The Paint Job

Paint is important—and not just for the external appear­ance of an air­craft. Paint­work has many functions, which include protecting aircraft against cor­ro­sion and lengthening their service lives. In the future, it will even improve environ­mental per­for­mance.

11.2016 | Text: Monika Weiner

Aircraft manufacturers are keen to reduce weight when­ever they can. There are good reasons for this wrest­ling over every gram: the lighter the air­craft, the less fuel it consumes, the more passen­gers it can accom­modate and the better its carbon foot­print. Conse­quently, engine manu­facturers are constantly opti­mizing their designs: fuselage parts are in­creas­ingly being made from ex­pen­sive, carbon-fiber-reinforced plastics; for the interior design, solid metals are being replaced by fiber-reinforced composites. And then the painters come along and spray almost a ton of paint on an Airbus A380. Is that really necessary?

“Absolutely,” explains Maike Timm, production manager for ­air­craft painting services at Lufthansa Technik. “Without the ­protective layer of paint, the air­craft components would get ­damaged very quickly—whether they are made from metal or plastic, it makes no dif­fer­ence.” In every­day flight service, the sur­faces are exposed to huge strains: ice crystals, dust par­ti­cles, ash and grains of sand pelt into the materials at speeds of 1,000 kilo­meters an hour. On top of this, there is UV ra­dia­tion and ­tem­pera­ture swings of -55 to +100 degrees Celsius. Unless the sur­faces are sealed, de-icing agents, kerosene and ­lubri­cation oil residues cause metals to quickly corrode and destroy the ­material compounds of fiber-reinforced plastics. For cost ­reasons, the first passen­ger air­craft took to the skies ­unpainted—there was a war on in Europe. But the alu­minum alloys were soon tar­nished and had to be fre­quent­ly re­pol­ished. Today not even the Junkers Ju 52 flies without a pro­tec­tive paint layer—even if it is not ­apparent at first sight. The outer shell sports a coat of metallic paint.

Protection coat by coat

Without paint, nothing works in aviation. However, the cans of paint you find in your local home im­prove­ment center are not up to the job—air­craft paint has to be ex­treme­ly thin, scratch­proof, dirt-resistant and environ­mentally friendly. Summing up the situa­tion, Timm says: “Thanks to new de­vel­op­ments, we can now work very eco­nomi­cally. Almost a ton of paint for the Airbus A380 may sound like a lot, but it’s spread over a surface of nearly 4,000 square meters. The four to five coats of paint that must be applied have a combined thick­ness of only a fraction of a millimeter.”

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Perfect fit A Lufthansa Technik employee working on a window band on D-AIRX, an Airbus A321-131, which was painted in early 1950s style to celebrate 50 years of Lufthansa in 2013.

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Perfect fit A Lufthansa Technik employee working on a window band on D-AIRX, an Airbus A321-131, which was painted in early 1950s style to celebrate 50 years of Lufthansa in 2013.

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Top coat The aircraft gains color with the third coat of paint—as here at Airbus.

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Top coat The aircraft gains color with the third coat of paint—as here at Airbus.

Decades of experience are contained within these layered systems. In the simplest case, two dif­ferent com­po­nents are required: painters first apply an anti-corrosion primer to the cleaned and sanded sur­faces; then the paint itself can be sprayed onto the primer. At this stage, there are also one-coat and two-coat systems avail­able. The paint manu­facturer AkzoNobel has de­vel­oped a two-coat system composed of a base coat and a clear coat. “The base coat/clear coat system has various ad­van­tages,” explains Thomas Böttcher, sales manager for air­craft paints at AkzoNobel Aerospace Coatings. “The clear coat protects against UV radiation and in­creases the gloss and paint retention. In ad­di­tion, the sur­faces are so smooth that the air­craft is less sus­cep­tible to dirt and has to be washed on av­er­age only half as often. And, lasting six to eight years, the life­times of the two-coat paint­work are sig­nifi­cantly longer than most other paint systems, which last an aver­age of only five years.”

“Without the protective layer of paint, the aircraft components would get damaged very quickly—whether they are made from metal or plastic, it makes no difference.“

Maike Timm, Lufthansa Technik

Detailed work on aircraft wing Painting at Airbus paint shop in Toulouse.

