5. As soon as the surfaces are dry, restore the original protective paint.
Treatment of Installed Magnesium Castings Magnesium castings, in general, are more porous and more prone to penetrating attack, than wrought magnesium skins. However, treatment is, for all practical purposes, the same for all magnesium areas. Engine cases, belleranks, fittings, "umerous covers, plates, and handles are the most common magnesium castings.
When attack occurs on a casting, the earliest practicable treatment is required if dangerous corrosive penetration is to be avoided. In fact, engine cases submerged in salt water overnight can be completely penetrated. If it is at all practicable, parting surfaces should be separated to effectively treat the existing attack and prevent its further progress. The same general treatment sequence in the preceding paragraph for magnesium skin should be followed.
If extensive removal of corrosion products from a structural casting is involved, a decision from the manufacturer may be necessary to evaluate the adequacy of structural strength remaining.
Specific structural repair manuals usually include dimensional tolerance limits for critical structural members and should be referred to, if any question of safety is involved.
TREATMENT OF TITANIUM AND TITANIUM ALLOYS Attack on titanium surfaces is generally difficult to detect. Titanium is, by nature, highly corrosion resistant, but it may show deterioration from the presence of salt deposits and metal impurities, particularly at high temperatures. Therefore, the use of steel wool, iron scrapers, or steel brushes for cleaning or for the removal of corrosion from titanium parts is prohibited.
If titanium surfaces require cleaning, hand polishing with aluminum polish or a mild abrasive is permissable, if fiber brushes only are used and if the surface is treated following cleaning with a suitable solution of sodium dichromate. Wipe the treated surface with dry cloths to remove excess solution, but do not use a water rinse.
PROTECTION OF DISSIMILAR METAL
CONTACTS Certain metals are subject to corrosion when placed in contact with other metals. This is commonly referred to as electrolytic or dissimilar metals corrosion. Contact of different bare metals creates an electrolytic action when moisture is present. If this moisture is salt water, the electrolytic action is accelerated. The result of dissimilarmetal contact is oxidation (decomposition) of one or both metals. The chart shown in figure 6-60 lists the metal combinations requiring a protective separator. The separating materials may be metal primer, aluminum tape, washers, grease, or sealant, depending on the metals involved.
Contacts Not Involving Magnesium All dissimilar joints not involving magnesium are protected by the application of a minimum of two coats of zine chromate primer in addition to normal primer requirements. Primer is applied by brush or spray and allowed to air-dry 6 hours between coats.
Contacts Involving Magnesium To prevent corrosion between dissimilar-metal joints in which magnesium alloy is involved, each surface is insulated as follows:
At least two coats of zinc chromate are applied to each surface. Next, a layer of pressure-sensitive vinyl tape 0.003-inch thick is applied smoothly and firmly enough to prevent air bubbles and wrinkles. To avoid creep-back, the tape is not stretched during application. When the thickness of the tape interferes with the assembly of parts, where relative motion exists between parts, or when service temperatures above 250° F. are anticipated, the use of tape is eliminated and extra coats (minimum of three) of primer are applied.
CORROSION LIMITS Corrosion, however slight, is damage. Therefore, corrosion damage is classified under the four standard types, as is any other damage. These types are: (1) Negligible damage, (2) damage repairable by patching, (3) damage repairable by insertion, and (4) damage necessitating replacement of parts.
The term "negligible," as used here, does not imply that little or nothing should be done. The corroded surface should be cleaned, treated, and painted as appropriate. Negligible damage, generally, is corrosion which has scarred or eaten away the surface protective coats and begun to etch 180
CONTACTING Aluminum alloy Cadmium plate Carbon and alloy steels Copper and alloys Nickel and alloys Titanium and alloys Chromium plate Corrosion resisting steel Magnesium alloys METALS Zinc plate Lead Tin coating Aluminum alloy Cadmium plate Zinc plate Carbon and alloy steels Lead Tin coating Copper and alloys Nickel and alloys Titanium and alloys Chromium plate Corrosion resisting steel Magnesium alloys Shaded areas indicate dissimiliar metal contacts FIGURE 6-60. Dissimilar metal contacts that will result in electrolytic corrosion.
Corrosion damage extending to classifications of "repairable by patching» and "repairable by insertion" should be rapaired in accordance with the applicable structural repair manual.
When corrosion damage exceeds the damage limits to the extent that repair is not possible, the component or structure should be replaced.
PROCESSES AND MATERIALS USED IN
CORROSION CONTROL Metal Finishing Aircraft parts are almost always given some type of surface finish by the manufacturer. The main purpose is to provide corrosion resistance;
however, surface finishes may also be applied to increase wear resistance or to provide a suitable base for paint.
