What happened
The aircraft was involved in a forced landing within a corn field following an unexpected loss of engine power. Prior to departure, the pilot reported that the engine appeared to be running roughly during start-up. After adjusting the mixture and performing a run-up check, the engine operation smoothed out sufficiently for takeoff.
Approximately twenty minutes into the flight, the engine began to vibrate noticeably. The exhaust gas temperature (EGT) climbed to 1379 degrees Fahrenheit. In response, the pilot enriched the fuel mixture, which caused the EGT to drop to 1349 degrees; however, the vibration intensified rather than subsiding. As the pilot reduced power in an attempt to manage the situation, the number one cylinder detached from the engine and penetrated the cowling. The pilot subsequently shut down the engine and managed to land the aircraft safely.
The investigation
Post-accident examination of the engine revealed that the crankcase halves were not fastened together securely. A critical maintenance error was identified regarding the hold-down nuts on the number three and number four cylinders. These nuts had been torqued to only 35 foot-pounds, which is significantly below the required specification of 55 foot-pounds.
Maintenance records indicated that this specific engine crankcase had been replaced when the engine reached 1137 hours of operation. The last major overhaul was performed at 3969 hours, and the most recent annual inspection occurred at 4393 hours. The discrepancy in torque values suggests a failure to adhere to standard maintenance procedures during one of these recent service intervals.
Findings
The primary mechanical failure was caused by the improper torque applied to the cylinder hold-down nuts. The under-torqued fasteners allowed the crankcase halves to separate under operational stress, leading to the loss of the number one cylinder and subsequent power loss. This case highlights the critical importance of verifying torque specifications during engine maintenance and overhaul procedures.