GFC500 problem

[Larry]

I had a GFC500 installed driven by a G5 with a G500 (legacy).  I'm getting a slight porpoising of 1/2 degree up and down every +/-15 seconds in level flight smooth air. I believe that I'm also getting it during climb and descent.  The pitch change is reflected in the G5 and G500, but don't know if this is the cause or reflection of the autopilot's action.  The installer claims he can find nothing wrong.  Has anyone experienced anything of this nature?

[81X]

Try using alternate static to see if it goes away.  If so, you have water or some other issue in the static system.

also, try configuring the AP to run without engaging it and see if the FD is commanding the pitch up and downs. If not, it could be a servo or control lube issue. 
 

It’s also worth a call to Garmin to ensure it’s configured correctly from a software standpoint.   

[Tom 4536]

1 hour ago, Larry said:

I had a GFC500 installed driven by a G5 with a G500 (legacy).  I'm getting a slight porpoising of 1/2 degree up and down every +/-15 seconds in level flight smooth air. I believe that I'm also getting it during climb and descent.  The pitch change is reflected in the G5 and G500, but don't know if this is the cause or reflection of the autopilot's action.  The installer claims he can find nothing wrong.  Has anyone experienced anything of this nature?

My newish GFC500 does the same thing. I have GI275's rather than a G5. My installer doesn't know of a fix.

[StevenL757]

2 hours ago, Larry said:

I had a GFC500 installed driven by a G5 with a G500 (legacy).  I'm getting a slight porpoising of 1/2 degree up and down every +/-15 seconds in level flight smooth air. I believe that I'm also getting it during climb and descent.  The pitch change is reflected in the G5 and G500, but don't know if this is the cause or reflection of the autopilot's action.  The installer claims he can find nothing wrong.  Has anyone experienced anything of this nature?

First - start by ensuring your firmware/SW versions are current across all your Garmin devices where applicable.  This is a best-practice that your installation shop should’ve done by default before releasing the aircraft back to service.  So, verify this with them first.

Second - I know it sounds crazy, but this has helped several people in the past.  Clean both control column shafts, and then lubricate both with silicone spray…and ONLY silicone spray.  Do not use Triflow LPS or anything like that, as it can cause buildup in the guide blocks under the panel that surround the shafts.  This will eventually cause increased sticking and friction.  Also, if you have any RAM mount or other device connected to one or both control columns, remove them temporarily and go fly again to test.  It’s possible - yet unlikely - that this is the culprit, but should be ruled out.  The added weight on the shafts by one or more devices is an excess force that an autopilot needs to counteract when in command of the aircraft.  Fly a multitude of headings - including with headwinds, tailwinds, and crosswinds during testing.  If the problem still persists in those scenarios, then notify your shop and make arrangements to have them look into it further.  Have them engage Garmin for guidance - don’t accept the shop’s “we don’t know” stance.

Steve

[Tom 4536]

12 minutes ago, StevenL757 said:

First - start by ensuring your firmware/SW versions are current across all your Garmin devices where applicable.  This is a best-practice that your installation shop should’ve done by default before releasing the aircraft back to service.  So, verify this with them first.

Second - I know it sounds crazy, but this has helped several people in the past.  Clean both control column shafts, and then lubricate both with silicone spray…and ONLY silicone spray.  Do not use Triflow LPS or anything like that, as it can cause buildup in the guide blocks under the panel that surround the shafts.  This will eventually cause increased sticking and friction.  Also, if you have any RAM mount or other device connected to one or both control columns, remove them temporarily and go fly again to test.  It’s possible - yet unlikely - that this is the culprit, but should be ruled out.  The added weight on the shafts by one or more devices is an excess force that an autopilot needs to counteract when in command of the aircraft.  Fly a multitude of headings - including with headwinds, tailwinds, and crosswinds during testing.  If the problem still persists in those scenarios, then notify your shop and make arrangements to have them look into it further.  Have them engage Garmin for guidance - don’t accept the shop’s “we don’t know” stance.

Steve

I tried silicone spraying the control column shafts and had all my Garmin software updated in the past month. I don't have anything mounted on my control columns, but my problem persisted after all these measures.

[StevenL757]

For @Larry and @Tom 4536, I missed this earlier, but did you try either of @81X' s first couple of recommendations also?

[Tom 4536]

Somehow I missed 81X's post. Thank you. All my servos have been replaced, but the idea of trying alternate static source and also seeing what happens to the FD without the autopilot engaged is a great idea. As soon as my plane is out of annual I will try those ideas.

It does appear that the autopilot is overcorrecting. With my Century 41 AP, it was just a matter of adjusting a gain control.