In most instances the original finishes cannot be restored in the field due to nonavailability of equipment or other limitations. However, an understanding of the various types of metal finishes is necessary if they are to be properly maintained in the field and if the partial restoration techniques used in corrosion control are to be effective.
Surface Preparation Original surface treatments for steel parts usually include a cleaning treatment to remove all traces of dirt, oil, grease, oxides, and moisture.
This is necessary to provide an effective bond between the metal surface and the final finish. The cleaning process may be either mechanical or chemical. In mechanical cleaning the following methods are employed: wire brush, steel wool, emery cloth, sandblasting, or vapor blasting.
Chemical cleaning is preferred over mechanical since none of the base metal is removed by cleaning. 'There are various chemical processes now in use, and the type used will depend on the material 181
being cleaned and the type of foreign matter being removed.
Steel parts are pickled to remove scale, rust, or other foreign matter, particularly before plating. The pickling solution can be either muriatic (hydrochloric) or sulfuric acid. Cost-wise, sulfuric acid is preferable, but muriatic acid is more effective in removing certain types of scale.
The pickling solution is kept in a stoneware tank and is usually heated by means of a steam coil.
Parts not to be electroplated after pickling are immersed in a lime bath to neutralize the acid from the pickling solution.
Electrocleaning is another type of chemical cleaning used to remove grease, oil, or organic matter. In this cleaning process, the metal is suspended in a hot alkaline solution containing special wetting agents, inhibitors, and materials to provide the necessary electrical conductivity. An electric current is then passed through the solution in a manner similar to that used in electroplating.
Aluminum and magnesium parts are also cleaned by using some of the foregoing methods.
Blast cleaning is not applicable to thin aluminum sheets, particularly alclad. Steel grits are not used on aluminum or corrosion-resistant metals.
Polishing, buffing, and coloring of metal surfaces play a very important part in the finishing of metal surfaces. Polishing and buffing operations are sometimes used when preparing a metal surface for electroplating, and all three operations are used when the metal surface requires a high-luster Electroplating Electroplating is the process of transferring metal from one object to another by chemical and electrical means. Several reasons for applying plated coatings are:
1. To protect the base metal (metal being plated) against corrosion. Tin, zinc, nickel, and cadmium are some of the metals used to form a protective coating on another metal by electrolytic action.
2. To protect the base metal against wear, caused by abrasion or fretting corrosion.
Chromium plating is extensively used for wear resistance on gages, dies, oleo pistons, and cylinder barrels. Nickel plating can also be used for this purpose.
3. To produce and retain a desired appearance (color and luster), as well as improve resistance to tarnish. Gold, nickel, or chromium plating can be used in this application.
4. To protect a base metal against some special chemical reaction; for example, copper plating is sometimes used to prevent certain parts of a component manufactured of steel from absorbing carbon during casehardening.
5. To increase the dimensions of a part. This process, known as "building up," may be applied to parts accidentally made undersize, or to worn parts. Nickel or chromium plating is commonly used for this purpose.
6. To serve as a base for further plating bright de bright deposits of nickel or nickel and operations, reduce buffing costs, and ensure chromium. Copper is commonly used for this purpose.
All electroplating processes are basically similar.
The equipment used consists of a tank or bath containing a liquid solution called an electrolyte, a control panel, and a source of direct current.
When a current is passed through the circuit, the plating material is the positive electrode or anode of the circuit. The part on which the plating is deposited is the negative electrode or cathode of the circuit. The source of power anode, cathode, and electrolyte form the plating electrical circuit and cause tiny particles of the plating material to be deposited on the surface of the part being plated. The process is continued until a plating of the required thickness is obtained. The electrolyte, anode, cathode, and current setting will vary with the type of plating material being used.
Some plating operations do not use anodes of the metal being deposited but obtain the metal from the electrolyte alone. Chromium plating is an example of this type plating. Lead anodes, instead of chromium anodes, which are unsatisfactory, are employed to complete the electrical circuit. The chromium for the plating comes from the chromic acid in the electrolyte.
Metal Spraying Metal spraying, or metallizing, is the surface application of molten metal on any solid base material. It is possible to spray aluminum, cadmium, copper, nickel, steel, or any of several metals using this process. In aircraft work the process is used primarily to spray a coat of pure aluminum on steel parts to improve their corrosion 182
The base material must be roughened (usually by sandblasting) and perfectly clean in order for the sprayed metal to adhere to the surface of the base material.
Metal-spraying equipment consists of a supply of oxygen and acetylene piped to the spray gun, which ends in a nozzle. At this point they can be ignited as in a welding torch. A supply of compressed air is also piped to the spray gun. This compressed air operates a feeding mechanism that draws the wire through the spray gun. The wire is melted by the hot oxyacetylene flame and is thrown against the surface being metallized by the compressed air.
