78J. Electric Trim stops when trimming up

[Jon M]

My electric trim freezes about half way through nose up trim.  Manual trimming is fine and free of any resistance.  So I am assuming to start with the motor/clutch assembly? It slows to a stop. Has anyone had a similar experience?   

[N201MKTurbo]

What kind of electric trim is it?

 

[PT20J]

What autopilot do you have? Electric trim is part of the autopilot if an autopilot is installed. Usually trim problems are due to friction in the mechanical trim system. Does the trim move easily when you manually rotate the trim wheel from stop to stop? Is the problem only on the ground? It is harder to trim up than down on the ground because the trim jack screw in the tail has to raise the empennage against gravity. The Bendix-King servo is pretty robust. The Garmin servo is barely adequate because the gearing necessary to make it move  fast enough reduces the torque.

 

[IvanP]

If you have Bendix-King autopilot, e.g., KFC 150, etc., and the failure is intermittent without any noticeable mechanical issues in teh system, you may want to look at the A/P disconnect switch on the yoke. These are known to fail and your electric trim will not work if that happens. I just had this issue recently - intermittent failure of manual electric trim in flight, but everything was fine during ground test. Fortunately, these are easy to find and relatively cheap (~$15). It could also be the split trim switch. These are hard to come by and rather expensive, if you can find one. The good news is that they can be rebuilt with sufficient skill and patience.  

[Yetti]

Start with the easy stuff.   When was your trim system lubed?   Starting with the trim wheel chain, to the jack screw and including all the u joints in between.

[Jon M]

22 hours ago, N201MKTurbo said:

What kind of electric trim is it?

 

POH shows the system is the KFC 200

[Jon M]

12 hours ago, IvanP said:

If you have Bendix-King autopilot, e.g., KFC 150, etc., and the failure is intermittent without any noticeable mechanical issues in teh system, you may want to look at the A/P disconnect switch on the yoke. These are known to fail and your electric trim will not work if that happens. I just had this issue recently - intermittent failure of manual electric trim in flight, but everything was fine during ground test. Fortunately, these are easy to find and relatively cheap (~$15). It could also be the split trim switch. These are hard to come by and rather expensive, if you can find one. The good news is that they can be rebuilt with sufficient skill and patience.  

Thanks for the reply. POH shows the system is the KFC 200.  The switch works fine.  It is the trim up that goes about half way then stops.  

[Jon M]

14 hours ago, PT20J said:

What autopilot do you have? Electric trim is part of the autopilot if an autopilot is installed. Usually trim problems are due to friction in the mechanical trim system. Does the trim move easily when you manually rotate the trim wheel from stop to stop? Is the problem only on the ground? It is harder to trim up than down on the ground because the trim jack screw in the tail has to raise the empennage against gravity. The Bendix-King servo is pretty robust. The Garmin servo is barely adequate because the gearing necessary to make it move  fast enough reduces the torque.

 

Thanks for the reply. POH shows the system is the KFC 200.  The switch works fine.  the manual trim operation is fine. It feels like the trim motor can't pull the jackscrew any further. 

[IvanP]

3 minutes ago, Jon M said:

Thanks for the reply. POH shows the system is the KFC 200.  The switch works fine.  It is the trim up that goes about half way then stops.  

When the trim up stops, does the trim down command work at that point or does it stop completely and you have to use the mechanical wheel? What happens when you do A/P test? Does it happen in flight or on the ground or both and every time you trim up or only occasionally? You mentioned in your original post that there does not appear to be any mechanical issue and the trim moves freely when turned manually so that would tend to indicate an electrical issue. Nevertheless, it may be worth comparing the force needed to turn the wheel with another Mooney to see if it truly is moving freely through the entire range without needing excesive force. 

