By Tim Hepher
DUBLIN (Reuters) - Airbus acknowledged a combination of internal manufacturing and design flaws as more examples of wing cracks arose during checks on the A380, while insisting the world's largest airliner is safe to fly.
A top executive said the European planemaker had established how to repair the cracks on a small number of parts inside the superjumbo's wings, which prompted European safety authorities to order inspections last week.
Airbus and one of the leading operators, Singapore Airlines, confirmed a Reuters report that more examples of the cracks had been discovered during compulsory inspections.
"The A380 is safe to fly," Tom Williams, executive vice president of programs at Toulouse-based Airbus, said.
The cracks were caused by factors including the choice of aluminum alloy for some of the 4,000 brackets in the wings as well as a type of bolt that put too much stress on the metal.
Engineers have ruled out metal fatigue, Williams said.
Airbus moved to shore up confidence in the world's largest jetliner amid a drip-feed of disclosures about cracking on components used to fix the outside of the wing to its ribcage.
Williams flew to Dublin to give an unscheduled address at an industry conference to dampen any concerns about safety.
European authorities have ordered inspections on almost a third of the superjumbo fleet, or 20 aircraft, after two types of cracks were discovered within weeks of each of other.
Since then, similar cracks have been found inside the 9,100-square-foot wings on aircraft at Singapore Airlines.
Airbus officials said that having understood the problem, they expected most of the aircraft being tested would show similar evidence of cracks, but that repairs were simple.
Singapore Airlines said it had found cracks on all four aircraft it has inspected so far, part of six which it must examine by Friday. One of the jets has already been repaired and is back in service, spokesman Nicholas Ionides said.
The airline opened up the debut A380 service in December 2007.
Checks involve emptying and venting fuel checks for about 24 hours followed by a visual check via a manhole under the wing.
The cracks first came to light during repairs, lasting over a year, on a Qantas A380 severely damaged by a dramatic engine blowout in November 2010 that punched holes in the wing.
At first engineers were unsure what had caused the cracks but the initial microscopic flaws led to the discovery of a second and potentially more serious type of crack, some of them up to two inches long, in the central part of the wing.
The findings caused concern at the European Aviation Safety Agency which turned down Airbus's request for limited extra time to examine the data and ordered mandatory inspections last week.
Designed just before the latest generation of mainly carbon-composite jetliners like Boeing's newly delivered 787 Dreamliner, the A380 is about 60 percent aluminum, the main material used for making aircraft for decades.
"All aluminum structures have cracks. It is the nature of the beast. Each component is designed and modeled according to the desired capacity," Williams said. If one part breaks the structure is designed so that the load is spread elsewhere.
To deal with the unforeseen cracking problem, Airbus is changing its manufacturing processes to ensure smooth operation until at least the next four-year maintenance checks.
Longer term, it plans to switch to a different alloy, restoring the aircraft to its normal lifespan of 25 years-plus.
The wings were designed and built in Britain, which prides itself on state-of-the-art wing assembly. Unions there recently objected to some work being outsourced to South Korea.
Industry executives at the Dublin conference welcomed the clarification and said the problem had been understood.
"When they had the second round of cracks, that got more people's attention and a few airlines were asking questions," an executive said, speaking on condition he was not identified.
An Australian engineering union last week called for all A380s to be grounded pending more investigation. Airbus has dismissed this by saying regulators would be quick to ban flights if they believed safety to be at risk.
(Additional reporting by John Crawley, Harry Suhartono; Editing by Elaine Hardcastle, Gary Hill)