CHEMICAL TREATMENTS Parco Lubrizing Parco Lubrizing is a chemical treatment for iron and steel parts which converts the surface to a nonmetallic oil-absorptive phosphate coating. It is designed primarily to reduce wear on moving parts.
The process is a modification of Parkerizing, and consists of a precleaning treatment in which vapor degreasing, acid pickle, or spray emulsion is used, followed by a 15-minute dip in a solution of water and 10 percent by volume of Parco Lubrite. This is followed by a water rinse and a dip in watersoluble oil. The phosphate surface soaks up oil and retains it.
Anodizing Anodizing is the most common surface treatment of nonclad aluminum alloy surfaces. The aluminum alloy sheet or casting is the positive pole in an electrolytic bath in which chromic acid or other oxidizing agent produces an aluminum oxide film on the metal surface. Aluminum oxide is naturally anodizing merely increases thickness and density of the natural oxide film.
When this coating is damaged in service, it can only be partially restored by chemical surface treatments.
faces, including corrosion removal, should avoid unnecessary destruction of the oxide film.
The anodized coating provides excellent resistance to corrosion. The coating is soft and easily scratched, making it necessary to use extreme caution when handling it prior to coating it with Aluminum wool, nylon webbing impregnated with aluminum oxide abrasive or fiber bristle brushes are the approved tools for cleaning anodized surfaces. The use of steel wool, steel wire brushes, or harsh abrasive materials on any aluminum surfaces is prohibited. Producing a buffed or wire brush finish by any means is also prohibited. Otherwise, anodized surfaces are treated in much the same manner as other aluminum finishes.
In addition to its corrosion-resistant qualities, the anodic coating is also an excellent bond for paint. In most cases parts are primed and painted as soon as possible after anodizing, The anodic coating is a poor conductor of electricity; therefore, if parts require bonding, the coating is removed where the bonding wire is to be attached.
• Alclad surfaces that are to be left unpainted require no anodic treatment; however, if the alclad surface is to be painted, it is usually anodized to provide a bond for the paint.
Alodizing is a simple chemical treatment for all aluminum alloys to increase their corrosion resistance and to improve their paint-bonding qualities. Because of its simplicity, it is rapidly replacing anodizing in aircraft work.
The process consists of precleaning with an acidic or alkaline metal cleaner that is applied by either dipping or spraying. The parts are then rinsed with fresh water under pressure for 10 to 15 seconds. After thorough rinsing, alodine is applied by dipping, spraying, or brushing. A thin, hard coating results which ranges in color from light, bluish-green with a slight iridescence on copperfree alloys to an olive green on copper-bearing alloys. The alodine is first rinsed with clear, cold or warm water for a period of 15 to 30 seconds. An additional 10- to 15-second rinse is then given in a Deoxylyte bath. This bath is to counteract alkaline material and to make the alodyzed aluminum surface slightly acid on drying.
Chemical Surface Treatment and Inhibitors As previously described, aluminum and magnesium alloys in particular are protected originally by a variety of surface treatments. Steels may have been Parco Lubrized or otherwise oxidized on the surface during manufacture. Most of these coatings can only be restored by processes which are completely impractical in the field. But, corroded areas where such protective films have been destroyed require some type of treatment prior to refinishing. The following inhibiting materials are particularly effective in the field treatment of aluminum, are beneficial to bare magnesium, and are of some value even on bare steel parts.
The labels on the containers of surface treatment chemicals will provide warnings if a material is toxic or flammable. However, the label might not be large enough to accommodate a list of all the 183
possible hazards which may ensue if the materials are mixed with incompatible substances. For example, some chemicals used in surface treatments will react violently if inadvertently mixed with paint thinners. Chemical surface treatment materials must be handled with extreme care and mixed exactly according to directions.
Chromic Acid Inhibitor A 10-percent solution by weight of chromic acid, activated by a small amount of sulfuric acid, is particularly effective in treating exposed or corroded aluminum surfaces. It may also be used to treat corroded magnesium.
This treatment tends to restore the protective oxide coating on the metal surface. Such treatment must be followed by regular paint finishes as soon as practicable, and never later than the same day as the latest chromic acid treatment. Chromium trioxide flake is a powerful oxidizing agent and a fairly strong acid. It must be stored away from organic solvents and other combustibles. Wiping cloths used in chromic-acid pickup should either be rinsed thoroughly after use or disposed of.
Sodium Dichromate Solution A less active chemical mixture for surface treatment of aluminum is a solution of sodium dichromate and chromic acid. Entrapped solutions of this mixture are less likely to corrode metal surfaces than chromic acid inhibitor solutions.
