Autopilot has me stumped

[TomR]

 My kap 150 autopilot has been a learning experience form the get go and a new issue has cropped up. Since getting the plane I had the autopilot lateral adjustment go out slightly but with a little advice from the avionics shop its dead nuts perfect. Now though I’m getting two distinct issues in regards to elevator/vertical control. First issue has only happened twice which was some extreme proposing +/- 20 degrees at speed and at altitude. The autopilot would just runaway and would run the trim up and down thru nearly 50% of its travel before I disconnected it. Only happened once on New Year’s Day but feeling a bit of g got me nervous. Now however I’m seeing minor altitude excursions in cruise. The autopilot will cruise nice and level then a minute later allow the plane to speed up, climb about 100 feet, run out of energy, the descend 10 or more knots slower. From what I can gather when this is happening the autopilot is not compensating at all just allowing the slow proposing up and down. I imagine I need it looked at just wondering if it’s something dumb that’s easily addressed. I’ve also noticed in cruise that my elevator is not level with the horizontal stabilizer. It appears to be about four or five degrees down while in cruise. I wonder if this has something to do with it. Thanks in advance to all 

Two details... when fixing the lateral adjustment only the roll null setting was adjusted. Second I have no flight director and no pre select if that matters.

Edited March 6, 2018 by TomR

[carusoam]

Tom,

Any idea how many hours are on the servos?

You might be Seeing a loss of altitude control related to the motor that drives the elevator.

the KAP computer seems to work barely fast enough to maintain control, but, with age and turbulence the oscillations seam to get started.

Are you familiar with autopilots central?

PP thoughts only, not a mechanic...

Best regards,

-a-

[N201MKTurbo]

How is your trim system? Does it move easily by hand?

[TomR]

22 minutes ago, N201MKTurbo said:

How is your trim system? Does it move easily by hand?

No binding at all and trim works as expected through the electric trim switch.

45 minutes ago, carusoam said:

Tom,

Any idea how many hours are on the servos?

You might be Seeing a loss of altitude control related to the motor that drives the elevator.

the KAP computer seems to work barely fast enough to maintain control, but, with age and turbulence the oscillations seam to get started.

Are you familiar with autopilots central?

PP thoughts only, not a mechanic...

Best regards,

-a-

I’m gonna look into Servo age and report back here tomorrow. 

[Mark89114]

I don't know what a KAP 150 is but AP central seems to have quite a following around here and that is where I took my KFC225 to have a pitch deviation repaired.  Needless to say it was an absolute disappointment.  I spent about $4K while they chased the issue.  First diagnosis was faulty tach generator which they replaced, problem still existed, tested the KI-256 twice and it was always stable and proper signals, they tested and "blew out" the static system, finally they just tightened up some springs in the control system to Mooney specs.  The problem still existed but now the up and down pitch oscillation is less.....  Their final answer was there is an acceleration sensor in the AP control head that might be bad.  I gave up till I took the plane out for a long trip, engaged the AP and the pitch oscillations are now wild out of control.  Hit the Big Red Button.  Hand flew the next 5 hours.  Found a local avionics shop who I described the previous problems and the new out of control situation.  This shop had another theory that the motor was taking too much voltage to get started on the pitch servo and the feedback to the computer created the up/down deviation.  First thing they found was the "new" tach generator soldered in had broken loose, so AP central didn't even do that right.  There was also evidence of carbon dust around the motor, which led credence to the theory that it was the motor, not the other associated systems.  He applied voltage to motor and it started at the high end of acceptable and on other positions the motor was out of tolerance on startup voltage.  I don't have the airplane back with overhauled pitch servo yet, but will keep everybody in the loop on this. The diagnosis on the front end seems to be much better than my experience with AP Central, actually talked to the technicican who is going to do the work versus some go between guy who I would best describe as a customer service sales tech, his knowledge wasn't impressive.   My summary is not happy with the guys from Tulsa.  

 

[DAVIDWH]

Hang on, the Calvary is on the way.

Garmin's GFC digital 500 autopilot, not yet STC'd  for Mooney application, but their horses are galloping down hill.

My KFC-150  has multiple issues, but instead of spending more $$ and making  them worse, I am drinking coffee and calling Garmin's avionics support hotline on a monthly basis.

