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Old 06-25-2012, 11:48 PM
Tim Lewis Tim Lewis is offline
Join Date: Aug 2006
Location: Bristow, VA
Posts: 178
Default Tru Trak Yaw Damper Problem On Preflight - solved.

Earlier this week during preflight I found the rudder was difficult to move. When swung vigorously from side to side, the rudder would fight the applied force, jerking through it's range of motion. I pulled the baggage panel wall and observed the Tru Trak yaw damper bellcrank was jumping around whenever the rudder was moved. Removed TT YD servo, bellcrank then moved smoothly, rudder moved smoothly. Servo, however, seemed nearly jammed -- very difficult to move servo torque enhancer rod. Removed servo from plane, servo magically became easy to move -- smooth operation, no problem. Hmmm. Reinstalled servo wiring and reinstalled servo in plane -- servo difficult to move again. What the....?

Cause/solution: Although the master switch was off during preflight, I'd left the avionics master switch and the landing light switches "on" from a previous flight. Every time I moved the rudder (with the servo installed) the servo motor generated voltage which flowed to the avionics bus, and crossed from the avionics bus to the main bus via the closed avionics master switch. (Servo motor apparently has permanent magnets, so it acts like a generator when it is turned.) The voltage generated a current to ground through the loads on the main bus (landing lights, etc). So the servo motor (generator) output saw a path to ground, causing the servo motor to fight any applied force. When I turned the avionics master switch off, the problematic rudder jerkiness went away. Or, when I turned off the yaw damper power switch, the problem went away. Path to ground removed, problem goes away.

Lesson learned: turn the avionics master switch off (like I normally do) during shut-down.

Problem solved, went flying.
Tim Lewis
RV-10 N31TD - 1580 hrs
RV-6A N47TD - 1100 hrs (sold)
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Old 06-26-2012, 02:49 AM
Sig600 Sig600 is offline
Join Date: Nov 2005
Location: KRTS
Posts: 1,798

That's some serious Sherlock Holmes sleuthing to come up with that answer! Well done.
Next?, TBD
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Old 06-26-2012, 09:15 AM
Norman CYYJ Norman CYYJ is offline
Join Date: Feb 2005
Location: Victoria B.C.
Posts: 1,444

All autopilot installations should have their own power switch and fuse/breaker. This is one of the reasons.
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Old 06-26-2012, 09:14 PM
sstellarv10 sstellarv10 is offline
Join Date: Feb 2009
Location: Southbridge,MA
Posts: 193

I got a headache just reading the solution.
Steve Stella
#40654 Finishing
N521RV reserved
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