Accident Overview
While descending and deviating around weather, the pilot of an airplane observed oscillations in fuel flow and manifold pressure on the left engine. Although the propeller continued to turn, the pilot chose not to feather it. The aircraft subsequently could not maintain altitude, prompting the pilot to extend the landing gear and ditch the airplane in a swamp.
Engine Performance
During the descent, the left engine exhibited fluctuations in fuel flow and manifold pressure. The pilot noted that the propeller was still rotating and therefore decided against feathering. However, the airplane was unable to sustain altitude, leading to the ditching.
Examination Findings
Post-accident examination of the left fuel tank revealed that the fuel float valve actuators and float arms were excessively worn. This wear could allow unmetered air to enter the fuel lines. Additionally, one of the left fuel valves could jam open, potentially causing the fuel pickup to become unported and introduce excessive air into the fuel system.
Probable Cause
The official probable cause is the failure of the pilot to feather the propeller when the engine lost power, and his failure to follow the emergency procedure checklist. A contributing factor was the inadequate design of the fuel tank float valves, which allowed for excessive wear and subsequent induction of unmetered air into the fuel lines.