NTSB Identification: DFW06LA154.
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Accident occurred Tuesday, June 06, 2006 in Pocahontas, AR
Probable Cause Approval Date: 11/29/2006
Aircraft: McClung Christian Eagle I, registration: N181DM
Injuries: 1 Uninjured.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
The 18,700-hour pilot was piloting his single-engine experimental airplane at an altitude of 4,500 feet mean sea level (msl) when he started to feel a "slight vibration" or "light buzz" from the engine. The vibrations gradually became more severe until there was a loud bang and the engine seized. The pilot made a forced landing to a hay field. Metallurgical examination of the engine revealed the crankshaft had fractured through the crank cheek, between the #5 and #6 cylinder journals as a result of fatigue cracking. Further examination revealed that the crankshaft was susceptible to early subsurface fatigue failure due to the presence of "honeycomb" structure and microcracking. The mechanical damage was from contact of the connecting rod with the crankshaft thrust faces, and the "honeycomb" structure was most likely a result of overheating during the forging process. The crankshaft had been installed new in the engine in 2001,and two years later was modified with 11:1 compression ratio pistons. As a result of this and other modifications, engine power was increased by 30 percent. Due to the fact that these modifications altered the engine's original certification criteria; the engine was re-classified as an experimental. The serial number of the fractured crankshaft was listed in Lycoming's Mandatory Service Bulletin (SB) 569A, titled Crankshaft Retirement for Certain Lycoming Engines, which recommended the crankshafts be replaced at the first instance the engine crankcase was separated or no later than February 21, 2009. The SB was generated as a result of several subsurface fatigue failures of similar crankshafts used on higher horsepower engines (greater than 300 HP). Those crankshafts were subject to Airworthiness Directive (AD) 2002-04-51, which would have required them to be removed before the date of this accident. Since the engine was classified as experimental, the crankshaft did not have to be removed as a result of this guidance. The AD and SB were issued as a result of a slight change in the specified composition of the crankshaft material, which was a small vanadium addition intended to lower the tempering temperature and reduce warpage; however, this change increased the susceptibility to "honeycomb" and microcracking when slightly over-heated during the forging process. The "honeycomb" and microstructure significantly reduced fatigue properties. The loss of fatigue life was most pronounced in the higher HP applications, and the experimental modifications made to this engine would have made the crankshaft more susceptible to fatigue fracture.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The loss of engine power as a result of a fractured crankshaft due to fatigue. A factor was the engine modifications to increase engine horsepower. Full narrative available
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