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autopilot control wheel hard over/shuddering


Hasteman
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I and trying to troubleshoot a pre 85 bird with AP-105 and FD-109 systems. The issue is that with INS as a vertical reference and autopilot engaged both control wheels will experience a frequent uncommanded movement left and right in a shuddering fashion. It doesnt move a lot but it will go from steady and smooth to rapid shudders in the blink of an eye. With VG as a vertical reference it mainly functions correctly but will radomly experience a sharp and rapid right hard-over. This is with AP coupled regardless of pilot or copilot side or uncoupled with AP engaged and in both HDG or NAV selected.

So far we have changed several AP AMPS, Aileron servo motor, Aileron drum and bracket assy, K-relays associated with HDG, swapped FD computers, swapped mode couplers, changed 2 AP controllers, and shot wires associated with hdg and roll data with no loss of continuity. Also slaved in an Airspeed sensor, and changed an elevator servo.

Any ideas or inputs that may be causing this? Am I overlooking something obvious or not thinking out of the box enough?

Any help would be appreciated.

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Are all of your inputs getting their power from the same AC bus? I vaguely recall a warning that said to avoid using the INS coupled when using the inverter for heading power on our USCG birds with LTN-101. Just a quick thought, probably not your situation or configuration with SCNS.

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Are all of your inputs getting their power from the same AC bus? I vaguely recall a warning that said to avoid using the INS coupled when using the inverter for heading power on our USCG birds with LTN-101. Just a quick thought, probably not your situation or configuration with SCNS.

The only issue with using INS as a vertical reference is the configuration of the AC power bus on the two inverters. The wrong combination will cause the ADI's to roll uncontrollably. That happens when the two inverters are in the I positions, if they are in - - position it functions properly.

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Never got to work on that system in all my years on the C-130. Are the ADIs and other NAV signals stable when this is happening? Sometimes it's hard to see when the pilots are worrying about the movement of the aircraft.

I would go in and check all the terminal board connections, even going so far as to loosening and tightening. Don't forget TB101 at FS 617.

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Just a status update. The issue was fixed by repairing a bad splice on the #1 HSI wiring bundle. I believe the splice was a heading feedback from the HSI to flight director and autopilot. Once the splice was repaired all associated issues went away. Thanks for taking the time to reply and hopefully you can add this to your toolbag also if you ever hear of a similar issue elsewhere.

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That leads me to a story. When I got to Norton AFB in 1981, a lot of the guys were telling me that the cleared-covered splices sucked. I hadn't even seen them at Rhein Main, we only had pink & blue for avionics work. I started checking out the problem, small wires easily broke loose of the crimp. I decided to go into the T.O., yeah, imagine that, and found that the clear splice required a different crimp die than was used for pink and blue splices.

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