Airworthiness Directives; Pilatus Aircraft Ltd. Models PC-12 and PC-12/45 Airplanes

This amendment clarifies information contained in Airworthiness Directive (AD) 98-11-01 R1, which currently requires replacing the fuel tank vent valves and drilling a 4.8 millimeter (0.1875 inch) hole in each fuel filler cap on certain Pilatus Aircraft Ltd. (Pilatus) Models PC-12 and PC-12/45 airplanes. AD 98-11-01 R1 also requires inserting a temporary revision in the Pilot's Operating Handbook (POH) that specifies checking to assure that the fuel filler cap hole is clear of ice and foreign objects. The intent of AD 98-11-01 R1 was to provide the option of incorporating new modified fuel tank vent valves instead of the replacement, drilling, and POH insertion requirements. The new modified vent valves cannot be incorporated without removing the fuel tank vent valves installed as originally required by AD 97-23-04 and carried over in the current AD. This provision of removing those vent valves is currently not provided for. This document clarifies and corrects this procedure. The actions specified in this AD are intended to continue to prevent moisture from entering the fuel tank inward vent valve and then freezing after a cold soak at altitude, which could result in wing airfoil distortion and structural damage with consequent degradation of the airplane's handling qualities.