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Regulators have said preliminary investigations show an oil fire broke out in the section of the Qantas engine that houses the turbines, which are spun at great speeds by combusting jet fuel. An oil pump and network of tubes lubricate and cool the turbines.

Experts say the fire could have caused the rotor to which the turbine blades are attached to expand, bringing the blades into contact with the casing that encloses the engine. Part of a shattered turbine disc was found in the wreckage of the engine, and another part flew off in the disintegration and hasn’t been recovered, the ATSB said.

Qantas, which grounded its six A380s for more than three weeks after the blowout, said Thursday it would conduct one-time checks on its superjumbos. Spokesman Simon Rushton said the inspections were not expected to take long or disrupt service.

Qantas replaced 16 Trent 900s before putting just two of its A380s back into the skies five days ago. The others are still undergoing tests.

“After discussions with the Australian Transport and Safety Bureau and Rolls-Royce, it was decided it was prudent to conduct further inspections of engine components, although there is no immediate risk to flight safety,” Qantas said in a statement.

The ATSB finding strengthens Qantas CEO Alan Joyce’s stated belief the engine blowout was caused by a manufacturing problem, not a maintenance issue, for which the airline could bear responsibility.

Rolls-Royce has remained largely silent about the issue since it happened, and the London-based company’s share price has seesawed. Rolls-Royce shares were down 1.4 percent at 610 pence ($9.51) in the first hour of trading on the London Stock Exchange on Thursday.

Qantas said Thursday it had filed a statement of claim in a federal court that will allow it to pursue possible legal action against Rolls-Royce if it isn’t satisfied with a compensation offer from the engine manufacturer.

“Today’s action allows Qantas to keep all options available to the company to recover losses, as a result of the grounding of the A380 fleet and the operational constraints currently imposed on A380 services,” Mr. Joyce said in the statement. He has refused to specify how much compensation the airline wants.

On Nov. 12, the company said its full-year earnings would be “slightly lower than previously guided” because of the problem with the engines. In July, it forecast underlying profits would grow by 4 percent to 5 percent compared with 2009.

John Page, an aircraft designer and senior lecturer in aerospace engineering at the University of New South Wales, said one possible explanation for a misalignment of the oil tubes was a programming error in computers used in the manufacturing process.

“The problem with it is if there is an error in the coding of the machining, then (to) some extent it’s not as obvious — because nobody’s actually doing it by hand,” he said. “As designs become more and more automated, people are expecting more perfection. It’s much closer to perfection — it just ain’t perfect.”

The ATSB is due to publish its preliminary report into the Qantas incident on Friday.

Chief Commissioner Martin Dolan said the agency pre-empted the report with Thursday’s safety recommendation because it realized only on Wednesday in discussions with Rolls-Royce the significance of the counterboring issue.

“We considered it was a sufficiently significant safety issue that we should immediately release it to parties who were operating with these engines,” Mr. Dolan told the Associated Press.

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