CDI deviation

[tedbain]

I have D1000 classic with Dynon autopilot & GTN650.
Flying LPV approach with the AP - CDI centered and glide slope captured.  Half way in from the FAF the aircraft started drifting left of center line and continued until CDI was full deflection to the right.  The AP then tracked the 0.3 NM offset to DA.  The AP flew the glide slope perfectly.  In the missed approach the AP centered the CDI and off we went.
I had not checked the RAIM before the approach.
Any ideas what happened? 

 

[Dynon]

The AP should be centering the needles, end of story. RAIM or other integrity monitoring elements of the GPS shouldn't make a difference here, but I believe that we change from GPS's roll steering to our own GPS guidance based on the CDI around the FAF. Have you done the autopilot tuning guide by chance? If not, that's always a good place to start.

 

[tedbain]

I appreciate the reply Dynon.
I did tune the AP upon installation 5 years ago but will do it again.

 

[cmgolden]

You might also check your autopilot servos on the ground. My aileron-servo sheer screw broke and the AP behaved a bit like this in the air. It seemed to be tracking sometimes, but other times not. It is obvious on the ground when your pinky finger can easily fight against the servo.

 

[tedbain]

Wow, OK I will check it.
Thanks.

 

[tedbain]

Shear screw is fine.
I followed the A/P tuning guide and everything was fine.
The only thing to do is to monitor it and assume that was a one time gremlin.