Porpoising With my KFC-150

[RJBrown]

This is not a request for help but a report of what happened. I hope that others may save on some unnecessary repairs.

When I first bought my MSE the auto pilot would porpoise in level flight. Not always but intermittently. I could not intentionally duplicate the problem but some where on a long flight it would return. The only fix was to turn it off and hand fly. It was annoying to me and caused motion sickness issues for some. I spent a bit of money checking and rechecking the system to no avail. The avionics shop and the local Auto Pilot specialist were willing to bill me for their time but were unable to find a solution. Without any repair input from those willing to charge the problem eventually disappeared.

 In a later conversation with the seller he told me that he never seen the problem because he never used the auto pilot. He had also never updated the database in the now irrelevant KNL88 loran.  The plane had a 20 year old database in the loran. He was not instrument rated and always hand flew the plane and had never used the loran. 

 It appears to me that the porpoising was caused by auto pilot non use and the repair was to use the auto pilot. I wonder if anyone else has experienced a similar issue? I could have spent thousands unnecessarily had I been a little quicker to get it fixed. I had  it looked at by 2 local shops including taking the "specialist" up for a thorough test flight. No answers. I spent a lot of time on the computer researching possible fixes and looked for someone to take it to. I was committed to finding solution and was ready to spend. Before I found a place I was comfortable taking it to the problem simply stopped occurring   

[PTK]

Interesting!! I'll remember that if mine ever decides to do the purpoise! I do use it quite a bit though. Always when IFR. It has been very reliable and solid.

[David Mazer]

My KFC 150 would also porpoise but it didn't stop.  It turned out the autopilot has had a number of software updates and I needed them.  You may find your problem isn't really solved.

[RJBrown]

It has been over 2 1/2 years since the problem occurred. I chased the problem with the resources on Centennial (APA) until they were exhausted. Was planning on spending to get it fixed as I consider an operating auto pilot a real safety feature for IFR. But before I could decide who to have look at it it magically fixed itself. 

[carqwik]

Lucky...

[richardheitzman]

most of the auto pilot porpoising I have repaired I traced to the altituide hold system. Static leaks being the number one failure, then auto pilot servo. I also had an issue with an aircraft that the wiring harness for the auto pilot was shorting out only in flight, and I was able to find the issue by visually inspecting the wiring harness from the servo to the computer.

 

You might have an ittermidiate leak in the static system. Check your drains, fittings, etc. visually inspect the system and replace static lines / fittings if they seem to be or looked cracked.

[Piloto]

I had the same problem with my Century 31 A/P. After exhaustive troubleshooting found the cause. It turns out that the yoke shaft slightly binds occasionally in-flight. This cause the pitch servo to over react and overshoot the corrective command. To fix the problem I just sprayed silicon lubricant on both shaft fully extended and push and pull the yoke a couple of times. After this the pitch performance has been rock solid. Try it before spending a fortune.

José

[RJBrown]

most of the auto pilot porpoising I have repaired I traced to the altituide hold system. Static leaks being the number one failure, then auto pilot servo. I also had an issue with an aircraft that the wiring harness for the auto pilot was shorting out only in flight, and I was able to find the issue by visually inspecting the wiring harness from the servo to the computer.

 

You might have an ittermidiate leak in the static system. Check your drains, fittings, etc. visually inspect the system and replace static lines / fittings if they seem to be or looked cracked.

For all I know an IFR check on the altimiter may have been the fix. Testing the static system might have been what it took. I can see how a few inches of vaccum line could be replaced "just because" on an IFR check without a note. Good info to think about thanks.

