What happened
On July 31, 2003, a privately owned Eurocub Mk IV ultralight, registration SE-YVV, was performing a routine flight from Lake Mälaren toward Dalarö. The pilot intended to land and dock the aircraft at Korsholmen. The landing itself was executed normally; however, immediately following touchdown, the aircraft experienced a rapid deceleration and tipped forward. This movement caused the propeller to strike the right-hand float.
While the pilot was able to evacuate the aircraft without injury, the impact caused significant damage to the float. Although the aircraft initially remained upright, the damaged right-on pontoon began to fill with water. Eventually, the weight of the water caused the aircraft to capsize, leaving it floating upside-down with only the left pontoon and a wingtip visible.
The investigation
Technical investigators examined the construction of the floats, which are manufactured in the United States and assembled by the aircraft manufacturer. The investigation focused on the structural integrity of the fiberglass pontoons and the adherence to assembly instructions.
Findings revealed that the right-hand float had compressed at the landing gear attachment points, leading to broken joints between the float deck and the body. Specifically, the 150-mm wide fiberglass reinforcement required by the assembly manual was missing. Furthermore, the distance between the landing gear attachment points and the nearest internal rib exceeded 500 mm, far surpassing the maximum allowable distance of 150 mm specified in the manual's "Helpful Hints" section.
Evidence of discoloration within the joints suggested that the adhesive bond may have been gradually loosening during previous flights. The aircraft manufacturer noted they had replaced the fiberglass reinforcement with a longer aluminum fitting to distribute loads, but they had not noticed the specific rib-placement instructions in the manual.