Chemical Surface Treatments Several commercial, activated chromate acid mixtures are available under Specification MIL-C- 5541 for field treatment of damaged or corroded aluminum surfaces. Precautions should be taken to make sure that sponges or cloths used are thoroughly rinsed to avoid a possible fire hazard after drying. (See figure 6-61) STEP 1 STEP 2 STEP 3 STEP 4
TYPE OF CLEANING TO PAINT STRIPPING CORROSION SURFACE TREAT-
CORROSION REMOVE FOREIGN (WHEN APPLI- REMOVAL MENT (WHEN
MATTER CABLE) APPLICABLE) Light or heavy pitting Remove foreign mat- Readily accessible Remove corrosion Chromate conversion or etching of alu- ter with cleaner, Spec areas: Strip with with brightener, coating Spec MIL- minum (clad) MIL-C-25769 stripper, Spec MIL- Spec MIL-C-25378 C-5541 R-25134 Confined areas:
Strip with solvent Light or heavy pitting As above As above Remove corrosion by As above or etching of mechanical method aluminum (clad) - or - Remove corrosion with brightener, Spec MIL-C-25378 Intergranular or As above As above Remove corrosion by As above exfoliation corrosion mechanical method of aluminum Light or heavy Painted Parts Not Required Remove corrosion Immersion chromate corrosion on small Clean and strip in If cleaning accom- and oxide film by conversion coating, aluminum parts solution of paint and plished with paint immorsion ot parts Spec MIL-C-5541 which can be varnish remover, and varnish remover in phosphoricremoved for Spec MIL-R-7751 Spec MIL-R-7751 chromate acid treatment Unpainted Parts solution Clean with compound, Spec P-C- 426, MIL-C-5543 or vapor degrease Stress corrosion Not Applicable See Step 1 See Step 1 See Step 1 cracking of aluminum FIGURE 6-61. Typical corrosion removal and treatment procedures for aluminum alloys.
PROTECTIVE PAINT FINISHES A good, intact paint finish is the most effective barrier between metal surfaces and corrosive media. The three most common finishes are a nitrocellulose finish, an acrylic nitrocellulose finish, and an epoxy finish. In addition, highvisibility fluorescent materials may also be used, along with a variety of miscellaneous combinations of special materials. There may also be rainerosion-resistant coatings on metal leading edges, and several different baked enamel finishes on engine cases and wheels.
AIRCRAFT CLEANING Cleaning an aircraft and keeping it clean are extremely important. A cracked landing gear fitting covered with mud and grease may be easily overlooked. Dirt can hide cracks in the skin. Dust and grit cause hinge fittings to wear excessively.
A film of dirt if left on the aircraft's outer surface, reduces flying speed and adds extra weight. Dirt or trash blowing or bouncing around the inside of the aircraft is annoying and dangerous. Small pieces of dirt blown into the eyes of the pilot at a critical moment can cause an accident. A coating of dirt and grease on moving parts makes a sort of grinding compound that can cause excessive wear.
Salt water has a serious corroding effect on exposed metal parts of the aircraft, and should be washed off immediately.
There are many different kinds of cleaning agents approved for use in cleaning aircraft. It is impractical to cover each of the various types of cleaning agents since their use varies under different conditions, such as the type of material to be removed, the aircraft finish, and whether the cleaning is internal or external.
In general, the types of cleaning agents used on aircraft are solvents, emulsion cleaners, soaps, and synthetic detergents. Their use must be in accordance with the applicable maintenance manual. The types of cleaning agents named above are also classed as light- or heavy-duty cleaners. The soap and synthetic detergent type cleaners are used for light-duty cleaning, while the solvent and emulsion type cleaners are used for heavy-duty cleaning.
The light-duty cleaners, which are nontoxic and nonflammable, should be used whenever possible.
Exterior Cleaning There are three methods of cleaning the aircraft exterior: (1) Wet wash, (2) dry wash, and (3) polishing. Polishing can be further broken down into hand polishing and mechanical polishing. The type and extent of soiling and the final desired appearance determine the cleaning method to be used.
Wet wash removes oil, grease, or carbon deposits and most soils, with the exception of corrosion and oxide films. The cleaning compounds used are usually applied by spray or mop, after which high-pressure running water is used as a rinse. Either alkaline or emulsion cleaners can be Dry wash is used to remove airport film, dust, and small accumulations of dirt and soil when the use of liquids is neither desirable nor practical.
This method is not suitable for removing heavy deposits of carbon, grease, or oil, especially in the engine exhaust areas. Dry-wash materials are applied with spray, mops, or cloths, and removed by dry mopping or wiping with clean, dry cloths.
Polishing restores the luster to painted and unpainted surfaces of the airplane, and is usually performed after the surfaces have been cleaned.
Polishing is also used to remove oxidation and corrosion. Polishing materials are available in various forms and degrees of abrasiveness. It is important that the aircraft manufacturer's instructions be used in specific applications.
The washing of aircraft should be performed in the shade whenever possible as cleaning compounds tend to streak the surface if applied to hot metal, or permitted to dry on the area. Install covers over all openings where water or cleaners might enter and cause damage.
