KFC-150 autopilot issues

[r0ckst4r]

1995 Mooney M20R

I have a KFC-150 autopilot that is giving me issues.  I'm 99% it's the unit itself but I want to see what you guys have done in the past for this.
On a cross county flight I noticed the plane starting to steadily climb with altitude hold on.   No matter what I did the plane would start to nose up a little when autopilot was engaged.  This was an hour or two into an uneventful flight.  On the way back it worked no problem.
Next flight different issues.   About 20-30 min in the autopilot would just disconnect and shut off.   If I tried to hit the "test" button it would just blink and beep constantly until I filled the fuse.
The next flight everything works normal again until about 10-20min in then it disconnects.  Now things are all screwy.  It go black without when the test button light like it should at startup.   Then lights will flicker and beeps then blank again or maybe test lights up again.   It might go off again or if I hit it nothing happens or it beeps and goes off.  Seems like the computer is cooked.    The Attitude indicator is spot on.  No delays on power up and not slow at all during maneuvers.  Trim switch works great, not sticking or anything like that and works perfectly.
My question I suppose it have you guys replaced this in the past?  Are there any "easy" upgrades for this or can it just be repaired/replaced?  I don't know if I want to shell out 30 grand for a garmin autopilot and avionics.  I like my setup and it works for me, really just want it fixed.

[wombat]

This does not sound like a wiring or servo issue.

It could realistically be one of about three things that I can think of.  I assume there are more possible problems.

1. The KC-192 has some sort of internal failure.  This is what I think you are suspecting too.    Maybe find/buy a replacement and just swap it in?
2. Your attitude source is no longer sending the correct information.  What is your attitude indicator?
3. There is a power or ground problem.

[Fly Boomer]

49 minutes ago, r0ckst4r said:

I like my setup and it works for me, really just want it fixed.

Take it to a King KFC-150 expert.  Sometimes the parts cannon can work, but diagnosis is helpful.

[r0ckst4r]

33 minutes ago, wombat said:

This does not sound like a wiring or servo issue.

It could realistically be one of about three things that I can think of.  I assume there are more possible problems.

1. The KC-192 has some sort of internal failure.  This is what I think you are suspecting too.    Maybe find/buy a replacement and just swap it in?
2. Your attitude source is no longer sending the correct information.  What is your attitude indicator?
3. There is a power or ground problem.

Attitude indicator was the first suspect before the weird flickering lights. I can't imagine how a bad attitude indicator input to the autopilot would cause it to spaz out like that. I am fairly certain it is option one or option three.

[wombat]

15 minutes ago, r0ckst4r said:

Attitude indicator was the first suspect before the weird flickering lights. I can't imagine how a bad attitude indicator input to the autopilot would cause it to spaz out like that. I am fairly certain it is option one or option three.

I'm in agreement with you.    I was thinking that maybe the 'valid attitude' signal was intermittent and when it goes away and comes back the autopilot does some sort of self-check that turns the lights off and then back on.    It's pretty far-fetched though.

[redbaron1982]

I had some intermittent issues with my KFC 150 and @Jake@BevanAviation helped me troubleshooting it, and after isolating the issue to the flight computer itself, repairing it.

As an additional data point, the previous owner went through 3 shops trying to fix the issue without success, and Jake was able to fix it and haven't failed again.

BTW: In my case I also discovered that the avionics fan was not working, so I think the failure mode was: avionics fan broke, the KFC 150 ran for some months without any cooling until something started acting funny. This is to say, check that your avionics fan is working properly.

[r0ckst4r]

1 hour ago, redbaron1982 said:

I had some intermittent issues with my KFC 150 and @Jake@BevanAviation helped me troubleshooting it, and after isolating the issue to the flight computer itself, repairing it.

As an additional data point, the previous owner went through 3 shops trying to fix the issue without success, and Jake was able to fix it and haven't failed again.

BTW: In my case I also discovered that the avionics fan was not working, so I think the failure mode was: avionics fan broke, the KFC 150 ran for some months without any cooling until something started acting funny. This is to say, check that your avionics fan is working properly.

I would be headed there ASAP but Witchita is pretty far from me in coastal NC.   Going to have to find someplace closer.  That sounds like a absolute possibility since it seems to perform normally cold but fails after time.

