Surely that line is a CDFA which you check against your altimeter.
I understood that a + V just happened in the GPS based on WGS84 whereas the CDFA is based on ordnance survey and positions can differ quite considerably.
A +V would also need a start point, ok way enough in this case KA20F but what would be your end point. The CDFA line finishes at the Missed approach point which is at a point 40ft above the threshold. Is that point in the +V database or is the threshold the zero point in the database?
A quick Google shows that in South Cornwall lines of latitude on GPS WGS84 are 70metres south of the same lines of latitude on Ordnance Survey maps which use OSGB36.
These differences are one of the reasons behind SBAS and GBAS (WAAS and EGNOS) AIUI.
Apparently (allegedly?), CAA insisted that GNSS VNAV was not in Jepp database for the UK.
Yes, same as the CAA inspector insisted the KLN94 is VFR only…
It could be ILS, +V or LNAV/VNAV(VNAV could be Baro or GNSS VNAV). While the procedure would have to be designed as BaroVNAV, nothing stops you from flying it GNSS VNAV (I’ve seen reports elsewhere).
What is LNAV/VNAV? It doesn’t exist in GA AFAIK, as a published procedure. The only vertical guidance with GPS is LPV or +V. There is Baro-VNAV but it is almost nonexistent in GA.
I understood that a + V just happened in the GPS based on WGS84 whereas the CDFA is based on ordnance survey and positions can differ quite considerably.
I don’t understand the relevance provided you respect the SDFs, and those are checked using the altimeter. And +V is supposed to do that.
Peter wrote:
What is LNAV/VNAV? It doesn’t exist in GA AFAIK, as a published procedure. The only vertical guidance with GPS is LPV or +V. There is Baro-VNAV but it is almost nonexistent in GA.
Peter, AFAIK it is the same mechanism as used by LPV, but as it is not down to LPV minima, it does not require an agreement with EASA for SBAS use as LPV does.
If you are checking using the altimeter then it becomes an ordinary CDFA which could even be from an NDB approach with the addition of some sort of vertical guidance which is not to be totally relied on.
Even in old portable Garmin GPSs you could set a top of descent to arrive at x feet above point B at eg 500ft. Would you really fly an IMC approach on that. It is possible but it doesn’t take into consideration tall radio masts that might be in the way of that GS. If it is in a database as +V at least there shouldn’t be, in theory, a mast in the way but how accurate is it? Especially, if you don’t have SBAS correcting the difference between the IAP designer using OSGB36 and the GPS using WGS84.
The degree of accuracy to which an approach can be flown is very significant. A DIY IAP means it is up to you to weigh up all the safety problems and to mitigate them.
These include the degree of accuracy of your nav system. For instance you might need to go to the threshold with your GPS and check out the WGS84 position there.
You may need to work out to how many degrees of latitude and longtitude your GPS is capable of in terms of metres. The same goes for any high obstacle (height and position) on your GPS, not on the map. That also includes refresh rate at the speed of your aircraft.
Remember in a DIY IAP you will set your own DH and your own degree of accuracy.
Eg can you couple your AP to your DIY IAP in order to gain greater flying accuracy especially in IMC.
But, by allowing DIY approaches the regulators have quite rightly said “it’s your responsibility and your life. You are in command.”
IMHO vast sums of money wouldn’t have been spent on WAAS and EGNOS if it didn’t add something to aviation.
gallois wrote:
If you are checking using the altimeter then it becomes an ordinary CDFA which could even be from an NDB approach with the addition of some sort of vertical guidance which is not to be totally relied on.
If it is in a database as +V at least there shouldn’t be, in theory, a mast in the way but how accurate is it?
The +V is nothing more than a more accurate way of doing a CDFA. You still have to go around at the DA (or MAPt) and respect every SDF so the accuracy is not really important.
Exactly, and everything else is basically irrelevant.
I understood the debate to be that +V was as good as having LPV.
LPV v +V
I was, perhaps clumsily, trying to point out that that is dangerous thinking.
I am well aware of the differences between LPV200 LPV 250 LNAV/VNAV and +V..
I understood the debate to be that +V was as good as having LPV.
Within the limits of the different DH figures, they are practically the same.
Especially if you don’t have LPV 
Peter wrote:
I don’t understand the relevance provided you respect the SDFs, and those are checked using the altimeter. And +V is supposed to do that.
The +V provided by GPS should nominally clear any SDF, but with temperatures above ISA, following the +V can place the aircraft below the indicated altitude. It is not the +V that moves, rather is the altimeter indication that will be below the minimum required. In some cases, the +V does not begin until a point past the FAF because of a SDF, so that the VDA will nominally clear the SDF.
Peter wrote:
I think almost nobody in light GA in Europe has baro-VNAV. If you want vertical guidance (coupled to autopilot or hand flown) it is LPV or +V.
Probably true regarding piston, it will be hard to find one with Baro-VNAV vertical guidance for approaches (up to the FAF VNV using Baro-NAV is more widely available and is supported by GTN and G1000), but when LNAV/VNAV minimums are published on an RNP procedure, the Vertical guidance provided by a TSO C145/146 system GPS is not +V or advisory, it is approved for flying the LNAV/VNAV vertical to the DA/H. Although this is only implemented in existing SBAS type GPS navigators, it is not considered as a an approach requiring SBAS because the lateral final approach path is +/- 0.3 NM wide and the vertical guidance is an alternative to using Baro-VNAV. I did not find many RNP with LNAV/VNAV, but they do seem to exist in the UK. An example I found was EGNT RNP 7 and RNP 25. Both have a lower DA for LNAV/VNAV than for LNAV using CDFA where +V would apply. A GTN/GNS should have these procedures in their database.
