ATL85LA227

While the acft was flying over wooded terrain, the eng lost power. The plt saw a plowed field aprx 3 mi away & began an emerg apch to the field. The acft struck the tops of trees on final apch, but continued flying until the plt made a gear up landing. An exam revealed that the exhaust muffler had failed at a weld where a ball joint was connected. This allowed hot exhaust gases to enter the eng compartment, burn the magneto wires & render the magnetos inoperative. An airworthiness directive, AD 76-23-03, which required an inspection of the exhaust sys each 100 hrs, had been complied with on 10/7/83, but had not been performed during the last annual inspection on 12/7/84. The acft had accumulated 68 hrs of flt time since compliance with AD 76-23-03.