Jumping HSI GS indicator on JF PA28R with LPV approach (standalone mode)

[Zulu]

Using the JustFlight Piper Turbo Arrow IV I was just flying the "RNAV 08L Z gps LPV" approach.
I expected to get the Glideslope on my HSI since this is an LPV approach.

Just before reaching the FAF (PIKMI), the Glideslope actually became alive but was jumping from bottom to the correct position up and down all the time instead of staying steady. 
Same for the vertical guidance needle on the NAV1 instrument.
It was just like the GS was turned off and on all the time.
This behavior remained the same for the whole final approach.
My coupler dial for the autopilot was on NAV (even though LOC is recommended for localizers), but I don't think this is an issue.

Did someone experience this behavior? Is the HSI GS not supported on the JF PA28R with the TDS GTN750?
When switching to NAV1 ILS frequency, the GS worked as usual.

If that matters: I was running the TDS GTN750 as standalone, no VC integration (GNS530 selected in the cockpit).

Just noticed the Known Issues section:

The HSI displays only horizontal deviation when in GPS mode, unfortunately vertical deviation is not yet supported.

Is this jumping GS needle the current expected behavior for that reason, so I can't fly LPV (or LNAV/VNAV) approaches at all?

[adminOwner2]

Hello,

When the TDS GTNXi is running in standalone mode, we do not set any GS needle values, because of this, the GS needle jumping is not being sent by us, but by MSFS. Can you confirm that you have the MSFS Community folder tds-gtnxi-gauge removed?

[Zulu]

Hey, thanks for the quick reply!

When the TDS GTNXi is running in standalone mode, we do not set any GS needle values

Ah, is this due to the limitation of the API to MSFS? I always thought that standalone and VC were technically identical in terms of what features are available.

So does this mean using it standalone or with other aircraft than JustFlight Piper/Warrior, we have no means to fly an RNAV (with LNAV/VNAV or LPV) approach at all? I mean looking on the VSR on final approach isn't appropriate, especially in IMC.
There are many airports that have RNAV procedures but do not support ILS on all their runways, it's a great help for IFR.
Or is there some other way to see some kind of glideslope for these kind of approaches?

I guess same limitation applies to the visual approaches where the GTN750 is able to create an aid in terms of a glideslope (but on visual it's not that crucial)?

Would it be possible to have some window containing the VDI written from the Garmin Trainer?
I think of something that can be opened from the toolbar as a small window showing just a small VDI which is loaded from your product and available no matter if it is running in standalone or in VC mode. That would help a lot to overcome this crucial limitation.
There are ways to display some URL in the sim by opening a window from the toolbar, maybe your addon could render such a tiny website containing just a basic VDI and be accessible this way?

Can you confirm that you have the MSFS Community folder tds-gtnxi-gauge removed?

Yes, the tds folder was removed, so I was running it in a standalone window which worked totally fine.
I initially had the PMS50 running in the cockpit just to have something active (switching to the very basic radio stack disables GPS functionality in the JF PA28R, so I at least need to have GNS530 active) and there I also selected the same RNAV approach. Switching to the GNS530 did not change this jumping behavior.
Maybe the selected RNAV approach in the PMS50/GTN530 was driving the jumping GS needle?

I will try to reproduce it later with no flight plan/approach selected in MSFS and TDS GTNXi running in standalone and then also try the VC integrated variant to see if then the LPV approach shows proper GS indicator.


Oh and one more question:
When reaching the FAF, I expected the GTN750 to show "LPV" in the status bar in green. But instead it decided to show a blinking OM (Outer Marker) and IM (Inner Marker) all the way, so I was not really sure if LPV was actually active.
I guess this is normal behavior? I learned that one needs to confirm the current mode at the FAF, but that's not possible due to the OM/IM status at the same location in the status bar.


EDIT: Just tried it out:
- Spawned just before PIKMI
- TDS GTNXi in standalone, selected CYVR as destination and loaded the RNAV 08L Z gps LPV approach and activated it, activated the leg to PIKMI (faf)
- CDI set to GPS
- Continuing after PIKMI, the GS needle (on the NAV 1 instrument as well as on the HSI) start jumping again

Attached is a screenshot - this time only the GNS530 was selected, no flight plan and no approach loaded in MSFS, so the needle has to be driven from the TDS GTNXi.

As soon as I shut down TDS GTNXi (via task icon), the GS needle stops moving and a red NAV marker is shown in the HSI which shows that no GPS signal is available.
So the GS needle has to be operated by the TDS GTNXi, I have no other explaination to this behavior.

---

I tested the VC mode, there the GS is showing up correctly (see screenshot).
So in this mode, the GS works fine.

Another observation: As you can see I'm quite far off the localizer, but the lateral needle on NAV1 and the HSI still shows quite centered. I don't think this is how it should behave on an LPV final approach leg since it should be MUCH more sensitive.
Is this a known limitation right now?
The way it is now, I would not be able to fly a centered approach in IMC, simply impossible with the low deviation of the vertical needle.
This behavior is the same, in the standalone and the VC mode.

So while using the VC mode at least gives me the GS, I still couldn't fly an LPV approach despite all criteria met.