Originally Posted by
EEngr
Just guessing here: The fan hub to shaft coupling is designed to withstand quite a bit of thrust (produced by the fan). But the fan hub to LP compressor barrel coupling not nearly so much. It's more for the torque of spinning the compressor. So an initial failure of the shaft to fan (where a jagged piece remains) would allow the fan to pull forward. The 24 (?) bolts holding the compressor stage failed under tension (photo in #194). That may be by design. To let the fan go forwardvwithout trying to pull the compressor rotor through the stator.
Like 'lomapaseo' I'm inclined to think that purposeful design of the fan to separate is unlikely, but to me your sequence of failure sounds right. I do find it surprising however that the bolts at the compressor flange all failed.
My armchair opinion is that the bolts in the flange were not tightened / sized / put in correctly.
A failure of the driveshaft would pull the whole LP compressor forward which would either pull it through the guide vanes and out the front of the engine with the fan. Or the guide vanes (stators) would hold (as they have done) and prevent the LP compressor from moving out of the engine. In this case the deceleration of the LP compressor and the fan would be high, but I doubt high enough to severe the bolts at the LP compressor flange as cleanly as what has occurred.