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Autopilot woes


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Hi all,

 

I have an Ovation with a KFC150 that's been a bit of a pain. There have been two main problems with it: Bad roll servo and computer failing the test at startup.

 

Right now, I'm shopping for a roll servo. The motor has fallen apart. I had it repaired a few years ago, but that repair was highly questionable and the motor has fallen apart again. According to my avionics shop, Honeywell (Bendix/King) wants $7K for the motor and $10K for a new servo! They're quoting me about $3400 for an overhauled servo from another (reputable) source. With installation, that'll cost more than my last annual!

 

Is there a reputable place to get a servo overhauled, or find a used, serviceable one for less?

 

As for the computer (KC192), it has had issues passing its test occasionally, but we haven't nailed down a cause. Last time it was in the avionics shop, they hooked up the test rig, they saw it fail (so they know I'm not crazy), but while they were messing with it, it just started working and they couldn't get it to fail again. I'd noticed it seems to fail the test more often in the winter so they stuck it in the freezer for a day, but it still worked just fine when cold-soaked. Now it's back in the plane and working fine, but I'm expecting it to start to fail again at some point. Has anyone else had this issue? How did you resolve it?

 

Thanks in advance for any help or guidance!

 

Kent

 

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Autopilots Central in Tulsa, OK.  A/P's are highly specialized and just bench testing is not good enough to find root problems... BTDT.  Just document the specifics of failures, call them, get an appointment.  I highly recommend them after my experience there.

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Don't know where you are located but Lancaster Avionics helped me.The last owner went to a different shop each time and was ready to replace the whole system prior to selling.

I sent the computer to Lancaster and they replaced a transistor. Worked fine for a while then it happened again. Sent the plane to them and same transistor was blown. They checked it out and found the cause was a frayed wire to the elevator trim servo .Been rock solid for over a year now. Wasn't that much money.

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Autopilots Central in Tulsa, OK.  A/P's are highly specialized and just bench testing is not good enough to find root problems... BTDT.  Just document the specifics of failures, call them, get an appointment.  I highly recommend them after my experience there.

 

$2,500 for an overhaul exchange pitch servo from Autopilots Central last summer.

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  • 3 weeks later...
  • 3 years later...
On 5/7/2015 at 4:50 PM, Awful_Charlie said:

Failing the self test is very often the trim servo or wiring/connections for it

Winner, winner, chicken dinner.

Sorry for the necropost, but I figured I'd follow up now that I ran across this again. ;) Avionics shop finally was able to get the trim servo to show a fault (about 2 years ago now). They went looking for a replacement - Honeywell wanted $9600 for an overhaul exchange! :o And even at that price, they didn't have any. Luckily, our avionics shop has a fantastic component-level repair guy who went in and was able to repair it himself for under half an AMU, and it's been working perfectly ever since.

However, with Honeywell's "fuck you" pricing, we're done with the KFC 150. If it ever fails again, it's getting ripped out and replaced with a GFC 500. I did, however, find out that you can buy a warranty on them, even after they fail, so if someone has a reason they can't or don't want to replace the autopilot, you can do that instead.

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37 minutes ago, Lance Link said:

Glad to see these posts, I learned a few things.  My KFC 150 started failing the test a month ago, and the plane goes to Executive Autopilots on Monday. Hopefully the fix will not break the bank.    

Luckily mine was easy, it had fried the yoke disconnect switch. Probably the cheapest possible fix. Basically as soon as power was applied it started beeping didn't even get to the beginning of the test part.

Edited by Steve W
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On 3/16/2019 at 10:21 PM, Steve W said:

Luckily mine was easy, it had fried the yoke disconnect switch. Probably the cheapest possible fix. Basically as soon as power was applied it started beeping didn't even get to the beginning of the test part.

Yes... For those who don't already know this, DO NOT use the yoke switch to disconnect the autopilot unless you really have to. The current for the trim servo flows through that switch, but it isn't actually rated to handle that much so it can easily arc and fry. For normal autopilot shutoffs, just hit the "AP" button instead.

This applies to all King autopilots, FWIW, not just the KFC 150.

At least that switch is relatively cheap - $150 or so IIRC. The trim switch is $800... Or you can order the exact same parts online and build it yourself for $13. Grrrr.

Edited by flyingcheesehead
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An argument in favor of using the red button to disconnect AP: If the autopilot fails you want your trained reaction to be “push and hold” the red disconnect switch.   

When my KFC225 system trim “ran away” the red button stopped the problem immediately. 

 

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52 minutes ago, Jerry 5TJ said:

An argument in favor of using the red button to disconnect AP: If the autopilot fails you want your trained reaction to be “push and hold” the red disconnect switch.   

When my KFC225 system trim “ran away” the red button stopped the problem immediately. 

 

YES - 

but we should ALL also have the A/P breaker marked with a colored plastic breaker marker as it is one more line of defense to quickly find in case of such an emergency.

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5 hours ago, Jerry 5TJ said:

An argument in favor of using the red button to disconnect AP: If the autopilot fails you want your trained reaction to be “push and hold” the red disconnect switch.   

When my KFC225 system trim “ran away” the red button stopped the problem immediately. 

 

That is what it's designed for - Just be aware that every time you disconnect using the red button, if you happen to do it at the same time a servo decides to move, you will be causing damage to the disconnect switch. 

I understand why they did it, but they need a more robust switch, and making one that's hefty enough to actually withstand servo operation when you hit it is going to make it too big to fit on your yoke.

