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PA46 autopilot KFC150 fails

Hello, everyone!

Maybe somebody can help me out.

I have a PA46 with a autopilot KFC-150 installed. I have a intermittent, occasional problem with the autopilot. After staying for sometimes on the parking, the autopilot fails the startup test: the trim indicator fails and the autopilot disconnects.

After resetting the breakers, the problem persists. I have to fly the plane manually.

This is how it goes:

The avionic specialists from Piper have spent quite some time on fixing this mistake, but no success. The wiring and servos are Ok. The autopilot itself was tested on a stand: no mistakes. The HSI and AI were tested as well.

I appreciate any comments or advice to solve the problem.

Can't you connect a laptop via the RS232 console and record the diagnostic messages when the failure occurs?

I would suggest reading through this (my website) and seeing if anything in there looks similar.

It looks like the people who had a look at your aircraft didn't have a clue, so they tried everything they knew about.

The servos are easy to bench test, and don't even have to be removed from the aircraft for that. You just connect 28V, and connect a control voltage to them. However I doubt it is the servos.

One of the things that often causes really wierd problems is the red button disconnect switch. This is a 2-pole switch which has two poles. One tells the autopilot computer that you pressed it (and carries a tiny current) and the other interrupts the power to all the servos so that if you have an emergency where the autopilot has gone mad you can hand fly provided you hold the button down continuously (and that pole carries a heavy current, due to the inductive loads of the servo clutches, and burns out fairly quickly - the switch is too small for the job). So you get a situation where the autopilot computer thinks all is well but actually the servos are not getting power, or are getting it intermittently. This fault is sometimes visible in the fault log which can be downloaded via the RS232 connection; this is all in the KFC150 installation manual. I think I have a copy somewhere... but you do need to be competent to do this. If you don't have aircraft systems / electronics competence then you need to get somebody to do this for you.

When the autopilot passes the power-up test, try waggling the red disconnect switch button side to side (don't actually press it) and see what happens. A replacement of that switch is always a good idea and is easy to do by anybody who can solder properly. Mine failed after about 800 hrs. You can buy them from, etc, for about €10. I have the P/N but not handy. It is made by NKK in Japan.

Shoreham EGKA, United Kingdom

A similar intermittent problem in our TB20 (KAP150 autopilot) turned out to be the disconnect switch.

EGBJ / Gloucestershire

I've deleted my confusing comments about N71GT which is actually a TB20

But it does look like this pilot may be based in Poland (googling on N71GT).

Shoreham EGKA, United Kingdom

Won't go thru self test. Suggest trying to self test with alternator off. Try self test at low voltage. Autopilot sensing Trim overspeed. Possible servo issue. Possible trim switch logic problem.-

Does anybody have info how OP’s problem was solved?

LDZA LDVA, Croatia

The S-TEC 30+ autopilot in my Warrior had a similar problem when the wires from the switches on the yoke through the steering column had some light chafing after a couple of years. We now installed a new (spiral) cable that does not go through the column. Not as elegant but no chafing.

Check the switches on the yoke and all cables!

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