Aircraft: Cessna 150. Injuries: 2 Minor. Location: Corinth, Miss. Aircraft damage: Substantial.
What reportedly happened: According to the pilot, after about two hours of uneventful flight a loud noise came from the engine compartment and the engine cowling began shaking violently.
The engine lost power. He was not able to restart the engine during the descent, so he selected a field for a forced landing.
During the landing roll the landing gear became entangled with vegetation and the airplane nosed over and came to rest on its back.
Examination of the engine revealed that the crankshaft gear bolt locking tab washer and the alignment dowel for the gear mounting flange were fractured due to fatigue. This allowed the gear to rotate on the crankshaft.
Investigators determined the fatigue was from improper torque application on the crankshaft gear bolt during the last overhaul, which had been completed 26 years and 643 total airplane flight hours before the accident.
In service bulletins and service instructions, the engine manufacturer warned that improper installation of the crankshaft gear may result in “complete engine stoppage” and recommended that engines be overhauled after 12 years in service, even if the engine had not accumulated the manufacturer-specified 2,000-hour time-between-overhaul during that period.
Probable cause: The improper torque application on the crankshaft gear bolt by maintenance personnel during an engine overhaul, which resulted in the fatigue failure of the locking tab washer and gear alignment dowel and subsequent engine stoppage. Contributing to the accident was the owner’s exceeding the engine overhaul calendar year limits recommended by the engine manufacturer.
NTSB Identification: ERA12LA574
This August 2012 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.