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

This amendment supersedes existing airworthiness directive (AD) 96-24-09, applicable to Allison Engine Company Model 250-C47B turboshaft engines, that currently requires replacing the engine main electrical harness assembly with an improved assembly, disabling the overspeed solenoid, inspecting the engine control unit (ECU) internal PW10 voltage to determine electrical noise characteristics, and replacing units not considered serviceable. In addition, the existing AD requires adding a placard to the helicopter instrument panel notifying the pilot that the overspeed protection system is disabled and removes a placard which was required by priority letter AD 96-21- 12; revises the Bell Helicopter Textron, A Division of Textron Canada Ltd. (BHTC) Model 407 Rotorcraft Flight Manual (RFM); and requires maintenance actions to clear the ECU of faults prior to each flight. This amendment continues to require replacing the engine main electrical harness assembly with an improved assembly, but adds the requirements to install a new hydromechanical unit (HMU) and ECU, removing the placard notifying the pilot that the overspeed protection system is disabled, and revises the BHTC Model 407 RFM. This amendment is prompted by the development of overspeed protection system modifications to reactivate the overspeed solenoid in conjunction with raising the power turbine overspeed trip point and revising the overspeed system to default to a minimum fuel flow in the event of its activation. 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.