applied in two coats over the modified primer: the first a mist coat, and the second a wet, full-hiding crosscoat, with 20 to 30 min. drying time allowed between the two coatings. Once the paint finish has set, paint stripper is necessary for its removal.
Acrylic nitrocellulose lacquer thinner is used in thinning acrylic nitrocellulose lacquers to spray consistency.
When rebuilding acrylic finishes, use two separate thinners: (1) Cellulose nitrate dope and lacquer thinner to thin the modified primer, and (2) acrylic nitrocellulose lacquer thinner to reduce the topcoat material. Make sure that the thinner materials are used properly and that the two are not mixed.
Touchup of Acrylic Nitrocellulose Finishes After removal of damaged paint, the first step before application or touchup acrylic nitrocellulose lacquer is preparing the old coat to receive the new.
Acrylic nitrocellulose lacquer thinner may be effectively wed to wipe small areas prior to painting.
This will soften the edges of the base paint film around damaged areas, which in turn will assure improved adhesion of the touchup coating. However, the thinner contains toluene and ketones and should never be used indiscriminately for cleaning painted surfaces.
When softening old, good-condition acrylic nitrocellulose finishes with thinner, care should be taken to avoid penetration and separation of the old primer coats. The new acrylic lacquer coat should be applied directly over the softened surface without the use of primers between the old and new coats.
EPOXY FINISHES Another type of paint becoming increasingly common is a Military Specification epoxy finish or proprietary epoxy primer and topcoats. These finishes ordinarily consist of a conventional wash primer coat and two coats of epoxy material. However, in some cases it may consist of a three-coat finish that includes wash primer plus epoxy-polyamide primer with an epoxy-polyamide topcoat.
The high gloss inherent with this system is primarily due to the slow flowing resins used. The thinners flash off quickly but the resins continue to flow for three to five days. It is this long flow-out time and the even cure throughout the film that gives the pigment and the film time to form a truly flat surface, one that reflects light and has the glossy "wet" look which makes them so popular.
Polyurethane finish is used on agricultural aircraft and seaplanes because of its abrasion reșistance and resistance to chemical attack. Phosphateesten (Skydrol) hydraulic Auid, which quite actively attacks and softens other finishes, has only minimal effect on polyurethanes. Even acetone will not dull the finish. Paint strippers must be held to the surface for a good while to give the active ingredients time to break through the film and attack the primer.
The epoxy material presently in use is a twopackage system that consists of a resin and a converter which must be mixed in definite proportions just before application. Since the proportions will vary between colors used and also with sources of procurement, it is important that instructions on the specific container be observed carefully. The converter should always be added to the resin, never resin to the converter. Also, do not mix materials from two different manufacturers. The mixture should be allowed to stand at least 15 min, before initial use.
In this time the curing action is started. The primary purpose of this waiting period is to aid in the application and actually has little to do with the results of the finish itself. After this induction period, the material is stirred and mixed with reducer to the proper viscosity for spraying. When you have the proper viscosity, spray on a very light tack coat, lighter than with a conventional enamel.
Allow it to set for about 15 minutes so the thinner can flash off, or evaporate, and spray on a full wet cross coat. The main problem with the application of polyurethane lies in getting it on too thick. A film thickness of about 1.5 mils (one and a half thousandths of an inch) is about maximum for all areas except for those subject to excessive erosion, such as leading edges. Too thick a film which might build up in the faying strips can crack because of loss of flexibility. A good practical way to tell when you have enough material is to spray until you feel that one more pass will be just right, then quit right there, before you make that ione more pass." The high solids content of polyurethane, its slow drying, and low surface tension allows the finish to crawl for an hour or so after it has been put on. If you can still see the metal when you think you have almost enough, don't worry; it will flow out and cover it. Almost no polyurethane job will look good until the next day, because it is still flowing. It will actually flow for about 3 to 5 days. It will be hard in this time, and the airplane may be flown in good weather, but the paint below the surface is still moving.
Masking tape may be applied after 5 hours under the most ideal conditions, but it is far better if you 120
can wait 24 hours after application of the finish;
it should be removed as soon after the trim is sprayed as possible. If it is left on the surface for a day or so, it will be almost impossible to remove.
Both the polyurethane enamel and the epoxy primer which bonds the film to the surface are catalytic materials. They should be mixed and used within 6 hours. If they are not applied within this time, they will not have the full gloss because of the reduced flow time. If it is impossible to spray all of the polyurethane within the 6 hour time period, careful addition of reducer can add a couple of hours to the useful life of the material.
