Jump to content

KFC150/ Random Trim Failure


elimansour

Recommended Posts

Hello all.  My M20J has a KFC150 which has been very solid and reliable notwithstanding being 21 years old now.  I wanted to share an infrequent KFC150 trim failure mode that I have now experienced twice and wanted to see if our awesome collective can shed some light at possible root causes.

Both failures occurred while I was adjusting the trim by actuating the yoke trim controls - once when leveling off from a climb and the second time while making adjustments on a descent to final.  In both cases the TRIM annunciator light on the KC192 illuminates (steady - no flash) and the KC192 aural alerter beeps continuously.  The only way to eliminate the condition is by reseting the Trim rocker switch.  Following the reset, trim works just fine without issue and can be adjusted by actuating the yoke trim controls with no further problem.  These 2 events occurred on different flights with several flights in between with no such failure.   

My understanding is that this error mode is normally indicative of some kind of trim failure so I asked my avionics tech. to take a look as I was curious why the above happened twice without any permanent issue in trim functionality.  He checked the autopilot wiring harness to the Trim Servo, voltages between the KC192 and the Trim Servo and the wiring to the Trim rocker switch and found no issues.  He was able to replicate the error by engaging the pitch trim servo and turning the trim wheel so that it overpowers the trim servo clutch thereby causing the TRIM annunciator light on the KC192 to illuminate and the KC192 aural alerter to beep continuously which makes sense - but could not recreate the error by just adjusting the trim using the yoke trim controls.   

I will keep flying and will note conditions associated with any further such errors but was wondering if anyone experienced anything like this and or is aware of what could cause such random and infrequent trim errors? 

Thank you!

  • Like 1
Link to comment
Share on other sites

So things that will cause a TRIM fail annunciation are.

  • No trim feedback within a time period after the computer has sent a auto-trim command.  
  • Trim feedback of the wrong polarity
  • Missing regulated trim voltage at P2 pin W of the flight computer
  • Auto trim drive with no command from the flight computer

If you have been able to consistently duplicate the fail condition while using the MET (manual electric trim) switch on the yoke I would start there.  Pay attention to the left side of the split rocker as it is the regulated voltage and disconnect signal side that is sent to the computer.  There are small return springs in the MET assembly that can go bad over time or break.  I have seen something similar before with after using MET the unit will not engage or will not pass PFT.  After replacing the assembly the issue goes away.  

  • Like 2
  • Thanks 1
Link to comment
Share on other sites

On 9/11/2020 at 11:41 AM, Jake@BevanAviation said:

So things that will cause a TRIM fail annunciation are.

  • No trim feedback within a time period after the computer has sent a auto-trim command.  
  • Trim feedback of the wrong polarity
  • Missing regulated trim voltage at P2 pin W of the flight computer
  • Auto trim drive with no command from the flight computer

If you have been able to consistently duplicate the fail condition while using the MET (manual electric trim) switch on the yoke I would start there.  Pay attention to the left side of the split rocker as it is the regulated voltage and disconnect signal side that is sent to the computer.  There are small return springs in the MET assembly that can go bad over time or break.  I have seen something similar before with after using MET the unit will not engage or will not pass PFT.  After replacing the assembly the issue goes away.  

Just to add to Jake's comments. I have been chasing the same problem with my KAP150 for the past couple of years. Very random when the problem would show up but mostly during manual trim during takeoff. Some times during manual trim entering downwind. It would appear to get better when I removed the KC191 and cleaned the connectors but always returned.

In researching how the trim system works I found that the KC191/192 monitors the manual trim system even when it isn't engaged (I was surprised). From the KC191 Maintenance/Overhaul Manual section 4.2.7: "The monitor is capable of detecting wrong way trim  operation.  The KC 191 incorporates a manual trim sensor which can sense the proper voltage level of the manual trim engage line. Thus, it can be used as a means of monitoring the manual trim speed which previously had to be separately checked by the pilot. In the event of failure in either the auto trim or manual trim systems, the trim engage signal to the KS179 trim servo is terminated, and a trim warning light flashes."

