divergence. The direction of free-flowing ink will frequently assist in determining grain direction.
b. Wavy, curly, and interlocked grain. Acceptable, if local irregularities do not exceed limitations specified for spiral and diagonal grain.
c. Hard knots. Sound hard knots up to g-inch in maximum diameter are acceptable providing:
(1) they are not in projecting portions of I-beams, along the edges of rectangular or beveled unrouted beams, or along the edges of flanges of box beams (except in lowly stressed portions); (2) they do not cause grain divergence at the edges of the board or in the flanges of a beam more than specified in column 3; and (3) they are in the center third of the beam and are not closer than 20 inches to another knot or other defect (pertains to ⅜-inch knots-smaller knots may be proportionately closer). Knots greater than ¼-inch must be used with caution.
d. Pin knot clusters. Small clusters are acceptable providing they produce only a small effect on grain direction.
e. Pitch pockets. 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 ⅛-inch width by g-inch depth and providing they are not along the projecting portions of l-beams, along the edges of rectangular or beveled unrouted beams, or along the edges of the flanges of box beams.
f. Mineral streaks. Acceptable, providing careful inspection fails to reveal any decay.
Defects Not Permitted.
g. Cross grain. Not acceptable, unless within limitations noted in a. above.
h. Wavy, curly, and interlocked grain. Not acceptable, unless within limitations noted in b.
i. Hard knots. Not acceptable, unless within limitations noted in c. above.
i. Pin knot clusters. Not acceptable, if they produce large effect on grain direction.
k. Spike knots. These are knots running com:
pletely through the depth of a beam perpendicular to the annual rings and appear most frequently in quartersawed lumber. Reject wood containing this defect.
I. Pitch pockets. Not acceptable, unless within limitations noted in e. above.
m. Mineral streaks. Not acceptable, if accompanied by decay (see f.).
n. Checks, shakes; and splits. Checks are longitudinal cracks extending, in general, across the annual rings. Shakes are longitudinal cracks usually between two annual rings. Splits are longitudinal cracks induced by artificially induced stress. Reject wood containing these defects.
o. Compression wood. This defect is very detrimental to strength and is difficult to recognize readily. It is characterized by high specific gravity; has the appearance of an excessive growth of summer wood; and in most species shows but little contrast in color between spring wood and summer In doubtful cases reject the material, or subject samples to a toughness machine test to establish the quality of the wood. Reject all material containing compression wood.
p. Compression failures. This defect is caused from the wood being overstressed in compression due to natural forces during the growth of the tree, felling trees on rough or irregular ground, or rough handling of logs or lumber. Compression failures are characterized by a buckling of the fibers that appear as streaks on the surface of the piece substantially at right angles to the grain, and vary from pronounced failures 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 reliable.
q. Decay. Examine all stains and discolorations carefully to determine whether or not they are harmless, or in a stage of preliminary or advanced decay. All pieces must be free from all forms of Glues used in aircraft repair fall into two general groups: (1) Casein and (2) resin glues. Any glue that meets the performance requirements of applicable U.S. 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.
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 yeara, 230
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.
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, resorcinol-formaldehyde, 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 glue line 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.
For those not familiar with the terms used relating to synthetic resin adhesives and their application, a glossary follows.
(1) Cold setting adhesive. An adhesive which sets and hardens satisfactorily at ordinary room temperature, i.e., 50° F.
to 86° F. (10° C. to 32° C.) within a reasonable period.
(2) Close contact adhesive. A nongap-filling adhesive suitable for use only in those joints where the surfaces to be joined can be brought into close contact by means of adequate pressure, and where glue lines exceeding 0.005 in. can be avoided with certainty.
(3) Closed assembly time. The elapsing between the assembly of the joints and the application of pressure.
(4) Double spread. The spread of adhesive equally divided between the two surfaces to be joined.
