KFC 150 Autopilot Woes

[PT20J]

16 minutes ago, Bravoman said:

On 5/18/2019 at 10:33 AM, carusoam said:

Some of the older BK autopilots have a couple of large-ish capacitors in the control box...

Capacitors don’t age very well after a couple of decades... and in BK’s boxes, they sometimes fall out of their mounts...

Based on things I have seen around MS...

Best regards,

-a-

Flux capacitors?

One of the problems with old electronics is the aluminum electrolytic capacitors. They really have a useful life of 15 years or so and do best when voltage is applied continuously. With time and disuse, the electrolyte can leak (it's acidic) which damages the printed circuit boards to which they are affixed. 

[carusoam]

Hmmmm...

Going back in time... Flux capacitors came out in 1985...

Some BK AP boxes predate that by a few years... 

I was busy reading the internet, while Skip was writing the answer to ‘how do capacitors fail’...

Back atcha,

-a-

[flyboy0681]

11 hours ago, PT20J said:

One of the problems with old electronics is the aluminum electrolytic capacitors. They really have a useful life of 15 years or so and do best when voltage is applied continuously. With time and disuse, the electrolyte can leak (it's acidic) which damages the printed circuit boards to which they are affixed. 

Here's a picture I took of my KAP 150 before sending it out to Autopilots Central a couple of years ago. While flying along I heard a pop then smoke came out of the head, turned out the capacitor blew its top, literally. 1.5 AMU's later it's still going strong.

[Greg_D]

I wanted to provide an update.  I sent the KC192 out to Bevan in Wichita.  Jake replaced various capacitors, resistors, and transistors.  He also cleaned up the electrical traces that were corroded from the burst capacitor electrolyte.  A bench test on power for about 6 hours showed that everything worked.  Total cost was just a little north of $800.  I can't recommend Jake and Bevan highly enough!  Prompt and courteous emails and phone calls throughout the repair process and the entire transaction was completed in less than a week.

After reinstalling in the plane and testing it, I found some additional problems.  The AP will hold a heading, hold altitude, fly a nav source, couple to a localizer/glide slope, couple to a GPS/LPV approach, etc.  The problem comes when I try to use the KAS297B altitude preselector.  Multiple issues here.  It won't capture a preselected altitude, either going up or down.  It won't hold a vertical speed, either up or down.  And when I press the CWS button to capture the present VS, the number that it is capturing is not accurate.  It sometimes shows a climb of 2400fpm.  I'm wondering if the issue is with altitude preselector.  But there is something else weird going on too.  I have a Ryan TCAD device installed.  It shows my current altitude after the altimeter setting is input.  I've noticed that the altitude will sometimes flash from the correct altitude to some crazy number like FL330, and then back.  This makes me wonder if there is something wrong with the altitude encoder or pitot-static system.  Any thoughts?

[jaylw314]

22 hours ago, Greg_D said:

I wanted to provide an update.  I sent the KC192 out to Bevan in Wichita.  Jake replaced various capacitors, resistors, and transistors.  He also cleaned up the electrical traces that were corroded from the burst capacitor electrolyte.  A bench test on power for about 6 hours showed that everything worked.  Total cost was just a little north of $800.  I can't recommend Jake and Bevan highly enough!  Prompt and courteous emails and phone calls throughout the repair process and the entire transaction was completed in less than a week.

After reinstalling in the plane and testing it, I found some additional problems.  The AP will hold a heading, hold altitude, fly a nav source, couple to a localizer/glide slope, couple to a GPS/LPV approach, etc.  The problem comes when I try to use the KAS297B altitude preselector.  Multiple issues here.  It won't capture a preselected altitude, either going up or down.  It won't hold a vertical speed, either up or down.  And when I press the CWS button to capture the present VS, the number that it is capturing is not accurate.  It sometimes shows a climb of 2400fpm.  I'm wondering if the issue is with altitude preselector.  But there is something else weird going on too.  I have a Ryan TCAD device installed.  It shows my current altitude after the altimeter setting is input.  I've noticed that the altitude will sometimes flash from the correct altitude to some crazy number like FL330, and then back.  This makes me wonder if there is something wrong with the altitude encoder or pitot-static system.  Any thoughts?

Did you do a static system test, since it was opened up when you removed the autopilot?  Maybe some blockages?

If the KAS297B is getting no encoder data, you get a series of dashes on the display instead of the wrong altitude, so that also suggests something upstream of the encoder?

[Greg_D]

Did not do a static test.  I don’t see any dashes on the 297, but the altitude readout on the TCAD device does jump around at times.

[amillet]

@Jake@BevanAviation  My KC 192 was last updated in 2016 by Capital Avionics in FL when I was having trim issues. It’s been working great since. Is there any advantage to preventatively replacing the capacitor that often fails and leaks?.