S-Tec 55X porpoise issue
I have a 2 year old S-Tec 55X system in my Twinkie. Worked perfect until after aircraft paint job, then I noticed the aircraft proposing. I will not swear the issue happened immediately after the re-paint, because I didn’t really notice it until 10 flights +/- after the paint job.

Issue: Porpoise +/- 20 feet at 100/200 FPM. Basically slow oscillations. The +/- 20 feet is consistence. FPM varies between 100 -300. Issue occurs if auto trim on or off or if in ALT hold of VS set to zero. Troubleshooting to date with shop and S-Tec via email and phone: Burn in servo, both pitch and trim. Check voltage on servos, check all airframe and cable tension, replace Alt transducer with temp, remove static tubes on alt transducer.
Question- We are now sending the computer back to S-Tec per their suggestion and S-Tec wants me to remove paint from static ports and the small block that is just forward of the static ports. Static ports are unique to the S-Tec (originals still feed panel instruments). Static ports look fine and have been pressure tested. S-Tec says the paint could disrupt air flow just enough to cause the small oscillation as the computer / ALT transducer are very sensitive.

Does this make sense? Before I start paint removal wanted to check. Not sure how a layer of paint can cause air flow issues over the static ports since we know the hole and lines are free of foreign material.
Thanks Greg

Greg,

It seems strange that it would be fine for several flights after paint and then generate a problem via the static ports.

Have you already talked to the paint shop about this? If they have seen it before they may have a thought or two.

Do know that my Century 2000 started to porpoise because the attitude indicator where it gets its altitude hold info was unhappy. Fixed that and the porpoise went away.

I believe S-Tec gets its sensing info from a different source so your problem may be different.

Other things to check might be the stab rigging, trim drum and cables associated with them.

If I recall correctly, when painting is done control surfaces are required to be balanced. Did the paint shop do this?

Let me also add that I have seen airflow mess with altitudes on two different aircraft. A GlasAir III and a Diamond Katana (I think). The situation was a tad unusual in that they were in cross country races I organized where the en route and finish lines to stop the clock were at 200' AGL over a runway. When the aircraft descended at full power, their altimeters lagged so that even the 'sharpest of pilots' flew at 40' to 100' AGL even though they had current altimeters. Obviously these were fiberglass vs the Comanches. Did not see this on any other aircraft in 4 decades of racing and 1.5 decades of organizing 21 races.

Hope this helps in some way.

Because the STEC-55 X is an electric rate based autopilot with a very sensitive altitude pressure sensor the problem you describe is quite common. It's usually found in new installations where it brings attention to pre-existing conditions like improper cable tensions. If it worked great before the paint job you need to look at things that might have happened during the paint job. Are the static lines completely free of moisture? Cleaning the port itself from paint could be helpful.

I fought with a STEC-55X in a T210 for a very long time before taming it after the computer was sent to STEC. I've been told, but have never confirmed that in some aircraft it's necessary to adjust the "gain" set in the computer. Apparently, there is no way to change the gain in the field and it was STC'd that way. Apparently, the have to change resistors on some circuit to make the change and won't allow it to be made in the field because it shouldn't be required.

All of the things they have asked you to do are things which have been known to cause trouble in some cases. Sending the computer to them is their last resort.

Still, if it didn't happen before painting I'd look to how that process could have altered things.

Interesting you mention this. I flew to St Louis from Palwaukee on Sunday and back and had the same issue in a Piper Archer 3. Couldn't figure out what was going on. I disconnected the AP hand flew and trimmed and then reset the AP in altitude hold. It seemed to do the trick. Spoke to my CFI today during my sim training and he said that is common with the STec 55x in the Archer.

I have an S-Tec 30 in my '59 - 250. Had the plane painted exactly a year ago. On the trip home from the paint shop I found that each time I engaged Alt Hold it would begin to porpoise in ever-increasing oscillations that would have resulted in disaster within a minute. (Another reason for my personal rule: "Always make sure the first flight after any work is done occurs in daylight VMC.") Upon my return home, I called the paint shop who explained that despite their best efforts to plug the static holes, minute traces of water could have crept into the static system. They had needed to use much more paint stripper and hence more water than average to strip off the old paint because in their words "the guy who painted your plane 30 years ago did too good a job!"

Other static-related instruments (airspeed, altitude, vsi) all displayed normal behaviour. (I have since learned that S-Tec recommends a separate static system for their autopilot, this was not done when mine was installed many years ago. While this omission did not have an effect on the problem, I mention this now before someone else jumps on it.) At this point I did not wish to disconnect any static lines with the attendant need to have the static system recertified. So, before doing anything drastic (or expensive) I placed a small electric space heater in the tailcone for a couple of days to hasten the evaporation of any moisture that might be in the static lines. Problem Solved!

I should mention that since the control surfaces had all been removed for painting, I felt the possibility existed that some of the cable tensions could have been compromised a bit by these abnormal activities. So I had my regular maintenance shop inspect all the tensions and readjust where necessary. They found nothing glaringly out of spec but after a careful going over the plane now holds altitude (in fact, all attitudes) even better.

And now for the rest of the story: After a couple of dozen flights, I could detect the growling sound that indicates a bad wheel bearing. The repeated pressure washings during the paint stripping process had caused water to seep into the wheel bearings, causing them to rust. We now have six new wheel bearings and things are fine. An important thing for all of us to remember whenever washing our planes.

Don Ostergard
c-flhv@magtech.ca

Don,

If you are referring to landing gear wheel bearings, I believe this is not an uncommon situation where paint stripper and other stuff contaminates the grease. In my case I had to replace one bearing and race after painting. Consequently, I would always recommend cleaning and re-greasing the landing gear wheel bearings after painting.

PatD