RXP units and HSI CDI (XP11.55)

[youretootall]

Proud new Moo owner here.  First off, I am still in XP11.  I was under the impression that the latest installer will detect this correctly and install 2.0.4, however the version file in the aircraft folder shows 2.1.0.  FYI.

The issue I am having is that the HSI CDIs (both sides) are not responding correctly to any of the RXP units when the CDI mode is set to GPS.  Ground-based navigation is working as far as I have tested (briefly).  I have tried all-GNS and all-GTN configurations.  

The deviation bar does not deflect (except during the initial test where it correctly shows half-left).  During the test, the CDI course also sets itself to 150, but it does so regardless of my auto-slew preference on the RXP menu.  After continuing past the test page, the course re-sets to an arbitrary value (024 or something, already forgot), and will be frozen.  Rolling the course knob increments the digital readout but not the needle.

Also, FYI, I didn’t touch any of the shipped GTN/GNS.ini settings at first, but I did check them while troubleshooting.

[tkyler]

noted and logged. not sure how that one got past me,  but it did!....  Thx for reporting.

-tk

Edited May 30, 2023 by tkyler

[youretootall]

Thanks, Tom.  Just to confirm, is it correct that my version file is showing 2.1.0 when the installer was pointed to an XP11 directory?

[pm_pilot]

Looking forward to the fix for XP12, too. I assume that’s why the autopilot went in circles after clicking the GPSS steering button and using the GTN 750 in CDI mode? XP 12.05, latest version of the MU.

Edited June 2, 2023 by pm_pilot

[tkyler]

1 hour ago, pm_pilot said:

I assume that’s why the autopilot went in circles after clicking the GPSS steering button and using the GTN 750 in CDI mode?

Not very likely pm_pilot.  "CDI mode" is only an indicating mode and has no input as to the flight path.    I couldn't say what might have caused the path you observed without seeing all the autopilot settings and also the flight plan in the GPS.

-tk

[pm_pilot]

Thanks for the quick reply. It was just a direct to airport in the GTN 750 using HDG, ALT on the autopilot as far as I remember and the GPSS roll mode button activated. Is that the correct way? I will try to replicate on my next flight.

Edited June 2, 2023 by pm_pilot

[pm_pilot]

@tkylerHi I tried it again, and instead of direct to It turned almost 180° to the left and stopped at heading north. Ap and GTN modes as I mentioned.

Edited June 6, 2023 by pm_pilot

[tkyler]

@pm_pilot

I'm sorry, I don't have much to offer here at the moment, may be someone else has experience with the RXP units?  I provide the RXP 3D integration as a courtesy for RXP owners and have no real experience with operating these units.  I can't speak as to how they interface with the default X-Plane GPS leg data.   My 'tests' for GPSS behavior is to use the default GNS unit flight plans and if you were seeing this behavior with those default GNS units, I may have more to offer.  If the exact same route and tracking behavior works with the default GNS units, but not with the RXP, we'd probably have to ping RXP support.  I'd recommend trying the same route with those units as a baseline data point for the debugging effort.

I do have a possible theory though.   In ealier versions of X-Plane,  you could enable GPSS and it would simply turn to track the magenta no matter the aircraft orientation relative to the path.   Philipp from Laminar mentioned that he was going to implement more stringent "intercept" criteria for tracking GPS and it would not automatically capture the route at engagement in all cases.  It could be such 'intercept criteria', either by X-Plane or the GTN simulations could be getting in the way...however, if you were heading in the general direction of your "direct to" waypoint at engagement, I wouldn't expect this to be a problem. 

What were the particulars of this setup?  Departure airport?   flight plan routing?  waypoint?   sequence etc...I'd be curious to see if this happens for all RXP users in the same scenario.

-tkyler

Edited June 6, 2023 by tkyler

[pm_pilot]

I departed from UIII, but didn’t use a flight plan, just went direct to airport (ZMCK), when airborne.

