AC 43.13-1A Page 3 1972 not diverge from the longitudinal axis of the material more than specifed in column 8. A check of all four faces of the board is necessary to determine the amount of divergence. The direction of free-flowing ink will frequently assist in determining grain direction.
b. Wacy, ourly, and interlocked grain. Acceptable, if local irregularities do not exceed limitations specifed for spiral and diagonal grain.
c. Eard knots. Sound bard knots up to ¼-inch In maximum diameter are acceptable prorlding: (1) they are not in projecting portions of I-beams, niong the edges of rectangular or beveled unrouted beams, or along the edges of manges of box beams (except in lowly stressed portions); (2) they do not cause grain divergence at the edges of the board or in the fanges of a beam more than specifed in column 8; and (B) they are in the center third of the beam and are not closer than 20 inches to another knot or other defect (pertains to %-inch knots-smaller knots may be proportionately closer). Knote greater than ¾-inch must be used with caution.
d. Pin knot olusters. Small clusters are acceptable providing they produce only a small effect on grain direction.
e. Pitoh pookots. Acceptable, in center portion of a beam providing they are at least 14 inches apart when they lie in the same growth ring and do not exceed 1½ inch length by %-inch width by ⅛-inch depth and providing they are not along the projecting portions of I-beams, along the edges of rectangular or beveled unronted beams, or along the edges of the flanges of box beams.
f. Afineral streaks. Acceptable, providing careful inspection falls to reveal any decay.
2. Defects Not Permitted.
a. Cross grain. Not acceptable, unless within limitations noted in 1a.
D. Wavy, curly, and interlocked grain. Not acceptable, unless within limitatlons noted in 1b.
c. Hard knots. Not acceptable, unless within limitations noted in 1c.
d. Pin knot oluaters. Not acceptable, If they produce large effect on grain direction.
e. Spike knots. These are knots running completely through the depth of a beam perpendicular to the annual ringe and appear most frequently in quartersawed lumber. Reject wood containing this defect.
1. Pitch pockets. Not acceptable, unless within limitations noted in le.
g. fineral streaks. Not acceptable. If accompanied by decay (see 1f).
b. Checks, shakes, and aplits. Checks are longitudinal cracks extending, in general, across the annunl rings.
Shakes are longitudinal cracks usually between two annual rings. Splits are longitudinal cracks induced by artificially induced stress. Reject wood containing these defects.
1. Compression cood. This defect is very detrimental to strength and is difficult to recognize readily. It la characterized by high specific gravits; has the appenrance of an excessive growth of summer wood; and in inost species shows but little contrast in color between epring wood and summer wood. In doubtful cases reject the material, or subject samples to a tonghness machine test to establish the quality of the wood. Reject all materlal containing compression mood.
j. Compressio» failures. This defect ia caused from the wood belng overstressed in compresslon due to natural forces during the growth of the tree, felling trees on rough or irregular ground, or rough handling of logs or lnmber.
Compression failures are choracterized by a buckling of the fbers that appear as streaks on the surface of the piece substantially at right angles to the grain, and rary from pronounced fallures to very fine hairlines that require close inspection to detect. Reject wood containing obvious failures. In doubtful cases reject the wood, or make a further inspection in the form of microscopic examination or toughness test, the latter means being the more rellable.
k. Decay. Examine all stains and discolorations carefully to determine whether or not ther are harmless, or in a stage of preliminary or advanced decay. All pleces must he free from rot, dote, red heart, purple heart, and all other cised that the repairs do not involve the addition of weight aft of the hinge line. Such a procedure may adversely disturb the dynamic and static balance of the surface to a degree which would induce flutter. As a general rule, it will be necessary to repair control surfaces in such a manner that the structure is identical to the original so that the weight distribution and resulting mass balance are not affected in any way.
4. GLUING PRECAUTIONS. Satisfactory glue Chap 1 joints in aircraft will develop the full strength of wood under all conditions of stress. To produce this result, the gluing operation must be carefully controlled so as to obtain a continuous, thin, uniform film of solid glue in the joint with adequate adhesion to both surfaces of the wood. Some of the more important conditions involve:
o. Properly prepared wood surfaces.
b. Glue of good quality, properly prepared.
c. Good gluing technique.