The catalysts used for these primers and finishes are highly reactive to moisture, and the cans should be recapped immediately after using. If a can of the catalyst is allowed to remain open for a period of time, and is then resealed, the moisture in the can will activate it, and swell it up so much there is danger of the can bursting. High humidity and/or heat accelerates the cure.
All catalyzed material must be removed from the pressure pot, the hose, and the gun, immediately upon completion of the spraying operation, and the equipment thoroughly washed.
terial is allowed to remain overnight, it will solidify and ruin the equipment.
Precautions must be taken to assure respiratory and eye protection when mixing the two-part resin and activator. Gloves and aprone should also be used to prevent skin contact. Smoking or eating in the mixing area should be specifically prohibited, and mixing should be accomplished in a well-ventilated area. The uncured resins and catalysts contained in these mixtures can cause skin sensitivity similar to a poison ivy reaction.
FLUORESCENT FINISHES Fluorescent paint finishes are available in two types of equal fade- and weather-resistant qualities:
(1) A removable finish which is designed for ease of removal and (2) a permanent finish which ordinarily may not be removed without stripping the entire paint finish down to bare metal. These fluorescent paints are applied over full-hiding, clean, white undercoats for maximum reflectance.
Replacement of Existing Finish For optimum weather resistance and film properties, the dry film thickness must be at least 3 mils for the fluorescent body coat and 1 mil for the clear topcoat. A clear topcoat of from 1 to 1-1/2 mils is necessary to screen out ultraviolet rays from the sun and prevent early or spotty fading of the fluorescent finish. The use of clear lacquers, other than those provided with the fluorescent paint, may also promou fading.
When the permanent finish is white and a fluoreecent finish is needed, the permanent white finish may serve as the undercoat. If the permanent finish is any other color, a white lacquer should be used under the fluorescent paint.
When applying fluorescent paint to epoxy finishes, first coat the epoxy surface with white nitrocellulose lacquer, since the fluorescent finish does not adhere too well to the epoxy films. These highvisibility finishes are effective for periods of 6 to 8 months.
Touchup of Fluorescent Finishes Touchup of fluorescent finishes is difficult to control and should seldom be attempted. Any touchup will be noticeable because of the variations in shad- Touchup of Epoxy Finishes Epoxy coatings may be applied directly over bare metal in small areas. Minor damage such scratches and abrasions should be repaired by applying the epoxy topcoat directly to the damaged area, whether or not the damage extends through to the bare metal, The area should be thoroughly cleaned and the edges of the old coating roughened to assure adherence. This material builds up very rapidly. Coats that are too heavy are easily produced and are particularly subject to poor adhesion and cracking.
Larger areas of damage should be repaired by stripping to the nearest seam line and building a complete epoxy finish.
Minor damage in fluorescent coatings is repaired by masking, stripping with toluene down to the white undercoat, and repainting with fluorescent paint. This should include one or more touchup coatings of fluorescent paint finish and then overcoated with a clear topcoat sealant.
ENAMEL FINISHES Enamels frequently are used for the topcoats in finishing aircraft. Practically all aircraft enamels are made by mixing a pigment with spar varnish or glycerol phthalate varnish.
Most enamel finishes used on aircraft components are baked finishes that cannot be duplicated under field conditions. Some are proprietary (patented) materials that are not available in standard stock.
However, for touchup purposes on any enameled surface, standard air-drying, glossy enamel or quick-drying enamel may be used.
High-gloss enamel is thinned with mineral spirits, can be applied by brushing, and should ordinarily be used over a zinc chromate primer coat base.
Quick-drying enamel is best thinned with aromatic naphtha. In situations where a primer is not available, either of these enamels may be applied directly to bare metal.
If no enamel is available for touchup purposes, epoxy topcoat material may be substitued. The use of acrylic nitrocellulose lacquer for enamel repairs usually is not satisfactory.
PAINT SYSTEM COMPATIBILITY The use of several different types of paint, coupled with several proprietary coatings, makes repair of damaged and deteriorated areas particularly difficult, since paint finishes are not necessarily compatible with each other. The following general rules for constituent compatibility are included for information and are not necessarily listed in the order of importance:
(1) Old type zinc chromate primer may be used directly for touchup of bare metal surfaces and for use on interior finishes.
It may be overcoated with wash primers if it is in good condition. Acrylic lacquer finishes will not adhere to this material.
(2) Modified zinc chromate primer will not adhere satisfactorily to bare metal. It must never be used over a dried film of acrylic nitrocellulose lacquer.
