The FAA tomorrow will issue a final airworthiness directive (AD) requiring operators of 11 General Electric GEnx turbofan engines to perform ultrasonic inspections of the two-spool engines every 90 days.

Although the AD also calls for U.S. operators to perform the inspections “before further flight,” GE reported that as of Sept. 18, all GEnx-1B and GEnx-2B engines in operational service had been inspected.

The directive comes after an “urgent recommendation” from the U.S. National Transportation Safety Board (NTSB) for immediate and repetitive inspections on the engine family, which powers the Boeing 787 and 747-8.

The NTSB request followed an initial investigation into a contained GEnx-1B engine failure on a 787 while in ground testing in July at Boeing’s final assembly facility in Charleston, S.C. The failure involved cracks in the fan mid-shafts (FMS) of the engine’s low-pressure turbine. An inspection of a second GEnx-1B on-wing in August also turned up an FMS crack.

GE issued a service bulletin on Sept. 14 calling for initial inspections within 30 days, followed by repetitive inspections every 90 days thereafter. The engine-maker says the inspection can be done on-wing and takes “two to three hours” to complete.

While a root cause has not yet been found, the NTSB says the fractures indicate an “environmentally assisted cracking,” where crack initiation and growth “is a product of galvanic corrosion in a moist environment which may generate hydrogen.”

GE says it has introduced changes in the production process that “address environmentally assisted cracking, including changes to the dry-film coating applied to the mid-shaft at the manufacturing stage, as well as changes to the assembly lubricant used when the retaining nut is clamped to the mid-shaft.” The engine maker says the “change to a different coating, which has already been certified on other GE engines, is FAA-approved for GEnx production.”

A GEnx-2B engine failure on a 747-8F taking off in Shanghai on Sept. 11 has also been found to have damage in the low-pressure turbine, though GE says it has “not been determined whether the event involved a fracture of the fan shaft.”