KFC 150 Autopilot misbehaving

[IvanP]

Good morning all,

I am seeking collective wisdom from MS members on a rather strange behavior of my KC 150 autopilot that I have observed on last couple of XC flights.

Plane: 1990 M20M with factory installed KFC 150 system, nav data from GTN 750. 

Symptoms: On startup AP passes its checks, after takeoff AP works in FD, HDG, and NAV modes during climb. After leveling off at altitude (15-22k), AP holds ALT and works in NAV and HDG modes as expected. After about 1-2 hours at altitude, the AP beeps and disconnects (no turbulence was encountered and no pilot input on controls or manual trim). FD bar remains visible on AI, but the plane does not hold altitude or direction. If I hit the AP disconnect button after this, FD bar goes out of view (as it should). Re-engaging AP in HDG or NAV modes without ALT works for few seconds and then it beeps again and disconnects. 

The AI gyro (vacuum) and HSI appear to be working fine (back-up electric AI agrees with the vacuum one driving the AP). Manual electric trim is also working fine. I suspect that this behavior may have something to do with ambient temps (-10 to -25 OAT when I observed this). I did not observe this behavior at lower altitudes/higher OAT.    

Any thoughts or guidance would be greatly appreciated before I start ripping things apart. I found bunch of posts on KFC 150 issues, but none that would describe similar behavior. Thank you in advance for any guidance you may be able and willing to offer.

Regards,

Ivan

[LANCECASPER]

I'm guessing the KC-192 board, but @Jake@BevanAviation is the expert.

I had a '96 Bravo with the exact same symptoms while under warranty. Bendix-King would send a loaner KC-192 A/P computer and it would work great and when I would get mine back, same symptoms. After a few times I called Bendix-King and told them I was keeping their loaner this time. They looked up the history and said that would be fine. 

[cbarry]

This may sound odd, but it is very likely the  yoke mounted  disconnect switch that is in need of replacement.  I had a very similar issue on a KFC150 and after researching the possible solutions, the disconnect switch seemed to be the best place to start.

[Fritz1]

I had a similar issue with my Bravo last year, KFC 225 would disconnect at high altitude and low temp, aircraft would zoom up 100-200ft until I caught it, push down and trim down manually, thaught it was the torque sensor in the pitch servo that failed to run the trim servo, torque sensor in pitch servo was fine, scratched my head, lubed the trim pivot bearings for the entire tail with mouse milk, ran trim back and forth at nose down end a couple of times, problem went away, keep fingers crossed, think pivot bearings were dry, TKS fluid washed them out, trim ran heavy at high alt low temp high power cruise and trim servo sensed overload, autopilot disengaged. This may not be the cause of your problem, however lubing the tail pivot bearings can't hurt, if it works cheap fix for aggravating problem

[Jake@BevanAviation]

@IvanP There are multiple things that could cause the system to disconnect in flight.  The key is being able to identify which one.  Listed below are some examples of failures that will cause a disconnect.

• Faulty/intermittent disconnect switch
• Faulty/weak return springs for MET (manual electric trim) specifically the left side of the split rocker.
• Faulty current sense resistor (thermal related).
• Capacitor leakage in power-supply circuit (thermal related), most common in units manufactured between 1991-1998.
• Faulty rate monitor circuit.
• Faulty MET voltage regulator.
• Faulty voltage monitor circuit.

If it was me I would check the disconnect switch, MET switch, and see if you can identify when the unit was manufactured.  Normally there is a red or black 4 digit stamp on the unit around the transducer port.  For example 3688 would be the 36 week in 1988.  You can look on eBay to get a idea of what the stamp looks like and location.  If you are within 1991-1998 there is a good chance you have some leaking capacitors. 

[IvanP]

@Jake@BevanAviation thanks for the points to look at. When I get back from my trip, I will try to find the manufacture date of the unit. I do not recall seeing any notes in the logbooks about switching out the main unit when I bough the plane about a year ago, so my guess is that it was made before 1990 given the date the plane was made. 

The MET switch in new (replaced about 10 months ago when I had the yokes off as a precaution as the springs in the old switch were a bit worn out) and seems to be working fine. We checked the disconnect switch at the time the MET switch was changed and there were no signs of failure. Nevertheless, I cannot positively rule out the D/C switch. What puzzles me is that it takes 1-2 hours at higher altitude  for the symptoms to show up. My guess is that if the issue was caused by the switch(es), it would be more random and not dependent on altitude and length of operation. Also, I forgot to mention in my initial write-up that on the first trip when I noticed this, I re-engaged the A/P after descending to about 4k and flew a coupled approach in the APR mode with no issues - AP followed both lateral and vertical guidance from GTN 750 with no issues until I disengaged it just short of arriving at the FAF. 

31 minutes ago, Jake@BevanAviation said:

@IvanP There are multiple things that could cause the system to disconnect in flight.  The key is being able to identify which one.  Listed below are some examples of failures that will cause a disconnect.

• Faulty/intermittent disconnect switch
• Faulty/weak return springs for MET (manual electric trim) specifically the left side of the split rocker.
• Faulty current sense resistor (thermal related).
• Capacitor leakage in power-supply circuit (thermal related), most common in units manufactured between 1991-1998.
• Faulty rate monitor circuit.
• Faulty MET voltage regulator.
• Faulty voltage monitor circuit.

