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The test comes after engineers last week checked the cable runs to the flight control surfaces and discovered some areas where they needed to be tightened. This work is also set to take place next week, after completion of the GVT.
Boeing says the delay to first flight from a target date in early December to a more likely date in January cropped up because the "build-up of tolerances between structure designed with older tools and the new design tools has caused fit-up issues that have been discovered and are being resolved in the final assembly process." The 'fit-up' issues are thought to have concerned areas such as the doors for the drop-down ram air turbine, an emergency hydraulic power system not used previously on the 747, as well as the Kreuger leading edge flaps on the new wing.
source: aviationweek.com