Airworthiness Directives; Bombardier Model CL-600-2B19 (Regional Jet Series 100) Airplanes

This amendment supersedes an existing airworthiness directive (AD), applicable to certain Bombardier Model CL-600-2B19 (Regional Jet Series 100) 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 amendment adds 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 terminates the requirements of the existing AD. This amendment also limits the applicability of the existing AD. This amendment is prompted by issuance of mandatory continuing airworthiness information by a foreign civil airworthiness authority. The actions specified by this 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.