Following a number of accidents and serious incidents involving Piper Chieftain PA-31-350 aircraft where a failure of one of the engine turbochargers had been central to the occurrence events, the Australian Transport Safety Bureau (ATSB) initiated a safety investigation into the broader issue of PA-31-350 turbocharger operational reliability.
In all of the principal occurrences, the turbocharger turbine wheel had separated from its central shaft. Metallurgical examination of the separated turbine wheel assemblies did not reveal any material/manufacturing anomalies that may have contributed to the failures.
During the course of the investigation, a number of other turbocharger related occurrences were identified. Most of the occurrences had resulted in a reduction in engine power which led to a range of outcomes, including engine shutdown, air returns, and diversions.
While in some occurrences, failure was the result of the separation of the turbine wheel from the turbine shaft, the investigation showed that turbocharger failure could arise from a number of causes, including lubrication issues and foreign object damage. It is likely that some of these mechanisms are interrelated, i.e. fatigue failure of the turbocharger shaft following bearing damage from an interruption or contamination of the oil supply.
No single contributory factor or common set of factors was identified across the failures examined.
Published literature has shown that turbocharger reliability can be significantly enhanced by ensuring that engine, aircraft and turbocharger manufacturer's operational procedures are closely followed - particularly in respect of the application and/or reduction of engine power levels. Specific maintenance attention to the turbocharger lubrication system is also important to ensure preservation and reliable operation of the turbocharger bearings.
Pilots of aircraft powered by turbocharged powerplants are reminded that a failure of the turbocharger system should not result in the complete loss of power from the affected engine. Attention is drawn to a US Federal Aviation Administration, Special Airworthiness Information Bulletin (SAIB) CE-09-11, which provides information for air crew on what to do in the event of a turbocharger malfunction or failure. A copy of that bulletin is included as Appendix B to this report.
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|Investigation level:||Complex - click for an explanation of investigation levels|
|State:||Other||Occurrence type:||Powerplant/propulsion - Other|
|Release date:||15 December 2011||Occurrence class:||Technical|
|Report status:||Final||Occurrence category:||Accident|
|Highest injury level:||None|