NYC85LA181

The helicopter with cfi/owner and student who was flying from the left side over high terrain at about 70 mph reported a gradual loss of power. The CFI took control and attempted to effect an airstart. This was without success. The plt then attempted to arrest the descent but acft impacted the ground with such force it broke up on impact. Both left and right skids collapsed, tail boom and rotor were separated at aprx the 180 station, and located aprx 25 ft from the main wreckage. Dual controls were installed, except on the right side there were no torque pedals attached to the torque control stubs. Inspection of the acft and eng disclosed one magneto was off on it's timing and the magneto had lost aprx 43% of it's magnetism. Both magnetos contained oil and carbon brush wear.