(3) Nitrocellulose coatings will adhere to acrylic finishes, but the reverse is not true. Acrylic nitrocellulose lacquers may not be used over old nitrocellulose fin-
(4) Acrylic nitrocellulose lacquers will adhere poorly to both nitrocellulose and epoxy finishes and to bare metal generally. For best results the lacquers must be applied over fresh, successive coatings of wash primer and modified zinc chromate. They will also adhere to freshly applied epoxy coatings (dried less than 6 hrs.).
(5) Epoxy topcoats will adhere to all the paint systems that are in good condition and may be used for general touchup, including touchup of defects in baked enamel coatings.
(6) Old wash primer coats may be overcoated directly with epoxy finishes. A new second coat of wash primer must be applied if an acrylic finish is to be applied.
(7) Old acrylic finishes may be refinished with new acrylic if the old coating is thoroughly softened using acrylic nitrocellulose thinner before paint touchup.
(8) Damage to epoxy finishes can best be repaired by using more epoxy, since neither of the lacquer finishes will stick to the epoxy surface. In some instances, airdrying enamels may be used for touchup of epoxy coatings if edges of damaged areas are first roughened with abrasive paper.
METHODS OF APPLYING FINISHES There are several methods of applying aircraft finishes. Among the most common are dipping, brushing, and spraying.
Dipping The application of finishes by dipping is generally confined to factories or large repair stations.
The process consists of dipping the part to be finished in a tank filled with the finishing material.
Prime coats are frequently applied in this manner.
Brushing Brushing has long been a satisfactory method of applying finishes to all types of surfaces. Brushing is generally used for small repair work and on surfaces where it is not practicable to spray paint.
The material to be applied should be thinned to the proper consistency for brushing. A material that is too thick has a tendency to pull or rope under the brush. If the materials are too thin, they are likely to run or will not cover the surface adequately.
Spray Painting All spray systems have several basic similarities.
There must be an adequate source of compressed 122
air, a reservoir or feed tank to hold a supply of the finishing material, and a device for controlling the combination of air and finishing material ejected in an atomized cloud or spray against the surface to be coated.
There are two main types of spray equipment. A spray gun with integral paint container is satisfactory when painting small areas. When large areas are painted, pressure-feed equipment is usually preferred, since a large supply of finishing material can be provided under constant pressure to a pressure-feed type of spray gun.
The air-pressure supply must be entirely free from water or oil to obtain good spray painting. Oil and water traps as well as suitable filters must be incorporated in the air pressure supply line. These filters and traps must be serviced on a regular basis.
The spray gun can be adjusted to give a circular or fan type of spray pattern. Figure 4-1 shows the spray pattern at various dial settings. When cover.
ing large surfaces, set the gun just below maximum width of the fan spray. The circular spray is suitable for "spotting-in" small areas.
The gun should be held 6 to 10 in, away from the surface and the contour of the work carefully fol.
lowed. It is important that the gun be kept at right angles to the surface. Each stroke of the spray gun should be straight and the trigger released just before completing the stroke, as shown in figure 4-2.
The speed of movement should be regulated to deposit an even, wet, but not too heavy, coat.
Each stroke of the gun should be overlapped to keep a wet film, thus absorbing the dry edges of the previous stroke.
The spray should be applied as an even, wet coat that will flow out smoothly and be free from spray dusti Inadequate coverage results from spraying too lightly and "runs" and "sags" from spraying too heavily.
To aid in obtaining good results, make sure the air pressure to the spray gun is between 40 and 80
p.s.i., depending on the material being used. With air pressures below 40 p.s.i. spraying is slow and tedious. Also, with viscous materials, full atomization is not obtained. Above 80 p.s.i. "dust" and blowback become troublesome.
When using pressure-feed equipment, adjust the air pressure in the container according to the viscosity of the paint and the length of the fluid hose used. The pressure must be such that the material reaches the spray gun head in a gentle and continuous flow. Generally, a pressure between 5 and 15
p.s.i. should be used. Higher pressures lead to runs and sags caused by the delivery of too much paint.
PREPARATION OF PAINT Before paint is used, it must be stirred thoroughly so that any pigment which may have settled to the bottom of the container is brought into suspension and distributed evenly throughout the paint. If a film, called "skinning," has formed over the paint, the skin must be completely removed before stirring. Mechanical stirring is preferable to hand stirring. A mechanical agitator or tumbler may be used. However, as tumbling does not always remove pigment caked at the bottom of the container, a test with a stirrer should be made to ensure that the pigment is completely held in suspension. For hand stirring, a flat-bladed, nonferrous stirrer should be used.