[Cruiser]

_{I recently had my trim servo repaired for a different type of problem on a KAP150 autopilot system. Which was fixed, but now I have this intermittent porpoising problem as related above. Both the avionics shop and AutoPilot Central that did the repairs think it could be caused by a faultly Attitude Indicator. I should know this week if the attitude indicator is the cause.}

_{Seems the A/P will work perfectly for long periods but then starts over correcting for altitude which it will hold but with excessive pitching of +/- 200 feet. It continues in this manner until the A/P is disengaged.}

_{Engaging the A/P again and it may or may not do the same thing. Last flight it was rock solid for almost an hour until I arrived at the destination.}

[Seth]

I never had a porposing issue before but did experience it in my Missile over the weekend between MD and SC both ways.  I'll test it again soon and see if it was due to the passenger load or if I do have an issue.  It only occured after the annual (literall days before) when an IFR check was indeed performed.

 

I have a KFC 200 however, not a 150.

 

-Seth

[Piloto]

If the porpoising is there with no ALT hold enabled then the problem is not related to the altitude transducer but to pitch control. If the yoke can be push or pull when the AP is engaged you may have worn out servo clutch. Check this on the ground

José

[Aron Peterson]

I too have a porpoising problem.  My flight director also takes a jump when it porpoises.  Autopilot Central seems to think that it's the gimbles in the ADI.

Rather than having my ADI overhauled I am installing the Aspen 1000 EFD.  I hope that fixes the problem

 

Aron

 

KFC200

[1964-M20E]

A different system but I have the Stec 30 system with Alt hold and I'll get some proposing as in the nose will go go up and down on approximately a 2 hertz cycle. I find that if I trim the plane slightly nose low it will go away or have a much lower amplitude. It holds altitude very tight though.

[David Mazer]

I too have a porpoising problem.  My flight director also takes a jump when it porpoises.  Autopilot Central seems to think that it's the gimbles in the ADI.

Rather than having my ADI overhauled I am installing the Aspen 1000 EFD.  I hope that fixes the problem

 

Aron

 

KFC200

My AI needed to be repaired twice in a relatively short period of time.  Each repair was $2,500.  On the second go around, I elected to go with the Aspen unit (it wasn't available when the AI tumbled the first time) for $5,000 ($2,500 more than the AI repair would have been but hopefully only once).  I'm hoping the Aspen unit is more reliable but it certainly isn't as smooth.  Each turn sees about a 100 ft climb and some hunting for heading and pitch.  When I brought this to the avionics shop's attention, their answer was the wholley unsatisfying - its within specs.  I hope you have better luck than I.

[Cruiser]

I too have a porpoising problem.  My flight director also takes a jump when it porpoises.  Autopilot Central seems to think that it's the gimbles in the ADI.

Rather than having my ADI overhauled I am installing the Aspen 1000 EFD.  I hope that fixes the problem

 

Aron

 

KFC200

 

Of course you know that in addition to the Aspen PFD you need the EA100 box to control the A/P

[aviatoreb]

My AI needed to be repaired twice in a relatively short period of time.  Each repair was $2,500.  On the second go around, I elected to go with the Aspen unit (it wasn't available when the AI tumbled the first time) for $5,000 ($2,500 more than the AI repair would have been but hopefully only once).  I'm hoping the Aspen unit is more reliable but it certainly isn't as smooth.  Each turn sees about a 100 ft climb and some hunting for heading and pitch.  When I brought this to the avionics shop's attention, their answer was the wholley unsatisfying - its within specs.  I hope you have better luck than I.

 

Which AI was it that you had repaired that needed to be repaired so quickly?  Surely that was a sign of a "not repaired" repair.  IRAN or overhaul?

[David Mazer]

Which AI was it that you had repaired that needed to be repaired so quickly?  Surely that was a sign of a "not repaired" repair.  IRAN or overhaul?

It was a KSC 55A.  I was told the life expectancy was highly variable and it can last months to decades with no ability to predict which.  That is when I decided to add the EA-100.

[aviatoreb]

It was a KSC 55A.  I was told the life expectancy was highly variable and it can last months to decades with no ability to predict which.  That is when I decided to add the EA-100.

 

Ouch - really?  Months for a properly repaired device?!  That stinks.

[fantom]

David...what shop are you using?

 

$2,500 for an AI repair or O/H is way high in my experience, and your Aspen issues sounds like an installation problem.

