What happened
During cruise flight, the pilot of a single-engine amphibious float-equipped airplane noticed that the propeller RPM began to fluctuate. Upon attempting to adjust the RPM, the engine made several clunking sounds and suffered a total loss of power.
The pilot subsequently performed a forced landing in a grassy field. During the landing, the main float wheels contacted the surface and dug into the soft ground. As the nosewheels touched down, the airplane nosed over onto its back.
According to the aircraft's pilot operating handbook, landings on land without engine power should be conducted with the landing gear up when encountering soft or rough ground, and with the gear down only on firm or smooth ground.