KFC200 (or KI256?) Wing Rocking Problem

[johnw413]

Hey Ya'll,

  My KFC200 has developed a new feature. After recently changing the KI256 it has begun to exhibit a tendency for the wings to start rocking. I can engage the AP while things are nice and level but any roll change - e.g. turbulence, a turn, etc - will cause the wings to start rocking about once a second, like a metronome, which won't stop until I disengage the AP, stop the rocking and re-engage. It's very annoying. The roll is small, about 2-4 degrees.

  Has anyone seen this movie? The tech in the avionics shop has made adjustments but I've begun to wonder if it's an oscillation problem between the KI256 and KFC200. I'm thinking of trying another KI256 to see if that solves it but wonder if it's a gain setting in the roll controller. The flight director bars do not show it as a command. Even when a turn is commanded it rocks during the bank.

  Any ideas or solutions?

TIA, John W

1981 M20J

 

[Jake@BevanAviation]

Double check that the gyro alignment was done on the new KI256 and have them look at the tach feedback from the roll servo.  Tach generators in the roll servo can go bad and produce roll oscillations.

[johnw413]

  Thanks for the reply! They've had the KI256 on a roll table while running the AP/FD but that won't easily simulate a feedback problem like what I've seen. The tach in the roll servo does sound like a definite possibility. I've already had an issue with the roll servo, which has been a tendency to stick on, which naturally happened while trying to intercept an ILS LOC in IMC. I had to struggle turning onto course until it let go. Apparently the pinion gear which connects to a capstan would not retract when the AP was disconnected. Not sure if the problem is the servo or the capstan. The tech showed me some metal filings from the pinion gear in the servo.

  I'll ask about the KI256. It was not overhauled just a used inst from a Navaho undergoing glass panel upgrade. I had hoped to replace the KI256 w/a KI300 but I'd like to hear that people are using it and not having the problems I've read about with the Sandia 340A, not to mention that the AP adapter has yet to get FAA approval.

[Jake@BevanAviation]

For the sticking solenoid I would have the tech check the travel of the solenoid and possibly adjusted the solenoid alignment so it doesn't get stuck when releasing.  I have seen this happen in the older style of solenoids but in the revision they added a plastic insert to try and prevent this.  

[johnw413]

  I'll ask about that. He did remove it and tried to improve its alignment to the capstan. I'm pretty sure at this point the servo will get pulled again and sent to (probably) Duncan for OH. I'm not sure about AP Central in Tulsa. Some people think they're great but my avionics shop seems to prefer Duncan, perhaps in part because Duncan bought the servo biz from BK.

[aviatoreb]

I had this exact problem with my KFC200 upon purchase, when I first purchased the plane about 10 years ago.  I called autopilot central who diagnosed the problem on the phone by my description. They told me to send in the servo and the computer box which I did and they came back overhauled and its been rock steady ever since.

[johnw413]

  So, what did it cost, if you don't mind my asking? I've never had AP problems before and they can be pretty scary in IMC. 

[aviatoreb]

  On 9/23/2019 at 3:53 PM, johnw413 said:

  So, what did it cost, if you don't mind my asking? I've never had AP problems before and they can be pretty scary in IMC. 

Expand  

I'm sorry - I just can't remember - it was like 10 years ago.

But I can say I was supremely impressed with AP central.  When they overhaul parts they come back literally better than new.  When new these had brass gears.  Now they have stainless steel gears.  All the electronic components are replaced.  Old school 70's and 80's era capacitors used to be get leaky with age and lose their functionality.  The new grade of stuff is much more robust and precise.  

But call them.  They do overhauls on a fixed price basis - they can tell you.  Let them diagnose your problem on the phone. See if you like them before you consider using them.

[johnw413]

  I spoke w/AP Central and they're pretty sure it is the tach generator, with maybe a solenoid problem related to its sticking. I'll get it yanked and sent off to maybe put my AP worries to rest.

Thanks!