AP74 loses heading hold.

Jan A

New Member
Joined
Aug 9, 2019
Messages
2
The D100 and AP74 in my LSA (WT9 Dynamic) will only hold the heading for about half an hour.
After the heading is lost there is no noticeable action from the roll servo.
  • Might this behaviour have anything to do with the roll servo shear screw?
  • How would the Servo Calibration and Servo Test routines be affected if a shear screw has been broken?
 

Dynon

Dynon Staff
Staff member
Joined
Jan 14, 2013
Messages
13,305
Location
Woodinville, WA
So first, what do you mean by "heading is lost"? Is heading mode still seemingly engaged, and represented by the light on the AP74 and on the on-screen AP status?

If the shear screw is broken, you'd expect the AP to not have any real definitive control of the airplane. What happens when you adjust heading when you're in this state? Is the AP able to control the airplane?

Regardless, the calibration and test routines are really there to determine the direction of the servo motors. There isn't any calibration data that would be affected by the shear screw breaking or its replacement. IE, after fixing that issue (if that's your issue), you'd be able to just go fly. It doesn't hurt to do a fresh servo calibration /test though, just to make sure everything is still electrically connected and that the servo is able to drive the control surfaces.
 

Dynon

Dynon Staff
Staff member
Joined
Jan 14, 2013
Messages
13,305
Location
Woodinville, WA
Actually, I just realized that you may have meant "what does the cal/test routine do if the shear screw has broken". The answer is that it shouldn't be able to drive the axis with the broken screw in any appreciable way. Friction between the arm and the rest of the stack up might cause it to seemingly have some force on the control surface, but it should be obvious that it's not able to move the control surface in earnest.
 

Jan A

New Member
Joined
Aug 9, 2019
Messages
2
Thank you very much for the feedback. I'll be in the hangar over the week-end and will repeat the servo test carefully.
I used the words "heading is lost" to describe that on many flights the AP will initially hold the heading aligned with the bug, but that after half an hour or so, the heading will start drifting away from the bug without there being any noticeable action from the roll servo on the stick.
 
Top