I think there has to be a fatality to have the NTSB involved. The FAA classifies accidents as incidents and accitents on some criteria, what I'm not sure, but its quite easy to hide accidents/incidents as George has mentioned.
If the repair did not fall in the Major repair category, then a Form 337 would not even need to be done, however, a log entry is a must anytime work of any sort is preformed on an aircraft. I'll admit that alot of people won't make an entry just to hide damage, and thats certainly wrong, thats where a good prebuy, with someone who knows what to look for can spot tale tale signs of repairs, re-riveting, etc.
Charles
