NTSB Identification: SEA02LA130.
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Accident occurred Friday, July 12, 2002 in Laurel, MT
Probable Cause Approval Date: 11/25/2003
Aircraft: Thurmond Moni, registration: N9104S
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The pilot of the homebuilt aircraft and installer of the wooden propeller assembly reported that during a performance test/rate of climb test with the new propeller, he smelled a wood burning smell followed by a "real bad vibration." The pilot initiated a forced landing to an open field. During the landing roll on the soft terrain, the landing gear sheared off. After the accident, the pilot noted that one of the blades of the wooden propeller was missing. The remaining blades and two hub plates were sent to the NTSB Materials Laboratory for examination. The metallurgist reported that the fretting observed on the hub plates suggested that the hub mounting bolts were not as tight as required allowing the hub to oscillate on its mounting flange. The motion in the pitch block of both blades produced the heat that charred the wood and softened the pitch block material allowing it to be displaced and imprinted with the pattern of the wood plies. The compression failure and the frictional features on the hub portion in the pitch block indicated a fatigue-like failure in wood which propagated until the remaining wood could not withstand the stresses of operation and subsequently failed in a tensile mode.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

Undertorqued mounting bolts which resulted in fatigue to the wooden propeller blade and subsequent in flight separation of one of the blades while in cruise flight. An improper installation of the propeller assembly and soft terrain encountered during the landing roll were factors.

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