Stec 50 autopilot altitude hold problems

STEC 50 AUTOPILOT ALTITUDE HOLD PROBLEMS MANUAL

I painted the contacts with Stabilant 22. First thing I tried was crawling under the panel and finding the connector between the pressure switch and the control module. But the intermittent nature of the problem made me think it was just a loose contact somewhere. 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. checking electrical contacts to the pitch servo checking cable tensions on the pitch servo looking for moisture in the static lines Simple stuff I looked at that failed to identify or fix the issue:

Per the STEC folks at Oshkosh this year, there are about a dozen causes for this problem. I wondered if Boeing had secretly upgraded theВ firmware without telling me The oscillations recently got so bad that the altitude hold would slam the nose 10 degrees down suddenly for no apparent reason. Smooth air or turbulence made zero difference. It would often do both on the same flight. To recap the problem - my pitch servo would either work perfectly or oscillate wildly - no rhyme or reason as to when and where. 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. Interaction between GNS-530, HSI, GPSS, and. GPSS and altitude hold (like in an S-TEC 50 autopilot) have to do with this. It seems like there might be something very simple that's wrong, and I might be about to blow 2amu for nothing. To remedy this, I have become accustomed to simply setting the Altitude Hold twice, whenever I use it. Failures here indicate a problem with the autopilot system altitude sensor. I try to troubleshoot for over an hour and give up, convinced it's permanently busted.В 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. The altitude holdВ oscillates wildly by 150 feet or more.В Resetting it does nothing. (2) Flight 2: The very next day, I cruise at 8000 in mostly smooth air, OAT ~-13. (1) Flight 1: Cruise at 9000, OAT ~-10,В sometimes smooth, someВ patches of significant turbulence encountered over 2 hours, and altitude hold works perfectly. Before I do that, I thought I would describe the behavior of the system on my last twoВ flights and see if anyone has ideas:

I'm may be looking at well over 2amu to go down this road. Next step would be to pull the tail servo, bench test, possibly send back to STEC to overhaul / replace.

STEC 50 AUTOPILOT ALTITUDE HOLD PROBLEMS MANUAL

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

Stec 50 autopilot altitude hold problems