KFC 150 Pitch Trim Servo Limits

[moosebreath]

I have just had the KS179 removed, repaired, and reinstalled.  During this process the servo was run without being installed.  How does the KFC 150 know where the trim stops are?  The servo was removed and replaced without attempting to retain any datum point.  Does the servo simply run the trim until it hits the mechanical stop and then disengage the clutch?  This seems rather simple minded but perhaps that is how it works.  The trim currently runs from stop to stop just as it did before.

I have the installation, service, and operating manuals and I still can't figure this out.  Anyone out there know for sure?

[moosebreath]

I also have the KFC 150 Mooney installation STC.  This details installation and testing of the entire system.  It has no mention anywhere concerning setting a pitch trim servo datum.  There is trim position feedback.  Perhaps the computer remembers where the clutch disengages at the end of travel.

[Jerry 5TJ]

The pitch servo sends a signal to the computer that it is holding up or down force. In response the computer sends a signal to the pitch trim servo to run until the pitch servo signals that the force is reduced (the plane is “in trim”).  The servos themselves can spin in either direction without limit.  

[moosebreath]

What you say is all true.  The question I had remaining is what happens when the trim goes all the way to the mechanical stop in one direction or the other.  Clearly the servos can not spin without limit then.

Being an engineer, I could not stand not knowing any longer.  If anyone else wants to discover exactly what your KFC150 is doing, and I understand if you don't, what you need is a copy of King Manual 006-5539-00, King KC 190,191,192 Autopilot Computer Maintenance / Overhaul.  There are no free sources that I could locate.  eFlightManuals.com has them or perhaps you can give your local shop a couple of six packs and take theirs to Staples.

However you acquire the manual turn to Section 4 for your computer and begin to learn how the system actually works in detail.  Expect that this will take a while and might not be helpful if you are not an engineer and/or electronics enthusiast.  Here is a crib if you are interested in the trim servos.  The King terminology for what I would call the "trim system" is the "autotrim system".  I thought initially that was an option I did not have installed.

The answer to my question is the following.  The trim servo has no stops or position encoding, which is why you can simply replace it and the system still works.  What it does have is tachometer feedback.  The computer monitors whether there is a command for the trim motor and then whether the trim motor is moving, at what speed, and in what direction.  It does this whether the autopilot / flight director is engaged or not.  King specifically says it monitors for actuation in a reverse direction and movement with no command signal.  In these cases it generates a trim fault.

What King does not say, explicitly, is what happens when the trim hits the mechanical stop.  It clearly senses a command to move with no tachometer feedback in this case.  I was curious as to whether the servo motor is commanded to stop or whether it has a clutch that releases and it keeps moving until you release the trim switch.  I did not want to buy the trim servo manual to find out.  The quest for the right manuals had already begun to make me feel like Groucho Marx in "A Day at the Races".

For the answer, I made a trip to the hanger and ran the trim to the stop.  I could hear the motor run once I silenced all the noisy panel fans and gyros.  When the trim hits the mechanical stop the motor stops.  Clearly there is logic in the computer to do this as it is perfectly clear from the schematics that there are no limit switches or encoders in the system.

I will make a separate slightly different post to document that the manual Theory of Operation section is just full of interesting things to know about how the system functions that are not documented in the Pilot's Guide such as how to calculate the lead for level off when you hit altitude hold (15% of the rate of climb in feet, I tested it yesterday, it works perfectly).  How the system recaptures after a lost glideslope (it depends on the delay before the signal is again recieved, did you know that?).  How it engages NAV mode once it is armed, (it depends on exactly how you engage NAV mode and the offset error when you do).  And yes, that old chestnut, does the glideslope capture from above and below.  The answer is YES.

Perhaps some other knowledge seeker will see this someday and also discover the path to KFC 150 enlightenment.  Why this information is not simply in the back of the Pilots Manual is a mystery to me.  King must have thought only engineer pilots would care.

[N201MKTurbo]

The Century servo has a slip clutch. When it gets to the end of travel, it just slips. The servo has no reason to know where it is.

Perhaps, the king servo either senses the slip, or monitors motor current and trips it off when it gets to the limit.

Edited June 21, 2021 by N201MKTurbo

[N201MKTurbo]

This excerpt from the operators manual indicates that the trim servo can generate faults. I assume it faults when it gets to the end of travel because it is being commanded to move and isn't moving because it hit the hard stop.

[moosebreath]

No it does not fault.  Hitting the end of travel does not generate a TRIM warning light or audible warning.  It just stops.  I speculate the computer senses a demand signal and no movement from the tach feedback and stops the motor.  There is no mention of how the end stops work in the manual that I could find.