Electric elevator trim switch Failed

[Alan Maurer]

Hello Mooney People,

Flying with an instructor at Mooney Review Course in Lakeland yesterday (really great course) the electric elevator trim quit and trim manual wheel was stuck. Nose was trimmed down and had to pull hard. With force we managed to trim with the wheel in Ovation M20R and flew home today  with trim switch off. It is broken but I wondered if anyone else had this problem and what needed a repair.

Thanks Everyone

Alan

 

[0TreeLemur]

Scary.  I thought autopilot servos were supposed to have a clutch that can be overcome by control forces, slipping to alleviate those control forces?

I can't answer your question but an obvious question is which autopilot do you have?

[Alan Maurer]

The autopilot is the Garmin GFC 700 . usually works perfectly. Took lots of force to turn the manual trim. 

[Pinecone]

The electric trim trims the tail.  So just like manual trim, if you are out of trim, you have to apply a force.

This has nothing to do with the autopilot and over riding it.  

[0TreeLemur]

33 minutes ago, Pinecone said:

The electric trim trims the tail.  So just like manual trim, if you are out of trim, you have to apply a force.

This has nothing to do with the autopilot and over riding it.  

Help me understand.   I know how electric elevator trim works.  It seems to me that the O/P is implying that the inop trim switch (A/P malfunction) is somehow related to causing difficulty in manually turning the elevator trim.   Is that not A/P related?   What am I not understanding about the original post?  

[Alan Maurer]

Hi Guys, To clarify...The thumb trim switch did nothing and the manual wheel between the seats would not turn until we forced it, which was not easy.

Now the thumb switch on the yoke does nothing and the trim wheel is still hard to turn. I have left the panel AP/trim switch off until I have whatever it is repaired. The AP is the GFC700 that is part of the G1000 system in the Ovation M20R.

Thanks

Alan

[rickseeman]

The title says the electric trim switch failed. If you can't turn it by hand I don't think that little motor would turn it. The switch might be fine. The trim system needs to be checked out. By the way my Ovation trim is pretty stiff also. 

[hypertech]

It should not be that hard to move.  Its not the easiest thing given the location but I've never had any trouble manually trimming my ovation.  It sounds like something is binding or not greased (or the actuator has failed in a creative way).

[A64Pilot]

Is the trim wheel hard to turn with the Master off?

It should not be hard, it may be the switch failure if it’s failed is from the trim being so hard. The jack screw is so difficult to lube that I think often it doesn’t get lube

[rickseeman]

15 minutes ago, A64Pilot said:

Is the trim wheel hard to turn with the Master off?

It should not be hard, it may be the switch failure if it’s failed is from the trim being so hard. The jack screw is so difficult to lube that I think often it doesn’t get lube

The switch is just an electrical switch. It doesn't know. I thought it may have smoked the motor but that is supposed to be protected by the circuit breaker. I was wondering if my jack screw needed lube. I have a list of things to ask Don Maxwell at annual time in a few months. I want a good annual by a Mooney savvy guy because I am not familiar with the shop that has been doing the maintenance.

[Fly Boomer]

26 minutes ago, rickseeman said:

I was wondering if my jack screw needed lube. I have a list of things to ask Don Maxwell at annual time in a few months.

If my trim wheel was difficult to turn, I would look at it sooner rather than later.

[rickseeman]

I'll look in the MM and see if it is something I can get to. I didn't know hard stiff it should be. But it feels hard to turn.

[A64Pilot]

36 minutes ago, rickseeman said:

The switch is just an electrical switch. It doesn't know. I thought it may have smoked the motor but that is supposed to be protected by the circuit breaker. I was wondering if my jack screw needed lube. I have a list of things to ask Don Maxwell at annual time in a few months. I want a good annual by a Mooney savvy guy because I am not familiar with the shop that has been doing the maintenance.

Of course the switch doesn’t “know” but the motor isn’t that strong if the trim is binding the motor can’t move the trim, the CB should pop before the motor quits, but of course over time excessive load will wear the motor out.

You need to get a licensed mechanic to see what’s wrong, trim is a flight control, and flight controls need to be taken seriously.

It’s my understanding thr trim switch is made from un-obtanium, but as I’ve not tried to source one I don’t know that.

It should not be so hard to turn that you think it’s hard to turn

[rickseeman]

I agree.