If it was me I would check the disconnect switch, MET switch, and see if you can identify when the unit was manufactured.  Normally there is a red or black 4 digit stamp on the unit around the transducer port.  For example 3688 would be the 36 week in 1988.  You can look on eBay to get a idea of what the stamp looks like and location.  If you are within 1991-1998 there is a good chance you have some leaking capacitors. 

 

[IvanP]

Hello All,

Additional observations from today's return flight. This time, I took notes, rather than relying on memory.

Ground check - pass. After departure, AP was engaged in NAV mode while climbing to 16.5k. Everything worked as it should. Tested D/C switch, MET operation, CWS button, course intercept, variations in climb angle - all was well. When reaching 16.5k - ALT was engaged and AP auto-trimmed for level flight. OAT -8 C, SKC, no turbulence. 

After approximately 1 hr and 15 min of continuous operation in NAV and NAV/ALT mode, the system beeped and disconnected lateral steering (NAV indicator on controller went off), but the FD bar remained in view and the plane continued to hold altitude (same pattern as on the flight few days ago). ALT indicator remained on. I disconnected AP via the D/C button on the yoke and tried to re-engage in NAV or HDG modes - both modes disconnected in less than 1 minute.

Next, I tried to engage the AP in ALT mode only - hit the ALT and AP-ENG buttons. The FD bar came into view and the plane maintained wings level and set altitude. I used the UP/DOWN control on the A/P controller to change altitude and the plane followed the commands. I maintained lateral control by using the CWS button to make minor course corrections for wind. No disconnects noted for approx. 1 hour of continuous operation in this mode. 

Shortly after starting descent from cruise altitude I tried to engage the AP in NAV mode again and it continued to track the programmed path and selected descent angle. After about 30 min, I switched to APR mode and flew simulated coupled LNAV/VNAV GPS approach to minimums. AP performed flawlessly - lateral nav spot on, GS intercept as expected, auto trimmed for changes in power and configuration (gear & flaps) and maintained GS right on the money until I manually disconnected it.  Total AP operation time in descent (NAV and APR modes combined) was about 40 min and not a single disconnect was observed. 

I think that we can probably rule out faulty switches at this point as the issue appears to be related to time in continuous operation (3 flights with identical pattern - NAV/HDG modes failure after more than 1 hour of continuous operation, functionality is restored after disengaging the NAV/HDG modes for 30-60 min or so.). It would seem unlikely that a failing mechanical switch/button would consistently exhibit such failure pattern, regardless of the number of actuations and various modes. Jake @Jake@BevanAviation, any thoughts on this strange gremlin?   

Thank you in advance for any advice you may be willing to offer. 

Best,

Ivan

 

 

[Jake@BevanAviation]

Possible compass valid issue?  If you lost the compass valid input it will disconnect the system when you have a lateral control mode selected.  However, if the valid was still missing it should not allow you to select a lateral control mode and engage the system.  Vertical modes will work without compass valid. What compass system do you have in your aircraft?  It is odd that you were able to re-engage the system in FD and ALT hold and not have any issues for a extended period of time.

[Brad Reid]

@IvanP were you able to get this fixed?  I'm having the same issue with a KFC-150.  tests pass after startup but autopilot disconnects after some time and then won't engage in heading/nav modes after that.  Flight director and altitude modes will work.  At first this was infrequent but seems to be happening more.  

[IvanP]

Never found what exactly was causing the system to go haywire. Jake at Bevan Aviation tested my AP computer and did not find anything wrong. My AI gyro was in need of overhaul so I had that done and I changed the A/P disconnect switch. System works fine now. 

[LANCECASPER]

On 9/23/2025 at 11:32 AM, Brad Reid said:

@IvanP were you able to get this fixed?  I'm having the same issue with a KFC-150.  tests pass after startup but autopilot disconnects after some time and then won't engage in heading/nav modes after that.  Flight director and altitude modes will work.  At first this was infrequent but seems to be happening more.  

What are the instruments in your panel that is giving the KFC-150 it's attitude and heading?  Bendix-King steam gauges?   Aspen with an EA-100?

[Brad Reid]

6 hours ago, LANCECASPER said:

What are the instruments in your panel that is giving the KFC-150 it's attitude and heading?  Bendix-King steam gauges?   Aspen with an EA-100?

The Bendix-King KI 525A PNI 

[LANCECASPER]

7 hours ago, Brad Reid said:

The Bendix-King KI 525A PNI 

OK that displays heading, then you probably have a KI-256 attitude indicator that supplies pitch and roll, which may need to be overhauled.

@Jake@BevanAviation commented back in 2024 on this and @IvanP had him test his autopilot computer. He would be the guy I would call. I'm sure he could help you get it sorted out.

[Jake@BevanAviation]

@Brad Reid If the system will engage with FD and ALT hold only and when selecting a lateral control mode like heading, nav, approach the system disconnects.  I would check the compass valid input to the computer.  If the compass valid fails and you have a lateral control mode selected it will instantly disconnect the system.  It will not allow you to engage the system with a lateral control mode if the compass valid has failed.

[Brad Reid]

Thanks for all the suggestions, It is working well again and seems very likely it was the red disconnect switch.  The red disconnect has a screw (round aluminum) on base that was loose and apparently causing it to fail a self check in the air.  It may have been just loose enough to pass on the ground but fail in the air.  simple twist fixed it.  If it ends up having a problem again and is something else I'll post here.