Page 18 AC 43.13- JL _ JL
FRONT SPAR
•TRIMMED OPENING -MINIMUM RADIUS ST
- RIBS • DAMAGE →Д
SADDLE GUSSET -BEAB SPAR 30 т PATCHI -12 T-→ PLYWOOD SKIN (I" MINIMUM 8T ST It MIN PLYWOOD SADDLE GUSSET RIB CAP MINIMUM THICKNESS • T.
NAILED - GLUED IN PLACE.
SECTION A-A 12 T. - PATCH /ІГРАТОН *4T UNSUPPORTED -RIB. CAP LAP
SECTION B-B SECTION C-C FIGURE 1.14.-Surface patches.
Par 24 Chap
Saddle Gusset Hailing Strips Maximum Diameter 251 Minimum Distance 15T
• Temporary Backing 30T- Patch - 12T- Minimum Plywood Saddle Gusset Minlmum Thickness =T Nail-Glued in Place
SECTION A-A 12T ~ - Patch - 121--- - Patch Backing 31 Clamp and Glue Backing Clamp and Glue Backing to Frame and Skin Framing Member to Frame and Skin
SECTION B-B SECTION C-C
• Hailing Strips-
- 12T - 7 LEA Tomporary Backing Block Shape to fet Skir Cellophane or Paper
SECTION D-D FIGURE 1.15.-Scarf patches. back of akin accessible.
Per 24
Page 20 AC 43.1:
FRONT SPI SADDLE GUSSET+
REAR SPAR
• 30 T -12T- - PLYWOOD SKIN T 3T (· MINIMUM)- 8T (1" MINIMUM) RIb CAP
SECTION A - A PLYWOOD SADDLE GUSSET
PLYWOOD (MINIMUM THICKNESS SKIN- PATCH -12 T-.- PATCH RIB CAP • 3T "SPAR" PLYWOOD OR SPRUCE- (* MINIMUM)
PLYWOOD SKIN
SECTION B - B SECTION C-C FIGURE 1.16.-Scarf patches--back of akin not accessible.
Par 24 Chap
AC 43.13-1A Page 21 1972
- OUTER EDGE OF DOUBLER
- BUTT JOINT OF PATCH TO SKIN -INNER EDGE OF DOUBLER
- NAIL HOLES -
SCREW HOLES -
TO BE FILLED BEFORE FINISHING.
PATCH DIMENSIONS _G SMALL 27112 LARGE 5 7
TWO ROWS OF SCREWS AND
NAILS REQUIRED ON LARGE
• BUTT JOINT OF PATCH TO SKIN PATCH.
SPLUG PATCH (GRAIN PARALLEL TO SKIN) а шатика
PLYWOOD DOUBLER (GRAIN PARALLEL TO SKIN)
PLYWOOD SKIN FIOURE 1.17.-Oval plug patch assembly.
with the face grain carefully oriented to the same direction as the original skin. Orientation of the face grain direction of the round plug patch to that of the skin surface is no problem, as the round patch may be rotated until grain Hirections match.
a. Steps in Making Oval Plug Patch.
(1) Explore the area about the hole to be sure it lies at least the width of the oval doubler from a rib or a spar. Refer to figure 1.17 'or repair details.
(2) Lay a previously prepared oval plug ratch over the damage and trace the patch.
Saw to the line and trim the hole edges with a knife and sandpaper.
(3) Mark the exact size of the patch on one surface of the oval doubler and apply glue to the area outside the line. The oval doubler hould be made of some soft-textured plywood, Chap 1 such as yellow poplar or spruce. Insert doubler through the hole and bring it, glue side up, to the underside of the skin with its pencil outline of the patch matching the edges of the hole. If the curvature of the surface to be repaired is greater than a rise of 1/8 inch in 6 inches, the doubler should be preformed by hot water or steam bending to the approximate curvature.
(4) Apply nailing strips outlining the hole to apply glue pressure between doubler and akin. Use bucking bar to provide support for nailing. When two rows of nails are used, stagger nail spacing.
(5) Apply glue to remaining surface and to an equivalent surface on the patch.
(6) Lay the patch in position over the doubler, and screw the pressure plate to the patch assembly using a small nail to line up the holes Par 24
Page 22 AC 43.13-' 19:
GRAIN DIRECTION OF SKIN, PATCH, AND DOUBLER.
SAW OUT IN DOUBLER
BUTT JOINT OF PATCH TO SKIN.
