martin-esmi wrote:
I meant that the altitude pre-select is transmitted (I believe it was).
Oddly enough there’s no explicit mention of this in the C172S AFM(S). There is one supplement that lists the various standards and TSOs that the installation meets and it does mention “enhanced surveillance”, but that’s all.
Airborne_Again wrote:
What behaviour? That both the altitude preselect and the VNAV target are transmitted? Does your 172S have the KAP140 or the GFC700 A/P? I’ve also got queries from ATC regarding the altitude preselect when flying a 172S, but not about the VNAV target.
I meant that the altitude pre-select is transmitted (I believe it was). We have the GFC700 A/P.
martin-esmi wrote:
I have seen the same behavior in our Cessna 172S when flying in Germany, I never change the altitude select nowadays before I’m cleared to the new altitude/FL.
What behaviour? That both the altitude preselect and the VNAV target are transmitted? Does your 172S have the KAP140 or the GFC700 A/P? I’ve also got queries from ATC regarding the altitude preselect when flying a 172S, but not about the VNAV target.
I have seen the same behavior in our Cessna 172S when flying in Germany, I never change the altitude select nowadays before I’m cleared to the new altitude/FL.
lionel wrote:
Yes, but the transponder doesn’t know which one the AP follows… And it is the transponder that radiates the information.
AFAIU the documentation, if the transponder doesn’t know it should transmit that fact and in that case the ATC radar system should use pilot selected altitude.
Ibra wrote:
Yes all aircraft installation is Garmin, if there is something fishy with install it would be from STEC3100 which is not made by Garmin but denopa mentioned it follows the blue bug, so AP/FD knows which one to fly and ATC should get “MCP selected altitude” on their screen…
Yes, but the transponder doesn’t know which one the AP follows… And it is the transponder that radiates the information.
Ibra wrote:
naybe they (or radar system) does not care where the blue or magenta bugs sits as long as it’s bugged outside en-route airspace
Could be !
Airborne_Again wrote:
As both the PFD, navigator and transponder are made by Garmin, they should be able to explain this.
Yes all aircraft installation is Garmin, if there is something fishy with install it would be from STEC3100 which is not made by Garmin but denopa mentioned it follows the blue bug, so AP/FD knows which one to fly and ATC should get “MCP selected altitude” on their screen…
Airborne_Again wrote:
I find it hard to believe that the ATC radar system does this wrong as then every airliner with VNAV would have the same problem
Yes I agree primary en-route ATC radar units (LTC & Scottish) will have these things well sorted on their radars as every airliner with VNAV would have same problem, but maybe they rarely bug VNAV with 600ft amsl for visual approach on magenta as their SOP in TMA is HDG/VLOC and VS/ALT/GPV 
Secondary ATC units (e.g. Southend, Cardiff, Solent) are just starting to upgrade their kits and they could be running on different hardware with few bugs, for instance some had issues with long reg characters when ModeS ID it was introduced on display
denopa wrote:
I’ve done many flights under London Control, never had a remark
Did you load GTN Visual to EGTF or VNAV direct to EGTF at FL400 while talking to them 
? naybe they (or radar system) does not care where the blue or magenta bugs sits as long as it’s bugged outside en-route airspace
