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.