Closer inspection of a General Electric GEnx-2B engine that failed last month on a Boeing 747-8F indicates that a wrongly assembled turbine part was the likely culprit, rather than the contamination suspected as the cause of a very similar GEnx-1B problem on a 787-8 in July.

The -2B failed as an Air Bridge Cargo-operated Boeing 747-8F was making its takeoff run in Shanghai, China, on Sept. 11. A visual inspection revealed damage to the low-pressure turbine (LPT), which at first appeared closely related to the problem experienced on a sibling -1B engine on July 28. Both engines were operating at high thrust at the time, and suffered a failure of the fan mid-shaft, which fractured just aft of the coupling nut. The LPT rotor shifted axially backwards, resulting in blade and vane clashing.

In both cases, the LPT case contained the failure and debris was released out of the tailpipe. In the case of the first incident, there also was “no engine over-speed as the LPT rotor remained coupled to the fan rotor at the FMS spline,” says the U.S. National Transportation Safety Board. GE, meanwhile, is not commenting on the issue, but operators say that plans are under way to make the engine serviceable with a new LPT section.

Coincidentally, the engine involved in the Shanghai incident also was one of the few GEnx units remaining to be inspected following the 787 event in July. Ultrasonic inspections of the fan mid-shaft for all GEnx engines in service were being completed at the time as part of efforts to provide additional engineering data for the NTSB-led investigation of the 787 engine failure.

The FAA subsequently issued an airworthiness directive (AD) requiring repetitive ultrasonic inspections of fan mid-shafts (FMS) of GEnx-1B and -2Bs. The agency comments the AD also is being 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.” This is believed to be a reference to a new type of lubricant briefly introduced in the engine, but subsequently replaced with an earlier form of the product.