KAP 200 autopilot fails with electric trim on (was: failed with altitude hold)

[MooneyAcolyte]

Hi fellow Mooniacs, 

happened to me yesterday: When switching the KAP 200 autopilot to altitude hold mode, it started trimming down continuously until the trim forces became too great and the AP disconnected. Tried 3 times with reset in between no joy. The electric trim works when the AP is off. The AP works without the altitude hold. 

I couldn't find anything in the archives, so I wonder if any of you had the same problem and might know what's wrong. Any help is appreciated. Model: 1984 M20J, original panel.

[carusoam]

Sounds like the AP’s trim servo or possibly the elevator servo has gone rogue…

Let’s invite @Jake@BevanAviation to the conversation…

AP servos see a lot of use and have a tendency to get dirty as they wear…

Are you seeing any altitude oscillations to go with that, any before now?

 

PP thoughts only…

Best regards,

-a-

[MooneyAcolyte]

Sounds like the AP’s trim servo or possibly the elevator servo has gone rogue…
Let’s invite [mention=17169]Jake@BevanAviation[/mention] to the conversation…
AP servos see a lot of use and have a tendency to get dirty as they wear…
Are you seeing any altitude oscillations to go with that, any before now?
 
PP thoughts only…
Best regards,
-a-

Thanks for your thoughts. I had very small and erratic as in non-periodic pitch excursions before, but so small that I ignored them. I was thinking perhaps the static line to the KC 295 broke. Would that be a possibility? I go check after Easter and post here if I find anything.


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[Jake@BevanAviation]

If the issue is related to alt hold functions only it might be an issue with the transducer board.  This board only does alt hold functions.  If you can set a climb and descent rate and the aircraft auto-trims properly with power changes I would guess the servos are probably ok.  A good test of the auto trim function might not be a bad idea as well.   Typically when the transducer boards fail you get a hard up or a hard down when selecting alt hold and they will not level out.

[MooneyAcolyte]

If the issue is related to alt hold functions only it might be an issue with the transducer board.  This board only does alt hold functions.  If you can set a climb and descent rate and the aircraft auto-trims properly with power changes I would guess the servos are probably ok.  A good test of the auto trim function might not be a bad idea as well.   Typically when the transducer boards fail you get a hard up or a hard down when selecting alt hold and they will not level out.

I‘lll test that - thanks. Question: wouldn’t a failed transducer board show up as a failed self-test? When push the test button, I get all 5 beeps. Blue skies, Markus


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[Jake@BevanAviation]

Nope, you can have a failed transducer board and it will pass PFT.

[MooneyAcolyte]

Nope, you can have a failed transducer board and it will pass PFT.

Good to know. I‘ll continue to investigate and have the avionics shop have a look at it. I‘ll report here.


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[PT20J]

I had a KFC 200 that did wonky things in ALT hold. I pulled the transducer board and found that the transducer itself had water in it. I emptied it by shaking it and let it drain for a couple of days and it worked fine after that. 

[carusoam]

+1 for being on the look out for water in the static system…

Water has an innate ability to be carried by air all the way to where the sensor is hiding…

See if the altimeter is showing any funky behavior as well…

PP thoughts only… not an instrument guru…

Best regards,

-a-

[MooneyAcolyte]

48 minutes ago, PT20J said:

I had a KFC 200 that did wonky things in ALT hold. I pulled the transducer board and found that the transducer itself had water in it. I emptied it by shaking it and let it drain for a couple of days and it worked fine after that. 

Please excuse my ignorance. The transducer board is inside the FC 295, correct?

[carusoam]

23 minutes ago, MooneyAcolyte said:

Please excuse my ignorance. The transducer board is inside the FC 295, correct?

Look at the back of the AP’s box… (helps to have a flexible camera…)

If you see a tube going there…

One end of the tube goes to the static system, the other goes to the sensor…

Somebody the other day reported their static sensor being independent of their auto pilot….

