What happened
On October 22, 2008, a Cessna C208B, registration PT-OGE, was performing a flight from Marília to São José do Rio Preto, Brazil. The aircraft was operated by Vera Cruz Táxi Aéreo Ltda. and was carrying one pilot and two passengers.
While cruising at 5,500 feet under Visual Flight Rules (VFR), the pilot detected an intermittent abnormal noise from the engine, accompanied by fluctuations and a subsequent drop in oil pressure. Following established emergency procedures, the pilot reduced power, feathered the propeller, and shut down the engine. Unable to reach an alternative runway, the pilot executed an emergency landing in a recently planted sugarcane field. The aircraft sustained light damage, but the pilot and both passengers were unharmed.
The investigation
CENIPA's investigation focused on the mechanical failure of the PT6A-1 and the maintenance records of the engine. Upon inspection, investigators found excessive metal shavings in the oil filter and chip detector. A teardown of the engine's reduction gearbox and turbine shaft housing revealed that the shaft had seized due to the failure of three sun gear components.
Testing confirmed that the failure was caused by fatigue in the gear set components and an overload failure in the sun gear. The investigation also uncovered significant discrepancies in maintenance documentation. Records regarding the total flight hours of the engine were inconsistent between the annual maintenance inspection forms and the aircraft logbooks. Furthermore, investigators found that the origin of several engine components could not be clearly traced due to conflicting documentation, with some parts being attributed to different engine serial numbers. There was also evidence that a previously overhauled item lacked the required release-to-service documentation.
Findings
- Maintenance oversight: The operator and the contracted maintenance facility failed to detect documentation conflicts regarding the airworthiness of engine components during the receipt of overhauled engines.
- Inadequate supervision: The operator accepted a justification from the maintenance shop regarding oil pressure operating at the lower limit of the green arc without independent verification, which compromised safety margins.
- Component fatigue: The primary cause of the engine failure was fatigue failure in the gear set components, likely exacerbated by an overload on the sun gear.
- Traceability issues: Discrepancies in maintenance logs and the lack of reliable component origin tracking made it impossible to verify the service life of certain parts, potentially allowing components with unknown or excessive service hours to remain in use.