Autopilot disconnect

[Tommy]

Hi I have noticed that my autopilot KFC 200 will disconnect automatically after flashing Trim on the warning panel few times.

My J will then either pitch up or down significantly after disengagement. So I assume that the autopilot was disengaged because the plane is quite out of trim.

Other than that the AP is working perfectly.

Initially I just disengage the ap completely then trim then re start ap again but I have found a much better and safer way to fix the problem: Engage the CWS then trim the aircraft then release CWS. The trim flash will disappear and the ap works normally again.

Is this normal? Will AP disengage when the plane is really out of trim?

Thanks.

[Bob_Belville]

Why don't you just trim the pitch as required? Flying with the A/P using the elevator to offset the epennage (mis)position slows you down and it is possible the A/P runs out of elevator.

 

I have not had my STEC50 disengage, but unless I am really busy when I arrive at desired altitude I manually adjust trim as plane accelerates/decelerates before engaging ALTHLD. 

[Bob - S50]

Sounds like your auto trim function is not working.  Try performing the pre-flight test procedure from the book:

 

"With no modes engaged, depress the Preflight Test button on the Mode Controller.  All modes will be annunciated on the Annunciator Panel, including Marker lights, and the red Autotrim light will flash. At least four flashes are needed to indicate proper Autotrim monitor operation.  The pilot first engages the Flight Director, either by depressing the FD button or Pitch Sync (CWS) button. This will synchronize the Command Bars with the existing aircraft pitch and command wings level. Next, engage the Autopilot and apply force to the controls to determine if the Autopilot can be overpowered.  NOTE: The Autopilot will not engage when the Flight Director is not operating.  To confirm proper operation of all servos (except Yaw Damper), synchronize the Flight Director for wings level. Command nose up with FD Vertical Trim control. After 3 seconds you should observe the elevator trim wheel turning in the direction commanded. Re-synchronize the FD for wings level by using the CWS button, then command nose down with FD Vertical Trim control. After 3 seconds you should again observe the elevator trim wheel turning in the direction commanded. Re-sync the FD.  Now set the heading bug under the lubber line on your PNI and engage HDG SEL mode. Move the heading bug to the right and to the left and observe if the controls operate as commanded.  Disengage the AP and check aircraft manual pitch trim. Set trim to takeoff position.  This concludes the preflight test."

 

The selections in bold are by me.

 

I do not know how much it costs to be repaired but I'll bet Autopilot Central can tell you.

 

Bob

[Tommy]

I guess I was too inpatient to wait for the airspeed to stabilise before I trim and engage autopilot. Also I had been playing a bit with the MP and RPM and Mixture lately!

[carusoam]

The biggest challenge that pitch trim has is a collection of hardened grease that grows on the screw over the years. It can affect all Mooneys. And it can be cleaned relatively easily.

While on the ground use the trim from one stop to the other. Do this by hand to sense any resistance. This will be the resistance that is sensed by the AP when it turns off...

Check with your mechanic for proper grease and procedure if you find some resistance and chunks of old grease in there...

I'm not a mechanic, but I've been here a while....

Who knows why a Mooney climbs or descends when the AP is turned off after flying level? What happens in this situation?

Best regards,

-a-

[Awqward]

Why don't you just trim the pitch as required? Flying with the A/P using the elevator to offset the epennage (mis)position slows you down and it is possible the A/P runs out of elevator.

 

I have not had my STEC50 disengage, but unless I am really busy when I arrive at desired altitude I manually adjust trim as plane accelerates/decelerates before engaging ALTHLD.

I don't know much about the KFC200 setup, I have a KAP150 in my J....I only have a servo on the pitch trim....not on the elevators

[Awqward]

I stand corrected! I do indeed have both a Pitch Trim Servo and a Pitch Servo....

[Bob_Belville]

I don't know much about the KFC200 setup, I have a KAP150 in my J....I only have a servo on the pitch trim....not on the elevators

Are your sure?

 

I believe that the reason Tommy sees the trim light flashing is that the A/P can only move the elevator, not the whole empennage which is what the trim wheel moves. The A/P needs some help holding the nose down or up as the case may be. 

[Bob_Belville]

My STEC50 does not move the empennage. But I have a separate STEC electric trim, a double switch on the yoke, that does move the empennage to trim pitch. As it moves you can see or feel the manual wheel between the seats moving. But the ALTHLD feature of the A/P does not have access to that system. I do not have altitude pre select or vertical speed select or glide slope lock on. Only altitude hold, which works quite well.

[jlunseth]

Another thing to check if your autotrim is not working correctly is to look for "stuff" under the trim wheel or for a seatbelt impairing the wheel.  In my aircraft the clearance between the trim wheel and the floor is quite small, the carpet is cut out around that area and that forms a well into which things tend to fall.  It does not take much under the trim wheel to hold it in place when the AP is asking it to turn.  On the seat belts, they can particularly be a problem if the pilot and/or right seat passenger cross buckle their belts, the pilot buckles to the base of the right seat belt for example.  Or if there is too much slack or loose ends around the trim wheel.  I have had the belts pin the wheel, and then of course the autotrim and the AP both get mad at you.

 

There are also several things in the control "line" that can do it.  I am no AP expert, but I understand from my avionics shop that the control wire connects to a chain that drives a sprocket to adjust the trim, and if that gets out of adjustment the chain binds on the sprocket - the sprocket teeth do not align with the holes in the chain, they align with the link connectors, and that stops the chain or causes it to bind and jerk free.  The AP does not like that either and will tell you so.

[preinke]

I think most points were covered above. My trim was stiff when I purchased the Mooney. Bu cleaning and greasing the jackscrew and cleaning and greasing the initial gearbox and lubricating all places along the torque tube helped a lot . If it is hard to turn the trim wheel when the master is off or if the trim clutch slips when moving it with the trim switch. then lubrication should fix it. If it is easy to trim with master off and no clutch slipping then some part of the automation circuit to minimize elevator torque isn't working and needs to be addressed as it can be a big pucker when the autopilot disconnects.