MINNEAPOLIS // Three months before a superjumbo jet engine blew apart and forced an emergency landing, European safety regulators had relaxed their inspection order for the same section of the engine implicated in the accident.
In January, the European Aviation Safety Agency (Easa) required airlines to inspect for wear on the shaft that holds one of the Rolls-Royce Trent 900 engine's turbine discs. The more wear they found, the sooner future inspections would be required.
In August, after Rolls-Royce had inspected several engines, Easa revised its directive. Previously, airlines had to calculate how worn out the part was based on the worst spot. Under the revised directive they calculate the average wear over the entire part.
And previously they had to assume the part was wearing out at a worst-case rate. The new rule allows them to calculate the wear rate on each engine. That meant less frequent inspections, which the revised directive said were "sufficient to prevent unacceptable wear."
The European directives warned of the potential for "in-flight shut down, oil migration and oil fire." The US Federal Aviation Administration went further in adopting a version of the European directive in September, warning of an "uncontained failure of the engine, and damage to the airplane."
Some of the parts inside jet engines rotate faster than the speed of sound. Engines are designed so that even if part of one shatters, pieces of metal are not sent rocketing away from the engine. An "uncontained engine failure" with shrapnel-like engine pieces that can damage other parts of the plane is rare and extremely dangerous.
That is what happened on November 4. Investigators have said that leaking oil caused a fire in the engine of a Qantas A380 that heated metal parts and made the motor disintegrate over Indonesia before the jetliner returned safely to Singapore. Experts say the fire damaged vital systems on the plane, which had been bound for Sydney.
The safety order was not addressing the exact same problem that caused the Qantas engine to disintegrate, but is very similar and involved a turbine next to the one that broke apart, said Chuck Eastlake, a former professor of aerospace engineering at Embry-Riddle University in Daytona Beach, Florida.
The decision to relax the Easa order was likely based on inspections that gave engineers confidence that the wear on parts that could cause an oil leak was predictable enough to allow more time to elapse, Eastlake said. In hindsight that appears not to have been the case, he said.
"That kind of stuff is always a judgment call based on experience," Mr Eastlake said. "It's hard to specifically justify a decision like that because it isn't a matter of plugging numbers into a calculator and out comes an answer."
John Cox, an aviation safety consultant and former airline pilot, said it's a question of balancing "what is reasonable to ask the airlines to do against safety."
"The problem is we had a catastrophic failure. It turned out that apparently at least one engine had substantial wear that inspections didn't pick up," he said in a telephone interview from London.
No one from Easa was available to talk about the directive.
The agency usually takes the lead in issuing safety orders involving aircraft, engines and other equipment made by European manufacturers, just as the FAA does in cases that involve US manufacturers.
The Qantas spokesman, Tom Woodward, said Qantas has complied with all safety orders.
Rolls-Royce Group PLC said in an update to investors on Friday that the Qantas engine incident was due to failure in a specific component that caused an engine fire and "the release of the intermediate pressure turbine disc." The company will be replacing the relevant part "according to an agreed programme" as inspections on the engine continue in association with aviation regulators, it said. The company did not provide details.
The disc, a plate that holds the turbine blades that move air through the motor, broke apart.
Three airlines - Qantas, Singapore and Lufthansa - fly the 20 A380s that use Trent 900 engines. Qantas and Singapore have grounded several of those after each found oil leaks on three engines.
Lufthansa spokesman Thomas Jachnow said the German airline has been told "that Rolls-Royce will gradually replace a modular part of the engine on all Trent 900 engines." He added that the "exact parts to be replaced haven't been finalised yet."
The Qantas chief executive, Alan Joyce, said yesterday that he was pleased with the progress of the Rolls-Royce investigation. But he would not give a timeline on when Qantas' six A380s, grounded since the failure, would be back in the air.
* Associated Press