Maintenance: SB 18-09-17 Aft bottom skin cracking.

Van's released SB 18-09-17 to address some cases of the aft fuse bottom skin cracking (examples of the cracks in flying aircraft can be found here and here). Following inspection of my airframe, I found no evidence of cracks.  However, I chose to install the stiffeners as described in the service bulletin.  The process took less than an hour with two people: One in the empennage and other underneath.

  

Here are the parts associated with the service bulletin.

Because the F-14148A Skin Doubler (the diamond) is affixed to the exterior of the dimpled F-01478 Bottom Skin, the dimples would remain unfilled and preserve voids under the six rivets which attach the Skin Doubler. So I chose to create fillers for the dimples by using drilled out rivet heads for the purpose.  Thus, I drilled out about 25 rivets on scrap to make sure I could obtain 6 rivets with well-centered holes.  I then sanded them smooth.  These six fillers were cleco'd into place on the Skin Doubler prior to cleco'ing on the skin (after all, we're working upside down here!), as shown below.

Removing the F-01406F Baggage Bulkhead Corrugration, placing boards and towels in the baggage area and a blanket in the tail, it was trivial to access the area required to install the stiffeners. 

A 12 inch #30 drill bit was used to match drill the bottom holes in each of the F-14147 Stiffener Clips.  There was adequate space available to squeeze the AN470AD4-4 universal rivets with my longeron yoke.   Tungsten bucking bars were used to buck the AN426AD3-x rivets.  It's worth noting that I increased the length of each AD3 size rivets by 0.5 as compared to what the SB calls out.

Following drilling through the F-01408-L/R Side Frames to attach the F-14147 Stiffener Clips.

Shop heads of the rivets, on the aft side of Side Frames, holding the Stiffener Clips.

Forward side of the Side Frames showing forward interior stiffeners in place.

Aft side of Side Frames showing single aft interior stiffener in place.

Exterior views of diamond Skin Doubler.  I need to clean up the area with acetone as I chose to prime the associated surfaces.