Summary
This action revises an earlier proposed airworthiness directive (AD), applicable to Pratt & Whitney Canada PT6A series turboprop engines, that have certain turbine exhaust ducts that were modified by Standard Aero Limited (SAL) of Winnipeg, Canada before September 1, 1997. That proposal would have required initial and repetitive inspections for cracks and, if necessary, replacing the turbine exhaust duct if the cracks exceed allowable limits. That proposal was prompted by reports of cracks along the weld seams of certain turbine exhaust ducts. This action revises the proposed rule by requiring inspections for low-quality welds and cracks, of a larger population of turbine exhaust ducts than those modified by SAL. The actions specified by this proposed AD are intended to prevent failure of the turbine exhaust duct due to cracking that could result in possible separation of the reduction gearbox and propeller from the engine, and possible loss of control of the airplane.