CEN18LA159

The commercial pilot reported that he heard an unusual noise from the engine during cruise flight and shortly afterward, the engine experienced a total loss of power. The pilot performed a forced landing to an open field. Visual examination of the engine revealed a hole in the crankcase. A teardown examination revealed fretting on the crankcase mating surfaces, shifting of the No. 2 main bearing within the journal support, thermal damage consistent with a loss of oil lubrication, and separation of the crankshaft. The observed fretting damage was consistent with relative movement of the crankcase halves during operation due to a lack of through-bolt torque. The main journal bearing damage, the separation of the crankshaft, and the crankcase damage were secondary and a result of the loss of lubrication. An engine overhaul was completed in about 11 years 5 months before the accident; about 849 flight hours had accumulated since the overhaul. Two cylinders were replaced 6 years before the accident, and one cylinder was replaced during the most recent annual inspection about 9 months before the accident. It is likely that improper torque of the through-bolt nuts was applied during one of the cylinder replacement events, though it could not be determined which event.

On May 4, 2018, about 1930 central daylight time, a Beech A36 airplane, N36LP, was substantially damaged during a forced landing following a loss of engine power near Shullsburg, Wisconsin. The pilot was not injured. The airplane was registered to and operated by a private individual as a Title 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed, and the flight was operating on an instrument flight rules flight plan. The flight originated from the Okmulgee Regional Airport (OKM), Okmulgee, Oklahoma, about 1537. The intended destination was Bucky's Airport (WN09), Plainfield, Wisconsin.The pilot reported that the airplane was in cruise flight at 7,000 ft mean sea level when he heard an unusual noise from the engine. Shortly afterward, the engine lost power and he executed a forced landing to an open field. He recalled thinking that oil may have escaped from the engine because he observed smoke. The airplane sustained substantial damage to the lower fuselage/nose landing gear wheel well structure. A postaccident engine examination revealed a hole in the crankcase above the no. 1 cylinder. A teardown examination revealed wear on the mating crankcase surfaces consistent with fretting of the case halves. The no. 2 main journal bearing was shifted within the bearing support, the lock tab was elongated, and the bearing was extruded. Bearing fragments were recovered from the oil sump. The crankshaft was separated between the no. 2 main bearing journal and the no. 2 connecting rod journal. The crankshaft was discolored adjacent to the separation consistent with thermal damage due to a lack of oil lubrication. The remaining portions of the crankshaft displayed normal operating and lubrication signatures. The no. 2 connecting rod bearing exhibited damage consistent with lubrication distress. The oil pump housing and impellers exhibited scoring consistent with hard particle passage. Airplane maintenance records indicated that an engine overhaul was completed in November 2006. The no. 2 and no. 4 cylinders were replaced in June 2012. The most recent annual inspection was completed in August 2017. The no. 5 cylinder was replaced during the annual inspection. At the accident site, the airplane recording hour (Hobbs) meter and tachometer indicated 4,698.4 hours and 7.951.79 hours, respectively. At the time of the accident, the engine had accumulated 848.9 hours since overhaul and 9.8 hours since the annual inspection. The time between overhaul (TBO) interval recommended by the engine manufacturer was 1,700 hours or 12 years, whichever came first. In October 2016, the NTSB issued a safety alert regarding improper torque of engine fasteners during maintenance activities.