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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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Post Options Post Options   Thanks (1) Thanks(1)   Quote compasst Quote  Post ReplyReply Direct Link To This Post Topic: GFC 500 dropping NAV coupling with GPS>VLOC switch
    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
+1
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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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Post Options Post Options   Thanks (0) Thanks(0)   Quote dmtidler Quote  Post ReplyReply Direct Link To This Post Posted: 31 Aug 2020 at 12:48pm
Have you had any issues with the GFC 500 flying GPS approaches slightly offset? 

Another forum I came across yesterday indicated there is a difference in the navigation signal from a GNS versus an IFD on GPS approaches (dynamic vs static) fed to the GAD 29B that may may cause the GFC 500 to not make a deliberate return to the centerline of a GPS approach with a lateral deviation of up to several hundred feet when driven by an IFD. Just curious if anyone on this forum has seen this behavior?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Warrenwhis Quote  Post ReplyReply Direct Link To This Post Posted: 01 Nov 2020 at 12:40pm
I have had and continue to have this problem.  I do not know what is needed to correct it.  Generally it wiggles around til it gets itself straightened out
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 01 Nov 2020 at 4:11pm
You have read the AFMS haven't you, and know that the GFC500 requires GPS position to track NAV/VLOC (i.e. ILS/LOC/VOR)? ;-)
If it is lost, use of the autopilot is limited to ROL/PIT mode. Otherwise you need to fly the approach manually using normal VLOC guidance from the IFD on the HSI.

Section 3A NON-NORMAL PROCEDURES, page 3-8 of Rev 8 of the GFC500 AFMS for a 172 says the following. It sounds like this is your situation, and is standard GFC 500 behaviour in case of lost GPS position.

AFAIK (I don't have the GFC500/G5/IFD combo (yet)) and per the wording below, the GFC500 needs GPS position for its aiding, not GPS course guidance which of course would disappear with the switch over from GPS to VLOC mode. Since it sounds like the IFD didn't lose the GPS signal, this might point to a wiring issue (GPS position feed to the G5 from the IFD, eg. on RS232 separate from ARINC that is normally used for tracking info... don't know if GPS position is included in the ARINC stream in addition to tracking info). Just a thought.

LOSS OF GPS INFORMATION

(GPS position information is lost to the autopilot.)

NOTE

If GPS position data is lost while the autopilot is tracking a GPS, VOR, LOC or BC course, the autopilot will default to roll mode (ROL). The autopilot will default to pitch mode (PIT) if GPS information is lost while tracking an ILS. The autopilot uses GPS aiding in VOR, LOC and BC modes.

1. Autopilot ................................................SELECT different lateral and vertical mode (as necessary) If on an instrument approach:

  1. AP DISC / TRIM INT button .............................................PRESS, Continue the approach manually Or

  2. Missed Approach Procedure.................................................................... EXECUTE (as applicable)



Edited by chflyer - 01 Nov 2020 at 4:41pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mfuesting Quote  Post ReplyReply Direct Link To This Post Posted: 01 Nov 2020 at 5:33pm
I have this combo of dual G5's, GFC500,IFD and have no problem with the switch from GPS to VLOC on such ILS approaches.  I do activate the approach mode on the GFC early and well before the FAF after completing the procedure turn inbound and lining up on the inbound course.  I have the IFD do the auto-tuning of the ILS frequency, but always verify it during the procedure turn inbound - double check to see that this feature is active and working as expected.

On the slight off course laterally on the GPS approaches and center-line of the runways, I too experience this all the time. For me it is about 2 to 3 widths of the runway at half a mile out, and then gradually corrects itself as I get closer to the threshold. Generally it wiggles around til it gets itself straightened out.   I do get a consistent error message about GAD needs servicing on IFD boot-up, but it has been checked and there is no problem with it.  I have been told that this and the side step are inter-related and is due to Garmin changing some of the data format on a recent Garmin upgrade.  Avidyne told me they are aware of this and is working to resolve it in a future upgrade.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gring Quote  Post ReplyReply Direct Link To This Post Posted: 01 Nov 2020 at 7:24pm
Originally posted by mfuesting mfuesting wrote:

...I have been told that this and the side step are inter-related and is due to Garmin changing some of the data format on a recent Garmin upgrade.  Avidyne told me they are aware of this and is working to resolve it in a future upgrade.

This is exactly why Avidyne needs to produce its own PFD and Autopilot servos.  Right now, you are one software upgrade from things not working.  It’s a frustrating time to be a consumer.  Those of us who think Avidyne has superior products get stuck with whatever else we can piece together.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 02 Nov 2020 at 4:28am
Avidyne should probably put something in place for Garmin (and other?) products like ForeFlight has done for Apple software upgrades, where they give users a green light for upgrading to a new iOS/PadOS when it comes out. This type of issue is likely to increase in frequency, and Avidyne needs to ensure enough priority is given to ongoing compatibility.

Fully agree that we don't want to have working installations break because of a Garmin software update that we could have avoided if we only knew ....


Edited by chflyer - 02 Nov 2020 at 4:31am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ddgates Quote  Post ReplyReply Direct Link To This Post Posted: 02 Nov 2020 at 9:50am
Originally posted by Gring Gring wrote:

Originally posted by mfuesting mfuesting wrote:

...I have been told that this and the side step are inter-related and is due to Garmin changing some of the data format on a recent Garmin upgrade.  Avidyne told me they are aware of this and is working to resolve it in a future upgrade.

This is exactly why Avidyne needs to produce its own PFD and Autopilot servos.  Right now, you are one software upgrade from things not working.  It’s a frustrating time to be a consumer.  Those of us who think Avidyne has superior products get stuck with whatever else we can piece together.

I agree with Going (and have for a long time).....even a strategic partnership or two would give us a pseudo-secure pathway for upgrades....now it is hit/miss, and no security for substantial longer term investment in panel upgrades.

The servo issue is very much still at issue in the autopilot sector...
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Post Options Post Options   Thanks (1) Thanks(1)   Quote MarkZ Quote  Post ReplyReply Direct Link To This Post Posted: 02 Nov 2020 at 5:54pm
Never trust Garmin to play well with others. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rallylancer122 Quote  Post ReplyReply Direct Link To This Post Posted: 02 Nov 2020 at 7:01pm
Originally posted by mfuesting mfuesting wrote:

I


On the slight off course laterally on the GPS approaches and center-line of the runways, I too experience this all the time. For me it is about 2 to 3 widths of the runway at half a mile out, and then gradually corrects itself as I get closer.

If it makes you guys feel any better I have the same problem with my stec 3100.  It flies an rnav approach perfectly.....1 dot to the right. Stec had me disable roll steering during approach on the ifd but that introduced some other problems, so now I'm waiting on a software update.   Interesting that you guys are having a similar problem. 
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