Incident Details
During cruise flight at 17,000 feet mean sea level, the left turbocharger failed, resulting in a partial engine power loss. The pilot declared an emergency and executed an uneventful landing at an alternate airport.
Investigation Findings
Examination of the failed turbocharger revealed that the turbine wheel had separated from the shaft. Three prior incidents involving similar failures of the same part number turbocharger had occurred in May 2007. Examination of all four failed turbochargers showed that they displayed the same discrepancy: the compressor wheel backface perpendicularity to the bore exceeded the maximum permissible value.
Production Change
The source of the perpendicularity error was traced to a change in the machining process for the compressor wheel, implemented in March 2007. This change involved a switch from a two-machine process to a three-machine process. The production change was made without formal documentation or coordination with other functional areas, and no formal first-article inspection was performed for the new process. In April 2007, the machine shop returned to the original two-machine process. The compressor wheels manufactured during the time the three-machine process was in use were installed in turbochargers produced from March 20 to April 20, 2007, including the four turbochargers involved in the subject incident and the three May 2007 incidents.
Corrective Actions
The manufacturer issued service bulletins in July 2007 identifying the affected turbochargers by serial number and mandating their replacement. In August 2007, the Federal Aviation Administration issued an airworthiness directive requiring compliance with the service bulletins.