Lots of talk here about HUMS not detecting the problem that led to this catastrophy.
The AS356 gearbox has had occurances of cracks being found in the epicyclic gearing, during overhaul. There was no history of 'chips' as a result of the cracking.
The manufacturer implemented a 50 hr boroscope inspection of the gearing.
My point is that HUMS is an additional tool/ aid to early fault detection, and not the all seeing, answer to everything that some people think it is.