MarkerInbound,
." Because of the immediate threat of multiple engine failures on a single aircraft and the availability of an appropriate inspection procedure, there is an urgent need for the FAA to act immediately."
NTSB...
The threat would appear that the crack can progress without load, withut even being on wing. It is "environmental" in nature, the report claims that the steel used in this Fan shaft is susceptible to atmospheric Hydrogen causing an accelerated galvanic corrosive propagation of cracking. They also are cautious that ETOPS is affected, should the shaft fail, and the remaining engine may be subject to 5.5 hours of operation OEI.
This would appear on the face of it to be at least as problematic as the RR TRENT problem with spline wear at the shaft efface.
The link is instructive... and direct, no soft pedalling, as it should be.
regards