Europe's Airbus has ordered more frequent inspections of the wings of the world's largest passenger jet after discovering unexpected levels of metal fatigue during testing on an A380 factory mock-up, industry sources said on Thursday.

The planemaker has asked airlines to inspect the wing's "spars" or main internal beams during regular major overhauls carried out after six years in service, and then again at 12 years, instead of waiting for the 12-year overhaul, they said.

The move comes as Airbus emerges from a painful two-year programme of modifications and hundreds of millions of euros of financial charges triggered by the discovery of cracks on brackets inside the wings of A380 jets already flying.

This time, however, Airbus has not so far been able to duplicate the fatigue test results on any aircraft in service.

The double-decker A380 entered service in 2007 with Singapore Airlines followed by the largest customer, Emirates, which would be first in line to carry out the increased checks.

An Airbus spokeswoman confirmed the discovery of unspecified "fatigue findings" on a factory test plane.

"This will be addressed during routine maintenance inspections and the aircraft remains safe to fly," she said.

Most aircraft undergo a regular pattern of checks from small daily ones to heavy maintenance checks every five or six years.

Aircraft industry experts have known for decades that metal fatigue cannot be eliminated, but have worked out a system for monitoring it backed up by mandatory maintenance schedules.

The test plane used for the A380 fatigue tests is used to replicate the wear and tear that the superjumbo would endure during three times its normal life, the spokeswoman said.

"Fatigue tests are a normal part of the design and certification process," she said.

Airlines nonetheless face unanswered questions over whether the inspections could lead to further expensive repairs.

"There may eventually have to be some kind of an upgrade at the overhaul stage but no action is required now," an industry source said, speaking on condition of anonymity.

Airlines are also sensitive to the amount of time an aircraft has to remain offline for each maintenance visit.

While the brackets that caused the company's 2012 wing cracks crisis are not individually viewed as critical parts, wing spars are beams that run outwards from the fuselage and are a fundamental part of the airplane's structure.

Each A380 has three spars holding up the wing.

To deal with the earlier problem of cracked brackets, Airbus devised a temporary fix followed by a permanent one that has been gradually working its way through the production line.

Qatar Airways, which refused to take aircraft with the temporary patch in order to avoid having to put them back for further work, is due to take its first three fully modified aircraft in June.

The airline's chief executive said earlier he may exercise options for three more A380s on top of 10 already ordered, and may order even more if the jet performs well in service.

But he expressed doubts over the A380's ability to fly profitably on some of the world's longest routes.

Qatar Airways plans to introduce A380 services this summer starting with Doha to London, which is well inside the range of the 525-seat passenger jet. – Reuters