What happened
Approximately 10 minutes after departing, while maintaining cruise, level flight, the pilot of the unspecified aircraft observed a light sheen of oil forming on the windshield. Due to this leak, the pilot elected to return to the departure airport.
As the aircraft approached within two miles of the airport, the heavy accumulation of oil made it difficult to see through the windshield. During the approach, the propeller RPM increased for approximately 20 seconds, after which the propeller detached from the engine. The pilot was able to glide the airplane to the runway and completed the landing without further incident. All occupants were not injured.
The investigation
Post-incident investigators found that the engine had been rebuilt and zero-timed by Teledyne Continental Motors only three days prior to the accident, with only 9.0 hours of operation recorded since the overhaul.
A Safety Board metallurgist performed an examination of the fractured crankshaft flange. The visual examination revealed that roughly one-half of the fracture occurred on a flat plane and contained crack arrest positions, which are features indicative of fatigue cracking. The metallurgist further noted that the primary fatigue initiation appeared to originate from a very large number of individual initiation sites located in the center of the fatigue region.