Sudden pitch-up attitude in altitude hold mode

[M252]

I am operating a Mooney M20K 252TSE, mostly IFR, that is equipped with conventional King avionics (e.g. KI-256, KCS55A, a KAP150 autopilot).

In Altitude hold mode but not in Heading mode I frequently experience a sudden and violent pitch-up attitude (this happens from time to time, on almost any flight after some time). This is quite frightening and obviously immediately I disengage  the autopilot. The plane always takes the nose up never down. After disengaging the autopilot the plane is still in trim.

Does anybody have an idea what may be the cause of the problem? I hear that either the KAP150 computer or the KI-256 could be the problem, however, maybe others have experienced the same problem.

[carusoam]

What year is your plane? If the system was not original, what year was the system installed?

Best regards,

-a-

[M252]

1990, all orignial avionics

 

Thanks

 

U

[FAST FLIGHT OPTIONS LLC]

Call autopilots central in Tulsa. They will tell you exactly what the problem is as well as what it's going to take ($$$) to fix it

[M252]

Call autopilots central in Tulsa. They will tell you exactly what the problem is as well as what it's going to take ($$$) to fix it

Thank you - will do.

 

Still keen to hear from others who have had a similar problem..

[Cruiser]

The best solution would be to find an avionics shop that can test your A/P components before pulling anything out. BK made a test computer with all the cables that a knowledgeable tech can connect and test all the servos and the computer for correct operation. Hard to say if they can find an intermittent condition.

 

AutoPilot Central may tell you to get the AI overhauled but I would pull the pitch servo and send it to them to get checked.

[Aron Peterson]

I have same problem. Does your F/D show the pitch up command as well?

[Nathan Peterson]

I have same problem. Does your F/D show the pitch up command as well?

Are you the one that is getting the panel upgrade?

[M252]

I have same problem. Does your F/D show the pitch up command as well?

I only have a KAP150 w/o FD. I have not been able to see if there is anything happening on the AI (too much focused on keeping plane and passengers calm...)

[M252]

The best solution would be to find an avionics shop that can test your A/P components before pulling anything out. BK made a test computer with all the cables that a knowledgeable tech can connect and test all the servos and the computer for correct operation. Hard to say if they can find an intermittent condition.

 

AutoPilot Central may tell you to get the AI overhauled but I would pull the pitch servo and send it to them to get checked.

That is a good point. Does seem to be difficult to find. I am actually based in Europe and there is one that can do that (Avionik Straubing in Germany).

[Piloto]

Before you spend any money. I had the same problem with a Century 31 autopilot and spend a lot of time trying to find the cause. In my case the cause was the yoke shaft binding momentarily with the panel guide. I lubricated both yoke shaft with silicon lubricant and problem solved. When the shaft bind the autopilot tries to correct. As the A/P servo put force on the linkage the shaft gets released and the plane pitch up. You may or may not notice some minor binding on the ground and don't pay attention to it. But in-flight any minor binding will cause the autopilot hiccup.

 

José   

[M252]

Before you spend any money. I had the same problem with a Century 31 autopilot and spend a lot of time trying to find the cause. In my case the cause was the yoke shaft binding momentarily with the panel guide. I lubricated both yoke shaft with silicon lubricant and problem solved. When the shaft bind the autopilot tries to correct. As the A/P servo put force on the linkage the shaft gets released and the plane pitch up. You may or may not notice some minor binding on the ground and don't pay attention to it. But in-flight any minor binding will cause the autopilot hiccup.

 

José   

Thank you José, I will try.

[fantom]

The best solution would be to find an avionics shop that can test your A/P components before pulling anything out. BK made a test computer with all the cables that a knowledgeable tech can connect and test all the servos and the computer for correct operation..

 

Excellent advise!

 

I had the same issue some 15 years ago with my KAP 150. It was a defective trim servo, repaired under warranty.

 

AutoPilot Central is an excellent resource, especially for hard to trace AP issues, BUT you've got to get lots of parts pulled, shipped, insured, installed again, tested, aligned, and signed off. Better, most times, to find a good avionics shop some place close, if at all possible.

 

BTW, welcome to MooneySpace M252. In the future, it would help you and us if you posted your model, year, and location, either in your signature block or to the left.

 

Good luck killing that porpoise.

[Piloto]

Forgot. Also check the static alternate air source knob. Make sure is closed. A leak on the static line inside the cabin will cause changes on the baro altitude, specially when air vents are adjusted. At altitude you should see about 100 to 200ft change when the alternate air source knob is pulled and pushed. If you do not see an altitude change then you have a static leak.

 

José 

[Alan Fox]

The King autos use the Horizon to establish pitch , Check the horizon if all else fails... 

