DCA12IA114 · Boeing 787 8

During a taxi test with no intention for flight, as the airplane was approaching 40 knots speed and prior to the engine N1 accelerating beyond 70%, the right engine N1 began to rollback and an EICAS advisory message was displayed indicating an N2 (high pressure rotor speed) exceedance. The flight crew reduced engine power to idle thrust, concluded the taxi test, and shut down the right engine during taxi back to the ramp. Examination of the right engine revealed extensive damage to the low pressure turbine (LPT).

The fan mid shaft (FMS) retaining nut was still connected to the FMS threads but the forward part of the FMS, from the rear of the forward threads, was separated from the remainder of the FMS. Separation of the rotating LPT section from the fan, allowed the LPT section to translate rearward resulting in extensive secondary engine damage.

Examination confirmed separation of the FMS adjacent to the lock nut face located at the aft most full thread root. The fracture exhibited features indicative of multiple failure modes: one progressive, and one instantaneous. About 85 percent of the fracture surface exhibited features consistent with progressive fracture. The remaining fracture surface showed signs consistent with instantaneous failure by overstress. Further examination revealed features consistent with environmentally assisted cracking (EAC) specific to GE 1014 ultrahigh strength steel.

The FMS threads and the retaining nut were coated with a dry film lubricant, and grease or engine oil was used as an assembly aid. Although a lead based dry film lubricant was previously used on GE engine fan mid shafts, during development of the GEnx engine, the design was changed to incorporate a lead free dry film lubricant, and graphite grease instead of the previously used engine oil as an assembly aid.

Testing of specimens taken from the FMS, and comparison to other dry film lubricants used previously on GE 1014 ultrahigh strength steel indicated that the dry film lubricant used on the incident FMS absorbed moisture at a higher rate. Additionally, the combination of dry film lubricant and graphite assembly grease was shown to increase the corrosion rate of GE 1014.

Following the FMS separation that occurred during this taxi test, GE developed an ultrasonic inspection to scan the forward end of the FMS in the area of the threads where the fracture occurred. While conducting the inspections, a GEnx-1B engine installed on another airplane was found to have an indication of a similar crack on the FMS. This airplane had not yet been operated in flight. Follow up testing indicated the crack exhibited features consistent with progressive environmentally assisted cracking.

Verbatim NTSB analysis and probable cause from the NTSB dataset