[PT20J]

The places where the trim system usually binds are: The jackscrew in the tail hasn't been lubricated recently, the trim servo is binding, the chain from the trim wheel to the front gearbox is too tight. Also, with a trim servo installed it is good to ensure that the trim system stop nuts are the newer stepped type to prevent the trim servo from jamming the trim system at either extreme.

The trim being "tight" is kind of a relative determination. Because you are turning a jackscrew and moving the entire empennage, rather than a tab on the elevator the Mooney trim wheel is harder to turn than say a C-172. But it shouldn't require undue force. I make it a habit to always return the trim to the takeoff position after landing manually so that I can tell that the trim feels normal. And, occasionally I run it from stop to stop manually just to check.

Skip

 

[Alan Maurer]

Skip

The trim wheel is usually very easy to turn. This time Could really not turn it. Finally we got it to turn. Before the other day, always easy to turn. 

After landing I always re trim for the next time out. Spoke to Sarasota Avionics/maintenance....they are going to look at the servo motor and the jack screw. 

Did not have a circuit breaker pop out. As of now the electric trim is not working. Stay tuned.....

[Pinecone]

If it was easy to turn, and very hard, SOMETHING CHANGED.  And in a not good way.

You need this looked at before further flight.

[LANCECASPER]

21 hours ago, Alan Maurer said:

Hello Mooney People,

Flying with an instructor at Mooney Review Course in Lakeland yesterday (really great course) the electric elevator trim quit and trim manual wheel was stuck. Nose was trimmed down and had to pull hard. With force we managed to trim with the wheel in Ovation M20R and flew home today  with trim switch off. It is broken but I wondered if anyone else had this problem and what needed a repair.

Thanks Everyone

Alan

 

Usually has to do with the jackscrew grease not being looked at in years. This is overlooked in many annuals.

[Fly Boomer]

On 1/30/2023 at 11:37 AM, PT20J said:

The places where the trim system usually binds are: The jackscrew in the tail hasn't been lubricated recently, the trim servo is binding, the chain from the trim wheel to the front gearbox is too tight. Also, with a trim servo installed it is good to ensure that the trim system stop nuts are the newer stepped type to prevent the trim servo from jamming the trim system at either extreme.

The trim being "tight" is kind of a relative determination. Because you are turning a jackscrew and moving the entire empennage, rather than a tab on the elevator the Mooney trim wheel is harder to turn than say a C-172. But it shouldn't require undue force. I make it a habit to always return the trim to the takeoff position after landing manually so that I can tell that the trim feels normal. And, occasionally I run it from stop to stop manually just to check.

Skip

 

Is there a Service Bulletin or other documentation regarding the stop nuts you are referring to?  I have been through my logs over and over, and I don't recall seeing anything like that.

[EarthboundMisfit]

Two years ago, the electric trim failed on our 2005 Ovation 2GX with STEC AP.  The AP no longer turned the trim wheel (but we could move it manually), and the stepper/pitch motor would still move the control wheel forward and aft. 

Apparently AP computer was fried, so we had it replaced.  The elec trim failed again on next flight as AP was engaged.  Hmm...

AP was fried again, and this time encouraged avionics folks to find root cause first.  This is what they reported:

Electric trim reported still inoperative following previous repair of flight computer. Removed computer for bench assessment/repair. Computer found unserviceable, investigate aircraft for source of damage. Nil defects apparent during initial testing, able to run electric trim in both directions without excessive current draw and no presence of shorts on interfacing wiring. Manipulate/disturb wiring harness in attempt to reveal intermittent short, nil defects apparent. Found fault internal to trim servo, fault intermittent, found drawing excessive current.

So, the trim servo had zapped two AP computers. 

Had the servo removed and overhauled, and the AP computer repaired (including replacing the blown internal fuse!).  The servo had to go back to STEC, so we became better practiced at manually trimming for the month it was gone.

No issues since.

Echo the couple of comments on jackscrews - they need to stay greased/lubed. 

Edited February 1, 2023 by EarthboundMisfit

[PT20J]

2 hours ago, Fly Boomer said:

Is there a Service Bulletin or other documentation regarding the stop nuts you are referring to?  I have been through my logs over and over, and I don't recall seeing anything like that.

SIM20-88A and SBM20-325

[Fly Boomer]

12 hours ago, PT20J said:

SIM20-88A and SBM20-325

Thanks for those references.  According to my records the SI/SB has not been complied with on my airplane.  I haven't removed the belly to look for myself yet, but I will -- my airplane is near the end of the s/n range, and sometimes these documents don't get the range exactly right.

