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GFC 500 dropping NAV coupling with GPS>VLOC switch

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compasst View Drop Down
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    Posted: 25 Jul 2020 at 6:15pm
I don't know if this is reported elsewhere on this forum, but I discovered a glitch in my new GFC 500 operation with dual G5s and IFD 540 today. I was able to replicate this three times today, so I feel it is a real issue. I also believe it happened during a series of coupled approaches a week ago when I was having problems with NAV tracking on some of the approaches (the ILS and VOR ones). My system tracks and executes all GPS approaches attempted to date - LPV, LNAV+V, LNAV).

Today I took a video of this so I have the details of what happened. While letting the autopilot fly (via GPS mode) to a VOR and then fly from that VOR on a VOR-A approach (ACO, KPOV), the GFC 500 accurately followed what the IFD wanted. This included navigating to the VOR, making a rather tight turn to follow the procedure turn outbound (on one of the trials, I intentionally set this up to see what navigator and autopilot would do) and then do the approach. As would be expected, the GPS>VLOC appeared on the IFD. As the instant that the GPS>VLOC changed to VLOC and the HSI changed from magenta CDI to green CDI, the G5 showed a yellow blinking GPS to the left of a ROL indicator, showing that the GPS nav signal had been lost. At this point, the NAV indicator on the autopilot also dropped off, leaving it in the ROLL mode as the green CDI drifted away from centered. 

I pressed the NAV button and re-coupled in the NAV mode, but the autopilot didn't change heading to recapture the CDI. Instead, it stayed on the heading it was when it went into ROL mode above. I switched to HDG mode, set an intercept heading for the 'desired track' of the plotted course of the VOR approach, The autopilot dutifully followed the heading to intercept the track, switched to NAV mode by itself, and flew the remainder of the approach as it should.

In reflection, I believe this issue also happened on the attempts to fly two coupled ILS approaches in my earlier practice session - when the GPS>VLOC auto sequences to VLOC, the NAV function of the GFC drops, leaving it in the ROL mode and allowing the aircraft to drift off course. Of course, during these ILS approaches, the GFC failed to capture and track the GS.

I will double-check this in the coming days.

Anyone else experience this? I'll also send this to TJ along with the video. I'll trim the video and see if I can figure out how to post it or at least a link for y'all to view.

 https://youtu.be/hjxg_NOq3mk


Edited by compasst - 26 Jul 2020 at 10:49am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote molokaisteve Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 2020 at 8:57am
Compasst,
Thank you for the post.  I'm also looking to install two G5's and the GFC500 in a Cessna 182.  Would you please keep us posted on what TJ says.  I was also considering installing the G3X with a G5 but I can't find where Avidyne shows that it is compatible.  The G3X is compatible with the GNS530W that I replaced so you would think it would work with the IFD540.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dmtidler Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 2020 at 9:06am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote compasst Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 2020 at 11:16am
I'm beginning to think that the autopilot's reaction is proper and to be expected. First off, Garmin 530 doesn't auto-switch GPS>VLOC as the Avidyne devices do. I will be flying a G3x with 760Xi and GFC500 in a few days and will see how this setup responds. According to the documentation, this navigation system will autoswitch GPS to LOC on an ILS at the FAF and that the autopilot will follow that changeover to maintain the course and follow the GS.

My current thinking is that GFC 500 sees that the GPS signal is 'lost' (thus the yellow message) during the IFD's auto-switch GPS>VLOC function and the autopilot doesn't know that the new VLOC signal is to be followed. Thus, the pilot is supposed to re-press the NAV or APR button(s) to re-educate the GFC that the new signal is to be followed. 

I think my next test will be a GPS to ILS with APR selected before I get to the FAF and see what happens, maybe later today. Stay posted!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dmtidler Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 2020 at 3:39pm
Originally posted by compasst compasst wrote:

I'm beginning to think that the autopilot's reaction is proper and to be expected. First off, Garmin 530 doesn't auto-switch GPS>VLOC as the Avidyne devices do. I will be flying a G3x with 760Xi and GFC500 in a few days and will see how this setup responds. According to the documentation, this navigation system will autoswitch GPS to LOC on an ILS at the FAF and that the autopilot will follow that changeover to maintain the course and follow the GS.

My current thinking is that GFC 500 sees that the GPS signal is 'lost' (thus the yellow message) during the IFD's auto-switch GPS>VLOC function and the autopilot doesn't know that the new VLOC signal is to be followed. Thus, the pilot is supposed to re-press the NAV or APR button(s) to re-educate the GFC that the new signal is to be followed. 

I think my next test will be a GPS to ILS with APR selected before I get to the FAF and see what happens, maybe later today. Stay posted!

After some research, I think this paragraph from section 3.3.2.2 (Flight Director Modes) of the G5 Pilot Guide describes the situation:

If the information required to compute a flight director mode becomes invalid or unavailable, the flight director automatically reverts to the default mode for that axis. A flashing yellow mode annunciation and annunciator light indicate loss of sensor or navigation data required to compute commands. When such a loss occurs, the system automatically begins to roll the wings level (enters Roll Hold Mode) or maintain the pitch angle (enters Pitch Hold Mode), depending on the affected axis. The flashing annunciation stops when the affected mode key is pressed or another mode for the axis is selected. If after 10 seconds no action is taken, the flashing annunciation stops. The flight director is automatically disabled if the attitude information required to compute the default flight director modes becomes invalid or unavailable.

The initial GFC 500 lateral navigation mode described was GPS indicated in green on the upper left of the G5 ADI before the IFD automatic switch to VLOC. When VLOC was automatically selected by the IFD connected to the G5s, per above, the GPS indication on the G5 ADI should have flashed yellow because VLOC steering information is now being provided to the G5 and that is incompatible with the active GPS lateral steering mode of the GFC 500 at the time; hence the ROL (Roll Hold Mode) annunciation until another roll mode is selected. 

If this is all correct, seems like there might be a learning curve to smoothly handling the GPS to VLOC switching be it manual or automatic with the GFC 500.




Edited by dmtidler - 26 Jul 2020 at 3:39pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote compasst Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jul 2020 at 7:04pm
Thanks, dmtidler, for posting (and reminding me of) the product of your research. And this is exactly the way it worked. Now to see how the G3x and the GTN 650 handle this issue. It is my understanding from the documentation that this pair introduces auto GPS > VLOC in Garmin devices. I'll be flying this combo in a week or so and will post results.

It would be nice if a 'bridge were passed along with the source switch to keep the autopilot following the valid navigator behavior. That said, I wonder if a similar situation using the 430/530 navigator with it's manual switch of source would also result in loss of AP following a desired course such as intercepting an ILS, for example.

As you pointed out, a pilot needs to be aware of this behavior when flying non-GPS approaches using a GPS navigator. Heads up, y'all.
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