Par 3
Pago 4
5. PREPARATION OF WOOD SURFACES FOR So more than GLUING, It is recommended that no more than 8 hours be permitted to elapse between final surfacing and gluing. The gluing surfaces should be machined smooth and true with planers, jointers, or special miter saws. Planer marks, chipped or loosened grain, and other surface irregularities will not be permitted. Sandpaper must never be used to smooth softwood surfaces that are to be glued. Sawed surfaces must approach well-planed surfaces in uniformity, smoothness, and freedom from crushed fibers.
a. Tooth-planing, or other means of roughening smooth well-planed surfaces of normal wood before gluing is not recommended. Such treatment of well-planed wood surfaces may result in local irregularities and objectionable rounding of edges. While sanding of planed surfaces is not recommended for softwoods, sanding is a valuable aid in improving the gluing characteristics of some hard plywood surfaces, wood that has been compressed through exposure to high pressure and temperatures, resin-impregnated wood (impreg and compreg), and laminated paper plastic (papreg).
b. Wood surfaces for gluing should be free from oil, wax, varnish, shellac, lacquer, enamel, dope, sealers, paint, dust, dirt, oil, glue, crayon marks, and other extraneous materials.
c. Wetting tests are useful as a means of detecting the presence of wax. Drops of water placed on the surface of wax-coated wood do not spread or wet the wood. At present, preliminary gluing tests appear to be the only positive meana of actually determining the gluing characteristics of plywood surfaces.
6. GLUES. Glues used in aircraft repair fall into two general groups: casein glues and resin glues. Any glue that meets the performance requirements of applicable United States military specifications or has previously been accepted by the FAA is satisfactory for use in certificated civil aircraft. In all cases, glues are to be used strictly in accordance with the glue manufacturer's recommendations.
Par 5 AC 43,13-1A 1972
a. Caseln Glues. Casein glues have been widely used in wood aircraft repair work. The forms, characteristics, and properties of water-resistant casein glues have remained substantially the same for many years, except for the addition of preservatives. Casein glues for use in aircraft should contain suitable preservatives such as the chlorinated phenols and their sodium salts to increase their resistance to organic deterioration under high-humidity exposures. Most casein glues are sold in powder form ready to be mixed with water at ordinary room temperatures.
b. Synthetic Resin Glues. Synthetic resin glues for wood are outstanding in that they retain their strength and durability under moist conditions and after exposure to water. The best known and most commonly used synthetic resin glues are the phenol-formaldehyde, resorcinolformaldehyde, and urea-formaldehyde types.
The resorcinol-formaldehyde type glue is recommended for wood aircraft applications. Materials, such as walnut-shell flour, are often added by the glue manufacturer to the resin glues to give better working characteristics and joint-forming properties. The suitable curing temperatures for both urea-formaldehyde and resorcinol glues are from 70° F. up. At the 70° F. minimum temperature it may take as long as 1 week for the glueline in a spar splice to cure to full strength. Thinner pieces of wood and/or higher curing temperatures shorten curing time considerably. The strength of a joint cannot be depended upon if assembled and cured at temperatures below 70° F.
7. MIXING OF RESIN GLUES. Liquid resin glues may come ready for use or in a form which requires only the addition of a hardener. In all cases, the mixing, glue consistency, assembly time, etc., should comply with the glue manufacturers' recommendations and instructions.
Cold-setting synthetic-resin glue, when prepared for use, is limited in working life, and care should be taken to discard the glue and clean the equipment before the end of the working life period. In very warm weather it may be found advisable to keep the gluepot in a bath of cool water, approximately 70° F., to prolong the working life of the mixture.
Chap 1
AC 43.13-1A Page 7 1972 1 6A RECOMMENDED •-
DA MINIMUM ‡
- 12 A RECOMMENDED- 6A RECOMMENDEO 10A MINIMUM 5A MINIMUM
NO FITTINGS WITHIN THESE LIMITS 1/4 A +
DIRECTION OF GRAIN IF SPRUCE
OR OUTER FACE GRAIN IF PLYWOOD REINFORCEMENT PLATES TO BE SPRUCE OR PLYWOOD AND SHALL BE GLUED ONLY.
SOLID SPARS MAY BE REPLAGED WITH LAMINATED ONES OR VICE VERSA, PROVIDED THE MATERIAL IS OF THE SAME HIGH QUALITY.
FIoURE 1.4.-Method of splicing solid or laminated rectangular spars.
but not necessarily, at a bolthole or cutout and spread in each direction until, in most cases, they extend a short distance beyond the ends of the plates where the resistance to spar shrinkage disappears. Other factors which have been found conducive to the formation of cracks, due to spar shrinkage in the region of plywood plates, are poor protective finishes, large cutouts, and metal fittings which utilize two lines of large diameter bolts.
