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What would cause the HSI to show half up half right instead of half left on GTN650 startup self test?

RescueMunchkin

By RescueMunchkin
in Avionics/Panel Discussion

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EricJ Grand Master

EricJ

Posted
Posted
4 hours ago, RescueMunchkin said:

It's not a new installation, but not my plane (yet)...owner says it worked fine before but hasn't used it in a while

I don't know what interface is used with those, but I suspect it's either a miswire or a software configuration issue in whatever is driving it.

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65MooneyPilot Collaborator

65MooneyPilot

Posted
Posted

I checked mine and the OBS and DTK is the same in the test 150 degrees. When I turn the OBS knob the OBS in the test does not change. It stays 150 degrees. In the OP’s picture it has 003 degrees and DTK 150 degrees. So the needle going to the right is correct. If the OBS knob in the OP’s changed the degrees in the test then if you turned it to 150 degrees it would be on the left.

30FCBC2B-5450-48EF-8458-9503EA06E4B0.jpeg

Vance Harral Mentor

Vance Harral

Posted
Posted
3 hours ago, 65MooneyPilot said:

If the OBS knob in the OP’s changed the degrees in the test then if you turned it to 150 degrees it would be on the left.

You are confused about what the GNS/GTN/GNX startup test does.  This is common, and understandable.

With respect to a traditional CDI such as in your and the OP's pictures, the startup test simply applies a voltage to the input pins of the CDI that control the lateral needle position relative to center, with a voltage that should drive the needle halfway across the display to the left side.  That's all it's testing: the ability of that voltage to drive the needle left.  It doesn't matter what you set the OBS knob to during the startup test sequence, it will stay in the same "half left" position regardless of the OBS setting.

You appear to be under the impression that the startup test creates a situation where it simulates your position being east of a course of 150 TO a waypoint.  Again, that's not what the test does.  The test is much simpler: it just drives the needle halfway left across the display.

On 11/11/2023 at 9:25 AM, 65MooneyPilot said:

your DTK is 150 degrees and if you dial up 150 it will be left of course

More confusion here.  A traditional CDI such as the Garmin GI-106 in the OP's picture and the King indicator in your picture does not transmit, receive, or display desired track (DTK).  The DTK=150 value shown in the test is irrelevant in your and the OP's airplanes.  It's there for more advanced indicators such as the HSI on a G5/GI275/G3X.

The startup test does display an OBS value, which is the value it's reading from the OBS setting on the external CDI.  If you rotate your OBS knob, you'll see the OBS value on the GNS/GTN/GTX screen change accordingly.  This is good to check on startup because it verifies that OBS output from your CDI is being correctly received as an input to the GNS/GTN/GTX.  Again, however, you can spin that knob all you want and the lateral needle position won't change during the startup test.

I'd bet a dollar that the OP's lateral CDI pins are hooked up backwards, and no one has ever noticed, because the people operating the airplane aren't actually using it for navigation.  Most of the VFR pilots I fly with these days don't even want to talk about CDIs, they just steer back and forth across the magenta line on their moving map.  It irritates me a little, but I don't really have a problem with that for VFR ops.  I try not to be an "old man" about it, and save the rants for IFR students.

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65MooneyPilot Collaborator

65MooneyPilot

Posted
Posted

Ok, great explanation! The test being a simple voltage says a lot. Getting into the nuts and bolts of how it works is always interesting. The installer should have noticed the indicator error when it was initially checked but nothing surprises me these days. I have caught several wiring errors since I have been installing all my new equipment.

Vance Harral Mentor

Vance Harral

Posted
Posted
2 hours ago, RescueMunchkin said:

Unfortunately nobody took that bet..but you do win the warm fuzzy feeling of saying you told us so!

That's appreciated, but mostly I'm glad it turned out to be something simple.  To give credit where it's due, Pete Mc was the first to suggest it.

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Pinecone Grand Master

Pinecone

Posted
Posted

The big take away is that it is IMPORTANT to check this in your Before Taxi checks.

How many just push the Continue and don't actually check the indicators.

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Vance Harral Mentor

Vance Harral

Posted
Posted
9 hours ago, Pinecone said:

How many just push the Continue and don't actually check the indicators.

Roughly 80% of all the pilots I've flown with who have GTN/GNS navigators.  Most of them have never had the startup test sequence explained to them, and they don't understand what it's intended to verify.  They also don't understand why they should care about it, because they don't understand that the connectivity between the navigator and the CDI is just a pedestrian set of relatively vulnerable, small wires, that bounce around and fatigue with every bump of turbulence.

Some installations are more robust than others, of course, and I've never actually seen a GTN/GNS-connected CDI go belly up in the middle of a flight.  I'm probably extra sensitive to it because I wired my own.  It just seems more sketchy when you're crimping the pins and routing the wires yourself; and thinking, "Really?  That's it?"

PeteMc Veteran

PeteMc

Posted
Posted
On 11/21/2023 at 3:05 PM, Vance Harral said:

To give credit where it's due, Pete Mc was the first to suggest it.

Well... for years my staff hated it when they came to me with a problem and I'd once again say to them "is it plugged in" meaning did you trace everything back to make sure something didn't get swapped or come loose.  

Typically the startup check is to make sure the GTN (or whatever) is still sending out the voltages it needs to give the expected result.  My guess in this case the original installation was faulty.  I doubt there is any way the wires were swapped if they didn't disassemble and resolder the connector for it to show a reverse sensing.  But sure glad it was a relatively quick fix!

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Vance Harral Mentor

Vance Harral

Posted
Posted
8 hours ago, PeteMc said:

I doubt there is any way the wires were swapped if they didn't disassemble and resolder the connector for it to show a reverse sensing.

The wires aren't soldered into the connector, they use pins which are (somewhat) removable.  And I don't think Garmin ships pre-built harnesses for navigator/CDI connections.  They are generally built at the avionics shop, since the distance between the navigator box and the CDI varies from airplane to airplane.

The connection from the navigator to the CDI (or to a GAD29 in the case of a G5 HSI) is just a bunch of individual small-gauge wires, terminated with pins that "click" into the connectors you get from Garmin.  It is definitely possible to get an individual pair of wires associated with a single differential signal backwards.  You can probably guess how I know this.  :lol:

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PT20J Grand Master

PT20J

Posted
Posted

One nice thing about the G3X is that I no longer have to worry about this bit of preflight checking. In fact, the GTN allows eliminating that startup page with a G3X installation.

Vance Harral Mentor

Vance Harral

Posted
Posted
13 minutes ago, PT20J said:

One nice thing about the G3X is that I no longer have to worry about this bit of preflight checking.

Is this because the GTN interfaces to the G3X via HDSB rather than discrete signals?  If so, I presume there is still some sort of integrity check associated with that connection.  Maybe that integrity check is continuous, rather than performed once at startup?

PT20J Grand Master

PT20J

Posted
Posted
1 hour ago, Vance Harral said:

Is this because the GTN interfaces to the G3X via HDSB rather than discrete signals?  If so, I presume there is still some sort of integrity check associated with that connection.  Maybe that integrity check is continuous, rather than performed once at startup?

Garmin doesn’t really explain it. I think the connection is CAN Bus via the GAD 29B. 

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