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Thread: Service Instruction SK-S1004

  1. #1
    Senior Member Andy's Avatar
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    Default Service Instruction SK-S1004

    Service Instruction SK-S1004 calls for replacement of the Garmin pitch servo bridle clamps because the plastic ones may allow the cables to slip. However, the service instruction assumes that the replacement clamps will be fitted in the same positions as the original clamps. If the clamps have slipped the positions may be wrong!

    What is the full procedure for positioning the replacement bridle clamps if the originals have already slipped?

    The question is not hypothetical. My FX-3 autopilot worked in pitch modes up to October 11th but after that flight all the symptoms, and data log analysis, seem to indicate the pitch servo is no longer connected to the elevator cables.

    The G3X installation manual seems to require the servo capstan to be centered when the elevator is in the middle of its control range. Is that correct? (The middle of the range is not zero elevator deflection).

    I plan to take the belly panel off tomorrow and hope someone can point me to an approved procedure.

    Thanks,
    Andy

  2. #2
    Senior Member Andy's Avatar
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    Default Re: Service Instruction SK-S1004

    Quote Originally Posted by Andy View Post

    My FX-3 autopilot worked in pitch modes up to October 11th but after that flight all the symptoms, and data log analysis, seem to indicate the pitch servo is no longer connected to the elevator cables.

    Andy

    My diagnosis was correct. The bridle cable had detached from the forward clamp, jumped the capstan, and was trapped between the capstan and the servo bracket. Photo shows what I saw when I removed the belly cover panel.

    IMGP0080.jpg

    This is a potential control jam failure although the jam could probably be overcome with enough stick force to break the other end of the bridle cable free of its clamp.

    I encourage compliance with Service Instruction SK-S1004 before you have this problem.


    Andy

  3. #3
    Senior Member Andy's Avatar
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    Default Re: Service Instruction SK-S1004

    Repair was completed and test flown a while ago. While checking the AFCS configuration I noticed that Pitch and Roll torques were set to 90% by the factory. During AFCS post repair ground test, and with the pitch servo driving to close to the nose down elevator stop, I found the stick force to overpower the AFCS to be much higher than I thought reasonable.

    I set the pitch and roll torque limits to 50% and have observed satisfactory pitch and roll AP performance on several test flights. What torque limits do other FX-3 or EX-3 owners use and why are the factory torques set so high?

  4. #4
    Senior Member 40m's Avatar
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    Default Re: Service Instruction SK-S1004


  5. #5
    Senior Member Andy's Avatar
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    Default Re: Service Instruction SK-S1004

    Quote Originally Posted by 40m View Post
    That document is presented like it was thumbed by a teenage texter but I was able to extract FX-3 roll torque of 70% and FX-3 pitch torque of 85%. The email is dated March 26, 2020. Thanks.

    My FX-3 was delivered June 18, 2020. After my pitch servo bridle failure I found that my pitch and roll torque limits were set to 90%.

    Garmin installation recommendation is (ref installation manual rev AM):

    "Autopilot tuning should be done with the maximum acceptable amount of servo torque. The servo torque should be set to the maximum value that can still be overpowered by the pilot in an emergency situation.

    Set Initial Servo Torque Limits:
    1. In configuration mode go to the Autopilot page
    2. Set the “Max Torque” value for each servo to the maximum percentage value that can still be overpowered by the pilot."


    My ground test after pitch bridle repair seemed to give much higher overpower forces with stick near the forward stop that it did with stick centered. I set the torque limit to a value I could overpower if I experienced a nose down hardover. The lower value should also reduce the probability of another bridle cable failure.

    I'm not suggesting any FX-3 owner should change the factory installed configuration. I'm only reporting what I did and why I did it.

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