Upgrading to a GNC 355...

Discussion in 'Avionics and Upgrades' started by BrianNC, Nov 19, 2019.

  1. BrianNC

    BrianNC Pattern Altitude

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    Our club is going to upgrade our 172 initially with a G5 (in the HSI version) and a GNC 355. We have an STEC 30 with altitude hold installed already. I know the G5 has a built in GPSS. Is that all that's needed for the 355 and STEC to work with each other since there is a GPSS, or do you still need the Garmin GAD 29B to make them work together? I have seen on this forum some posts quoted from Garmin about the GAD 29B, but didn't know if you still needed that with the G5 installed.

    Or what is needed for this setup for the STEC 30, G5, and 355 to play together?

    Thanks for clearing this up.
     
    Last edited: Nov 19, 2019
  2. AA5Bman

    AA5Bman Pre-takeoff checklist

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    I’m not an expert but I’ve been through the wringer trying to figure out how the same basic setup would work with my Century IIB. Suffice to say, I’m pretty confident you need the Gad29B, but once you do that, you get GPSS out of your existing AP. Since the Stec is not attitude-based, you can pull out your whole vacuum system and replace your AI with a G5 too (this is what I wanted to do, but it’s not going to be possible because my AP relies on attitude input from my AI that the G5 won’t provide).
     
  3. Salty

    Salty En-Route PoA Supporter

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    You still need the GAD 29B
     
  4. Salty

    Salty En-Route PoA Supporter

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    From the G5 install manual. You will also need the GMU11. The G5 manual isn't updated for the 355 yet, but this picture is still accurate. I just installed a 355 last week.

    Screen Shot 2019-11-20 at 11.07.48 AM.png
     
    Last edited: Nov 20, 2019
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  5. Salty

    Salty En-Route PoA Supporter

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    Here's the page from the 355 manual.

    Screen Shot 2019-11-20 at 11.22.03 AM.png
     
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  6. Salty

    Salty En-Route PoA Supporter

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    But this page confuses me. It implies you can do it without a G5 at all.

    Screen Shot 2019-11-20 at 11.24.35 AM.png
     
  7. Salty

    Salty En-Route PoA Supporter

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    And just to round it out here's the page from the G5 manual on S-TEC integration

    Screen Shot 2019-11-20 at 11.30.45 AM.png
     
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  8. asicer

    asicer En-Route

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    It would seem to me that hooked up like above, there's no real way for the A/P to track a heading and only the 55X would have GPSS. For autopilots without a dedicated GPSS input, the G5 normally does it by steering the heading bug input.
     
  9. N1120A

    N1120A Line Up and Wait

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    It basically works the same was as the STEC GPSS module works, with the GPS essentially providing heading information.
     
  10. Ryan Klems

    Ryan Klems Pre-takeoff checklist

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    You actually need all of those connections. The 355 to GAD29B and GAD29B to S-tec and the 355 to S-tec.
     
  11. asicer

    asicer En-Route

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    Right, and that's not a real heading (i.e. magnetic reference) but rather a fixed GPS track, right?
     
  12. N1120A

    N1120A Line Up and Wait

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    It is the GPS ground track heading. Which, despite protestations from people who think they are smarter than they are, is what ATC is looking at the vast majority of the time.
     
  13. chemgeek

    chemgeek Cleared for Takeoff

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    If you have G5s and the GMU-11 and GAD29B a legacy AP will absolutely track a magnetic heading using the heading bug on the G5 HSI. If following a flight plan in the GPS unit, the GAD29B will emulate GPSS by providing analog inputs to your AP in heading mode. This include intercepts, anticipating turns, flying holds, etc. The AP will also follow VOR or LOC signals in tracking mode (on an STEC 20/30 for example). The GAD29B is required to translate the digital output from the GPS unit into analog signals for your legacy autopilot.

    I just installed dual G5s in my plane, and my STEC 20 works flawlessly with a GNS-430 for enroute, GPS, or VOR/LOC/ILS with the GMU-11 and GAD29B. It will also fly magnetic headings assigned by ATC.
     
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