So to this dumb old pilot, this is what it sounds like happened:
They were doing high speed single-engine cuts. During the last one, the MRRPM decreased to 91%. The pilot lowered the collective but the NR only came back up to 92%. The blade spacing got out of phase (which we know happens anyway with multi-blade rotors in forward flight) and a bad vibration began, apparently a vertical because they got into "biomechanical feedback" (which is I guess a fancy way of saying PIO) in the collective channel.
Things happen fast when they go wrong in helicopters.
The pilot of the chase helicopter radioed that the 525 was getting some serious blade flapping, but apparently it was too late. The thing came apart.
Part of the problem was that the 525 pilots couldn't quickly and easily get the second engine back online to get the MRRPM up. Computers, gotta love 'em! In a normal helicopter they could've just rolled the throttle up or pushed the lever forward. Not in these newfangled crash-by-wire birds! Not only that, apparently they couldn't see how low the NR had gotten because the cues on the display are rather small. Not that it would've helped if the vibration was preventing them from pushing whatever (presumably small) button to take them out of "simulated OEI" operation.
But it makes you wonder, doesn't it? I mean, how does an R-22 pilot get his MR back if it droops too low? Right, he pulls back on the cyclic and loads the rotor! Sometimes just lowering the collective doesn't do the trick. And so, because the 525 ON A TEST FLIGHT *inexplicably* did not have a working CVR, we will never know what the pilots were talking about and why they didn't "simply" raise the nose and get the speed down and the rpm up.
Bell says that they did not anticipate how bad the vibration would get at high-speed and low NR. Really? Hmm. I wonder why not? Maybe they should've have consulted with Sikorsky. Those guys up in Connecticut have much more experience designing and building multi-blade, high-speed rotors, something that Bell seems kind of new and inexperienced at.
Heh. "Biomechanical feedback."