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.
Bolted repairs can be detrimental, cutting the composite structure’s residual strength by up to 50% and adding extra weight. “We have made the first step—not in convincing the authorities—but in increasing the reliability and acceptance of bonding as a repair method,” says Christian Sauer, Lufthansa Technik engineering manager for airframe-related component services, speaking at Aviation Week’s MRO Europe Conference.
“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. The problem is [that] bonding—which we believe is the best way to repair composite aircraft—is not allowed by the regulators,” he says.
This is because bonded repairs are largely done manually and it is hard to prove the strength, quality and durability of the joint without breaking it again. There is also a lack of specialized training for bonding, fueling regulators’ concerns, says Sauer.
“The quality of the repair depends on the skill of the mechanic. There is no system that can tell you how much it can withstand. All of these uncertainties mean the authorities don’t allow us to do this on primary structures,” adds Sauer. “What we need to do is take all uncertainties out of the process. We need an industry standard for bonding, which has started; but we are not there yet.”
Using technologies from the CAIRE project, Lufthansa Technik 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 all the dimensions have been met.
CAIRE also has been adapted from a stationary in-shop system to a mobile robot that can be secured to any surface using suckers. “You can move it where you want, and it can do this process wherever it is needed,” says Sauer.
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 at least 60% longer than the automated process. Tests on the automatically produced repairs have proven they are 5-15% stronger than the manually ground alternative.
A version of this article appears in the November 3/10 issue of Aviation Week & Space Technology.