The importance of cleaning in an aircraft maintenance programme

Citation

Wright, T. (2005), "The importance of cleaning in an aircraft maintenance programme", Aircraft Engineering and Aerospace Technology, Vol. 77 No. 5. https://doi.org/10.1108/aeat.2005.12777eaf.001

The importance of cleaning in an aircraft maintenance programme

The importance of cleaning in an aircraft maintenance programme

Keywords: Aircraft, Cleaning, Maintenance

An airliner is designed to have an in- service life of around 20 years and to achieve this, care needs to be taken in the overhaul and maintenance procedures followed throughout the plane's lifetime. When on the ground most aircraft appear to be strong and robust, they are in fact very delicate pieces of engineering.

Aircraft designers are constantly trying to achieve lighter, stronger and more efficient structures and to do this they rely more and more on lightweight alloys and exotic materials. In the basic state these materials are mostly quite resilient to corrosion attack, however, when mixed or joined together they are prone to rapid deterioration and try to revert back to their natural state.

To help prevent this deterioration the materials are treated with a variety of protective films and coatings. This process starts before the pieces are even assembled together. The surfaces of the aluminium sections to be incorporated in the structure are pretreated by anodising and chromate conversion coatings. Before these coatings can be applied the metal surfaces need to be free from oil, grease and corrosion products. This involves an alkaline cleaner stage to give a water break free surface followed by a dexoidiser to give an oxide and smut free surface. This will help to ensure that the coatings will react in a proper manner with the substrate and form a protective coating.

To maintain the integrity of the materials and treatments used in the construction of the aircraft, all maintenance products intended for use on aircraft will have been put through a stringent series of tests to gain the approval of the relevant authorities.

Contamination causes corrosion

Aircraft cleaning is essential to maintain safety and efficiency. The amount of contamination an aircraft picks up during its normal day-to-day operations is staggering. Though a plane may look clean to the naked eye, with every journey the surface will collect a thin layer of chemical contaminants, which if left may cause corrosion.

Corrosion can be regarded as the slow destruction of metals by electro-chemical action. Aircraft operate under widely varying climatic conditions in all parts of the world, sometimes in environments highly conducive to the production of corrosion. Sea salt and salt laden sand are two of the commonest and most corrosive contaminants an aircraft can pick up as it flies around the world. Other chemical pollutants in the atmosphere, such as sulphur/oxygen compounds can react with water vapour to form very corrosive products like sulphuric acid. An aircraft will also pick up general atmospheric contamination such as dirt and soot. A layer of this contamination although only a few microns thick, can equate to a significant amount of extra weight on larger aircraft, which in turn will result in extra drag, inhibit engine efficiency and thereby increase fuel consumption.

Aircraft engines also produce their own contaminants in the form of exhaust gases that are deposited on the aircraft surface. The nature of these by- products is such that they will react with airborne chemicals and produce corrosive compounds.

Maintaining a clean aircraft will aid in the detection of any problems, such as leaks which may occur on the underside around hydraulic, water and overflow holes and drains. It will also inspire confidence in passengers about to board the plane that the plane is well maintained (Plate 1).

Plate 1 Aircraft cleaning in progress

In the past the exteriors of aircraft were cleaned by using copious amounts of water, detergent and solvents. Such methods are now frowned upon and with the advent of tougher environmental legislation outlawing the use of certain agents used in the cleaning process, new products and procedures have had to be developed.

Problems also arise from the fact that many of the components used in aircraft manufacture are made from toxic materials such as cadmium and chromium. Wear from these components, which can sometimes be evident as a powdery residue on the surface of the plane, was also washed off into the river systems leading to a build up of toxic metals in the environment. Over the past few years the environmental agencies have become much stricter about the amount of contamination allowed in watercourses, so cleaning processes which minimise the risk of contaminating the environment are being adopted.

Dry washing/cleaning

In recent years several chemical companies have been developing an effective alternative to wet washing; and so the term dry washing/cleaning was born. Although not strictly a dry process, the ARDROX 6017 is about the best possible alternative and is recommended by certain paint manufacturers. It neither produces any liquid effluent to pollute drains or watercourses, nor does it cause problems for the operators, it also does not damage painted surfaces and has the advantage that it is easy to use.

The product is basically made up of safe surfactants, these ensure the aircraft surface is sufficiently wet and that the cleaner is not repelled into small droplets on the surfaces to be cleaned, together with safe solvents which penetrate any oil, grease, carbon, etc. and a small amount of abrasive powder to assist in the removal of oxides and light particle contamination. Also present is silicone to provide the lubrication for the cloths, pads, etc. and also leave a slight residue, both to produce a lustre and prevent adhesion of soils.

