Composite materials:
Repairing the "Black Gold", part 2

Lufthansa Technik is ready for future materials

Composite materials challenge the personnel in Lufthansa Technik's painting hangars. Unintentional chippings and cracking paint are more often found on CFRP structures than on comparable aluminium surfaces, mostly due to differences in thermal expansion. Another reason is a sudden evaporation of residing resins that can occur even long after the curing process. Even an intentional paint stripping is more difficult on composite surfaces than on metals. The use of solvents is strictly forbidden on CFRP, which is why to date the complete depainting has to be done by hand. The manual grinding process used therefore is not only more time-consuming, but also more dangerous because the composite fibre structure underneath the paint can be damaged if too much force is used. Although trials with solvent-resisting barrier varnishes are already carried out by aircraft manufacturers, the paint stripping process for the present will remain pure handcrafting. For future airplanes featuring a nearly all composite fuselage, new methods and procedures need to be developed. Lufthansa Technik is currently evaluating the use of a new depainting technology based on laser beams. It works absolutely solvent-free and is moreover capable of working colour-selective.

The steady increase in the use of composite materials holds many challenges for MRO-companies like Lufthansa Technik. Especially the complicated detection of damages and their professional repair and painting procedures require very special equipment. One of the most important examples are the autoclave ovens which are essential for the curing of composite parts from a certain size onwards. While smaller structures can be "cold cured" at room temperature, bigger components have to be heated up in an autoclave at 250 or 350 degrees Fahrenheit in order to gain their full strength. Within the Lufthansa Technik Group, four such ovens are already in use, one in Hamburg/Germany, two in Tulsa/Oklahoma and another one in Shenzhen/China. To cure even bigger parts a new autoclave with 16 feet in diameter and 26 feet in length has been constructed in Hamburg in 2009. 

Highly trained personnel for a safe handling of composite materials

Despite all technology, even more important for the handling of composites are highly qualified technicians. They are trained by Lufthansa Technical Training (LTT) in two different courses to prepare them perfectly for working with the new materials. The first training course "Composite Fundamentals" teaches the basic knowledge about all different composites in use, from production methods and material characteristics to special requirements regarding the handling of such materials. The second course "Composite Advanced Repair" moreover provides practical exercises to learn about the detection of certain damages and complex repair procedures of all kind. Additional training courses from LTT and external providers further increase the abilities of Lufthansa Technik's employees.

This self-acquired experience is very important, because for the use and handling of composite materials only very few norms and regulations exist to date. But since international standards would make the work on composites much easier, Lufthansa Technik actively participates in numerous boards and committees that work on such standardizations. Internally the "LHT composite council" is held once a month, where representatives of numerous departments discuss their experience with these materials. As one of only very few MRO-providers, Lufthansa Technik is also a member of the Commercial Aircraft Composite Repairs Committee (CACRC), in which for example aircraft manufacturers, research institutes and airlines work on international standards for the handling of composites. Moreover, Lufthansa Technik participates in the German aviation research program (LUFO4), which grants the company research funds of up to 1.2 million Euros for the development of an automated repair procedure for composite structures. Through all the research conducted Lufthansa Technik is well prepared, even for future airplanes. Those will feature composite materials not only in secondary structures, but nearly in the complete airframes of Boeing 787 and Airbus A350.