What was found
TURBINE (MAINSHAFT) RETENTION NUT FAILED (CRACKED IN HALF) DURING OPERATION OF ENGINE ON THE GROUND. IT WAS OBSERVED THE PROPELLER WOULD NOT ROTATE FREELY AND THE ENGINE HAD SEIZED. UPON DISASSEMBLY OF ENGINE, OBSERVED THAT ONE THREAD OF THE NUT (IN TWO PIECES) WAS COMPLETELY MISSING. SUBMITTER STATED FAILURE OF THIS NUT DURING FLIGHT OPERATION WILL RESULT IN ENGINE FAILURE AND/OR UNCONTAINED FAILURE OF THE ENGINE. ENGINE TSN: 3,012.0 HRS; CSN: 4,525 HRS; TSO: 179.1 HRS. CSO: 190.