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KAP 140 - roll axis problem

Today on my way back from Germany the Kap 140 roll axis developed a fault. I’m quite sure it was working just after takeoff, but then failed.
There was no fault visible on the Kap 140.
The altitude was maintained fine.
ROL HDG and NAV mode where all affected. It seemed like the autopilot was not moving the ailerons at all.
When engaged the normal resistance was felt when overpowering the autopilot.
Tried to cycle the power and also had a fuel stop, where the airplane was stopped. No change.
The plane has more than 1000 h under current ownership with no autopilot faults.
I’m fearing it is the roll servo.
How do I diagnose further?

pmh
ekbr ekbi, Denmark

I have now talked to our normal workshop. They can get an overhauled servo, but since they don’t have any avionics specialist it will be a bet if that is the problem.
Could use some hints if it is worth guessing it is the servo, or we need to travel longer to get to a proper avionics shop.

pmh
ekbr ekbi, Denmark

It is probably the roll servo.
A failure after 1000 h would be typical, at least that is when most roll servos in Cirrus aircraft fail. Depending on the design of the servo it might work to take it apart and clean it of all carbon dust. Some people do that with good results. Other than that I can only recommend contacting Martin Scheifl at Avionik Straubing, whi is the best autopilot specialist I know.

To diagnose a KAP 140 fault you need to plug in a laptop to the diagnostics socket which is located above the parking brake in a Cessna. The laptop needs to be running Hyperterm through a serial port and the required settings are in the KAP140 line maintenance manual. With the laptop connected you can engage/disengage the servo clutches, provide drive commands in all directions, read the servo feedback signals etc and read any fault diagnostic history from the in-built continuous monitoring. It’s a pretty reliable system for finding issues with this system.

Avionics geek.
Somewhere remote in Devon, UK.

wigglyamp wrote:

To diagnose a KAP 140 fault you need to plug in a laptop to the diagnostics socket which is located above the parking brake in a Cessna. The laptop needs to be running Hyperterm through a serial port and the required settings are in the KAP140 line maintenance manual. With the laptop connected you can engage/disengage the servo clutches, provide drive commands in all directions, read the servo feedback signals etc and read any fault diagnostic history from the in-built continuous monitoring. It’s a pretty reliable system for finding issues with this system.

Thank you very much.
Will need to find a avionics shop capable of testing like you describe.

pmh
ekbr ekbi, Denmark

pmh wrote:

Will need to find a avionics shop capable of testing like you describe.

If the referenced maintenance manual is available, a software/hardware engineer who has worked with serial port communication could do this test based on what wigglyamp wrote.

Last Edited by JnsV at 07 Sep 21:48
Hajdúszoboszló LHHO

I did manage to find a copy of the KAP 140 FLIGHTLINE MAINTENANCE MANUAL.
We will create the cable and then access the error log.

pmh
ekbr ekbi, Denmark

As wigglyamp says, this is obvious enough with a laptop running Hyperterminal.

Note that win7 or later doesn’t come with Hyperterminal, but a much easier to use free terminal program is Teraterm. Choose the Serial Port option on it.

The servos are also easy to bench test once you have the MM for them. You apply a variable voltage to the control input (as well as the supply voltage) and it should move. Servos wear out as sure as death and taxes – it’s only a matter of time. I have not seen the inside of a KAP140 servo but it is probably similar to the KFC225 one here. You need someone who understands electronics… even basic electronics.

Administrator
Shoreham EGKA, United Kingdom

wigglyamp wrote:

To diagnose a KAP 140 fault you need to plug in a laptop to the diagnostics socket which is located above the parking brake in a Cessna.

What connector? DSUB 9 or 25 pin?

ESKC (Uppsala/Sundbro), Sweden

pmh wrote:

I did manage to find a copy of the KAP 140 FLIGHTLINE MAINTENANCE MANUAL.

In electronic form or hardcopy?

ESKC (Uppsala/Sundbro), Sweden
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