The AP/disconnect switch can fail in a rather strange manner. it is DPDT and if the NC contacts start to wear out you can see intermittent trim failure, albeit it would probably not be limited to any specific trim position. It will still disconnect the A/P when pressed so it may seem as if it is working fine. I believe that all B/K autopilots in Mooneys used the same D/C and trim control switches. It is not very hard to get to and you can easily check for continuity with multimeter. 

[PT20J]

I would expect a bad switch to cause the servo to stop suddenly. Slowing to a stop seems more like a mechanical issue. If it trims easily manually, it might be a slipping clutch in the servo. @Jake@BevanAviation might have some ideas.

[Jake@BevanAviation]

I would have someone listen to the motor speed when operating the trim system on the ground.  If the motor is slowing down I would expect a increased load on the servo.  If the load is too much eventually the slip clutch would start to slip.  When was the last time the trim controls were inspected and lubed?  Running trim by hand do you notice a area where there is increased friction?  The servo would have to work against the increased load when using MET (manual electric trim).  We have seen several Mooney's where there was excess friction causing the trim drive to slow when using MET.  Cleaning and proper lubrication of moving parts tends to resolve this issue.

If the motor speed doesn't slow I would expect a problem with the slip clutch assembly.  I can tell you what the slip clutch torque limit should be if I have the model and SN of the aircraft.  There is a slight possibility that there is a issue with the voltage regulator in the trim servo.  Normally if the voltage regulator is soft the trim servo drive is very slow or INOP.

[Jon M]

14 hours ago, IvanP said:

When the trim up stops, does the trim down command work at that point or does it stop completely and you have to use the mechanical wheel? What happens when you do A/P test? Does it happen in flight or on the ground or both and every time you trim up or only occasionally? You mentioned in your original post that there does not appear to be any mechanical issue and the trim moves freely when turned manually so that would tend to indicate an electrical issue. Nevertheless, it may be worth comparing the force needed to turn the wheel with another Mooney to see if it truly is moving freely through the entire range without needing excesive force. 

The AP/disconnect switch can fail in a rather strange manner. it is DPDT and if the NC contacts start to wear out you can see intermittent trim failure, albeit it would probably not be limited to any specific trim position. It will still disconnect the A/P when pressed so it may seem as if it is working fine. I believe that all B/K autopilots in Mooneys used the same D/C and trim control switches. It is not very hard to get to and you can easily check for continuity with multimeter. 

 

4 hours ago, Jake@BevanAviation said:

I would have someone listen to the motor speed when operating the trim system on the ground.  If the motor is slowing down I would expect a increased load on the servo.  If the load is too much eventually the slip clutch would start to slip.  When was the last time the trim controls were inspected and lubed?  Running trim by hand do you notice a area where there is increased friction?  The servo would have to work against the increased load when using MET (manual electric trim).  We have seen several Mooney's where there was excess friction causing the trim drive to slow when using MET.  Cleaning and proper lubrication of moving parts tends to resolve this issue.

If the motor speed doesn't slow I would expect a problem with the slip clutch assembly.  I can tell you what the slip clutch torque limit should be if I have the model and SN of the aircraft.  There is a slight possibility that there is a issue with the voltage regulator in the trim servo.  Normally if the voltage regulator is soft the trim servo drive is very slow or INOP.

Thanks Jake.  I will Look into the VR and the other suggestions here.  I will check with my other Mooney folks at the airport to get a feel for their trim manually.  Have a great day!

[Jon M]

5 hours ago, PT20J said:

I would expect a bad switch to cause the servo to stop suddenly. Slowing to a stop seems more like a mechanical issue. If it trims easily manually, it might be a slipping clutch in the servo. @Jake@BevanAviation might have some ideas.

Yeah the trim up just slowly chokes but the trim down works regardless.  Both in the air and on the ground.  It is very slow though?  So I will check the voltage regulator as Jake suggested.  Cheers.  

[IvanP]

I would think that if the clutch was slipping, the motor would keep going and you would hear it. If the whole system stops, I would suspect other things than clutch. The voltage regulator seems plausible culprit.