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

This amendment adopts a new airworthiness directive (AD) that is applicable to Allison Engine Company Model 250-C47B turboshaft engines. This action supersedes priority letter AD 96-21-12, applicable to Bell Helicopter Textron, A Division of Textron Canada Ltd. (BHTC) Model 407 helicopters, that currently prohibits further flight, due to uncommanded inflight engine shutdowns. This action 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, this 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 BHTC Model 407 Rotorcraft Flight Manual (RFM); and requires maintenance actions to clear the engine electronic control unit (ECU) of faults prior to each flight. Accomplishment of these actions will enable operators to resume flight operations. This amendment is prompted by investigation into the causes of the inflight engine shutdowns. 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.