7/25/91 АС 43-4А
(3) High gloss spot cleaner conforming to MIL-C-85570, Type III, is recommended for cleaning exhaust track areas of high gloss paint systems. This material contains solvents, detergents, and suspended abrasive matter to remove soil by wearing away the surface that holds it.
Thixotropic (viscous) cleaner conforming to MIL-C-85570, Type V, is recommended for cleaning wheel wells and replacement of some solvent cleaning where water rinsing can be tolerated. This cleaner contains solvents, detergents and some thickening agents. When applied undiluted to an oily or greasy surface, the cleaner clings long enough to emulsify the soil (about 5 to 15 minutes) and can then be rinsed away with fresh water.
NOTE: The use of solvents for cleaning operations is becoming more and more limited due to environmental regulations. Determine local requirements for waste disposal.
85704 become emulsions when diluted. (5) Solvent emulsion cleaners conforming to MIL-C-43616 and MIL-C- The solvent contained in the cleaner softens oily soils so that they can be emulsified by the detergent and rinsed away.
(6) Detergent solution cleaners conforming to MIL-D-16791 dissolve grease, but are excellent cleaners for interior lightly soiled areas, plastics, in water and clean by dissolving soluble salts, emulsifying low viscosity oils, and suspending easily removed dirt and dust. and instrument glass covers. . They are not very effective on can be easily wiped away or absorbed on a cloth. Solvents differ significantly Cleaning solvents dissolve oily and greasy soils so that they in cleaning ability, toxicity, evaporation rate, effect on paint, and flammability. The use of cleaning solvents is intended for localized spot application only. A dry cleaning solvent conforming to P-D-680, Type II, is the most common cleaning solvent used on aircraft, due to its low toxicity, minimal effect on paint, and relative safety. Other solvents, such as alcohols, ketones, chlorinated solvents, and naphtha, are specialized materials and have restricted use. Refer to the manufacturer's maintenance and cleaning procedures for specific applications.
(8) Miscellaneous cleaning agents include:
Plastic polish which contains mild abrasive matter to polish out scratches in canopy materials;
(її) Alkaline chemicals used to neutralize specific acidic soils;
(iii) Sodium bicarbonate for electrolyte spills from sulfuric acid batteries; and Monobasic sodium phosphate and boric acid for electrolyte spills from nickel-cadmium batteries.
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(9) Steam cleaning is not recommended for general use on aircraft.
It erodes paint, crazes plastic, debonds adhesives, damages electrical insulation, and drives lubrication out of bearings.
C. Cleaning Procedures.
(1) The following cleaning procedures are recommended:
(ї) Remove/disconnect all electrical power;
(її) Ground aircraft;
(iii) Aircraft wash personnel should wear protective gear (gloves, goggles, aprons, etc.);
(close doors, openings, cover vents, pitot static openings, cover wheels, etc.); (Iv) Protect against water/cleaning compound intrusion (V) Accomplish pre-wash lubrication. Lubricate in accordance with applicable maintenance manual;
(vi) Mix cleaning solution to manufacturer's recommendation;
(vii) Use spray not a stream of water during aircraft wash;
(viii) Do not use abrasive cleaning pads; and (їх) Rinse aircraft with fresh water to remove all cleaning compounds.
(2) The following postcleaning procedures are recommended:
(i) Remove all covers, plugs and masking materials;
(¡i) Inspect and clear all drain holes;
(jii) Inspect open and all known water trap areas for water accumulation and proper drainage;
(TV) Lubricate aircraft in accordance with applicable maintenance manual; and (V) Apply operational preservation.
d. Preservation. The day-to-day application of corrosion preventive compounds is used to protect metal aircraft parts and components. They function by preventing corrosive materials from contacting and corroding bare metal surfaces. Many of these compounds are also able to displace water and other contaminants from the surfaces to be protected. Some also provide lubrication as well as corrosion protection. Corrosion preventive compounds vary in appearance and consistency from thick, black types to light oils. Some are Chap 4 80 Par 412
7/25/91 AC 43-4A water displacing and others are not. The thicker compounds provide the best corrosion protection, are longer lasting, and are more difficult to remove. The must be removed and replaced regularly to provide continuing protection. thinner materials provide some lubrication and do not crack, chip, or peel but
e. Surface Treatment.
