Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Dornier Model 328-100 and -300 series airplanes. This proposal would require inspecting the identification plate on the fire extinguisher bottle of the auxiliary power unit (APU), and replacing the existing actuating cartridge of the fire extinguisher bottle with a correct actuating cartridge, if necessary. This proposal also would require removing the fire extinguisher bottle equipped with the actuating cartridge from the APU, and reinstalling the fire extinguisher bottle equipped with the correct actuating cartridge into the APU. This action is necessary to prevent failure of the actuating cartridge on the APU fire extinguisher, which could result in the inability to extinguish an APU fire in-flight, and consequent reduced structural integrity of the airplane. This action is intended to address the identified unsafe condition.