I used two search engines to look for "stepped stop nut", and the only references I found were the Mooney SI and the SB that incorporates the SI.  I also looked at a reference of all AN, MS, and NAS fasteners and parts, and couldn't find that term.  "Stepped stud" seems to be a thing, but not "stepped stop nut".  Is this a Mooney-specific part that can only be bought from the factory or through an MSC?  On my airplane, the part number is 740086-005.  The illustration in the IPC looks about the same as the illustration in the SI -- looks like a regular jamb nut.  Did you source the parts for your airplane from Mooney?
 

[PT20J]

54 minutes ago, Fly Boomer said:

Thanks for those references.  According to my records the SI/SB has not been complied with on my airplane.  I haven't removed the belly to look for myself yet, but I will -- my airplane is near the end of the s/n range, and sometimes these documents don't get the range exactly right.

I used two search engines to look for "stepped stop nut", and the only references I found were the Mooney SI and the SB that incorporates the SI.  I also looked at a reference of all AN, MS, and NAS fasteners and parts, and couldn't find that term.  "Stepped stud" seems to be a thing, but not "stepped stop nut".  Is this a Mooney-specific part that can only be bought from the factory or through an MSC?  On my airplane, the part number is 740086-005.  The illustration in the IPC looks about the same as the illustration in the SI -- looks like a regular jamb nut.  Did you source the parts for your airplane from Mooney?
 

My airplane had them installed at the factory. I believe it’s a Mooney part, not a standard part. The original design was just jam nuts that could, well, jam. The step nut design engages the step to stop rotation without adding friction to the trim system.

[DContreras96]

On 1/31/2023 at 8:17 PM, EarthboundMisfit said:

Two years ago, the electric trim failed on our 2005 Ovation 2GX with STEC AP.  The AP no longer turned the trim wheel (but we could move it manually), and the stepper/pitch motor would still move the control wheel forward and aft. 

Apparently AP computer was fried, so we had it replaced.  The elec trim failed again on next flight as AP was engaged.  Hmm...

AP was fried again, and this time encouraged avionics folks to find root cause first.  This is what they reported:

Electric trim reported still inoperative following previous repair of flight computer. Removed computer for bench assessment/repair. Computer found unserviceable, investigate aircraft for source of damage. Nil defects apparent during initial testing, able to run electric trim in both directions without excessive current draw and no presence of shorts on interfacing wiring. Manipulate/disturb wiring harness in attempt to reveal intermittent short, nil defects apparent. Found fault internal to trim servo, fault intermittent, found drawing excessive current.

So, the trim servo had zapped two AP computers. 

Had the servo removed and overhauled, and the AP computer repaired (including replacing the blown internal fuse!).  The servo had to go back to STEC, so we became better practiced at manually trimming for the month it was gone.

No issues since.

Echo the couple of comments on jackscrews - they need to stay greased/lubed. 

I picked up my 2005 Ovation 2 GX in October 2024.  Within a month during a flight I smelled a puff of electric smoke that dissipated and after that, my electric trim was inoperable.  It was discovered that the STEC 55X board had a fried trace line on one of the boards.  Got a replacement unit and that was supposed to solve the problem.  Installed the new STEC 55X and with the first push of the electric trim, it fried the second board.  I have a third unit that I am not installing until we figure out what is going on.  My uneducated guess is the trim servo is shot but I am wondering if the trim lubrication should be checked.  The manual trim works and I can move the trim full range with 2 fingers and a thumb, but it is a little tough to get through some parts of the travel.  Annual was at the Mooney factory in September.  I'm hoping it is limited to the servo.  Any thoughts? 

[MikeOH]

From your description I doubt it is the servo if the failure is IMMEDIATE after pushing the trim switch.  It sounds more like there is a short in the wiring to the trim switch which comes into play when you press the trim switch.  I would want the shop to look at the schematic for the failed board and identify the part of the circuit that is getting damaged.  That knowledge would help in deducing what might be going on with the trim switch wiring.

You could also disconnect the servo and check it for shorts with an ohmmeter.  I would think, but do not know, that the drive circuitry for the servo is protected against shorts.  Again, not certain on that.  Hopefully, others with more detailed knowledge can weigh in, too.