The Australian Transport Safety Bureau (ATSB) has ruled that creep rupture in the stage-1 low pressure turbine blades of a CFM56-3C-1 turbofan was the likely cause of an abnormal engine problem encountered on a Qantas Boeing 737-400 in 2009.
"Engine disassembly and inspection revealed signicifcant damage to the stage-1 LPT," ATSB said in a statement.
"Analysis of the stage-1 LPT blades show that some blades had sustained levels of thermally-induced microstructural degradation, which may have affected the creep resistance of the alloy and resulted in the blades being suspceptible to failure by creep rupture."
As a result of the finding, CFM will revise its service bulletin SB 72-1113 and expand the range of blade manufacturing batch numbers that are predisposed to creep-related failure, ATSB said.
Blades in identified batches will be withdrawn from service when next removed from the engine, the statement added.
The aircraft, registered as VH-TJY, departed Brisbane for Melbourne at approximately 1900 EST on 10 November 2009.
As the aircraft ascended 24,000ft (7,315m), the flight crew heard a loud thump from the right side of the aircraft, followed by a rise in the exhaust gas temperature and engine vibrations beyond normal limits.
Engine indications returned to normal after the crew retarded the right engine thrust lever. By this time, a non-normal checklist for engine limit, surge and stall had been completed.
The aircraft landed safely at Brisbane Airport. As a precaution, the right engine was shut down during taxi.
Preliminary borescope inspection by the operator's engineering staff revealed significant damage to the low pressure turbine (LPT) assembly.
The engine was returned to an overhaul facility in Melbourne for disassembly and inspection, overseen by ATSB investigators.
ATSB added that creep rupture was identified as the likely mechanism failure in previous stage-1 LPT blade failures in the CFM56 engine.