What happened
On 5 January 2004, a Piper PA-2SO-201T, registration G-DIZY, experienced a loss of engine power while operating near Buckmore Park, Blue Bell Hill, Rochester. Initial suspicions during the investigation focused on a potential failure of the engine-driven fuel pump; however, subsequent examinations of the pump revealed no mechanical faults.
The investigation
Following the clearance of the fuel pump as a cause, the aircraft's maintenance organisation performed a deeper inspection of the powerplant. This secondary investigation identified that the turbine wheel within the turbo-charger unit was stiff and resistant to manual rotation. The unit was dispatched to an FAA-approved overhaul agency in the United States for detailed analysis.
The inspection report from the US facility confirmed that there was no mechanical damage to the internal components and that the nut on the turbine/compressor shaft had been correctly torqued. However, investigators discovered a significant accumulation of carbon deposits situated between the turbine wheel and the bearing housing seal area. This buildup created an interference that restricted the movement of the rotating assembly.
Findings
Because a turbo-charger relies on extracting energy from exhaust gases to boost manifold pressure, the restricted movement of the unit caused an obstruction in both the inlet and exhaust manifolds. This resulted in increased exhaust back-pressure and a simultaneous drop in inlet manifold pressure, which directly led to the loss of engine power.
While such carbon deposits are typically the result of failing to follow engine cooling-down procedures—whereby oil is allowed to idle and cool before shutdown—the operators of G-DIZY were noted to be highly diligent in following all standard shutdown protocols. The turbo-charger had been installed in August 2001 and had only completed 275 operating hours, making the rapid accumulation of carbon difficult to account for.