HELP another STEC 30 altitude hold oscillation thread

[DXB]

I've read the info on this topic here and have tried the simple stuff that people have said can fix this. Specifically: 

(1) The system ground tests fine per the manual  (2)  the cables from the tail servo are at correct tension levels (tested at annual)  (3) pitch controls are well-lubed and not binding anywhere (4) avionics guy sprayed contact cleaner in the servo

Next step would be to pull the tail servo, bench test, possibly send back to STEC to overhaul / replace.  I'm may be looking at well over 2amu to go down this road. Before I do that, I thought I would describe the behavior of the system on my last two flights and see if anyone has ideas:

(1) Flight 1: Cruise at 9000, OAT ~-10, sometimes smooth, some patches of significant turbulence encountered over 2 hours, and altitude hold works perfectly.

(2) Flight 2: The very next day, I cruise at 8000 in mostly smooth air, OAT ~-13.  The altitude hold oscillates wildly by 150 feet or more. Resetting it does nothing. I try to troubleshoot for over an hour and give up, convinced it's permanently busted. Then I get dropped down to 6000 feet, still in pretty smooth air, and it works perfectly for the rest of the flight at multiple lower altitudes !  WTF!!  It's not a high altitude, thin air, less resistance thing because it just worked fine at 9000 the previous day.

 It seems like there might be something very simple that's wrong, and I might be about to blow 2amu for nothing.  This is also infuriating for a system that I installed for >15amu about 4 years ago, particularly when there is a 7amu Trutrak system that is superior just around the corner.  

 

 

[ArtVandelay]

Try not trimming it too perfectly, adjust it so it’s slightly out of trim.


Tom

[Marauder]

Dev - make sure there is no water in the static system. There is a pressure switch somewhere in the system used for the STEC altitude hold. Check to make sure there is no leaks there and more importantly no signs of water in the line leading to it.

The gizmo is about 1.5” round and an inch thick. It will have a static line going to it. I would replace that before I messed with replacing servos.

Also, these servos have a maintenance routine that may need to be conducted. It’s some sort of exercise routine. Paul (user handles is something like kortespate)I think once described it.


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[Bob_Belville]

@kortopates

[Marauder]

Dev - it’s the blue gizmo in the photo.




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[DXB]

9 minutes ago, ArtVandelay said:

Try not trimming it too perfectly, adjust it so it’s slightly out of trim.


Tom

Good idea.  Sadly I already tried that one, forgot to mention above.

3 minutes ago, Marauder said:

Dev - make sure there is no water in the static system. There is a pressure switch somewhere in the system used for the STEC altitude hold. Check to make sure there is no leaks there and more importantly no signs of water in the line leading to it.

The gizmo is about 1.5” round and an inch thick. It will have a static line going to it. I would replace that before I messed with replacing servos.

Also, these servos have a maintenance routine that may need to be conducted. It’s some sort of exercise routine. Paul (user handles is something like kortespate)I think once described it.
 

Ha - that static moisture and/or switch problem sounds like the kind of intermittent problem that might cause my issue.  Also @kortopatesI am willing to put my servos on an exercise/ weight loss regimen if you think it would help.

[rbridges]

keep me posted.  my altitude hold has been erratic for years.  I have just learned to trim it well.  

[MIm20c]

I agree with @Marauder  

With your system being so new I would be surprised if the servo was the problem.  I can’t remember if the blue housing for the pressure transducer is a good thing or bad (one color has a higher failure rate iirc). Make sure the sensor is secured down correctly and is not vibrating. Might want to try the alternate air source while it’s misbehaving to see if that helps or changes the behavior. 

Edited February 12, 2019 by MIm20c

[DXB]

2 hours ago, MIm20c said:

 Might want to try the alternate air source while it’s misbehaving to see if that helps or changes the behavior. 

Another great suggestion.

[M20S Driver]

Have you tried the procedure pasted below?  I had the same problem and doing this routine  from the Stec manual before flight fixed it.  I now do it once  a month regardless if it it holding or not.  My issue may have been related to the contact resistance of the motor brushes and this action helps it.

