Power Loss Incident
The pilot of a single-engine, turboprop-equipped agricultural airplane was en route to a spray site when the engine began losing power. The pilot described the power loss as a "min-flow" condition and performed an emergency landing in a field.
Examination Findings
After the incident, the fuel control unit (FCU) was removed from the engine for examination. Inspection of the rod guide revealed a small leak between the compressor discharge pressure (CDP) port and the overboard drain; however, the leak was insufficient to cause a significant decrease in fuel flow. Further examination of the bellows within the FCU showed a small hole in the outer ply. This hole was determined to be significant enough to reduce fuel flow. The hole was caused by pitting corrosion, but the corrosive agent could not be identified, making it impossible to determine when the corrosion began.
Service Bulletin History
On July 3, 2006, Pratt and Whitney Canada issued Service Bulletin 13408, titled "TURBOPROP ENGINE FUEL CONTROL UNIT - REPLACEMENT OF." This bulletin applied to PT6A-65AG and PT6A-65B engines. Its purpose was to reduce the chance of a power rollback due to reduced fuel flow resulting from a leak in the FCU bellows caused by material irregularities. The bulletin instructed operators to replace the hydromechanical FCU with a new unit featuring a more robust P3 bellows design and an air flow deflector to prevent P3 air impingement on the bellows. Pratt and Whitney recommended replacing the FCU the next time the engine was disassembled and access to the necessary subassembly was available. A review of the aircraft logbooks revealed that this service bulletin, while not mandatory, was applicable to the engine but had not been complied with at the time of the incident.