I just had my avionics shop install a Shadin ADC-200 (yellow tag) to enable air data on my IFD-540. The HSI is a King KCS-55A. I thought everything was working on the test flight -- the air data definitely works -- but now I'm getting GPSS "fail" on my STEC 55X when I try to enable GPS steering.

Before I just toss it back in the avionics shop's lap, where should they be looking for the culprit -- at the HSI end of things or at the IFD end? The STEC works fine in other modes, HDG, ALT, etc.

I'm hoping this is just some parameter or speed mis-match but maybe someone here has some experience with something similar. I've opened a ticket with Avidyne but haven't been able to get back to the airport to download logs yet. I've emailed STEC support, with no response.

Thanks for any pointers!

Rolfe

I'm using an S-Tec 60-2, which means I have an external GPSS steering converter, the S-Tec 55 I believe incorporated it internally.

If I am correct the GPSS steering module can accept either serial or ARINC 429 for steering information, mine is using ARINC 429, the setting for me is on the "Main ARINC 429 Config" page, I have "Out 1" speed is set to "Low" and Data is set to "GAMA 429"

I'm using an IFD440 and it only has one ARINC 429 out.

Thank you! This is very helpful as a place to start looking! I'm hoping it is something so simple as wrong protocol or wrong speed.

Rolfe

I used to have a similar setup and experienced those same issues, which baffled both me and my installer. Each time this happened I would recycle the 55X and the GPSS would work fine.

I also discovered that on the ground following powering up the Avionics Master switch, I had to allow the IFDs to fully initialise first before the 55X was switched on and initialised resulting in no further GPSS failure.

My installer could determine why this solution worked and it may not be the solution to your issue, but it is a simple, cost free method to try.

Victor,

Thanks for this -- yes, it is a simple thing to try and I believe I've heard of someone else having a similar power-up sequence issue.

Rolfe

That is precisely the solution.

If you let the STEC 55x boot up first, it will set and lock a speed for the RS232 line, and will not adjust again. If that doesn't match the transmitted speed that the IFD is using for that Rs232 out, then you will get GPS FAIL on the STEC.

However, if the IFD is already running and transmitting, the STEC will sync correctly.

So, just remember to TURN OFF the A/P before shutdown (not a bad idea anyway), and then do not power it up until the IFD has booted already.

* Orest

I'm guessing I don't have that issue with mine either because I'm using a 60-2 with the GPSS module, and/or because it gets steering information via ARINC and not RS-232.

To you statement, I wonder what speed the S-TEC 55X is expecting when it defaults to a given speed, maybe set the output of the IFD to be that speed?

Our issue was that if we matched the 55x default RS232 speed, it would be affect another part of our setup unfavorably.

If has been a while, but I think it related to the inability (early on in the IFD firmwares) to have different speeds on in/out of the IFD port, or we needed HI speed for something else, something like that. That may no longer be a limiting (install) issue.

* Orest

I know Avidyne had issues early on with different speeds in vs out on the same channel, but I believe that has been fixed long ago.

If I had to guess the S-Tec is probably expecting 9600 bps for the speed.

My install was a long time ago too!

Now that I think about it, I believe that it was like this ... we wanted to reserve HI speed for ASPEN / IFD comm. When the STEC boots up, if it finds HI speed transmissions it will keep the HI speed, else if all is quiet, it will downstep to LO and stay there. It never checks again, unless it is power cycled of course.

There was (is?) no way to change this STEC behavior or set a default speed (unlike the IFD or ASPEN).

So booting up IFD first, then STEC 55x, allows for them to handshake. It is just automatic now for me, and there is perhaps some value to having the A/P OFF on initial startup, so I have never bothered to explore any further solutions.

But, it is a real gotchu, if you don't know what is going on.

* Orest

Since I started this thread, I thought it would be appropriate to end it with the solution.

It turns out that recent avionics work (air data computer connection from HSI to IFD) required removing IFD and the STEC 55X which sits above it. The STEC55X computer is a very tight fit in the tray, and the shop did not completely reseat it. But, complicating matters, they reseated it just enough to work on the flight home where everything was flawless. It was the next flight when it failed GPSS and the next flight after that wouldn't power up at all. That was the clue that led us to take both units out and reseat both. The bezel on the 55X sits further recessed from the lip of the IFD. If they are even, the autopilot appears seated correctly, but isn't. There is a second push of about 1/4 inch to go.

Hopefully, this saves someone else some time when troubleshooting a similar issue!

Rolfe

What a satisfying fix. The best ones are simple ones.

* Orest