As the U.S. National Transportation Safety Board (NTSB) continues its investigation into the July 28 failure of a General Electric GEnx-1B engine on a Boeing 787, operators of 747-8s powered by sister version GEnx-2B are halfway through inspections with no further issues discovered.

GE in August issued service bulletins for voluntary inspections of the 92 GEnx engines in service on the 787 and 747-8 fleets, prompted by the failure of an engine on a 787 at Charleston, S.C. Only eight of these engines were in service at the time, on four Japan Airlines 787s, and those powering Ethiopian Airlines’ first 787 were inspected before the aircraft’s recent delivery.

The remaining 84 are -2Bs powering 747-8 passenger and freighter models.

The service bulletins called for ultrasonic inspections of the fan midshaft, which the NTSB earlier identified as the component at the heart of the July 28 event.

The fan shaft connects the fan stage with the low-pressure (LP) turbine and is made up of two main sections. Evidence from the damaged engine indicates the likely source of the failure was close to the thread for the torque-retaining nut that connects the two shaft sections.

The safety board says the engine “fractured at the forward end of the shaft, rear of the threads where the retaining nut is installed.” The fracture allowed the rotating LP turbine to move aft, clashing with the LP stators and causing parts to be jettisoned from the exhaust.

Metallurgical inspections and review by a team of experts from the NTSB, the FAA, Boeing and GE discovered the small fracture leading to the aft part of the Ishikawajima Heavy Industries-made shaft assembly.

Sources close to the NTSB investigation say metallurgical and design issues have now largely been discounted. The focus is thought to be shifting to potential surface contamination as a prime suspect.

GE says the inspections are being performed on-wing, adding that the checks “are not driven by a safety concern, but to provide GE Aviation with additional engineering data to use in the investigation with the NTSB.”

The engine maker says it has no plans to redesign the shaft or the thread.