N441TB

After completing several practice landings and autorotations at the airport, the Certified Flight Instructor (CFI) and student pilot departed for the practice area. The CFI demonstrated straight in and 180-degree autorotations; the first 180-degree attempt began at 6,000 feet mean sea level and 55-60 knots. During the second 180-degree attempt, he started about 200 feet lower. The airspeed bled off to about 50 knots during the turn, and the main rotor blades’ revolutions per minute (rpm) began decaying rapidly. About 200 feet above ground level (agl), the CFI attempted a power recovery, but the rpm did not increase with an increase in throttle. He attempted to flair to bring the rpm back, but this did not work either. He attempted a full down autorotation, and put the helicopter in a level attitude prior to contact with the ground. The helicopter bounced; on the second contact, the right skid dug into the muddy ground, and rolled onto its right side. The pilots were not injured, and extricated themselves from the wreckage. The helicopter sustained substantial damage to the airframe and rotor blades. The pilots stated that the helicopter had no mechanical failures or malfunctions.