KFC200 Autopilot or trim issues

[milotron]

Hi, I am working through some new AP or trim issues and trying to understand the baseline condition that I might have never noticed in the past.

 

The issue:

AP drops out with a flashing 'trim' alert when trying to maintain in ALT mode or in attitude mode. Electric trim works fine when no on AP. Heading and Nav modes work fine. Note that I don't seem to get the four beeps when pushing the test button anymore. Not sure what the beep codes mean.

When on AP and before the drop out, the manual trim wheel cannot be turned by hand; is this normal as I have never noticed. It cannot be overwhelmed by hand, it feels locked and not the same feeling as pulling through the clutch when fighting against it in electric trim mode. This same sensation can be simulated on the manual wheel when only the left hand trim switch is activated, which I understand engages the clutch in the servo.

 

Thoughts:

Is this an AP issue or a trim/servo issue?

The Trim indicator no longer comes on when I turn the elec trim switch/breaker on, but again unsure if that is normal or not.

I have been poking at the bulkhead transition that the wiring pass through beside the battery box as it had a big gap and I am still chasing a potential CO issue. Broken or chafed wire perhaps?

 

Any thoughts on where to look or likely culprits? Going into annual soon, but also would like to ensure I can point in the right direction versus random guess and test.

 

 

Thanks,

 

[tmo]

I was having similar issues, and haven't managed to trace the root cause yet. Mine would randomly start trimming down for no apparent reason.

@Jake@BevanAviation was very, very, very helpful in tracking down the gremlins, but weather got in the way.

I would suggest you start with a through and proper ground test (as suggested to me by Jake):

• With a level attitude gyro engage FD and A/P on.
• Hold CWS and position the yoke to about mid travel then release CWS.
• While supporting the yoke use the up or down modifier button in the KC-290 to provide a pitch command.  
• Resist the movement of the servo and see how long it takes for auto trim to start.
• Note the effort on the yoke from the pitch servo and the direction of auto-trim.
• Hit CWS and reposition the yoke if needed.
• Do the same test but in the opposite direction and note the results.
• Hit CWS and reposition the yoke.
• Without using the Up/Down modifier in the KC-290 apply pressure on the yoke with a push/pull for constant pressure.
• Note the time it take for auto trim and the direction.
• Repeat the test in the opposite direction and note the results.

Quoting Jake: When supporting the yoke with no input command from the KC-290 or pressure on the yoke there should be no auto-trim.  If you notice a disparity in the effort needed to start auto trim we might have a switch or spring that has moved inside the pitch servo. For this installation the pitch servo needs 11inlb of torque to close a micro switch per the MM for the servo.

Also, make sure you're doing this on level ground, so that the plane's attitude is "sane" (no bank). Pitch will, obviously, be different than in cruise.

Interestingly, my AP, also a KFC200, also in a M20K, never beeped during the self tests, but I didn't trace that down yet, either.

For me, I'm focusing on some stiffness in the trim jack screw - it takes more force to turn the manual trim in some spots / trim settings than in others, and I'm thinking this might be confusing the auto-trim feature.

Good luck!

[Jake@BevanAviation]

With the A/P system engaged the trim solenoid pulls in the trim servo and remains engaged until the system is disconnected and this is all normal.  With the A/P system off you should be able to freely trim the aircraft by hand or you could used MET(manual electric Trim) on the yoke to trim the aircraft.  NOTE: you can not have auto-trim and MET at the same time.  If the A/P system is engaged and you trigger the MET function the A/P will disengage.

Typically a flashing trim annunciation is due to the system detecting a trim fault.  MET and auto-trim work independently from each other, you could have a failure for auto-trim but have normal operation of MET.  There is a rare instance that it could be a component level failure for the flasher circuit inside of the KC-295 flight computer.  When this happens you will have a random flashing trim annunciation that will vary in speed and consistency.  Another possibility is the MET switch in the yoke specifically the left side of the split rocker.  If the return springs in the MET assembly are weak or broken there is a possibility the left side of the switch is causing the system to disconnect in flight. 

[milotron]

Thanks for this. I was unsure how the electric trim and elevator servo work with each.

It seems that the elevator trim cannot achieve what is needed then is activating electric trim and running. This seems to be a consistent pitch down at the end of these events.

[tmo]

1 hour ago, milotron said:

This seems to be a consistent pitch down at the end of these events.

Does your AP eventually end up pitching down as well?

[milotron]

In the brief testing I did, I did notice the down pitch to limits, but don't recall testing the other direction. I will do so this weekend.  I also have not been able to make it 'fail' on the ground yet.

 

[tmo]

How does your elevator look when on the ground? Mine points down, as in yoke is all the way in towards the panel, and it does take some effort to pitch it up (I probably should measure it and maybe adjust the balancing weight under the panel). It is fine when flying, in all regimes, but prop wash alone won't raise it when taxiing.