INNER EDGE OF
DOUBLER
OUTER EDGE OF
DOUBLER
NAIL HOLES -
SCREW HOLES -
TO BE FILLED BEFORE FINISHING
• BUTT JOINT OF PATCH TO SKIN E PLUG PATCH PLYWOOD SKIN • SAW CUT IN DOUBLER
PLYWOOD
DOUBLER
DIMENSIONS 8 SMALL CIRCULAR PLUG PATCH 2 LARGE CIRCULAR PLUG PATCH 3· 3 (TWO ROWS OF SCREWS AND NAILS REQUIRED FOR LARGE PATCH.)
FIGURE 1.18.-Round plug patch assembly.
Par 24
AC 43.13-1A 1972 that have been previously made with patch and plate matching. No. 4 roundhead screws are used. Lead holes in the plywood doubler are not necessary. Waxed paper or cellophane between the plate and patch prevents glue from sealing the plate to the patch. No clamps or further pressure need be applied as the nailing strips and screws exert ample pressure. Hot sandbags, however, may be laid over the patch to speed the setting of the glue. Finish in the usual manner.
b. Round Plug Patch. The steps in making a round plug patch shown in figure 1.18 are identical with those for making the oval patch except the insertion of the doubler. In using the round patch, where access if from only one side, | Chap 1 Page 23 the round doubler cannot be inserted unless it has been split.
25. FABRIC PATCH. Small holes not exceeding 1 inch in diameter, after being trimmed to a smooth outline, may be repaired by doping a fabric patch on the outside of the plywood skin. The edges of the trimmed hole should first be sealed and the fabric patch should overlap the plywood akin by at least 1 inch.
Holes nearer than 1 inch to any frame member, or in the leading edge or frontal area of the fuselage, should not be repaired with fabric patches.
26.-36. RESERVED.
Par 24
Page 24 AC 43.13-1 197 Section 2. FINISHING WOOD STRUCTURES
37. GENERAL, Any repair to spars, ribs, skin surfaces, or other structural parts of the airframe involves finishing as the final step of the job. The time and effort spent during the preparatory phase of the refinishing process will be reflected in the appearance of the finished surface. Adherence to the instructions issued by the finish manufacturer is necessary to obtain the appearance desired and protective characteristics for the product used.
38. PRECAUTIONS TO BE OBSERVED. When making repairs, avoid excessive contamination of surfaces with glue squeezeout at joints and on all surfaces. Excess glue should always be removed before applying finish. Because paints and glues are incompatible, even a slight amount of glue underneath the finish may cause premature deterioration.
a. Soiling substances, such as oil and grease, should be removed as completely as possible.
Naphtha may be used to sponge off oil and grease. Markings that are made by grease pencils or lumber crayons containing wax are harmful and should be removed, but marks made by ordinary soft graphite pencils and nonblotting stamp pad inks may be safely finished over. All dust, dirt, and other solid particles should be cleaned off.
b. Sawdust, shavings, and chips must be removed from enclosed spaces before they are sealed off by replacement of akin. A vacuum cleaner is useful for such cleaning.
6. Since no satisfactory gluable sealer has yet been developed, it is necessary to avoid applying sealer over the areas where glue will be applied. Mark off areas to receive glue, allowing an additional 1/4 inch on each side of the glue area to provide for misalinement when mating the parts. It is preferable to leave some unsealed areas rather than risk weakening the glue joint by accidental overlap of the seale into the glued areas.
d. Finish is likely to crack when applied oven fush-driven nails and screws. To avoid this, a strip of tape may be applied over the heads finish is applied. after application of sealer and before the finely
e. Fill all holes left from nail-strip gluing or countersunk nails and screws with a wood filler before finishing the surface. It may be necessary to cover with a patching putty the slight depressions left after applying filler, if a completely smooth surface is desired; but, as rule, patching putty may be dispensed wit safety.
f. Treat surfaces which are likely to come i contact with fabric during the doping proces with a dope-proof paint, cellophane tape, etc to protect them against the action of the so vents in the dope.
39. FINISHING OF INTERIOR SURFACES. Finig repaired ribs, spars, interior of plywood ski and other internal members, including areas c contact between metal and wood, by applyi at least two coats of spar varnish. Prot built-up box spars and similar closed stru tures on the interior by at least one heavy cor of spar varnish or lionoil. Where better protection is required, as on the surfaces of wheel wells and the bottoms of hulls below the foo boards, an additional coat of aluminized sea consisting of 12 to 16 ounces of alumir paste per gallon of sealer may be applied.