"Engage altitude hold mode. ALT lamp is illuminated on AP display.  Move A/C Control Wheel as far forward as possible. After 3 seconds, TRIM UP lamp becomes illuminated on AP  and audible alert sounds a steady tone. After 7 seconds, TRIM UP lamp  flashes and audible alert becomes periodic.  Do the same for aft control wheel.   Move A/C control Wheel forward until TRIM DN lamp is extinguished. Audible alert is squelched"

[kortopates]

Definitely follow through with Chris' @Marauder advice on the Static line checks and the Alt hold sensor.

If nothing shows up, before you pull servo's or any parts to send in for bench checking I highly recommend you take the time to make a flight to a shop that has full capabilities on working on your auto-pilot. I don't have any suggestions in your part of the country but you want someone that has both the expertise and equipment to diagnose the issue. Sending in one part pf after another gets very expensive without fixing it. You need the expertise of Stec AP specialist not an avionics tech. 

[mike_elliott]

I would also clean the trim servo armature. You have a sub min. D connector that has 4 pins connected. One is + one - one servo clutch one ground. I cant remember the pinouts right off. I made a "connector" pigtail I could plug into the servo and could put 12V and run the motor in each direction for a minute or 2. This is a fix that was given to me by an STEC tech back in '05. Every annual I would "clean" the servo armatures and that kept it rock solid. I am not saying this is your issue, but this is a known problem with STEC servos, right Barry?

[DXB]

13 hours ago, M20S Driver said:

Have you tried the procedure pasted below?  I had the same problem and doing this routine  from the Stec manual before flight fixed it.  I now do it once  a month regardless if it it holding or not.  My issue may have been related to the contact resistance of the motor brushes and this action helps it.

"Engage altitude hold mode. ALT lamp is illuminated on AP display.  Move A/C Control Wheel as far forward as possible. After 3 seconds, TRIM UP lamp becomes illuminated on AP  and audible alert sounds a steady tone. After 7 seconds, TRIM UP lamp  flashes and audible alert becomes periodic.  Do the same for aft control wheel.   Move A/C control Wheel forward until TRIM DN lamp is extinguished. Audible alert is squelched"

Interesting - I tried this manual procedure a while back and it seemed fine -  I didn't realize the potential for it to be both diagnostic and therapeutic - will try again.

4 hours ago, mike_elliott said:

I would also clean the trim servo armature. You have a sub min. D connector that has 4 pins connected. One is + one - one servo clutch one ground. I cant remember the pinouts right off. I made a "connector" pigtail I could plug into the servo and could put 12V and run the motor in each direction for a minute or 2. This is a fix that was given to me by an STEC tech back in '05. Every annual I would "clean" the servo armatures and that kept it rock solid. I am not saying this is your issue, but this is a known problem with STEC servos, right Barry?

Thanks for this - great expertise here.  I will run it by the avionics guy next time it's in the shop -probably more than I should be messing with.  

[DXB]

Follow up on thread -  fixed !!

To recap the problem - my pitch servo would either work perfectly or oscillate wildly - no rhyme or reason as to when and where. It would often do both on the same flight. Smooth air or turbulence made zero difference.  The oscillations recently got so bad that the altitude hold would slam the nose 10 degrees down suddenly for no apparent reason.  I wondered if Boeing had secretly upgraded the firmware without telling me 

Per the STEC folks at Oshkosh this year, there are about a dozen causes for this problem.  Simple stuff I looked at that failed to identify or fix the issue:

- “exercising” the servo per the  manual

- looking for moisture in the static lines

- trying it on alternate static

- leaving it slightly out of trim

- checking cable tensions on the pitch servo

- checking electrical contacts to the pitch servo

So I figured that I was looking at an issue with the pressure switch, the control module, or the servo itself that was going to cost me serious $$$ to get sorted.  But the intermittent nature of the problem made me think it was just a loose contact somewhere.  First thing I tried was crawling under the panel and finding the connector between the pressure switch and the control module. I painted the contacts with Stabilant 22.   Hurray!!!  Blind avionics squirrel finally got a nut on his own  

 

 

 

 

[chrixxer]

Mine was behaving exactly the same way, I had an S-Tec shop (Tucson Aeroservice Center) diagnose it. It was the pitch servo itself. Sent it out for flat-rate service and it's been rock solid ever since. 2AMU.