What happened
On May 4, 2018, at approximately 19:30 CDT, a Beech A36, registration N36LP, was conducting a flight from Okmulgee Regional Airport (OKM) in Oklahoma to Bucky's Airport (WN09) in Wisconsin under instrument flight rules. While cruising at 7,000 feet MSL, the pilot heard an unusual noise from the engine, followed by a total loss of engine power. The pilot observed smoke and suspected oil loss before executing a forced landing in an open field near Shulls andburg, Wisconsin. The pilot was not injured, though the aircraft sustained substantial damage to the lower fuselage and the nose landing gear wheel well structure.
The investigation
An examination of the engine revealed a hole in the crankcase located above the number 1 cylinder. A teardown of the engine showed wear on the mating surfaces of the crankcase halves consistent with fretting. Investigators found that the number 2 main journal bearing had shifted within its support, the lock tab was elongated, and the bearing had been extruded. Fragments of the bearing were found in the oil sump.
Further analysis showed the crankshaft had separated between the number 2 main bearing journal and the number 2 connecting rod journal. Discoloration near the separation point indicated thermal damage caused by a lack of oil lubrication. Additionally, the oil pump housing and impellers showed scoring from the passage of hard particles. Maintenance records showed the engine overhaul was completed in November 2006, and the most recent annual inspection occurred in August 2017. The engine had accumulated 848.9 hours since its last overhaul.
Findings
Engine analysis indicated that the primary failure was caused by improper torque of the engine through-bolt nuts. This improper torque allowed for relative movement between the crankcase halves, leading to the observed fretting. This movement subsequently caused damage to the number 2 main journal bearing and a loss of oil lubrication, which ultimately resulted in the crankshaft separation and catastrophic engine failure.