[jlunseth]

1 minute ago, tmo said:

How does your elevator look when on the ground? Mine points down, as in yoke is all the way in towards the panel, and it does take some effort to pitch it up (I probably should measure it and maybe adjust the balancing weight under the panel). It is fine when flying, in all regimes, but prop wash alone won't raise it when taxiing.

Mine also, at rest. It is how they are built, not an issue. Actually, it serves as a sort of gust lock because it would take a heck of a wind to pick the elevator up.

[tmo]

Thanks for confirming; this bears on the ground checks of the AP - I propose that it might be difficult to be able to hold it level enough for the pitch trim to not kick in. Ah, the joys of our hobby...

[jlunseth]

The pitch trim in the AP test is very slow. You have to wait for it to happen, but it will.

[milotron]

OK, some updates and more questions:

 

I got the AP to fail during ground testing now.  Got two different beepy/blinky codes. First was 5 blinks/beeps before failure. Second was 2 after the failure.

 

With pitch trim and yoke held i a mid position it was fine. When calling for climb yoke would move, but would trip out and fail after five or so second when the electric trim started moving. Similar in the down mode.

Question: should the pitch trim be able to hold the yoke in position on the ground, no engine running against the yoke counterweights? It seems this is the issue with mine, it pulls in pitch but perhaps not enough or quick enough.

 

[carusoam]

See if @Jake@BevanAviation is cruising by...  (update on observations above)

-a-

 

[PT20J]

58 minutes ago, milotron said:

 

Question: should the pitch trim be able to hold the yoke in position on the ground, no engine running against the yoke counterweights? It seems this is the issue with mine, it pulls in pitch but perhaps not enough or quick enough.

 

The M20K elevator has a downspring which is why the elevator hangs down to the stop when the airplane is on the ground. The trim primarily changes the angle of incidence of the tail and to a lesser extent adjusts the tension on the downspring to improve longitudinal stability. On M20J and earlier models with trim bungees, the trim would move the yoke quite a bit on the ground. I've never tried it on a M20K, but since it is always a downspring, I would expect that the yoke would not move (but just be held nose down with a varying force) as the trim is moved from full up to full down. You can check this easily by simply moving the trim wheel from stop to stop.

This has nothing to do with the autopilot operation. Engaging the autopilot on the ground can be confusing because the autopilot is a closed loop system and without the airplane responding to servo commands, the feedback loop is broken. For instance, say the airplane isn't exactly level and the attitude indicator shows a 1/2 degree right bank. The autopilot will command left aileron. But, sitting on the ground, the airplane will not bank. So, the autopilot will command more left aileron. The airplane still cannot respond. This continues until eventually the ailerons will be fully deflected to the left.

Skip

[carusoam]

+1 for the wind not having much of an affect on the controls while on the ground...

In my old C, trimming also caused the yoke to move...

As the tail moves up, the elevator moves down... (fuzzy old memory)

Easily viewed while on the ground.

The O... the elevator is always hanging down... and yoke pressed against the panel...

Pretty convenient for getting in and out of the plane... 

PP observations about elevator bungees only... not a mechanic...

Best regards,

-a-

[milotron]

1 hour ago, PT20J said:

The M20K elevator has a downspring which is why the elevator hangs down to the stop when the airplane is on the ground. The trim primarily changes the angle of incidence of the tail and to a lesser extent adjusts the tension on the downspring to improve longitudinal stability.

Hi, I don't think this is true for the K. There are pretty large lead weights under the glareshield on the yoke mechanism that force the yoke into a full nose-down state. 

What I was saying was the servo that pulls this doesn't seem to be able to counter this weight on it's own, but not sure that is a measure of anything useful.

I do seem to have this isolated to the pitch servo mechanism so hopefully just a clutch adjustment or something simple.

[PT20J]

19 minutes ago, milotron said:

Hi, I don't think this is true for the K. There are pretty large lead weights under the glareshield on the yoke mechanism that force the yoke into a full nose-down state. 

What I was saying was the servo that pulls this doesn't seem to be able to counter this weight on it's own, but not sure that is a measure of anything useful.

I do seem to have this isolated to the pitch servo mechanism so hopefully just a clutch adjustment or something simple.

Perhaps I misunderstood you. Yes, there is a bobweight, but that’s part of the  elevator control system not the trim system. Bobweights are generally used to improve maneuvering stability by increasing stick force per g. But thinking about it, it is an additional static nose down force on the stick. The spring is part of the trim system.

Skip

[milotron]

17 minutes ago, PT20J said:

Perhaps I misunderstood you. Yes, there is a bobweight, but that’s part of the  elevator control system not the trim system. Bobweights are generally used to improve maneuvering stability by increasing stick force per g. But thinking about it, it is an additional static nose down force on the stick. The spring is part of the trim system.

Skip

All good. We are both correct! I just spent some time poking through the manual and saw the spring arrangement.

 

I just should just leave it alone and let my mechanic fix the damn thing week after next...

