The European Aviation Safety Agency (EASA) issued an emergency airworthiness directive (AD) for the Rolls-Royce Trent 1000 engine on July 25, several days after All Nippon Airways was forced to ground five of its Boeing 787 Dreamliners when the engine maker flagged up problems during product development testing. The Trent 1000 powers all of the 787s operated by the Japanese carrier. EASA said the AD action, requiring replacement of a specific gearbox assembly on some engines, was taken after testing at Rolls-Royce indicated possible transfer gearbox (TGB) failures on Trent 1000s. An engineering investigation determined that the crown wheel gear of the TGB had failed due to corrosion apparently made possible as the result of a change to the engine's manufacturing process. Rolls-Royce was able to identify the specific engines involved and focus the AD on only those requiring the new parts. By July 30, all five 787s had been returned to service, according to an All Nippon Airways spokesman.