The pilot, who was conducting a flight in the local traffic pattern, was just turning onto the downwind leg when the airplane's engine suddenly quit without warning. Being unable to get the engine to restart, he selected what looked to him like a nearby level field of tall grass for his emergency forced landing. Just after touching down in the field, the airplane hit a number of tree stumps that had been hidden by the tall grass. The impact sequence resulted in extensive damage to the fuselage, wings, and landing gear. A post-accident inspection of the engine, fuel system, and air induction system determined that the fuel tanks were nearly full, and that there were no apparent anomalies that would have contributed to a loss of engine power.