Dial Locking bolt Spreaderadjustment valve
- Air valve Fluidneedle - Gun body valve Dial at 8 Dial at 10 FICURE 4-1. Spray patterns at various dial settings.
6 to 10 inches Do not arc stroke arcing causes uneven application.
Move gun in straight line.
Begin stroke Release trigger before then pull trigger. completing stroke.
FICURE 4-2. Spray gun stroke.
The degree of thinning depends on the type of spray equipment, air pressure, atmospheric conditions and the type of paint being used. No hard and fast rule for thinning ratios can be applied. Because of the importance of accurate thinning, some manufacturers recommend the use of viscosity control.
This is usually accomplished by using a viscosity (flow) cup. When the right proportion of thinner is mixed into the material, a cupful of material will flow out completely in a designated number of seconds. The finishing manufacturer can specify the number of seconds required for a given material.
Material thinned using this method will be of the correct viscosity for best application.
In many cases manufacturers recommend that all materials should be strained before use. A 60- to 90-mesh strainer is suitable for this purpose. Strainers are available in metal gauze, paper, or nylon mesh.
COMMON PAINT TROUBLES Poor Adhesion Paint properly applied to correctly pretreated surfaces should adhere satisfactorily, and when it is thoroughly dry, it should not be possible to remove it easily, even by firm scratching with the fingernail. Poor adhesion may result from one of the following:
(1) Inadequate cleaning and pretreatment.
(2) Inadequate stirring of paint or primer.
(3) Coating at incorrect time intervals.
(4) Application under adverse conditions.
(5) Bad application.
Spray Dust Spray dust is caused by the atomized particles becoming dry before reaching the surface being painted and thus failing to flow into a continuous film. The usual causes are incorrect air pressure or the distance the gun is held from the work.
Sags and Runs Sags and runs result from too much paint being applied causing the film of wet paint to move by gravity and presenting a sagging appearance. Incorrect viscosity, air pressure, and gun handling are frequent causes. However, inadequate surface preparation may also be responsible.
Spray Mottle Sometimes known as "orange peel" or "pebble," spray mottle is usually caused by incorrect paint viscosity, air pressure, spray gun setting, or the distance the gun is held from the work.
Blushing Blushing is one of the most common troubles experienced and appears as a "clouding" or 124
"blooming" of the paint film. It is more common with the cellulose than synthetic materials. It may be caused by moisture in the air supply line, adverse humidity, drafts, or sudden changes in temperature.
PAINTING TRIM AND IDENTIFICATION NUM- BERS When an aircraft is being painted, the predominate color usually is applied first over the entire surface. The trim colors are applied over the base color after it dries. When the top of the fuselage is to be painted white with a dark color adjoining it, the light color is applied and feathered into the area to be painted with the dark color. When the light color has dried, masking tape and paper are placed along the line of separation and the dark color is then sprayed on.
Allow the paint to dry for several hours before removing the masking tape. Remove the tape by pulling slowly parallel to the surface. This will reduce the possibility of peeling off the finish with the tape.
All aircraft are required to display nationality and registration marks. These marks may be painted on or afhxed using self-adhering plastic figures. The marks must be formed of solid lines using a color that contrasts with the background. No ornamentation may be used with the markings, and they must be affixed with a material or paint that produces a degree of permanence. Aircraft scheduled for immediate delivery to a foreign purchaser may display marks that can be easily removed. Aircraft manufactured in the United States for delivery outside the U.S. may display identification marks required by the State of registry of the aircraft.
The aircraft may be operated only for test and demonstration flights for a limited period of time or for delivery to the purchaser.
Aircraft registered in the United States must display the Roman capital letter "N" followed by the registration number of the aircraft. The location and size of the identification marks vary according to the type of aircraft. The location and size are prescribed in the Federal Aviation Regulations.
DECALCOMANIAS (DECALS) Markings are placed on aircraft surfaces to provide servicing instructions, fuel and oil specifications, tank capacities, and to identify lifting and leveling points, walkways, battery locations, cr any areas that should be identified. These markings can be applied by stenciling or by using decalcomanias.
Decalcomanias are used instead of painted instructions because they are usually cheaper and easier to apply. Decals used on aircraft are usually of three types: (1) Paper, (2) metal, or (3) vinyl film. These decals are suitable for exterior and interior surface application.
To assure proper adhesion of decals, clean all surfaces thoroughly with aliphatic naphtha to remove grease, oil, wax, or foreign matter. Porous surfaces should be sealed and rough surfaces sanded, followed by cleaning to remove any residue.
The instructions for applying decals are usually printed on the reverse side of each decal and should be followed. A general application procedure for each type of decal is presented in the following paragraphs to provide familiarization with the techniques involved.