surface treatment of the metal with a prescribed chemical to form a protective (1) An important step in the corrosion control process is the film. Chemical surface treatments properly applied provide corrosion resistance to the metal and improve the adhesion of subsequently applied paints. These surface treatments, also known as chemical conversion coatings, chromate conversion coatings, chemical films, or pretreatments, are aqueous acid solutions of active inorganic compounds which convert aluminum or magnesium surfaces to a corrosion resistant film.
materials conform to MIL-C-81706. (ї) Aluminum and aluminum alloys chemical conversion (її) Magnesium alloy chemical conversion materials conform to MIL-M-3171.
(iii) Ferrous metals, stainless steel, and titanium treatment prior to painting are limited to corrosion removal and cleaning.
conversion coatings. (2) The surface should be prepared for application of the chemical that have been chemically stripped prior to pretreatment and repainting to chemically stripad Feather the edges of paint along the edge of areas ensure a smooth, overlapping transition between the old and new paint surfaces.
mat saturated with water. (ii) Clean the area with a fine or very fine clean abrasive attention should be given to fasteners and other areas where residues may become entrapped. At this stage in the cleaning, the surface should be water break- Rinse by flushing with fresh water. Particular usually contaminated with grease or oil, which will later interfere with conversion coating, sealing, and painting. A surface showing water breaks (water beading or incomplete wetting) is the area with a solution of aircraft cleaning compound using the abrasive mat. Rinse thoroughly with water. If the surface is not free of water breaks, reclean moistener, or nonatomizing sprayer to achieve a yellow to gold color for (3) Chemical conversion coating is applied by brush, sponge stick for magnesium (30 seconds to 2 minutes). aluminum (usually 2 to 4 minutes) or greenish brown, brassy or brownish-yellow Chap 4 Par 412 81
(4) Immediately rinse the part thoroughly with fresh water. Do not wipe with a rag or cloth.
(5) Allow the chemical conversion coated surface to dry a minimum of 1/2 hour and a maximum of 4 hours before painting.
f. Sealants.
(1) Sealants are one of the most important tools for corrosion prevention and control. They prevent the intrusion of moisture, salt, dust, and aircraft fluids, which can lead to extensive corrosion. For sealants to be effective, it is critical that the correct sealant be chosen for a specific area/situation and that it be applied correctly.
(2) Sealants are used for the following reasons:
(і) Fuel tank sealing;
(ii) Pressure areas;
Weather sealing;
(iv) Firewalls;
(V) Electrical;
(VT) Acid-resistant areas;
(vii) windows;
(viii) High temperature applications; and (їх) Aerodynamic sealing.
properties and intended use. Refer to the aircraft manufacturer's manual for (3) There are numerous sealing compounds available with different specific information concerning selection of the sealing compound and proper application. Observe the warning and cautions of the manufacturer when using sealing compounds. Sealing compounds generally are divided into two major types, those requiring a curing agent and those which cure in air.
Polysulfide, polythioether, and polyurethane sealing compounds consist of the base (prepolymer) and the accelerator (curing agent).
When thoroughly mixed, the catalyst cures the base to a rubbery solid. Rates of cure depend on the type of base, catalyst, temperature, and humidity. A full cure may not be achieved for as long as 7 days.
Silicone sealing compounds generally consist of one component which cures by reaction with moisture in the air. If silicones are applied too thick or in such a way as to prevent moisture from entering the material, they may not cure at all. In addition, many silicone sealing compounds produce acetic acid (vinegar smell) while curing, which can lead to severe corrosion problems. The use of silicone sealing compounds on aircraft Chap 4 82 Par 412
1/25/91 AC 43-4A (MIL-A-46145). should be limited to those non-corrosive products conforming to Specification Some sealing compounds may require the application of a special primer or adhesion promoter prior to sealant application in order to develop a good adhesive bond with the surface. Use only those primers or adhesion promoters recommended by the product manufacturer.
