S-TEC 55x Altitude Hunting

[Jeff_S]

Went out for a scenic cruise with my brother last weekend and my always-trusty S-TEC 55x A/P altitude hold was on the fritz as soon as I activated it.  It started porpoising pretty badly while it hunted for altitude.  I did a hard reset of the A/P system and tried again, and it held altitude for a minute or so and then the oscillations started back up.

When I had the same problem (and same A/P) in my J-model we discovered the static lines were cracking with age and the leaks were causing this behavior, but it was much more gentle than what I experienced last weekend.  But these lines are now 14 years old, and it's been very hot here lately even in the hangar, so that's my first guess.  Any other thoughts or ideas on this?  I've got a request in with our field maintenance shop to take a look, but I always like to give them specific ideas so they don't spend all my money doing their OWN hunting!

[carusoam]

See if the AP has a rubber hose connecting it to the static system...

Rubber hoses have a tendency to age poorly...

One minute it is using static data, next it is using internal cabin pressure (slight vacuum compared  static line)

This effect has been documented around here... unfortunately I don’t remember what AP it was...

Not that it would matter much... the rubber hose should be pretty easy to find.

Good luck with the hunt.

Best regards,

-a-

[mike_elliott]

2 minutes ago, Jeff_S said:

Went out for a scenic cruise with my brother last weekend and my always-trusty S-TEC 55x A/P altitude hold was on the fritz as soon as I activated it.  It started porpoising pretty badly while it hunted for altitude.  I did a hard reset of the A/P system and tried again, and it held altitude for a minute or so and then the oscillations started back up.

When I had the same problem (and same A/P) in my J-model we discovered the static lines were cracking with age and the leaks were causing this behavior, but it was much more gentle than what I experienced last weekend.  But these lines are now 14 years old, and it's been very hot here lately even in the hangar, so that's my first guess.  Any other thoughts or ideas on this?  I've got a request in with our field maintenance shop to take a look, but I always like to give them specific ideas so they don't spend all my money doing their OWN hunting!

Jeff, run the servo motor in each direction for about a minute to clean the armature. You will need a sub min. D connector, I believe pins 2,3,4,and 7 are used, but you can look at  your existing wiring to find out. 7 is ground. One of the pins is clockwise + , one is counter clockwise + one is clutch engage +.  I built a pigtail and did this ea. annual on both the pitch and roll servo using the old elt  battery when I had a STEC 30 and experienced the problem once. The solution came from the Stec field rep who lives in your neck of the hills.

[kmyfm20s]

When I had this issue the cable was loose and slipping on the servo. They tightened it up and all was good. 
 

Here is a quick read that might be helpful.

https://genesys-aerosystems.com/blog/the-ups-and-downs-of-autopiltot-troubleshooting

[Jeff_S]

This is all good feedback, and it gives me some things to discuss with maintenance. Thanks!

[Jeff_S]

To close the loop (for now!) on this, I considered all this advice, took out the manual and (finally...I know, should have done it long ago) went through the full pre-flight routine on the ground. In addition, I spoke to our local avionics guy who suggested doing an in-flight check to see if the pre-select function worked in both climb and descent, as well as testing out the Alternate Static source to see what that did.  And you've guessed by now, but the A/P performed flawlessly yesterday.  One other bit of advice that was passed to me yesterday was that S-TEC says during the pre-flight routine, it's important to actually do the clutch-override process as that helps "clear the mechanism" (my term) to keep them performing better.

So I don't really know the cause, but it's working now and I have steps to diagnose if it comes back.  Thanks again to all.

Jeff

[mike_elliott]

man, I hate to hear about magical fixes. Just as you say, you can guess what will happen now....in IMC on the glideslope of course.

[buddy]

Jeff, I hope you don’t need a servo. I had to replace my roll servo last year and Stec didn’t have any in stock, it took at least a month to get and it wasn’t to much fun to replace either.

[mike_elliott]

4 minutes ago, buddy said:

Jeff, I hope you don’t need a servo. I had to replace my roll servo last year and Stec didn’t have any in stock, it took at least a month to get and it wasn’t to much fun to replace either.

There is that one Bi*ch bolt on the roll servo in the pilot's wing. It can separate the wannabes from the real mechanics for sure. A new or yellow tagged servo also has to be "approved" for your STC by Genesis (read: pay a vig) if I am not mistaken, at least that was the case when I *thought* I had a bad servo until I learned of the clean the armature trick.

[Jeff_S]

On 7/10/2020 at 5:55 PM, mike_elliott said:

Jeff, run the servo motor in each direction for about a minute to clean the armature. You will need a sub min. D connector, I believe pins 2,3,4,and 7 are used, but you can look at  your existing wiring to find out. 7 is ground. One of the pins is clockwise + , one is counter clockwise + one is clutch engage +.  I built a pigtail and did this ea. annual on both the pitch and roll servo using the old elt  battery when I had a STEC 30 and experienced the problem once. The solution came from the Stec field rep who lives in your neck of the hills.

Mike, the problem came back a few weeks ago so I spoke with Brian Kendrick about all the possibilities.  He suggested much the same procedure, although just doing this by running the VS at +10 and -10 for the full travel in each direction.  After listening to all the symptoms, he felt there was probably just some gunk that built up on the brushes of the pitch servo due to lack of exercise and the humid/salty climate.  So I've done this and so far things seem to be working better!

[DXB]

Painful topic - my own personal saga with the STEC-30, which I think has the same basic altitude hold system, is in the thread below. There are many potential causes. In my case there was a happy ending after I found the connector between the pressure switch and the control module and painted the contacts with Stabilant 22.