Suggested retail $6995.                    ( Best guess is early 2019 availability )

Best

[Andy95W]

40 minutes ago, DAVIDWH said:

Hang on, the Calvary is on the way.

Calvary is where Jesus was crucified.

Cavalry are Army soldiers on horses.

(Sorry- pet peeve of a former soldier.  It's like when someone says "nucular".)

[AH-1 Cobra Pilot]

41 minutes ago, Andy95W said:

Calvary is where Jesus was crucified.

Huh.  I thought it was supposed to be through his arms.

[jlunseth]

What regime(s) of flight were you in?

I have had similar issues with my KFC200, but there are a couple of flight regimes where the two things you describe are normal.  

The "major excursion" issue first.  If you are flying a coupled instrument approach and you somehow couple from too far below the glideslope, the AP will pitch strongly up to get to the GS.  If you are not ready for it, you can find yourself in a strongly nose-up slow flight and we all know what that means.  So if it was a coupled approach, the AP may have been doing what it thought it was supposed to.  Its not a genius.  If the major excursion was in cruise, then this is not it.  The AP itself needs to be looked at.

On small, 100 foot deviations, these can be caused by atmospheric conditions, small waves in the atmosphere.  I have experienced them over flat plains, like the Dakotas.  Normally, they manifest just as airspeed deviations, the plane pitches up to overcome a down wave, so you lose ten knots of airspeed or more.

You might just trying lubing the chain.  If it gets dry it will bind and can form a wrinkle or kink, which then may unwrinkel and the AP has to overcome the change.  

[Oldguy]

Seems similar to a problem I had crop up about 2 years ago. Turned out to be a combination of water in the static line and a slight hole in it as well. Before jumping into expensive items and parts, make sure your lines are clear and connected. If you have been out in the rain (flying or not) good chance you need to drain your static lines.

Yes, very disconcerting when it occurs the first (and every subsequent) time. Of course mine always started with the dive portion....

[LANCECASPER]

On 3/6/2018 at 11:44 AM, Mark89114 said:

I don't know what a KAP 150 is but AP central seems to have quite a following around here and that is where I took my KFC225 to have a pitch deviation repaired.  Needless to say it was an absolute disappointment.  I spent about $4K while they chased the issue.  First diagnosis was faulty tach generator which they replaced, problem still existed, tested the KI-256 twice and it was always stable and proper signals, they tested and "blew out" the static system, finally they just tightened up some springs in the control system to Mooney specs.  The problem still existed but now the up and down pitch oscillation is less.....  Their final answer was there is an acceleration sensor in the AP control head that might be bad.  I gave up till I took the plane out for a long trip, engaged the AP and the pitch oscillations are now wild out of control.  Hit the Big Red Button.  Hand flew the next 5 hours.  Found a local avionics shop who I described the previous problems and the new out of control situation.  This shop had another theory that the motor was taking too much voltage to get started on the pitch servo and the feedback to the computer created the up/down deviation.  First thing they found was the "new" tach generator soldered in had broken loose, so AP central didn't even do that right.  There was also evidence of carbon dust around the motor, which led credence to the theory that it was the motor, not the other associated systems.  He applied voltage to motor and it started at the high end of acceptable and on other positions the motor was out of tolerance on startup voltage.  I don't have the airplane back with overhauled pitch servo yet, but will keep everybody in the loop on this. The diagnosis on the front end seems to be much better than my experience with AP Central, actually talked to the technicican who is going to do the work versus some go between guy who I would best describe as a customer service sales tech, his knowledge wasn't impressive.   My summary is not happy with the guys from Tulsa.  

 

This guy chased problems in the KFC225 and learned a lot in the process.  http://www.peter2000.co.uk/aviation/kfc225/index.html

I had a KFC225 and a dual Aspen (so no KI-256) in an Ovation and never had a problem.

[jonhop]

Tom,

I too had a problem with vertical oscillation with my KAP-150 that was an intermittent roller coaster ride. I replaced my trim switch, overhauled the KG-258, bench tested the trim servo and later the whole system and the problem could not be diagnosed over a week at Executive Autopilots in Sacramento. The system would not fail on the ground but would in the air 90 percent of the time.  As a last resort, sent the KC-191 to Autopilots Central after discussing the whole situation with them...They ended up replacing 5 bad capacitors, a flex cable, and directly soldered the pitch microprocessor from the socket to the board. The KAP-150 is now rock solid, although I have not tested it on an approach. 