(4) Sealant application procedures:
Following the removal of corrosion and application of a chemical surface treatment, prime all surfaces, except internal fuel tank surfaces. If the surfaces have been contaminated following surface treatment, clean the area with cleaning solvent and a clean cloth. Dry the surfaces immediately with a clean cloth. Do not allow solvent to evaporate from the contacting adjacent areas during application and postapplication smooth-out exterior surface lap and butt seams. operations. Examples of where masking may be beneficial are fillet sealing of Mask off the area being sealed to prevent sealant from coating of an adhesion promoting solution. Allow to dry by evaporation without When required by the manufacturer, apply a thin touching the area for 30 minutes to 1 hour before applying sealant.
Spatula type sealants may be applied with a nonmetallic spatula or scraper. Avoid the entrapment of air. Work sealant into recesses by sliding the edge of the scraper firmly back over the recesses dipped in water. between the tape. Smoothing will be easier if the non-metallic scraper is first smoothed until the desired thickness is reached. (v) Sealant to be applied with a brush is applied and usually require masking and is especially adaptable to filling seams or the application of form-in-place gaskets. Sealant to be applied with a caulking gun will not
(vii) Sealant applied with a spray gun should be applied in a solid, continuous pattern.
recommendations. (viii) Allow sealant to dry or cure to manufacturer's feels tacky, then topcoat as necessary. (ix) When required, prime sealant as soon as it no longer
(5) Faying surface sealant is applied between the contacting surface † two or more parts, and is the most effective seal that can be produced. It should be used for all assembly and, where possible, reassembly. where at al possible it should be used in conjunction with fillet sealing. types of faying surface seal installations, removable and permanent. There are two Chap 4 Par 412 83
7/25/91 AC 43-4A panels, inspection plates, windows, etc. The sealant is normally applied to the (ї) The removable type is for access doors, removable substructure and a parting agent applied on the removable panel during sealant cure.
(ii) The permanent type is for sealing between parts of a structure that is permanently fastened together with a high adhesion sealant.
(6) The fillet, or seam, seal is the most common type found on an aircraft. Fillet seals are used to cover structural joints or seams along stiffeners, skin butts, walls, spars, and longerons, and to seal around fittings and fasteners. It should be used in conjunction with faying surface sealing and in place of it if the assembly sequence restricts the use of faying surface sealing.
(7) Injection sealing is used primarily to fill voids created by structural joggles, gaps, and openings. Use only those sealants recommended by the aircraft/equipment manufacturer. sealant gun. This method is a means of producing a continuous seal where it Force sealant into the area using a becomes impossible to lay down a continuous bead of sealant while fillet sealing. Clean the voids of all dirt, chips, burrs, grease, and oil before injection sealing.
(8) The fastener sealing method depends on the type of fastener.
Fasteners are sealed either during assembly or after assembly. To seal a permanent fastener during assembly, apply the sealant to the hole or dip the fastener into the sealant, and install the fastener while sealant is wet. • For removable fasteners, start the fastener in the hole and apply sealant to the lower side of the fastener head or countersink. To seal after assembly, apply sealant to the fastener head after installation.
(9) Sealing of fuel cells should be accomplished per the aircraft manufacturer's maintenance manual procedures.
g. Paint Finishes and Touchup Procedures.
(1) The primary objective of any paint system is to protect exposed surfaces against corrosion and other forms of deterioration. Operational uses for particular paint schemes include:
(ї) High visibility requirements;
(ї1) Identification markings;
(111) Abrasion protection; and
(iv) Specialty coatings (i.e., walkway coatings).
(2) The paint system on aircraft consists of a primer coat and a topcoat. The primer promotes adhesion and contains corrosion inhibitors. The topcoat provides durability to the paint system, including weather and chemical resistance, along with the coloring necessary for operational requirements.