Edited June 6, 2023 by pm_pilot

[pm_pilot]

@con7inuumHi, I hope it’s okay to ping you. You mentioned some problems with the GTN750, too? what was your concern, did it not follow flight plan, either using the GPSS steering switch?

Edited June 7, 2023 by pm_pilot

[pm_pilot]

I have the suspicion that it is a database problem, ZMCK is fairly new. Will report back: no luck, tested at a more common area, LSZA- LIME as well.

Edited June 8, 2023 by pm_pilot

[youretootall]

@tkyler Need to refine the feedback from my OP.  The GTN and GNS will show proper deflection on the CDI provided that "Connect GPS to VOR" is selected on the RXP menu.  So that is an issue with the GTN/GNS.ini as shipped.  However, the course slews to to the DTK regardless of any RXP options, so that's a problem for the round-gauge cockpit where you'd be having to set the HSI course yourself (but you already knew that).

Am I correct in assuming that the GPS/LPV glideslope does not integrate with the Sperry avionics for an RNAV LPV approach?  My recollection from flying a C501 IRL with a GNS530 (Sperry panel and AP otherwise) is that the GNS still drove the G/S indication to "half-up" on the test page.  That makes sense to me as all the NAV/GS inputs are run through the Garmin.  Unfortunately, I can't remember if we ever flew LPV approaches on the 501...

Edited June 8, 2023 by youretootall

[tkyler]

5 hours ago, youretootall said:

GPS/LPV glideslope does not integrate with the Sperry avionics for an RNAV LPV approach?

Thats a correct assumption,  the Sperry AP GS is terrestrial ILS signal only; however, I don't currently deflect the GS indicator on the HSI based on a LPV guidance signal.....that's a good question in my mind though...   It stands to reason that if the Garmin units can drive the analog inputs of the HSI CDI, it could do the same for the GS indicator....but I don't recall ever coming across that info for the GS, either IRL or within X-Plane, so did not implement it.  

Id have to confirm with an avionics buddy if the Garmin units can drive the GS needle of an HSI (your experience and logic indicates it probably does)..so it'd just be reinforcing confirmation on my part,  but I'll look into if/how X-Plane implements this "GPS driving of the GS needle".  

-tk

Edited June 8, 2023 by tkyler

[Graeme_77]

The issue with these old systems is getting the vertical signal in to the HSI and the AP.

The course deviation can be output to the HSI provided the system was designed with an external resolver. (The resolver takes the VOR signal and compares with the OBS setting to generate the course bar deflection - an external resolver system allows a NAV/GPS to feed course bar information directly and to process the OBS selection within the radio - i.e. the resolving happens inside the radio for VOR so a "pure" course bar defection is sent to the HSI)

In some cases, similarly, the GPS itself can generate analog GS output signals even when performing LPV or LNAV+V approaches. 

Given the analog nature of the Mu2 instruments I'd be fairly happy to see the LPV glideslope presented on the avionics when in LPV or LNAV+V.  I'm sure a competent avionics shop could make this work, either directly or with some electronic magic.  The real thing would probably end up with a flight manual limitation specifying lateral AP coupling only, but it's probably technically capable of making it work anyway.

• Some avionics setups make it harder and so make use of what's called an LPV Converter. This reads the GPS digital output parameters for the LPV approach and converts them into digital LOC and GS parameters which the avionics ILS display understands and can feed AP inputs. This allows older systems to fly LPV approaches by basically tricking the avionics into thinking they have a real ILS signal. There's a changeover button on the panel somewhere to swap between LPV and ILS. Conceptually, it's like the roll steering button that's used to give GPS steering on the heading bug input, although the implementation is different. 

GNS Install manual extract - note the analog limit - Garmin could prove their analog outputs wouldn't cause any harm and could be trusted to be standards compliant just like any other VOR/LOC/GS, but if the random old airplane autopilot listens to the digital data who knows what crazy stuff could happen.

Edited June 8, 2023 by Graeme_77