 

I, for one, want to stay far away from your avionics guy. Who is it?

[PMcClure]

For reference, I had the same problem on my B36. We went through 4 AI's in 2 years. Three times the original was sent out for overhaul and reinstalled. Once was a new unit. I thought it may be the avionics shop but asked around and found 2 other pilots having trouble with the same units, but using different shops. Could have been the same overhaul shop, though. These units must be poorly constructed.

 

I am having the same trouble on my Ovation. Additionally, if you disrupt the pitch, by pushing hard on the yoke, it oscillates 300-400ft before recovery. It acts exactly like the problems in the B36. I plan on a little further investigation before taking it to the shop. If it is the AI, I will probably replace it with the Aspen system. PS - some days (seams when CG is a little more forward) it is solid as a rock. 

 

Anyone have good experience (or bad) with the Aspen? I have a 530w/430 with KFC 225 A/P. 

[M016576]

<blockquote class='ipsBlockquote'data-author="fantom" data-cid="89103" data-time="1360001005"><p>

David...what shop are you using?<br />

<br />

$2,500 for an AI repair or O/H is way high in my experience, and your Aspen issues sounds like an installation problem.<br />

<br />

I, for one, want to stay <em class='bbc'>far</em> away from your avionics guy. Who is it?</p></blockquote>

+1. Overhaul on my century 41 A.I. two years ago was $600. A brand new mechanical AI is less than $2500, isn't it?

[David Mazer]

Yes, my replacement was in the $600 range but it is far less capable and wouldn't drive the A/P.  I don't need the additional capabilities since I upgraded the Aspen so we aren't comparing apples to apples.  The shop was Sarasota Avionics.  They haven't been the cheapest shop and every installation seems to need some additional work in follow up but they have, IMO, been honest and straightforward with me and stood behind their problems.  I did check on the AI's cost of repair and durability and was dismayed to find it was about right for the market and the number of choices for repair was extremely limited.  Hence, my decision to go a different direction even at a higher initial cost.

[Marauder]

I too have a porpoising problem. My flight director also takes a jump when it porpoises. Autopilot Central seems to think that it's the gimbles in the ADI. Rather than having my ADI overhauled I am installing the Aspen 1000 EFD. I hope that fixes the problem Aron KFC200 My AI needed to be repaired twice in a relatively short period of time. Each repair was $2,500. On the second go around, I elected to go with the Aspen unit (it wasn't available when the AI tumbled the first time) for $5,000 ($2,500 more than the AI repair would have been but hopefully only once). I'm hoping the Aspen unit is more reliable but it certainly isn't as smooth. Each turn sees about a 100 ft climb and some hunting for heading and pitch. When I brought this to the avionics shop's attention, their answer was the wholley unsatisfying - its within specs. I hope you have better luck than I.

David -- The Aspen PFD 1000 has different setup configurations for the various APs. As well, depending on what is driving the Nav, it will dictate how smooth the AP will function. With GPS and using the Aspen GPSS, my STEC 60 has never been that smooth. The Aspen has also improved the VOR tracking for me. If you are experiencing worse performance since upgrading, something is not right. On the STEC, the roll computer needed to be modified. Perhaps a similar modification is required.

[PMcClure]

David -- The Aspen PFD 1000 has different setup configurations for the various APs. As well, depending on what is driving the Nav, it will dictate how smooth the AP will function. With GPS and using the Aspen GPSS, my STEC 60 has never been that smooth. The Aspen has also improved the VOR tracking for me. If you are experiencing worse performance since upgrading, something is not right. On the STEC, the roll computer needed to be modified. Perhaps a similar modification is required.

David/Marauder - Can you confirm your installation costs for the Aspen and who did it? This is probably my next move for my Ovation. 

[David Mazer]

Total cost for conversion to the Aspen was in the $5,000 range.

 

There is a software update for the Aspen units.  Maybe I can ask them to check the A/P software at the same time.  Thanks Marauder.