Various areas of aircraft, such as the sections housing radar and the area forward of the cockpit that are finished with a dull paint, should not be cleaned more than necessary and should never be scrubbed with stiff brushes or coarse rags. A soft sponge or cheesecloth with a minimum of manual rubbing is advisable. Any oil or exhaust stains on the surface should first be removed with a solvent such as kerosene or other petroleum-base solvent.
The surfaces should be rinsed immediately after cleaning to prevent the compound from drying on the surface.
Before applying soap and water to plastic surfaces, flush the plastic surfaces with fresh water to dissolve salt deposits and wash away dust particles. Plastic surfaces should be washed with soap and water, preferably by hand. Rinse with fresh water and dry with chamois or absorbent cotton. In view of the soft surface, do not rub 185
lar materials).
ash eas ımi urin of the aircraft. contacts, etc.).
ean just terior surface. the inside difficult bolts, bits dropped an disolytic corrosion.
inside the airdebris should make cleanparticles an areas, dropto catch this debris.
pick up dust pit and cabin.
blems during taken from tion (NFP. A) aning Operation".
blem the constitute by their ility of reup of flamre is any gents or also irce the or static 186
(4) Deodorizing or Disinfecting Agents.
number of proprietary agents useful for aircraft cabin deodorizing or disinfecting are nonflammable.
Most of these are designed for spray application (aerosol type) and have a nonflammable pressurizing agent but it is well to check this carefully as some may contain a flammable compressed gas for pressurization.
(5) Abrasives. Some proprietary nonflammable mild abrasive materials are available for rejuvenating painted or polished surfaces. They present no
(6) Dry Cleaning Agents. Perchlorethylene and trichlorethylene as used at ambient temperatures are examples of nonflammable dry cleaning agents.
These materials do have a toxicity hazard requiring care in their use. Fire retardant treated materials may be adversely affected by the application of these agents as is true of the water-soluble agents.
Flammable and Combustible Agents
(1) High Flash Point Solvents. Specially refined petroleum products, first developed as ¿Stoddard Solvent" but now sold under a variety of trade names by different companies, have solvent properties approximating gasoline but have fire hazard properties similar to those of kerosene as commonly used (not heated). Most of these are stable products having a flash point from 100° F.
to 140° F. with a comparatively low degree of toxicity.
(2) Low Flash Point Solvents. Class I (flash point at below 100° F.) flammable liquids should not be used for aircraft cleaning or refurbishing.
Common materials falling into this "Class" are acetone, aviation gasoline, methyl ethyl ketone, naphtha and toluol.
In cases where it is absolutely necessary to use a flammable liquid, high flash point liquids (those having a flash point of 100° F. or more) should be used.
(3) Mixed Liquids. Some commercial solvents are mixtures of liquids with differing rates of evaporation such as a mixture of one of the various naphthas and a chlorinated material. The different rates of evaporation may present problems from both the toxicity and fire hazard viewpoints and such mixtures should not be used unless they are stored and handled with full knowledge of these hazards and appropriate precautions taken.
Container Controls Flammable liquids should be handled only in approved containers or safety cans appropriately labeled.
Fire Prevention Precautions During aircraft cleaning or refurbishing operations where flammable or combustible liquids are used, the following general safeguards are recom-
(1) Aircraft cabins should be provided with ventilation sufficient at all times to prevent the accumulation of flammable vapors. To accomplish this, doors to cabins shall be open to secure maximum advantage of natural ventilation. Where such natural ventilation is insufficient under all conditions to prevent the accumulation of flammable vapors, approved mechanical ventilation equipment shall be provided and used. The accumulation of flammable vapors above 25 percent of the lower flammability limit of the particular vapor being used, measured at a point five feet from the location of use, shall result in emergency revisions of operations in progress.
(2) All open fame and spark producing equipment or devices that might be brought within the vapor hazard area should be shut down and not operated during the period when flammable vapors may exist.
Electrical equipment of a hand portable nature used within an aircraft cabin shall be of the type approved for use in Class I, Group D, Hazardous Locations as defined by the National Electrical Code.
Switches to aircraft cabin lighting and to the aircraft electrical system components within the cabin area should not be worked on or switched on or off during cleaning
(5) Suitable warning signs should be placed in conspicuous locations at aircraft doors to indicate that flammable liquids are being or have been used in the cleaning or refurbishing operation in progress.
Fire Protection Recommendations During aircraft cleaning or refurbishing operations where flammable liquids are used the following general fire protection safeguards are recommended:
(1) Aircraft undergoing such cleaning or refurbishing should preferably be located outside of the hangar buildings when weather conditions permit. This provides for added natural ventilation and normally assures easier access to the aircraft in the event of fire.
