Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to all Boeing Model 777 series airplanes. This proposal would require either a one-time inspection or a review of the airplane maintenance records for both stabilizer trim control modules (STCM) of the trim system of the horizontal stabilizer to determine if STCMs having certain serial numbers are installed; and follow-on corrective actions, if necessary. This proposal also would require eventual replacement of affected STCMs with new or reworked STCMs, which would terminate the follow-on actions. This action is necessary to prevent an uncommanded stabilizer trim due to simultaneous failure of two static seals on one STCM, combined with failure of the automatic shutdown function of the stabilizer trim system. Such failures could result in loss of pitch control and consequent loss of control of the airplane. This action is intended to address the identified unsafe condition.