A crack in a component connecting the fan booster to the low-pressure turbine led to the contained failure of a General Electric GEnx-1B engine during a Boeing 787 taxi test on 28 July, the US National Transportation Safety Board (NTSB) says today.
The NTSB continues to look for a root cause of the crack, but released the first facts of the highly-publicised engine failure on the runway at North Charleston, South Carolina.
The fan mid-shaft assembly, which is supplied by Japan's IHI, cracked in the region where the shaft extending from the low-pressure turbine connects with a torque nut, the NTSB says. On the other side of the torque nut are the fan booster stages, which are driven by the shaft power produced by the low-pressure turbine.
The fractured mid-shaft assembly moved the shaft slightly backward, causing the rotating low-pressure turbine blades to collide with the stationary stator vanes immediately behind. The fragments of the blades and vanes were ejected out the back of the engine, igniting a small grass fire immediately behind the 787.
The NTSB noted the crack led to a "contained failure," meaning all the fragments were ejected out of the back of the engine as designed, and not out of the side of the fan case.
GE explains the failure mode for the cracked mid-shaft assembly performed as designed. By moving slightly backward, the action prevents an overspeed condition, which could have led to an uncontained ejection of fragments through the engine fan case.
The NTSB is still analysing the metallurgy of the mid-shaft assembly to determine the cause of the fracture that led to the contained failure. The agency's investigators are also checking the engine's manufacturing and assembly records.
In hundreds of millions of service hours on GE engines, the company has recorded only five or six incidents of cracking in the fan mid-shaft assembly, GE says. The failures have been traced to a variety of causes, including contamination caused by poor handling and oil leaks, as well as other components rubbing against the shaft, it adds.
GE also emphasises that it remains unaware of any issues that could affect the entire GEnx fleet, which currently powers two Japan Airlines 787s and more than 20 747-8s.
The engine involved in the incident belonged to one of three 787s being readied for delivery to Air India. It was manufactured to the original Block 4 certification standard. But the newer certificated standard, called the performance improvement package (PIP)-1, shares the same design for the fan mid-shaft assembly, although the low pressure turbine blades have been redesigned.
The NTSB is also analysing the cockpit voice recorder and flight data recorder for clues to the root cause of the incident.