Incident Overview
A pilot reported that during flight, the aircraft encountered turbulence. Concurrently, the engine began to run rough. The pilot initiated troubleshooting and observed that the roughness was directly proportional to throttle movement; advancing the throttle increased the roughness. The propeller control appeared to have no effect on the roughness. The engine RPM began to drop regardless of manifold pressure settings.
Decision to Land
Faced with diminishing engine performance, the pilot elected to land on an interstate highway. During the landing maneuver, the aircraft struck a motor vehicle and subsequently impacted an embankment.
Post-Accident Findings
After the landing, examination revealed that the propeller blades were in the feathered position. An inspection of the propeller system determined that the blade actuating pin on one blade had separated from the blade butt at a pre-existing crack. The propeller had been overhauled on January 6, 1978, at 932.8 hours of operation. The recommended overhaul period for the propeller was 1,200 hours or five years, whichever occurred first.
Mechanical Details
The separation of the blade actuating pin from the blade butt, due to a pre-existing crack, led to the propeller blades assuming a feathered state. This condition likely caused the observed engine roughness and loss of RPM, as the propeller could not maintain proper pitch control. The pilot's troubleshooting indicated that the roughness correlated with throttle movement, consistent with a propeller malfunction rather than an engine issue.
Conclusion of Investigation
The investigation focused on the propeller system failure, specifically the separation of the blade actuating pin at a pre-existing crack. No other causal factors were identified in the source material.