KfC-150 test fail question

[Will.iam]

I have a KFC150 autopilot system with yaw damper on a M20K 252. I have on occasion had it fail the trim test which keeps beeping until you reset the system with the circuit breaker. In the past i assumed that it failed when the manual trim switch was not centered due to weak springs and when i got it over hauled and reinstalled it cured the problem but it might also have been coincidental as it has now come back. I do notice it seems temperature related as i had the worse time with it last winter and when i finally got the manual trim switch finally repaired it was spring time and warmer weather. I also noticed even when it fails after flying for awhile about 30 mins or longer it will pass the test and then the system works fine until the next random time later on another startup flight usually cold temperature. I’m starting to think maybe the grease on the horizontal trim system or in the trim motor itself is so cold on these cold mornings it is inhibiting the trim motor from being able to move during the test which then fails. Once i manually move the trim a few times it seems to pass later when it’s warmed up. Anybody have this type of problem and know what the fix is? Maybe clean out the old grease and apply some fresh new grease? Maybe a section that gets dry and needs grease that would help make it smoother? Maybe my elevator trim motor is on its way out?

any insight would be helpful thanks. 

 

ok on further search i did find 2 threads on mooneyspace of which of them i had already seen and had greased the jackscrew at last annual i normalky manually trim when i hand fly and the trim wheel doesn’t seem stiff to me but that coukd be subjective. I did find another thread that stated this:

There is a pillow bearing adjacent to the chain sprocket on the trim drive shaft.  That bearing was in need of lubrication.  Cleaning and lubricating the bearing solved the problem.  Should be three to four hours of labor.

is this accessible from the belly with the belly pan removed? And what type of lubricant goes on the pillow bearing?

Edited December 22, 2022 by Will.iam
Added additional info

[Fly Boomer]

15 hours ago, Will.iam said:

There is a pillow bearing adjacent to the chain sprocket on the trim drive shaft.  That bearing was in need of lubrication.  Cleaning and lubricating the bearing solved the problem.  Should be three to four hours of labor.

If we are talking "Stabilizer Trim Control Shaft", my book says:
     Universal Joints     Δ
     Guide Blocks         α

Δ translates to MIL-L-7870, MIL-L-7870-B, superseded by MIL-PRF-7870-C Low Temperature Oil (General Purpose)
- Royco 363
- AeroShell Fluid 3

α translates to Graphite and MIL-G-3278 Grease or MIL-G-23827, superseded by MIL-PRF-23827-C Type 2
- Royco 27
- Mobil Grease 27
- Mobil Grease 33

[PT20J]

16 hours ago, Will.iam said:

ok on further search i did find 2 threads on mooneyspace of which of them i had already seen and had greased the jackscrew at last annual i normalky manually trim when i hand fly and the trim wheel doesn’t seem stiff to me but that coukd be subjective. I did find another thread that stated this:

There is a pillow bearing adjacent to the chain sprocket on the trim drive shaft.  That bearing was in need of lubrication.  Cleaning and lubricating the bearing solved the problem.  Should be three to four hours of labor.

is this accessible from the belly with the belly pan removed? And what type of lubricant goes on the pillow bearing?

The B-K servo has a lot of torque. If you can move the trim wheel manually with normal effort, lubrication is not the problem.

The B-K carrier bearing is a needle bearing that is supposed to be lubricated annually. You have to remove the trim servo chain and disconnect the torque tube and slide the bearing apart and then lube with a grease needle using MIL-PRF-81322 grease.

Skip

[Will.iam]

34 minutes ago, PT20J said:

The B-K servo has a lot of torque. If you can move the trim wheel manually with normal effort, lubrication is not the problem.

The B-K carrier bearing is a needle bearing that is supposed to be lubricated annually. You have to remove the trim servo chain and disconnect the torque tube and slide the bearing apart and then lube with a grease needle using MIL-PRF-81322 grease.

