GEnx low-pressure turbine problems create high-pressure issues for the engine maker
There are generally few silver linings in a situation where two related engine types, both brand new, fail within weeks of each other.
Yet, which is dealing with simultaneous issues on the -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 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 onwere 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 -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.”
Further investigations of the 747-8 engine anomoly then revealed that the -2B's FMS was not broken and that damage was limited to the LPT.
The NTSB, which was leading the Shanghai investigation with the Chinese aviation authority, then issued a statement to correct the impression the two were linked. It says “the preliminary examination of the engine revealed damage that was primarily limited to the LPT hardware at the rear of the engine. Aside from minor collateral damage, the engine hardware forward of the LPT (including the neighboring high-pressure turbine) does not exhibit damage.”
The NTSB also adds “there was no indication of damage to the fan mid-shaft. We are continuing to work with the CAAC and GE to determine the reason for these observations.”
GE then issued a service bulletin on Oct. 3, calling for -1B and -2B operators to do a one-time, fleet-wide borescope inspection of the LPT stage-one static nozzles—which direct air into the rotating blades—to ensure they are properly installed. The investigation seems to indicate that a misassembled LPT stage-one nozzle was the culprit.
In the meantime,CEO Akbar Al-Baker was quoted by Reuters on Oct. 1 as threatening to further delay formal acceptance of his airline's first 787 until the new aircraft ready for delivery receive the new coating. By Oct. 3, however, it appeared this threat was an empty one, and preparations were underway for an expected delivery ceremony in mid-October. Qatar's 787 deliveries were originally expected to begin earlier in the summer but were delayed by issues with integration of the inflight entertainment system.