IFD540 loss of gps Integrity. What that means for the also installed IFD440?

[Flyxaos]

This is a question that is maybe specific to the IFD 540 and 440 dual installations.

On a practice LPV approach the other day the 540 lost gps integrity and downgraded to RNAV+V. At the same time the 440 retained integrity and said it was still LPV.

In that instance, I just stopped using the guidance on the PFD that the 540 is linked to and started using the guidance the 440 is linked to, a "steam" CDI.

But it occurs to me that maybe the CDI wasn't using the guidance from the 440, but instead that of the 540. The reason being, I think the guidance and output to the PFD and CDI are linked between the two when it is using GPS for guidance. Is the guidance on the CDI that is linked to the 440 coming from the 440 and was it really getting LPV from it?

If it is on an ILS or VOR they are separate, but I'm not clear on how it interacts when on GPS.

I am also wondering how the 440 is actually giving redundancy in the install. If I shut off the 540 the 440 would happily operate the CDI, but not the PFD on my main display. It isn't connect to the main PFD.

Issue is fixed, btw. It was a failing GPS antenna on the 540.

This is installed in Glasair Sportsman, so there is no manufacturer to ask these questions, really.

Thoughts appreciated...

James

 

[Martin Pauly]

James,

I'm glad you found the reason (failing antenna). An antenna or antenna wiring is the most likely culprit when integrity downgrades differ between two GPS units.

As for which unit (540 or 440) is linked to your "steam" CDI, that is determined by your installation. It could be the 540, or the 440, or there could be a switch linking it to one or the other.

When you write "the guidance and output to the PFD and CDI are linked between the two when it is using GPS for guidance", are you suggesting the steam CDI automatically switches to the same source used by your PFD? Nope, that doesn't happen.

Without knowing how your panel is wired, it's impossible to tell you exactly what's going on. But it's important to know, as you just found out! This would be a good topic to research/experiment with a little, e.g. by turning the 540 and 440 through different flight plans (turn off auto-sync for that), or by powering one unit off at a time and seeinghow your HSI and CDI behave.

- Martin

 

[Flyxaos]

I was hoping you would answer... hah. I have watched your videos on the subject of the IFD's, helpful. Thank you.

The 440 is linked to the steam cdi, for sure. It is not linked to the PFD, the 540 is. There is no switching mech or second adhrs to be able to link the 440 to the main PFD. There is also no switching mechanism for the 540 to select the steam CDI.

Mostly the question is because the Avidyne manual says that when it is on GPS guidance in a 540/440 linked system, you cannot tune the 540 and 440 to different waypoints and then get one set of guidance, gps, out of the 540 to the PFD and one set of guidance to another GPS point on the 440 and it's CDI.

I can have each tuned to a different VOR and have different indications. But in the case of an RNAV LPV approach on a glideslope they are both giving vertical and horizontal guidance to the same place. 540 to the PFD and 440 to the CDI.

I "Think" the guidance to the CDI from the 440 is from the 440. But am not positive on that.
Again, I "Think" the linking is more to do with both being programmed to the same course. But again, not sure.

Thus the question. I will investigate more, including shutting one off/failing to see what I get. Avidyne is actually quite helpful, but I'm not to the tech support phase just yet.

 

[Martin Pauly]

I "Think" the guidance to the CDI from the 440 is from the 440. But am not positive on that.
Again, I "Think" the linking is more to do with both being programmed to the same course. But again, not sure.

Next time you are in the plane, turn the IFD540 off. See what the "steam" CDI does - if it continues to provide guidance, then it's hard-wired to the 440.

- Martin

 

[Jim_R]

Avidyne is actually quite helpful, but I'm not to the tech support phase just yet.

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