"Approved Data" or TSO, or whatever isn't really the issue. The REAL issue is the FAA and it's singular inconsistency in interpreting the regulations,policies, etc. And, it's NOT a safety issue. Good grief, we have had how many thousands of planes running around with field approvals for decades and you hardly ever hear of an issue from those.
But, try to get someone to issue a field approval these days. Every FSDO is different, but the primary response is that it's a lot easier to say NO than to work it out. And, this seems to be true of Engineering as well.
Meantime, our maintenance professionals put their signature, which means their certificate, on the line every time they sign something, ANYthing off. What if the next mechanic decides your plane isn't legal, just cause Joe AI thought that was a "Minor", and the new guy disagrees. The point being, mechanics are just as inconsistent as the FAA, wonder why?
There are indeed cases where the regulations and policies APPEAR to be straightforward, but there are too many Philidelphia lawyers out there.
I once wanted to put a set of German wheel skis on a Husky. These were JAA certified. Turns out the FAA jointly approves most JAA approved stuff. So, no sweat, right? Nope. I finally wound up talking to the head of the Denver Aircraft Certification Office. He informed me that, with JAA approval, we could install the skis, BUT, they'd have to be signed off on a 337 by an IA AND a FSDO Inspector. I asked why a FSDO type, since they are approved, AND STC'd. His response: "That's the way it is.".
Fortunately, we found a very cooperative FSDO Inspector, who signed them off.....he was just as confused why he needed to sign them off as I was.....so took some convincing.
MTV