Service Bulletin SB 18-02-02: Cracks in the stabilator spar box

A high-time RV-12 was discovered to have a crack in the front spar box in the vicinity of where the stabilator horn is connected.  I don't know how the hell anyone discovered this crack considering that

dye penetrant was used to make it visible.  After dye penetrant was applied, the part was sanded to bare metal and application of a bright light revealed the crack.  Amazing!  I'm trying to imagine what prompted them to suspect that a crack was there. Anyway, the Mothership came up with a fix consisting of doubler plates which go between the horn bracket and the top and bottom of the box spar, doubler plates which go between the stabilator hinge brackets and the front of the box spar, and gussets which connect the hinge brackets to the stabilator skin and the spar, top and bottom.




The result of all this is an upgrade which is massively stronger than the original part.  I don't mean simply 50% stronger. I mean massively stronger.  The original stabilator hinge brackets simply bolted to the spar web, and the stabilator horns bolted to the spar box flange.  The new system rivets doubler plates to the stabilator horns, which then get bolted and riveted to the spar, top and bottom.

New, two-piece hinge brackets are riveted together and then bolted and riveted to the spar.  New doubler plates go between the hinge brackets and the spar.  Gussets tie the new hinge brackets to the stabilator skins and the spar, top and bottom.  The six #40 holes seen in the top of the spar (also on the bottom) are left open.






In the finished product (below), the original hinge brackets (green pieces on the table) are shown for comparison.

Quite a few rivets had to be drilled out, with their mandrels punched into the center of the box spar.  When I'm moving the stabilator around, I can hear them rattling around in there.  This tipped the scales in favor of installing the new fiberglass tips on the stabilator (what's another $130 at this point?).  Installation of the tips requires drilling out all the rivets in the end ribs and reversing them.  When they're out I can dump out all the debris.  Ordered the kit today.