GI 275 /Century 21 - HDG mode fails after a while

[NicoN]

Our GI275 drives a Century 21 A/P.

the wiring semms to be correctfor me. Basically it does what is expected.

on ground, i can move the heading bug left or right from the lubber line and the control wheel starts turning. Same if I am taxiing; the control wheel moves if i change direction

I did this a thousnd times:

- In the ghangar

- taxiing on the airport

 

After take-off, it normally works also, but after 10 or 15 minutes, the A/P does not follow the Hdg bug any more. It simply continues flying the last heading. Whatever I do on the hdg bug - nothing happens; It keeps the wings level perfect but tha's it.

Yesterday I was doing test turn using the hedg bug. and I think i had a moment where A/P stopped turning, returned to a wings level atttude

After landing and waiting a couple of miuntes it worked again

We already had an echange of several capacitors on the board ( It hink not all). It is the same situation, not better not worse

Now the question: Can a failed vibration test be the reason that the heaidng mode quits (i.e. the GI 275 stops siganlling to the A/P)?

I know, that sounds crazy

[Fly Boomer]

3 hours ago, NicoN said:

Our GI275 drives a Century 21 A/P.

the wiring semms to be correctfor me. Basically it does what is expected.

on ground, i can move the heading bug left or right from the lubber line and the control wheel starts turning. Same if I am taxiing; the control wheel moves if i change direction

I did this a thousnd times:

- In the ghangar

- taxiing on the airport

 

After take-off, it normally works also, but after 10 or 15 minutes, the A/P does not follow the Hdg bug any more. It simply continues flying the last heading. Whatever I do on the hdg bug - nothing happens; It keeps the wings level perfect but tha's it.

Yesterday I was doing test turn using the hedg bug. and I think i had a moment where A/P stopped turning, returned to a wings level atttude

After landing and waiting a couple of miuntes it worked again

We already had an echange of several capacitors on the board ( It hink not all). It is the same situation, not better not worse

Now the question: Can a failed vibration test be the reason that the heaidng mode quits (i.e. the GI 275 stops siganlling to the A/P)?

I know, that sounds crazy

You may have been on the right track replacing capacitors, but just didn't get to the failing component.  Or simply a bad connection either inside the autopilot or outside.  Can the autopilot be pulled out and re-racked a few times to scrape off corrosion from the connectors?

[N201MKTurbo]

All the different signal sources are combined in the radio coupler. It sounds like the problem is there somewhere. It could be the radio coupler or the GAD 29, or most likely the wiring between those. I would especially look at the little transformers for the heading signals. They are handcrafted by the installer. A bad connection anywhere in that assembly will cause your symptoms.

[LANCECASPER]

Paging @Jake@BevanAviation. Dr. Jacob Kinsey please report to the ER...lol

(https://bevanaviation.com/our-company/)

 

[N201MKTurbo]

The GI275 doesn't use the GAD 29 (my bad) but still has the transformers wired by the installer.

[Jake@BevanAviation]

Odd question but when the system stop working and only does wings level have you ever tried to manual force the system off level and see if it returns to level flight?  This would tell you a couple of things.  The computer is still working for servo drive, attitude reference is still functional, and the roll servo is still responding to commands.  The only thing you would be missing at that point is the heading bug error from the GI275.  I am going to assume you don't have any messages on the GI275 indicating some type of heading failure.  Also, when heading stops functioning have you tried to see if the other lateral control modes like NAV/APPR are functional?

One last thing. Century used this analog gate IC's for signal routing and I have seen these become intermittent with time.  Do you know if any of these IC's were replaced when the unit was worked on?  Century P/N would be 48s114 or MC14016BCP.

[Jake@BevanAviation]

8 minutes ago, N201MKTurbo said:

The GI275 doesn't use the GAD 29 (my bad) but still has the transformers wired by the installer.

 

Transformer isolation is not needed with the Century 21 system.

[N201MKTurbo]

OK, here is the correct interconnect. (My bad again)

[NicoN]

Oaky, thank you.

I forgot to say that NAV-Mode always works fine - forever. It is only heading mode which sucks.

The C21 computer has a stack of 3 PCB. On the upmost Board, 8 caps (looking like grenades have been replaced. The rest not, as someone invested a lot of work in placing the original parts with their values facing down.

AFAIK the C21 system is only the computer in the panel.

- the motor is always working

- I do not see any error messages

-  NAV/APPR work

- wings level function is always perfect

So, I think it is really a problem of the heading part.

 

AFAIK, we only have the C21 flight computer 1D772 which is driven with the signals from  the GI275

No transformers, no Radio-coupler

I have the service manual and I am still not sure whichCD-xxx is where.

