Bonded Repairs Prevailing On Composites

Debate continues on merits of bolt-on versus bonded composite repairs

There is still some way to go before bonded repairs will be accepted on primary composite structures, according to executives speaking at Aviation Week’s MRO Europe Conference.  

Regulators only allow bolt-on metal repairs to primary composite structures on the latest generation of aircraft—just like standard metallic-build versions—because of concerns over the strength, quality and durability of manually bonded repairs.

“There is still the debate between bonded and bolted repairs. We need both,” says James Kornberg, AFI KLM E&M aerostructures product and business development general manager.

While Lufthansa Technik is working with Airbus and other industry stakeholders to automate and standardize bonded composite repairs as part of the Composite Adaptable Inspection and Repair (CAIRE) project team (see sidebar), Kornberg is unconvinced. “It is very interesting, but practically this system cannot be used because it is not validated. Today we don’t think it is necessary to have a robot to repair these aircraft,” he says.

Kornberg argues that in the field it is too tricky to get the proper tooling for bonded repairs and recreate the conditions needed, so bolted repairs will continue to take the lead. “You cannot do bonded repair on very large areas. It is technically not possible, so there is a limit. It is case-by-case. You have the right to push for bonded repairs, but it will not always be possible.”

Bombardier’s director of engineering & component services Michael Curran agrees. “We’re doing a huge amount of work on bonded repairs. For instance, if something needs fixing during the manufacturing process it’s not going to be done with a bolted repair. But we have the advantage of clean rooms, autoclaves and perfect vacuums. The difficulty is reproducing that in the field,” he says.

Both agreed that the prime objective is getting the aircraft back into service as quickly as possible, with a minimum cost for repairs, adding that bonded repairs lend themselves more to secondary structures such as nacelles and fan cowlings.

“You don’t want to touch it again. That works with bolted repairs. There is no limit to how many bolt-on repairs you could do,” says Curran. The metal fix, which carries a weight and strength penalty, will then fly with the aircraft until the end of its life, or until the structure is replaced. Curran described this as “fit and forget.”

Bombardier is establishing bolt-on repair procedures for the CSeries, testing potential events such as heat exposure and lightning strikes. “The big message to take out of this is we’re not going to be doing carbon-type repairs. These are metallic repairs that bolt onto the structure, which should make it easier for MROs and airlines. It is not necessary at this point to do carbon repairs everywhere,” he says. 

A version of this article appears in the November 3/10 issue of Aviation Week & Space Technology.

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