40. FINISHING OF EXTERIOR SURFACES. Exteri surfaces should first be sealed with at least t coats of sealer or spar varnish. The surfa finish should then be completed by the applid tion of enamel, aluminized varnish, or oth special finish as required to duplicate the orig Par 37 Chap
AC 43.13-1A 1972 nal finish. If dope or lacquer is used to complete the finish, the sealer coats should be dope-proof. Spar varnish or sealer conforming to Specification MIL-V-6894 is satisfactory.
41. FINISHING OF END GRAIN SURFACES. End grain surfaces, such as edges of plywood skins and holes in spars and other primary structural members, require careful protection.
Sand these surfaces smooth. Apply two coats of a highly pigmented sealer, or one coat of wood filler, and one coat of clear sealer to end grain interior surfaces and cut holes. Exterior end grain surfaces (except those covered with doped fabric) require an additional (third) coat of clear sealer. A final coat of aluminized varnish may be applied to end grain surfaces.
If the surfaces are to be finished with dope or lacquer, a dope-proof sealer similar to Specification MIL-V-6894 should be used.
Exposed end grain includes such surfaces as those around ventholes, inspection holes, fittings, and exposed scarfed or tapered surfaces such as those of tapered blocking.
42. FINISHING WITH FABRIC OR TAPE. To refinish with fabric or tape, it is first necessary to insure that paint has been removed from an Chap 1 Page 25 area greater than that to be covered by the fabric.
a. Apply two brush coats of a dope-proof sealer similar to Specification MIL-V-6894, allowing the first coat to dry 2 hours and the second coat at least 6 hours. Follow with one coat of clear dope, and allow it to dry 45 minutes.
Apply a second coat of clear dope and lay into the wet film a piece of pinked-edge airplane cloth. All air bubbles should be worked out by brushing to insure maximum adherence. Allow this to dry 45 minutes. Apply one brush coat to insure proper penetration, and at least one spray coat of clear dope, allowing each to dry 45 minutes. The dried spray coat may be sanded with fine sandpaper to obtain a smoother finish. Complete the refinishing of the surface by application of lacquer, enamel, or aluminized varnish as required to match the adjacent area.
b. The size of the fabric patch should be such as to extend at least 1/2 inch on each side of any crack or group of cracks, at least 1 inch on each side of & scarfed joint glue line, and at least 2 inches beyond any edge of a skin patch to insure proper adhesion.
43.-53. RESERVED.
Par 40
C 43.13-1A Page 27 972 Chapter 2. AIRCRAFT METAL STRUCTURES Section 1. REFERENCES AND PRECAUTIONARY MEASURES I. REFERENCES. The following chapters of this aC should be referred to when accomplishing repairs to aircraft metal structures:
a. Identification and Inspection of Materials.
(dentification and inspection of materials should be conducted in accordance with Chaper 7.
b. Corrosion Protection. Corrosion protection reatment, cleaners, and paint removing should e accomplished in accordance with Chapter 6.
c. Aircraft Hardware. Acceptable means of ttachment are listed in Chapter 5.
b. Materials and Construction Techniques. The development of new materials and techniques has made possible the use of control surfaces of less mass weight for a given area than some aircraft of older design. The effect of repair or weight change on the balance and center of gravity is proportionately greater on lighter surfaces than on the older designs. Since control surfaces on some models are balanced for flutter-free operation up to maximum speed for which the aircraft was originally designed, special attention, therefore, must be given to such surfaces relative to the effects of structural repairs and rework on their balance con- 5j. FLUTTER AND VIBRATION PRECAUTIONS. To revent the occurrence of severe vibration or utter of flight control surfaces during flight, recautions must be taken to stay within the esign balance limitations when performing aintenance or repair.
a. Balance Changes. The importance of retainr the proper balance and rigidity of aircraft entrol surfaces cannot be underestimated. As reneral rule, repair the control surface in ch a manner that the structure is identical to : original so that the weight distribution is t affected in any way. In order to preclude he occurrence of flutter of the control surface n flight, a degree of static and/or dynamic balnce is established for each model of aircraft.
certain conditions, counter-balance rht is added forward of the hinge line to tain balance. Remove or add balance ught only when necessary in accordance h the manufacturer's instructions, or obtain A approval. Flight testing may be required.