(5) Gap-filling adhesive. An adhesive suitable for use in those joints where the surfaces to be joined may or may not be in close or continuous contact, owing either to the impossibility of applying adequate pressure or to slight inaccuracies of machining. Unless otherwise stated by the manufacturer, gap-filling adhesives are not suitable for use where the glue line exceeds 0.050 in. in thickness.
(6) Glue line. The resultant layer of adhesive joining any two adjacent wood lay.
ers in the assembly.
(7) Hardener. A material used to promote the setting of the glue. It may be supplied separataly in either liquid or powder form, or it may have been incorporated with the resin by the manufacturer.
It is an essential part of the adhesive, the properties of which depend upon using the resin and hardener as directed.
(8) Open assembly time. The period of time between the application of the adhesive and the assembly of the joint components.
(9) Single sproad. The spread of adhesive to one surface only.
(10) Spread of adhesive. The amount of adhesive applied to join two surfaces.
(11) Synthetic resin. A synthetic resin (phenolic) is derived from the reaction of a phenol with an aldehyde. A synthetic resin (amino plastic) is derived from the reaction of urea, thiourea, melamine, or allied compounds with formaldehyde.
(12) Synthetic resin adhesive. A composition substantially consisting of a synthetic resin, either the phenolic or amino type, but including any hardening agent or modifier which may have been added by the manufacturer or which must be added before use, according to manufacturer's instructions.
Synthetic resin adhesives are used extensively for joining wooden structures to avoid the localized stresses and strains which may be set up by the use of mechanical methods of attachment, The strength of such structures depends largely on the efficiency of the glued joints, and cannot be verified by means other than the destruction of the joints. Acceptance must be governed by adequate precautions throughout the gluing process and by the results obtained by representative test pieces.
Synthetic resin adhesives usually consist of two 231
separate parts, the resin and the hardener. The resin develops its adhesive properties only as a result of a chemical reaction between it and the hardener. With some adhesives, an inert filler is added to increase viscosity and to improve the gap-filling properties of the mixed adhesive.
Synthetic resins can be obtained either in liquid or powder form. In general, powder resins have the longest storage life, since they are less susceptible to deterioration from high ambient temperatures.
Powder resins must be mixed with water in accordance with the manufacturer's instructions before they can be used in conjunction with a hardener. To obtain satisfactory results, it is essential that they be properly mixed. Once mixed, the adhesive must not be diluted unless this is permitted by the manufacturer's instructions. In many instances, manufacturers specify a definite period of time which must elapse between the mixing and the application of the adhesive. During this period, the adhesive should be covered to prevent contamination. When resins are supplied in liquid form, they are ready for immediate use in conjunction with the hardener. Liquid resin must not be diluted unless this is permitted by the manufacturer's instructions.
When mixing the hardener with the resin, the proportions must be in accordance with the manufacturer's instructions. Hardeners should not be permitted to come into contact with the resin except when the adhesive is mixed prior to use.
GLUING The surface to be joined must be clean, dry, and free from grease, oil, wax, paint, etc. It is important that the parts to be joined have approximately the same moisture content, since variations will cause stresses to be set up because of swelling or shrinkage which may lead to the failure of the joint.
The moisture content of wood can be determined by taking a sample of the wood to be glued, weighing it, and then drying it in an oven at a temperature of 100° C. to 105° C. Calculate the moisture content by using the formula:
• × 100 where, W: = the weight of the sample prior to drying.
W›= the weight of the sample after drying.
Example Substitution and solution of above formula:
2 - 15 x 100 = 33 or 33%.
The approximate moisture content can also be determined by using a moisture meter. When this instrument is used, its accuracy should be checked periodically.
The wood to be glued should be at room temperature. The surfaces to be joined should not be overheated since this affects the surface of the wood and reduces the efficiency of most synthetic resin adhe- Synthetic resin adhesives are very sensitive to variations in temperature. The usable (pot) life of the adhesive, proportion of hardener to use, and clamping time all depend largely on the temperature of the glue room at the time of gluing.
