What happened
While maneuvering an agricultural airplane to enter a field for the purpose of commencing spraying operations, the aircraft's engine lost power at an altitude of 120 feet AGL. The pilot attempted to restart the engine, but the attempt was unsuccessful. During the subsequent forced landing, the aircraft contacted circular rows of cotton before coming to rest in an upright position. There were no injuries reported.
The investigation
The engine underwent examination at the manufacturer's facility. Investigators discovered silicon sealant on the crankcase mating surfaces and on the base flanges of the #1, #2, #4, and #6 cylinders. Evidence of fretting was found on the main bearing bosses, the mating surface of the crankcase halves, and the cylinder base flanges.
Mechanical inspection revealed that the #2 main bearing was fragmented, with a cracked and damaged bore. Furthermore, the crankshaft was found to be fractured between the #3 crankshaft cheek and the #2 main bearing. A metallurgist from TCM determined that the origin of this fatigue fracture was located at the aft radius of the #2 main bearing.
Findings
The investigation concluded that the presence of silicon sealant prevented the bolts mating the crankcase halves from being properly torqued. This improper assembly led to vibrations at the crankcase mating surfaces, which caused the #2 main bearing to shift and ultimately fail, resulting in the loss of engine power.