Summary
This document proposes the supersedure of an existing airworthiness directive (AD), applicable to certain Bombardier Model CL-600-2B19 (Regional Jet Series 100) series airplanes, that currently requires a revision to the Airplane Flight Manual (AFM) to prohibit the use of mach trim and to add speed restrictions if the autopilot is disengaged or inoperative. That AD also requires installation of an associated placard. This proposed AD would add requirements for replacement of the horizontal stabilizer trim control unit (HSTCU) with a new HSTCU, and reactivation of the mach trim engage/disengage switch/light (if deactivated). Accomplishment of these actions would terminate the requirements of the existing AD. This proposed AD also would limit the applicability of the existing AD. This proposal is prompted by issuance of mandatory continuing airworthiness information by a foreign civil airworthiness authority. The actions specified by the proposed AD are intended to prevent deficiencies of the HSTCU, which could result in a nose-up trim runaway when a single component in the mach trim circuit fails.