Paper Decals Immerse paper decals in clean water for 1 to 3 min. Allowing decals to soak longer than 3 min. will cause the backing to separate from the decal while immersed. If decals are allowed to soak less than 1 min., the backing will not separate from the decal.
Place one edge of the decal on the prepared receiving surface and press lightly, and then slide the paper backing from beneath the decal. Perform minor alignment with the fingers. Remove water by gently blotting the decal and adjacent area with a soft, absorbent cloth. Remove air or water bubbles trapped under the decal by wiping carefully toward the nearest edge of the decal with a cloth. Allow the After the decal has dried, coat it with clear varnish to protect it from deterioration and peeling.
Metal Decals with Cellophane Backing Apply metal decals with cellophane backing adhesive as follows:
(1) Immerse the decal in clean, warm water for 1 to 3 min.
(2) Remove it from the water and dry carefully with a clean cloth.
(3) Remove the cellophane backing but do not touch adhesive.
(4) Position one edge of the decal on the prepared receiving surface. On large foil decals, place the center on the receiving 125
surface and work outward from the center to the edges.
(5) Remove all air pockets by rolling firmly with a rubber roller, and press all edges tightly against the receiving surface to assure good adhesion.
Metal Decals with Paper Backing Metal decals with a paper backing are applied similarly to those having a cellophane backing.
However, it is not necessary to immerse the decal in water to remove the backing. It may be peeled from the decal without moistening. After removing the backing, apply a very light coat of cyclohexauone, or equivalent, to the adhesive. The decal should be positioned and smoothed out following the procedures given for cellophane-backed decals.
Metal Decals with No Adhesive Apply decals with no adhesive in the following
(1) Apply one coat of cement, Military Specification MIL-A-5092, to the decal and prepared receiving surface.
(2) Allow cement to dry until both surfaces
(3) Apply the decal and smooth it down to remove air pockets.
(4) Remove excess adhesive with a cloth dampened with aliphatic naphtha.
Vinyl Film Decals To apply vinyl film decals, separate the paper backing from the plastic film. Remove any paper backing adhering to the adhesive by rubbing the area gently with a clean cloth saturated with water;
remove small pieces of remaining paper with mask- Place the vinyl film, adhesive side up, on a clean porous surface, such as wood or blotter paper.
Apply cyclohexanone, or equivalent, in firm, even strokes to the adhesive side of decal.
Position the decal in the proper location, while adhesive is still tacky, with only one edge contact.
ing the prepared surface.
Work a roller across the decal with overlapping strokes until all air bubbles are removed.
Removal of Decals Paper decals can be removed by rubbing the decal with a cloth dampened with lacquer thinner.
If the decals are applied over painted or doped surfaces, use lacquer thinner sparingly to prevent removing the paint or dope.
Remove the metal decals by moistening the edge of the foil with aliphatic naphtha and peeling the decal from the adhering surface.
Vinyl film decals are removed by placing a cloth saturated with cyclohexanone or MEK on the decal and scraping with a Micarta scraper. Remove the remaining adhesive by wiping with a cloth dampened with dry-cleaning solvent.
Methods of repairing structural portions of an aircraft are numerous and varied, and no set of specific repair patterns has been found which will apply in all cases. Since design loads acting in various structural parts of an aircraft are not always available, the problem of repairing a damaged section must usually be solved by duplicating the original part in strength, kind of material, and dimensions. Some general rules concerning the selection of material and the forming of parts which may be applied universally by the airframe mechanic will be considered in this chapter.
The repairs discussed are typical of those used in aircraft maintenance and are included to introduce some of the operations involved. For exact information about specific repairs, consult the manufacturer's maintenance or service manuals.
BASIC PRINCIPLES OF SHEET METAL REPAIR The first and one of the most important steps in repairing structural damage is "sizing up" the job and making an accurate estimate of what is to be done. This sizing up includes an estimate of the best type and shape of patch to use; the type, size, and number of rivets needed; and the strength, thickness, and kind of material required to make the repaired member no heavier (or only slightly heavier) and just as strong as the original. Also inspect the surrounding members for evidence of corrosion and load damage so that the required extent of the *cleanout" of the old damage can be estimated accurately. After completing the cleanout, first make the layout of the patch on paper, then transfer it to the sheet stock selected. Then, cut and chamfer the patch, form it so that it matches the contour of that particular area, and apply it.
Maintaining Original Strength In making any repair, certain fundamental rules must be observed if the original strength of the structure is to be maintained. The patch plate should have a cross-sectional area equal to, or greater than, that of the original damaged section.
