My goal tonight was to rivet the trim tab spar onto the bottom trim tab skin, so I can work on gluing the ribs in place over the weekend. On the inboard end of the trim tab, the trim tab horn is riveted onto the outside of the skin, and in two places it shares rivets with the spar. I started by back riveting the spar, and everything was looking good. I left out the two rivets that are shared with the horn, and then came back to them at the end (masked over area in the picture below).
Because the horn is on the outside of the skin and it protrudes out, I decided to squeeze these rivets. The horn is in two parts, so I riveted on the first one, then started on the second. After setting the second of the three rivets, I gave it a quick inspection and realized that the first two rivets had twisted and on one, a small crack had opened up on the side of one of the dimples (bottom left rivet below).
Unfortunately I’ll need to order a new skin, and will need to remove the 30-odd rivets I set tonight.
I’m not yet 100% sure why this happened, but my guess is that for some reason the horn was not completely flush with the skin. Then as the rivets set, they pulled the skin and horn together, causing the skin to bend up on one side. The horn couldn’t bend with the skin because it was sitting flush against the other half of the horn that was already riveted. As I remove all these rivets, I should be able to confirm the issue.
When I do this again, I’ll try to back rivet these on the corner of my back rivet plate, which is likely to produce a better result.