More KFC-150 Autopilot Woes

[Greg_D]

Just looking to gather some thoughts/recommendations.  The plane is a 94 Ovation.  Autopilot has worked rock solid since the KC-92 was repaired about two years ago, including a flight yesterday.  It passes the preflight test just fine.  Today, it won’t track the heading bug or capture a course.  The servo is engaged as I can feel force holding the yokes when I try to turn them with the AP engaged.  The flight director command bars move properly when the heading bug is turned.

I’m thinking the roll servo may be shot or just bad enough that it will hold the everything steady, but won’t supply enough force to turn.  
 

When flying yesterday, I thought I may have smelled some smoke in the cockpit, but didn’t see any popped breakers and everything worked fine.  I triple checked everything.

 

Thoughts?  And I’m scared to ask how much roll servos run these days.

[PT20J]

You can test the roll servo power by engaging the AP on the ground with the engine running and the KI 256 spun up and moving the yoke left and right. You should feel the servo pulling against you as the clutch slips. You can also move the heading bug back and forth and the yoke should follow.

Since both course and heading are not working, I would check connections between the HSI and KC 192. One thing that happens with the old B-K products is that sometimes the connector pins lose tension and don't make good contact with the edge connector on the unit. I have fixed some issues buy taking a long narrow blade screwdriver and gently flexing the contacts in the tray connector to make better contact with the circuit board edge connector. Here's a manual that may help. Good luck.

Skip

KFC150 Installation Manual.pdf

[Greg_D]

19 minutes ago, PT20J said:

You can test the roll servo power by engaging the AP on the ground with the engine running and the KI 256 spun up and moving the yoke left and right. You should feel the servo pulling against you as the clutch slips. You can also move the heading bug back and forth and the yoke should follow.

Since both course and heading are not working, I would check connections between the HSI and KC 192. One thing that happens with the old B-K products is that sometimes the connector pins lose tension and don't make good contact with the edge connector on the unit. I have fixed some issues buy taking a long narrow blade screwdriver and gently flexing the contacts in the tray connector to make better contact with the circuit board edge connector. Here's a manual that may help. Good luck.

Skip

KFC150 Installation Manual.pdf 2.21 MB · 0 downloads

Skip, thanks.  I did the test on the ground.  The servo is definitely grabbing something, because the yokes are firm. Edit:  I can’t overpower the roll servo when it is engaged.  And the heading bug does not turn the yokes.  The flight director command bars do respond to the heading bug though.  Overpowering the servos should kick the AP off, right?

 

You think it may be an HSI or pin issue?  Where are these contacts you mentioned?  Are the in the back of the KC-192 tray?

Edited November 10, 2021 by Greg_D

[PT20J]

2 hours ago, Greg_D said:

Skip, thanks.  I did the test on the ground.  The servo is definitely grabbing something, because the yokes are firm. Edit:  I can’t overpower the roll servo when it is engaged.  And the heading bug does not turn the yokes.  The flight director command bars do respond to the heading bug though.  Overpowering the servos should kick the AP off, right?

 

You think it may be an HSI or pin issue?  Where are these contacts you mentioned?  Are the in the back of the KC-192 tray?

Contacts are on the connector in the back of the KC 192 tray.

You should be able to overpower the autopilot in roll and pitch. There are slip clutches in the servos so you can control the airplane by brute force if it goes nuts until you disconnect the autopilot. Overpowering the servos shouldn't cause the autopilot to disengage. The autopilot will disengage by pressing the red trim interrupt/disengage button on the yoke, operating the manual electric trim switches, turning off the autopilot on the KC 192, or pulling the autopilot breaker.

The FD command bars give a visual indication of what the autopilot computer is computing for roll and pitch. So a good test would be to fly it in all modes with just the FD and see if the command bars are commanding properly for heading and course corrections. If that works then the problem must be between the computer and the servo. Could be a connector issue or a servo issue or something in the circuitry within the KC 192 that drives the servo.

Skip

[LANCECASPER]

11 hours ago, Greg_D said:

Just looking to gather some thoughts/recommendations.  The plane is a 94 Ovation.  Autopilot has worked rock solid since the KC-92 was repaired about two years ago, including a flight yesterday.  It passes the preflight test just fine.  Today, it won’t track the heading bug or capture a course.  The servo is engaged as I can feel force holding the yokes when I try to turn them with the AP engaged.  The flight director command bars move properly when the heading bug is turned.

I’m thinking the roll servo may be shot or just bad enough that it will hold the everything steady, but won’t supply enough force to turn.  
 

When flying yesterday, I thought I may have smelled some smoke in the cockpit, but didn’t see any popped breakers and everything worked fine.  I triple checked everything.

 

Thoughts?  And I’m scared to ask how much roll servos run these days.

Call Bob Weber @Bob Weber

https://webairconsulting.com/

[Jake@BevanAviation]

If the FD bars respond to heading bug changes with HDG mode selected my guess is roll servo problem possibly bad motor or transistors.  If you are handy with a voltage meter you can check the drive voltage from the computer at the roll servo connector pins D and L.  With the computer on, a level attitude gyro, and hdg mode selected moving the bug should create a positive or negative DC drive voltage (left or right of lubber line) if the meter is across pins D and L.  Under the lubber the voltage should be close to 0VDC.  This would break it down to bad computer if you get no voltage or bad servo if you get the drive voltage at the meter.

[Greg_D]

2 hours ago, Jake@BevanAviation said:

If the FD bars respond to heading bug changes with HDG mode selected my guess is roll servo problem possibly bad motor or transistors.  If you are handy with a voltage meter you can check the drive voltage from the computer at the roll servo connector pins D and L.  With the computer on, a level attitude gyro, and hdg mode selected moving the bug should create a positive or negative DC drive voltage (left or right of lubber line) if the meter is across pins D and L.  Under the lubber the voltage should be close to 0VDC.  This would break it down to bad computer if you get no voltage or bad servo if you get the drive voltage at the meter.

Jake, thanks.  Do you have anything that shows which pins are D and L?

I have a friend with an Ovation very close to mine.  His doesn't have a KI256 though.  Would it be possible to drop his "computer" in my panel to rule that out as the problem?  As I mentioned above, the servo does engage.  Maybe the motor or transistors are bad in the servo.  It looks like there are a bunch of them out there for sale at a reasonable price.

Edited November 10, 2021 by Greg_D

[Jake@BevanAviation]

@Greg_D See image for connector pin-out.  If you were going to swap computers you would have to make sure the part numbers were the same, the same voltage, and the same adapter chips if you were going to try and fly on it.

Edited November 10, 2021 by Jake@BevanAviation

[Greg_D]

13 minutes ago, Jake@BevanAviation said:

@Greg_D See attachment for connector pin-out.  If you were going to swap computers you would have to make sure the part numbers were the same, the same voltage, and the same adapter chips if you were going to try and fly on it.

Thanks for the photo.  Not going to fly with the other computer, just a ground check.

[Jake@BevanAviation]

For a ground check as long as the model number and voltage are the same you should be fine.

[Bob Weber]

I'm glad you're on this one Jake, you know what you get to fix now...