Very scary stuff. Its a great pity to see HUMS fail to spot this one coming. The procedure of finding a small metal particle on a chip detector then putting the component onto "close monitoring" is totally standard, but in this case not good enough.
Perhaps it will be a large crack such as was found in the bevel gear of a CHC 332L some years ago, totally un-spotted by HUMS. Since cracks don't release much debris (to be picked up by a chip detector etc) we rely on HUMS, but really HUMS has not evolved much since inception in 1990 or so. We need to get the Neural Network concept being trialled by GE, into service asap.
HUMS was a great invention and step forward, but for the last 20 years the industry has been resting on its HUMS laurels. Not good enough!
HC