It is generally desirable to apply adhesive to both surfaces of the material. This applies particularly where the glue line is likely to be variable or when The adhesive can be applied by a brush, glue spreaders, or rubber rollers that are slightly grooved. The amount of adhesive required depends largely on the type of wood and the accuracy of machining. Dense wood requires less adhesive than soft or porous types. Adhesive should be applied generously to an end grain. Smooth, side-grained surfaces may be satisfactorily glued with a thin spread. The general rule is that the adhesive should completely cover the surfaces to be glued and remain tacky until pressure is applied to the joint.
Difficult gluing conditions may occur when a soft wood is to be glued to one much denser because the adhesive tends to flow into the more porous wood.
In such instances, unless otherwise specified by the manufacturer of the adhesive, precoating and partial drying of the softer surface, before normal spreading, is suggested.
Care should be taken before applying the adhesive to ensure that the surfaces will make good contact and that the joint will be positioned correctly. The interval between the application of the adhesive and assembly of the joint should be kept as short as possible. Some adhesives contain solvents which should be allowed to evaporate before the joint is assembled. If this is not done, bubbles.
may be created and result in a weak joint. For adhesives of this type, the manufacturer will specify a time interval which should elapse before the joint is closed.
To ensure that the two surfaces bind properly, pressure must be applied to the joint. The strength of the joint will depend to a great extent upon how evenly the force is applied. The results of evenly 232
and unevenly applied pressure are illustrated in figure 5-109.
Pressure is used to squeeze the glue out into a thin continuous film between the wood layers, to force air from the joint, to bring the wood surfaces into intimate contact with the glue, and to hold them in this position during the setting of the glue.
Pressure should be applied to the joint before the glue becomes too thick to fow and is accomplished by means of clamps, presses, or other mechanical devices.
Non-uniform gluing pressure commonly results in weak and strong areas in the same joint. The amount of pressure required to produce strong joints in aircraft assembly operations may vary from 125 to 150 p.s.i. for softwoods and 150 to 200
p.s.i. for hardwoods. Insuficient pressure to poorly machined wood surfaces usually results in thick glue lines, which indicates a weak joint, and should be carefully guarded against.
The methods used in applying pressure to joints in aircraft gluing operations range from the use of brads, nails, screws, or clamps to the use of hydraulic and electrical power presses. Hand-nailing is used rather extensively in the gluing of ribs and in the application of plywood skins to the wing, control surfaces, and fuselage frames.
On small joints, such as those found in wood ribs, the pressure is usually applied only by nailing the joint gussets in place after spreading the glue.
Since small nails must be used to avoid splitting, the gussets should be comparatively large in area to compensate for the relative lack of pressure. At least four nails (cement-coated or galvanized and barbed) per sq, in, are to be used, and in no event Arrows indicate pressure gap Pressure block FIGURE 5-109. Results of uneven and even pressure.
must nails be more than ¾ in, apart. Small brass screws may also be used advantageoulsy when theparticular parts to be glued are relatively small and do not allow application of pressure by means of clamps.
Apply pressure using cabinet maker clamps, parallel clamps, or similar types. Use handspring clamps in conjunction with softwood only. Because of their limited pressure area, they should be applied with a pressure-distributing strip or block at least twice as thick as the member to be pressed.
High clamping pressures are neither essential nor desirable, provided good contact between the surfaces being joined is obtained. When pressure is applied, a small quantity of glue should be squeezed from the joint. This should be wiped off before it dries. The pressure must be maintained during the full setting time. This is important since the adhesive will not reunite if disturbed before it is fully set.
The setting time depends on the temperature at which the operation is carried out. An increase in temperature results in a decrease in the setting period. Conversely, a decrease in temperature causes an increase in the setting time.
Full joint strength and resistance to moisture will develop only after conditioning for at least 2 days.
Again, this depends on the ambient temperature and the type of hardener used. Usually when repairs are made, the joint will be of sufficient strength after 1 day Testing Glued Joints Frequent tests should be made to ensure that the joints are satisfactory. Whenever possible, tests should be carried out on pieces cut from the actual component. The test samples should be 1 in. wide and at least 2 in. long. The pieces should be joined with one member overlapping the other by ⅜ in.
