You make an important point about exceedance monitoring for non-FADEC engines. I admit I have been very spoiled since everthing I work on these days is FADEC or at least electronically controlled. I also don't deny that there are certain faults that HUMS will luckily catch. But that's not what HUMS was designed for (or priced for), and it's somewhat like using a sledge hammer to drive a thumb tack. There are cheaper systems that will do those things. (A low cost engine monitor for example.)
There will always be the odd anecdotal thing that happens to get caught by the HUMS. I'm sure the HUMS salespeople don't peddle the system as a tubine shi.. ingestor detector. But even there, you note that the problem was significant enough to be detected an reported by the pilot. You mention workmanship errors and some other odd problem conditions. I know everyone will jump on me for this - but it's still dumb luck. If we look at the spectrum of defects likely to escape the process already in place, there are very few of those which would likely be picked up by HUMS. I can find a lot of very tragic incidents in that same vein that wouldn't have been caught by HUMs.
As for Nick, he always goes me one further. (He's more spoiled than me) where it's easy to sensor everything. I'm still working enough on the lights and mediums where sensors and wire (lightning protected wire these days) adds up in dollars and pounds pretty fast. That regime recognition is still a really useful way to keep down the expense and make the technology accessible.
Fact remains that even with regime recognition, HUMS is a major intrusive system on a "conventionally instrumented" (steam gauge) aircraft. On a data-bussed aircraft, regime recognition-based HUMS can be added for next to nothing - and could be done as basic equipment. That's where I see HUMs finally being able to deliver on its promise.
On "regime recognition." Nick, who was that PhD at Sikorsky that coined the term. I recall him as the genuine stereotypical mad scientist, and I always consider him the father of HUMs. I was rivetted by his presentation at Sikorsky back in '81 or '82 was it? In a way, more of that concept has worked its way into how our cars deploy air bags than into our helicopters. shame.
I hope ben123 has gotten what he wanted out of this.
Later...