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The Airbus A380 wing cracks: an engineer’s perspective

The European Aviation Safety Agency (EASA) has ordered the inspection of all Airbus A380 superjumbos following the detection of cracks in the wings of several aircraft. Cracks were first discovered in…

All 68 of the world’s Airbus A380 have to be inspected. Telstar Logistics

The European Aviation Safety Agency (EASA) has ordered the inspection of all Airbus A380 superjumbos following the detection of cracks in the wings of several aircraft.

Cracks were first discovered in December last year, in a Qantas-owned Airbus A380 that was being repaired after an engine explosion in Singapore.

On January 20, the EASA issued an Airworthiness Directive (AD) grounding 20 A380 for visual inspections.

The 20 affected aircraft were early-delivery airframes (aircraft bodies without engines) – ten with Singapore Airlines, seven with Emirates, one with Air France and two Airbus test A380.

This initial directive allowed up to six weeks for a detailed visual inspection to be carried out on A380 that had completed between 1,300 and 1,799 flights. A380 that had completed more than 1,800 flights had to be inspected within four days.

According to EASA spokesperson Dominique Fouda, the initial round of checks “found cracks in almost all of the planes inspected”.

Peter Pearson

As a result, the EASA revised their AD, requiring the inspection of all 68 Airbus A380 in operation worldwide, and the use of high-frequency eddy current equipment for crack detection. This is a commonly-used non-destructive evaluation technique used to detect small surface cracks.

But just how much do we know about these cracks? And do the cracks have the potential to make the A380 unsafe to fly?

For a start, there are two different cracks being investigated:

  1. Hairline cracks around fastener holes in the internal wing structure, and
  2. Cracks at the edges of the vertical web of the feet.

Airbus

The fastener holes are located in “rib feet” which are L-shaped brackets extending from the wing rib for attaching to the aircraft skin, using bolts through these holes. It is my understanding that it is the rib-feet attached to the lower skin which have this problem.

A number of wing ribs are distributed along the wing – more than 40 on each wing of the Airbus A380 – and these run from the front to the back of the wing’s internal structure.

These ribs help maintain the aerodynamic shape of the wing and transfer the aerodynamic and fuel loads to the spars – beam-like structures running along the length of the wing that are the main load-carrying structure of the wing.

The cracks found around these holes are being attributed to the manufacturing process used at Broughton, UK, where the wings are made.

This manufacturing process may be related to the combination of the use of “interference-fit” fasteners and the loads being applied when the wing skin is attached to the wing.

Guillaume Horcajuelo/EPA

Interference-fit refers to the process whereby the fastener hole is made slightly smaller than the fastener diameter. When the fasteners are inserted, this has the effect of inducing stresses which offset the tensile stresses on the lower wing skin during flight, increasing fatigue life.

If the high loads being applied to the fasteners during assembly are not adequately accounted for, they will combine with the stresses arising from the interference fit, potentially leading to cracking. The particular type of aluminium alloy used will also affect this joint behaviour where a balance has to be achieved between stiffness, strength and fracture toughness.

The cracks at the edges of the vertical section of the L-shaped rib feet have been reported as more severe than those observed around the fastener holes (Type II, using Airbus terminology, as opposed to Type I for the fastener hole cracks).

These cracks are likely to be at the corners of the rib feet where stresses are higher. These are of greater concern and could be fatigue driven. The AD also states that this condition, if not detected or corrected, could potentially affect the structural integrity of the aircraft.

That said, the cracks observed so far have not been on the main load-bearing structure of the wing. It would take multiple simultaneous structural failures along a number of ribs to give rise to a structural integrity issue. It’s worth noting that there are around 4,000 rib-feet on each A380.

So what can be done about these cracks?

Well, once detected, these faults may be repaired by various means, depending on the extent of damage. For small surface cracks, metallic or carbon-fibre composite patches can be used and there is always the more-costly alternative of part replacement.

