Federal investigators said an aging part fractured and caused a vintage plane to crash into the ocean off Portland Head Light last summer, killing the doctor piloting the aircraft. The National Transportation Safety Board said in its report released earlier this month that the 1946 Stinson Voyager suffered ‘‘a fatigue failure of the No. 3 piston skirt,’’ causing a total loss of engine power. The Sun Journal reported that the NTSB said that although the piston was overhauled about five years before the accident, the part was more than 65 years old. The pilot, 60-year-old Dr. Louis Hanson of Durham, died.