Repairing the "Black Gold", part 1
Maintenance of composite materials
In the aviation business they are often called the „black gold": Composite materials. As their name suggests, they combine several positive characteristics of different substances in a single material. Hence, the so called "Composites– often provide features superior to those of metals. While providing the same stability and stiffness, composite materials are much lighter than for example aluminium, which to date is still predominantly used by aircraft manufacturers. Moreover, composites are nearly completely insensible to issues like corrosion and metal fatigue.
Previous developments led to a large number of different composites, from glass- and aramid-fibre reinforced plastics (GFRP, AFRP) to rolled aluminium and glass-fibre plates (GLARE). But if aeronautic engineers are talking about "Composites–, they are mostly referring to carbon-fibre reinforced plastics (CFRP). Its woven black mats of carbon-fibre have mainly contributed to the term "black gold– and its characteristics made CFRP the most utilized composite material in aircraft construction to date. Imbued in epoxide resins, these mats are deployed layer by layer to form a certain structure, which will permanently keep its shape after curing in large autoclave-ovens. While the percentage of such material in secondary structures like control surfaces or engine cowlings was constantly increased over the years, their use in primary structures like the fuselage is yet very limited but will be significantly increased with the introduction of new aircraft types in the near future.
As more and more composites are used in aircraft structures, the number of damages occurring to them will increase as well. In 2006 Lufthansa Technik's engineers already documented 1648 structural damages to composite parts on the 243 aircraft in Lufthansa's fleet. Three out of four were caused by mechanical impact, for example in collisions with ground servicing vehicles, jet bridges or runway debris. Lightning and bird strikes were mostly responsible for the remainder, as were, to a smaller amount, hail strikes or overheating issues. A narrowbody aircraft suffered structural damage statistically every 4600 flights, a widebody aircraft every 1000 flights on average.
Difficult detection of sometimes severe damages
If a composite part is damaged and how serious its structure is affected, is hard to tell with the naked eye. Unlike on aluminium, impacts on CFRP parts can not always be detected by visible inspections, but they might have caused severe damages to the inner structure which hence remain undetected by untrained personnel. Delaminations, water ingressions or damaged honeycomb structures inside a CFRP structure are also hard to locate by inexperienced workers. For this reason, all personnel of Lufthansa Technik, who are responsible for inspection and maintenance of composites, pass a special training that teaches them to safely detect damages on the inside and outside of these materials. Besides technical measures like tap-tests, ultrasonic or thermography, nearly 80 percent of all composite inspections at Lufthansa Technik are still carried out visibly.
If damage is detected, a decision has to be made, whether to repair a structure or not. In some cases it makes more sense to replace the damaged part due to safety and economic reasons. To avoid the mostly high cost of a new composite part, Lufthansa Technik also offers engineering services that develop equivalent parts from alternate materials. In the past this method was used to replace a composite fan cowl door for the IAE V2500 engine. Its optimized construction and advanced fabrication made the new aluminium cowling up to 60 percent more cost effective than a new composite part, thereby requiring much less maintenance and offering no disadvantage in terms of weight. The newly constructed aluminium cowl is hence only slightly more expensive than a repair of the existing composite structure.
Such repairs are carried out with a huge variety of different procedures. For a ferry flight to the maintenance hangar, a temporary fixture with special speed tape or a new filling with resin might be enough for a safe "return home–. Permanent solutions for secondary structures can later be provided by special riveting or bonding repairs. In the first case, Lufthansa Technik's trained personnel use a special reinforcement structure made of metal or composite material as well, a so called "doubler–. Metal doublers are made of titanium instead of aluminium, because the latter has a different thermal expansion coefficient than the CFRP structure which can lead to serious tension in the part to be repaired. Furthermore, carbon-fibres tend to react chemically with aluminium which leads to corrosion problems not existent in combination with titanium. To perfectly match the material characteristics of a composite part to be repaired, doublers can also consist of composite materials. Depending on the accessibility of the repair area, these can be fastened in a pre-cured or in a "wet– condition to be cured after assembly.
Bonded repairs are often carried out by completely rebuilding the structure with so called "pre-preg repairs– or by using a "wet lay up"-procedure. The latter begins with a complete detachment of the damaged area. The clean cavity remaining will then be covered from one side with a forming tool and laid up layer by layer with freshly imbued CFRP-plies from the other side. If necessary, a honeycomb structure will also be inserted into the work piece before laying up new repair layers. After one side has reached the desired thickness, an extra ply is laid up to cover the repair area and the forming tool can be removed. The same repair principle can now be used on the opposite side of the cavity. After all repair layers are attached to the work piece, the repair area is cured by electric blankets at 190 degrees Fahrenheit. The "pre-preg repairs– on the other hand reproduce the lamination methods of the original part which is thereby completely rebuilt to an airworthy condition.