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EXCESSIVE ELEVATOR TRIM TAB PLAY

skyyking

Active Member
I have 1/4” of play at the elevator trim tab. The play is not at the trim tab horn clevis, but in the shaft inside the cable. As you move the trim tab, one can see that this play is caused by fore and aft movement internal to the cable.
Aircraft has 160 hrs TT.
Anyone else have this, any fixes?

Thank you.
RV-9
 
Trim tab play

Common issue with the trim tab cable. Over the course of the distance from the trim tab control to the aft tab, that little bit of internal ‘slop’ adds up. I’ve dealt with it for 8 years and don’t believe it to be an issue. I did squeeze some viscous grease into the shaft to help fill in the space with questionable success.
As a side note, the regional FSDO Inspector that performed my inspection did not like the tab wiggle but ultimately signed it off. Hope that helps.
Jim Diehl 7A
#1976
Dues Paid
 
1996 RV-6 here with ~700 hours on it and it's been exactly like that for the ~350 hours i've owned it. The manual trim cable has slop in it.
 
I've had this play in my trim tab too. I think it has a lot to do with the 180 degree turn the cable makes in the front. It's never been a problem.
 
AC 23-629-1b Ch 3.d.2.
Okay, chapter and verse. As designed, the Van's manual trim system has free play that cannot be mitigated. There is ample (but only anecdotal) evidence the system has not contributed to elevator flutter. If I were concerned about it and wanted to avoid an electric trim conversion I would connect the trim control forward to a bellcrank and then aft using a straight throttle cable, thus avoiding the bend.
 
The free play in the elevator trim tab can be caused by wear or looseness between the cable core and sleeve at the rear end. It is easy enough to test by gently wiggling the trim tab up and down and watching for side-to-side slop in the cable where it exits the elevator. I expect this part of the cable is more likely to wear because it is exposed to dust and moisture. The fix for a local older RV that had this problem was to replace the cable.

Vans have previously been quoted in VAF as saying that 1/4" of play is acceptable in the trim tab, which is more than I would've considered to be OK before reading that information.
 
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Slop mitigation

The trim tab slop can be mitigated by installing a spring between washers at the trim tab end of the cable. This takes up the slack.

Ed Holyoke

Okay, chapter and verse. As designed, the Van's manual trim system has free play that cannot be mitigated. There is ample (but only anecdotal) evidence the system has not contributed to elevator flutter. If I were concerned about it and wanted to avoid an electric trim conversion I would connect the trim control forward to a bellcrank and then aft using a straight throttle cable, thus avoiding the bend.
 
Define excessive play?

In section 15 Inspection and Preparation of the Airframe under flight controls it says that "trim tabs must be free of excessive play." I would think the information in the AC was purpose written, but if we're not abiding by that what is meant by excessive play?

This part of my build is coming up and I'de like to know if there's something I can do in the build process to mitigate this. I'de hate to get into the final inspection and have my ETT flopping around.
 
The trim tab slop can be mitigated by installing a spring between washers at the trim tab end of the cable. This takes up the slack.

Ed Holyoke

I would never do that. If the cable fails there will be a sudden tab deflection that will cause a big pitch change and possibly excessive g-loading until you are able to compensate for it.
 
Failure mode?

The cable is way, way robust for the amount of wear on it and the forces that it can put on a trim tab and vice versa. The most likely spot for a failure would be at the tab itself. The doubled sheetmetal horn is subject to wear from the clevis pin. Mine was ovaled out quite a bit, so I drilled and reamed it larger and pushed a flanged bushing into it. Now the clevis pin is riding in bronze and the stresses on the edges of the hole in the horn have been mitigated. That's when I added the spring to take the slack out. My thinking was that the slack in the system was causing the wear in the horn in the first place.

There are indeed places on an aircraft where a hardware failure would be catastrophic. Prop bolts come to mind. I'm not so sure that this is in that category and making the installation as sound and secure as possible was my approach to it.

Ed Holyoke

I would never do that. If the cable fails there will be a sudden tab deflection that will cause a big pitch change and possibly excessive g-loading until you are able to compensate for it.
 
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