[kortopates]

Find an Bendix King autopilot specialist with the necessary BK test harness. Without this gear and expertise you can spend a fortune repairing things in the system that doesn't fix the issue. But since it's failing the start up test with less beeps a BK AP specialist should be able to figure it out pretty quick. But not a typical A&P.

 

 

Sent from my iPhone using Tapatalk

[kortopates]

BTW, if you want to change pitch with AP engaged in pitch mode, press the CWS button and then just adjust pitch with yoke. When you release the CWS, the AP will engage trim to hold the pitch you had when you released the CWS. Works the same in altitude hold too, such as after making a altimeter baro setting and you want to gain or loose a few feet.


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[tmo]

On 1/17/2021 at 3:41 AM, milotron said:

Question: should the pitch trim be able to hold the yoke in position on the ground, no engine running against the yoke counterweights?

Does it even make sense to test the AP without the engine running? The AI (attitude indicator, KI256) is a reference for the AP and without the engine running, you have no vacuum, hence no reference for the AP to use. Or do you have electric / standby vacuum? My notes say "with engine running and AI fully spun up".

[Jake@BevanAviation]

After doing some additional testing on the KTS 151 with a KC295 flight computer.  The computer will flash a trim fail if it sees the following condition.  

• microswitch closed in the pitch servo.
• auto trim command sent to the trim servo
• no feedback from trim servo within a period of time
• flashes trim fail as long as the microswitch is closed.

The test procedure says to wait at least 20 seconds for the fail to be issued and should clear within .82+/-.3 seconds after removing the command for auto trim. NOTE: the computer issuing a trim fail annunciation did not result in the system being disconnected.  If I jumper the feedback to be the wrong polarity the trim fail is issued within 3-5 seconds of the microswitch being closed.

Any shop experienced with using the KTS150 flight line tester should be able to quickly isolate the issue in the aircraft.  If the aircraft was here, I would start troubleshooting the trim servo first.

[kortopates]

7 hours ago, tmo said:

Does it even make sense to test the AP without the engine running? The AI (attitude indicator, KI256) is a reference for the AP and without the engine running, you have no vacuum, hence no reference for the AP to use. Or do you have electric / standby vacuum? My notes say "with engine running and AI fully spun up".

Virtually all AP troubleshooting is done without running the engine. That's why its so important to seek out a AP specialist with the test harness Jake mentions above in order to efficiently troubleshoot the system. The test harness gear fits between each of the servos being tested and allow simulating AP functions and measuring response to isolate a failure. When it can't be repeated on the ground it may well require a test flight to try to duplicate it, but like anything else intermittent issues are the hardest to troubleshoot.

Also, the pilot too can do a lot to trouble shoot the AP performing all the preflight actions without the engine running but using the electric standby vacuum and testing both heading left/right, pitch up/down with trim and AP disconnect. 

Edited January 18, 2021 by kortopates

[Emmet]

Hi, I am working through some new AP or trim issues and trying to understand the baseline condition that I might have never noticed in the past.
 
The issue:
AP drops out with a flashing 'trim' alert when trying to maintain in ALT mode or in attitude mode. Electric trim works fine when no on AP. Heading and Nav modes work fine. Note that I don't seem to get the four beeps when pushing the test button anymore. Not sure what the beep codes mean.
When on AP and before the drop out, the manual trim wheel cannot be turned by hand; is this normal as I have never noticed. It cannot be overwhelmed by hand, it feels locked and not the same feeling as pulling through the clutch when fighting against it in electric trim mode. This same sensation can be simulated on the manual wheel when only the left hand trim switch is activated, which I understand engages the clutch in the servo.
 
Thoughts:
Is this an AP issue or a trim/servo issue?
The Trim indicator no longer comes on when I turn the elec trim switch/breaker on, but again unsure if that is normal or not.
I have been poking at the bulkhead transition that the wiring pass through beside the battery box as it had a big gap and I am still chasing a potential CO issue. Broken or chafed wire perhaps?
 
Any thoughts on where to look or likely culprits? Going into annual soon, but also would like to ensure I can point in the right direction versus random guess and test.
 
 
Thanks,
 

It looks exactly like the issue I had with mine. Long story short - the cause was the bearing on the trim torque tube below the trim servo. It seldom sees some lube during annuals, gets stuck and produces what you are experiencing.

[milotron]

All good feedback guys, thanks. I will pass this on to my  AME next week.

 

As for the CWS function, mine has been disconnected since before I bought the plane due to an intermittent wiring issue. I was going to get it rewired and reinstated during this annual anyways.

 

Clearly an issue with the pitch servo which is a great starting point for troubleshooting.

[skykrawler]

2 hours ago, Emmet said:

It looks exactly like the issue I had with mine. Long story short - the cause was the bearing on the trim torque tube below the trim servo. It seldom sees some lube during annuals, gets stuck and produces what you are experiencing.

It's in the manual...100hr inspection.  A sticker is supposed to be placed near the trim servo saying it was lubed.   My airplane was 34 years old when I bought it and has no sticker and it looked like it at never been touched.  I've lubed it every year at annual.