GFC700 Autopilot oscillation issue

[agmolnar]

Hello,

I am new (to posting) on the forum, although I have read many posts over the past half year or so with great interest. I have been flying DA40s for about the same time, and have recently been in the market to purchase a used one (2004 - 2007 Garmin 1000).

Currently, I have a 2007 in escrow but there is one issue with it that is being investigated/fixed during the pre buy inspection process. On the test flight it became apparent that the aircraft hunts for the correct pitch attitude when the AP is engaged (mostly in altitude capture mode, but also somewhat in altitude hold mode), and ends up over and under shooting the desired altitude, resulting in pitch oscillations with fpms of +/- 750. So far, the shop (on Garmin's recommendation) swapped out the GIA63s, but that did not solve the problem.

I have read extensively about the KAP140 oscillation issues, and have also read about the static port issues from a 2011 post. I managed to take a video of the PFD while the aircraft was exhibiting this behavior.

Does anyone have any experience with what this issue might be?

Here is a video of the oscillation, I hope the link works:
https://www.dropbox.com/s/ear1ncn02jxlb ... n.mov?dl=0

Best,
Arpad

[Steve]

Make sure your mechanic cleans the elevator trim damper rod and checks the friction setting. One that is binding/dirty/misadjusted could cause the symptoms you describe.

Steve

[agmolnar]

Thank you Steve. They did make some adjustments to the trim wheel and related area (there was some slop in the trim), but that didn't solve the issue unfortunately.

[Steve]

The part I'm speaking of is in the tail. Most (non-Diamond familiar) A&Ps know nothing about it. Adjustment instructions are in the MM.

Steve

[mdieter]

We had a kink in the pitot / static tubing inside the wing. Seems that excess tubing bunched up and eventually kinked and caused a restriction. It caused the autopilot to make more severe pitch changes as it tried to 'find' the right altitude.

[Tim M]

Our airplane is doing the exact same thing. When it begins to hunt (usually in mild turbulence) it calms down when I switch to alternate static source.

[Charbie]

The pitch servo was replaced on mine in 2009 before I owned it. It took two hours to diagnose the issue. That's all I could find in the receipts.

[agmolnar]

Hello all, thanks for your various inputs. Turned out to be a weak motor on the AP pitch servo which only showed itself in flight and not during ground testing. Probably an issue the shop should have identified a bit earlier in the process.

Looks like escrow should close shortly and I'll be hopefully back online with a new N # and bird, and looking forward to posting and sharing info as I collect it.

Best,
Arpad