Summary
The FAA proposes to supersede an existing airworthiness directive (AD) that applies to all Boeing Model 777 airplanes. The existing AD requires, for the drive mechanism of the horizontal stabilizer, repetitive detailed inspections for discrepancies, repetitive lubrication of the ballnut and ballscrew, repetitive measurements of the freeplay between the ballnut and the ballscrew, and corrective action if necessary. This proposed AD would revise the compliance times of the existing AD. This proposed AD results from a report of extensive corrosion of a ballscrew in the drive mechanism of the horizontal stabilizer on a Boeing Model 757 airplane, which is similar in design to the ballscrew on Model 777 airplanes. We are proposing this AD to prevent an undetected failure of the primary load path for the ballscrew in the drive mechanism of 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.