The speed of the manual trim system is set based on the voltage generated by the pitch trim servo. This voltage is output from the trim servo on P1791 pin A (Manual Trim Voltage), goes to the Trim Arm switch, to P2 pin W on the KC191/192, through the Up/Down switches and back to the trim servo as a drive signal. Different aircraft configurations have different dash numbers that specify the voltage output. For my aircraft it is a KS 179 Pitch Trim Servo Part # 065-0052-03, Manual Trim Voltage: 11 +/- 1 VDC, Clutch Torque: 20 +/- 2 LBF-IN, Manual Trim Speed: 11.9 to 15.2 RPM. What this means is that the trim servo generates the manual trim voltage that goes through the engage switches and back to the trim servo to drive the servo at the correct speed. The KC191/192 senses the voltage and alarms if it is wrong or missing. Looking at the KC191 schematic I can't tell if it is looking for the proper voltage or just that the voltage is there. The signal goes through an op amp and then into a micro processor. I don't have the micro processor software available so I can't follow it any further.

I have included a drawing I did of the KAP150 wiring in my aircraft that shows the connections. I found the BK installation manual hard to follow so I did drawing to better understand how everything is connected.

So far to try to correct the problem I have replaced all the pins on the back of the KC191 connectors that have anything to do with manual trim and the manual trim switches. The first attempt was P1911 H and F (manual trim in and out pins). I still had the problem return. The second attempt a few weeks ago was the Manual Trim Voltage (P2-W), Trim Fdbk +Up (P2-X) and Trim Fdbk -Dn (P2-20) pins. I only have five hours on the plane since then so too early to tell but so far no problems. The next step is to check the voltage output from the trim servo to see if it is out of specs. If I still don't find anything I will take another shot at the trim switches even though they are only a few years old.

That is probably a lot more detail than what you are looking for but this problem has been bugging me for a while.

Jim

KAP150 Schematic.pdf

  • Thanks 1
Link to comment
Share on other sites

Jim.  Thank you.  That is very helpful. I will share with my avionics tech.  I was not aware that it constantly monitors the trim system but now it makes sense why the TRIM error was generated when the autopilot was off.  After the avionics folks removed and tested the KC192, and reinstalled it, I have not had any issue in any further flights...not sure if it is due to reseating the connectors or just part of the random failure mode.  I will keep flying to see if/ when it happens again.

Thanks again.

Eli

Link to comment
Share on other sites

The voltage monitor at P2 pin W is set by the adapter modules in the flight computer that are aircraft specific.  There is a voltage range that the computer looks for when doing the PFT and will monitor this voltage during operation.  If the voltage goes to low or drops out the computer will issue a fail annunciation as flashing trim.  I have seen the voltage regulators in the trim servo get soft under a load like MET operation.  If the regulator is in a no load environment the output will be sufficient to satisfy the computer.  When the regulated voltage is used to drive up or down trim during MET operation the computer will sense this low voltage and issue a fail annunciation.

Link to comment
Share on other sites

23 minutes ago, Jake@BevanAviation said:

The voltage monitor at P2 pin W is set by the adapter modules in the flight computer that are aircraft specific.  There is a voltage range that the computer looks for when doing the PFT and will monitor this voltage during operation.  If the voltage goes to low or drops out the computer will issue a fail annunciation as flashing trim.  I have seen the voltage regulators in the trim servo get soft under a load like MET operation.  If the regulator is in a no load environment the output will be sufficient to satisfy the computer.  When the regulated voltage is used to drive up or down trim during MET operation the computer will sense this low voltage and issue a fail annunciation.

Jake

Thanks for confirming that. I thought that the actual voltage was being monitored as there is an analog/digital chip in the circuit. I suspect that my problem is that the trim servo voltage regulator is at the lower end of the spec. I think that by cleaning and reseating the AP computer I am getting things flowing well enough the computer is happy for a while. I suspect that you will be seeing my trim servo in a few months for a rebuild when I have confirmed where the problem is.

Jim

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.