Comm/autopilot problems

Bruce Crain

For you builders and flyers hopefully this will help you avoid problems with respect to RMI or RFI even when you think the RG400 coax is bullet proof. My comm coax is run down the right side of the fuz and nothing else is run down that side with it. The only place it came close to is where the problem began which is the autopilot servo. In the past if I had a problem with a certain freq (I now understand it is the higher freqs above 127.0 on the comm freqs that were giving me the problems) I would just ask Center or approach if they had another freq as this one is messing with my autopilot.

I did some trouble shooting with the comm and autopilot interaction with respect to the higher frequencies on the radio. Particularly on Wichita approach. The frequency 134.85 (Wichita approach) would bank the Q left on this frequency. I did some testing and it seemed that anything above 127.0 comm frequency would make the autopilot turn and bank. I took a look in the tail cone and noticed 2 holes in the bulkhead before the split line in the fuz. The hole I was using to run the RG400 comm coax through was only about 4" away from the autopilot servo. So I drilled another hole about 10 to 12 inches away from the autopilot and re-ran the RG400 to the comm antenna in the tail cone. Before I flew I tested the autopilot with the comm set at the offending 134.85 freq. It seemed solid!

Then I flew up toward Wichita and tuned in their freq (134.85) and talked with them and the autopilot stayed solid as a rock. Even RG400 has it's limits I suppose.

Well that's it in a "nut shell".

Bruce Crain

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