paint or polish
AERONAUTICAL FUEL EFFICIENCY
PAINTING VERSUS POLISHING AIRCRAFT EXTERIOR SURFACES
The decision to paint or polish the metal surfaces of airplanes is based on marketing, economic, and environmental considerations. Although the net operating cost of polished airplanes is slightly more than that of painted airplanes, no compelling reason generally exists to choose one type of livery over the other. The result is a world fleet made up of airplanes with surfaces that are mostly painted, mostly polished, or both painted and polished.Operators of commercial airplanes base their decisions to paint or polish exterior airplane surfaces on several considerations. Short- and long-term business requirements, as well as the availability of financial, labour, and material resources, must be evaluated when considering the following factors:
Because the colours, patterns, and symbols on the exterior of airplanes convey an image to the public, marketing considerations hold substantial weight in the decision to paint or polish. While some operators believe that their image is best presented with a decorative paint scheme, others believe that a polished surface works best. Once established, the markings become the most visible identifier of an operator at any airport, and they are often retained for many years.
Operators must consider all the factors that contribute to total cost when deciding between painting or polishing their airplanes:
▪ Airplane purchase price.▪ Maintenance.▪ Corrosion protection.
AIRPLANE PURCHASE PRICE
Decorative painting is included in the base price of an airplane. However, full-fuselage painting, unusual markings, and late revisions may cost more.All exterior airplane paint can be classified either as decorative, which includes an operator's markings, or as protective, which is light grey in colour. Protective paint is used in certain areas to prevent corrosion, and it is used on all composites to prevent erosion and moisture ingress. These composite areas include wing fairings, control surfaces, radomes, tail cones, engine nacelles, and large portions of the empennage. For this reason, even polished airplanes use a considerable amount of protective paint.
Decorative paint schemes generally use a minimum of 3 or 4 colours and a maximum of 14 or 15 colours applied to the upper half of the fuselage and to the vertical stabilizer and rudder. These schemes are also applied to the horizontal stabilizer and elevator on Douglas-designed airplanes. A base colour is applied first, followed by stripes, lettering, and logos. Polished airplanes forgo the base colour, restricting the use of decorative paint to stripes, the operator's name and registry number, and logos.
Beyond the need for washing, the comparative cost of maintaining painted and polished surfaces is significantly affected by the policies of individual operators. Most repaint their airplanes every four years, often during a scheduled C- or D-check, but do not completely strip the paint during each cycle. Instead, they alternate between complete stripping and merely scuff-sanding the existing paint layer and applying a new topcoat. Painting costs include labour, stripper, paint, primer, masking materials, and proper disposal of consumables.
Airplanes should never carry more than two layers of paint. With more than two layers, operating efficiency drops, inspections become more difficult, and corrosion can start in chips that remain under a fresh topcoat. Excessive paint build-up is a particular concern on aging airplanes, as the build-up may cause difficulty during inspection of the rows of rivets and lap splices that connect fuselage panels.
Maintaining the appearance of a polished airplane requires repolishing up to three times a year with a special compound applied with mechanical buffers, as well as regular washing to clean oxidation buildup from unpainted surfaces. Both activities require a considerable investment in buffing equipment and personnel. Periodic maintenance can be performed while a polished airplane is being repolished, but not while a painted airplane is being stripped and repainted.
While the lighter weight of a polished an aircraft saves fuel costs, as shown in (table 1), this savings is more than offset by the higher cost of washing, polishing, and painting a polished fuselage throughout its service life refer(table 2).The net operating cost of polished airplanes, calculated as a percentage of the total operating cost, is between 0.06 percent and 0.30 percent more than the total operating cost of fully painted airplanes.
Polished and painted airplanes both need to be washed regularly to preserve their exterior surfaces. However, for the sake of appearance and image, it is not uncommon for polished airplanes to be washed twice as often as fully painted airplanes. Regular washing protects against corrosion by removing contaminants. It also gives maintenance personnel the opportunity to assess the surface condition of an airplane, which permits operators to predict the date and extent of future maintenance required for corrosion and erosion. A mild alkaline detergent and pure warm water should be used. It is particularly important to wash new airplanes, because the protective oxide film that naturally forms and grows on aluminium with age is relatively thin and provides little protection. Both painted and polished surfaces can be adequately protected from corrosion. Fuselage skins are made from Alclad aluminium that consists of a high-strength core alloy bonded to a thin layer of pure aluminium or aluminium alloy. Wing skins are made of bare aluminium and are protected by an impact-resistant paint system. Polished surfaces are protected from corrosion by regular buffing after washing. Painting protects against oxidation, salts, and jet fuel spills. However, unrepaired chips and cracks in paint collect dirt and moisture and so may become corrosion sites. Painted surfaces are also susceptible to filiform corrosion, or worm corrosion, which begins between metallic surfaces and paint and erodes both. It creates hydrogen and lifts up the paint layer as it travels across the surface.
Though the weight of paint adds to fuel consumption, the fuel-cost savings offered by polished surfaces is outweighed by the cost of maintaining the polished surfaces. However, because this difference is a very small percentage of operating cost, many operators decide to paint or polish their airplanes based on marketing and environmental impact considerations. Some believe that a distinctive image can best be achieved with a full paint scheme, while others believe the image can be projected best by mostly polished surfaces. The availability of safe solvents and facilities that comply with environmental laws can also play a role in the choice between painting and polishing.
Table 1: Decorative Paint Scheme Weights, Lb (Kg)
Table 2: Annual Operating Cost Differences Related to Paint Scheme (1998 U.S. Dollars)