filure to check and retain control surface bale within the original or maximum allowavalue could result in a serious flight haz-
directed to the effect indiscriminate application of extra coats of dope or paint has on the balance of control surfaces. Proper maintenance of control surface balance may require removal of dope or paint, down to the base coat, prior to application of finish coats. Consult the aircraft manufacturer's instructions relative to finishing and balance of control sur-
have occurred from unbalanced conditions caused by the collection of water or ice within the surface. Therefore, ventilation and drainage provisions must be checked and retained when maintenance is being done. Certain construction designs do not provide for ventilation and may collect moisture through condensation which will affect balance. In the event this condition is found, refer to the manufacturer's instructions for moisture removal.
e. Trim Tab Maintenance. In addition to unbalanced control surface, loose or vibrating trim tabs will increase wear of actuating mechanisms and hinge points which may de- Par 54
velop serious flutter conditions. Most trim tabs are not balanced separately from the control surface. Minimum tab flutter is maintained through rigid actuating mechanisms. Trim tabs and their actuating mechanism are constructed as lightly as possible to keep the weight aft of the hinge line of the control surface as low as possible. Actuating mechanisms are highly susceptible to wear, deformation, and fatigue failures because of the buffeting nature of airflow over the tab mechanism.
Trailing-edge play of the tab may increase, through wear of the mechanism, to an unsafe condition. Careful inspection of the tab and its mechanism should be conducted during overhaul and inspection periods. Compared to other systems on the aircraft, only a minor amount of tab-mechanism wear can be tolerated. In the absence of a specified limit for tab trailing edge movement in the manufacturer's manual, it is acceptable to use a limit of 2 1/2% of the chord, measured at the trailing edge of the tab.
For example, a tab that has a chord of 4" would have a maximum permissible free play of 4" x .025 or 0.100 inches (total motion up and down) measured at the trailing edge. Correct any free play in excess of this amount.
Care must also be exercised during repair or rework to prevent stress concentration points or areas which could increase the fatigue susceptibility of the trim tab system.
Par 55 AC 43.13.
56. BRAZING, Brazing may be used for repa to primary aircraft structures only if brazi was originally approved for the particular & plication. Brazing is not suitable for repair welds in steel structures due to lower streng values of the brazed joint as compared welded joints. Brazing may be used in the 1 pair of secondary structures.
Due to the large number of brazing allo used, it is difficult to be certain that the mai rial selected for repairing a brazed joint w result in a joint having the same streng characteristics as the original. In cases whe it is necessary to apply copper alloy brazil material, more than once on a steel surfa‹ and particularly if temperatures over 2,000° are reached, there is a possibility that brazi!
metal may penetrate between the grains in t steel to an extent that may cause crackin Copper brazing of steel is normally accor plished in a special furnace having a controlk atmosphere, and at a temperature so high th field repairs are seldom feasible. If copp brazing is attempted without a controlled a mosphere, the copper will probably not con pletely flow and fill the joint. Therefore, col per brazing in any other than appropriatel controlled conditions is not recommended.
57-67. RESERVED.
Chap
C 43.13-1A Page 29 1972 Section 2. WELDING
8. GENERAL. This section covers weld repairs aircraft and component parts thereof with he exception of welding on powerplants and propellers which is provided in chapters 12 and 14 respectively. Observe the following when using such equipment as tungsten inert gas (TIG), metal inert gas (MIG), plasma arc, shield carbon arc, and oxygen-acetylene gas.
a. Equipment Selection. Use the welding equipment manufacturers' information to determine if the equipment will satsify the requirements lor the type of welding operation being underaken. Disregarding such detailed operating intructions may cause substandard welds. For xample, when using TIG equipment, a weld an be contaminated with tungsten if the proper size electrode is not used when welding with lirect current reverse polarity. Another example, the depletion of the inert gas supply below the critical level causes a reduction in the gas flow and will increase the danger of atmospheric contamination.
(1) Electric welding equipment versatility requires careful selection of the type current and polarity to be used. Since the composition and thickness of metals are deciding factors, the selection may vary with each specific application. Metals having refractory surface oxide films, i.e., magnesium alloys and aluminum and its alloys, are generally welded with A.C., while D.C. is used for carbon, low alloy, noncorrodible, and heat-resisting steels, copper, etc. General recommendations covering current and polarity are shown in figure 2.1.
(2) Oxygen-acetylene gas equipment is suitable for welding most metals. It is not, how- FIGURE 2.1.-Current and polarity selection for inert gas welding.
