Aircraft piston engine crankcase 30-year repair defect pattern

Tom's observation that for thirty years, periodic investigators have flagged the under-matching 4043 weld as the cause of failure, despite ~100,000 successful repairs over forty years. Used to make the methodological point that failure analysts mistake difference for cause.

I had to help them respond to that. For thirty years, every now and then someone would investigate one of these and find their weld metal was softer than their base metal, and say that's the cause. They didn't go through any fracture mechanics and determine what the stresses were. I don't know if the original manufacturer knows what the stresses are, other than that they're low. It's just a container for oil up there. No one's really done a big analysis on it, and they don't need to — these things are sort of proven by about seventy years of history.