To strip or not to strip Aircraft painting
What about painting ? This factor is rarely consideration when small to medium operator is buying an airplane, operators only realising the requirement after a need for painting, such as skin peeled off, faded colour, and dirt absorbent etc. becomes apparent.
Painting and repainting process
Aircraft painting is a simple process, the process is similar to automotive painting, some significant difference are other detail requirement from cleanliness, future impact as well as cost. Now we take a closer look at paint removal processes
Paint removal process
Various method of paint removal process have been utilised on metal and composite substrates, each having their own advantages and disadvantages, as well as its environmental impact. Some operator consider more on how to reduce labour cost rather than evaluating the best method of painting. The most common paint removal process used is manual stripping, though there are several more which are environmentally acceptable method, who provides alternatives phenol-based methylene chloride chemicals, from use chemical softeners to media blast to erode the paint layer; as well as using high intensity light or pulsed-light energy to strip the surface coating.
Manual Stripping, Revisiting
Manual chemical stripping is a process of stripping the paint manually with the help of chemical liquid or acid to destroy the paint layer and strip it manually, for typical 70 seat airplane this normally takes two days until the aircraft is ready for painting.
Preparation for this process consist of detail stripping and careful application of remover with small brush in sensitive area such as door, windows, flight control surfaces, and any other parts subject to clearances which may contribute to the penetration of unwanted paint stripped particles. Proceeds with preparing to protect the sensitive area such as composites, Rubber boots, sealant, gaps, flexible hoses, electrical wiring etc by aluminium foil tape, some process use reverse method by masking process preparation, stripping, and detail stripping.
Stripping process is simply applying the remover liquid or acid, and manually stripping it. This process may require several application of paint removal and stripping, depending on paint age, layers and adhesive quality of paint. Some primer coating require manual sanding and scotch briting to prevent acid penetration to the structure. In cleaning process another liquid is required to rinse this paint remover residue by solvent, and some distilled water, however usage of water have to be minimised concerning environmental and waste cost.
Painting process, in general after stripping and cleaning the aircraft is ready for primer coating and after drying, finishing paint layer can be applied commonly poly urethane top coat, although some operator use different way of layers.
British Airways is currently using polymide nylon process as intercoat to paint their fleet. This is one of the most time saving and cost effective approach, polymide nylon coating is applied in between the primer and top coat paint. This nylon will be acting as paint remover chemical stopper when stripping is required, light acid chemical stripper will be resist by this polymide nylon, then further process is continuing normal cleaning and ready for another repaint with another application of nylon coating and top coat. This process will certainly save time compare to manual stripping to metal, and other advantage is preventing potential corrosion in skin area as well as structural area, because it utilise lighter acid, and the stripping just stop until remover reaching polymide nylon.
What are the other process other than manual striping ?
Plastic Media Blasting
Plastic media blasting (PMB) is a process of coating removal with a plastic as a media to strip the paint layer or powder coating. Similar with a sand blasting process, but it uses recyclable plastic particles as the blasting media. PMB can remove paint layers without harming sensitive material such as aluminium, brass, copper, magnesium, thin steel and titanium since the plastic granules are harder than coatings but softer than underlying substrates. However this require proper training and considerably high skill required.
Since many variable can affect the successful stripping, there are special training required which covers: preparing the aircraft and configuration, protecting vital openings and areas by proper masking techniques, skin structure mapping, distance from the work, nozzle size, adjusting media flow rates, adjusting pressures range, selection of blast media, angle of impingement, and finally cleaning up and final preparation of the aircraft for painting.
This technique can be used for stripping honeycomb, plastic, composites, as well as unreachable components such as structure paint removal or system components such as landing gear. Having in mind that this process requires authority approval as well as manufacture approval and recommended procedures.
Flashlamp / Carbon Dioxide (CO2) Pellet Blasting
The Flashlamp / CO2 pellet blasting process is a combination of two current technologies: Pyrolyzation /ablation of surface coatings by impingement of pulsed-light energy from a xenon flash lamp combined with Dry ice (CO2) pellet blasting. Basically the pulsed-light energy bursts ablate the surface coating while a continuous stream of dry ice pellets sweeps away the carbonized residue and maintains the temperature of the substrate below boiling temperature 90o Celcius.
The whole process occurs within a closed loop with continuous high volume and high speed vacuum flow maintained. The vacuum flow carries paint residue into a series of High Efficiency Particulate Air (HEPA) filters and an activated charcoal scrubber effectively rendering the system discharge non-hazardous.
