posted on Jul, 26 2013 @ 09:54 AM
The investigation has taken a turn. The FAA has issued an emergency AD to all 777 operators to check the gearbox of the GE90-115B. Until recently,
the engine has been incredibly reliable, but recently they have been seeing problems in the gearbox.
It was believed that the problem stemmed from a materials failure and was limited to a small number of gearboxes, manufactured after last September,
through about March of this year. GE performed an ECI (Eddy Current Inspection) to check for gear separation within the gearbox. The inspection
showed two more cases of cracking, but didn't highlight materials failure as the problem.
GE began looking at the heat transfer process used in manufacturing the gearboxes as a possible cause, and operators were told to change either the
gearbox in at least one engine, or one engine itself, to keep from having "twinned" gearboxes on one aircraft. That way if one failed, there was
still a separate gearbox operating from a "good" batch.
The investigation was thrown a curveball on July 2nd, when an Incheon bound Korean Airlines 777 was forced to divert to Russia, after a failure that
was almost identical to the prior failures on other aircraft. This aircraft was brand new, and the gearbox for both engines had been produced in June
of this year, several months after
the suspect batch of materials.
The AD orders 20 more 777-300ERs to be "de-twinned" and have at least one gearbox or engine changed with either an earlier produced one of the same
type, or a modified gearbox. GE says the suspect gearboxes are down to approximately 29, and should all be out of service by September.
AvWeek Gearbox article