I can see that, right up to the point that surely the airframe is designed to cope with this? and adding mass to the wing will lower it's resonant frequency?
The airframe was not designed to handle the load from the winglets...the winglets came later...
I find it interesting that only pax ac have found cracks so far...
The issue was found by Boeing Shanghai on the new C2F line July 2019).....What about all of the previous C2F conversions done by others..., why was nothing found there?
Are freighters (and C2F) not included in the group the FAA wants tested?