[Robg]

I have the exact problem in my Bravo with all king original avionics including the EFIS and KAP150 with the FD of about the same vintage as yours. Mine will do sudden excursions either up or down. Sometimes it is fine, others it's all over the place. Mine is definitely being commanded by the FD,though. I can have it in FD mode only without the autopilot engaged and see the command bars make uncommanded climbs and descents. I've had the ap computer checked, AI overhauled, a bearing on the trim shaft replaced and the trim servo replaced but nothing has fixed it. We are still trying to troubleshoot it, although it seems to only happen in colder, by no means necessarily freezing, weather. It's driving me crazy.

[Marauder]

I have the same bloody issue. It's driving me insane. It all started over 2 months ago when my original A/I went tango uniform, and I had it replaced (it would show me in a climb or descent even though I would be in stable flight). The replacement worked for one flight, and then after departing Charlotte for Atlanta I had the same issue. They (Epps Aviation at KPDK) said I had a static leak, and they replaced the A/I again, and I had to buy a brand new airspeed indicator from Mooney (that's where the static leak was according to the avionics folks at Epps). Now, the F/D commands unusual pitch up commands. I've engaged the A/P with altitude preselect on, and after three seconds it would climb from about 7-800fpm to over 1600. I've also been in level flight and selected ALT and engaged the A/P, and the airplane will either descend or pitch up.

I can't make any sense out of it - but what I can't do is spend more money on the damn thing. It feels like the A/I (which is under warranty since it was recently installed) and F/D isn't properly calibrated, or something else. Every other function works just fine. After the last replacement, I flew it to Chattanooga for dinner and that's when I realized the problem. No issues, I just hand flew it in. Coming back to Atlanta, I tested it out again, and it worked just fine.

I'm so aggravated.

Forgive me - but can you be a bit more specific in where the shaft would bind?

I did NOT check the alternate static source, and considering the fact that I had a static leak and my airspeed indicator was replaced with a brand new one, perhaps the mechanics didn't "reset" things, or perhaps they didn't connect hoses and wires tightly enough.

It's a KI-256 with the KFC150 auto flight system.

I can't comment on what the AP is doing but what Piloto is talking about is the yoke binding at the spot it goes into the panel. There is a guide there and it will bind periodically. Like him, I have used silicone to free it up.

His other comment about the alternate static may not play a role if the pitch information is obtained through the AI as Alan is indicating. If however, the AP uses static pressure to set pitch or hold altitude (like my STEC), then I would definitely check it. My alternate static port valve had a leak in this year caused by a deteriorating O ring.

[flyboy0681]

Forgot. Also check the static alternate air source knob. Make sure is closed. A leak on the static line inside the cabin will cause changes on the baro altitude, specially when air vents are adjusted. At altitude you should see about 100 to 200ft change when the alternate air source knob is pulled and pushed. If you do not see an altitude change then you have a static leak.

 

José 

 

I was once flying along and inadvertently pulled the static air partially out and got the jolt of my life, so there may be something to this.

[schule]

I had a similar problem going into last annual. I would sometimes get pitch oscillations of 10-15 degrees in rapid up and down cycles.

After and AI and computer rebuild which failed to fix it....we were able to isolate it to a worn/corroded pitch servo. It was sticking and over voltage on its attempts to adjust pitch. Works great now.

Autopilots Central is my recommendation.

[philiplane]

It's either the attitude indicator providing improper input, or the altitude chamber is suspect. 

 

The pitch servo only does what it is told.

[aaronk25]

Loose wire pin connector in wiring?? +1 for auto pilots central. But you have to get the bird there. They have the equipment to isolate all the systems and diagnose the problem.

[Guest]

Most autopilots have an annual servo check to be done on the bench. Most GA owners never have this done due to cost.

I too have seen control column shaft which stick and have also seen one bent to the point it was dragging in the panel bearing block.

Clarence

[HeyChuck]

Does the violent pitch up typically occur at a particular altitude, or is it random?

The optical encoder on an encoding altimeter can generate an erroneous output at certain

altitudes if it is not working correctly. I have seen this during a "bi-annual certification" test.

That could cause the autopilot to think it was at the wrong altitude and attempt to correct if

in altitude-hold mode.

[Danb]

I had the same issue in my 1987 J..mine ended being a computer board..I had runaway trim in Ifr going into Myrtle beach and was scared for a second..amazing how fast your brain remembers where the circuit breaker is...good luck

[John Pleisse]

Auto trim mode or trim servo. You are not getting trim and the AP moves your elevator to full travel. But you should have an annunciator that should warn you of the condition. Alt hold or your Alt Can wouldn't create this, unless it continually provides different barometer values (for whatever reason).