FTW04LA238 · Piper PA-31-350

The 1,800-hour commercial pilot reported that while in cruise flight, the left engine stopped producing power and the left propeller fell forward approximately six inches. The pilot secured the engine and made a precautionary landing. The airplane sustained substantial damage during the forced landing to a field. A review of aircraft maintenance records revealed that the airplane experienced a gear-up landing in January 2002, which resulted in a propeller strike. The left engine was removed and inspected per the manufacturer's overhaul manual and returned to service. The engine had accrued approximately 440 hours since it was overhauled. Examination of the engine revealed that the entire three-bladed propeller assembly and the #1 cylinder had separated from the engine. The engine was disassembled and the crankshaft, along with an intact #3 connecting rod assembly and the fractured #2 connecting rod assembly were examined by a Safety Board Materials Engineer. The crankshaft was fractured in two places; one fracture was just forward of the #3 rod journal in the crank cheek, and the other fracture was in the aft crank cheek adjacent to the #2 main journal. The interior surface of the #1 and #2 main bearing halves exhibited significant rubbing and a portion of the bearing liner material was delaminated and was missing from the forward end of the bearing. On the oil slinger face, just forward of the #1 main journal, 360 degree rubbing damage was noted just outboard of the radius. In the rub region, metallic transfer was present with a color consistent with aluminum material. No evidence of radial cracks was observed in this area. An exact determination for the crankshaft failure could not be determined; however, evidence supported two possibilities that could have attributed to its demise. One possibility was that the crankshaft fracture initiated from rubbing damage between the crank cheek aft of the #2 main journal (thrust face) and the crankcase, with the other fatigue region being secondary. The exact cause for the rubbing was not clear, but one possibility was that the bearing liner delaminated and liberated a section causing a reduction in the clearance between the crankshaft and the crankcase as it was rubbing/exiting through the oil slinger. The other possibility was that the propeller strike caused the rubbing/cracking to occur, which was not detected during the subsequent overhaul/inspection.

Verbatim NTSB analysis and probable cause from the NTSB dataset