MADRID—Bolt-on metal repairs to primary composite structures could become a thing of the past, as work to improve the acceptance and standardization of bonded repairs progresses, experienced repair providers suggest.

Bolted repairs can be detrimental, cutting the composite structure’s residual strength by up to 50% and adding extra weight. But regulators do not yet accept bonding.

“We have made the first step … in increasing the reliability and acceptance of bonding as a repair method,” Christian Sauer, Lufthansa Technik’s(LHT) engineering manager for airframe-related component services, announced at MRO Europe 2014. 

LHT is a member of the Composite Adaptable Inspection and Repair (CAIRE) project team, which includes Airbus and other industry stakeholders. CAIRE is aiming to automate and standardize bonded composite repairs to help tackle certification and quality assurance issues.

“The only [primary structure] repair that is certifiable at the moment [for visible damage] is bolted, meaning that just like a standard metallic aircraft you put a bolted patch on it,” he explained. “The problem is bonding—which we believe is the best way to repair composite aircraft—is not allowed today by the regulators.”

This is because bonded repairs are largely done manually, and their strength and durability are hard to verify without breaking the repair. There is also a lack of specialized training for bonding, fueling regulator concerns, Sauer said.

Using technologies from the CAIRE project, LHT and its partners have developed an automated system for assessing, designing and repairing damage to composite structures. It optimizes the repair before scanning the surface for contaminants, grinding out the damaged material and creating the fix, which is applied manually. Afterwards, the new geometry can be checked to ensure that all the dimensions have been met.

CAIRE has also been adapted from a stationary in-shop system to a mobile robot, which can be secured to any surface using suckers. “You can move it where you want, attach it to any surface and it can do this process wherever it is needed,” Sauer said.

This “very precise, very quick” automated process could replace the current system of manual layer-by-layer grinding to create a bonded fix, which lacks standardization and is extremely time intensive – taking a minimum of 60% longer than the automated process, depending on the structure. Tests on the automatic repairs have also proven 5-15% stronger than the manually ground alternative.

Sauer said CAIRE can be applied to any shape of patch on any type of surface, although admits that steps are still proving “a little problematic” for the new technology. Also, several sensors are needed to cover a wide range of potential surface contaminants and there are no rules yet on acceptable levels of surface contamination.

“We have to continue this work, especially on contamination and on training. We are not yet in a position to go to the authorities and say we can do bonded repairs on primary structures, but the first steps have been made and we continue to follow this,” he said.