Of course, I can’t recall what AP we were discussing at the time… 

Best regards,

-a-

[MooneyAcolyte]

7 minutes ago, carusoam said:

Look at the back of the AP’s box… (helps to have a flexible camera…)

If you see a tube going there…

One end of the tube goes to the static system, the other goes to the sensor…

Somebody the other day reported their static sensor being independent of their auto pilot….

Of course, I can’t recall what AP we were discussing at the time… 

Best regards,

-a-

Thanks, I'll try that.

[PT20J]

31 minutes ago, MooneyAcolyte said:

Please excuse my ignorance. The transducer board is inside the FC 295, correct?

Yes

[MooneyAcolyte]

Latest update: still scratching my head.

Based on the suspicion of water in the static line and/or the pressure transducer board, I took the KC 295 out, pulled the transducer board and inspected: no water. To be on the safe side, I put the board in the oven at 50degC for an hour and blew warm, dry air through the static lines. I put everything back together - no change.

I tested again and found that the trim-down runaway occurs also without the altitude hold mode. The electric trim works without the autopilot.

Next sep: test-fly autopilot with electric trim off.

Any ideas to solve this mystery are welcome.

Blue skies!


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[PT20J]

1 hour ago, MooneyAcolyte said:

Latest update: still scratching my head.

Based on the suspicion of water in the static line and/or the pressure transducer board, I took the KC 295 out, pulled the transducer board and inspected: no water. To be on the safe side, I put the board in the oven at 50degC for an hour and blew warm, dry air through the static lines. I put everything back together - no change.

I tested again and found that the trim-down runaway occurs also without the altitude hold mode. The electric trim works without the autopilot.

Next sep: test-fly autopilot with electric trim off.
 

It's been a long time since I flew a KAP/KFC 200, but I believe the logic is the same as the 150 and it will not engage with the trim off. @Jake@BevanAviation will know for sure.

Skip

[Jake@BevanAviation]

It will engage but it will not be happy and will have a constant trim warning.  

[MooneyAcolyte]

12 hours ago, Jake@BevanAviation said:

It will engage but it will not be happy and will have a constant trim warning.  

OK, I'll try and share how it went. It's rather a part of the diagnosis than a standard operating procedure. While I have your attention @Jake@BevanAviation, my preliminary conclusion given the observation

* electric trim works w/o AP

* AP works w/o electric trim

* AP passes PFT

* FD works w electric trim

* Pitch trim runs away pitch-down with AP enabled in any mode

is that the issue is still the KC 295, not the trim servo. Would you agree?

 

Thanks in advance for any guidance you can provide.

[Jake@BevanAviation]

MET (manual electric trim) has separate circuitry vs autotrim.  MET can be functional but have a issue with autotrim functions and still be related to the trim servo.  Easiest way to see where the issue is by unplugging the trim servo and monitor the drive lines from the computer for autotrim functions with a DC volt meter. Do a autotrim test (can be push/pull or use the UP/DN switch in the KC290) and monitor the DC volt meter. Ideally without the micro-switches being closed in the pitch servo the computer should generate no drive.  When a micro-switch in the pitch servo is closed and remains closed there is a 3-5 second delay before the computer will issue a autotrim command.  The autotrim command from the computer will stop when the micro-switch is opened. 

If the computer checks out with the closing of the pitch servo micro-switches and autotrim drive being issued, the issue is with the trim servo. In normal operation if the computer detects a trim runaway condition (trim feedback) without a autotrim drive command being issued it will issue a trim fail warning.  Copies of the IM can be found in the download section for interconnect reference.

