Incident Overview
During the initial climb at 16,000 feet, the pilot noticed oil droplets forming on the windscreen. Upon observing the oil, the pilot contacted air traffic control, declared an emergency, and initiated a standard rate, 180-degree turn back to the airport. As the turn was completed, a "boom" was heard, accompanied by a shudder, and the windscreen became covered with oil. The pilot reported a possible pitch-up at the sound but immediately obtained the best glide speed of 90 knots and retraced the ground track using the global positioning system (GPS). The airplane landed uneventfully.
Post-Landing Examination
An examination of the airplane revealed minor damage to the front of the cowling. The engine showed a fractured crankshaft aft of the flange. The propeller, spinner, and forward part of the crankshaft were missing. Several months after the incident, the separated propeller assembly and crankshaft were located.
Engine and Crankshaft Analysis
Examination of the engine determined that the crankshaft was fractured through aft of the propeller flange. The fracture was caused by a fatigue crack that originated from the aft relief radius of the propeller flange. The surface of the aft radius showed no evidence of mechanical damage, and the fatigue crack emanated from multiple origins over a wide area, indicating that significant stress was required to initiate the crack. The reason for the crack initiation could not be determined.
The fatigue crack origin area was located in the relief radius in line with the axis of the propeller blades, consistent with forward and aft bending stresses imposed by the blades during flight. This location suggests the possibility that the blades were out of balance while rotating or that the crankshaft had been damaged in the past. No evidence of fretting was noted between the faces of the crankshaft propeller flange and the alternator pulley. The propeller assembly was attached firmly to the crankshaft.