On September 26, 1999, approximately 1430 Pacific daylight time, a Piper PA-28-180, N9395J, registered to and being flown by a private pilot, sustained substantial damage when a six-inch section of outboard propeller tip separated during cruise flight near Vaughn, Washington. The pilot and three passengers were uninjured. Visual meteorological conditions prevailed and no flight plan had been filed. The flight, which was personal, was to have been operated under 14CFR91, and originated from the Bremerton National airport, Bremerton, Washington, approximately 1400. The pilot reported that while in cruise at 2,000 feet altitude and 2,000 RPM there was a sudden loud noise and vibration. He reduced power and executed an emergency landing to a private airstrip nearby. Post-landing examination of the aircraft revealed that approximately six inches of one propeller blade had separated from the fixed-pitch propeller (refer to photographs 1 and 2). The propeller, manufactured by Sensenich Corporation, was a model 76EM8S5-0-60, fixed pitch, aluminum unit (serial number 25629K). The total time on the propeller was unknown. The owner, however, reported that this propeller had been on this aircraft when he bought the plane about six years previously. The missing tip was not recovered, however, the remainder of the propeller was shipped to the Safety Board's Materials Laboratory, Washington, DC, for further metallurgical examination. The examination revealed a flat, chordwise fracture area that contained crack arrest positions and radial marks typical of fatigue cracking. A portion of the blade, containing the fracture surface, was cut from the remainder of the blade and examined more closely. The fracture features within the fatigue region emanated from an origin area at the leading edge of the blade and adjacent to the fracture surface. The portion showing fatigue cracking features was about 75% of the overall separation surface face. The remaining portion of the fracture surface showed inclined planes, typical of overstress. Bench binocular microscopy revealed that the origin area was located within a notch at the leading edge at the flat side of the blade. The depth of the notch was about 0.008 inch. The leading edge of the blade contained sanding marks, typical of a blend repair, in the vicinity of the notch. The sanding marks did not extend into the deepest part of the notch, directly adjacent to the fracture surface at the origin area. After cleaning, the fracture surface was examined by means of a scanning electron microscope (SEM). Examination of the fracture surface under high magnification revealed striations typical of fatigue. X-ray energy disperse spectroscopy (EDS) of the blade material generated a large peak for aluminum and smaller peaks for copper, silicon and manganese, which were consistent with the specified 2025-T6 aluminum alloy (refer to attached metallurgical report). FAA Advisory Circular 43.13-1A, Section 3 "Repair of Metal propellers," paragraph 585(a) discusses procedures applied to the repair of aluminum propellers. Specifically, this reference addresses rounding out of nicks, scars, cuts, etc. by advising to "...exercise care to remove the deepest point of the injury..." (refer to ATTACHMENT FAA AC 43.13). The propeller and sectioned fracture surface were returned to the owner.