NTSB Identification: DFW07LA203.
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Accident occurred Friday, September 14, 2007 in Floydada, TX
Probable Cause Approval Date: 05/28/2008
Aircraft: Air Tractor AT 602, registration: N8522P
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 was headed toward a spray site in his single-engine, turboprop-equipped agricultural airplane when the engine began to lose power. He described the power loss as a "min-flow" condition, and he was forced to make an emergency landing to a field. As a result, the fuel control unit was removed from the engine and the bellows and rod guide were examined. Examination of the rod guide revealed there was a small leak between the CDP and overboard drain; however, the leak was not enough to cause a significant decrease from the fuel control. Examination of the bellows revealed there was a small hole in the outer ply that was significant enough to reduce fuel flow. The hole was due to pitting corrosion, but since the corrosive agent could not be determined, it was not possible to determine at what point the corrosion began. On July 3, 2006, Pratt and Whitney Canada issued Service Bulletin 13408, titled TURBOPROP ENGINE FUEL CONTROL UNIT - REPLACEMENT OF, which applied to the PT6A-65AG and PT6A-65B engines. The purpose of the Service Bulletin was to reduce the chance of a power rollback due to reduced fuel flow as a result of a leak in the Fuel Control Unit (FCU) bellows from material irregularities. The Service Bulletin told the operator to replace the hydromechanical FCU with a new one which incorporated a more robust P3 bellows design and an air flow deflector to prevent P3 air impingement on the bellows. Pratt and Whitney recommended that the FCU be replaced the next time the engine was disassembled and access was available to the necessary subassembly. A review of the aircraft logbooks revealed that this Service Bulletin (which was not mandatory) was applicable to this engine, but had not been complied with at the time of the accident.

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

A leak in the outer ply of the BeCu bellows. This hole appeared to be the result of pitting corrosion, but the corroding agent was not determined.

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