Dean Lewins/AAP

It’s worth noting that it’s not unusual for new aircraft to experience relatively minor structural (or other) problems once in service. These issues are bound to be minor, because the development of new aircraft entails a tremendous intellectual effort by thousands of engineers using advanced analytical techniques, sophisticated numerical simulation and extensive physical testing.

Airframers (such as Airbus) also work closely with national aviation certification authorities at all stages of development, to ensure the highest possible safety standards are met.

But for now, the EASA is sticking to its guns:

“This condition, if not detected and corrected, may lead to a reduction of the structural integrity of the aeroplane.”

And Airbus representatives are sticking to theirs:

“These brackets [in which the cracks have been detected] are located on wing ribs which are not a main load-bearing structure and, thus, the safe operation of the aircraft is not affected.”

It remains to be seen which analysis is closer to the truth, but it’s unlikely we’ve heard the last of the Airbus A380 saga.

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5 Comments sorted by

  1. Carl A. Paez

    Retired Program Engineering Director

    I have been following with great interest the unfortunate development associated with the A380 wing substructure (wing ribs) cracking.
    Most of the publish articles do little to address the causative factors and potential repercussions if these cracks are not corrected in short order.
    I have been in the aerospace industry since the mid 1960's and became very aware of Stress Corrosion Cracking (SCC) on the Lunar Module (back in 1967). The 7449 aluminum alloy is well known to be susceptible to SCC…

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  2. Brian Falzon

    Professor of Aerospace Engineering at Monash University

    Hello Carl

    Thank you for your e-mail. Indeed, another important function of ribs is to limit the bucking length of skin-stringer wing sections. Incidentally, I have conducted extensive research, published in academic journals, into the failure mechanisms of skin-stringer carbon-fibre composite panels. My understanding of the A380 cracks is that these have only been reported in the rib feet attached to the lower wing skin which is predominantly in tension during flight. I would be interested to know if your findings indicate otherwise.

    Brian

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  3. Carl A. Paez

    Retired Program Engineering Director

    Brian,
    from what I read the rib feet are made from 7449- T651 and that is a very bad SCC material with a threshhold of about 7,000 psi for cracking to start. I was under the impression that most cracks were near the upper skin and was due to the pull up stresses that were developed during fastener attachemnt trying to close out the gap that excisted between the skin and the rib feet. The skin is machined and then hot formed, and by Airbus admission it does not always fit propely, thus a pre-stress is left after rib feet are attached. In addition, the interference fit fasteners used to attach the rib feet leave a high tension residual that causes the cracks in the flange to initiate. Therefore whether by interference fit fasteners, or by flange bending the residual stresses will cause cracking. When the upper skin losses its rib support, then compression buckling at much reduced loads will take place due to much larger beam column lengths; and therefore reduced alloawbles.
    Carl Paez

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  4. Brian Falzon

    Professor of Aerospace Engineering at Monash University

    Hello Carl

    A copy of Flight International has just landed on my desk. An article by David Kaminsky-Morrow reports that the problem is '..in the area of a butt-strap joint used between different lower skin panels. In the region of rib 26 and stringer 21....' I do agree, that if this problem occurs on the upper skin panels, buckling becomes an issue, but this has not been reported yet.

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  5. Keith Bentley

    retired Structures testing and analysis

    Brian,

    Thanks for this very interesting article. I was curious how the holes are drilled. I assume the cracked holes are in a mixed stack (CFRP/Aluminum) and hence are probably line drilled. Installing an interference fit fastener in a mixed stack line drilled hole up can be difficult. The CFRP material will lose it's matrix (epoxy glue) strength in the hole as the interference fit fastener is forced through both the metal and CFRP. This can make for a "loose" fit in the CFRP while the metal receives temporary fatigue life benefit of the interference fit. The loose fit in the CFRP however can promote a bending moment to the radius of the metal bracket as well as induce heavy fretting crack nucleation as a result of loss of clamp-up forces. Of course any KISCC issues will only aggravate the damage tolerance by increased da/dN properties.

    There are sound solutions and I'm sure Airbus will solve this issue.

    Please keep up the open communication.

    keith

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