Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to all Boeing Model 757 series airplanes. For certain affected airplanes, this proposal would require repetitive testing of the secondary brakes of the horizontal stabilizer trim actuator (HSTA). For all affected airplanes, this proposal would require repetitive overhauls of the primary brake, ballscrew assembly, and differential assembly of the HSTA, which would constitute terminating action for the repetitive testing of the secondary brake. This action is necessary to prevent grease contamination on the primary HSTA brake and consequent loss of the primary brake function, which, in combination with the loss of the secondary HSTA brake function, could result in loss of control of the airplane. This action is intended to address the identified unsafe condition.