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servo failure after corrosion treatment


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An altitude hold issue can indeed show up with intermittent and/or erratic symptoms. I've dealt with 3 different issues with my STEC AH in the past 12 months. When it works, it works great, but...

My latest issue involved the pitch servo and we were able to revive it for a yet-to-be determined amount of time with some in-plane maintenance (with the guidance of Autopilots Central - great guys!). Using a variable DC power supply unit we were able to disconnect the servo wiring harness and fabricate a new DE-9 connector/test harness to check for the motor starting voltage and amperage draw. Then we ran the motor both forward and reverse several times to assure good brush contact. It's been a few weeks now working great and cost less than $5 + labor.

When/if it starts causing problems again we'll yank it out and send to AC for an overhaul and solenoid upgrade (to new style).

One more thing... did your clutch "clean-up" involve completely removing the clutch plates for cleaning or did they just remove the cover and spray it down with solvent? You won't get it resolved IMO without disassembly. I hope this helps.


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Clutch plates are probably closed with a solenoid.... hmmm...

Is there a way to put an eye on these two devices to watch the operation?  Or inspect the clutch part for meat left on the bone? Or the solenoid for its strength in closing?

Sorry to add more questions to the pile and less answers...

I expect one day, I will be OH'ing my trim servo motor.  It does a lot of work compared to the other servos...

Best regards,

-a-

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Anthony, it's been a great learning experience for me, and here's one lesson I learned that may benefit you and/or others.

 

The S-TEC pitch servo employs a simple friction clutch that's nothing more than some tension and shim washers adjusted appropriately tight to a "slip torque" specific to each individual airframe. The solenoid engages or disengages the motor from the capstan gearing as commanded. A newer style solenoid was instituted some time ago to more positively disengage the drive and older servos can have this upgraded. With the old style you may have to relieve yoke pressure on the capstan before it effectively disengages.

 

I've attached a screenshot of the clutch diagram for your viewing pleasure. I'll also include a pic of my clutch in the plane with its cover removed.

9d4d438a46eb4c1bcbe3c89fb453c9f6.jpg

3c046ec38e53e515a5dcccef7aafc15d.jpg

 

 

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3 hours ago, Marauder said:

Dev - the only thing that would or could be different at night or dusk is the electrical load on the plane. Since you are dealing with a weird problem, try this weird troubleshooting; turn off the Nav (and landing light if you leave it on), pull the autopilot breaker and reset it.

See if lightening the electrical load makes a difference.
 

 

It's a reasonable thought Chris - but since I have an LED landing light, the only additional thing I have on at night are  panel lights - I can't image 2 LED nulites an 2 post lamps for the trim and flap indicators make the difference, but it's certainly worth a try.

3 hours ago, cnoe said:

\My latest issue involved the pitch servo and we were able to revive it for a yet-to-be determined amount of time with some in-plane maintenance (with the guidance of Autopilots Central - great guys!). Using a variable DC power supply unit we were able to disconnect the servo wiring harness and fabricate a new DE-9 connector/test harness to check for the motor starting voltage and amperage draw. Then we ran the motor both forward and reverse several times to assure good brush contact. It's been a few weeks now working great and cost less than $5 + labor.

When/if it starts causing problems again we'll yank it out and send to AC for an overhaul and solenoid upgrade (to new style).

One more thing... did your clutch "clean-up" involve completely removing the clutch plates for cleaning or did they just remove the cover and spray it down with solvent? You won't get it resolved IMO without disassembly. I hope this helps.

Good info thanks.  Yeah it's only been cleaned in place, not disassembled - I was hoping to avoid since it's much more $$ and it seems to work fine except on pretty rare evening outings, but I'm certainly moving in the direction getting it pulled now :( 

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Good info thanks.  Yeah it's only been cleaned in place, not disassembled - I was hoping to avoid since it's much more $$ and it seems to work fine except on pretty rare evening outings, but I'm certainly moving in the direction getting it pulled now  


Well Dev, you're in the world between make believe and reality. If you are absolutely not seeing the symptoms during the day, you'll need to figure what the difference is.

It is pointing to something with the electrical system since that would be the only logical factor that could change between day and night flight. The autopilot doesn't know the difference between day and night.

What is your buss voltage showing during all this?


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Does the JPI measure a difference in either volts or amps between night and day?

In case there is something coming on with the switch that isn't being remembered... like the old mechanical, motor driven, anti-collision light....

Best regards,

-a-

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I wouldn't be too quick to yank the servo out unless you have to. The motor performance can be somewhat determined on the plane as noted above, and a servo OH will put you out likely 1.5 AMU or thereabouts + R&R labor.

I also didn't mean to sound the alarm about the clutch contamination. If you KNOW it was contaminated then the clutch should be disassembled for cleaning, but I'd be inclined to nail down a diagnosis first if at all possible. Have you completed the S-TEC "functional ground test"? If the control wheel moves okay during the test your clutch may be okay.

A high starting voltage (requirement) of over 2 volts might point to a faulty motor (or dirty/sticking brushes). Also make sure your PFGC is grounded properly (ground lead from harness).


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1 hour ago, Marauder said:

Well Dev, you're in the world between make believe and reality. If you are absolutely not seeing the symptoms during the day, you'll need to figure what the difference is.

It is pointing to something with the electrical system since that would be the only logical factor that could change between day and night flight. The autopilot doesn't know the difference between day and night.

What is your buss voltage showing during all this?

 

I swear it's only a problem at dusk or night, which makes up maybe 10% of my flying at most, so it's taken a while for me to become convinced of this fact. And now seems to happen every single time at night and otherwise works perfectly  - a very specific phenomenon. I will check volts/amps - easy enough by dumping the JPI. Panel lights on is the only difference in that scenario. Come to think of it, the STEC-30 itself has a uselessly dim night light built in and annoyingly bright indicators that don't dim adequately, but they run off the same panel lights switch and dimmer knob. I wonder if that input into the unit could be interfering with the altitude hold function somehow.  I've reset in flight and done the ground ops test multiple times after putting away in the hangar. It always test fine and I had assumed because the clutch is more easily overpowered with loads on the elevator in the air.  However pitch pressure on the control wheel in flight during malfunction gives no resistance at all, and one should hear the trim horn I would think if the clutch was slipping?

Hmm...I'm intrigued now.  I've only done the ground test with everything else off. I will have to try it with the panel lights on now and report back.  If that's not it, @cnoe's algorithm seems the way to go before pulling it.

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  • 1 month later...
On ‎19‎/‎11‎/‎2016 at 0:39 AM, cnoe said:

 


Once the servo clutch is contaminated it must be disassembled, cleaned, and reassembled with the proper torque value. They don't do this for free either. Note that you can't simply spray it down with a solvent/cleaner and expect it to work again.

And let me re-emphasize that you must SEAL the servo to prevent this problem and not simply cover it up. If the applying facility doesn't understand this you'll very possibly have trouble post-application.


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How do you seal the servo?

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By "seal" I mean put a bag or plastic around it and seal any openings with tie-wraps, tape, safety wire, or whatever it takes to prevent ANY Corrosion-X from penetrating. Keep in mind that it's designed to penetrate everything and will even get between lap joints. I plan to actually remove my roll servo next time I fog the wings.


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