October 08, 2012
Credit: Credit: Guy Norris/Aviation Week
Guy Norris London
There are generally few silver linings in a situation where two related engine types, both brand new, fail within weeks of each other.
Yet General Electric, which is dealing with simultaneous issues on the GEnx-1B and -2B, can take what appears to be a shred of comfort from the fact that the two problems are unrelated. While this may seem to be even worse news on the surface, the circumstances behind both failures initially indicate the issues are isolated events that do not require a fundamental redesign.
However, like a good detective story with a twist at the end, it seemed until recently that the two events were closely linked. Initial reports from investigators looking into the Sept. 11 failure of a GEnx-2B powering a Boeing 747-8 in Shanghai seemed to tally uncannily with the circumstances surrounding the July 28 failure of a GEnx-1B powering a 787-8 at Charleston, S.C.
The 787 engine problem was caused by the failure of the GEnx-1B fan mid-shaft (FMS), which was traced to surface contamination. This was thought to have been the inadvertent consequence of a recent switch to a new environmentally friendly coating, and resulted in the low-pressure turbine (LPT) rotor shifting backward, clashing blades and vanes. The shattered parts of the LPT exited the tail pipe, but were contained.
Operators of both GEnx-1B on 787s and the closely related -2B engines on 747-8s were asked to perform voluntary shaft inspections as a result, though GE pointed out the ultrasonic inspections of the fan mid-shaft were aimed at providing additional engineering data for the NTSB-led investigation of the 787 engine failure.
Within days of these checks being completed, however, there was a second failure—the -2B event at Shanghai. This incident, which occurred at high power during an aborted takeoff run, also resulted in LPT failure and parts exiting the engine. However, contrary to the initial impression left by the smoking evidence on the runway at these events, the two were not related.
The FAA issued an airworthiness directive (AD), requiring repetitive ultrasonic inspections of the fan mid- shafts of all -1B and -2Bs. The AD was also introduced following the discovery of a cracked shaft on a second -1B during the inspections. It adds the “root cause is still under investigation, but the failure of the FMS is likely due to environmentally assisted cracking; a type of corrosive cracking that is time- dependent.”