The 1D772 has only ONE 37pin D-SUB connector - nothing else

Is it labeled as CD-186 in the manual? Where is CD-175 or CD-194?

 

According to the different manuals ist should be Pin 17 on CD-175 where the signal for Hdg comes in.

But it could also be Pin 14 /CD-186

 

[Jake@BevanAviation]

@NicoN With the other lateral modes still working it shouldn't be a servo issue.  You are down to checking the heading error from the GI275 for the C21 and possibly the IC's for signal processing inside the C21.  Be very careful if you are going to open the unit as all boards are interconnected with ribbon cable that are known to cause issues.  CD-175 would be the DG or HSI connector, CD-194 would be incoming power/ground, compass jumper select, and disconnect switch.  CD-186 would be the connector at the back of the rack for the C21. 

What compass system did you replace with the GI-275?

[NicoN]

Correct.

This was an issue when the plane came back from the installer of the GI275s:

They removed a DG hooked to the C21 (NOT HSI!!!!!)

And obviously they did NOT change the previous jumpering . It took me a while to convince the installer that the GI 275 is "talking" always HSI and not DG.

After changing that (but where exactly) the hdg mode worked just fine. That is why I am searching for the problem in the C21 and not in the wiring itself

So, the question is where the wires from our GI275 really go - directly into the 1D772? Or is something in between (a magic box?).

After a couple of flights my co-owners started telling me that the HDG mode is failing.

As I wrote:

- the ground test is always fine

- even in flight the hdg mode seems to work at least for a while

- I tested the hdg mode even while on ground with external power for 30 minutes (!) moving the hdg bug left and right - fine!

What I cannot test on ground is if engine vibration has some negative impact.

That's why I am trying to understand the vibration test.

Can the diagnostic logs tell me more?

So far I could not check them. Some seem password protected.

 

 

 

[Jake@BevanAviation]

@NicoN for a HSI the most common was a NSD-360.  Excitation to the NSD-360 would have been 9.6VDC.  The output error under the lubber was around 4.8VDC as the Century 21 uses a half of supply for reference and the supply regulation is 9.6VDC.  Going left or right of the lubber line should produce a heading error that goes above or below the 4.8VDC reference.  Depending on how the installer wired the aircraft it could be to CD-175  or to CD-186.  See print below for details.  The Garmin IM show the GI275 P2752 pin 13 to CD-175 pin 17 for the HI and GI275 P2752 pin 52 to shield ground for the LO. 

No magic box would be needed for the GI-275 install, it should be a shielded twisted pair from the GI275 to the C21. If the installer followed the Garmin IM they should have wired it to CD-175.  It might be worth it to have Garmin look at the logs for the heading error to see if it is dropping out over time. After discussing this with our installers we think if the system was failing the vibration test you would have several messages and the system would have to re-align.  Another thing they discussed was what color is the heading bug text is?  Do you know if the installer used a GMU-11 or 44? Also, if you have picture/video of the system when it is failed that might help. 

 

[John Car]

Was this issue ever fixed?

[NicoN]

No. We still have the same problem.

The GI 275s seems to work perfect - no error messages, no aligning. The heading/compass is stable and super precise, but the calibration was another nightmare with our installer. Finally we did it on our own.

Last weekend I had 2 flights in F120 for  4 hours - and believe it or not: HDG mode was perfectly working at cruise altitude and failed again in low altitudes.

I tested it several times and HDG mode was working with manual HDG changes and also with GPSS activated.

When it was working I reached under the panel and the tray was really cold. So, maybe we have a temperature problem.

I quickly made an extension cable to operate the computer off the tray, but did not try it in real life so far.

 

Another question: has someone a schematic of the boards inside the computer available? I have the service manual but this ends always with "replace unit".

For me, the upmost board drives the heading function, the middle seems to be responsible for NAV mode and the lower seems to have some power supply function and the power transistors for the motor.

Edited June 4 by NicoN

[jetdriven]

It could be the ribbon cables inside the unit unit. We had the unit to do this and we sent it to auto pilot Central. I think it was and they went through it and messed with it and cleaned it up and did whatever it cost 1000 bucks though and it works now it’s been two years. Another thing also was those stupid circular connectors will act up in flight. Ours was randomly ignoring the bug at times and then halfway ignoring it if you can imagine that the servo would just kinda go to the neutral position and you could override it left or right it would just stay there. And then we figured out what it was. It was a cold solder joint to the AC transformer inside the Magic box. Of course I soldered  this wire and for some reason the an ionic decided to resolder  it and that’s when it was broken, but it was barely contacted. It was getting some AC through and it was kind of fading in and out and the way it was responding to was responding to head g