N/A Airport
Aircraft Accident/Incident Report

Otter Lake, New York 13338
Thursday, August 16, 2012 8:39 EDT

While climbing to 6,500 feet mean sea level, about 45 minutes into the flight, the engine began to run rough “like it was developing carburetor ice,” so the pilot applied carburetor heat, since the meteorological conditions were conducive to carburetor ice. The engine, however, continued to run rough and began to “cough” like it was starved of fuel, so the pilot made sure that both fuel valves were open, and turned on the electric fuel boost pump. The pilot then assessed that the engine might stop running, so he called for the high altitude engine failure checklist. However, before he and the other crewmember began the checklist, the propeller came off the airplane, struck the right side of the cowling, struck the right lift strut, and fell away. The pilot then declared an emergency with air traffic control, began evaluating landing sites, and elected to ditch in a nearby lake. Examination of the wreckage revealed that the entire propeller assembly, including the propeller spinner and propeller flange extension, had separated from the propeller flange, which was part of the forward portion of the engine crankshaft. Examination of the engine revealed that the bolts that held the propeller flange extension onto the engine crankshaft had sheared off and that the No. 6 cylinder exhaust valve and valve seat were missing from their installed positions. Further examination revealed that the exhaust valve head had come to rest in the oil pan, and parts of the exhaust valve seat had been captured in the muffler.Examination of the No. 6 cylinder exhaust valve revealed that the valve stem had fractured near the transition from the head radius to the cylindrical stem and that lead deposits were on the stem next to the fracture. Multiple ratchet marks were present around the perimeter of the stem. The presence of the ratchet marks and the fracture surface were consistent with fatigue crack initiation and propagation. Examination of the other cylinders and pistons also revealed the presence of heavy lead deposits, which can act as an insulating layer and can interfere with the normal heat transfer process; such deposits indicate that the engine may have been run at some point at excessively high temperatures. According to the engine manufacturer, they were aware of about eight previous in-service failures of exhaust valves. They advised that the exhaust valves appeared to be intolerant of temperatures over about 750 degrees C and that heat stress was the most common issue that they had identified. They further advised that around that temperature, the exhaust valve would start showing indications of stress corrosion/cracking at the base of the stem.Examination of the crankshaft revealed that it had six evenly spaced threaded holes and three smooth bored holes. Each threaded hole contained the shank of a capscrew that had fractured flush with the face of the forward end of the crankshaft. The three smoothed-bored holes showed no evidence of them containing anything that had been inserted or installed in the bore. The fracture faces on the capscrews had a smooth appearance with smear marks orientated in the direction of rotation, which are consistent with an overstress fracture in shear.According to the manufacturer, at the time of the accident, they had identified about 8 instances where the propeller flange attachment had separated from airplanes due to varying circumstances. To alleviate some of the problems that they observed, they took several steps ensure that a flange would be properly installed. These steps included issuing two service bulletins and changes to the configuration of new engines to have the longer flange as standard. They also altered the maintenance requirements of the engine for more detailed monitoring of the propeller flange area, and altered the design of the engine to incorporate three 8mm dowels between the crankshaft and the propeller flange; in July 2011, they began including these propeller flange dowels into the engines.Comparison of the engine’s propeller flange to other propeller flanges indicated that the propeller flange diameter of the engine was markedly smaller than others that were in service. Since torque is the product of force and distance, and the distance of each of each capscrew from the center of the flange was small, the torque on each capscrew was very high. Furthermore, since the capscrews were fully threaded along their length, they had no bearing portion for shear loads, and the threaded portions had stress raisers in the form of a radius at the root. FAA documentation also indicated that assembly and maintenance errors have contributed to hazardous or catastrophic conditions with propeller assemblies. No indication however of the propeller flange design being designed to mitigate the effects of maintenance errors was discovered, though the propeller and propeller flange were required to go through frequent maintenance. Examination of the propeller flange design also did not indicate that it was designed to facilitate maintenance and correct reassembly or if the incorrect assembly of parts could result in hazardous propeller effects, or if the parts were designed to minimize the risk of incorrect assembly. Review of the experimental amateur built rules promulgated by the Federal Aviation Administration and the light sport aircraft consensus standards also did not reveal any guidance regarding design precautions to minimize the hazards that could occur to an airplane if a propeller blade failed or was released by a hub failure, requirements of a safety analysis to ensure that the risk to the aircraft from propeller failure conditions was within an acceptable range, or whether parts were required to be designed to minimize the risk of incorrect assembly.

A failure of an engine cylinder exhaust valve due to the buildup of lead deposits on the valve stem and fatigue cracking of the valve stem, resulting in a total loss of engine power, and the loss of the propeller. Contributing to the accident was the lack of design standards or guidance for assembly and maintenance of light sport aircraft.