Analysis of failed turbocharger turbine wheel shafts, Garrett model TH08A

On 4 August 2005, a Piper PA-31-350 Chieftain aircraft, registration VH-MZM, departed Dubbo Airport, NSW, for a local post-maintenance acceptance flight following an engine change. Shortly after departure, the pilot reported that the right engine manifold pressure fluctuated then dropped and maintained 28 inHg. The aircraft was returned to Dubbo with reduced power to the right engine.

Examination of the aircraft by the overhaul engineers revealed the newly-installed turbocharger turbine wheel had failed. The turbocharger was supplied to the ATSB for further examination and analysis.

During the course of the investigation, a second, similar turbine wheel failure occurred in a Piper PA-31-350, registration VH-TZY. Similarly to the first incident, the pilot observed significant manifold pressure fluctuations during cruise flight of the aircraft and the right engine was shut down. The aircraft was returned to the departure aerodrome. The turbocharger had been in service for approximately 241 hours and was subsequently forwarded to the ATSB.

Analysis of both turbine wheels revealed fracture characteristics indicative of a fatigue cracking mechanism. The failures had both occurred through a sealing ring groove, adjacent to the friction-welded joint between the turbine wheel and shaft components. While the post-failure damage of the fracture surfaces in both instances prevented the identification of the prime factor/s giving rise to the initiation of fatigue cracking, it was likely that the stress-raising effect of the ring groove location, geometry and associated microstructure, when combined with pre-existing cyclic loading conditions, was sufficient to initiate fatigue cracking and consequent turbine wheel failure.