On September 28, 2008, at 1525 central daylight time, a Beech C23 single-engine airplane, N6714E, sustained substantial damage when it impacted terrain following a loss of engine power while maneuvering near Duncan, Oklahoma. The private pilot, who was the registered owner, and passenger sustained minor injuries. Visual meteorological conditions prevailed for the Title 14 Code of Federal Regulations Part 91 personal flight. The local flight departed Duncan approximately 1500.

The pilot, who purchased the airplane in February 2008, flew the airplane from Guthrie, Oklahoma, to Duncan on a 1.3 hour flight with no problems noted. The passenger boarded the airplane in Duncan, and the pilot performed a run-up prior to the local flight. At an altitude of 2,500 mean sea level (msl), the engine RPM dropped, and the engine "shook violently." The pilot attempted to troubleshoot the engine problem; however, the engine vibration continued. The pilot elected to shut down the engine and execute a forced landing to a field. During the forced landing, the airplane impacted a fence and terrain. Examination of the airplane revealed the firewall and left wing were bent and buckled. The tachometer showed 1,688 hours at the time of the accident.

A review of the Lycoming O-360-A4K (s/n: L26749-36A) engine maintenance records revealed the engine underwent its most recent "annual" inspection on December 20, 2007, at a tachometer time of 1,661.8 hours. During the inspection, the engine oil cooler lines were replaced and no additional anomalies were noted. On January 21, 2008, at a tachometer time of 1,664.8 hours, the engine underwent maintenance due to an engine oil leak caused by a crack oil cooler elbow fitting. Included in that maintenance was the disassembly of the engine and replacement of the rod bearings, main crankshaft bearings, and piston rings. The engine was test run and leak checked with no anomalies noted. The engine maintenance records stated that the maintenance was done in accordance with the Lycoming Overhaul Manual.

On October 21, 2008, the engine was examined and disassembled at the facilities of Air Salvage of Dallas, Lancaster, Texas, under the supervision of a NTSB investigator, and three Federal Aviation Administration (FAA) inspectors. According to a FAA inspector's report, examination of the engine revealed the wrong gasket was installed on the oil filter adapter assembly, the oil filter contained metal debris, the piston rings were worn and did not exhibit etched part numbers, and the oil rings were different between the 4 pistons. Internal examination of the engine revealed the crankshaft was fractured into 3 sections, 2 of the 8 tappets exhibited pitting, the aft main bearings and rod bearing part numbers matched the January 2008 engine maintenance record entry, however, the front main bearings did not match the part number indicated in the record. The fractured crankshaft was retained for metallurgical examination.

The fractured crankshaft was examined by the NTSB materials laboratory. Examination of the crankshaft revealed the crankshaft was fractured in two places; one fracture occurred forward of the number 2 crankshaft journal and one fracture occurred aft of the number 3 crankshaft journal. The features on both fracture surfaces were consistent with fatigue crack propagation with crack initiation at the journal radii. Circumferential crack arrest lines were observed on both fracture surfaces and ratchet marks were visible on the forward fracture surface, consistent with fatigue crack growth. The crack arrest lines and ratchet marks pointed towards the journal radius as the initiation site for both fatigue cracks.

Size measurements of the crankshaft journals were consistent with material removal from some of the journals. Three of the four crankshaft journals were undersized. Journal numbers 2 and 4 were undersized by 0.003 inch, and number 3 was undersized by 0.005 inch. According to the Lycoming Overhaul Manual, the crankshaft journal diameters may be reduced by 0.003 inch by polishing without renitriding the crankshaft. In that event, it is advised that all journals be polished to the same size. Grinding is not advised as it may penetrate the nitride layer at the journal radius leaving the crankshaft prone to fatigue failure. If a journal is polished to undersize, the code "M03P" should be stamped as a suffix to the part number on the flange. There was no code suffix found on the flange. If a journal is reduced by more than 0.003 inch, the journals should be ground to undersize, the shaft renitrided and the code "RN" stamped as a suffix to the serial number on the flange. There was no code suffix found on the flange. In addition, there was no documentation noted in the maintenance records that the shaft had been renitrided.