Garmin G5, Century 21 wild rolling autopilot stumper

[jkarch]

I have a bit of a stumper.  I brought my plane up to an avionics shop to do an installation of a dual G5 system connected to my M20J’s Century 21 Autopilot.  When I got the plane back I found that the autopilot in DG mode could not hold a heading and would hunt with rather extreme bank angles around a heading point.  Namely, it would go into steep turns (+/-45 degrees+) left and right as it crossed the heading bug in HDG mode and I would have to take over the controls by force and disable autopilot to prevent this undesirable operation.  NAV mode in GPSS also shows the same behavior.  The Attitude indicator had to be retained (probably for this very reason).   On the ground, when the engine was running and the attitude indicator was level, engaging the autopilot where the HDG=HDG bug and HDG mode enabled, the roll motor would hold ailerons level.  When the engine shuts down, the AI often settles in a left banking position.  When this happens, the ailerons respond by rolling to the right, so it’s clear that the autopilot is responding appropriately to the attitude input, and therefore the attitude connection, which was unmodified on connector CD176 seems correctly connected.   Before the G5 upgrade the autopilot worked fine with the original 52D54M gyro.

I made sure that the jumper connecting pins 13 and 14 (DG select pin) on connector CD-194 was removed since the 52D54M gyro was replaced by a G5 HSI as per Century Flight System’s recommendation.  The G5 install manual incorrectly targets different jumper pins on CD-185, which does not exist on a C21 autopilot.  This makes me question the reliability of the G5 install manual, but that’s another issue.

It appears that on connector CD175, someone took a grey wire out (perhaps it is the HDG signal that is on pin 17 but I don’t know) and wired it to a non-shielded 22 AWG wire. I would probably prefer that it be shielded but I’m sure it’s not the issue as it’s probably heavily filtered.  Pin 2, however, is a blue wire that is called course select and it is also supposed to be connected to the GAD29B.  Both signals need to be connected and shielded to the GAD29B as per Garmin manuals, though I wonder if there are potential erratas as the information provided is incorrect.   CD175 is left hanging and while the grey wire (suspected to be Pin 17, HDG signal) is connected, Pin 2 is left floating.  When powered on, the blue wire pin 2 measures 4.825V using a Fluke 87 IV meter wrt airframe ground.  Pulling the GTN750 and GAD29B circuit breakers did not change this voltage measurement, making me question if this signal is connected to anything of interest.

 

I’m supposed to bring the plane next weekend in to the avionics shop for a fix, but if anyone has experience working with the C21 and G5s / GAD29B and can chime in on potential issues, it would be helpful because the shop is trying to do the work on it over the weekend while I wait (it’s about 100 miles away from me).

 

I know that the CIII autopilot has radio couplers, etc, but this connection is direct.  While there is a GPSS scaling factor, there is no option for HDG bug scaling factor so I wonder if the voltage outputs going to the Century 21 are correct.  I also have heard that the C21 is supposed to be limited to +/-29.5 degrees of bank in spite of any inputs on HDG or CRS which is a bit strange too.  Any ideas?

I did notice this: https://www.beechtalk.com/forums/viewtopic.php?f=21&t=71222  .  It does seem the attitude input is suspect because it should be clamped to max deflection, but on the ground it seems to respond, so I'm not 100 percent sure, but perhaps as one poster suggests the signals are intermittent after being relocated from one location to another.

Thank you!

Edited February 2, 2020 by jkarch

[tigers2007]

How old is the actual Century box? I’ve had similar issues with my C2b doing the random rocking and then a deep roll to the left. I’ve had the connectors cleaned and both the AI and DG overhauled and it randomly does that. It makes me wonder if the 40+ year old main circuit board needs a rework. I’m hesitant to ship it out due to the fear of a ridiculously priced estimate to basically reflow solder joints.


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[OR75]

I wired the G5 to the century IIB and it has the opposite effect : it tracks the heading bug very very very slowly 

I reverted back to the DAC 31 which tracks great 

[jkarch]

So I have an update and should have a better one next week once the fix is made.  Seems the installers somehow clipped/disconnected the excitation wire to the attitude indicator, so the attitude indicator control response remained flat.  That enabled the aircraft to roll left and right with impunity (I stopped it at a steep turn).  The signal wire is connected, as is the excitation return, but the excitation from the tray to the connector may have been disconnected during the G5 install.  I'm biding my time until I install a GFC500 and 86 the vacuum system and gyro, but my gut feel is this will fix it.   In the meantime, I took the advice of many and contact cleanered the heck out of the connections and made sure everything was ok.  It's definitely useful to have some Pomona test pins and a volt meter for this kind of debugging.

-J

[jkarch]

On 2/2/2020 at 9:08 AM, OR75 said:

I wired the G5 to the century IIB and it has the opposite effect : it tracks the heading bug very very very slowly 

I reverted back to the DAC 31 which tracks great 

The G5 has a GPSS scale factor for gain setting.

[OR75]

9 hours ago, jkarch said:

The G5 has a GPSS scale factor for gain setting.

It had no impact when I adjusted it 

try to go slow first like setting it to 1.1 ... no change 

went up to 3 ... no change 

Garmin was not able to tell what type of adjustment I should be typically making 

 

[jkarch]

Was it in GPSS or DG heading mode?  It seems the scale factors are different.  For DG heading mode the adjustments could the the potentiometers on the autopilot unit itself.  For example the L and R adjustments that bank for standard rate turns at full deflection (90 degrees) of the DG.  I think the GPSS gains determine how you steer in to intercept a course.  Of course your unit may behave differently than the C21.  The C21 has a centering potentiometer, followed by left and right bank adjustments.

[OR75]

25 minutes ago, jkarch said:

Was it in GPSS or DG heading mode?  It seems the scale factors are different.  For DG heading mode the adjustments could the the potentiometers on the autopilot unit itself.  For example the L and R adjustments that bank for standard rate turns at full deflection (90 degrees) of the DG.  I think the GPSS gains determine how you steer in to intercept a course.  Of course your unit may behave differently than the C21.  The C21 has a centering potentiometer, followed by left and right bank adjustments.

Heading mode is what I did most testing.   GPSS was also very sluggish 

[jkarch]

10 hours ago, OR75 said:

Heading mode is what I did most testing.   GPSS was also very sluggish 

Some people had good results with a gain circuit. I heard there was a radiocoupler or something involved, maybe that was an issue?

[Bob Weber]

Think of it this way, I believe it may help. That system must be set up to form a balance between the attitude signal and the roll mode signal. Old school training taught us "10° of HDG should equal 10° of bank". There is a flight adjust procedure that should have been done on an initial flight after the upgrade. I can help you thru the flight adjust, but I have a few questions that may help first.

Bob Weber

webairconsulting.com 616 822 1999

Edited February 4, 2020 by Bob Weber

[N201MKTurbo]

I think you should double check your wiring.

[Bob Weber]

1 minute ago, N201MKTurbo said:

I think you should double check your wiring.

Hence the few questions...

[OR75]

42 minutes ago, Bob Weber said:

Hence the few questions...

I did double check and double the wiring

On the other hand I am reluctant to start messing with the Century IIB console adjustments (fear of ending up with neither of the GPSS systems working)

[Bob Weber]

12 minutes ago, OR75 said:

I did double check and double the wiring

On the other hand I am reluctant to start messing with the Century IIB console adjustments (fear of ending up with neither of the GPSS systems working)

Your system would be easy to troubleshoot, I can certainly help with it as well.

Century Flight Systems was the first factory school I attended nearly 30 yrs ago and I still talk to my old instructor from time to time.