Good luck...

Jon

[takair]

All of the stuff above, but a couple of thoughts. The trim is activated by switches in the servo when they sense a load.  You can check the basic feature by engaging on the ground with the elevator held neutral.  Now push or pull against the servo and see if your trim goes opposite.  I don’t recommend doing this in flight unless you are really on the yoke, as you can get severe pitch excursions.  This will tell you the basic health of the switches, but not if they are in spec.  Another way to tell is when disengaging the AFCS is the aircraft generally in trim?  It should be.  If not, the elevator servo may need work.  Friction in the elevator and links can also cause this issue.  A common source of friction on early Mooney’s is the yoke panel feedthrough.  If it is not smooth, the AFCS can chase itself and the trim and you can be in for a wild ride.  A little lube can go a long way.

[TomR]

Guys first off thanks too everybody for all of your advice. Amazing the wealth of knowledge here. Was gonna take up the bird tonight but a flat nose tire was what welcomed me when I got to the hangar. In any case I’m definitely going to keep troubleshooting this. Again thanks so much I’ll report back when I find out more.

[carusoam]

Need an idea for the flat tire?

Look up  Michelin Air stop tubes... creases in the tube... broken/separated valve stems... proper installation ideas...

Ideas that have been shared by MSers over the years...

PP sharing old memories, not a mechanic...

-a-

[cliffy]

Do you have a long or short valve stem tube? Very hard to find a short stem tube anymore.That is what is needed. The long stem pushes on the wheel hub cap and stays bent all the time. That's where the leak usually starts. You might look for a 90 degree metal valve stem tube to fit the wheel. They work great.  If you can find a short stem tube even better. 

IIRC 150AP sometimes have the AI pickups dirty and giving false readings to the computer.  200s are susceptible to bad ground wires all over the system. 

[nels]

22 hours ago, TomR said:

Guys first off thanks too everybody for all of your advice. Amazing the wealth of knowledge here. Was gonna take up the bird tonight but a flat nose tire was what welcomed me when I got to the hangar. In any case I’m definitely going to keep troubleshooting this. Again thanks so much I’ll report back when I find out more.

Be thankful you saw your nose tire was flat inside your hangar. Sure beats having it flat during landing or at a strange airport. Always look at the good side if possible.

[TomR]

2 hours ago, nels said:

Be thankful you saw your nose tire was flat inside your hangar. Sure beats having it flat during landing or at a strange airport. Always look at the good side if possible.

Yes absolutely. Always use Michelin now but this tube is original since I bought the plane so who knows what it is. Thankfully there is a great A&P onsite at my airport who's great for minor fixes like this. But yes I'll second Michelin stuff it is by far and away the best in my experience

[Releew]

Call Steve at Autopilot Central.......

Rick

[Rick Junkin]

I had similar pitch oscillations with my KAP150 last year, and simply lubing the yoke panel pass-through phenolic bushing made it stop. But after a weekend of formation flying the pitch oscillations are back (lots of yoke movement on the wing to be sure). I’m going to try lubing the pass-through again and see what happens. I searched and found this thread to get other ideas, hopefully one of the no/low cost remedies will tide me over for another year. It certainly wouldn’t hurt to spray some contact cleaner on the parts mentioned above.

Cheers,

Rick

[TomR]

That’s what the conclusion was during my annual. It seemed to only happen during the bitterly cold weather. Fortunately it seems like nothing was wrong with the autopilot. Just that the servos were fighting binding in the flight controls. Picking up my airplane Thursday so hopefully this will solve the issue for now

[Rick Junkin]

On 5/8/2018 at 6:38 AM, TomR said:

That’s what the conclusion was during my annual. It seemed to only happen during the bitterly cold weather. Fortunately it seems like nothing was wrong with the autopilot. Just that the servos were fighting binding in the flight controls. Picking up my airplane Thursday so hopefully this will solve the issue for now

@TomR, What's the verdict? Did lubing the flight controls solve your problem? Re-reading the thread I see that I repeated @takair's suggestion, my bad.

Cheers,
Rick