porpose on Auto Pilot?

[AdamZ]

What could make a plane pourpose while on Auto pilot. I have heard that the problem could be a sticky yoke in need of some lubricant. Situation is plane is at 3000' with alt hold and it will porpose between 2900 and 3000. I have never had this happen on the autopilot that is installed with the Tiger but only on the Pipers ( Archer III and Lance) with S-Tec auto pilots FBO says just put approved lubricant on the yoke column?

 

[Lance F]

I talked to an STEC rep at AOPA Expo about this very issue. He suggested 3 possibilities:
1) static leak
2) sticky servo
3) improper tension on the bridle cable
Really for any of those you'd have to see your friendly STEC service center.
I don't see how lubrication of the yoke could cause this at all.

 

[gismo]

IME the most common causes of pitch oscillation are 1) pitch servo needing overhaul, and 2) water in the static system. If you have an alt static valve try opening that next time. If it helps chances are really good that you have water in the static line.

 

[wsuffa]

What Lance said.

If the AP is tapped off the airplane static system and you see no other issues in the instruments fed from static, it's probably the servo.

If this is a problem that's become steadily worse over time, it's probably the servo. Had the hold servo rebuilt last year on my STec 60... similar symptoms degrading to the point where it put the plane into a dive when altitude hold was engaged. Repair shop said "servo is 'toast' ".

 

[AirBaker]

My STec 50 has a gentle porpose if the airplane isn't trimmed correctly. I typically don't use alt hold until I need it.