Signs of flaws before A380 disintegration

2010-11-12 13:31

kalahari.com

London - Rolls-Royce engines on the world's largest airliner malfunctioned four times before one of them disintegrated during flight last week, and aviation experts said that the earlier mishaps may hold clues to design or construction flaws.

Four problems dating to 2008 led to two warnings for airlines to check parts of the Rolls-Royce Trent 900, a technologically advanced model used by 20 Airbus A380s around the world.

Three of the four previous problems centred on the turbines or oil system. An oil leak is suspected in last week's disintegration, and since then several A380s have been grounded after oil leaks or stains were discovered in another six Trent 900 engines.

The number of problems identified in the Trent 900 is not unusual for a jet engine, and analysts, pilots and former safety investigators caution that there is no obvious link between the earlier mishaps and last week's failure.

But the experts also said on Thursday they suspect there may be critical problems that weren't detected before the engine went into use. Several experts pointed to the tubes that pump oil around the spinning turbines of the house-sized engine as a possible culprit.

Based on what authorities have said so far, it appears the problem with the Trent 900 is a "fundamental unforeseen flaw" that will likely be very expensive to fix, said Michael Barr, who teaches aviation safety at the University of Southern California.

Problematic certification process

Joerg Handwerg, a spokesperson for the pilots' union for German airline Lufthansa, said that minor problems are routine for any jet engine, but it is possible that the issues are an indication that regulators did not adequately check the engine before approving it for commercial use.

"When you see we have a problem with not just one of these engines but several then it points towards that we have a problem in the certification process," Handwerg said.

Barr said the Trent 900 is "out there right at the edge of the envelope" of new engines that are lighter, more energy efficient, less polluting and less noisy.

"The engine has been out there for a few years, they've got some miles on it, and now they're finding things they didn't foresee," he said.

The European air-safety regulator pointed to the oil system as a culprit in last week's incident, saying on Thursday that leaking oil may have caught fire in the Qantas Trent 900 that disintegrated and sent pieces of machinery slicing through vital control systems in the wing.

With the final determination of what caused the Qantas accident months away, experts said it was too early to know if regulators had missed clues that could have helped prevent the midair disintegration.

Premature wear

The European Aviation Safety Authority (EASA) issued an emergency order requiring airlines to re-examine their Trent 900s and ground any aircraft with suspicious leaks. It said a preliminary probe showed that an oil fire had broken out in the section housing the turbines, shafts that power the engine when they are spun at great speeds by combusting jet fuel. An oil pump and network of tubes lubricate and cool the turbines.

EASA said the blaze may have caused the breakup of the intermediate pressure turbine disc, a heavy metal plate that holds the blades of the middle of three turbines.

Turbine engines are known to generate vibrations that can cause parts to wear prematurely. The EASA order, known as an airworthiness directive, indicates that the oil tubes may have fractured as a result of such vibrations and spewed oil in a very hot section of the engine, causing a fire. The resulting heat could have caused the rotor to which the turbine blades are attached to expand, bringing the turbine blades into contact with the casing that encloses the engine.

"Once these things start rubbing, catastrophic failure can occur rapidly," said William Voss, head of the Flight Safety Foundation from Alexandria, Virginia.

There was no comment from Rolls-Royce Group PLC, a London-based aerospace, power systems and defence company separate from the car manufacturer. The company and the safety agencies probing last week's incident have released few details of their investigation and have not said whether they are re-examining the earlier mishaps.

Of the four earlier mishaps, two were considered too minor to investigate at the time. Another two were serious enough that regulators ordered airlines to check their Trent 900s for possible danger.

August warning

In 2008, EASA warned of potential cracking on the outer surfaces of the metal plates that direct hot air from the jet-fuel combustion chamber onto the spinning turbine blades. The debris could block the gas flow and crack the turbine blades, leading to an in-flight engine shutdown, it said.

In August, the agency warned of unusual wear in the metal slots along the outer surface of the drive shaft in the intermediate pressure turbine - the same part of the engine identified in Thursday's European directive. These slots fit into the grooves on the rotor, allowing it to drive the engine's turbines. With the rotor becoming increasingly wobbly on the shaft, changes in thrust could generate stresses that could lead to catastrophic failure by the rotating turbine rearward into the engine's stationary casing.

In both cases, EASA recommended repeated inspections to check for cracks and unexpected wear. The agency did not release details about the incidents that led to the 2008 and the August directives.

Also in August, a Lufthansa crew on a Tokyo-Frankfurt A380 carrying 526 passengers shut down one of its engines after seeing a warning of low oil pressure. The flight landed safely and mechanics discovered a break in a shaft responsible for pumping oil into the engine, the German air safety agency said.

