Problem with GFC700 pitch servo?

Any DA40 related topics

Moderators: Rick, Lance Murray

Post Reply
User avatar
linzhiming
3 Diamonds Member
3 Diamonds Member
Posts: 105
Joined: Wed Dec 31, 2014 1:14 am
First Name: Wolfgang
Aircraft Type: DA40
Aircraft Registration: N799DS
Airports: EGLK
Has thanked: 9 times
Been thanked: 29 times

Problem with GFC700 pitch servo?

Post by linzhiming »

Hello,

I would like to see whether anyone has seen the following problem with their GFC700 pitch servo:

- AFCS initialises without any error after going through standard startup sequence
- When AP button is pressed on the ground, ROL and PIT modes are engaged as confirmed by the green annunciations
- However, when I try to move the stick, I can't move it to the side but unexpectedly can move it forward and backward
- HDG mode on the ground works as expected, however there is no reaction to VS +/- modes, i.e. neither the stick nor the trimwheel would move
- When in the air, there is no issue with any of the roll modes but pitch modes are not working initially. It leaves me with the odd situation of working the stick forward and backward whilst the movements to the left and right are locked
- AP annunciations (actually, more precisely, FD annunciations) show altitude hold mode being captured with FD command bars moving the right way but AP does not react
- The interesting thing however is that after handflying the selected AP pitch mode for a couple of minutes, the AP pitch modes (all main modes, FLC, VS, ALT) suddenly start working again and the AP behaves as normal, including through turbulence and for the duration of an entire 4-hour flight
- Same issue happens (all of the above) on two subsequent flights
- FD behaves normal throughout

It looks like a problem with the pitch servo. Does anyone have an idea what the problem could be as the problem seems to occur only at the beginning of the each flight?

Wolfgang
User avatar
PIERRELFAI
2 Diamonds Member
2 Diamonds Member
Posts: 12
Joined: Sun Apr 09, 2017 8:17 am
First Name: Pierre
Aircraft Type: DA40D
Aircraft Registration: FHRLE
Airports: LFAI

Re: Problem with GFC700 pitch servo?

Post by PIERRELFAI »

Hi I found exactly the same problem but with a KAP 140 AP ! That happen soon the Weather is cold as now in france , after the first flight and a warm airplane the problem desapeared . We face this problem only during winter time. We suspected the pitch servo , we checked several time but never clearly found the problem
User avatar
Rick
5 Diamonds Member
5 Diamonds Member
Posts: 1575
Joined: Tue Jul 06, 2010 10:09 pm
First Name: Rick
Aircraft Type: DA40
Aircraft Registration: NONE
Airports: KROA
Has thanked: 107 times
Been thanked: 297 times

Re: Problem with GFC700 pitch servo?

Post by Rick »

Wolfgang, very interesting! I have exactly the same GFC700 problem, only with the roll axis. In cold weather, when I engage the AP, it immediately captures the pitch axis, but I am free to move the stick left and right. If I follow the FD cues by hand, eventually (after maybe a couple of minutes, or even more if it's really cold) I can feel the servo kick in and take over the roll axis. As long as I manually follow the FD cues, all is well, but if I let it drift too far off of the HDG or NAV course and don't correct by hand, I get an autopilot disconnect and failure (the "CHECK ATTITUDE"" box appears). It is definitely temperature-related - it only happens when it gets cold. It always engages immediately when either the plane or the OAT is warm. We've always figured it was an issue with the roll servo, but we haven't looked too hard for it yet. I've never had it fail to work after a couple of minutes. It's like the servo needs to get warmed up before the clutch will grab...
Roanoke, VA (KROA)
User avatar
Rick
5 Diamonds Member
5 Diamonds Member
Posts: 1575
Joined: Tue Jul 06, 2010 10:09 pm
First Name: Rick
Aircraft Type: DA40
Aircraft Registration: NONE
Airports: KROA
Has thanked: 107 times
Been thanked: 297 times

Re: Problem with GFC700 pitch servo?

Post by Rick »

I just found this thread from back in 2013 where I had already posted this problem (oops!). I have re-racked the LRU's (a couple of times) since then, but it still happens. And the FD works correctly, but the AP just doesn't follow it. Feels like a servo problem. Hopefully somebody else has already solved this one...
Roanoke, VA (KROA)
User avatar
linzhiming
3 Diamonds Member
3 Diamonds Member
Posts: 105
Joined: Wed Dec 31, 2014 1:14 am
First Name: Wolfgang
Aircraft Type: DA40
Aircraft Registration: N799DS
Airports: EGLK
Has thanked: 9 times
Been thanked: 29 times

Re: Problem with GFC700 pitch servo?

Post by linzhiming »

Rick wrote:I just found this thread from back in 2013 where I had already posted this problem (oops!). I have re-racked the LRU's (a couple of times) since then, but it still happens. And the FD works correctly, but the AP just doesn't follow it. Feels like a servo problem. Hopefully somebody else has already solved this one...
Thanks, Rick. I read that thread but I am not sure whether it is a similar issue as, oddly, there is no error message at all! I am surprised that the self-check doesn't find anything upon startup and that no error message is shown at all.

Wolfgang
User avatar
Rick
5 Diamonds Member
5 Diamonds Member
Posts: 1575
Joined: Tue Jul 06, 2010 10:09 pm
First Name: Rick
Aircraft Type: DA40
Aircraft Registration: NONE
Airports: KROA
Has thanked: 107 times
Been thanked: 297 times

Re: Problem with GFC700 pitch servo?

Post by Rick »

linzhiming wrote:Thanks, Rick. I read that thread but I am not sure whether it is a similar issue as, oddly, there is no error message at all! I am surprised that the self-check doesn't find anything upon startup and that no error message is shown at all.

Wolfgang
I don't get any error message either, as long as the AP thinks it is managing the heading ok (even though it's actually me). In your case, I wonder what would happen if you engage the AP in FLC mode, then hit DOWN a bunch of times to increase the set speed, while manually keeping the actual speed constant (assuming you have control of the pitch and not the servo). I would think this would upset the Flight Director, and maybe produce an error once it decides it can't hold the speed you requested.

It still feels to me like the same problem, just a different axis. The AP thinks it is in control, but the servo is not providing any control resistance or making any corrections in one axis, while correctly controlling the other axis. And then, after a couple of minutes, it "fixes" itself. Possibly the GFC error detection is more tolerant of PITCH issues then it is of ROLL issues, so you're not getting an error as easily?

Have you ever had this problem when it's not cold outside?
Roanoke, VA (KROA)
Post Reply