On March 21, 1997, about 1715 Alaska standard time, a wheel equipped Beech G-18S airplane, N555CG, sustained substantial damage when approximately eight inches of a propeller tip separated from a propeller blade on the left engine and penetrated the nose of the airplane, coming to rest in the right engine nacelle. The solo airline transport certificated pilot was not injured. The 14 CFR Part 135 cargo flight departed Nome, Alaska, about 1715, and was en route to Gamble, Alaska. According to the operator's chief pilot, the pilot of the accident airplane reported the propeller tip separation occurred shortly after reaching cruise altitude. The pilot shut down the left engine, returned to Nome and landed without incident. An examination of the remaining portion of the propeller blade disclosed a small nick on the surface of the blade that coincided with the juncture of the fracture plane. The accident airplane received damage to a former in the nose section, just in front of the radio bay.