The presence of cracks does not necessarily mean that the spar must be discarded. If the crack is not too long or too close to either edge and can be reinforced properly, it will probably be more economical and satisfactory to effect a repair than to install a new spar or section. However, a generally acceptable proce dure suitable for all airplane models cannot be Chap 1 described here. It is recommended the manufacturer or the Federal Aviation Administration be contacted for specific instructions before making repairs not in accordance with the manufacturer's approved instructions or the recommendations of this advisory circular, because of the possibility of strength deficiencies.
16. ELONGATED HOLES IN SPARS. In cases of elongated boltholes in a spar or cracks in the vicinity of boltholes, splice in a new section of spar, or replace the spar entirely, unless the method of repair is specifically approved by a representative of the HAA. In many cases, it has been found advantageous to laminate the new section of the spar (using aircraft plywood for the outer faces, particularly if the spar roots are being replaced.
Par 15
Page 8 АС 43.13-1А 1972 IF SPLICE IS MADE WHERE ROUTIN 18 FEATHERED TO FULL
WIDTH OF SPAR, TAPERED PLATES OONFORMING TO THE CONTOUR OF THE ROUTINS, SHOULD FIRST BE ADDED.
OTHERWISE THE SPLICE N THE SAME AS SHOWN.
БА
- 6А - • 6А
- 12A RECOMMENDED -
IDA MINIMUM
NO FITTING WITHIN THESE LIMITS
DIRECTION OF GRAIN
REINFORCEMENT PLATES MAY BE OF PLYWOOD, THE SAME MATERIAL AS THE SPAR, OR MATERIAL OF EQUAL OR HIGHER QUALITY AND SHALL BE GLUED ONLY.
FIGURE 1.5.-Method of splicing solid "I spars.
17. RIB REPAIRS. Make replacement ribs from & manufacturer's approved drawing, or from a drawing made by the repair agency, and certified by the manufacturer as correct. The original rib may be used as a pattern in making the new rib if it is not too seriously damaged to permit comparison. Wood ribs should not be attached to wood spars by nails driven through the rib capstrips, as this weakens the rib materially. The attachment should be by means of glue with cement-coated, barbed, or spiraled nails driven through the vertical rib members on each side of the spar. The drawing or pattern should be retained by the repair agency for use by the FAA inspector when making his inspection. (Acceptable methods of repairing damaged ribs are shown in figure 1.10.)
a. Compression Ribs. Acceptable methods of repairing damaged compression ribs are shown in figure 1.11. (A) illustrates the repair of a compression rib of the "I" section type; i.e., wide, shallow capstrips, a center plywood web with a rectangular compression member on Par 17 each side of the web. The rib is assumed to be cracked through capstrips, web member, and compression member. Cut the compression member as shown in (D), remove and replace the shortest section, adding the reinforcement blocks as also shown in figure 1.11(D). Cut and replace the aft portion of the capstrips and reinforce as shown in figure 1.10 excep that the reinforcement blocks are split in the vertical direction to straddle the center web.
The plywood sideplates, as indicated in figure
1.11(A), are glued on. These plates are added to reinforce the damaged web. (B) illustrates a compression rib of the type that is basically a standard rib with rectangular compression members added to one side and plywood web to the other side. The method used in this repair is essentially the same as in (A) except that the plywood reinforcement plate shown in solid black in section B-B is continued the full distance between spars. (C) illustrates a com pression rib of the "I" type with a rectangular vertical member each side of the web. Th Chap
I€ 43.13-1A Page 9 972 10 д
NO FITTING WITHIN THESE LIMITS
- DIRECTION OF GRAIN IN - 1/28
PLYWOOD REINFORCEMENT
PLATES TO BE SAME AS ORIGINAL WEB.
PLYWOOD
A-SOLID WOOD FILLER BLOOKS
SECTION A-A FIGURE 1.6.-Repairs to built-up "I spar.
method of repair is essentially the same as in (A) except the plywood reinforcement plates on each side shown in solid black in section -C are continued, as in (C), the full distance between spars.