(2) It is recommended that during such cleaning or refurbishing operations in an aircraft outside of the hangar that portable fire extinguishers be provided at cabin entrances having a minimum rating of 20-B and, at minimum, a booster hoseline with an adjustable water spray nozzle be available capable of reaching the cabin area for use pending the arrival of airport fire equipment. As an alternate to the previous recommendations a Class A fire extinguisher having a minimum rating of 4-A plus or a Class B fire extinguisher having a minimum rating of 20-B should be placed at aircraft cabin doors for immediate use if required.
NOTE 1: All-purpose (dry chemical) type extinguishers should not be used in situations where aluminum corrosion is a problem.
NOTE 2: Portable and semi-portable fire detection and extinguishing equipment has been developed, tested and installed to provide protection to aircraft during construction and maintenance operations. Operators are urged to investigate the feasibility of utilizing such equipment during aircraft cabin cleaning and refurbishing operations.
(3) Aircraft undergoing such cleaning or refurbishing where the work must be done under cover should be in hangars equipped with automatic fire protection equipment.
POWERPLANT CLEANING Cleaning the powerplant is an important job and should be done thoroughly. Grease and dirt accumulations on an air-cooled engine provide an effective insulation against the cooling effect of air flowing over it. Such an accumulation can also cover up cracks or other defects.
When cleaning an engine, open or remove the cowling. Beginning with the top, the engine and accessories are washed down with a fine spray of kerosene or solvent. A bristle brush may be used to help clean some of the surfaces.
Fresh water and soap, and approved cleaning solvents may be used for cleaning propeller and rotor blades. Except in the process of etching.
caustic material should not be used on a propeller.
Scrapers, power buffers, steel brushes, or any tool or substances that will mar or scratch the surface should not be used on propeller blades, except as recommended for etching and repair.
Water spray, rain, or other airborne abrasive material strikes a whirling propeller blade with such force that small pits are formed in the blade's leading edge. If preventive measures are not taken, corrosion causes these pits to rapidly grow larger.
The pits may become so large that it is necessary to file the blade's leading edge until it is smooth.
Steel propeller blades have more resistance to abrasion and corrosion than aluminum alloy blades. Steel blades, if rubbed down with oil after each flight, retain a smooth surface for a long time.
Propellers should be examined regularly because cracks in steel or aluminum alloy blades can become filled with oil which tends to oxidize. This can readily be seen when the blade is inspected Keeping the surface wiped with oil serves as a safety feature by helping to make cracks more Propeller hubs must be inspected regularly for cracks and other defects. Unless the hubs are kept clean, defects may not be found. Steel hubs should be cleaned with soap and fresh water, or with an approved cleaning solvent. These cleaning solvents may be applied by cloths or brushes. Tools and abrasives that scratch or otherwise damage the plating should be avoided.
In special cases in which a high polish is desired, the use of a good grade of metal polish is recommended. Upon completion of the polishing, all traces of polish must be removed immediately, the blades cleaned, and then coated with clean engine oil.
All cleaning substances must be removed immediately after completion of the cleaning of any propeller part. Soap in any form can be removed by rinsing repeatedly with fresh water.
After rinsing, all surfaces should be dried and coated with clean engine oil.
After cleaning the powerplant, all control arms, belleranks, and moving parts should be lubricated according to instructions in the applicable maintenance manual.
SOLVENT CLEANERS In general, solvent cleaners used in aircraft cleaning should have a flashpoint of not less than 105° F., if explosion proofing of equipment and other special precautions are to be avoided.
Chlorinated solvents of all types meet the nonflammable requirements but are toxic, and safety precautions must be observed in their use. Use of carbon tetrachloride should be avoided.
Dry-cleaning Solvent Stoddard solvent is the most common petroleum-base solvent used in aircraft cleaning. Its 188
flashpoint is slightly above 105° F. and can be used to remove grease, oils, or light soils. Drycleaning solvent is preferable to kerosene for all cleaning purposes, but like kerosene it leaves a slight residue upon evaporation which may interfere with the application of some final paint films.
Aliphatic and Aromatic Naphtha Aliphatic naphtha is recommended for wipedown of cleaned surfaces just before painting. This material can also be used for cleaning acrylics and rubber. It flashes at approximately 80° F. and must be used with care.
Aromatic naphtha should not be confused with the aliphatic material. It is toxic and attacks acrylics and rubber products, and must be used with adequate controls.
Safety Solvent Safety solvent, trichloroethane (methyl chloroform), is used for general cleaning and grease removal. It is nonflammable under ordinary circumstances, and is used as a replacement for carbon tetrachloride. The use and safety precautions necessary when using chlorinated solvents must be observed. Prolonged use can cause dermatitis on some persons.
Methyl Ethyl Ketone (MEK) MEK is also available as a solvent cleaner for metal surfaces and paint stripper for small areas.
