Interesting technical discussion as to how the faults happen but as an operator I'm more interested in what I'm likely to see if the FCC starts going sideways and what I can do about it.
Okay first stipulating that the recent test failure had nothing to do with what happened to the accident aircraft, it does bring up an interesting issue. The test simulated a runaway trim with the A/P engaged. The first sign of this problem would likely be a "Stab Out of Trim" light illuminating on the forward instrument panel. This light indicates that the elevator deflection has exceeded a certain amount because the stabilizer is not properly trimmed. The structure of the non-normal checklist seems to assume that the problem may be that the A/P is not trimming sufficiently for the airspeed changes and NOT that there is a runaway trim. Specifically, if the trim wheel is moving the checklist basically says do nothing because it assumes that the A/P is catching up with the trim changes. Otherwise if the trim is not moving, then disengage the A/P and trim with the yoke switches. This action assumes the A/P was not trimming when it should have, like a change in speed, turn, etc., rather than trimming when it shouldn't have as in a runaway trim.
Thus the current procedures actually lead down the path of not intervening if the trim wheel is moving with the A/P engaged and a "Stab Out of Trim" light illuminated which would actually delay the pilot response to a runaway trim. At some point the pilot would have to make the determination that it is actually a runaway OR the A/P will eventually disconnect with a gross out of trim condition. I could see how this could create a potentially hazardous condition that would actually be harder to recover from than a runaway with the A/P off. Perhaps this NNC now needs to be reevaluated.