[PT20J]

Mooney used a several different guides for the control columns. Later models use an eyeball made of nylatron which should not be lubricated. They do seem to get stiff over time but it’s the bearing surface between the ball and socket that wears and the only solution is to replace them.

That said, they would have to be very bad to affect the autopilot and you would surely notice it when hand flying. 

Did the installer check the bridle cables for proper tension? I would also check to see if the trim servo is hunting which would cause the trim wheel to rock back and forth. 

I think @donkaye had some issues initially — maybe he has some ideas. 

Skip

[Tom 4536]

19 hours ago, PT20J said:

Mooney used a several different guides for the control columns. Later models use an eyeball made of nylatron which should not be lubricated. They do seem to get stiff over time but it’s the bearing surface between the ball and socket that wears and the only solution is to replace them.

That said, they would have to be very bad to affect the autopilot and you would surely notice it when hand flying. 

Did the installer check the bridle cables for proper tension? I would also check to see if the trim servo is hunting which would cause the trim wheel to rock back and forth. 

I think @donkaye had some issues initially — maybe he has some ideas. 

Skip

The servo cable tensions were checked. When the plane porpoises, the trim wheel does not move but the yokes move slightly. Not sure what the "bridle cables" are. Thank you for your input.

[PT20J]

Just now, Tom 4536 said:

The servo cable tensions were checked. When the plane porpoises, the trim wheel does not move but the yokes move slightly. Not sure what the "bridle cables" are. Thank you for your input.

Bridle cables are the cables between the servo sheave and the control push-pull tubes that translate the rotational motion of the servo motor to the linear motion of the controls. If they are loose, there will be slop in the system, and, depending on time constants involved, it can cause an oscillation.

[Tom 4536]

1 minute ago, PT20J said:

Bridle cables are the cables between the servo sheave and the control push-pull tubes that translate the rotational motion of the servo motor to the linear motion of the controls. If they are loose, there will be slop in the system, and, depending on time constants involved, it can cause an oscillation.

Those are the cables that were checked.

[PT20J]

@Tom 4536, does it oscillate in all pitch modes or only ALT hold? That might eliminate the static source as an issue. You've already replaced the servos and checked the cable tensions. If all the setup parameters are correct per the STC, then it might be something with the airframe. You mentioned having to adjust the gain on the old autopilot. I'm not sure of the circumstances, but are you sure that it wasn't tweaked to overcome some other problem in the airframe? The GFC 500 servos are not as robust as some legacy servos. Ultimately this is a problem that will need to be worked out with your installer and Garmin field support which has direct access to engineering.

Skip

[Tom 4536]

10 minutes ago, PT20J said:

@Tom 4536, does it oscillate in all pitch modes or only ALT hold? That might eliminate the static source as an issue. You've already replaced the servos and checked the cable tensions. If all the setup parameters are correct per the STC, then it might be something with the airframe. You mentioned having to adjust the gain on the old autopilot. I'm not sure of the circumstances, but are you sure that it wasn't tweaked to overcome some other problem in the airframe? The GFC 500 servos are not as robust as some legacy servos. Ultimately this is a problem that will need to be worked out with your installer and Garmin field support which has direct access to engineering.

Skip

It oscillates only in alt hold. I mentioned the gain control in the old autopilot only to mention that the gain control was available if it needed to be adjusted. It didn't need to be adjusted. To my knowledge the GFC 500 doesn't have a gain control to reduce the oscillations. The old AP worked perfectly until it died. After two attempted repairs by Executive Autopilot I gave up on the old Century 41 AP and had the GFC 500 installed.

[PT20J]

3 hours ago, Tom 4536 said:

It oscillates only in alt hold. I mentioned the gain control in the old autopilot only to mention that the gain control was available if it needed to be adjusted. It didn't need to be adjusted. To my knowledge the GFC 500 doesn't have a gain control to reduce the oscillations. The old AP worked perfectly until it died. After two attempted repairs by Executive Autopilot I gave up on the old Century 41 AP and had the GFC 500 installed.

If it only oscillates in ALT, I would definitely check out the static system. If that’s good, it might be the GI275.

There are adjustments to the GFC 500. They are set up in configuration mode. But the dealer is supposed to set them according to the STC. 

Skip

[carusoam]

There is a history of some APs using a rubber hose to connect to the static system…

The rubber hose would age, and break, causing the affect of an open alt static valve…

test your alt static valve to see if it is working… and showing a difference when open or closed…

Some static drains have also caused some issues by not closing properly…

This is probably best done during the pitot static check…

PP thoughts only, probably nothing to do with a GFC500 problem…

Best regards,

-a-

[philiplane]

You have the same problem that Piper Archers & Warriors have. In their case, it was attributed to over-sensitive reaction to static system pressure fluctuations. I believe Garmin did a software update to smooth the pressure sensor data.

