September 14, 2012
General Electric (GE) is preparing to dismantle and inspect a GEnx-2B engine that failed at high thrust during a Sept. 11 takeoff run of an Air Bridge Cargo-operated Boeing 747-8F in Shanghai.
Although GE says it is too early to connect this event with the failure in July of a GEnx-1B engine on a Boeing 787 at Charleston, S.C., the initial evidence indicates several similarities.
The crew rejected the takeoff and returned to the ramp, where visual inspection revealed damage to the low-pressure (LP) turbine. The damage was contained, though just as with the Charleston event, parts of the turbine assembly had exited from the tailpipe.
Inspections of the world fleet of GEnx-1B and -2B engines are almost complete, but the engine involved in the latest event had yet to be checked. “It was one of the few left,” says GE, which expects inspections of the final 747-8F engines to be completed by Sept. 16.
The U.S. engine maker says, “We don’t know if it is the same situation. We expect to have the engine returned in a matter of days for a teardown and investigation. The Air Bridge Cargo freighter had not been inspected.”
Ultrasonic inspections of the fan mid-shaft for all GEnx engines in service are being completed as part of efforts to provide additional engineering data for the National Transportation Safety Board (NTSB)-led investigation of the July 787 incident. This inquiry determined that the Charleston event was caused by the failure of a mid-fan shaft during high-power ground runs on an aircraft destined for delivery to Air India.
The fan shaft connects the fan stage with the LP turbine and is made up of two main sections. The NTSB disclosed that the failure was close to the thread for the torque-retaining nut, which connects the two shaft sections, and that it occurred “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.
However, no details have been released about why the failure occurred. Following metallurgical analysis that ruled out flaws in the shaft material, the focus is believed to have shifted to surface contamination as the prime suspect for the failure. While none of the parties involved are able to confirm this, a statement from GE that it has introduced an “improved coating process to the mid-shaft of new-production GEnx engines” would appear to define this as the most likely cause.