You can have all sorts of fancy diagnostic tools such as HUMS, AAD, chip detection, SOAP etc but they are no use at all if the operator's technicians do not pay attention to what they are trying to tell him!
HeliComparator,
Indeed, there are limitations to what types of failures monitoring systems like HUMS or magnetic chip detectors can prevent. Instrumentation like chip detectors or vibration monitors (accelerometers) are only capable of detecting failures after they have initiated. But these types of failures are usually not catastrophic in nature. Vibration monitoring systems can identify which drivetrain component (such as a gear or bearing) is failing based on meshing or passing frequency. But this also requires a detailed acoustic characteristic model of the drivetrain to compare against.
The approach used for flight critical parts like gears, main rotor shafts, hubs, etc. is to design, manufacture and maintain the part such that a structural failure in service is never going to occur (in theory). This means a having detailed fracture control plan for the service life of these components. Unfortunately, even the best QA and maintenance procedures are only effective if they are adhered to.