GE has finished the first round of checks on all in-service and spare GEnx turbofans ahead of the September 21 publication of a U.S. Federal Aviation Administration airworthiness directive (AD) that calls for ultrasonic inspection of early-build engines’ fan midshafts every 90 days.

GE has developed a field ultrasonic method to inspect the suspect area of the engines while they remain on the airplanes. The company said the entire process takes three technicians between two and three hours to complete, or nine work-hours per engine as indicated in the AD.

Boeing 787 customers have taken delivery of 14 GEnx-1B engines and 747-8 customers have taken 114 GEnx-2Bs.

The AD comes a week after the National Transportation Safety Board recommended that the FAA address separate incidents of fracture and a crack in a specific area of the fan midshaft in GEnx-1Bs installed on Boeing 787s. The first, involving an Air India 787 performing ground tests on July 28 in Charleston, South Carolina, resulted in a contained failure that caused a grass fire next to a runway at Charleston International Airport. On August 31 an ultrasonic inspection revealed cracking in the same area of the fan midshaft in a GEnx-1B on another 787 that hadn’t yet flown.

Both parts developed fractures and/or cracks at the forward end of the shaft, rear of the threads where assembly calls for installation of a retaining nut. The GEnx engine uses a “dual shaft” design, meaning one shaft connects the compressor spool at one end to the high-pressure turbine spool at the other end. A longer “fan shaft” connects the fan and booster in the front of the engine to the low-pressure turbine in the back.

Investigators have yet to determine the cause of a third GEnx failure, involving a GEnx-2B attached to an AirBridgeCargo 747-8 that lost power during a takeoff roll in Shanghai, China, on September 11. Although a visual inspection revealed damage to the low-pressure turbine, and the NTSB said the failure appeared “similar in nature” to the earlier incidents, investigators haven’t concluded whether or not it originated from a fracture of the fan midshaft.

As a result of findings to date, GE has introduced changes in the production process that address so-called environmentally assisted cracking, or cracks caused by galvanic corrosion in a moist environment. A GE spokesman told AIN that the company has returned to using the same midshaft dry-film coating it used for the GE90-115B. The company abandoned it for the GEnx in an effort to lessen environmental concerns associated with lead use. “The FAA is aligned with GE on the changes to the coatings,” he said, referring to the return to the older coating on the GEnx. The company also changed the lubricant used when technicians clamp the retaining nut to the mid-shaft during the assembly process.