I don't believe there has ever been a failure on any Grob spar
fitting. The British air academy cadets tested a G-103 with 50,000
simulated winch launches and got one weld to crack. To my knowledge
that is it, an AD was issued and a few cracks were found, but nothing
ever came apart due to spar fitting failure. I'd say to keep on with
the ultrasonic testing every 500 hours if your national aviation
authority allows this, the US doesn't, but how often have we been
right on anything recently?
JJ