[MooneyAcolyte]

MET (manual electric trim) has separate circuitry vs autotrim.  MET can be functional but have a issue with autotrim functions and still be related to the trim servo.  Easiest way to see where the issue is by unplugging the trim servo and monitor the drive lines from the computer for autotrim functions with a DC volt meter. Do a autotrim test (can be push/pull or use the UP/DN switch in the KC290) and monitor the DC volt meter. Ideally without the micro-switches being closed in the pitch servo the computer should generate no drive.  When a micro-switch in the pitch servo is closed and remains closed there is a 3-5 second delay before the computer will issue a autotrim command.  The autotrim command from the computer will stop when the micro-switch is opened. 
If the computer checks out with the closing of the pitch servo micro-switches and autotrim drive being issued, the issue is with the trim servo. In normal operation if the computer detects a trim runaway condition (trim feedback) without a autotrim drive command being issued it will issue a trim fail warning.  Copies of the IM can be found in the download section for interconnect reference.

Thanks, I‘ll try that and report here


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[MooneyAcolyte]

On 4/17/2023 at 2:42 PM, Jake@BevanAviation said:

MET (manual electric trim) has separate circuitry vs autotrim.  MET can be functional but have a issue with autotrim functions and still be related to the trim servo.  Easiest way to see where the issue is by unplugging the trim servo and monitor the drive lines from the computer for autotrim functions with a DC volt meter. Do a autotrim test (can be push/pull or use the UP/DN switch in the KC290) and monitor the DC volt meter. Ideally without the micro-switches being closed in the pitch servo the computer should generate no drive.  When a micro-switch in the pitch servo is closed and remains closed there is a 3-5 second delay before the computer will issue a autotrim command.  The autotrim command from the computer will stop when the micro-switch is opened. 

If the computer checks out with the closing of the pitch servo micro-switches and autotrim drive being issued, the issue is with the trim servo. In normal operation if the computer detects a trim runaway condition (trim feedback) without a autotrim drive command being issued it will issue a trim fail warning.  Copies of the IM can be found in the download section for interconnect reference.

Hi @Jake@BevanAviation, couple of clarification questions:

* the IM you are referring to is this the one at the link below, correct?

* I find the connector of the flight computer to the pitch trim servo on page 130. In it, the main command and return lines appear to be "TRIM SERVO CMD +" and "TRIM SERVO CMD REFERENCE" (screen shot below). 

* I assume the check is to measure the DC voltage on "TRIM SERVO CMD +" on the flight computer side, correct?

* I do not see a line that indicates the servo micro-switch. How can I test the command for a closed micro switch? Wouldn't that mean the computer must be connected to the servo?

I'm sorry if the questions are overly stupid. Thanks for any help you can offer. Blue skies, Markus

 

Edited April 18, 2023 by MooneyAcolyte

[Jake@BevanAviation]

Wrong IM,  you need the one for the KAP/KFC 200 system. 

[MooneyAcolyte]

On 4/18/2023 at 9:22 PM, Jake@BevanAviation said:

Wrong IM,  you need the one for the KAP/KFC 200 system. 

Thanks, found the right one now and have made a little test plan based on the schematics in the IM. Will report to when I will have had time to hook up the multimeter

KS272 pin	action
J,H	contact GND
N	+14V
D	check +14V for trim down
P	check +14V for trim up
A	check +14V if electric trim is ON
C	contact KC295 P2951/N
R	contact KC295 P2952/N, should have no command if KS272/F is open
K	contact KC295 P2952/F should have no command if JS272/L is open
F	contact KC295 P2952/T
L	contact KC295 P2952/d_
E	contact KC295 P2952/y_

Edited May 4, 2023 by MooneyAcolyte

[MooneyAcolyte]

Latest update: started the test and found that the KC295 is connected to a KS179, not a KS272. If someone could point me to the right installation manual, that would be wonderful. Blue skies!

[Jake@BevanAviation]

See attached for interconnect.

KFC200 M20J-K.pdf

[Arthur]

I had the exact same problem in November. Pitch servo has been in avionics shop (well known for King Auto pilot repair) since Feb. They can’t seem to find the problem. Servo works but under load draws too much current and overheats. My servo is the KS270-065-0027-04 Not the upgraded K207A.

Difficult to find a servo under $3000