18. PLYWOOD SKIN REPAIR. Make extensive repairs to damaged stressed skin plywood structures in accordance with apecific recommendations from the manufacturer. It is recommended hat repairs be made by replacing the entire panel from one structural member to the hext, if damage is very extensive. When damaged plywood skin is repaired, carefully inspect the adjacent internal structure for possible hidden damage. Repair any defective frame members prior to making skin repairs.
a. Types of Patches. Four types of patcheshe surface or overlay patch, the splayed atch, the plug patch, and the scarf patch-are acceptable for repairing plywood skins. Surface patches should not be used on skins over 1/8 inch thick. Splayed patches should not be used on skins over 1/10 inch thick. There are no skin thickness limitations for the use of scarf patches and plug patches.
b. Determination of Single or Double Curvature, Much of the outside surface of plywood aircraft is curved. On such areas, plywood used for repairs to the skin must be similarly curved. Curved skins are either of single curvature or of double (compound) curvature. A simple test to determine which type of curvature exists may be made by laying a sheet of heavy paper on the surface in question. If the sheet can be made to fit the surface without wrinkling, the surface is either flat or has single curvature. If, however, the sheet cannot be made to fit the surface without wrinkling, the surface is of double curvature.
op 1 Par 17
Page 10 AC 43.13-1A 1972
ANGLE OF FACE GRAIN SAME AS IN ORIGINAL WEB
NEW WEB STAGGER WEB SPLICES (See Fig. 1.8 for method of splicing plywood webs).
• 10 B C
FOR CLARITY IOA - 6A- NEW WEB NOT INOLUDED IN THIS VIEW.
- SPRUCE REINFORCEMENT PLATES. A, 8,0, D = ORIGINAL DIMENSIONS.
FoURs 1.7.-Method of splicing box-spar flanges (plate method).
19. REPAIRS TO SINGLE CURVATURE PLYWOOD SKIN. Repairs to skins of single curvature may usually be formed from flat plywood, either by bending it dry or after soaking it in hot water.
The degree of curvature to which a piece of plywood can be bent will depend upon the direction of the grain and the thickness. Figure
1.12 is presented as a guide in determining which process of bending should be used for the curvature being considered. Plywood, after softening, may be bent on a cold ventilated form, or it may be bent over the leading edge near the part being patched if space permits.
In either method it should be allowed to dry completely on the form. When bending plywood over a leading edge, drying may be hastened by laying a piece of coarse burlap over the leading edge before using it as a bending form.
A fan to circulate the air over the bent piece will speed the drying. In bending pieces of small radii, or to speed up the bending of a large number of parts of the same curvature, it Par 19 may be necessary to use a heated bending form. The surface temperature of this form may be as high as 149° C. (800° F.), if necessary, without danger of damage to the ply.
wood. The plywood should be left on the form, however, only long enough to dry to room conditions.
20. REPAIRS TO DOUBLE CURVATURE PLYWOOD SKIN. The molded plywood necessary for a repair to a damaged plywood skin of double curvature cannot be made from flat plywood unless the area to be repaired is very small, or is of exceedingly slight double curvature; there fore, molded plywood of the proper curvature must be on hand before the repair can be made.
If molded plywood of the proper curvature is available, the repair may be made following the recommended procedures.
21. SPLAYED PATCH. Small holes with largest dimensions not over 15 times the skin thickness in skins not more than 1/10 inch in thickness Chop
AC 43.13-1A Pogo 11 972 JOAT
- INSERTED WEB SECTION 46- (1/6 FLANGE DEPTH)
PLYWOOD COVER STRIPS (Same thickness as "A").
FILLER BLOCKS I. AFTER INSERTED WEB HAS BEEN GLUED AND NAILED IN PLACE,GLUE AND NAIL COVER STRIP OVER ENTIRE LENGTH OF SPLICE JOINTS.
2. SECTIONAL SHAPE OF FILLER BLOCKS MUST CONFORM EXACTLY TO TAPER OF SPAR. THEY MUST NOT BE TOO TIGHTLY FITTED OR WEDGING ACTION WILL LOOSEN EXISTING GLUE JOINTS OF WEBS TO FLANGES. IF TOO LOOSELY FITTED, CRUSHING OF WEB WILL OCCUR WHEN CLAMPING.
SIRIT TO SAME AS ORIGINAL PLY OF NEW PLYWOOD WEB AND COVEF FIGURE 1.8.-Method of splicing box-spar webs.
Par 21
NO FITTING WITHIN THESE LIMITS 5 TO 1 SLOPE
- LONGITUDINAL CRACK
AND LOCAL DAMAGE SPRUCE BLOCKY
DIRECTION OF GRAIN IF
SPRUCE OR FAGE GRAIN IF PLYWOOD FIGURE 1.9.—Method of reinforcing a longitudinal crack and/or local damage in a solid or internally routed apar. AC 43.13-1A 1972
$3.1$- Page 13
• 12F RECOMMENDED -1 1OF MINIMUM
DAMAGED AREA GUSSET - ORIGINAL Y - 8
PLYWOOD- C==
SPRUCE OR
BASSWOOD AT A SPAR *4А† = C=
•Од- ЗА AT TRAILING EDGE -10 A A, B,O, D, E, F, 0 • ORIGINAL DIMENSIONS•
REINFORCEMENT PLATES
SHALL BE PLYWOOD GLUED AND NAILED.