Flashlamp / CO2 pellet blasting process uses no solid particulate or liquid media; the lamp-focused-energy range is only a few inches and all the removed coating particles are contained and collected at the stripping source. This process can be implemented out-of-doors or inside large maintenance hangars in which other aircraft are being serviced, ambient temperature and relative humidity have no impact on the stripping process. However , there are safety equipment required for the personnel working within ten feet of the stripping head wear ultraviolet (UV) rated tinted glasses and ear protection (due to the noise level of the CO2 pellet blast nozzle).
Most of aeronautical advancement from aircraft system, to technology are generally derived from military application, paint removal system that is currently used in military is pulsed light system, the system combines pulsed light energy and a steady stream of dry ice pellets to remove up to 0.37m2 (4 ft 2) of paint per minute.
This system is used to strip Navy carrier based aircraft such as T45 Goshawk, Boeing says this is less than one-third of the cost of manual removal, and one-sixth the cost of chemical stripping. The system have been tested and verified to he safe for all types of metallic and composite substrates, including carbon fibre, Kevlar and boron/carbon fibre epoxy-based components.
Laser Paint Stripping Laser paint stripping, is similar to the flashlamp / CO2 pellet blasting process, but it uses high-intensity, pulsed-light energy bursts to peel off the paint film. This system also using closed loop principles to vacuum the ablated paint residue, up to this point, both process works similar. But the pulsed-light energy is generated by a pulsed CO2 laser. The flashlamp is pulsed 4 to 6 times per second, the laser is pulsed up to 1,000 times per second. By pulsing the laser rapidly and rastering it quickly over the surface yields a strip rate similar to that of the flashlamp/CO2 pellet blasting process. Because no cooling media is used in the laser process, substrate temperature is controlled by limiting the thickness of paint film removal with numerous passes to remove normal coating thickness.
High Pressure Water Blasting
The process is by application of soft chemical technology uses non methylene chloride-based chemicals (light Chemical) to soften surface coatings, which are then blasted away with high pressure (about 6,000 PSI) water jets.
Lufthansa engineers Reinhold Kaempf Introduced Aqua stripping in 1991. This high pressure rotating water Jets peel paint, waste water is filtered purified and most of it recycled. Paint and other residues are disposed of in an environmentally safe manner. This is a mounting jet on arms system and automatically controlled by a computer to maintain the appropriate distance from the aircraft configuration that is being stripped, aircraft configuration are stored in the computers memory.
To operate the Aqua stripping operation, Lufthansa built a large $114 million 2 bay paint hangar at the Hamburg base along with $25 million central waste water treatment plant. The hangar is big enough to accommodate a 747-400 and A300-600 simultaneously.
Wheat Starch Dry Media Blasting
Wheat starch blasting is a development from the Plastic Media Blasting (PMB) process. The primary difference of course is the stripping media. The blasting, recovery, and recycling system is essentially the same for both processes.
The primary advantage of wheat starch media is technology because it reduce damage potential especially in composite area, and it uses less media. Because the wheat starch that blasted onto the coating surface fractures into smaller pieces that are recyclable and reusable until they are too small to erode the paint film effectively.
Which paint removal system are suitable for your aircraft
To select the right product, firstly we have to eliminate whether a manual or automated process is desired, if operating small fleet with rarely paint required it is much better to use manual removal system, in non manual process selection is not as simple as those removal process itself, there are several issue need to be consider.
From time to accomplish job from preparation for stripping, until the aircraft is ready for painting, which is corresponds to AOG (aircraft on ground) cost because for medium to large aircraft, this cost is more than total painting cost, productivity cost as rate of its usage whether to be used in large fleet with wide body or small to air medium aircraft services with high rate of work such as painting services, actual cost per area stripped, environmental impact as waste recycle cost, special dedicated facility
Another consideration is evaluating the system. Those that involve pressure change or high temperature process as most of non manual system requires more attention, some civil manufacturer doubt of structural or skin buckling effect in most of non manual process because it requires so many variable, skill trained operator requirement, special equipment, and dedicated stripping facilities.
Therefore all system must be carefully examined, these describes here are just some major issue that may impact the usability of the method alone.
Secondly for non manual removal process the acceptability is similar with manual process with most important issue is to consider such: for airlines with variable fleet have to make sure that the product they opt to choose is approved by all of their fleet manufacturer. Boeing only allowed one process for their fleet with dry removal process. The product must be approved by aviation regulator or authority applicable for all fleet operation.
The last major factor that need to considered are technical issue, such acquisition cost, actual cost per stripped area, most of non manual process require dedicated stripping facility except for Flashlamp/dry ice or laser process. Finally decision should be based on Life Cycle Cost analysis for every product.
