HISTORY OF FLIGHTOn July 19, 2019, about 1010 Alaska daylight time, a float-equipped de Havilland DHC-2 (Beaver) airplane, N68083, was substantially damaged when it was involved in an accident near Seldovia, Alaska. The pilot and three passengers sustained minor injuries, one passenger was fatally injured, one passenger sustained serious injuries, and one passenger was uninjured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 135 on-demand commercial flight. The purpose of the flight was to transport passengers from a remote lodge located at Tutka Bay, Alaska, to Lake Hood Seaplane Base (PALH), Anchorage, Alaska. The pilot stated that, after loading the passengers and their baggage onboard, he started the airplane's engine and provided a passenger safety briefing. He then taxied out and initiated his takeoff. He stated that as the airplane was accelerating on the step, he saw the left float begin to move into his peripheral vision from the left cockpit window. He said that the airplane felt as if it had "lost its rigidity" on the floats and began to "yaw" to the left. In an effort to correct for the left turning tendency, he applied right aileron, to no avail. The left wing subsequently impacted the water and the airplane nosed over, separating the right wing from the fuselage. He stated that the airplane immediately began to fill with water as he struggled to exit the submerged wreckage. During hospital interviews, the passengers consistently reported that after the engine was started, the pilot asked them to remove their headsets while he provided the passenger safety briefing. They stated that the briefing was rushed and difficult to hear due to engine noise. They reported choppy water conditions, with one passenger reporting white caps on the ocean waves visible in the distance. The passengers said that, during the takeoff, the airplane impacted a swell or wave and nosed over abruptly, and the cabin rapidly filled with water. One passenger stated that the airplane briefly became airborne before impact. A witness located near the accident site stated that the airplane appeared to accelerate slowly and struggle into the air. He reported that the airplane climbed to an altitude of about 50 to 100 ft, briefly leveled off, then began a gradual descent toward the water. The airplane impacted the water on the nose of the left float and immediately cartwheeled, coming to rest inverted in the ocean waters. He alerted emergency services and drove his boat to the submerged wreckage to assist with the rescue. An Alaska State Trooper reported conditions at the accident site of seas less than 1 ft with a long wavelength and winds less than 10 knots. The trooper recalled that, while at the hospital, the pilot stated that he was taking off parallel to the swells when the airplane impacted a swell and became airborne, the left float broke, and the airplane cartwheeled. AIRCRAFT INFORMATIONThe airplane was equipped with Aerocet 5850 seaplane floats in accordance with Supplemental Type Certificate (STC) No. SA01722SE. AIRPORT INFORMATIONThe airplane was equipped with Aerocet 5850 seaplane floats in accordance with Supplemental Type Certificate (STC) No. SA01722SE. WRECKAGE AND IMPACT INFORMATIONThe airplane nosed over in ocean waters and came to rest inverted. All major components were recovered except for the right wing, right aileron, and right lift strut. Flight control continuity was verified from the control yoke to the right fuselage bellcrank push/pull tube, the left aileron and elevators, and from the rudder pedals to the rudder. An examination of the floats and their attachment rigging revealed that the front left flying wire attachment fitting (attached to the left float) fractured, and the associated flying wire exhibited compression buckling signatures near the upper attachment point. The right rear flying wire attachment strap (attached to the fuselage) fractured, and the associated flying wire exhibited compression buckling signatures near the lower attach point. (See Figure 1). No significant deformation was present on the underside or nose of either float. Figure 1- Float Assembly Diagram (courtesy of Aerocet) An examination of the left float struts and flying wire attachment components was completed by the National Transportation Safety Board (NTSB) Materials Laboratory personnel. The left float struts had fracture surfaces with rough matte-gray features and deformation consistent with ductile overstress with bending or torsion loads. An examination of the aluminum flying wire attachment fittings revealed that the bolts for attaching the two left flying wire attachment fittings to the left float were bent, and the attachment hole in the intact left rear attachment fitting was elongated. Fractures in the left front attachment fitting and right rear attachment strap were consistent with ductile overstress fracture. Small areas of corrosion were observed on the fractured left forward flying wire attachment fitting; however, the total area of corrosion was a small percentage of the total cross-section, and the remainder of the fracture and associated deformation of the lug exhibited rough matte-gray features consistent with a ductile overstress fracture. In addition, small amounts of corrosion were also observed on the three intact flying wire attachment fittings. A postaccident examination of the airframe and engine revealed no evidence of mechanical malfunction or failures that would have precluded normal operation. ADDITIONAL INFORMATIONOn October 8, 2012 Aerocet issued Service Letter SL05-56-15145, which states in part: Float Model Affected: Aerocet Model 5850 Twin Seaplane Float installation on DeHavilland DHC-2 "Beaver" (All) Symptom: Reported corrosion of aluminum fitting beneath flying wire clevis in parts with extensive marine service and prolonged inspection intervals. Regular inspection at normal intervals will prevent possible failures due to corrosion-weakened fittings. Approval: This SL reflects FAA Approved design changes and current recommended inspections, and does not require FAA approval. Corrective Action: NONE – no required action, suggested action only. During next regular inspection per Aerocet Service Manual, A-10034, Rev 1 or later, include partial disassembly of the lower ends of the flying wires at the Aerocet fittings for all flying wires (crossing wires). Particular attention should be focused beneath the flying wire clevises for indications of corrosion. Parts exhibiting corrosion should be replaced. Original 56-15145 (now designated with a "-1") is easily identified as painted aluminum. Replacement part, 56-15145-2, is constructed from corrosion resistant steel (stainless), not painted. Aerocet Test Report A-33162 The flying wires attachment design limit load was 3,453 pounds of force. Testing completed by Aerocet and documented in Test Report A-33162 revealed that buckling of flying wires was only produced at strap and fitting failure loads above 9,000 pounds of force.