SEATTLE -- The first airline to start flying the Boeing 787 still has two of them grounded due to issues with the gearboxes on the Rolls Royce engines powering the aircraft.

Five of All Nippon Airway's eleven 787s were briefly pulled out of service to check their engines for problems.Three are now back in service; how quickly the remaining two will be returned to service remains unclear.

The problem appears to involve corrosion found in some of the engines' gear boxes. Each engine has two gearboxes that drive electrical generators. The plane's fly-by-wire control system depends on the generators, as well as additional backups.

A spokesman for Rolls Royce said the problem was discovered by the UK-based engine maker during product development testing.

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