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

This amendment revises Airworthiness Directive (AD) 98-11-01, 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 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. This AD maintains the requirements of AD 98-11-01, and adds the option of modifying the fuel tank vent valves instead of the drilling and POH requirements carried over from AD 98-11-01. This AD is the result of mandatory continuing airworthiness information (MCAI) issued by the airworthiness authority for Switzerland. 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.