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56 minutes ago, flyingcheesehead said:

I understand why they did it, but they need a more robust switch, and making one that's hefty enough to actually withstand servo operation when you hit it is going to make it too big to fit on your yoke.

Behold the power of relays... And if one aerospace grade high reliability relay was not enough for the manufacturer, put another one in the power line in series controlled from the second pole of the switch.

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15 minutes ago, Aerodon said:

It would not take much to persuade me to replace my KFC150 with a dual G5 / GFC500 combination.

It was on my list for "as soon as approved" however an unexpected engine overhaul means I may have to settle for doing it incrementally.

Edited by Steve W
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I had the same intermittent startup self test failure with my KC192.  It is a common problem caused by faulty capacitors in the computer power supply.  The colder they are and/or the longer the unit sits unused the more likely the problem is to occur.  This explains why it usually happens on the first flight of the day.

A work around solution is to postpone the self test until after the engine has run for a while to allow the capacitors to charge.  Eventually the unit will fail without a self test sounding the warning tone as soon as the unit is powered on.  You can still get around the problem by pulling and resetting the autopilot breaker a few times but this is clearly not a long term fix.

Autopillots Central is very familiar with the problem and can quickly repair it for you if you pull and send them the KC192.  

The disconnect switch and trim servos can cause similar problems.  The clue that it is the power supply capacitors is the occurance of the failure being correlated to the time since the computer was powered up.

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9 hours ago, Bob - S50 said:

I believe in doing what the manufacturer recommends until proven that doing something else is better.

I don't think the manufacturer cares in this case... ?

9 hours ago, Steve W said:

Behold the power of relays... And if one aerospace grade high reliability relay was not enough for the manufacturer, put another one in the power line in series controlled from the second pole of the switch.

Yeah, but that relay would have to be pretty much fail-safe to please the FAA, I think. And everything fails eventually. I can see why they would want the ability to cut out the trim with a switch on the yoke.

1 hour ago, moosebreath said:

I had the same intermittent startup self test failure with my KC192.  It is a common problem caused by faulty capacitors in the computer power supply.  The colder they are and/or the longer the unit sits unused the more likely the problem is to occur.  This explains why it usually happens on the first flight of the day.

A work around solution is to postpone the self test until after the engine has run for a while to allow the capacitors to charge.  Eventually the unit will fail without a self test sounding the warning tone as soon as the unit is powered on.  You can still get around the problem by pulling and resetting the autopilot breaker a few times but this is clearly not a long term fix.

Autopillots Central is very familiar with the problem and can quickly repair it for you if you pull and send them the KC192.  

The disconnect switch and trim servos can cause similar problems.  The clue that it is the power supply capacitors is the occurance of the failure being correlated to the time since the computer was powered up.

The problem with self test failures is that there are numerous reasons why it can happen, so it doesn't point you toward a solution at all. You have to start throwing parts at it until it works. I mean, it is possible to troubleshoot, it's just not easy. It doesn't throw codes or anything to tell you why it failed.

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2 hours ago, Lance Link said:

Excellent report on Executive Autopilots at KSAC they fixed mine today for $500.  They were ready and waiting per my appointment.

Great! And, to give credit where credit is due, Skycom Avionics as KUES Waukesha, WI is the place that brought mine back to life.

We also did a major upgrade there in late 2017, pulling out a KLN 89B, KX155, KR 87, KN 64, KMA 24, Argus 7000/CE, and intercom and replacing with a GTN 750, GTX 345, GMA 350c, and FlightStream 510. They nailed it: On time, on budget, with every single thing working perfectly right off the bat. They also refurbished our KX 165 with all new electrolytic caps, cleaned and repaired various contacts and such, and it works better than new. 

Couldn't be happier, and highly recommend them. Hope to go back at some point for a TXi, G5, and GFC 500 if I can ever find the end of the rainbow... ;)

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I can’t recommend AP central.  They did a terrible job.  Pitch oscillation, supposedly it was the tach generator in the pitch trim servo, they replaced tach generator.   Didn’t fix it, chased down a bunch of other supposed issues, that didn’t fix it.  I gave up spending money on them.  To top it all off, they soldered the tach generator in and the connection failed, now no AP at all.  Took to a different shop, and he suspected that the voltage required to start the electric motor was too high, did some simple testing and the voltage required to start operation was all over the place, from within tolerance to outside of tolerance.  Rebuilt trim servo for several AMU and now it works.  If AP central did their job right the first time and knew what they were doing I would have saved money.  Not impressed with AP central at all.  

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On 3/16/2019 at 10:36 PM, flyingcheesehead said:

Yes... For those who don't already know this, DO NOT use the yoke switch to disconnect the autopilot unless you really have to. The current for all of the servos flows through that switch, but it isn't actually rated to handle that much so it can easily arc and fry. For normal autopilot shutoffs, just hit the "AP" button instead.

The red button has two functions: autopilot disconnect and trim interrupt. Only the trim servo power runs through the switch (see attached installstion manual p 2-37) The autopilot disconnect is just a low current signal to the computer. So, it is perfectly fine to use the red button to disconnect the autopilot. If you get a trim runaway, you hold the button down to interrupt power to the trim servo until you can turn off the trim breaker/switch, or pull the autopilot CB (the two are in series for powering the trim servo). 

What might wear out the button is testing the interrupt function by running the trim and interrupting it with the button. Better to hold down the button first and then check that the manual electric trim will not function. This way you are not breaking current flow with the switch and possibly arcing the contacts. 

Skip

KFC150 Installation Manual.pdf

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