CHI06LA165

The airplane was substantially damaged during a forced landing to a cornfield following a loss of engine power after takeoff. The pilot sustained serious injuries. Postaccident examination revealed that the number two cylinder exhaust valve had fractured at the transition radius between the valve stem and the valve head. The fracture surface exhibited arrest lines, and ratchet marks consistent with fatigue fracture initiating from multiple origins. The fatigue features had progressed across approximately 95 percent of the stem diameter. Measurements of the valve stem diameter, and the valve guide inside diameter indicated that the running clearance was 0.015 to 0.017 inch at the inboard end, and 0.007 to 0.013 at the outboard end. The maximum serviceable limit for valve stem to valve guide clearance was 0.007 inch.

On June 24, 2006, about 0815 central daylight time, a Luscombe 8A, N2890K, piloted by a private pilot, was substantially damaged during a forced landing following a loss of engine power after takeoff from Bowling Green Municipal Airport (H19), Bowling Green, Missouri. The personal flight was being conducted under 14 CFR Part 91 without a flight plan. Visual meteorological conditions prevailed at the time. The pilot sustained serious injuries. The intended destination was a private grass strip near Mexico, Missouri.

The pilot reported about one minute into the flight, during climb out after takeoff, the engine started "knocking." He stated approximately 5 to 7 seconds later the engine and propeller stopped. He secured the engine and executed a forced landing to a corn field.

The post accident engine examination revealed that the number two cylinder exhaust valve had fractured. Metallurgical examination of the exhaust valve determined the valve fractured at the transition radius between the stem and the valve head. The fracture surface was in a flat plane perpendicular to the longitudinal axis and exhibited arrest lines consistent with fatigue fracture. Several ratchet marks were observed, which is consistent with multiple fatigue origins. The fatigue features had progressed across approximately 95 percent of the valve stem diameter.

The valve stem diameter measured from 0.3450 to 0.3465 inch at the inboard end near the fracture surface. At the outboard end the diameter measured 0.3384 to 0.3394. The stem diameter referenced by the engineering drawing was 0.3395 to 0.3405 inch. The valve guide inside diameter measurements ranged from 0.361 to 0.362 inch at the inboard end, and 0.346 to 0.351 inch at the outboard end.

According to the engine manufacturer, the running clearance between the exhaust valve stem and valve guide for new parts was 0.0027 to 0.0047 inch. The specified maximum serviceable limit for valve stem to valve guide clearance was 0.007 inch.

The engine maintenance records indicated that an annual inspection was completed on November 4, 2005. The engine total time was 1,533.5 hours at the time of that inspection.