Trim slipping

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JU343

Trim slipping

Post by JU343 »

I have noticed that the trim wheel is unable to hold selected position during high speed descents (150KIAS). Unless I keep holding the trim wheel, it rolls backwards. Thus I need constantly to keep stick pressure forward.

Is this normal or is the trim clutch worn out?
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Steve
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Re: Trim slipping

Post by Steve »

Not normal, but may simply need adjustment.
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Re: Trim slipping

Post by CFIDave »

I experienced something similar on a DA40 last year, where the autopilot couldn't hold elevator trim when in "high-speed" cruise: The autopilot would add more nose-down trim, the wheel would then slip backwards to T/O trim, the autopilot would move the wheel forward again, and the cycle would repeat.

The first (and easiest) fix is to adjust (tighten) the friction block in the tail that's easily visible right under the elevator. If that doesn't work, then your mechanic/tech will need to take apart the center console to adjust the friction on the elevator trim wheel. A spring scale is used to measure the appropriate force (described in the DA40 AMM).
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Re: Trim slipping

Post by BRS »

I have been dealing with trim slippage for years. I've asked for it to be fixed a few times but finally I decided it was time for me to dive in and fix it myself. After all, I'm the one who really cares about it. It has been a learning exercise and I realize even the guys in London don't know everything. Or perhaps they do as the solution is not yet proven.

Like most folks I started out at he friction trim block on the tail. The AMM says to adjust it to "(3.4 - 6.7 lb)" with a spring scale. I found mine had been adjusted to over 16 lbs! That explains why I get spring back on the trim wheel when turning it. BTW - the AMM seems to indicate that the main reason for the tail friction block is if there is a cable failure then this will dampen out any flutter.

Seeing that adjusting the tail friction block tighter was not an option I pulled the seats and the center console cover. Something I was hoping to avoid. After studying the assembly visually and in the AMM I came to the conclusion that the so called "Friction Damper Nut" on page 2 of 27.30.00 has little to do with dampening or trim slippage. What that nut actually adjusts is the friction of the trim-wheel when it hits the stop (up or down). This allows the wheel to slip when too much force is being applied. BTW - the KAP140 bypasses this 'friction-clutch'. Diamond Tech felt sure adjusting this nut would help trim slippage and then encouraged me to disassemble the the wheel and related parts to compare them with the AMM. Which I did but then realized I don't have a drawing for this assembly when the KAP140 is installed. So I've emailed them the pictures and am waiting on a response.

In the mean time I took a close look at the diagram on page 2 and came to the conclusion that the only possibility to adding friction to the cable movement at the trim-wheel is to tighten the pivot bolt and nut that holds the whole thing together. In doing so, add pressure on the two "plastic friction washers" pictured just above the 'ball-stud'. I won't know for sure until I try it. After I hear back from Diamond. Here are some pics.
Attachments
Plastic Friction Washers.  Theory: Squish these (there is one on each side) with the pivot bolt to add friction to the cable movement.
Plastic Friction Washers. Theory: Squish these (there is one on each side) with the pivot bolt to add friction to the cable movement.
To adjust the so called "friction damper nut" you will need filed down 19mm or 3/4" wrenches.
To adjust the so called "friction damper nut" you will need filed down 19mm or 3/4" wrenches.
Hope you don't have the pleasure of taking this all apart.  Don't bother, it's not the cause of trim slippage.  It's a nice learning exercise though.
Hope you don't have the pleasure of taking this all apart. Don't bother, it's not the cause of trim slippage. It's a nice learning exercise though.
Another view.  The rubber bushing on the right was a surprise.  Not at all like the AMM
Another view. The rubber bushing on the right was a surprise. Not at all like the AMM
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Re: Trim slipping

Post by AndrewM »

Brock, how did this story end...? Referencing your most recent post on this, did tightening the pivot bolt and nut that holds the whole thing together in the center console resolve the issue? Conducting an annual right now, and during the last annual I believe the friction trim block in the tail was adjusted, but that did not fix the issue (which seems identical to what is described in this thread). Thanks!
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Re: Trim slipping

Post by BRS »

Andrew,
Thanks for asking. It was most interesting an the solution was not what I had expected. It's been awhile since figuring this out so I'm working off memory. As I recall, tightening the pivot bolt thus adding tension to the packing did NOT solve the problem. What worked in the end was tightening the "hing bolt" for lack of a better name. This is the bolt down at the bottom from which the trim assembly hinges on. It was not a bolt that an engineer would likely design as a friction point but it worked like a charm.
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Re: Trim slipping

Post by CFIDave »

I went through a similar experience about 2 years ago when the 2008 DA40 XLS we had taken in on trade (for a new DA40NG) wouldn't hold trim. As soon as I leveled the aircraft off in cruise to an Indicated airspeed above 130 knots, the trim wheel would start to turn (even if turning on the GFC700 autopilot). The plane simply wouldn't "hold trim" at higher airspeeds.

I worked with a local maintenance shop that began by adjusting the friction damper on the tail, but after another test flight we discovered this did not fix the problem.

So the mechanic took apart the center console trim assembly (per instructions in the AMM) and adjusted the friction there. A subsequent test flight showed that this fixed the problem, and the new owner of this plane has had no issues with the trim system.
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