The order by the European Aviation Safety Authority backed earlier indications from investigators that they suspect a turbine disc was the cause of last week's engine failure on the Airbus A380, but was the first official mention of an oil fire preceding the engine's disintegration.
The A380 engine failure shortly after takeoff from Singapore on Nov. 4 has raised concerns over the safety of the world's biggest passenger airplane three years after its debut. The failure sent shrapnel slicing through the plane's wing and hurtling down over an Indonesian island before pilots made a safe emergency landing with 466 passengers and crew aboard.
Qantas said this week it had found small oil leaks on Rolls-Royce Trent 900 engines on three of its other Airbus A380s during tests after the Nov. 4 incident. The airline, Australia's national carrier, said Thursday it was keeping its six A380s grounded until further checks were completed - extending an earlier deadline.
Singapore Airlines on Wednesday after checks prompted by the Qantas incident revealed what the company called oil stains in the Trent 900 engines. Lufthansa also uses the A380-Trent 900 combination, but said on Wednesday its checks had not turned up anything untoward.
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The European regulator said in a new "emergency airworthiness directive" posted on its website Thursday that airlines using Trent 900 engines should conduct "repetitive inspections" of them.
Twenty planes operated by Qantas, Germany's Lufthansa and Singapore Airlines use the Trent 900 engines. Nine have been grounded - six Qantas and three Singapore Airlines.
EASA said airlines should check several parts of the engines, including the oil service tubes, to ensure there is no "abnormal" leakage. If any such leaks are found, the airlines are prohibited from using the engines.
The directive was issued in response to the Qantas engine failure. EASA said an analysis of the investigation into the incident so far "shows that an oil fire" in part of the engine "may have caused the failure" of the engine's intermediate pressure turbine disc.
"This condition, if not detected, could ultimately result in uncontained engine failure potentially leading to damage to the aeroplane and hazards to persons or property on the ground," the directive said.
The Australian Transport Safety Bureau, which is taking the lead in the investigation, has focused on a mangled section of a shattered turbine disc that was recovered from the stricken plane. It has been sent to Britain for testing, with investigators coordinating with Rolls-Royce, the bureau says.
Airworthiness directives are issued by the European agency to advise airlines about extra inspections or repairs needed to deal with potential problems on planes, and are relatively common occurrences covering many different types of planes and engines.
However, those classified as emergency directives are unusual, said Jason Middleton, an aviation professor at the University of New South Wales in Sydney.
William Voss, head of the Flight Safety Foundation based in Alexandria, Virginia, said the EASA directive indicated the investigation into the Qantas incident had narrowed, and EASA was highlighting the oil service tubes as the likely source of the leak so airlines could more easily inspect for the problem.
"It appears the investigation has shown that oil contamination and burning in the very hot section of the engine where the energy is extracted from the fuel is a problem on that type of engine," he said. "I expect Rolls-Royce is working to fix this very quickly."
The latest directive was the third one issued this year on the Rolls-Royce Trent 900 engines.
In one of those directives, the European agency warned that unusual wear to parts of the engine could cause problems in the intermediate pressure turbine - the same part of the engine identified in Thursday's directive.
Too much wear could cause the turbine to move backward into a nonmoving part of the engine, the earlier directive said. That could eventually lead to an oil fire and an uncontained engine failure.
Middleton said the engine parts the agency has directed airlines to check all appear to be in the same area as the damaged disc.
The directive seems to confirm an oil fire erupted inside the engine, and suggests that the fire may have caused the disc to fail, Middleton said.
He cautioned that it's simply too soon to tell if the issues are related. Still, he said, it is intriguing.
"The original (directive) does point at an area which looks to be one of interest right now," Middleton said. "There COULD be a connection there."
Qantas spokesman Tom Woodward said the airline's checks were already complying with the new EASA order, and that there had been no new discoveries of any problems. Still, engineers were conducting further tests on the three engines where oil was found, and none of the A380 fleet would return to the air for the time being, he said.
"The objective is to get them back in service as soon as possible," Woodward said. "We don't want to attach a timeframe to that at this stage, because the situation is pretty fluid. The inspections are ongoing and it really depends on when our engineers are satisfied it is safe to bring them back into service."
Singapore Airlines said the new directive did not mean any disruptions to its services. The airline is replacing the engines on the three A380s that had oil stains and has deployed other types of jets to fill the gaps, spokesman Nicholas Ionides said.
"Singapore Airlines has been, and will be, in full compliance with the directive," Ionides said in an e-mailed statement. "Precautionary engine changes have been carried out for three engines, and we are inspecting our wider fleet in accordance with the directives set out by EASA and the recommendations from Rolls-Royce."
The Qantas and Singapore incidents are not the first problems Rolls-Royce has faced with its engines. In September 2009, a Singapore Airlines A380 was forced to return to Paris mid-flight after an engine malfunction. Last August, a Lufthansa crew shut down one of its engines as a precaution before landing in Frankfurt after receiving confusing information on a cockpit indicator.