ERA18LA168 · Stinson L 5 C

While in cruise flight, the pilot noticed an increase in oil temperature and loss of oil pressure. Shortly thereafter, the engine lost total power, and the propeller stopped rotating. The pilot performed a forced landing to a lake, which resulted in substantial damage. Examination revealed that the propeller could not be rotated by hand, and the crankshaft was fractured at the No. 2 crank pin. Review of maintenance logbooks revealed that the most recent engine overhaul was performed about 24 years and 711 flight hours before the accident. Since that time, the propeller had been removed and replaced on two separate occasions due to propeller strikes. There was no record of any crankshaft inspections following either of the propeller strikes, contrary to manufacturer guidance. Additional damage found on the Nos. 1 and 2 cylinders was caused by the crankshaft failure and pistons that impacted their respective rods and skirts. In addition to the main fracture, a separate crack was discovered slightly aft of the No. 1 main bearing and had propagated nearly the width of the crankshaft, but had not fully failed. The thrust nut, which required 375 ft-lbs of torque, was easily removed without effort or special tools, indicating that it was torqued to a value significantly below specification. This could cause axial play along the crankshaft and contribute to the progression of the crankshaft failure that was likely initiated during the previous 2 propeller strikes. The crankshaft likely failed after its structural integrity was compromised during previous propeller strikes. The improper torque on the thrust nut likely exacerbated the failure of the crankshaft.

Verbatim NTSB analysis and probable cause from the NTSB dataset