Incident Overview
A ski-equipped airplane nosed over while landing on a frozen lake. The pilot reported that prior to the accident, when departing another seaplane base, the surface was very rough due to frozen slush created by snowmobiles.
Sequence of Events
During touchdown on the frozen lake, the pilot heard a "loud grinding" noise and the airplane slowed down rapidly. The grinding noise stopped, then started again, much louder. Subsequently, the nose of the airplane dropped, and the propeller contacted the ice. The airplane then dropped to the right and flipped over onto its back.
Post-Accident Observations
The pilot reported that the ground track from the left ski was "rounded" at initial touchdown, followed by a space, then a much narrower and deeper track. Examination revealed that the four quarter-inch bolts sheared off between the axle and the gear leg on the left ski.