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(3) Some aircraft surfaces (teflon-filled, rain erosion, walkways, etc. require specialized coatings to satisfy service exposure and operational manual for the aircraft in question. needs. For these surfaces, refer to the specific manufacturer's maintenance
(4) The Environmental Protection Agency, as well as certain local air pollution control districts, has implementated rules which limit the volatile organic content (VOC), or solvent content, of paints applied to aircraft and ground support equipment. It is the responsibility of the user to CURRENT RULES CAN RESULT IN LARGE FINES. insure that these rules are understood and obeyed. FAILURE TO COMPLY WITH
(5) Much of the effectiveness of a paint finish and its adherence depend on careful preparation of the surface prior to touch up and repair.
Aged paint surfaces must be scuff sanded to ensure adhesion of overcoated, freshly applied paint. Sanding requires a complete roughening of the paint surface and can be accomplished by hand sanding or with the use of power tools.
For final preparation, ensure that surfaces to be painted are free of corrosion, have been prepared and the surrounding paint feathered, have been cleaned, and have been conversion coated. Replace any seam sealants when necessary. Mask areas as required to prevent overspray.
(6) Spray application for touchup, overcoat and total repaint:
as required and recommended by the paint manufacturer, stirred, and applied in even coats. Primer thickness varies for each type primer but generally the total dry film thickness is 0.6 to 0.9 mils (0.0006 to 0.0009 inch). You should be able to see through this film thickness. Allow primer to air dry prior to topcoat application in accordance with the paint manufacturer's recommendations.
Normally topcoat application should occur within 24 hours after primer application.
(ii) Topcoats should be thinned with the applicable thinner even coats. as required and recommended by the paint manufacturer, stirred, and applied in Topcoat thickness varies for each topcoat but generally the total dry film thickness is 1.5 to 2.0 mils (0.0015 to 0.002 inch). Allow the topcoat to air dry in accordance with the paint manufacturer's instructions.
applied over a primer in accordance with the manufacturer's instructions. (A) Teflon-filled (antichafe) coatings should be primer in accordance with the manufacturer's instructions. (B) Walkway compounds should be applied over a
413.-499. RESERVED.
Chap 4 Par 412 85 through 90
CHAPTER 5. INSPECTION REQUIREMENTS
500. GENERAL. Except for special requirements in trouble areas, inspection for corrosion should be a part of routine maintenance inspections; i.e., daily or preflight. Overemphasizing a particular corrosion problem when it is discovered and forgetting about corrosion until the next crisis is an unsafe, costly, and troublesome practice. Inspection for corrosion is a continuing requirement and should be accomplished on a daily basis. If corrosion control is assigned to a special crew or group, maintenance checks should be scheduled in such a way that these crews may accomplish their inspections and necessary rework while access plates are removed and components are disconnected or out of the way.
a. Most manufacturers' handbooks of inspection requirements are complete enough to cover all parts of the aircraft or engine, and no part or area of the aircraft should go unchecked. Use these handbooks as a general guide when an area is to be inspected for corrosion.
b. Irouble areas, however, are a different matter, and experience shows that certain combinations of conditions result in corrosion in spite of routine inspection requirements. These trouble areas may be peculiar to particular aircraft models, but similar conditions are usually found on most aircraft.
c. The flight routes and bases of operation will expose some airplanes to more corrosive conditions than others. The operational environment of an aircraft may be categorized as mild, moderate, or severe, with respect to the corrosion severity of the operational environment. The corrosion severity of the operational environments around the world are identified in Figure 4-15 through Figure 4-20. The corrosion severity of any particular area may be increased by many factors including: airborne industrial pollutants, chemicals used on runways and taxiways to prevent ice formation, humidity, temperatures, prevailing winds from corrosive environment, etc.
501. FREQUENCY OF INSPECTIONS. In addition to the routine maintenance inspections, the following special requirements should be observed:
a. Aircraft operating in a severe environment should be inspected every 15 days.
b. Aircraft operating in a moderate environment should be inspected every 45 days.
c. Aircraft operating in a mild environment should be inspected every 90 days.
d. The aircraft should be washed prior to any inspection for corrosion.
e. Checks should be performed by a crew familiar with corrosion problems and the nature of their treatment.
Chap 5 Par 500 91
f. Operators of low utilization aircraft should develop a corrosion inspection and repair program based on calendar time rather than flight hours.
