Airbus Adjusts A380 Assembly Process
Jan 26, 2012
By Robert Wall
Airbus has put in place changes to its A380 wing assembly process as part of its long-term fix to address cracking of wing components, but it also expects most of the A380s already built to require fixes as an interim measure.
Twenty of the more than 60 A380s in service worldwide are affected by a Jan. 20 European Aviation Safety Agency’s (EASA) airworthiness directive (AD) to inspect the aircraft for potential cracks of wing rib-feet. The findings by airline inspections that have unfolded in recent days are “in line with the expectations,” says Airbus Executive VP-Programs Tom Williams. The company expects to see cracks in most A380s already built.
Singapore Airlines, which has the most A380s affected, says four of its aircraft have undergone the process, and “there were findings during each inspection.” Of those, “one aircraft has already undergone repairs and is back in service. Repairs will be carried out before the remaining aircraft are returned to service,” says an airline representative.
The AD requires A380s with 1,800 flight cycles or more to undergo a detailed visual inspection within four days or 14 flight cycles, whichever occurs first. For aircraft with 1,300-1,800 flight cycles, the inspection has to take place within six weeks or 84 flight cycles, according to EASA. Some aircraft not yet affected by the directive because they have a lower number of flight cycles will undergo the process once they reach the EASA threshold or even before if they go into C Checks, Williams notes.
There are enough replacement wing rib-feet available to avoid a parts shortage that could affect the return of the aircraft to service.
At issue is an L-shaped bracket that attaches the wing skin to the ribs. Each wing has about 2,000 L-shaped brackets (30-40 per rib, with 60 ribs per wing), so the failure of one bracket is not seen as a safety issue. EASA says, “This condition, if not detected and corrected, could potentially affect structural integrity of the aeroplane.”
To avoid the problem on new-build aircraft, Airbus is already using shimming to reduce the strain applied when the wing skins are attacked to the ribs, thereby reducing the loads on the wing rib-feet. The gap between wing skin and rib, in the affected area, was about 1.5-2 mm, rather than 0.5 mm, causing unexpected strain on the rib-feet in the lower wing area near Rib 26 and Stringer 21.
Airbus also is changing the material of the part from an aluminum 7449 alloy to a stronger component. The overall effect will be to add 89 kg to the aircraft, Williams says.
Furthermore, Airbus is making changes to the interference fit fasteners because it judged the current arrangement contributory to the damage seen.
The inspection regime the airlines are undertaking involves draining the wing tanks and opening an access panel. Depending on local rules, the process takes a day or more. A repair action, if deemed necessary, can take several days.
Airbus stresses the issue is not a flight safety concern.
As part of the root cause analysis, Airbus instrumented one of its own aircraft to assess whether it had erroneously estimated the loads the wing would bear, leading to the cracks. But the aircraft maker determined that was not the case. EASA notes that this AD “is considered to be an interim action to immediately address this condition.” But, it adds, “Further mandatory actions might be considered” as a result of the ongoing investigation.
Wing changes being introduced on the A380 to boost the maximum takeoff weight to 574 from 569 metric tons should not create a new problem in this area, Airbus believes.