The glued test sample should be placed in a vice and the joint broken by exerting pressure on the overlapping member. The fractured glue faces should show at least 75% of the wood fibers evenly distributed over the fractured glue surfaces. A typical broken test piece is shown in figure 5-110.
Where repairs are to be made on old aircraft in which the wooden structure is joined with a casein cement, all traces of the casein cement must be removed from the joint, since this material is alkaline and is liable to affect the setting of a synthetic resin adhesive. Local staining of the wood by the casein cement can, however, be disregarded.
FIGURE 5-110. Typical broken test piece.
SPLICED JOINTS The scarf joint is generally used in splicing structural members in aircraft. As illustrated in figure 5-111, the two pieces to be joined are beveled and glued. The slope of the bevel should be not less than 10 to 1 in solid wood and 12 to 1 in plywood. Make the scarf cut in the general direction of the grain slope as shown in figure 5-111.
The chief difficulty encountered in making this type of joint is that of obtaining the same bevel on each piece. The strength of the joint will depend upon the accuracy of the two beveled surfaces, because an inaccurate bevel will reduce the amount of effective glue area. (See figure 5-111.)
New section to be spliced in
- Slope 10 to I in solid wood 297013 A - Correctly beveled pieces Cap B - Incorrecthy beveled pieces FIGURE 5-111. Beveling scarf joint.
One method of producing an accurate scarf joint is illustrated in figure 5-112. After the two bevels are cut, the pieces are clamped to a 2X4 backboard or some similar material. A fine-tooth saw is then run all the way through the joint. One of the pieces is then tapped on the end until it will move no farther, and the saw is again passed through the joint. This procedure is continued until the joint is perfect. A light cut of a plane is then used to smooth the surfaces of the joint.
Undamaged section Guide board FIcURE 5-112. Making a scarf joint.
It is recommended that no more than 8 hrs. 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 are not permitted. Sandpaper must never be used to smooth softwood surfaces that are to be glued. Sawed surfaces must be similar to well-planed surfaces in uniformity, smoothness, and freedom from crushed fibers.
Tooth-planing, or other means of roughening smooth, well-planed surfaces of normal wood before gluing are not recommended. Such treatment of well-planed wood surfaces may result in local irregularities and objectionable rounding of edges. Although 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), or laminated paper plastic (papreg).
PLYWOOD SKIN REPAIRS Most skin repairs can be made using either the surface or overlay patch, the splayed patch, the plug patch, or the scarf patch. Probably the easiest to make is the surface patch. Surface patches should not be used on skins over ½-in. thick.
To repair a hole by this method, trim the damaged skin to a rectangular or triangular shape depending upon the exact location of the damage relative to the framing members (figure 5-113). Where Front spar Minimum radius = Trimmed - 5 times openings skin thickness Rear spar FIGURE 5-113. Typical shapes for damage removal.
the framing members form a square corner and the damage does not extend to the next parallel member, a triangular opening should be made. The corners of the cutouts should be rounded with a radius of at least five times the thickness of the skin.
Doublers, made of plywood at least as thick as the skin, are reinforcements placed under the edge of the hole inside the skin. The doublers are nailed and gluded in place. The doublers are extended from one framing member to another and are strengthened at the ends by saddle gussets attached to the framing members.
A patch large enough to extend at least 12 times the skin thickness beyond all edges of the opening is cut from material of the same kind and thickness as the original panel. The edges of the patch are then beveled (scarfed), as shown in figure 5-114.
It is usually impossible to use clamps when gluing an external patch; therefore, the pressure is applied by some other means. It is usually done by placing heavy weights on the patch until it is dry.
Two or three small nails driven through the patch will prevent slipping during the drying.
After a surface patch has dryed, it should be covered with fabric. The fabric should overlap the original plywood skin by at least 2 in.