[Ragsf15e]

I know we discussed lubing the control column earlier, but @donkayehad a similar problem with porpoising that was resolved with fixing “stickyness” in pitch controls.  Might be worth reading his thread.

[whiskytango]

I have noticed the same problem occurring intermittently.  It seems to occur after I get a new altimeter setting from ATC while in altitude hold mode.  The GFC500 will cyclicly hunt about 40 feet above and 40 feet below the previously set altitude over about a 5 second period.  I have not tried lubing the controls or testing with Alt Static (I will try those on next flight).  What does seem to make it stop is to temporarily use Vertical Speed mode, set to 0 FPM.  It will then hold selected altitude perfectly, until the next altimeter setting change.  Not sure how to interpret this. 

[donkaye]

5 minutes ago, whiskytango said:

I have noticed the same problem occurring intermittently.  It seems to occur after I get a new altimeter setting from ATC while in altitude hold mode.  The GFC500 will cyclicly hunt about 40 feet above and 40 feet below the previously set altitude over about a 5 second period.  I have not tried lubing the controls or testing with Alt Static (I will try those on next flight).  What does seem to make it stop is to temporarily use Vertical Speed mode, set to 0 FPM.  It will then hold selected altitude perfectly, until the next altimeter setting change.  Not sure how to interpret this. 

While it shouldn't hunt, it definitely should change altitude if the altimeter setting has changed.  That is one of the benefits of the GFC 500 over the KFC 150.  It modifies the altitude automatically to account for altimeter setting changes.

[Tom 4536]

45 minutes ago, whiskytango said:

I have noticed the same problem occurring intermittently.  It seems to occur after I get a new altimeter setting from ATC while in altitude hold mode.  The GFC500 will cyclicly hunt about 40 feet above and 40 feet below the previously set altitude over about a 5 second period.  I have not tried lubing the controls or testing with Alt Static (I will try those on next flight).  What does seem to make it stop is to temporarily use Vertical Speed mode, set to 0 FPM.  It will then hold selected altitude perfectly, until the next altimeter setting change.  Not sure how to interpret this. 

This is great info. I know that the autopilot corrects the altitude as soon as I reset the altimeter setting, but what I don't know is whether the oscillations start when the altimeter setting is changed. I haven't found any way to stop the oscillations. My plane is in for annual so I won't be able to check this for a while. Setting the Vertical Speed to 0 will be an interesting experiment. My GFC 500 software was updated in the past month and the update did not solve the problem. Thanks for the ideas.

[thomas1142]

I have this issue on climb out. I set IAS climb at 90 knots and will see anywhere from 88 to 93 knots on the climb. At times the pitch correction to the correct airspeed is a bit excessive, especially in IMC. I started using VS climb out and it works much better. Kind of disappointed.

[Larry]

Thanks all for the suggestions.  I should be able to fly this W/E and I'll check the shafts and Alt static.  I get the pitch variations in climb also, but I don't know if that is just normal in IAS mode.  In cruise without the AP engaged, the G5 is steady and shows no uncommanded change in pitch.

[jetdriven]

Two friends of mine have this problem.  One found a way to adjust the gain in the G5 on the pitch and it fixed it. Both shops said they can’t do anything further. 

[rbp]

2 hours ago, thomas1142 said:

I have this issue on climb out. I set IAS climb at 90 knots and will see anywhere from 88 to 93 knots on the climb. At times the pitch correction to the correct airspeed is a bit excessive, especially in IMC. I started using VS climb out and it works much better. Kind of disappointed.

I’ve been testing my new gfc500 over the last two days. The northeast has been tremendously bumpy below 10,000 feet. When using indicated airspeed mode or vertical speed mode today it would pitch up And overshoot, then lower the pitch and under shoot, Back-and-forth a few times. I chalked this up to windshear, of which there was plenty. My plan was to test everything again in smooth air  when this front moves through. But generally I was disappointed in the performance versus the KFC 225

[donkaye]

2 hours ago, jetdriven said:

Two friends of mine have this problem.  One found a way to adjust the gain in the G5 on the pitch and it fixed it. Both shops said they can’t do anything further. 

If Garmin is monitoring this thread, and they have been known to do so, they are in big trouble.  Before i knew that it was illegal to change the settings on the G5 for a certified installation, I changed mine and was raked over the coals by Trek.  I quickly changed mine back.  These systems are significantly tested through all sort of tests, and, although it might seem to correct the problem under a certain set of circumstances, that doesn't mean it couldn't be unsafe under another untested set.

Bottom line, if you value your license, your insurance, and maybe your life, I wouldn't make ANY changes to your system that is not approved under the STC.