AT A JOINT TO TRUSS MEMBERS THE DIRECTION OF THE
FACE GRAIN MUST BE
THE SAME AS ORIGINAL GUSSET.
T +3А→ - 10 A- FIGURE 1.10.-Repair of wood ribs.
ар 1 Par 21
Page 14 AC 43.13-1А 1972 SEE FIG. 1.10 FOR REPAIR OF CAPSTRIPS. , REPAIR DAMAGE-
SEE (D) (A) -ЗА - ЗА SEE FIG. 1.10 FOR REPAIR OF CAPSTRIPS. - REPAIR DAMAGE- (B) SEE FIG. 1.10 FOR REPAIR OF CAPSTRIPS. → REPAIR DAMAGE (G) * 12 A RECOMMENDED→
IOA MINIMUM 6 A- (D) А-А C-0
PLYWOOD
REINFORCEMENT E SAME THICKNESS AND FACE GRAIN DIRECTION AS ORIGINAL.
FIGURE 1.11.-Typical wing compression rib repairs.
Par 21 Chap 1
Ju r'ercent moisture content, Thoroughly soaked in hot water 1972 AC 43.13-1A bent on cold mandrels and bent on cold mandrels PLywood At 900 to At 0° or 450 At 900 to At 00 or 450 thickness face grain to face grain face grain to face grain ·→ (I) (2) (3) (4) (5) (6) Inch No. plies Inches Inches Inches Inches 3 0.5 0.1 3 105 ole 3 2.6 •8 3 3.8 1.2 3 5.3 1,8 3 761 2.6 5 17 6 2 5 21 7 3 5 27 10 4 5 31 20 12 5 5 43 28 16 7 5 54 36 21 10 Columns (1) and (3) may also be used for determining the maximun thickness of single laminations for curvel members.
FIGURE 1.12.—Minimum recommended bend radii for aircraft plywood.
may be repaired by using a circular splayed patch as illustrated in figure 1.18. The term "splayed" is used to denote that the edges of the patch are tapered, but the slope is steeper than is allowed in scarfing operations. The steps shown in figure 1.18 should be taken in making a splayed patch.
a. Lay out the patch according to figure 1.18.
Tack a small piece of plywood over the hole for a center point, and draw two circles, the inner one to be the size of the hole, and the outer one marking the limits of the taper. The difference between the radii is 5T (5 times the thickness of the skin). If one leg of the dividers has been sharpened to a chisel edge, the dividers may be used to cut the inner circle completely through.
b. Taper the hole evenly to the outer mark with a chisel, knife, or rasp.
c. Prepare a circular tapered patch to fit the prepared hole and glue the patch into place with face-grain direction matching that of the original surface.
d. Use waxed paper between the patch and a AC 43.13-1А 1972 plywood pressure plate cut to the exact size of the patch. This prevents extruded glue from binding patch and plate together. Center the plate carefully over the patch.
•. Apply pressure. As there is no reinforcement behind this patch, care must be used so that pressure is not great enough to crack the skin. On horizontal surfaces, weights or sandbags will be sufficient. On patches too far in for the use of standard hand clamps, jaws of greater length may be improvised.
f. Fill, sand, and refinish the patch.
22. SURFACE PATCH. Plywood skins that are damaged between or along framing members may be repaired by surface or overlay patches as shown in figure 1.14. Trim the damaged skin to a rectangular or triangular shape and round the corners. The radius of rounded corners should be at least 5 times the skin thicknesa. Cover surface patches with fabric before finishing. Fabric should overlap the original skin at least 2 inches. Surface patches located TRIM TO CIRCULAR SHAPE (15 T MAXIMUM DIAMETER. MINIMUM DISTANCE TO FRAME = 15 T.)
-FACE GRAIN OF PATCH PARALLEL TO FACE GRAIN OF SKIN.
INCH WEIGHTS OR CLAMP OR 1585 PRESSURE PLATE fOR "PLYWOOD
PAPER OR CELLOPHANE
- PLYWOOD SKIN FlOURe 1.18.-Splayed patch.
Par 21 Chap 1