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S-tec 55 Autopilot altitude hold problem
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Paul
Posted 2007-09-24 2:34 PM (#566)
Subject: S-tec 55 Autopilot altitude hold problem



Regular

Posts: 65
2525
Most times my autopilot works fine.

But some flights it refuses to hold an altitude - dives or climbs at random at up to 1500fpm!

Tried pulling the circuit breaker to reset, even switching off the Avionics master to reset.

But nothing worked.

Landed. Refueled, took off and - everything back to normal!

Had this 3 - 4 times in past 4 years.

Anyone had similar problems and know the cure?


Paul
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frch
Posted 2007-09-26 8:13 AM (#567 - in reply to #566)
Subject: Re: S-tec 55 Autopilot altitude hold problem


New user

Posts: 2

Location: KSTS Santa Rosa, CA
Paul, I started having the same problem but in the 200 to 600 foot range. Bob Nicolas said it might have something to do with where the system is measuring altitude/pressure and that ( If I understood correctly) it could be connected to a different static port to solve the problem. Rob Howe owner of N210SH the first and only "super" Eagle produced
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Dan H
Posted 2009-10-22 2:49 AM (#633 - in reply to #566)
Subject: Re: S-tec 55 Autopilot altitude hold problem


New user

Posts: 3

I wonder how this was resovled, as the post is two years old. I have a 79 P210 with an S-TEC 55X that is about 12 years old. It was fine until last year when it wouldn't capture the preselected altitude by about 200 feet. Turned out the climb/descent had drifted out of alignment and was at zero descent rate when 200 feet above the selected altitude, so it didn't capture. Now that it was solved by an adjustment, I have now experienced sudden pitch-ups at 1500 FPM. Twice it happened as I hit ALT/VS in climb although a rate of 500 FPM was selcted, and once after being level for about 5 minutes I had a sudden pitch-up for no apparent reason. Still being investigated. Anyone have any similar experiences and can offer how it was resolved?
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Ojaiflyers
Posted 2009-10-22 9:24 AM (#635 - in reply to #566)
Subject: Re: S-tec 55 Autopilot altitude hold problem


Member

Posts: 16

I have had my 79 Silver Eagle since 11/03 My S-tec 55X was new then. My ongoing problem has been my that it kicks out at various times but mostly with altitude hold on. This has been going on and off for the last 4 years. Many shops I have taken it to thought it was a seating problem. I took the plane to S-tec a year and a half ago, no luck finding the problem. They thought it might have been a bad pin so they replaced the pins, but were never really sure this was the problem. The unit worked great for the test flight of about 30 minutes and about 50 miles out when I left the factory, then it started again. Many times after I turn everything off and back on a few times the problem goes away. The last shop to work on it changed all the wiring and disconnect switch thinking that was the problem, again works for a few flight and it was back. I had them put everything back including my original disconnect switch and the problem went away for several months. It is back again but not on every flight. Like the toothache when you go to the dentist. Very frustrating, I plan on another trip back to the factory next month.
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Dan H
Posted 2009-10-22 4:18 PM (#636 - in reply to #566)
Subject: Re: S-tec 55 Autopilot altitude hold problem


New user

Posts: 3

Those intermittent problems are really difficult to track down. At least my problems have been relatively constant, and it seems that others have experienced the same type of problem with the 55X also. Funny you should mention toothaches. I have a tooth that has had two root canals, and is now bothering me a lot. Went to the dentist and he said it looked fine.
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Ourieff
Posted 2009-11-03 6:27 PM (#637 - in reply to #566)
Subject: RE: S-tec 55 Autopilot altitude hold problem


New user

Posts: 3

Has anyone checked the current draw of your pitch servo as it begins to move? The servo should draw a very specific amount of current/volts. If it is gummy (from time, dust and use) it will require a considerably higher current/volt before it begins to move; this leads to a lag in its activation; which means the s-tec computer is constantly "behind" the servo and commands pitch up-pitch down-pitch up-pitch down. You should see this on the s-tec screen as the trim indicator continuously cycles. The flight manifestation is a porpoise up and down. It can be 20-60 feet or more. The "gunk" in the servo can cause it to jam periodically, the s-tec commands a pitch up or down; nothing happens so it cranks more in "command"; suddenly the servo moves and catches up and bam- 1500 fpm.


Blowing a light lubricating oil into the servo with shop air as you cycle the trim on the ground usually works and can be done on an as needed basis; or if it recurs quickly; then you will need a new servo. I am now in the process of buying a new servo; oil worked for 4 months x2. S-tec wants 3000 bucks for their 24 volt dc motor...

I initially also tried a "different static port". I disconnected the port and just used ambiant air from the plane at the location of the connection. Didn't work, and shouldn't work as a correction for a new problem. I am aware of some new installs where interior air was better than a static port; but wouldn't explain a "change" in computer function after the fact.

Bruce
N169JM
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Ojaiflyers
Posted 2012-10-03 9:52 PM (#918 - in reply to #566)
Subject: RE: S-tec 55 Autopilot porpoising


Member

Posts: 16

My auto pilot has had this issue for years now. When I engage the auto pilot and select a altitude to climb to it porpoises on the way up then at altitude it continues. We have adjusted the tension on the cables several times and replaced the capstans. Any other ideas would be appreciated.
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Ojaiflyers
Posted 2012-10-03 10:16 PM (#930 - in reply to #566)
Subject: RE: S-tec 55 AutopilotPorposing


Member

Posts: 16

My auto pilot porpoises on the way up to per selected altitude and also when I select alt hold. We have adjusted cable tension sever times and changed capstans to no avail. Any new ideas would be appreciated
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Wannabe
Posted 2014-06-24 8:55 PM (#1490 - in reply to #566)
Subject: RE: S-tec 55 Autopilot altitude hold problem


New user

Posts: 2

This thread has spanned some time.....I wonder if there is any resolution? I've been to 3 avionics shops. My P210's STEC 55x will not hold altitude, porpoising is not the issue. It will run the trim OPPOSITE the direction it needs to go to hold altitude. That can make for an exciting ride. Passes ground checks just fine. Replaced the altitude pressure tansducer. But the problem remains. The problem may (or may not) have started when I got an avionics upgrade. Is there something that could have been reversed that causes the altitude hold to trim the wrong direction when in the altitude hold mode?

Marc
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Ojaiflyers
Posted 2014-06-25 7:45 AM (#1491 - in reply to #1490)
Subject: RE: S-tec 55 Autopilot altitude hold problem


Member

Posts: 16

My problem was finally solved when my shop changed out my turn coordinator. I had a S-Tec turn coordinator that has been the problem all along.
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nozzle
Posted 2014-06-25 9:09 AM (#1492 - in reply to #1491)
Subject: RE: S-tec 55 Autopilot altitude hold problem


Member

Posts: 9

Location: KVPZ Valparaiso, Indiana
We had the same problem very recently on an aircraft that we just obtained from the West Coast. In our case we found that the trim actuator was years past its time limit and the chain was so loose between the dual actuators that turbulence could have knocked it off the sprockets. We overhauled the actuators and reset the chain as well as re-tensioned the cables and now it works perfectly. There is no hint of porpoise.
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