Skip

It’s the normal effort that I’m not sure. It’s not horrible and is harder to manually trim when i load up like when rolling full flaps and holding back pressure on the yoke to keep the nose from dropping its harder to turn but gets easier as i trim off the pressure and the manual trim switch runs the trim wheel fine. Is that the same motor that the autopilot uses or is there a separate trim motor?

[LANCECASPER]

3 hours ago, Will.iam said:

It’s the normal effort that I’m not sure. It’s not horrible and is harder to manually trim when i load up like when rolling full flaps and holding back pressure on the yoke to keep the nose from dropping its harder to turn but gets easier as i trim off the pressure and the manual trim switch runs the trim wheel fine. Is that the same motor that the autopilot uses or is there a separate trim motor?

You should have a separate manual electric trim motor and then a pitch servo for the autopilot.

Edit: @PT20J corrects me below

[PT20J]

44 minutes ago, Will.iam said:

It’s the normal effort that I’m not sure. It’s not horrible and is harder to manually trim when i load up like when rolling full flaps and holding back pressure on the yoke to keep the nose from dropping its harder to turn but gets easier as i trim off the pressure and the manual trim switch runs the trim wheel fine. Is that the same motor that the autopilot uses or is there a separate trim motor?

Since the trim is moving the whole empennage, aerodynamic loading will affect the trim force. 

The autopilot has a pitch servo and a trim servo. The autopilot uses the trim servo and it is also used for manual electric trim when the autopilot is off.

Lots of things can cause the trim fail indication. It's a good question for @Jake@BevanAviation.

Skip

[Fly Boomer]

1 hour ago, PT20J said:

The B-K carrier bearing is a needle bearing that is supposed to be lubricated annually. You have to remove the trim servo chain and disconnect the torque tube and slide the bearing apart and then lube with a grease needle using MIL-PRF-81322 grease.

Where can I find documentation on that?  Apparently not in SMM or IPC?

[PT20J]

5 minutes ago, Fly Boomer said:

Where can I find documentation on that?  Apparently not in SMM or IPC?

It's in the SMM for the M20J: 27-42-00 Electric Trim System Maintenance. 

[Fly Boomer]

1 hour ago, PT20J said:

It's in the SMM for the M20J: 27-42-00 Electric Trim System Maintenance. 

This is what I have.  Unless I'm asleep, I don't see anything specific about maintenance.  Stabilizer Trim System starts near the bottom of the first page.  Does the manual for your M20J have more detail?

M20K SMM MAR 2000 nbr 134 CHAP 27 - FLIGHT CONTROLS (partial).pdf

[PT20J]

29 minutes ago, Fly Boomer said:

This is what I have.  Unless I'm asleep, I don't see anything specific about maintenance.  Stabilizer Trim System starts near the bottom of the first page.  Does the manual for your M20J have more detail?

M20K SMM MAR 2000 nbr 134 CHAP 27 - FLIGHT CONTROLS (partial).pdf 161.38 kB · 3 downloads

Interesting. Here is the excerpt from the M20J manual.

[Fly Boomer]

5 minutes ago, PT20J said:

Interesting. Here is the excerpt from the M20J manual.

Thanks for sending this.  Seems like pretty important info.  I'll incorporate this into my annual inspection even though Mooney seems to have removed it from the SMM.

[Jake@BevanAviation]

The trim servo has a voltage regulator that generates a regulated voltage off the trim power.  This voltage is used for MET (manual electric trim) and is monitored by the flight computer in the event the regulator fails to annunciate a trim failure condition.  It is possible that in the colder temps the voltage is just out of mfg. specs for the monitor circuit in the computer.  As the components warm up, the voltage falls within specs and the failure warning on pft goes away.   If the regulator fails and goes INOP you have no MET functions and a constant warning from the flight computer.  

