have two adjacent airspaces, the classification of the boundary is that of the less strict of the two.
if touch a CAS while on your own – MOR, if you are in contact with an ATCO, then some tolerance is applied. Again, can’t find it.
Interesting, too, but it would have to be a radar controller. And, gosh, this means that ATC are allowed to do their job, and being easy with traffic which is not conflicting – like they do elsewhere in Europe Nooo, that will never happen
Peter wrote:
That’s very interesting, because allowing people to fly at the base of CAS implies making an allowance for TXP error.
Very fine but important difference: Are people allowed to fly “at the base of CAS” or “at an indicated altitude that if correct would indicate the base of CAS”?
Peter wrote:
If someone knows more detail, they should post it here where people will see it, instead of a private site read by (when I was there, 40 people)
Only during the last 7 days there were more than 250 active users on that forum.
Per ICAO, you are allowed to do what I posted e.g. fly at 2500ft in a 2500ft-base TMA.
What ATC see you at, depends on the various equipment errors, and those are supposed to be within certain limits, which ATC is supposed to allow for, and which airspace design is supposed to allow for (e.g. having CAT at 2600ft in a 2500ft-base TMA would be dumb).
250 active users
They must be counting hits… I saw the stats over 10 years. They had a few hundred members and only 40-50 ever visited the forum. It was a discussion topic, and was simply accepted. Most people just never visited. They joined up, and forgot about it. It’s the way these things run. Look at our fly-in telegram groups. You get 50 people, of whom 40 actually never participate.
Malibuflyer wrote:
Very fine but important difference: Are people allowed to fly “at the base of CAS” or “at an indicated altitude that if correct would indicate the base of CAS”?
My understanding is that the rules about boundaries are absolute. The offence, or non-offence, is where you actually are. What some tool (be it CAIT, or your altimeter, or your GPS) says about where you are may be useful, but is rarely definitive and will always be argued about in individual cases. But it’s quite clear to me because the offence is actually infringing controlled airspace, not causing a transponder output to be propogated which suggests an infringement.
It is a good thing that they’ve seemingly acknowledged transponder tolerances as an issue. That said, I don’t understand why any pilot isn’t paying close attention to how well his transponder FL output corresponds to his altimeter when set to 1013. I have once or twice flown an aircraft with a notable discrepancy (i.e. more than 100ft) and have, when liaising with ATC, proactively made them aware of it. They have always been clear that they’d rather you flew at the right altitude on the altimeter and they’ll deal with the erroneous transponder output.
Malibuflyer wrote:
Very fine but important difference: Are people allowed to fly “at the base of CAS” or “at an indicated altitude that if correct would indicate the base of CAS”
The indicated altitude (assuming correct altimeter setting). There is nothing else they could do, and nobody can tell the difference. There is no requirement to apply the permissible altimeter error.
In general, airspace and procedure design takes into account both instrument and flight operational error to ensure safe separation. Airways are (traditionally) not 4 or 5NM wide because aircraft inside fly close to the edge, but because they were designed around the errors or VOR navigation and later RNAV5; and the buffer from CAS boundary to the lowest usable level within CAS of 500ft exists for the same reason.
Graham wrote:
That said, I don’t understand why any pilot isn’t paying close attention to how well his transponder FL output corresponds to his altimeter when set to 1013. I have once or twice flown an aircraft with a notable discrepancy (i.e. more than 100ft) and have, when liaising with ATC, proactively made them aware of it. They have always been clear that they’d rather you flew at the right altitude on the altimeter and they’ll deal with the erroneous transponder output.
It is not always possible do check – see integrated avionics. The only way is to a transponed calibration from time to time with a Radar ATCO.
And about the Altimeter vs Transponder – it depens. For me it was the other way around – I’ve been asked to go down by 100ft (while I was flying night IFR in IMC!) so that I stay below LTMA on their radar screen, so as not to set off the alarm. It really depends…
Peter wrote:
Per ICAO, you are allowed to do what I posted e.g. fly at 2500ft in a 2500ft-base TMA.What ATC see you at, depends on the various equipment errors, and those are supposed to be within certain limits,
If all pilots would consider and respect that, altitude related busts would happen quite rarely.
Not only “what ATC see you at” but also “what you see you at” depends on the various equipment errors. Being aware of this errors, you obviously can not fly at an Alt- indication of 2500ft but need to stay below an indication of 2500ft minus all potential error margins. Only by doing this you can be sure that you actually stay at or below 2500ft and actually being at 2500ft is “the worst case”.
In that “worst case” it is quite likely that the altitude encoder of the transponder also shows something below 2500ft (as some of the instrument error is related to the static system of the plane and hence go in the same direction) but there is a slight possibility, indeed, that it shows above 2500. But quite unlikely.
I think that most of the altitude busts do rather come from the fact that pilots do not respect your ICAO definition but rather believe they are allowed to fly 2500ft indicated and therefore – due to equipment errors that make their altimeter show a too low altitude – are actually busting the airspace.
Graham wrote:
My understanding is that the rules about boundaries are absolute. The offence, or non-offence, is where you actually are.
Cobalt wrote:
The indicated altitude (assuming correct altimeter setting).
Interesting! Two opposing statements!
Despite all this, the vast majority of busts here are not people flying at 2500ft and showing up as 2600ft on ATC displays (and getting MORd). The vast majority are people who know where the CAS is but getting distracted.
It’s actually very hard to prevent, especially if flying with passengers. The simplest way is to plan a flight via database waypoints, all at one level, fly it on autopilot, and do no sightseeing.
And there are lots of ways to get bitten. Look at this proposition. You want this CAS transit, from bottom to the top.
You have to start below 3500ft because the EGMC airspace is overlaid by Class A 3500ft+ and under VFR that is closed. They probably have no authority over the overlying Class A anyway – or do they? When I do this transit I always record ATC comms, and of course the GPS track, and get ATC to confirm my txp altitude.
If you started a bit more west, say at Lamberhurst, then you will be heading for some 2500ft CAS, and they can’t clear you into that, even if you have the full IR. AFAIK they will never get an ok from London Control which owns all that Class A.
And similar cases near Solent (who are actually quite an unhelpful unit, nowadays, generally, on top of London Control refusing them to allow traffic in the CAS at FL70+ above them (which is not their property).
So many gotchas…
