A group of airlines operating mostly brand new Boeing 777-300ERs powered by General Electric GE90-115Bs are either replacing entire engines or transfer gearboxes on individual units after GE alerted them to the potential for a gear separation.

The engine manufacturer has issued two service bulletins (SBs) to GE90-115B operators after two in-flight shut downs due to gear separation within the Avio-made gearboxes. The first event occurred on Feb. 11 when an Aeroflot 777-300ER shut down an engine enroute to Moscow from Bangkok, and the second took place 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.

Although no specific root cause has yet been identified, GE says a material “anomaly” has been identified during early analysis of the transfer gearboxes concerned.

The problem is restricted to a batch of 118 recently manufactured transfer gearboxes made over a six-month period from September 2012 to March 2013. The immediate focus is on 26 aircraft which have both of their GE90-115Bs equipped with the suspect units.

The SBs call for the replacement of either the transfer gearbox or the entire GE90 engine in order to “de-twin” the affected fleet of 26 777s within five days. GE says “this action ensures that at least one engine has a transfer gearbox from outside of the suspect population or which has received enhanced eddy current inspections.” The gearbox swap-out takes between eight and 12 hours per engine.

GE says it is “aggressively shipping replacement transfer gearboxes to customers which have undergone an enhanced Eddy Current Inspection (ECI) to assist in the replacement efforts.” The engine maker is also developing an on-wing ECI to check for possible gear separation within the transfer gearbox, and along with its soon-to-be-acquired partner company Avio, has also developed an enhanced ECI process which is being used on all new transfer gearboxes installed on GE90-115B production engines.

Despite the large number of gearboxes in the suspect batch the actual number of aircraft impacted remains relatively small because of the distribution of the units among the engine fleet. Some 32 of the gearboxes are either on engines that are for spares or which have yet to be delivered. Aside from 26 of the engines that are subject to immediate removal or gearbox exchange, the remaining 60 engines include 34 ‘singles’ that are paired with engines not affected by the SB. The balance of 26 engines are those paired with the engines now being either removed or being fitted with replacement gearboxes. These will be subjected to subsequent on-wing ECI checks.

The FAA is widely expected to issue an airworthiness directive on the matter shortly.