Bob Turner; with all due respect....I love what you are doing and wholeheartedly agree with proposing an alternate solution. But to really attempt to swing the meter a bit...my opinion would be that to include a brief test and results...would be significantly more favorable than what is currently offered.
Most folks don't realize that the FAA doesn't have many subject matter experts left, those positions filled these days, with "compliance" folks, rather than actual engineering....you know...the kind with MATH involved...You will note that ALL of their findings are observations or anecdotal. No stress Calcs of any kind were performed. (or at least not included in the report).
It would not be difficult to build a simple test fixture, simulate the mounting points of the rudder on the aircraft, cycle test to failure a few test specimens. The results and accompanying engineering report would be valuable as gold...and increase the likelihood of a favorable AMOC decision many fold.
Test labs, are plentiful in the aerospace business, my experience is that testing like this, would probably be under $8k and a fixture could be built for under $1k.
Specimens for failure proving and specimens for AMOC acceptance would be required.
simulating the loads would not be overly difficult (from what I have seen) which is to state that the failures all have commonality...heavy on the end, some level of excitation during flight and linear, sideways-downstream yielding.
What I believe the FAA is concluding for themselves is that the 1025 substrate is suffering from probable work hardening, most likely from a combination of heat affected zone from welding, combined with the much more significant live loads imposed by the increased mass of the beacon. They are guessing, as Gordon eludes to..that the original design, is inside the failure modes safety factor. ( they are probably correct too ).
4130 is a much better material choice and particularly if post-weld stress relief was included in the fab process...I guarantee no-one stress relieved the 1025...Given that most, if not all of the 1025 rudders were gas welded...the variables keep adding up...but heat affected zone in the proximity near to the weld, is likely, as is work hardening over the life-cycle of the beam.
This would be a great time, to raise a little funding, build a fixture, test to failure coupons and demonstrate passing coupons to higher than required safety factor and offer a well planned AMOC to the community, via the FAA. Not only would this strengthen the community, but...could improve the FAA stance, if we demonstrated our willingness to be a part of the solution, along with the knowledge and acumen to resolve our aging needs.
Again...please don't take my two cents, as criticism...it is definitely not intended as such. I think your solution makes an absolute ton of sense and I think the FAA's proposed AD is over- reach.
They do, however have an obligation to address the issue, once it has been brought to light.
With much respect,
Steve.
( I am feeling lucky to have bought my oversized PA-14 Rudder out of 4130 last year for the Fat Beast).