I asked Stec technical support what the 60-2 would do if after the GS was intercepted and being tracked, and subsequently the vertical flag was removed. It would indicate FAIL on the annunciator and continue to track the GS up/down differential signal. Assuming that this would be neutral at the GPIP, the AP would continue the established descent.
Dave_Phillips wrote:
I’ll have a play next week as I’m calibrating a load of CAT III ILS which will give me plenty of opportunity to fly along a runway at 8-12ft.
Brilliant! I am looking forward to see the results.
I asked Stec technical support what the 60-2 would do if after the GS was intercepted and being tracked, and subsequently the vertical flag was removed. It would indicate FAIL on the annunciator and continue to track the GS up/down differential signal. Assuming that this would be neutral at the GPIP, the AP would continue the established descent.
That implementation makes sense.
Was that for analog FCS signals, or ARINC429?
The Stec autopilots thru the 55X series are analog for the pitch axis. So all vertical guidance is analog.
An update. Early days but I flew an AP coupled ILS approach to a CAT III runway last night (OTBD). The scenario was G1000NXi aircraft in APR mode with a DA of 233ft(206ft) set. As one would expect, the AP continued beyond DA and continued to track both the CL and GP. This resulted in a noticeable ‘fly down’ at 0.29nm DME (approx 0.31nm from notional touchdown point) and then a ‘fly-up’ at 0.22nm DME; that is consistent with the structure of the ILS GP signal and is entirely predictable. I disconnected the AP over the threshold before landing. Since I landed I didn’t get the opportunity to ascertain AP performance once the GP signal had been lost. I’ll have a go at that one on Monday.
Next we will fly the LNAV/VNAV to the same runway. There is no LPV here. 
You will note that the MAP is at 0.5DME so it is relatively safe to let the AP continue beyond MAP. The G1000NXi manual only states that at MAP either GA Mode should be selected or the AP should be disengaged. Like the ILS, I would expect the AP to continue to track the guidance given by the GNSS approach; this particular exercise will (dis)prove that hypothesis and confirm the nature of the guidance given. If it appears to continue guidance to GPIP we will follow to threshold (in this case 50ft RADALT) before disconnecting.
Once I get to an LPV airfield I will initially fly manual approaches following our standalone flight calibration guidance system with the LPV approach selected on the G1000NXi. Our own guidance system (accurate to 0.20m) provides a GP to just before GPIP before demanding an 8-12ft fly through along the remainder of the runway. This will give us the opportunity to compare the known guidance against that which the LPV approach may give. We have a number of these profiles to fly and will monitor the LPV guidance to see what it says when we reach GPIP and beyond. If it appears to give ‘sensible’ guidance, we may ultimately try an approach with the AP engaged throughout although this could be tricky as we fly all profiles ‘clean’ at 160-180kts and, ordinarily, the AP isn’t quick enough to give the corrections required to stay within our calibration tolerances; APs tend to try and smooth-out corrections. Furthermore, my instinct says the AP may well have a good go at trying to kill us. 
I would welcome any requests for profiles you may wish to see, within reason!
To pick up this old thread, what happens to the vertical guidance on a +V approach?
Does it disappear below the official IAP (say an NDB IAP) minima, or does it continue all the way to the tarmac like LPV (or ILS) does?
I have heard of a scenario of a GTN650 + G5 where the GS indication disappeared at a certain point, flying the +V on an NDB approach.
The GPS based GP vertical guidance appears when the FAF becomes the active fix and disappears when the GP hits the pavement. It does not matter the type of GPS approach annunciation, be it LPV, L/VNAV, LP+V, or LNAV+V.
+V does not work on an NDB approach. You are describing VNV which uses Baro VNAV to guide a descent using a flight plan that includes altitudes. It stops when the FAF is reached.
NCYankee wrote:
+V does not work on an NDB approach. You are describing VNV which uses Baro VNAV to guide a descent using a flight plan that includes altitudes. It stops when the FAF is reached
I think Peter is describing an LNAV “overlay” of an NDB approach. In that case I, too, would expect a +V GP all the way to the threshold – at least if the procedure has a well defined FAF.
I’ve tried an NDB approach with LNAV overlay on Garmin’s GTN650Xi simulator and I do get a GP all the way.
I have the GTN 750 in my Bonanza and it does not provide +V on conventional approaches, but I simulated with both the GTN and the GTN Xi. The Xi does appear to offer +V on conventional straight in approaches. On the GTN 750Xi simulating the KLKR NDB 24, I see +V, but on the GTN 750, just NDB 24 lateral is shown. Thanks, I wasn’t aware of this difference.
Edit, I think the +V is for straight in to a runway, and does not require a defined FAF fix as is the case for the KLKR NDB 24 procedure.
The +V is not part of any approach procedure, it is merely an allowed vertical path to get you to the MDA that provides guidance along a CDFA kind of path. The path has no use below the MDA and does not guarantee obstacle clearance below the MDA, even though it is provided to the surface.
