NTSB Identification: CHI04LA128.
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Accident occurred Saturday, May 29, 2004 in Hager City, WI
Probable Cause Approval Date: 03/30/2005
Aircraft: North American P-51C, registration: N61429
Injuries: 1 Fatal.

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 airplane collided with trees and terrain following a loss of engine power while maneuvering during an airshow. Witnesses reported hearing the pilot state that he had an engine malfunction as he was lining up for a low pass over the airport. They stated the propeller was turning, but the airplane was descending. The airplane passed under high tension power lines and collided with trees prior to coming to rest in the back yard of a residence. Examination of the engine revealed the upper vertical camshaft drive gear had backed off of the drive shaft. The camshaft gear fits on splines on the upper vertical drive shaft followed by a tab washer, a retainer washer, and a retaining nut. The retainer washer has two interior tabs that insert into smaller splines at the top of the shaft, which keep it from rotating. On the outer diameter surface, the retainer washer contains slots where tabs from the tab washer can be bent into place. A metallurgical examination of the gearbox cover, retaining nut, retaining washer, tab washer, and the camshaft drive gear revealed the components were installed in the correct order. The damage on the components indicates that the tabs on the tab washer were most likely not bent into place around the retaining washer and nut, allowing the nut to back off the shaft. It could not be determined if the nut had been properly torqued during its installation. The engine had been overhauled approximately 424 hours prior to the accident.

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

The improper installation of the camshaft drive gear assembly which resulted in the retaining nut backing off allowing the drive gear to move up the shaft. A factor associated with the accident was the low altitude at which the power loss occurred and the trees which the airplane contacted during the forced landing.

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