Accident Overview
During takeoff, approximately 10 feet above the ground, the aircraft began backfiring and losing engine power. The pilot aborted the takeoff but was unable to bring the aircraft to a stop before it struck a ditch located just off the departure end of the airstrip.
Post-Accident Inspection
A post-accident inspection of the engine performed by an FAA Airworthiness Inspector revealed that a portion of the number three cylinder head casting had fractured. This fracture included the area through which the exhaust valve rocker pin passes. As a result, the number three cylinder exhaust valve remained in the closed position during the period when it should have been open, leading to a loss of engine power.
Metallurgical Analysis
The fractured rocker box casting was sent to the NTSB metallurgical laboratory for further examination. The analysis determined that the fracture was caused by a fatigue crack. This crack originated in the wall of the hole in the casting through which the rocker arm pivot bolt passed. However, the specific cause of the fatigue crack's initiation could not be determined.
Conclusion
The accident resulted from an engine power loss due to a fractured cylinder head casting that prevented proper exhaust valve operation. The fracture itself was attributed to a fatigue crack of unknown origin.