Airworthiness Directives; Bombardier Model DHC-8-100, -200, and -300 Series Airplanes

This amendment adopts a new airworthiness directive (AD), applicable to certain Bombardier Model DHC-8-100, -200, and -300 series airplanes, that requires installation of a backup pressure regulating valve on the oil pump of the propeller control unit (PCU) on both engines. The actions specified by this AD are intended to prevent a build-up of oil pressure in the oil pump of the PCU should the existing valve fail. Such failure of the pressure regulating valve could lead to oil leaks, fracture of the pump, inability to maintain engine oil pressure, and inability to feather the propeller, with consequent reduced controllability of the aircraft. This action is intended to address the identified unsafe condition.