Airworthiness Directives; Boeing Model 747 Airplanes

The FAA is adopting a new airworthiness directive (AD) for all Boeing Model 747 airplanes. This AD requires the following actions for the drive mechanism of the horizontal stabilizer: Repetitive detailed inspections for discrepancies and loose ball bearings; repetitive lubrication of the ballnut and ballscrew; repetitive measurements of the freeplay between the ballnut and the ballscrew; and corrective action if necessary. This AD results from a report of extensive corrosion of a ballscrew in the drive mechanism of the horizontal stabilizer on a similar airplane model. We are issuing this AD to prevent an undetected failure of the primary load path for the ballscrew in the horizontal stabilizer and subsequent wear and failure of the secondary load path, which could lead to loss of control of the horizontal stabilizer and consequent loss of control of the airplane.