Occurrence
A pilot reported that while performing aerobatic maneuvers for approximately 10 minutes, the aircraft began to descend from 4,000 feet. After leveling off at about 1,000 feet above ground level (AGL), the aircraft started to vibrate or shake violently. The pilot reported reducing the throttle to idle, suspecting a propeller blade may have been lost, and to slow the aircraft. The pilot subsequently landed in a corn field, during which the aircraft nosed over.
Investigation Findings
Post-incident examination revealed that the propeller had separated from the engine upon ground contact, with all three blades still attached to the propeller hub. The pilot stated the belief that the aircraft may have experienced some form of structural failure.
Examination of Stabilizer Support Tubes
Inspection of the left and right stabilizer support tube assemblies showed no indications of preexisting cracking. However, deformation patterns on the right tube indicated that it had been subjected to overstressing loads. The pilot/owner had previously modified these tubes to make them adjustable.