If the member is subjected to compression or to CHAPTER 5
AIRCRAFT STRUCTURAL REPAIRS bending loads, place the splice on the outside of the member to secure a higher resistance to such loads.
If the splice cannot be placed on the outside of the member, use material that is stronger than the material used in the original member.
To reduce the possibility of cracks starting from the corners of cutouts, try to make cutouts either circular or oval in shape. Where it is necessary to use a rectangular cutout, make the radius of curvature at each corner no smaller than 1/2 in. Either replace buckled or bent members or reinforce them by attaching a splice over the affected area.
Be sure the material used in all replacements or reinforcements is similar to the material used in the original structure. If it is necessary to substitute an alloy weaker than the original, use material of a heavier gage to give equivalent cross-sectional strength. But never practice the reverse; that is, never substitute a lighter gage stronger material for the original. This apparent inconsistency is because one material can have greater tensile strength than another, but less compressive strength, or vice versa. As an example, the mechanical properties of alloys 2024-T and 2024-T80 are compared in the following paragraph.
If alloy 2024-T were substituted for alloy 2024-T80, the substitute material would have to be thicker unless the reduction in compressive strength was known to be acceptable. On the other hand, if 2024-T80 material were substitued for 2024-T stock, the substitute material would have to be thicker unless the reduction in tensile strength was known to be acceptable. Similarly, the buckling and torsional strength of many sheet-metal and tubular parts are dependent primarily upon the thickness rather than the allowable compressive and shear When forming is necessary, be particularly careful, for heat-treated and cold-worked alloys will stand very little bending without cracking. Soft alloys, on the other hand, are easily formed but are not strong enough for primary structures. Strong 127
alloys can be formed in their annealed condition and heat treated to develop their strength before assembling.
In some cases, if the annealed metal is not available, heat the metal, quench it according to regular heat-treating practices, and form it before age-hardening sets in. The forming should be completed in about half an hour after quenching, or the material will become too hard to work.
The size of rivets for any repair can be determined by referring to the rivets used by the manufacturer in the next parallel rivet row inboard on the wing, or forward on the fuselage. Another method of determining the size of rivets to be used is to multiply the thickness of the skin by three and use the next larger size rivet corresponding to that figure. For example, if the skin thickness is 0.040.
in., multiply 0.040 by 3, which equals 0.120; use the next larger size rivet, 1/8 in. (0.125 in.).
All repairs made on structural parts of aircraft require a definite number of rivets on each side of the break to restore the original strength. This number varies according to the thickness of the material being repaired and the size of the damage. The number of rivets or bolts required can be determined by referring to a similar splice made by the manufacturer, or by using the following rivet formula:
Number of rivets required - L × T x 75,000 on each side of the break S or B The number of rivets to be used on each side of the break is equal to the length of the break (L) times the thickness of the material (T) times 75,000, divided by the shear strength or bearing strength (S or B) of the material being repaired, whichever is the smaller of the two.
The length of the break is measured perpendicular to the direction of the general stress running through the damaged area.
The thickness of the material is the actual thickness of the piece of material being repaired and is measured in thousandths of an inch.
The 75,000 used in the formula is an assumed stress load value of 60,000 p.s.i. increased by a safety factor of 25%. It is a constant value.
Shear strength is taken from the charts shown in figure 5-1. It is the amount of force required to cut a rivet holding together two or more sheets of mate.
rial. If the rivet is holding two parts, it is under single shear; if it is holding three sheets or parts, it is under double shear. To determine the shear strength, the diameter of the rivet to be used must be known. This is determined by multiplying the thickness of the material by three. For example, material thickness 0.040 multiplied by 3 equals
0.120; the rivet selected would be 1/8 in. (0.125 in.) in diameter.
Bearing strength is a value taken from the chart shown in figure 5-2 and is the amount of tension required to pull a rivet through the edge of two sheets riveted together, or to elongate the hole. The diameter of the rivet to be used and the thickness of material being riveted must be known to use the bearing strength chart. The diameter of the rivet would be the same as that used when determining the shear strength value. Thickness of material would be that of the material being repaired.
Example:
Using the formula, determine the number of 2117-T rivets needed to repair a break 2-1/4 in.
long in material 0.040-in. thick:
Number rivets = L × T × 75,000 per side S or B *Single-Shear Strength of Aluminum-Alloy Rivets (Pounds) Composition Ultimate Strength of Diameter of Rivet (Inches) of Rivet Rivet Metal (Alloy) (Pounds Per Square Inch) 1/16 3/32 1/8 5/32 3/16 1/4 5/16 3/8 2117 T 27,000 83 186 331 518 745 1,325 2,071 2,981 2017 T 30,000 92 206 368 573 828 1,472 2,300 3,313 2024 T 35,000 107 241 429 670 966 1,718 2,684 3,865 *Double-shear strength is found by multiplying the above values by 2.
