On July 25, 2000, a Cessna 421A (registration N1477D) was involved in an aviation accident near Homewood, MS. Investigators recorded the probable cause as: The inadequate installation of the right propeller by the mechanic for his failure to properly torque the eight nuts resulting in fatigue failure of the studs and separation of the right propeller during cruise flight. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During cruise, a vibration preceded complete separation of the right propeller, which contacted the right horizontal stabilizer. The connecting rod of the No. 3 cylinder failed from fatigue and elevated temperatures. Installation issues included uncalibrated torque wrenches and outdated manual.
Incident Overview
During cruise flight, a slight vibration was felt before the right propeller completely separated and contacted the right horizontal stabilizer. The flight diverted and landed uneventfully.
Examination Findings
Investigation revealed that the right engine No. 3 cylinder connecting rod had failed, with evidence of exposure to elevated temperatures and higher stress fatigue crack propagation. Extensive fretting was noted on the faying surfaces of the right propeller gear and hub, and on both sides of the propeller bulkhead and fillet assembly. The holes in the propeller bulkhead assembly and propeller gear were elongated and egg-shaped. Fatigue was observed on all eight studs, with wear on the shanks of seven studs; the eighth stud was not examined. Four of the seven recovered spacers were damaged, shortening their length. Only two of the eight fractured studs were extended the specified minimum distance beyond the nut. No discrepancies were found with the threads in the hub holes of the left propeller, or with the threads in the right propeller hub holes that were checked. The fractured propeller blade from the right propeller exhibited overstress failure.
Maintenance Issues
The mechanic who installed the right propeller used uncalibrated torque wrenches and an outdated Service Manual. The airplane had accumulated approximately 50 hours since installation of the right propeller.