This is a very active solvent and metal cleaner, with a flashpoint of about 24° F. It is toxic when inhaled, and safety precautions must be observed during its use.
Kerosene is mixed with solvent-emulsion type cleaners for softening heavy preservative coatings.
It is also used for general solvent cleaning, but its use should be followed by a coating or rinse with some othes type of protective agent. Kerosene does not evaporate as rapidly as dry cleaning solvent and generally leaves an appreciable film on cleaned surfaces, which may actually be corrosive. Kerosene films may be removed with safety solvent, water emulsion cleaners, or detergent mixtures.
Cleaning Compound For Oxygen Systems Cleaning compounds for use in the oxygen system are anhydrous (waterless) ethyl alcohol, isopropyl (anti-icing fluid) alcohol, or a mixture of may ho wed freon and isopropyl alcohol. These may be used to clean accessible components of the oxygen system such as crew masks and lines. Fluids should not be put into tanks or regulators.
Do not use any cleaning compounds which may leave an oily film when cleaning oxygen equipment.
Avoid prolonged skin contact with the freon-alcohol mixture or breathing of the vapors. Instructions of the manufacturer of the oxygen equipment and cleaning compounds must be followed at all times.
EMULSION CLEANERS Solvent- and water-emulsion compounds are used in general aircraft cleaning. Solvent emulsions are particularly useful in the removal of heavy deposits, such as carbon, grease, oil, or tar. When used in accordance with instructions, these solvent emulsions do not affect good paint coatings or organic finishes.
Water-Emulsion Cleaner Material available under Specification MIL-C- 22543A is a water-emulsion cleaning compound intended for use on both painted and unpainted aircraft surfaces. This material is also acceptable for cleaning fluorescent painted surfaces and is safe for use on acrylics. However, these properties will vary with the material available, and a sample application should be checked carefully before general uncontrolled use.
Solvent-Emulsion Cleaners One type of solvent-emulsion cleaner is nonphenolic and can be safely used on painted surfaces without softening the base paint. Repeated use may soften acrylic nitrocellulose lacquers. It is effective, however, in softening and lifting heavy preservative coatings. Persistent materials should be given a second or third treatment as necessary Another type of solvent-emulsion cleaner has a phenolic base which is more effective for heavyduty application, but it also tends to soften paint coatings. It must be used with care around rubber, plastics, or other nonmetallic materials. Rubber gloves and goggles should be worn for protection when working with phenolic-base cleaners.
SOAPS AND DETERGENT CLEANERS A number of materials are available for mild cleaning use. In this section, some of the more common materials are discussed.
Cleaning Compound, Aircraft Surfaces Specification MIL-C-5410 Type I and II materials are used in general cleaning of painted and unpainted aircraft surfaces for the removal of light-to-medium soils, operational films, oils, or greases. They are safe to use on all surfaces, 189
including fabrics, leather, and transparent plastics.
Nonglare finishes should not be cleaned more than necessary and should never be scrubbed with stiff Nonionic Detergent Cleaners These materials may be either water-soluble or oil-soluble. The oil-soluble detergent cleaner is effective in a 3- to 5-percent solution in dry.
cleaning solvent for softening and removing heavy preservative coatings. This mixture's performance is similar to the emulsion cleaners mentioned
MECHANICAL CLEANING MATERIALS Mechanical cleaning materials must be used with care and in accordance with directions given, if damage to finishes and surfaces is to be avoided.
Mila Abrasive Materials No attempt is made in this section to furnish detailed instructions for using various materials listed. Some do's and don'ts are included as an aid in selecting materials for specific cleaning jobs.
Powdered pumice is used for cleaning corroded aluminum surfaces. Similar mild abrasives may also be used.
Impregnated cotton wadding material is used for removal of exhaust-gas stains and polishing corroded aluminum surfaces. It may also be used on other metal surfaces to produce a high Aluminum metal polish is used to produce a a sure foul ot be wed on.
anodized surfaces because it will remove the oxide Three grades of aluminum wool, coarse, medium, and fine, are used for general cleaning of aluminum surfaces. Impregnated nylon webbing material is preferred over aluminum wool for the removal of corrosion products and stubborn paint films and for the scuffing of existing paint finishes prior to Lacquer rubbing compound material can be used to remove engine exhaust residues and minor oxidation. Heavy rubbing over rivet heads or edges where protective coatings may be worn thin should be avoided.
Abrasive Papers Abrasive papers used on aircraft surfaces should not contain sharp or needlelike abrasives which can imbed themselves in the base metal being cleaned or in the protective coating being maintained. The abrasives used should not corrode the material being cleaned. Aluminum oxide paper, 300 grit or finer, is available in several forms and is safe to use on most surfaces. Type I, Class 2 material under Federal Specification P-C-451 is available in 1½- and 2-inch widths. The use of carborundum (silicon carbide) papers should be avoided, particularly on aluminum or magnesium.