Due to the uncertainties that may be encountered in various operating environments, adjustments to the calendar time inspection interval should be made after analysis of corrosion inspection findings.
RECOMMENDED DEPTH OF INSPECTION. Generally speaking, the applicability of inspection requirements provides a ready means to insure adequate inspection of all compartments and interior aircraft cavities. When such general requirements are observed, along with a periodic check of the list of common trouble areas, adequate maintenance should be assured for most operating conditions. To assist in assuring complete coverage, the following summary is included:
a. Daily and preflight inspection. Check engine compartment gaps, seams, and faying surfaces in the exterior skin. Check all areas which do not require removal of fasteners, panels, etc., such as bilge areas, wheel and wheel well areas, battery compartments, fuel cell and cavity drains, engine frontal areas, including all intake vents, and engine exhaust areas.
b. Indepth inspections. In addition to the more common trouble spots that are readily available for inspection, remove screw-attached panels, access plates, and removable skin sections as necessary to thoroughly inspect the internal cavities. Inspection should also include removal of questionable heavy internal preservative coatings, at least on a spot-check basis. Inspect the interior of the aircraft in corrosion prone areas such as around lavatories, galleys, under floors, baggage compartments, etc. (see Figure 5-1).
c. Corrosion inspections should be accomplished at each annual inspection or other scheduled indepth inspections in which areas of the aircraft not normally accessible will be available for corrosion inspection.
d. Corrosion preventive compounds such as LPS3, Dinol AV5, or equivalent products and later advanced developments of such compounds may be used to effectively reduce the occurrence of corrosion. Results of corrosion inspections should be reviewed to help establish the effectiveness of corrosion preventive compounds and determine the reapplication interval of them.
Chap 5 92 Par 501
FIGURE 5-1. CORROSION FOUND AFTER REMOVING CARGO DOOR THRESHOLD COVERS PRIMARY APPROACH. The primary approach to corrosion detection is corrosion inspections on a regularly scheduled basis. Early detection and treatment reduce costs, out of service time, and the possibility of flight or flight related incidents. All corrosion inspections should start with a thorough cleaning of the area to be inspected. A general visual inspection of the area follows using a flashlight, inspection mirror, and a 5-10x magnifying glass. The general inspection should look for obvious defects and suspected areas. A detailed inspection of damage or suspected areas found during the general inspection follows. The detailed inspection can be one or more of the following.
504. NONDESTRUCTIVE INSPECTION (NDI).
a. Visual Inspection. Visual inspection is the most widely used technique and is an effective method for the detection and evaluation of corrosion. Visual inspection employs the eyes to look directly at an aircraft surface, or at a low angle of incidence to detect corrosion. Using the sense of touch of the hand is also an effective inspection method for the detection of hidden well developed corrosion. Other tools used during the visual inspection are mirrors, borescopes, optical micrometers, and depth gauges. The following shows the type of corrosion damage detectable using the visual inspection method: Figures 5-2 and 5-3 show chipped, missing, and lifted paint;
Chap 5 Par 502 93
Figure 5-4 shows dished and popped rivets; Figure 5-5 shows skin bulges or lifted surfaces; Figures 5-6 and 5-7 show cracks; and Figures 5-8 and 5-9 show corrosion products.
(1) The indications of corrosive attack can take several forms depending on the type of metal and the length of time the corrosion has had to develop. Corrosion deposits on aluminum and magnesium are generally a white powder, while ferrous metals vary from red to dark reddish brown stains.
(2) Sometimes the inspection areas are obscured by structural members, equipment installations, or for some other reason are awkward to check visually. Adequate access for inspection must be obtained by removing access panels and adjacent equipment, cleaning the area as necessary, and removing loose or cracked sealants and paints. Mirrors, borescopes, and fiber optics are useful in providing the means of observing obscure areas. Figures 5-10 through 5-18 depict some of these conditions.
FIGURE 5-2. CORROSION UNDER CHIPPED AND LOOSE PAINT ON WING SKIN Chap 5 94 Par 504