Amateur-built STOL CH750 N904MS incident near Cassville, Missouri

Probable cause

The fatigue failure of the propeller hub retaining bolts due to insufficient clamping force .

NTSB analysis

The airplane was in cruise flight when its propeller separated. The pilot performed a forced landing to a snow-covered field, during which the nose landing gear collapsed causing minor damage. Postaccident metallurgical examination of the crankshaft extension attachment bolts revealed that they had fractured from fatigue due to insufficient clamping force. Dish-shaped washers were installed between the bolt head and extension, and thread locking compound was found trapped under the concave side of the washers. The maintenance manual in effect at the time of the incident called for application of thread locking compound to both the female threads in the crankshaft and to the male bolt threads. However, this procedure has the potential for inadvertently wiping a portion of the thread locking compound that is applied to the bolt onto the face of the propeller extension without the installer knowing, thereby trapping it between the washer and the extension face. In this incident, the torque applied to the attachment bolts at installation was insufficient to squeeze out the thread locking compound from under the washers and compress the washers flat, which would have produced the proper metal-to-metal contact. While the applied torque may have been less than specified, it is also possible that the trapped thread locking compound may have had a hydraulic effect that resisted the clamping force associated with a properly applied torque. In the former case, insufficient clamping force would have been present from the time of installation. In the latter case, clamping force could have lessened over time due to wear at the edges of the washer, compression creep, and wear of the trapped thread locking compound. Since this event, the manufacturer has implemented changes to the maintenance procedures with detailed instructions and warnings to prevent contamination of the crankshaft, propeller hub extension, washers, and bolt faces with thread locking compound. In addition, the design of the hub attachment has been modified to incorporate three 8 millimeter dowel pins between the hub and crankshaft to prevent movement between the propeller extension and crankshaft.

Text from the NTSB. Open the NTSB docket for CEN11IA235 for the full record.