Airbus believes three factors are conspiring to generate cracks in the wing-rib feet on A380s, after inspections ordered by Europe's safety regulator turned up problems on additional aircraft.

Twenty A380s - essentially all those delivered to the end of 2009 - are the subject of a European Aviation Safety Agency airworthiness directive requiring inspection of aircraft with more than 1,300 cycles within six weeks. A limited number of jets, with more than 1,800 cycles, were ordered to undergo inspection within four days.

Singapore Airlines, the recipient of the first five A380s, needed to inspect an initial six aircraft by 28 January, and the airline said it discovered "findings" in all four it had examined by 25 January.

"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 added. "We have been liaising closely with Airbus and are keeping the relevant regulatory authorities fully informed."

EASA's directive covered 10 Singapore Airlines A380s, plus seven with Emirates and one with Air France, as well as two Airbus test airframes. It ordered operators to conduct a detailed visual inspection of wing-rib feet, and to report findings to Airbus, after two types of crack were discovered in a Qantas A380 undergoing extensive repair in Singapore.

The airframer said the choice of alloy - designated 7449 - combined with a fastener interference-fitting process appeared to be generating the first type of crack in the feet. But a second type of crack - which EASA described as "more significant" - was also being created during the pull-down of wing skins, in the area of a butt-strap joint used between different lower skin panels.

In the region of rib 26 and stringer 21, larger-than-expected gaps - some 1.5-2mm rather than 0.5mm - between the sections involved in the pull-down had resulted in stresses being induced, leading eventually to cracking under the wear of normal airline operations.

Airbus executive vice-president for programmes Tom Williams said the interim fix being carried out on affected aircraft naturally relieved these stresses, and eliminated the problem.

"We have enough ribs and feet [to conduct the repairs]," he said. Williams added that a permanent solution would look at changing the alloy - although this would require thicker rib-feet and add some 89kg in weight - and amending the pull-down process.

"It's not a materials quality issue. It's not a production issue. It's the combination of both," said Airbus.

While 67 A380s have been delivered, some of the in-service fleet will be checked during routine maintenance visits before reaching the cycle threshold set by EASA.

"It has been determined that the [new type of] cracks may develop on other aeroplanes after a period of time in service," the directive states. The inspections are an "interim" measure, it adds, and further mandatory actions "might be considered".

Source: Air Transport Intelligence news