U.S. National Transportation Safety Board (NTSB) investigators believe the failure of a General Electric GEnx-1B engine on a taxiing Boeing 787 July 28 stemmed from the failure of a fan mid-shaft.

The fan shaft forms the low-pressure spool of the GEnx-1B engine and connects the fan stage with the low-pressure (LP) turbine. The shaft is made up of two main sections, and the failure is thought to have emanated in the torque-retaining nut connecting the two. The NTSB says the GEnx engine “fractured at the forward end of the shaft, rear of the threads where the retaining nut is installed.” It adds that the fan mid-shaft is “undergoing several detailed examinations, including dimensional and metallurgical inspections.”

Investigators and a team of experts from the NTSB, FAA, Boeing and GE specializing in engine systems and metallurgy found the small fracture leading to the aft part of the shaft, which is made by Ishikawajima Heavy Industries of Japan.

The fracture in the shaft assembly allowed the rotating LP turbine to move aft, clashing with the LP stators. The impact caused significant damage to the LP turbine section, pieces of which were jettisoned from the engine exhaust. The hot parts exited the engine and sparked a grass fire by the runway which caused the airport at Charleston, S.C., to be briefly shut down. The engine was powering a 787 on a pre-first flight high speed taxi run. The aircraft was the second to be completed at Boeing’s Charleston site and is destined for delivery to Air India.

The fan shaft is designed to separate in extreme stress events to avoid over-speeding the LP turbine and risking an uncontained failure.

GE, meanwhile, repeats that it “continually monitors and analyzes the performance of the GEnx fleet in service, and we are not aware of operational issues that would hazard the continued safe flight of aircraft powered by these engines.”

An analysis of other shaft-related events on around 25,000 GE and CFM engines in service over the past 10 years and 600 million flight hours shows just six shaft fracture incidents have taken place.

While no specifics are being discussed about potential causes of the fracture, previous incidents are known to have been caused by corrosion, rubs or pressure of the nut on the shaft.  

The NTSB adds that “investigators will continue the detailed examination of the engine and metallurgical analysis of its components. Investigators have also begun reviewing the engine manufacturing and assembly records.”