Future Changes in paint removal
Future changes that may affect painting industry concerning environmental factors would be driven by many factors such as increasing trend in waste cost or waste increment contain, environmental organisation, potential corrosion excitation within a certain period of time, and many other factors. There might be some changes in future, unless there are new invention in chemical waste technology that can convert or dissolve chemical waste safely efficiently and much cheaper than current system.
Aircraft painting service provider have different option in choosing their painting method either manual or fully automatic / robotics. Although there are some aircraft appear colourless, but generally all aircraft need to be paint and repaint. There are several current technology in painting aircraft.
BOEING have been trying to overcome time constraint, environmental impact by chemical stripping, and other factor by awarding contract to Boeing and 3M to develop paintless aircraft called Joint Strike Fighter Paintless Airplane Program (JPAP), by simply replacing paint with thin polymer films backed by pressure sensitive adhesive coating called appliques.
“Paintless”, this sounds very promising for airline operator as well as highly manoeuvrable combat aircraft, Boeing has developed paintless technology to reduce the toxic paints and solvents used in military airplane. Bob Burns, from Boeing Joint Strike Fighter Paintless Airplane Program (JPAP) team said that “This is very promising research,” “It is environmentally friendly, highly durable, will cut maintenance costs and improve supportability of the military fleet.” JPAP aimed at reducing the support costs of in-service aircraft. Painting, stripping and repainting airplanes contributes significant costs of maintaining military aircraft.
“As we progressed with the research, we began to discover other potential benefits of JPAP as well,” Burns said. “We learned that JPAP offers the services a way to control a phenomenon called ‘weight growth’.” In 1996 by Joint Strike Fighter program Office awarded the contract to Boeing & 3M to demonstrate the feasibility of replacing paint with thin polymer films backed by pressure sensitive adhesive coating called appliques.
JPAP’s primary objective is to quantify the reduction in aircraft support costs associated with appliqués by estimating the total life cycle costs for fleet use of appliqués and comparing them to paint and appliqué work can be done concurrently with other types of maintenance instead of in separate facilities at a different time. Other JPAP objectives include demonstrating the suitability of appliqués for maritime and carrier environments, and for supersonic aircraft.
How many layer of repaint can be applied ?
In the airline industry, deciding how many layer repaint can optimally be done without stripping varies. Geoff Bancroft surface finishing technical manager from British Aerospace Woodford recommend painting schedules as follows: one stripping required in every 2nd painting process, and the recommended average time is 5 years, however operator may opt to choose which one is the suitable. Considering waste cost, weight factor and time consuming the best way is to peel off the paint to the green or primer like British Airways process. This can optimally be done for one layer paint only.
If we look at the Boeing B737-200 that is still in airline operation with almost 30 years of service with average 6 painting process. To paint this aircraft in UK. It cost round about US$ 100000 it cost 1.5% of its price, quite significant for this class. Not to mention the Non productivity of aircraft, Lufthansa said jumbo jet sitting on the ground will cost its operator more than 30,000 US-Dollars interest alone per day, and it normally requires about two week of painting. Some operator see the need of stripping only from direct cost alone, but haven’t thought about the extra weight it carries.
Lets have al closer look for the cost difference between repaint with and without stripping in certain period of time, in this case we will discuss for 70 seat class. Painting this aircraft in England will cost about US $ 50000 in which 50 – 60 % is labour cost, say US $ 30000. While 40% of labour cost is stripping and 60% is for painting, it looks very attractive for an operator who can save US $ 12000 for not having it stripped plus 2 days saving out of a week painting, this 2 days saving from AOG (aircraft on ground) approximately 8 hrs x 2 days = 16 hrs @ US $ 2500/hr (excluding fuel) = US $ 40000 therefore it saves US $ 52000 without stripping the aircraft.
How about the extra weight carried, this paint will add .2 % of unnecessary weight and the pilot will normally try to maintain recommended company cruising speed to avoid delay, from aerodynamic point of view an increase of x % (up to 1%) of weight will required increase of 1.5 x % of power required, so additional power of .3% from average normal cruise consumption of round about 3000 lbs/hr it increase 9 Lbs/hr or only 6 ltr/hr, and with current fuel cost of 25 pence/litre (UK prices) it require an additional cost of US$ 2.5/hr to carry extra paint without stripping. If in one year the operator utilise 8 hr /day for 325 days its 2600 hr = US $ 6500 for a year and after 8 years the operator will start paying US$ 52000 for the additional weight. Still reasonable for keeping it un stripped.
How about additional 2 layers of paint ? It becomes US $ 5/hr extra fuel required and it correspond to US $ 13000 a year, therefore after 4 year the operator have to start paying US$ 52,000 for extra 2 layers paint carried. This support Geoff Bancroft / BAe reasons for recommending 2 layer of maximum layer of painting process.
