History of the Flight
A Beech 18 seaplane, equipped with independent floats, was departing a fishing camp on Bradburn Lake, Manitoba, for its third flight of the day to Selkirk. After boarding six passengers and baggage, the pilot taxied to the southwest shore and began the take-off run heading approximately 015 degrees true. During the run, while the aircraft was "on the step," the pilot noticed a yaw to the left. He corrected with rudders and differential engine power, but at about 60 mph, the yaw recurred and worsened. The left float separated, the left wing struck the water, and the aircraft turned sharply left, stopping in about 15 feet of water and coming to rest on its left wing tip. The pilot and six passengers were not injured; they exited the aircraft and swam to shore. The accident occurred at 1040 central daylight saving time in daylight.
Injuries and Damage
There were no injuries. The aircraft sustained substantial damage when the left float separated and the aircraft capsized. The baggage was damaged by water.
Aircraft and Pilot Information
The aircraft was a 1943 Beech 18 (model 3T, serial 43-35481) with two Pratt & Whitney R-985-AN-14B engines, total time 15,748 hours. It was equipped with EDO 56-7850A floats, each attached independently with five struts. The take-off weight was about 300 pounds below gross weight, with center of gravity within limits. The pilot held a Commercial Pilot License, had 9,000 total hours (6,000 on seaplanes) and 700 hours on type, and was certified for the flight.
Meteorological and Wreckage Information
Witnesses reported winds from the south at less than 10 knots, not gusty, with light rippled waves. Examination of the wreckage showed the left float's front and rear vertical struts still attached to the float but with twisted top fittings showing overload failure. The diagonal side strut eyebolt failed at the swivel fitting. The rear side strut upper attachment bolt was twisted, and the rear swivel fitting bolt was broken. The front side strut was still attached to the fuselage, but the float end of the front side strut swivel fitting and mounting pad showed no damage; the mounting bolt and nut were not recovered.
Float Installation and Maintenance
The Bristol Float Service Manual specifies castellated nuts with cotter pins for strut mounting bolts. The operator reported using castellated nuts and cotter pins, but only nine of ten recovered bolts for the left float had fibre self-locking nuts. Several bolts did not pass fully through the nut for secure locking. The floats and attachment struts were last removed for inspection in April 1988; the aircraft had flown about 1,160 hours since then. The operator's inspection approval did not include the Bristol requirement for removal every 500 hours or end of season.
Examination and Conclusions
The Transportation Safety Board Engineering Branch examined the front swivel fitting and mounting pad. They concluded that the attaching bolt likely did not break but moved out of position because the securing nut was either not installed or came off in service. The bore of the front float mounting pad showed circumferential marks consistent with an AN7 bolt threads, likely from side loads while the bolt was partially withdrawn. The rear swivel fitting bolt was bent and broken by shear and tensile loading, with no progressive failure. The diagonal strut eyebolt fractured from tensile overload. Moderate corrosion was present but not significant. The Board determined that the left front swivel fitting attachment bolt probably moved out of position because the securing nut was either not installed or came off in service, leading to a series of failures culminating in float separation.