I would be curious to see where the regulated voltage was when the unit was cold.  There is a zener diode in the trim servo that could be replaced to increase the regulation in cold temps if it is too low.  During PFT, the computer sends two up drive pulses and two down drive pulses to the trim servo and monitors the feedback lines to see if the motor is functioning.  If you are getting the 4 trim light flashes during PFT we can assume the computer is sending the commands to the servo and looking for feedback.  If for some reason the motor does not run the system will fail the PFT.  During this test the solenoid will not pull so you won't see the trim move in the aircraft.

[Will.iam]

45 minutes ago, Jake@BevanAviation said:

The trim servo has a voltage regulator that generates a regulated voltage off the trim power.  This voltage is used for MET (manual electric trim) and is monitored by the flight computer in the event the regulator fails to annunciate a trim failure condition.  It is possible that in the colder temps the voltage is just out of mfg. specs for the monitor circuit in the computer.  As the components warm up, the voltage falls within specs and the failure warning on pft goes away.   If the regulator fails and goes INOP you have no MET functions and a constant warning from the flight computer.  

I would be curious to see where the regulated voltage was when the unit was cold.  There is a zener diode in the trim servo that could be replaced to increase the regulation in cold temps if it is too low.  During PFT, the computer sends two up drive pulses and two down drive pulses to the trim servo and monitors the feedback lines to see if the motor is functioning.  If you are getting the 4 trim light flashes during PFT we can assume the computer is sending the commands to the servo and looking for feedback.  If for some reason the motor does not run the system will fail the PFT.  During this test the solenoid will not pull so you won't see the trim move in the aircraft.

Thanks. Jake. You looked at my airplane back when i had a comm issue with by garmin 530W to trouble shoot why my nav function was flying 30 degrees off. You had chased it down to a wiring harness issue as it worked fine when you had your computer checker harness hooked up. After the holidays when the temp is very cold i’ll fly up there and have you see if replacing that diode would help with more reliable/consistent trim tests as it does fail with the continuous beeping that can only be silenced with resetting the A/P circuit breaker. Yesterday flying it would not pass for 20 mins as i flew around but after landing i tested it and it passed so i was going to takeoff again just to see how well the autopilot would work after i had my original ki256 overhauled as i taxied back to takeoff I pressed the test button one more time thinking it had warmed up enough to pass and then it failed again had to reset and test another 10 times before it passed again then i took off and the autopilot works great just still have the known nav issue. 

[Jake@BevanAviation]

If you want to stay in the Dallas area you might reach out to Trevor at Tech Aero in Denton.  He is an amazing tech and knows how to efficiently troubleshoot these legacy systems.  You want to bring the aircraft up, just give us a call to make sure we have the room.  If Trevor wants to remove the servo and cold soak it then check the voltage regulation that is where I would start.

[Will.iam]

41 minutes ago, Jake@BevanAviation said:

If you want to stay in the Dallas area you might reach out to Trevor at Tech Aero in Denton.  He is an amazing tech and knows how to efficiently troubleshoot these legacy systems.  You want to bring the aircraft up, just give us a call to make sure we have the room.  If Trevor wants to remove the servo and cold soak it then check the voltage regulation that is where I would start.

Might have him test it as by the time i get to you it would not be cold soaked where as denton is just a 5 min hop from me. Right now we are down to 12 degrees. I might go over to the hanger and try a test in this cold climate. I guess since after passing the test it doesn’t matter that i climb up to below freezing cruise altitudes when flying just don’t do a test up there.

[Will.iam]

Well i went over and tried 10 times and all ten test failed. Will have to get it over to the experts while we have such cold wx to troubleshoot this issue. 

[LANCECASPER]

On 12/23/2022 at 12:33 AM, Will.iam said:

Well i went over and tried 10 times and all ten test failed. Will have to get it over to the experts while we have such cold wx to troubleshoot this issue. 

If it was me I would l fly it 269 nm on a heading of 356 to Wichita to see @Jake@BevanAviation

[Will.iam]

10 minutes ago, LANCECASPER said:

If it was me I would l fly it 269 nm on a heading of 356 to Wichita to see @Jake@BevanAviation

True. And they are colder up there than in texas.