An alternative way to save painting cost is chemical waste issue, to waste a drum of chemical treatment in US cost up to $1,000 and about US $ 3500 to waste post strip paint for 70 seat aircraft class in mainland UK and much cheaper if waste is deliver outside Mainland UK. Some painting service provider in Netherlands, West Germany and other European country with tighter policy of chemical waste offer stripping with cheaper option, the customer may carry their own strip waste to dump in cheaper waste cost area such as India, some of ex Russian country for European based fleet, as well as Indonesia or Thailand for Asian based fleet operator or South America for American based fleet.
So are you ready to carry this single layer paint waste some where else? Seems ok if routes deviation to home base destination in this class of aircraft is less than 420 Nm or one hour total because the operating cost for one hour to dump this is about the same save you can make.
In aerodynamic, Drag is one that most aerodynamicist is always trying to reduce, Skin friction and induced drag are the major contributors of an airplane. For a jet transport about 75% of the total drag derived from these drag, and skin friction drag contribute about more than 50% of these drag. Therefore skin or paint is a potential power or fuel saving factor, from kind of paint and the cleanliness of the airplane itself.
All aerobatics airplane are waxed regularly prior competition, Lufthansa aircraft are washed regularly, not only for appearance, but a clean aircraft burns less fuel. Apropos fuel consumption: should the instruments of a jumbo jet show that its speed is 10 km/h too slow, causing the pilot to advance the thrust levers, then the aircraft could consume an additional 800,000 liters of fuel annually.
Weight vs cost
Does weight contribute a great impact ? depends on how many increased weight is added. Approximate weight of 1 layer of paint (primer and top coat) will have an additional weight of 125 gr/m2. BAe 146 would have an increase of 150 lbs calculated weight for one layer of primer and topcoat, and seven layers would carry unnecessary 1000 Lbs additional weight increment or equivalent to 7 passenger or 1.2 % of increase weight, this is not very significant.
How about additional layer in Galaxy C 5 or Jumbo jet B 747, it weighs about 6 passenger or 1000 Lbs per layer, if operator have 5 layer of paint it’s almost two tons of extra weight or 27 passenger and it increased ½ % of fuel consumption.
And for Boeing 737 with normal cruising consumption of 3000 Lbs/hr and 7 layer of repaint (7x.16%=1.12%) without stripping will require an additional fuel of 35 Lbs/hr or 20 ltr/hr of extra fuel or US $ 8.5. If operator utilise B737 8 hrs /day for 325 day a year, in one year its utilisation is 2600 hrs its US $ 22100 per year of extra unnecessary weight carries.
Structural Impact Other consideration are structural consideration, Paint specialist found some corrosion or potential corrosion area just after stripping especially in composites area. Customer or operator should consider to strip the composite area or ask detail about its structural or paint history if buying second hand aircraft, bird strike, dent or skin repair, can be well hidden after painting. In contrary, when manual acid based stripping required the acid particles or liquid could enter some structure if not properly applied and it is a corrosion potential.
Flight control Flight control problem might be another factor need to have better consideration when deciding not to strip the airplane for painting. Flight control mechanical characteristics may change especially for mass & aerodynamic balance type of flight control, thicker paint will change its Inertia distribution. And control characteristic may change even it is very difficult to detect by pilot, especially most of current narrow to wide body operator which of 90% fly with auto pilot engage cannot really detect this and additionally autopilot servos will work harder as well as its hinges or bearings. So don’t think twice Stripping is very recommended.
Icing condition is similar to flight control impact, there will be no option whether to strip or not to strip the flight control, British Aerospace Woodford surface finishing apply different paint for flight control surfaces such PTFE / Teflon paint based to reduce ice accretion in flight control surfaces, as well as around flight control area.
Layer of paint acceptability
As previously discussed in technical aspect for the narrow body layer, it is shown that the optimum layer is 5 years. For two additional layers, it require about 4 years until the operator paid for the extra weight it carries, and just as a reminder the regulation require weighing 2 year after manufacture proceeds with 5 year interval with the cost of UD$ 800 – 1000 for 70 – 130 seat airplane consecutively.
Other consideration for repainting without stripping are: paint adhesive bonding quality, as well as more attention needed in potential peel off area, all aerofoil leading edges, engine air intake area, nose radome area and all movable painted area such landing gear door, flaps, flight control etc. Other peel off potential area that need more attention are under sides fuselage and underside wing for low wing airplane due to gravel from the runway as well as fuselage area within the circumferences of propeller for prop airplane.