Ok, understand the root cause is the discontinuity of the J-stiffener. Given the difficulty of the fix, please tell us you looked at an "external strap" (not a plate doubler) of considerable thickness as an alternative to fixing the discontinuity. If you did, fine. But you owe us that answer.
We considered a wide variety of methods to address this. An external strap does not address the issue. Adding more skin structure doesn't resolve the actual problem. The need here is to prevent the problematic flexing of the skin in the first place.
I am a bit confused why Greg or Scott have been so quiet on this issue. There are many questions being asked in this thread, they should address them.
1) Why it is this a mandatory repair even if no cracks are visible?
2) Show us how to support the tail cone skin to prevent additional damage from climbing inside.
3) What is the history of this SB?
4) Are we risking a crash if there is a 1" crack in this area?
We're pretty busy right now with a lot of work that we have to balance, so our apologies if we don't reply in a timely manner every time. The clips in the kit are pre-bent, the rivets are pretty easy to drill out, and it's all pulled rivets to install the new parts. Yes, getting in the tailcone is a bit awkward. To address your questions:
1) For SLSA it is mandatory. For ELSA and EAB you have the power of discretion. Cracks will eventually form. Every airplane is a little different. It will vary from one aircraft to another in terms of how long it will take, and whether you'll see them when they start to form.
2) To be honest, I think people are overthinking this a bit. Put a tail stand in place so the tail won't strike the ground when someone's in the tailcone. And the idea with the plywood is to prevent a knee or other body part from applying concentrated stress on a skin. The plywood described in the bulletin spreads the load over a large area and accomplishes this. If you want to use dense foam or some other method, your choice. I'll ask our engineering team for some additional input on this and see what they say.
3) It was just issued. Aircraft with both very low and very high hours (and in-between) have been found with similar cracks in the same locations. Engineering analysis says cracks will eventually form. Parts to address the stress concentrations and load carry-though were issued. Service bulletin and parts were released.
4) There's no way to answer that question for every airplane equally. A crack is not a crack is not a crack. It depends completely on the loads applied to the airplane with the 1" crack, where exactly the crack is located, and a bunch of other hypotheticals/variables. Once a crack starts, it represents a structural failure which was predicated by the discontinuity in the J-stiffener and the resulting transfer of loads into the skin/rivets. The whole point here is to
prevent the cracking. Once the skin has cracked, the skin's remaining ability to handle these loads is diminished. The important point here is that we are installing new parts with this SB to remove the problem loads from the skin itself. It can only get worse one it starts, and how much worse is the variable. It depends. The stresses on the top and bottom of the tailcone can be quite high, and if there are discontinuities and/or cracks where the load concentration is highest, it needs to be addressed. Proactively. That's why we issued this bulletin.
Note that we don't do risk analysis based on hypotheticals, and we certainly don't issue SBs for fun. We engineer and issue SBs to cover the potential risk for a fleet of aircraft based on real-world experience. Will your airplane fall out of the sky if there's a one-inch crack? That depends on a lot of things. We'd very much prefer to have you execute this simple SB rather than find out the hard way.
I'm an RV-12 owner with pretty bad arthritis, so I share in the awkward pain of climbing in a tailcone. If it's difficult physically for someone to get in the tailcone, I'd say get a helper. Teenagers work cheap and maybe you'll introduce someone new to experimental aviation. It's inconvenient, but not really all that difficult to get back there... That said, I know I sure wouldn't want to do it every 100 hours (as some have suggested they might do instead, and we do not advocate that) just to inspect, and please note that it
needs to be inspected from inside, not just from outside. When you review the SB you'll see the cracks in the photos are from inside the aircraft. And, when you climb back there once and you get the parts installed, you're done.
We appreciate the feedback, and we review it and consider it for context when creating the next SB or when adjusting an existing one.
Hope that helps.