The cracks (less than 1cm long) were found on the feet of the wing ribs. The feet attach the rib to the wing skins.
Airbus has traced the problem to the 7449 aluminium used in the wing ribs. 7449 is more sensitive to the way the parts are assembled on the wing. They ruled out flight loads or fatigue as causes.
A few years back,
Machine Design reported:
Servosystem deftly handles Airbus wings
MARCH 3, 2005 Staff
With a span nearly as long as a football field at 261 ft, producing a single Airbus A380 wing involves precisely positioning a massive structure to drill, rivet, and bolt approximately 180,000 holes.
During wing manufacturing, six servohydraulic axes move panels measuring up to 111 ft long and weighing 8,818 lb.
That presented a significant challenge for Airbus' manufacturing team in Broughton, U.K., and Electroimpact Inc., Mukilteo, Wash., the prime contractor for wing-assembly automation tools.
Wings are a structural framework of spars and ribs covered with metal panels. The panels consist of a curved, aluminumalloy skin reinforced by stringers that ensure shape and strength. The tricky part is moving assembled panels into four-story high jigs holding the ribs, spars, and leading and trailing edges, for subsequent assembly and fastening.
The huge size and flexible nature of a completed panel — up to 111-ft long and weighing 8,818 lb — creates a motion-control nightmare. According to Electroimpact's Ted Karagias, handling a wing panel with multiple support points isn't easy. Cranes won't work because the panels distort when suspended, he explains. "Basically you have a statically indeterminate system. The panels twist, bend, and kick as they react to the forces introduced by lifting equipment," he says.
To overcome this problem, Electroimpact devised a manipulator with six coordinated servohydraulic arms to maintain the panel's proper form and precisely control position. "Two of the six arms control the panel's vertical position," he says. "The other four act as slaves imparting a constant programmed force on the wing panel. That way, when the positioning arms are commanded to move up or down, the load-seeking arms follow along to maintain the panel's form."
Each panel-loader arm has four hydraulic-driven axes plus one passive axis, requiring the simultaneous coordination of 24 axes.