Accident Overview
A Lake Amphibian was being operated with two ferry tanks installed in the cabin. The pilot departed with the aircraft's fuel tanks full and an additional 15 gallons in the ferry tanks. The center of gravity (CG) at zero fuel weight was 3/4 inch past the aft limit. With the addition of fuel, the position of the CG remained unchanged, but the aft limit of the CG envelope moved forward as weight increased.
Fuel System Issue
A connection where the ferry tanks were plumbed into the aircraft's fuel system leaked. This leak allowed air to be sucked into the fuel system, leading to operational issues.
Engine and Flight Path
Witnesses reported that the engine became intermittent. Subsequently, the aircraft entered a spin from which it did not recover.
Center of Gravity Considerations
The aircraft's CG at zero fuel weight was beyond the aft limit by 3/4 inch. While the addition of ferry tank fuel did not shift the CG position further aft, the aft limit of the allowable CG envelope moved forward with higher weights, potentially resulting in an out-of-limits condition.