Incident Description
During cruise flight, the pilot felt a "bump" under the seat. Subsequently, the airplane pitched up erratically and became difficult to control. The pilot selected an open field and executed an emergency landing. The airplane came to rest in a pond.
Pilot's Account
The pilot reported that the crankshaft sheared behind the propeller flange and in front of the crankcase. This separation was associated with the loss of control.
Aircraft Examination
Examination of the airplane at the accident site revealed that one of the propeller blades separated at the flange. The nose bowl cowling sustained substantial damage due to the airplane nosing over into the pond. The firewall was bent slightly rearward at the base, and water damage was noted on the wing spar.
Propulsion System Investigation
Further examination of the partial crankshaft recovered showed that the separation had occurred between the oil slinger and the propeller mounting flanges, leaving a fracture face. The fracture face displayed arced crack arrest marks, consistent with fatigue propagation. The arcing of the crack arrest marks indicated that the fatigue origin was located further forward, towards the propeller-mounting flange on the unrecovered portion of the crankshaft.