Pocahontas, AR, USA
N181DM
McClung Christian Eagle I
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.
On June 6, 2006, approximately 0800 central daylight time, a homebuilt McClung Christian Eagle I single-engine airplane, N181DM, sustained substantial damage during a forced landing following a loss of engine power near Pocahontas, Arkansas. The airline transport rated pilot, the sole occupant of the airplane, was not injured. The airplane was registered to and operated by Red Eagle Aviation Incorporated, of Peachtree City, Georgia. A flight plan was not filed for the cross-country flight that originated from the Walker County Airport, near Jasper, Alabama, about 0638. Visual meteorological conditions prevailed for the personal flight conducted under 14 Code of Federal Regulations Part 91. According to the 18,700-hour pilot, the airplane was in cruise flight 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. He increased engine power by 50 RPM and monitored the engine via the JPI engine analyzer. All readings were "normal." The engine vibrations lasted for about 15-20 seconds before they became "sharper" and "more pronounced." The pilot attempted to navigate toward the nearest suitable airport; however, the engine vibrations increased and began to shake the airplane "violently." The pilot added that "some 45 seconds or so after the first indication of a problem, there was a loud bang of an explosion and oil covered my canopy, the engine seized (oil pressure confirmed at zero) and the propeller stopped completely." The pilot executed a forced landing in a hay field covered by three-foot-high grass. The airplane rolled about 900 feet before a wheel pant caught a rock, which caused the airplane to skid about another 30 feet on its nose. According to the pilot, the accessory section of the engine had "blown off," and one of the engine mounts failed. There was also damage to the lower engine cowl, the 3-bladed propeller, the wheel pants, and the landing gear box. Examination of the Lycoming AEIO-540-D4A5 engine revealed the crankshaft had fractured through the crank cheek, which contained the counterweights, between the #5 and #6 cylinder journals. The fractured sections of crankshaft were sent to the Safety Board's Materials Laboratory, in Washington, DC, for metallurgical examination. The examination revealed that a visual and magnified inspection of the fracture faces exhibited beach markings, which were indicative of fatigue cracking. The fatigue initiated in the vicinity of the aft radius of the #5 journal and propagated rearward through most of the crank cheek and balancer arms before separating the crankshaft. However, due to post-failure mechanical damage, the exact fatigue origins could not be determined and it could not be establish if the failures were initiated at the surface or subsurface. The crankshaft met the manufacturer's specified metallurgical requirements. 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. A review of the engine logbooks indicated that the crankshaft had been installed new in the engine in 2001, and had accumulated 263.5 hours at the time it failed. Between 2003 and 2004, Penn Yan Aero, Penn Yan, New York, modified the engine with 11:1 compression ratio pistons (certificated configuration was 8.5:1 compression ratio pistons). As a result, this modification (along with several other modifications) increased engine power from the certified 260 horsepower (HP) to 340 HP at full mixture setting, and 350 HP when leaned to best power, a 30 percent increase. Due to the fact that this modification 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 was released April 11, 2006. The SB recommended that these 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. Lycoming has since returned to the previous composition. 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 loss of engine power as a result of a fractured crankshaft due to fatigue. A factor was the engine modifications to increase engine horsepower.
Source: NTSB Aviation Accident Database
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