FICURE 5-1. Single shear strength chart.
Thickness of Diameter of Rivet (Inches) Sheet (Inches) 1/16 3/32 1/8 5/32 3/16 1/4 5/16 3/8
0.014 71 107 143 179 215 287 358 430 .016 82 123 164 204 246 328 410 492 .018 92 138 184 230 276 369 461 553 .020 102 153 205 256 307 410 412 615 .025 128 192 256 320 284 512 640 768 .032 164 245 328 409 492 656 820 984 .036 184 276 369 461 553 738 922 1,107 .040 205 307 410 512 615 820 1,025 1,230 .045 230 345 461 576 691 922 1,153 1,383 .051 261 391 522 653 784 1,045 1,306 1,568 .064 492 656 820 984 1,312 1,640 1,968 .072 553 738 922 1,107 1,476 1,845 2,214 ,081 622 830 1,037 1,245 1,660 2,075 2,490 091 699 932 1,167 1,398 1,864 2,330 2,796 .102 784 1,046 1,307 1,569 2,092 2,615 3,138 .125 961 1,281 1,602 1,922 2,563 3,203 3,844 .156 1,198 1,598 1,997 2,397 3,196 3,995 4,794 .188 1,445 1,927 2,409 2,891 3,854 4,818 5,781 250 1,921 2,502 3,202 3,843 5,125 6,405 7,686 .313 2,405 3,208 4,009 4,811 6,417 7,568 9,623 .375 2,882 3,843 4,803 5,765 7,688 9,068 11,529 .500 3,842 5,124 6,404 7,686 10,250 12,090 15,372 FIGURE 5-2. Bearing strength chart (pounds).
Given : Maintaining Original Contour L = 2-1/4 (2.25) in. Form all repairs in such a manner that they will T = 0.040 in. fit the original contour perfectly. A smooth contour Size of rivet: 0.040 x 3 = 0.120, so rivet is especially desirable when making patches on the must be 1/8 in. or 0.125. smooth external skin of high-speed aircraft.
S = 331 (from the shear strength chart).
B = 410 (from the bearing strength chart).
(Use S to find number of rivets per side as it is smaller than B.)
Keeping Weight to a Minimum Keep the weight of all repairs to a minimum.
Make the size of the patches as small as practicable and use no more rivets than are necessary. In many Substituting in the formula: cases, repairs disturb the original balance of the structure. The addition of excessive weight in each
2.25 x 0.040 x 75,000 6,750 repair may unbalance the aircraft so much that it 331 331 will require adjustment of the trim-and-balance = 20.39 (or 21) tabs. In areas such as the spinner on the propeller, rivets/side. a repair will require application of balancing Since any fraction must be considered as a whole number, the actual number of rivets required would be 21 for each side, or 42 rivets for the entire repair.
patches so that a perfect balance of the propeller assembly can be maintained.
GENERAL STRUCTURAL REPAIR Aircraft structural members are designed to per- 129
form a specific function or to serve a definite purpose. The prime objective of aircraft repair is to restore damaged parts to their original condition.
Very often, replacement is the only way in which this can be done effectively. When repair of a damaged part is possible, first study the part carefully so that its purpose or function is fully understood.
Strength may be the principal requirement in the repair of certain structures, while others may need entirely different qualities. For example, fuel tanks and floats must be protected against leakage; but cowlings, fairings, and similar parts must have such properties as neat appearance, streamlined shape, and accessibility. The function of any damaged part must be carefully determined so that the repair will meet the requirements.
INSPECTION OF DAMAGE When visually inspecting damage, remember that there may be other kinds of damage than that caused by impact from foreign objects or collision.
A rough landing may overload one of the landing gear, causing it to become sprung; this would be classified as load damage. During inspection and "sizing up of the repair job," consider how far the damage caused by the sprung shock strut extends to supporting structural members.
A shock occurring at one end of a member will be transmitted throughout its length; therefore, inspect closely all rivets, bolts, and attaching structures along the complete member for any evidence of damage. Make a close examination for rivets that have partially failed and for holes which have been elongated.
Another kind of damage to watch for is that caused by weathering or corrosion. This is known as corrosion damage. Corrosion damage of aluminum material is usually detected by the white crys.
talline deposits that form around loose rivets, scratches, or any portion of the structure that may be a natural spot for moisture to settle.