The grain structure of carborundum is sharp, and the material is so hard that individual grains will penetrate and bury themselves even in steel surfaces. The use of emery paper or crocus cloth on aluminum or magnesium can cause serious corrosion of the metal by imbedded iron oxide.
CHEMICAL CLEANERS Chemical cleaners must be used with great care in cleaning assembled aircraft. The danger of entrapping corrosive materials in faying surfaces and crevices counteracts any advantages in their speed and effectiveness. Any materials used must be relatively neutral and easy to remove. It is emphasized that all residue must be removed Soluble salts from chemical surface treatments such as chromic acid or dichromate treatment will liquefy and promote blistering in the paint Phosphoric-Citric Acid A phosphoric-citric acid mixture (Type I) for cleaning aluminum surfaces is available and is ready to use as packaged. Type II is a concentrate which must be diluted with mineral spirits and water. Skin contact should be avoided by wearing rubber gloves and goggles. Any acid burns may be neutralized by copious water washing, followed by treatment with a diluted solution of baking soda (sodium bicarbonate).
Baking Soda Baking soda may be used to neutralize acid deposits in lead-acid battery compartments and to treat acid burns from chemical cleaners and inhibitors.
STRUCTURAL METALS Knowledge and understanding of the uses, strengths, limitations, and other characteristics of structural metals is vital to properly construct and maintain any equipment, especially airframes. In aircraft maintenance and repair, even a slight deviation from design specification, or the substitution of inferior materials, may result in the loss of both lives and equipment. The use of unsuitable materials can readily erase the finest craftsmanship. The selection of the correct material for a specific repair job demands familiarity with the most common physical properties of various Properties of Metais Of primary concern in aircraft maintenance are such general properties of metals and their alloys as hardness malleability, ductility, elasticity, toughness, density, brittleness, fusibility, conductivity contraction and expansion, and so forth.
These terms are explained to establish a basis for further discussion of structural metals.
permits it to be permanently drawn, bent, or twisted into various shapes without breaking. This property is essential for metals used in making wire and tubing. Ductile metals are greatly preferred for aircraft use because of their ease of forming and resistance to failure under shock loads.
For this reason, aluminum alloys are used for cowl rings, fuselage and wing skin, and formed or extruded parts, such as ribs, spars, and bulkheads.
Chrome molybdenum steel is also easily formed into desired shapes. Ductility is similar to malleability.
Elasticity is that property which enables a metal to return to its original shape when the force which causes the change of shape is removed. This property is extremely valuable because it would be highly undesirable to have a part permanently distorted after an applied load was removed. Each metal has a point known as the elastic limit beyond which it cannot be loaded without causing permanent distortion. In aircraft construction, members and parts are so designed that the maximum loads to which they are subjected will not stress them beyond their elastic limits. This Explanation of Terms Hardness refers to the ability of a metal to resist abrasion, penetration, cutting action, or permanent distortion. Hardness may be increased by coldworking the metal and, in the case of steel and certain aluminum alloys, by heat treatment.
Structural parts are often formed from metals in their soft state and are then heat treated to harden them so that the finished shape will be retained. pereined, Hardness and strength are closely associated properties of metals.
Brittleness is the property of a metal which allows little bending or deformation without shattering. A brittle metal is apt to break or crack without change of shape. Because structural metals are often subjected to shock loads, brittleness is not a very desirable property. Cast iron, cast aluminum, and very hard steel are examples of brittle metals.
A metal which can be hammered, rolled, or pressed into various shapes without cracking, breaking, or having some other detrimental effect, is said to be malleable. This property is necessary in sheet metal that is worked into curved shapessuch as cowlings, fairings, or wingtips. Copper is an example of a malleable metal.
Ductility is the property of a metal which A material which possesses toughness will withstand tearing or shearing and may be stretched or otherwise deformed without breaking. Toughness is a desirable property in aircraft metals.
Density is the weight of a unit volume of a material. In aircraft work, the specified weight of a material per cubic inch is preferred since this figure can be used in determining the weight of a part before actual manufacture. Density is an important consideration when choosing a material to be used in the design of a part in order to maintain the proper weight and balance of the Fusibility is the ability of a metal to become liquid by the application of heat. Metals are fused in welding. Steels fuse around 2,600° F. and aluminum alloys at approximately 1,100° F.
Conductivity is the property which enables a metal to carry heat or electricity. The heat conductivity of a metal is especially important in welding because it governs the amount of heat that will be required for proper fusion. Conductivity of the metal, to a certain extent, determines the type of jig to be used to control expansion and contraction. In aircraft, electrical conductivity must also be considered in conjunction with bonding, to eliminate radio interference.
Contraction and expansion are reactions pro- 191