On August 26, 1995, at 0945 Alaska daylight time, a wheel equipped Piper PA-28-181 airplane, N2937X, registered to and operated by Skagway Air Service of Skagway, Alaska, experienced a severe vibration during climbout from the Juneau Airport, Juneau, Alaska. The airplane returned to the Juneau airport. The air taxi flight, operating under 14 CFR Part 135, departed Juneau and the destination was Gustavus, Alaska. A company visual flight rules flight plan was in effect and visual meteorological conditions prevailed. The airline transport certificated pilot and the two passengers were not injured and the airplane received minor damage. According to the operator, approximately 6 inches of one propeller blade separated from the propeller. As a result of the vibration, the alternator broke away from the engine case resulting in a small engine compartment fire. The separated propeller tip was recovered. The propeller tip and propeller were sent the NTSB's Materials Laboratory Division for examination. The examination revealed that the area of the fatigue origin was a wedge shaped cavity on the leading edge of the blade. The cavity was oriented approximately 35 degrees to the leading edge of the blade and extended below the surface about 0.03 inch. The total length of the cavity (on both separated pieces) was approximately 0.25 inch. The composition and hardness of the material from which the propeller was manufactured was checked and found to meet the manufacturer's specifications. According to a McCauley Service Letter 1995-4, dated February 10, 1995, it states that stress riser damage caused by stones or other small objects striking the propeller blade as it is rotating is the only damage that needs to be repaired in the field. During a telephone conversation with the pilot, she said she did not see any nicks or gouges in the leading edges of the blades during preflight.