The German agency said it did not investigate. Neither incident was serious enough for EASA to issue an airworthiness directive afterward, said Dominique Fouda, a spokesperson for the Europe-wide agency.

Lufthansa spokesperson Thomas Jachnow said the German airline was still investigating the Frankfurt incident and could not comment on the cause.

Different parts

In September 2009, a Singapore Airlines A380 flying from Paris to Singapore turned back after takeoff when the crew noticed unusual vibration in an engine and shut it down.

France's BEA transport investigation agency said no investigation was conducted because the plane was under the crew's control at all times and passengers' safety was never at risk.

Airbus spokesperson Stefan Schaffrath said the cause of the Singapore incident was identified and fixed.

"Clearly it wasn't the same part and not the same location as on the Quantas investigation," he said, without providing details.

The problem that aviation regulators face with incidents such as those that sparked the EASA's past airworthiness directives lies in determining how serious they are and how far they should go in requiring a fix. Raising the alarm in every case, including those that are easily fixable and not a threat to aviation safety, could force the unnecessary grounding of entire fleets of airliners.

Twenty A380s operated by Qantas, Lufthansa and Singapore Airlines use the Trent 900 engines.

Lufthansa all clear

Qantas said this week it found small oil leaks in engines on three of its other Airbus A380s in tests conducted after the November 4 incident. Australia's national carrier said on Thursday it was keeping its six A380s grounded until further checks were completed.

Singapore Airlines on Wednesday grounded three of its 11 A380s after checks prompted by the Qantas incident revealed what the company called oil stains in the Trent 900 engines. Lufthansa said its checks had not turned up anything wrong.

It's not clear from information released so far by safety officials whether there is a connection between last week's Qantas engine disintegration and problems identified in previous safety orders, said John Goglia, a former member of the National Transportation Safety Board and an expert on airline maintenance.

"We don't know enough yet to connect the dots," he said. But he added, "What we've seen so far may indicate this particular engine has a significant problem that may not be easily resolved."

The disintegration on the Qantas A380 was far more serious than the airline has implied in its public statements, Goglia said.

Damage from engine shrapnel to the wing over the engine occurred very close to the wing's front spar, one of two support beams in the wing that attach the wing to the plane, he said. If the shrapnel had hit the spar it could possibly have weakened the spar and even have caused the wing to fall off, he said.

Taking a hard look

As it was, the shrapnel appears to have damaged electrical cables and hydraulic lines inside the wing, Goglia said. Pilots were unable to close the landing gear doors, an indication of hydraulic damage, and had difficulty shutting down the engine next to the engine that disintegrated, an indication of an electrical problem, he said. The A380 has four engines.

"This accident is going to cause authorities to take an awful hard look at a lot of things that went wrong," Goglia said.

Photos and video of the incident and its aftermath show the shrapnel clearly ruptured a hydraulic line and an electric line in the wing, cutting off the pilots' control of half the brake flaps and the remaining engine on the affected wing, along with the door of the landing-gear compartment, Handwerg said.

"It's not a single failure, it's a multiple failure. That obviously creates a more risky situation," he said.

Investigators in Singapore have probed the stricken engine with borescopes - flexible tube-like microscopes with a light attached that are used for poking into hard-to-reach places, the Australian Transport and Safety Bureau (ATSB) said in a statement on Friday.

Some control and other components have been removed from the engine and sent to Rolls-Royce's repair and overhaul facility in Derby, England, for analysis, the bureau said in an update on the investigation, which it is leading but includes specialists from European and Asian agencies.

Search for parts and debris

 Investigators are waiting for a special cradle to be delivered - due sometime this weekend - from Germany so they can remove the huge engine.

Inspection of the engine had helped officials narrow the search area in Indonesia where they are still looking for engine parts and debris that fell off the plane. ATSB officials went to Batam Island on Friday to join Indonesian authorities looking for the missing parts, which include a chunk of a turbine disc that shattered in the explosion. The part of the disc that was recovered from the aircraft is being examined at the Rolls-Royce facility in Derby, under ATSB supervision.

Investigators have finished interviewing the flight crew about the blowout's effect on the aircraft systems and what the crew did to deal with it, and were starting to interview the cabin crew about safety procedures for passengers, the bureau said.

"It is anticipated that the examination of the management of the emergency will identify valuable insights into the handling of future emergencies in the A380," the bureau said.

- AP
Read more on:    singapore airlines  |  rolls-royce  |  lufthansa  |  qantas  |  air travel
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