General Electric (GE) says transfer gearboxes on additional GE90-115B engines are having to be replaced after a suspected failure in one such unit on July 2 forced a Korean Air Boeing 777-300ER to divert to Anadyr in Russia while enroute from Chicago to Incheon, South Korea.

The failure is the third such event for the GE90-115B so far this year and follows an in-flight shutdown in February on an Aeroflot 777-300ER enroute to Moscow from Bangkok, and a similar event on May 9, when an Air China flight from Beijing to Paris diverted to Stockholm after the left engine had to be shut down in flight. The affected part is in the transfer gearbox, which transfers energy from the engine core to drive the accessory gearbox.

Following the May incident, GE issued a service bulletin, which called for the replacement of either the transfer gearbox assembly or the entire GE90 engine in order to “de-twin” the affected fleet of 777s within five days. The action, which was later mandated by the FAA in an airworthiness directive (AD), ensured that at least one of the two engines on each 777 had a transfer gearbox from outside the suspect population or was equipped with one which had been inspected. Although no specific cause was identified at the time, GE and Avio, the GE-owned Italian supplier of the gear system, suspected a material “anomaly” as the culprit.

The problem resulted in radial gear cracking and separation and was thought to be isolated to a specific batch of gearboxes made between September 2012 and March 2013. However, with the recent Korean Air incident, which impacted a newly built 777, the suspect group has been expanded to include gears assembled as recently as June.

This also led GE to recommend that operators de-twin 20 more 777-300ERs by replacing the transfer gearbox assembly with an earlier model or with a modified version. That action, which was also mandated by a new FAA AD issued on July 12, is already virtually complete, with only five more aircraft remaining to undergo the process, according to GE.

When the materials anomaly was originally suspected as the cause earlier this year, GE employed an enhanced Eddy Current Inspection (ECI) process to check for possible gear separation within the transfer gearbox. According to the FAA, these inspections found two additional gears with cracks, prompting the issuance of another AD on June 26. Under this directive, operation of any GE90-110B-powered 777-200LR or GE90-115B-powered 777-300ER with affected transfer gearbox assemblies on both engines was prohibited from around July 1, the day before the Korean Air incident.

However the ECI approach did not pinpoint material issues as the cause, so attention switched to the heat-transfer process used in the making of the gearbox housing. Investigations pointed to the process as the possible cause of surface stress on the gear which, in turn, led to radial cracking and the separation. To counter this, from June onwards, Avio has been treating all transfer gearbox housings with a cold working process called shot peening, which entails impacting the surface with high-velocity shot that plastically deforms the metal, producing a surface more resistant to fatigue. The Korean Air aircraft involved in the July 2 diversion is the only one of the airline’s fleet of 15 777-300ERs that was fitted with transfer gearboxes from the suspect population of 132 gearboxes. Both units were replaced, and the aircraft has returned to service, adds GE.

The Korea Aviation and Railway Accident Investigation Board delegated the investigation of the July 2 in-flight shutdown to the NTSB, which is working with GE and the FAA to determine the root cause. GE adds the “final probable cause” for the July 2 event will be determined by the NTSB.