ARDROX 6017 can be applied either by brush, mop, roller, or spray. In normal conditions spray application is the best. It should be applied as a light (mist type) even coating to remove the soils visible on certain areas. Experience will show what amount to use on what degree of soiling. Another variable is the concentration of product required for the degree of soiling encountered. For heavily soiled areas or aircraft being dry cleaned for the first time then the product should be used at 100 per cent concentration. On subsequent applications or on areas with little soiling the cleaner may be diluted and well mixed with up to 30 per cent water.

Once in contact the product should be agitated with either special pads or a soft bristled brush, to ensure that all soils are loosened and held in suspension by the cleaner. During cleaning, the cleaning pads should be inspected regularly to ensure they have not picked up any grit or abrasives which may scratch the aircraft surface, etc.

A suitable dwell time should be allowed (experience will show normally 1-2 min) as this will ensure that any stubborn/heavy soiling is penetrated by the product prior to bulk soil removal.

The bulk of product residue and any soiling should be removed by using a suitable absorbent cloth or pad. Several cloths will be required if the aircraft is heavily soiled. Once used, the soiled cloths are collected for laundering.

After a short period of time the wiped area will dry sufficiently to allow the surface to be buffed to a lustre with a dry cloth or mop. After several areas have been buffed, then the contaminated mop head should be placed in the bin for laundering and reuse.

The fuselage of a medium sized aircraft can be cleaned with about 5l of dry cleaner, such as ARDROX 6017 and heavily soiled or oily areas should be pre-cleaned using a safe solvent. Transparencies should be cleaned with approved cleaner such as ARDROX 6077 transparency cleaner. These products should be used in accordance with the instructions detailed on the technical data sheets. A copy of the material safety data sheets (MSDS) should also be obtained as it will contain all relevant chemical and user information. It will also provide information on the “Environmental Requirements” and other factors regarding the use of the product and “Personal Protective Equipment”.

Cleaning undercarriage bays

The cleaning of undercarriage bays needs specialist attention. It is an important area to keep clean as it aids the detection of any hydraulic leaks and also helps to ensure the integrity of any electrical connections. In areas where water use is not restricted this process is normally carried out using a heavy-duty alkali cleaner such as ARDROX 1900B. In which case, after cleaning it may be beneficial to treat the whole area with a water displacing fluid such as ARDROX 3961. This not only drives away any water but also leaves a protective film on the surface. Then normally after this operation the whole undercarriage would then be lubricated.

In areas where water use is restricted solvent wipes such as ARDROX 9PR- 70 can be used instead. If required the metal structure can then be cleaned using ARDROX 6017. After this operation the operator should be asked if application of a Corrosion Inhibiting Compound (CIC) ARDROX AV-8 AV- 15 etc. is required.

Interior cleaning

It is not only the outside of planes that require attention on a regular basis, since the introduction of mass air travel specialist companies have been employed to clean the passenger compartments of aircraft. Originally it was just to empty the toilets, pick up litter, replace the seat headrests and layout the seat belts. However with the introduction of long haul intercontinental flights, both for passengers and freight, cleaning has become paramount for the aircraft operators.

Many travellers do not realise the intensity of action that takes place during an aircraft turnaround. It is necessary for the cleaning companies to not only pick up litter and arrange the seat belts, etc., but also to clean galleys, toilets and each individual seat. On overnight stops it may also be necessary to clean the cabin carpets and even deep clean/descale the toilet systems. If you consider that a large plane may have up to 17 toilets, maybe 6 galleys in addition to several hundred feet of carpet and 400 plus seats and windows, then the scale of the operation becomes clear.

All products used have to be approved by the authorities, thus specialist products exist designed for these conditions. Carpet and upholstery cleaners need to be applied by machine and also by hand where necessary. ARDROX 6082 can be used both in machines and also applied by hand to get rid of stubborn stains. The recirculatory toilet systems pose their own problem, but these days it is possible to find both formaldehyde based, such as ARDROX 1822, and non-formaldehyde based santisers, such as ARDROX 1823.

During the lifetime of a plane, the paint coating will be stripped on several occasions not only to allow an in-depth NDT (non-destructive testing) inspection to take place but also to reflect changes in aircraft livery and to repair the damage done to the coating from daily wear and tear. The exterior paint of a plane can be removed by using paint strippers applied by brush or spray. ARDROX 204 will remove a wide range of paint finishes including chromate primers. After application the paint will blister and then most of the paint can be removed with non-metallic scrapers before it is rinsed with water to leave a clean surface. Polyurethane and epoxy coatings will need a more specialised product such as ARDROX 2526.

Due to the specialised nature of the airline industry, the constraints on product use imposed by the manufacturers and also by the environmental regulations, it is always best to ask the advice of the local sales representative as to which product best fits the task in hand.

Plate 2 Trevor Wright

Details available from: Chemetall PLC. Tel: 01908 649333, Fax: 01908 373939, E-mail: ukinfo@chemetall.com

Trevor WrightTechnical Service Manager (Plate 2).