STec 30 Alt Hold Fail

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N645XL
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STec 30 Alt Hold Fail

Post by N645XL »

I've had about a 2 year running battle with my STec 30's altitude hold. The symptoms were all over the board from uncommanded climbs and descents to non-response holding altitude. The autopilot was installed in 2009 so the symptoms appeared after about 10 years.

My avionics shop pulled the pitch servo this week and found massive quantities of motor-brush carbon dust under the dust cover. The windings were getting hot and it wasn't generating any torque. They're sending it off for repair/replacement. I suppose my question is, why did it eat up the brushes?

I'll keep you posted if anything else pops.
GimpyPilot
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Re: STec 30 Alt Hold Fail

Post by GimpyPilot »

Bob,

Did your avionics technician also check the pressure transducer? If you have the old blue plastic one, keep in mind that they are failure prone. The newer one is aluminum, and much more robust. It's worth a look. The old ones are not serviceable, but Genesys/Cobham/S-Tec will send you a new one for about $100.

-Michael
N645XL
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Re: STec 30 Alt Hold Fail (Updated)

Post by N645XL »

Thanks Michael. I just sent my shop a note.

Update: (16 May 21) - I retrieved s/n 0115 from the avionics shop Thursday minus the altitude hold servo which is en-route to the service center. The shop put the servo on the bench and were able to duplicate the in-flight symptoms of uncommanded climbs and descents plus the lack of power to even hold altitude. Apparently the carbon debris from the motor brushes was shorting the adjacent motor windings which caused the pitch computer to apply more and more voltage for a response until the motor finally moved which drove it to be extremely aggressive; hence, the abrupt climbs and descents. Right now it's a theory with plausibility. They said they'd inquire about the $100 transducer exchange which would likely be the next step if the servo rebuild/replacement doesn't fix the problem. I expect the servo to be returned in June. If I don't post anything further next month, the servo fixed it; otherwise, we go into overtime with a transducer drama. :-)

While it was in the shop I also had them replace the GTX330ES with a GTX345 transponder. Lots more to learn with the GTX345 but so far I've flown with it twice and I'm very pleased with it. The cabin looks much less cluttered without the Stratus and charging cable sitting up front. I'm able with ForeFlight, to Bluetooth into both the GTX345 and PMA450 audio panel with my iPad. I still get my audio runway proximity warnings and audible traffic alerts through both headsets. I love the setup. I have yet to see any weather displayed on the GNS430W's since we've had unusually sunny skies the last week but weather is the iPad's primary job anyway. I had them run the wiring from the GTX345 to the Aspen display but I have no plans to activate that (traffic-display) Aspen feature which I'm told is about $800. Having traffic on the #2 430W is fine with me. There were no free RS-232 ports on the 430W's so I did not have them install the FlightStream 210. They gave me a refund on that device. I figured it was a luxury item anyway considering my flying generally doesn't involve complex flight plans.
Last edited by N645XL on Thu Jul 22, 2021 6:56 pm, edited 1 time in total.
N645XL
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Re: STec 30 Alt Hold Fail

Post by N645XL »

Update: we're in overtime. The repaired pitch servo has a lot of power and whatever was causing the uncommanded climbs and descents is now on steroids with a like-new pitch servo driving the stab with more power. Some people pay money for rollercoaster rides. All I have to do is hit alt hold. :-) Anyway, transducer has been ruled out when a good one was temporarily swapped-in and test flown. The pitch computer is slated for testing this week if the wx cooperates and I can get it to the shop.

GimpyPilot: FYI I do have the newer aluminum transducer.

Update for 20 July 21: A new avionics shop gave it their best shot. They disassembled connectors to take voltage readings with all system components online and operational. All voltages checked good. Only the dynamics of flight seem to trigger the malfunction. The shop has video of the panel indications to share with STec but I'm sure they'll just say to send in the pitch computer since that's the only box or wire that hasn't been examined under the cover.

Update: I surrendered the pitch computer 3 August for STec to examine.
N645XL
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Re: STec 30 Alt Hold Fail

Post by N645XL »

Another update: STec found a faulty pitch computer circuit board. They’re forecasting a repair no sooner than December but probably into 2022. I can only imagine the delay has something to do with the ships’ backed-up at the west coast ports.

1 Oct 2021: STec is sending a replacement unit. I guess mine will go to someone else at a later date.

9 Oct 21 - Pitch computer installed. Altitude Hold still fails.
N645XL
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Re: STec 30 Alt Hold Fail

Post by N645XL »

Final update: Short version - APinspector did my annual this month and looked at the autopilot issues. Last year when the avionics shop said it was the pitch servo, they were correct. The problem continued because when they reinstalled it, they put the bell crank on upside down and backward. The pitch computer never needed to go in for repair. Thanks to APinspector’s experience and keen eye, he spotted and corrected the problem. Autopilot works like new.
tafisama
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Re: STec 30 Alt Hold Fail

Post by tafisama »

It's not fair to go through the agony to finally come to the realisation.The good part is it has been solved.
GimpyPilot
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Re: STec 30 Alt Hold Fail

Post by GimpyPilot »

tafisama wrote: Sun May 22, 2022 3:34 am It's not fair to go through the agony to finally come to the realisation.The good part is it has been solved.
That's not agony, my friend. It's simply airplanes.
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