Burnishing-Polishing of one surface by sliding contact with a smooth, harder surface. Usually no displacement nor removal of metal.
Burr-A small, thin section of metal extending beyond a regular surface, usually located at a corner or on the edge of a bore or hole.
Corrosion--Loss of metal from the surface by chemical or electrochemical action. The corrosion products generally are easily removed by mechanical means. Iron rust is an example of corrosion.
Crack-A physical separation of two adjacent portions of metal, evidenced by a fine or thin line across the surface, caused by excessive stress at that point. It may extend inward from the surface from a few thousandths inch to completely through the section thickness.
Cut-Loss of metal, usually to an appreciable depth over a relatively long and narrow area, by mechanical means, as would occur with the use of a saw blade, chisel or sharp-edged stone striking a glancing blow.
Dent-Indentation in a metal surface produced by an object striking with force. The surface surrounding the indentation will usually be slightly upset.
Erosion-Loss of metal from the surface by mechanical action of foreign objects, such as grit or fine sand. The eroded area will be rough and may be lined in the direction in which the foreign material moved relative to the surface.
Chattering Breakdown or deterioration of metal surface by vibratory or "chattering" action.
Usually no loss of metal or cracking of surface but generally showing similar appearance.
Definition of Defects Types of damage and defects which may be observed on parts of this assembly are defined as follows:
Galling-Breakdown (or build-up) of metal sur:
faces due to excessive friction between two parts having relative motion. Particles of the softer metal are torn loose and "welded" to the harder.
Brinelling--Occurrence of shallow, spherical depressions in a surface, usually produced by a part having a small radius in contact with the surface under high load.
Gouge-Grooves in, or breakdown of, a metal surface from contact with foreign material under heavy pressure. Usually indicates metal loss but may be largely displacement of material.
Inclusion-Presence of foreign or extraneous material wholly within a portion of metal.
material is introduced during the manufacture of rod, bar or tubing by rolling or forging.
Nick-Local break or notch on edge. Usually displacement of metal rather than loss.
Pitting-Sharp, localized breakdown (small, deep cavity) of metal surface, usually with defined edges.
Damage Repairable by Patching Damage repairable by patching is any damage exceeding negligible damage limits which can be repaired by bridging the damaged area of a component with a material splice. The splice or patch material used in internal riveted and bolted repairs is normally the same type of material as the damaged part, but one gage heavier. In a patch repair, filler plates of the same gage and type of material as that in the damaged component may be used for bearing purposes or to return the damaged part to its original contour.
Scratch-Slight tear or break in metal surface from light, momentary contact by foreign material.
Score-Deeper (than scratch) tear or break in metal surface from contact under pressure. May show discoloration from temperature produced by friction.
Stain-A change in color, locally causing a noticeably different appearance from the surrounding Upsetting--A displacement of material beyond the normal contour or surface (a local bulge or bump). Usually indicates no metal loss.
CLASSIFICATION OF DAMAGE Damages may be grouped into four general classes. In many cases, the availability or lack of repair materials and time are the most important factors in determining whether a part should be repaired or replaced.
Negligible Damage Damage which does not affect the structural integrity of the member involved, or damage which can be corrected by a simple procedure without placing flight restrictions on the aircraft, is classified as negligible damage. Small dents, scratches, cracks, or holes that can be repaired by smoothing, sanding, stop drilling, or hammering out, or otherwise repaired without the use of additional materials, fall in this classification.
Damage Repairable by Insertion Damage which can be repaired by cutting away the damaged section and replacing it with a like section, then securing the insertion with splices at each end is classified as damage repairable by inser- Damage Necessitating Replacement of Parts Replacement of an entire part is considered when one or more of the following conditions exist:
(1) When a complicated part has been extensively damaged.
(2) When surrounding structure or inaccessibility makes repair impractical.
(3) When damaged part is relatively easy to replace.
(4) When forged or cast fittings are damaged beyond the negligible limits.
STRESSES IN STRUCTURAL MEMBERS Forces acting on an aircraft, whether it is on the ground or in flight, cause pulling, pushing, or twisting within the various members of the aircraft structure. While the aircraft is on the ground, the weight of the wings, fuselage, engines, and empennage causes forces to act downward on the wing and stabilizer tips, along the spars and stringers, and on the bulkheads and formers. These forces are passed on from member to member causing bending, twisting, pulling, compression, and shearing.
As the aircraft takes off, most of the forces in the fuselage continue to act in the same direction; but because of the motion of the aircraft, they increase 131