UPDATE 4-Checks ordered on one third of A380 fleet
* Cracks found in wings
* No A380s grounded, Airbus says safe to fly
* Airlines must check a third of the current fleet
* Deadline up to 6 weeks for checks
By Tim Hepher and Axelle du Crest
PARIS, Jan 20 (Reuters) - Europe's safety regulator on Friday ordered urgent inspections on almost a third of the Airbus A380 fleet after cracks were found inside the wings of the world's largest jetliner.
No superjumbos have been grounded but the most heavily used aircraft -- those subjected to at least 1,800 take-offs and landings that impose the most strain on an airframe -- must be examined within four days, authorities said.
The European Aviation Safety Agency acted after European planemaker Airbus disclosed two sets of cracks on its A380s just two weeks apart, and barely four years after the 525-seat double-decker passenger jet entered service.
The second type of cracks, which like the first appeared on a bracket joining the exterior to metal ribs inside the wings, was "more significant" and could develop on other aircraft if the problem is not addressed, the Cologne-based agency said.
"This condition, if not detected and corrected, could potentially affect the structural integrity of the aeroplane," EASA said in an airworthiness directive issued on Friday.
Inspectors ordered checks within six weeks for another category of aircraft with a history of 1,300 to 1,800 cycles.
In total, the two-speed recommendations cover 20 aircraft including 10 from Singapore Airlines, seven from Emirates, one from Air France and two test planes, according to a published list of serial numbers at a380production.com.
Experts say engineers must carry out visual checks by climbing inside the ribcage of each UK-built wing. The process is expected to take each jet out of service for about 24 hours.
Airbus said the order demonstrated the "airworthiness process is working" and reiterated the aircraft is safe to fly.
It has dismissed calls to ground the jets but the Australian engineering union which handles routine servicing and engine checks on superjumbos operated by Qantas Airways said it was concerned about the speed of the planemaker's response.
"They have described these as tiny cracks, but every crack starts off as a tiny crack and they can grow very quickly," said Stephen Purvinas, federal secretary of the Australian Licensed Aircraft Engineers Association who has called for the grounding.
The discovery comes 14 months after a Rolls-Royce engine blowout on a Qantas A380 triggered global headlines.
It was during a $130 million repair job on that same aircraft that hairline cracks in the wing first came to light.
Industry sources said the second set of cracks appeared on aircraft operated by Dubai's Emirates, the largest A380 customer. Emirates declined comment but said on Thursday it continued to monitor its fleet and safety was its top priority.
Air France and Singapore Airlines confirmed they would carry out checks and stressed their commitment to passenger safety.
Other operators include Korean Air, China Southern and Germany's Lufthansa.
AIRLINE, PASSENGER CONCERN
The cracks are another test of Airbus's morale just as the EADS subsidiary recovers from years of delays, having hit its A380 delivery target for the first time in 2011.
The mammoth jet was conceived in part as a European bid to impose itself on the global scene and outdo the instantly recognizable Boeing 747, but became mired in development problems that strained tensions between France and Germany.
Developed at an estimated cost of 12 billion euros in Britain, France, Germany and Spain, the A380 has room on its wings to park 70 cars and a wingspan of 79.8m (261ft 10in).
Airbus has sold 253 of the long-range aircraft, listed at $390 million each, and 68 A380s are currently in service.
Operators have boasted of strong demand for seats on the A380 as passengers test out its claim of quiet cabins or a spacious twin deck, but Airbus and airlines will be keen to ensure the latest incidents do not affect its popularity.
Such concerns would magnify if they affected the decision of airlines to buy the jet, though investors display little anxiety as yet over that happening. Shares in Airbus parent EADS dipped 0.5 percent in line with the French market.
South Korea's Asiana Airlines, which plans to induct six A380s into its fleet between 2014 and 2017, said it was not changing that schedule as yet, but could reconsider depending on the outcome of investigations into the cracks.
In European airports, some travellers began to fret.
"I fly all around the world on my job, and so far I've had the luck to avoid flying in an A380, even on my trips to Tokyo," said stage designer Olaf Zombeck, 67.
"I wouldn't want to get on board for at least another few years since all problems should have been discovered by then."
Joerg Ritter was seeing his teenage daughters off at Frankfurt airport.
"I had no idea about the cracks until just now, so this is certainly very worrying to hear. But thankfully my girls are only flying to London, where they don't use any A380s," he said.
Others put their faith in a system of careful maintenance.
"It's part of their job. Everything is checked. I trust them -- I'm going on holiday and I'm not going to start worrying," said Laurent Amelin, 34, a French auto maintenance worker at the A380 check-in counter at Paris' Charles de Gaulle airport.
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