[Jake@BevanAviation]

Pull the unit out of the rack (KC192) flight computer. Be sure to remove the static line to the transducer first.  On the back of the unit around the transducer port there should be a red or black 4 digit stamp.  This is the year and week the unit was manufactured.  Units made between 1991-1998 have capacitor leakage problems.  This is becoming more common with time.  I have 3 units here all with the same problem, leaking capacitors.  The capacitors that always fail are black and say IC on the side.  Depending on the severity of the capacitor leakage it can cause units to be beyond economical repair.  The leaking electrolyte can cause the system to not pass PFT or have weird annunciation/functionality.

[redbaron1982]

1 hour ago, r0ckst4r said:

I would be headed there ASAP but Witchita is pretty far from me in coastal NC.   Going to have to find someplace closer.  That sounds like a absolute possibility since it seems to perform normally cold but fails after time.

I live in South Texas, shipped Jake the unit for him to repair, and then installed it back. You just need to have an AP that can sign off this in the logbooks.

Edited October 4 by redbaron1982

[Will.iam]

I live in north texas and just flew up to Wichita, Kansas and had jake check my system out in the plane. I also had my ADI overhauled and that needs to be fine tuned to the kc-150 which he was able to do on the spot. 

[0TreeLemur]

Send it to Jake.   He'll fix it to good as new condition.   That's what I did and all's good.

[MikeOH]

2 hours ago, Jake@BevanAviation said:

...Units made between 1991-1998 have capacitor leakage problems.  This is becoming more common with time.  I have 3 units here all with the same problem, leaking capacitors.  The capacitors that always fail are black and say IC on the side.  Depending on the severity of the capacitor leakage it can cause units to be beyond economical repair....

Ah, yes, the "capacitor plague"; I remember this disaster well as a working EE back then:

https://en.wikipedia.org/wiki/Capacitor_plague

[r0ckst4r]

4 hours ago, Jake@BevanAviation said:

Pull the unit out of the rack (KC192) flight computer. Be sure to remove the static line to the transducer first.  On the back of the unit around the transducer port there should be a red or black 4 digit stamp.  This is the year and week the unit was manufactured.  Units made between 1991-1998 have capacitor leakage problems.  This is becoming more common with time.  I have 3 units here all with the same problem, leaking capacitors.  The capacitors that always fail are black and say IC on the side.  Depending on the severity of the capacitor leakage it can cause units to be beyond economical repair.  The leaking electrolyte can cause the system to not pass PFT or have weird annunciation/functionality.

PM sent.   You will be getting that unit as soon as I can send it.

[r0ckst4r]

8 hours ago, redbaron1982 said:

I live in South Texas, shipped Jake the unit for him to repair, and then installed it back. You just need to have an AP that can sign off this in the logbooks.

Does it just pull out with an Allen wrench like the other radios?  Any special procedure?

[LANCECASPER]

36 minutes ago, r0ckst4r said:

Does it just pull out with an Allen wrench like the other radios?  Any special procedure?

Like @Jake@BevanAviation mentioned in his post, "Be sure to remove the static line to the transducer first."

[PT20J]

10 hours ago, r0ckst4r said:

Does it just pull out with an Allen wrench like the other radios?  Any special procedure?

Crawl under the instrument panel and look at the back of the tray. There should be a short black rubber hose that connects to the static plumbing. Disconnect this and then remove the computer from the front.

[r0ckst4r]

5 minutes ago, PT20J said:

Crawl under the instrument panel and look at the back of the tray. There should be a short black rubber hose that connects to the static plumbing. Disconnect this and then remove the computer from the front.

Thank you.   I'm assuming the plane can still be hand flown with the unit out and the circuit breaker pulled.  Does this hose need to be capped or plugged?

[redbaron1982]

6 minutes ago, r0ckst4r said:

Thank you.   I'm assuming the plane can still be hand flown with the unit out and the circuit breaker pulled.  Does this hose need to be capped or plugged?

I'm not going to get into the legal aspects of your question, I'm not an expert. I'm sure that the "return to service" after removing the AP must be signed off by an AP.

But going to the more technical stuff, the hose you're referring to, has the static pressure used by the altimeter, VSI, and airspeed indicator. If the hose is left open, it is like having the alt static port open, the effect would be a higher altitude indication and a lower airspeed indication. To what extent, I'm not sure.

So, the short answer, yes, that "hose" should be terminated so the static pressure in the pitot-static system is not affected.

[PT20J]

Removing and installing panel-mount avionics is preventive maintenance. However, disconnecting the static line is not. So, if you plan to operate the airplane while the computer is removed, the strictly proper way to do this is to have a mechanic remove the computer, plug the static line, make a logbook entry, placard the circuit breaker and electric trim switch inop and, if you intend to fly IFR, perform a static leak test as required by 91.411.