NTSB Identification: SEA02LA121.
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Accident occurred Wednesday, July 03, 2002 in Shelley, ID
Probable Cause Approval Date: 03/30/2004
Aircraft: Grumman-Schweizer G-164B, registration: N297AL
Injuries: 1 Minor.

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 had just commenced a spray run when there was an abrupt loss of power accompanied by engine roughness and vibration. Unable to maintain flight, the pilot executed a forced landing into the field being sprayed, landing across the rough, furrowed terrain. Post crash examination revealed that the top of the number five cylinder had separated from the lower half of the cylinder. Further metallurgical examination revealed that the cylinder split circumferentially due to fatigue which originated at the first and second thread roots of the cylinder threads on the inside of the cylinder head. Dimensional examination revealed that the threads between the cylinder base and head had been improperly machined resulting in the cylinder's abutting surfaces not coming into complete contact during assembly of the two components.

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

A loss of engine power due to the manufacturer's improperly machined threads, which prevented the cylinder base/head halves from being properly assembled, leading to fatigue propagation and the ultimate separation of the cylinder. A contributing factor was the rough terrain.

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