Surface patches located entirely aft of the 10% chord line, or which wrap around the leading edge and terminate aft of the 10% chord line are permissible. The leading edge of a surface patch should be beleved with an angle of at least four times the skin thickness. Surface patches can have as much as a 50-in. perimeter and can extend from one rib to the next. The face-grain dircction of the patch must be in the same direction as the original skin. Surface patches should not be used on skins over ⅛-in.
thick.
Flush Patches In places where an external patch would be objectionable, such as on wing coverings or fuselage skin, etc., a flush patch can be used.
Plug Patches Two types of plug patches, oval and round, can be used on plywood skins. Since the plug patch is strictly a skin repair, it should be used only for damage that does not involve the supporting structure under the skin.
A plug patch has edges cut at right angles to the surface of the skin. The skin is cut out to a clean round cr oval hole with square edges. The patch is cut to the exact size of the hole, and when installed, 235
Front spar Trimmed opening іґ B Saddle gusset Rear spar -30 T -Plywood skin Patch 12 T (1" minimum) ЗТ (¼" minimum) -Rib cap
- Plywood saddle gusset Section A-A Minimum thickness = T Nailed-glued in place I21 • I platch - 12T- - Patch -Rib cap Spartas Spar Unsupported Section B-B Section C-C FICURE 5-114. Surface patches.
- Grain direction of skin, patch, and doubler
•Saw cut 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 I Plug patch
- Plywood skin - Saw cut in doubler
• Plywood doubler Dimensions A BC Small circular plug patch 2· , 2 1% Large circular plug patch 3% 3 2% (Two rows of screws and nails required for large patch) FIGURE 5-115. A round plug patch.
the edge of the patch forms a butt joint with the edge of the hole.
A round plug patch, shown in figure 5-115, can be used where the cutout hole is no larger than 6 in, in diameter. Large and small circular patches have been designed for holes of 6 and 4 in. in diameter.
The steps for making a circular or round plug patch are:
(1) Cut a patch of the correct dimension for the repair. The patch must be of the same material and thickness as the origina.
skin. Orientation of the face-grain direction of the round plug patch to that of the skin surface is no problem, since the round patch may be rotated until grain directions match.
(2) Lay the patch over the damaged spot and mark a circle of the same size as the patch.
(3) Cut the skin out so that the patch fits snugly into the hole around the entire perimeter.
(4) Cut a doubler of ¼-in. plywood so that its outside radius is g in. greater than the hole to be patched and the inside radius is ⅜ in. less. For a large patch, these dimensions would be ⅛ in, each. The doubler should be of a soft plywood such as poplar.
(5) Cut the doubler through one side so that it can be inserted through to the back of the skin. Apply a coat of glue to the outer half of the doubler surface where it will bear against the inner surface of the skin.
(6) Install the doubler by slipping it through the cutout hole and centering it so that it is concentric with the hole. Nail it in place with nailing strips, using a bucking bar or similar object for backing. Waxed paper must be placed between the nailing strips and the skin.
(7) After the glue has set in the installation of the doubler, apply glue to the surface of the doubler where the patch is to join and to the same area on the patch. Insert the patch in the hole.
(8) Apply pressure to the patch by means of a pressure plate and No. 4 wood screws placed at approximately 1-in. spacing.
Waxed paper or cellophane placed be- 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
ABCDE F Small 2% 1¾ 3 4% Large 2 3% 1% 5 7 Two rows of screws and nails required on large patch Butt joint of patch to skin
- Plug patch (grain parallel to skin) ....
- Plywood doubler (grain parallel to skin)
- Plywood skin FIGURE 5-116. An oval patch.
tween the plate and patch will prevent glue from sealing the plate to the patch.
(9) After the glue has set, remove the nails and screws. Fill the nail and screw holes, sand, and finish to match the original surface.
The steps for making an oval plug patch, figure 5-116, are identical with those for making the round patch. The maximum dimensions for large oval patches are 7-in. long and 5-in. wide. Oval patches must be prepared with the face grain carefully oriented to the same direction as the original skin.
