No aircraft have been grounded but the inspections must be carried out within six weeks for most of the 22 aircraft placed on a check-up list, which is expected to affect Singapore Airlines (SIAL.SI), Dubai's Emirates and Air France (AIRF.PA).

A handful of the most heavily used aircraft -- subjected to at least 1,800 take-offs and landings that impose the most wear and tear on an airframe -- must be examined within four days in a process likely to take each jet out of service for 24 hours.

The European Aviation Safety Agency acted after European planemaker Airbus disclosed two sets of cracks on its A380 superjumbos 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 the ribs inside the wings, was "more significant" and could develop on other aircraft if the problem is not addressed, the European 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.

There was no immediate reaction from Airbus, but while waiting for the EASA verdict its chief executive moved to reassure passengers over the safety of its best-known model.

"It is embarrassing, but we will do everything to ensure safety is not compromised," Chief Executive Tom Enders said.

"We have a pretty good understanding, but the investigation is ongoing. What we have developed already is a repair solution and this is what we will apply on the various aircraft if and where it is necessary," he told CNN television on Thursday.

The episode comes 14 months after a Rolls-Royce (RR.L) engine blowout on a Qantas (QAN.AX) A380 triggered global headlines. It was during a $130 million repair job on that same aircraft that the hairline wing cracks first came to light.

MIXED PASSENGER REACTION

Airbus and operator airlines 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 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 do not yet display any anxiety over that happening. Shares in Airbus parent EADS (EAD.PA) dipped 0.5 percent in line with the French market.

South Korea's Asiana Airlines (020560.KS), 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 the 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.

Airbus said on Thursday the cracked parts were "non-critical" and insisted the A380 was safe to fly. Both Airbus and U.S. rival Boeing are heavily regulated on safety matters.

The cracks are another test of Airbus's morale just as the EADS (EAD.PA) subsidiary recovers from years of delays, having hit its A380 delivery target for the first time in 2011.

Aviation's mammoth was conceived in part as a European bid to impose itself on the global scene and outdo the instantly recognizable Boeing (BA.N) 747 but became mired in development problems that strained tensions between France and Germany.

The A380 -- developed at an estimated cost of 12 billion euros in Britain, France, Germany and Spain -- 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.

Singapore Airlines took the first aircraft in December 2007 followed by Emirates and Qantas.

Because the timetable depends on the number of flight cycles, aircraft do not necessarily face inspections in the order they were delivered. Qantas uses the A380 on long Pacific routes with proportionately fewer take-offs and landings.

Air France placed the giant A380 at first on the short hop between London and Paris in order to perfect operations.

The Air France-KLM (AIRF.PA) subsidiary declined comment.

Singapore Airlines confirmed it would carry out checks.

"The safety of our customers and crew is our number one priority and we will ensure that we take whatever action is needed for the continued safe operation of our A380 fleet," it said in a statement.

Other operators include Korean Air (003490.KS), China Southern (600029.SS) and Germany's Lufthansa (LHAG.DE).

(Additional reporting by Harry Suhartono, Christiaan Hetzner, Philip Baillie and Paul Sandle.; Editing by James Regan and David Cowell)

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