Airbus has traced the source of the cracking in A380 wing structures to unexpected additional stresses imparted by the manufacturing process, and is confident that its original flight loading calculations for the type are accurate.

The airframer is in the process of changing the manufacturing process and has developed a fix for affected aircraft, as the European Aviation Safety Agency prepares to instruct operators to conduct precautionary inspections.

But Airbus emphasises that the cracking problem - while needing to be addressed, to avoid longer-term issues - is not a safety risk in the short- to medium-term.

The cracks were originally discovered in the rib feet of the Qantas A380 which suffered an uncontained engine failure in November 2010, and has since been under repair in Singapore.

Airbus carries out A380 wing manufacture at the UK plant in Broughton, before transferring the wings to the Toulouse final assembly line. An Airbus wing specialist on the A380 said the airframer's investigations indicated that parts were being stressed at some point during the manufacturing process, which involves drawing the wing skin over the built-up rib and spar assembly before attaching it.

"It's possible to get standing stresses that hadn't been expected," said the specialist, which translated into additional loading during flight. Airbus has already conducted verification flights to measure actual loading, and found that its original design calculations are correct.

"We've confirmed that ordinary flight loads are exactly as predicted," the specialist said, and added that the components' design would remain "completely unchanged". But the airframer said it was changing the build process, to ensure that wing assembly does not generate unforeseen stresses.

Rib feet are L-shaped brackets, about 9in (23cm) tall, which connect the wing skin to composite rib structures. There are some 30-40 on each rib spanning the wing.

While the Qantas aircraft sustained extensive wing damage during the engine failure, engineers carried out an intensive inspection to establish whether there was evidence of damage which could not be attributed to the event.

This inspection revealed faint hairline cracks in the area where the rib foot is attached to the wing skin, running from the bolt-hole to the edge of the foot. He said a "few" of these feet were affected, located about halfway along the span of the wing.

The Qantas aircraft is one of the early airframes, MSN14, and the finding prompted precautionary checks on other initial-production A380s.

Nine aircraft have been inspected. Checks on another A380, during a 2C maintenance visit earlier this month, found cracks on a rib foot, in a different position. An immediate remedial solution involves replacing the affected rib feet and dropping a new section into its place.

While Airbus insists that the cracks are "nothing of concern", and would only need to be addressed over a period of years rather than urgently, it expects EASA to release an airworthiness directive instructing operators to carry out inspections - although the precise extent of these has yet to be disclosed.

"In the long term it would have to be fixed, but in the short term it's not a problem," the specialist pointed out.

Airbus said it is confident that the crack problems, originally only noticed as a consequence of the Qantas repair, would easily have been detected during a later heavy check, even if it had not been identified during earlier routine maintenance.

Source: Air Transport Intelligence news