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keying the Com radio disengages the Auto Pilot

Postby clouddodger » Thu Jan 29, 2015 3:08 pm

I recently made a flight using the AP and when i keyed the mike to report my position the AP disengaged.
I have since made some tests and found on the lower channels it,s consistent, and intermittent on the upper channels.
I obtained some rf chokes but really don' t know where to best apply them.
The plane is a Lightning using the standard configuration GRT AP with GRT servos and a single engage/disengage switch.
The AP works fine until the radio (Garmin SL40) is keyed.
I would appreciate any suggestions.
"I fly the Stars and Stripes"
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Re: keying the Com radio disengages the Auto Pilot

Postby deej » Thu Jan 29, 2015 7:53 pm

Verify the integrity of your antenna coax all the way from the SL40 to the antenna. Check for breaks in the shield, sharp kinks, etc. Verify that the connectors on both ends of the coax are properly crimped and/or soldered (depending on type), and haven't come loose. Try to ensure that the coax doesn't run near any of the autopilot wires. is your antenna installed anywhere near the autopilot servos?

Somehow RF is getting out of your coax/antenna and feeding back into the aircraft. You may see other issues besides the AP.

-Dj
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Re: keying the Com radio disengages the Auto Pilot

Postby Bobturner » Thu Jan 29, 2015 10:50 pm

Is that a fiberglass airplane? If so, how large is the com antenna's ground plane? You may need to run the autopilot wires inside shielded cable. Do not randomly install an RF choke on those wires, it will affect the digital signals.
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Re: keying the Com radio disengages the Auto Pilot

Postby gacoon » Fri Jan 30, 2015 5:13 am

Look In the Jabiri mainteance manual section 11-11.6.

http://www.usjabiru.com/uploads/JSA_SM230SP-A2.pdf

Where is your com antenna mounted, is it similiar to one in Jabiru?
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Re: keying the Com radio disengages the Auto Pilot

Postby clouddodger » Fri Jan 30, 2015 4:21 pm

Thanks for the quick feedback. The info in the Jabiru aircraft manual appears to be what I was looking for.
The lightning is a fiberglass craft and the antenna is glassed to the right bulkhead just behind the baggage compartment. I will inspect the coax for any issues.
The ferrite chokes I have are clip on,I will start at the antenna which is connected with a BNC connector and apply a ferrite as close to the connector as possible.
Pending some decent weather here in Nebraska, I will report back on the results.
Again Thanks for your help.
"I fly the Stars and Stripes"
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Re: keying the Com radio disengages the Auto Pilot

Postby clouddodger » Sat Mar 28, 2015 3:15 pm

An Update on the problem;
I have since moved the comm antenna cable from the floor of the cabin to the bulkhead and this made the greatest improvement.
It is much better, but not perfect.
I have also added ferrites to the servo cables, the ground lead from the engage/disengage switch, the comm antenna cable at the antenna, and added another ground from the battery to the firewall.
My next step is to look at the headset jacks ground and cabling. I also have some larger ferrites supplied by GRT I can apply to the cables into the SX. It's been a long process because it doesn't fail on the ground so it's "try and fly".
I purchased an RF "sniffer" that attaches to a multimeter and will try it on the ground, my thinking is even though it doesn't fail on the ground I may be able to detect some errant RF on the various leads.
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Re: keying the Com radio disengages the Auto Pilot

Postby clouddodger » Fri May 22, 2015 3:49 pm

Well this has been a real challenge, but it now works correctly! I woke up one night and and realized that if it only failed in the air it must be something with the alternator. The Jabiru charges mainly at higher rpm. I applied ferrites to the cable from the alternator and the ground cables at the battery. Evidently the alternator on the Jabiru 3300 contains some spurious frequencies in its output. I flew for 2 hours yesterday with no disengagements from the AP.
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"I fly the Stars and Stripes"
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