Splayed Patch A splayed patch is a patch fitted into the plywood to provide a flush surface. The term "splayed" denotes that the edges of the patch are tapered, but the slope is steeper than is allowed in scarfing operations. The slope of the edges is cut at an angle of five times the thickness of the skin.
Splayed patches may be used where the largest dimension of the hole to be repaired is not more than 15 times the skin thickness and the skin thickness is not more than 1/10 in.
Lay out the patch as shown in figure 5-117. Tack a small piece of plywood over the hole for a center point. Draw two concentric circles around the damaged area. The difference between the radii of the circles is five times the skin thickness. The inner circle marks the limit of the hole and the outer one marks the limit of the taper.
Cut out the inner circle and taper the hole evenly to the outer mark with a chisel, knife, or rasp.
Weights or clamp Pressure plate %" T = 1/10" or i" plywood or less - Waxpaper or cellophane ST • 5T L plywood skin FICURE 5-117. A splayed patch.
Prepare a circular patch, cut and tapered to match the hole. The patch is of the same type and thick ness as the plywood being repaired.
Apply glue to the beveled surfaces and place the patch into place with the face-grain direction matching that of the original surtace.
After the patch is in place, a pressure plate cut to the exact size of the patch is centered over the patch, with waxed paper between the two, and pressed firmly against the patch with a weight (sometimes a sandbag) or clamp. Since there is no reinforcement behind the splayed patch, use care to prevent excess pressure. After the glue has set, fill, sand, and finish the patch to match the original surface.
Scarf Patch A properly prepared and inserted scarf patch is the best repair for damaged plywood and is preferred for most skin repairs. The scarf patch differs from the splayed patch in that the edges are scarfed to a 12 to 1 slope instead of the 5 to 1 used with the splayed patch. The scarf patch also uses reinforcements under the patch where the glue joints occur.
Much of the outside surface of plywood aircraft is curved. If the damaged plywood skin has a radius of curvature greater than 100 times the skin thickness, a scarf patch can be installed. Backing blocks or other reinforcements must be shaped to fit the skin curvature.
Figures 5-118 and 5-119 illustrate methods for making scarfed flush patches. Scarf cuts in plywood are made by hand plane, spoke shave, scraper, or accurate sandpaper block. Rasped surfaces, except at the corners of scarf patches and sawed surfaces, should be avoided since they are likely to be rough or inaccurate.
When the back of a damaged plywood skin is accessible (such as a fuselage skin), it should be repaired with scarf patches following the details shown in figure 5-118. Whenever possible, the edge of the patch should be supported as shown in section C-C. When the damage follows or extends to a framing member, the scarf should be supported as shown in section B-B.
Damages that do not exceed 25 times the skin thickness in diameter after being trimmed to a circular shape can be repaired as shown in figure 5-118, section D-D, provided the trimmed opening is not nearer than 15 times the skin thickness to a framing member. The backing block is carefully 239
• Saddle gusset Nailing strips
- Maximum diameter 25T Minimum . distance.
~ 15T Temporary backing 30T Patch -12T Backing Plywood saddle gusset (·" minimum ) Minimum thickness = T Nail-glue in place Section A-A 121 Patch • Patch Backing Framing member - Backing Clamp and glue backing to frame and skin Section B-B Section C-C Nailing strips
- 127.
Temporary backing block, shape to fit skin Section D-D Cellophane or waxpaper.
FIGURE 5-118. Scarf patches, back of skin accessible.
Front spar Ribs Saddle gusset Rear spar
- 30T -12T- f Plywood skin 81 3T (Ke minimum) - L Rib cap Plywood saddle gusset (1" minimum ) Section A-A Nail and glue in place (minimum thickness = T) Plywood skin 121- Patch -12T - Patch T -T Rib cap ЗТ (¼ minimum ) Plywood skin Plywood or spruce Section B-B Section C-C FICURE 5-119. Scarf patches, back of skin not accessible.