Airworthiness Directives; Allison Engine Company Model 250-C47B Turboshaft Engines

This amendment supersedes an existing airworthiness directive (AD) 97-21-09, applicable to Allison Engine Company Model 250-C47B turboshaft engines, that currently requires replacing the engine main electrical harness assembly with an improved assembly, installing a new hydromechanical unit (HMU) and electronic control unit (ECU), removing the placard notifying the pilot that the overspeed protection system is disabled, and revising the Bell Helicopter Textron, A Division of Textron Canada Ltd. (BHTC), Model 407 Rotorcraft Flight Manual (RFM). This amendment continues the requirements of the current AD, but adds the requirement to install ECUs with improved resistance to corrosion. This amendment is prompted by reports of ECUs with annunciated hard faults due to corrosion on internal connectors. The actions specified by this AD are intended to prevent uncommanded inflight engine shutdowns, which can result in autorotation, forced landing, and possible loss of the helicopter.