Further intermediate layers allow addi­tional functions to be inte­grated, such as se­lec­tive paint removability. This function is created by means of a thin sepa­rating layer applied to the anti-corrosion primer. If the air­craft needs a new paint­job after a few years, the old paint can be chemi­cally stripped along the sepa­rating layer, leaving the anti-corrosion coating beneath intact. “In this way, it is possible to cut the times needed for removing old paint and applying new paint by ten per­cent,” emphasizes Böttcher.

Colorful planes

After the 2016 Olympic Games in Rio, the German Olympic team flew home in a “Plane of Champions,” a Lufthansa Boeing 747-8 specially designed for the event. However, the air­craft was not repainted for the occasion, just covered with adhesive colored films. The air­craft under­neath still has the classic white design.

In principle, airc­raft can be painted in any color. However, white has the ad­van­tage that it does not heat up. Although a black air­plane could use a thinner coat of paint because dark colors provide better coverage, it would also absorb much more sunlight. The in­te­ri­or would then have to be constantly cooled.

Even colorful and complex designs can be used. That said, multi-colored paint­work is expensive, because every color has to be applied in a separate operation, and parts that are meant to become or remain a dif­fer­ent color have to be masked off by hand. To minimize the amount of work this entails, engineers are cur­rently de­vel­op­ing robots that can apply the various colored paints directly to the aircraft.

Using surfaces to save fuel

In the future, paints will even help save fuel—for example, by having ultra-smooth sur­faces over which the wind glides with scarcely any re­sist­ance. Engineers at Lufthansa Technik are cur­rently ex­pe­ri­menting with polishing agents containing nano­particles that smooth out all sur­face irregu­larities.

Another approach involves providing the sur­faces with a struc­ture that reduces drag. This is how sharks move so easily through the water: micro­scopi­cally thin grooves in their scales running parallel to the direction of the current reduce re­sist­ance. Re­search­ers in the EU’s Clean Sky project have explored how this riblet effect can be used for aviation. Engineers at the Fraunhofer Institute for Manu­facturing Technology and Advanced Materials IFAM de­vel­op­ed a technique by means of which fine grooves can be applied to an air­craft. The simul­taneous stamp hardening method makes it possible to imprint micro­structures using a paint-coated silicon film that bears a negative of the riblet structure. By means of simul­taneous UV irradiation, the paint sets, and then the silicon film can be taken off. “The paint offers several advan­tages,” explains Fraunhofer researcher Dr. Volkmar Stenzel. “It’s UV-resistant, it’s good at with­standing mechanical strains, it doesn’t create any addi­tional weight and it can be applied easily to non-flat surfaces.”

“For example, work is being conducted into special resins that make paints dirt-, water- and ice-repellent. Such paints would dramatically reduce the times and costs needed to clean and de-ice aircraft.“

Dr. Volkmar Stenzel, Fraunhofer IFAM

Paint from a gun Among other techniques, Airbus uses electro­static paint spraying systems, which generate less spray mist. Here the fuselage the first A350XWB for Finnair is painted in Toulouse.

As part of the Clean Sky project, Lufthansa Technik and Airbus engineers tested the resist­ance of 10x10 centimeter micro­structured areas of paint under real conditions in standard flight operation. Currently the re­search­ers are looking for solutions to apply the required structures in a fully auto­mated process, and are inves­ti­gating whether the cost is worth it. According to pro­jec­tions, it can save around one percent of the fuel used in aviation—with an annual kero­sene con­sump­tion of some 300 million tons in civil aviation, it certainly is worth it.

Reduced drag is just one of many expec­tations the aviation industry has for the aircraft paints of tomorrow. “The trend is toward the inte­gration of extra functions,” reports Fraunhofer expert Stenzel. “For example, work is being conducted into special resins that make paints dirt-, water- and ice-repellent. Such paints would dra­mati­cally reduce the times and costs needed to clean and de-ice aircraft.”

Autor

Text:
Monika Weiner, has been working as a science journalist since 1985. A geology graduate, she is especially interested in new developments in research and tech­nol­ogy, and in their impact on society.

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