ALTERNATING DIREO
CURRENT CURREN' With High- Frequency STRAIGHT REVERSE MATERIAL Stabilization Polarity Polarity dagnesium up to 1/8 in. thick 1 N.R. 2 ngnesium above 8/16 in. thick 1 N.R. N.R.
agnesium Castings N.R. 2 uminum up to 8/82 in. thick N.R.
luminum over 8/82 in. thick N.R. N.R.
luminum Castings N.R. N.R.
Stainless Steel N.R.
Brass Alloys N.R.
Silicon Copper N.R.
ilver OUUN: N.R.
ver Cladding N.R. N.R.
d-facing N.R.
st Iron N.R.
w Carbon Steel, 0.015 to 0.080 in. 2** N.R.
w Carbon Steel, 0.030 to 0.125 in. N.R. N.R.
ch Carbon Steel, 0.015 to 0.030 in. 2 * 1 N.R.
eh Carbon Steel, 0.030 in. and up 2 * 1 N.R.
Recommended 2. Acceptable N.R. Not Recommended Where A.C. is recommended as second choice, use approximately 25% higher current than is recommended for DCSP.
Do not use A.C. on tightly jigged part.
ap 2 Par 68
ever, the best method to use on such materials as stainless steels, magnesium, and aluminum alloys because of base metal oxidization, distortion, and loss of ductility.
NoTE: When oxyacetylene is used, all flux must be removed, as it may cause corrosion.
b. Accurately Identify the Type of Material to be Repaired. Reference to chapter 7 may provide this information. If positive identification is not possible, contact the aircraft manufacturer or subject the item to a metallurgical laboratory analysis. Before any welding is attempted, carefully consider the weldability of the alloy since all alloys are not readily weldable. The following steels are readily weldable: plain carbon of the 1000 series, nickel steel of the SAE 2800 series, chrome-nickel alloys of the SAE 8100 series, chrome-molybdenum steels of the SAF 4100 series, and low nickel-chromemolybdenum steel of the SAL 8600 series.
c. Preparation for Welding. Hold elements to be welded in a welding jig or fixture which is sufficiently rigid to prevent misalinement due to expansion and contraction of the heated material and which positively and accurately position the pieces to be welded.
d. Cleaning Prior to Welding. Clean parts to be welded with a wire brush or other suitable methods. When cleaning with a wire brush, do not use a brush of dissimilar metal; for example, brass or bronze on steel. The small deposit left by a brass or bronze brush will materially weaken the weld and may cause cracking or subsequent failure of the weld. In case members were metallized, the surface metal may be removed by careful sandblasting followed by a light buffing with emery cloth.
•. Condition of Complete Weld. Make sure that:
(1) The finished weld has a smooth seam and is uniform in thickness;
(2) The weld metal is tapered smoothly into the base metal;
(3) No oxide has formed on the base metal at a distance of more than 1/2 inch from the weld;
Par 60 AC 43.13-1 197
(4) The weld shows no signs of blowholes porosity, or projecting globules;
(5) The base metal shows no signs of pit ting, burning, cracking, or distortion;
(6) The depth of penetration insures fusion of base metal and filler rod; and
(7) Welding scale is removed by wire brush ing or sandblasting.
f. Practices to Guard Against. Do not file weldi in an effort to make a smooth-appearing job, a such treatment causes a loss of strength. D not fill welds with solder, brazing metal, ol any other filler. When it is necessary to rewel a joint which was previously welded, remov all old weld material before rewelding. Avoic welding over a weld because reheating may cause the material to lose its strength and be come brittle. Never weld a joint which hai been previously brazed.
g. Torch Size (Oxyacetylene Welding). The sizi of the torch tip depends upon the thickness of the material to be welded. Commonly user sizes proved satisfactory by experience are:
Thickness of steel Diameter of Drill size (in Inches) hole in tip
0.015 to 0.031 0.026 71
0.031 to 0.065 .031 68
0.065 to 0.125 .037 63
0.125 to 0.188 .042 58
0.188 to 0.250 .055 Б4
0.250 to 0.875 .067 51
h. Welding Rods and Electrodes. Use weldin rods and electrodes that are compatible wit the materials to be welded. Welding rods anc electrodes for various applications have special properties suitable for the application intended. Figure 2.8 shows the allowable strength for the weld metal. Figure 2.2 lists specifications for corresponding metals, ar the applicable specification will identify t use of A.C. or D.C., straight or reverse polal
1. Rosette Welds. Rosette welds are generali employed to fuse an inner reinforcing tube (liner) with the outer member. Where a ro sette weld is used, drill the hole, in the outside tube only, of a sufficient size to insure fusion o Chap