Boeing 777 222 N773UA

The airplane, a Boeing 777-222, experienced a full length fan blade fracture in the No. 2 (right) engine, a Pratt & Whitney (P&W) PW4077 turbofan, while in cruise flight shortly before top of descent. The examination of the No. 2 engine revealed most of the inlet duct and all of the left and right fan cowls were missing. Two small punctures were found in the right side fuselage just below the window belt with material transfer consistent with impact from pieces of an engine fan blade. The examination of the engine's fan blades revealed fan blade No. 11 was fractured transversely across the airfoil directly above the fairings that are between the base of each blade. The other fan blade, which was identified as fan blade No. 10 and was the adjacent trailing blade, was fractured across the airfoil at about midspan. Laboratory examination of fan blade No. 11 revealed a low cycle fatigue (LCF) fracture that originated on the interior cavity wall directly below the surface. The entire fan blade set, including fan blade No. 11 had last been overhauled by P&W's Overhaul & Repair (O&R) facility in July 2015. As part of the overhaul process, the blades underwent a fluorescent penetrant inspection (FPI) and a thermal acoustic imaging (TAI) inspection. The records for the TAI inspection in July 2015 as well as an earlier TAI accomplished in March 2010 revealed a thermal indication in the same location as where the LCF crack occurred. The records for the fractured fan blade's July 2015 TAI inspection was annotated 'paint' that, according to the inspector, was consistent with him accepting the indication because he thought it was an issue with the paint. P&W developed the TAI inspection process in about 2005 to be able to inspect the interior surfaces of the hollow core PW4000 fan blade.  P&W in keeping with NDI industry practice when implementing a new inspection process classified the TAI as a new and emerging technology and therefore did not have to develop a formal program for initial and recurrent training, certify the TAI inspectors, or have a Level 3 inspector on staff, as is done in other established NDI techniques.  But in 2015, and still in 2018 when the incident occurred, P&W was still categorizing the TAI as a new and emerging technology after having inspected over 9,000 fan blades.  At one point, P&W did provide training on the TAI, however, neither of the two inspectors were permitted to attend the training so that they could work to clear out a backlog of blades in the shop. The TAI inspector who worked on the incident fan blade stated that they never got any feedback from the engineers about the blades that they had rejected.  When they would reject a blade, it would go to an engineer for further evaluation.  However, they never got any feedback from the engineers if the rejection was a valid rejection or if it was a false positive. After it was determined that the two previous TAIs of the fractured fan blade showed thermal indications at the location of the fatigue crack, P&W initiated an over-inspection of all of the digital images of the TAIs accomplished on PW4000 112-inch fan blades. Because the aluminum versus the CFRP structure has the ability to yield while absorbing the same amount of energy, it can redistribute the FBO loads between the fan case and the inlet without causing failure to the inlet, or the fan case to inlet interface. The inlet and fan cowl structural analyses showed that the CFRP aft bulkhead design was less capable than the aluminum bulkhead that was tested during engine certification test and determined that multiple possible scenarios could have led to their separation; 1) the inlet aft bulkhead load path damage caused by the unanticipated magnitude of the displacements induced by the displacement wave following the FBO combined with the anticipated inner barrel fragment induced damage progressed under rundown loads, resulting in portions of the inlet departing within one second following the FBO, 2) the departure of portions of the inlet including the lower aft bulkhead caused the static and/or dynamic loads to increase beyond the fan cowls capability, that lead to the departure of large portions of the fan cowl, 3) the fan cowl honeycomb core strength was reduced below its capability to react rundown loads due to moisture ingression at the hinge points leading to large portions of the fan cowl departing prior to the inlets departure.

Quoted verbatim from the NTSB record.