The Australian Transport Safety Bureau (ATSB) says that a problem in the "manufacturing process" led to the 4 November uncontained engine failure on a Rolls-Royce Trent 900 powering a Qantas Airways A380.
Investigations at Rolls-Royce facilities in Derby revealed an "area of fatigue cracking" within a stub pipe that feeds oil to the High Pressure (HP)/Intermediate Pressure (IP) bearing structure.
That was associated with a misaligned region of counter-boring within the stub pipe outlet, producing a localised thinning of one side of the pipe wall. That led to an uncontained failure of the IP turbine.
"The turbine disc, blade and nozzle-guide vanes separated into a number of sections, rupturing the surrounding IP turbine casing and damaging the engine's thrust reverser. Damage was also evident to the engine cold stream duct and outer cowl panels. The No 2 engine thrust links were severed and extensive damage was evident to the LP turbine nozzle guide vanes and stage-1 LP turbine blades," says the ATSB.
Most damaging to Rolls-Royce is the ATSB's belief that this was a manufacturing defect.
"[The] misaligned stub pipe counter-boring is understood to be related to the manufacturing process. This condition could lead to an elevated risk of fatigue crack initiation and growth, oil leakage and potential catastrophic engine failure from a resulting oil fire," says the agency.
This "critical safety issue" requires immediate action by the airlines operating Trent 900-powered A380s, says the ATSB and Australia's Civil Aviation Safety Authority. They have recommended an additional one-off inspection within two flight cycles by the operators. These are Qantas, Singapore Airlines, and Lufthansa.
"In the event abnormal or eccentric counter-boring of the tubes described in the service bulletin is identified, this must be recorded as a major defect of the engine," says CASA.
Industry sources say that the problem affects only the "A mod" and "B mod" variants of the Trent 900.The latest variant, "C mod", is not a subject of the finding. This appears to indicate that Rolls-Royce may have identified the fault and fixed it on the latest engine variant. It is not clear, however, when the fix was implemented.
The engine manufacturer has spelt out the procedure for detecting the problem. It requires the "specialised examination, measurement and reporting of the HP/IP bearing structure stub pipe counter-bore geometry". A 20-flight cycle compliance limitation was specified for the examination.
It says in a brief statement that the ATSB's findings are consistent with its comments so far.
"Since 4 November, we have been working closely with the regulators, Airbus and our airline customers and implemented a regime of inspection, maintenance and removal which has assured safe operation and will progressively allow the whole fleet of Trent 900 powered Airbus 380s to return to service. Rolls-Royce will continue to work with the investigating authorities and the regulators to ensure compliance with safety standards," it adds.