Airbus is expecting up to 120 A380s to be delivered before the permanent fix for the wing-rib bracket cracking problem on the type is fully in place.

The airframer detailed its proposed solution during a briefing in Toulouse. Its initial retrofit, for aircraft already in service, centres on 23 hybrid ribs mounted in the more lightly-loaded sections of each wing.

Retrofit involves replacing the type-7449 aluminium with a more robust grade, type 7010. This retrofit also involves localised reinforcement and fitting thicker rib brackets. Two ribs in each wing-tip will also be replaced.

But the forward-fit solution will replace these ribs with all-aluminium components, also built from type-7010 material, from the outset. The region around the inspection manholes will be reinforced and the rib feet will be reshaped to make them more forgiving.

Parts will be available to perform the modification on in-service aircraft from the first quarter of 2013 while wing assembly from the end of this year will implement the rib design change. All ribs will be built from type-7010 aluminium, in a similar way to other Airbus programmes. A380s delivered from 2014 onwards will have the new rib design.

Airbus executive vice-president for programmes Tom Williams says the type 7449 aluminium was strong and lightweight but brought a "degree of brittleness". Eliminating this material and switching to the 7010 grade gives greater strength but also around 90kg additional weight.

The flight-test programme for certification has not yet been detailed but Williams says the airframer hopes to fly the demonstrator in the autumn. He stresses that the fix will restore the full life capability of the wing without impacting performance.

Williams estimates that 110-120 aircraft will have to be retrofitted. He says the cause of the cracking is "well established", the result of material choice plus thermal distortion at extremely low temperatures, and stresses generated during assembly.

"When we were designing the wing, we pushed hard for weight reduction," Williams says, adding that the original hybrid design saved 300kg.

But he admits that the airframer, during ground testing of the airframe, made "assumptions" on the use of the type-7449 material because it had been used on other programmes.

He adds that the linear, finite-element modelling used to model some ribs on the aircraft assumed that adjacent ribs would behave the same way. Some non-linear modelling tools, which might have helped the analysis, were not available at the time.

Williams also points out that fatigue testing cannot take into account every variation of temperature and pressure to which the aircraft will be subjected in service.

Intensive testing is under way, he says: "We only want to do this one time. It's very disruptive to customers. If we have to ground [a customer's aircraft] we only want to do it on time.

"It's not a complicated fix by any means, [but] it's about making 110% sure we have the right answer."

But the airframer is "still working its way" through the approach to implementing the fix on in-service aircraft. Williams says a "nose-to-tail" grounding is one option, enabling completion in one session, but some customers might opt to split the fix across C-checks.

Source: Air Transport Intelligence news