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Piper Malibu PA46 Autopilot problem

Hi everybody. Maybe somebody can help me out.

I have a PA46 with a autopilot KING KFC 150 installed. I have a problem with the autopilot. After a flight time of 30 to 120 minutes the ailerons begins to swaying. Also in a left or right turn the ailerons sway but the course correction is o.k. This is how it goes:



After turn off/on the autopilot, the problem persists. Otherwise the autopilot works very accurate, with and without this problem.
The following parts have been replaced: Flight director, HSI, KFC 150 panel and slaved gyrocompass (behind the luggage storage at the rear of the aircraft). Nothing has changed.

I appreciate any comments or advice to solve the problem.

Have you been in contact with Martin Scheifl from Avionik Straubing about this problem yet?

Do you have a video of the flight director during the oscilation?

Doe sit happen in all autopilot modes? I suggest switching the modes all off when it happens and trying in simple wings level, pitch hold mode. If you have this information it can narrow down the problem and save you lots of money For example switchung the HSi and the gyrocompass is completely useless if the problem also happens if the corresponding autopilot modes are switched off…

www.ing-golze.de
EDAZ

Excessive friction or mass in a closed loop control system will cause oscillation. One common way in which this happens with autopilots is that the start current on the servo becomes high, often caused by brushes/commutators, but also by friction in the drive system.

+1 for Martin Scheifl!

I reckon this is excessive open loop gain on the roll axis.

Same issue described here

See the case of the wire which came off the tachometer. That increases the proportional gain of the servo (RPM per volt of the input voltage) approx 3 to 5 times, and it predictably makes the control loop unstable.

In this case the wire may have come off, or the tachometer itself is shagged. They are DC brush motors, made by Faulhaber in Germany, and cost about £40.

The flight director, in this case, probably won’t be of much help because it might just show the “correcting” waveform, out of phase.

I am not sure whether the KFC150 servos do the servo gain internally (like the KS170C / 171C / 172C servos of the KFC225 do) but the principle is the same. All these King servos contain a tachometer which is a simple crappy DC motor (with metal brushes) which will wear out as sure as death or taxes.

If my hypothesis is right, the oscillation will be seen in the most basic mode i.e. ROL + PIT.

The servo can be easily bench tested.

It could also be a loose bridle cable around the servo capstan. I am a bit reluctant to believe this however because this is what almost every avionics shop will say and they spend a fortune of your money. But this is quick to visually check, and would have been done by now, I would hope.

One common way in which this happens with autopilots is that the start current on the servo becomes high, often caused by brushes/commutators, but also by friction in the drive system.

That’s very true but on the King servos the tacho feedback masks these effects. As a (desirable) result, the servos, when working within the control loop, have almost no “start current”. And the RPM per volt is really linear. In contrast, the STEC servos went backwards about 50 years, to pre-WW1. The King servos are WW2

@Jack – you will probably need to find a competent avionics shop, which has a real electronics engineer. Very few of them in Europe. Jesse here is in the Netherlands, if that’s any use to you.

The following parts have been replaced: Flight director, HSI, KFC 150 panel and slaved gyrocompass (behind the luggage storage at the rear of the aircraft)

That is really terrible. Most of those items cannot possibly cause this problem.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

That’s very true but on the King servos the tacho feedback masks these effects. As a (desirable) result, the servos, when working within the control loop, have almost no “start current”. And the RPM per volt is really linear. In contrast, the STEC servos went backwards about 50 years, to pre-WW1. The King servos are WW2

Thanks for the correction. My comments were indeed based upon S-TEC servos.

I think you might be wrong about pre-WW1 though. I’m pretty sure that much knowledge on servo control theory came out of WW1 and related to the accurate rotational control of large mechanical guns.

Last Edited by Alan_South at 05 Nov 10:18

I would start with the servos.

EGTK Oxford

I agree ,

I also would start with looking at the Roll servo (It looks indeed like a feedback tach problem)
The tach feedback from the servo is “sent” , to the computer where the servo speed is controlled (different from KFC225 )
If the computer does not get tach (servo RPM) information it wil increase the drive voltage to the maximum, causing the servo to overshoot al the time.
But it also could come from a high breakout voltage of the DC motor inside the servo or from the att gyro,or wiring (less likely)

To get the rollservo out of a PA46 isn’t fun, if it’s an pa46 jetprop conversion it’s even worse .
When it has to come out for the first time the LH side wall has to come loose to get acces to the servo connector
(the servo is in/under the pax seat behind the pilot)

Try to find and make an appointment with a good AVX shop with experience in KFC AP trouble-shooting and bring your aircraft ,in the end it will safe you money.
(most common KFC problems can besolved in one working day, when the needed parts are in stock)
.

Last Edited by Jetprop at 05 Nov 23:24

Not sure if age specific, but removing the roll servo from my PA46 was pretty straightforward. Admittedly, I fly with the seat behind the pilot removed, so did not need to remove the seat first to gain access.
No need to remove the left hand side wall on mine either, but you have to remove the pedestal frame the seat normaly screws into to access all the nuts retaining the servo. Once you have undone the servo, the connector is accessable. Mine is a 2000 model.

The flap actuator motor also lives inside that pedestal.

I replaced my roll servo (KF225 system) for exactly the same symptoms, and it was due to a tacho failure.

Cheers – E

eal
Lovin' it
VTCY VTCC VTBD
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