Originally Posted by
BFSGrad
The National Transportation Safety Board determines the probable cause(s) of this incident to be:
The fatigue failure of the right engine fan blade. Contributing to the fan blade failure was the inadequate inspection of the blades, which failed to identify low-level indications of cracking, and the insufficient frequency of the manufacturer’s inspection intervals, which permitted the low-level crack indications to propagate undetected and ultimately resulted in the fatigue failure. Contributing to the severity of the engine damage following the fan blade failure was the design and testing of the engine inlet, which failed to ensure that the inlet could adequately dissipate the energy of, and therefore limit further damage from, an in-flight fan blade out event. Contributing to the severity of the engine fire was the failure of the “K” flange following the fan blade out, which allowed hot ignition gases to enter the nacelle and imparted damage to several components that fed flammable fluids to the nacelle, which allowed the fire to propagate past the undercowl area and into the thrust reversers, where it could not be extinguished.
NTSB Final Report for UAL 328 / N772UA
I found this rather significant
Simulation studies indicated that the carbon fiber reinforced plastic (CFRP) honeycomb structure of the event engine inlet and inlet aft bulkhead was unable to dissipate and redistribute the energy of the loads imposed by the (Fan Blade Off) event in the same manner as the aluminum structure inlet that was used during certification tests.
This reads to me that the fundamental design of an element that is subject to certification testing was changed, but the new design was not subjected to certification testing.
How can such a thing happen? Was the FAA aware of the design change or was it signed off by a Boeing "FAA Representative"?