Accident Details
On September 28, 2008, at 1525 central daylight time, a Beech C23 single-engine airplane, registration N6714E, sustained substantial damage during a forced landing following a loss of engine power while maneuvering near Duncan, Oklahoma. The private pilot, who was also the registered owner, and a passenger sustained minor injuries. Visual meteorological conditions prevailed, and the flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight. The local flight had departed Duncan at approximately 1500.
The pilot, who had purchased the airplane in February 2008, flew from Guthrie, Oklahoma, to Duncan on a 1.3-hour flight without any issues. After the passenger boarded in Duncan, the pilot performed a run-up before departure. While at an altitude of 2,500 feet mean sea level (msl), the engine RPM dropped and the engine began shaking violently. The pilot attempted to troubleshoot the problem, but the vibration continued. He then shut down the engine and executed a forced landing to a field. During the landing, the airplane struck a fence and the terrain. Post-accident inspection revealed that the firewall and left wing were bent and buckled. The tachometer indicated 1,688 hours at the time of the accident.
Engine Examination
The engine was a Lycoming O-360-A4K, serial number L26749-36A. A review of its maintenance records showed that the most recent annual inspection was performed on December 20, 2007, at a tachometer time of 1,661.8 hours, during which the engine oil cooler lines were replaced and no other anomalies were noted. On January 21, 2008, at 1,664.8 hours, maintenance was performed due to an oil leak from a cracked oil cooler elbow fitting. This maintenance included disassembly of the engine and replacement of rod bearings, main crankshaft bearings, and piston rings. The engine was test run and leak checked with no anomalies noted. The records stated that the work was done in accordance with the Lycoming Overhaul Manual.
On October 21, 2008, the engine was examined and disassembled at Air Salvage of Dallas in Lancaster, Texas, under the supervision of an NTSB investigator and three Federal Aviation Administration (FAA) inspectors. According to an FAA inspector's report, the examination revealed that a wrong gasket was installed on the oil filter adapter assembly, the oil filter contained metal debris, the piston rings were worn and did not have etched part numbers, and the oil rings differed among the four pistons. Internal examination showed that the crankshaft was fractured into three sections. Two of the eight tappets exhibited pitting. The aft main bearings and rod bearing part numbers matched the January 2008 entry, but the front main bearings did not match the part number recorded. The fractured crankshaft was retained for metallurgical examination.
Crankshaft Analysis
The fracted crankshaft was examined by the NTSB materials laboratory. The crankshaft was found to be fractured in two places: one fracture forward of the number 2 crankshaft journal and one fracture 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 surfaces, and ratchet marks were visible on the forward fracture surface, consistent with fatigue crack growth. The crack arrest lines and ratchet marks indicated the journal radius as the initiation site for both fatigue cracks.
Measurements of the crankshaft journals showed that three of the four 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, crankshaft journal diameters may be reduced by up to 0.003 inch through polishing without renitriding the crankshaft, but all journals should 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. No such code suffix was 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 "RN" stamped as a suffix to the serial number on the flange. No such suffix was found, and no documentation of renitriding was noted in the maintenance records.