Par 1-18 Page 1-71 (and 1-10) move nails after assembly, the mechanic should examine adj acent structure to see whether nails remain from original manufacture.
On larger members (spar repairs for example), apply pressure by means of screw clamps, such as a cabinet-maker’s bar or “C-clamps.” Strips or blocks should be used to distribute clamping pressure and protect members from local crushing due to the
Par 1-18 Page 1-72 (and 1-10) limited pressure area of the clamps, especially when one member is thin (such as plywood). The strip or block should be at least twice as thick as the thinner member being bonded.
Immediately after clamping or nailing a member, the mechanic must exam ine the entire joint to assure uniform part contact and adhesive squeeze-out. Wipe away excess adhesive.
ASSEMBLY TIME IN BONDING. Where pieces of wood are coated and exposed freely to the air, some adhesives experience a much more rapid change in consistency than where the pieces are laid together as soon as the spreading has been done. The condition of free exposure is conveniently referred to as “open assembly” and the other as “closed assembly.”
When room temperature curing adhesives are coated on wood parts and left exposed to the atmosphere (open assembly), the time for co mplete assembly is appreciably reduced compared wi th closed assembly periods. Use assembly time and clamping pressures recommended by the adhesive manufacturer.
Resorcinol adhesive cures at “room temper atures,” but the proper cure temperature is crit ical. A minimum cure temperature of 70 °F must be main tained for several hours. Other types of adhesive require various times and temperatures for curing. Refer to manufacturer’s instructions.
Resorcinol, epoxy, and other adhesives increase in strength mainly as a result of a chemical reaction. The pressing time for room temperature curing adhesives should be 8 hours or mo re to ensure the cure is complete before handling or machining the bonded structure. The temperature of the wood and the surrounding air during the cure period should be at least 70°F for the entire pressing period unless the adhesive manufact urer’s directions state that a lower temperature is permissible.
CLAMPING PRESSURE. Use the recommended pressure to squeeze adhesive out into a thin continuous film between the wood layers. This forces air from the joint, and brings the wood surfaces into intimate contact with the adhesive, to hold them in this position during the curing of the adhesive. Too much pressure and not enough assembly time, is often as much of a problem as too little pressure, applied over a long time period.
Pressure should be applied to the joint before the adhesive becomes too thick to flow and is accomplished by means of clamps, presses, or other mechanical devices.
Non-uniform clamping pressure commonly r esults in both weak and strong areas in the same joint. The amount of pressure required to pr oduce strong joints in aircraft assembly oper ations varies with the type of adhesive used and the type of wood to be bonded. Typical pre ssures when using resorcinol or pl astic resin a dhesive may vary from 125 to 150 pounds per square inch for softwoods and 150 to 200 pounds per square inch for hardwoods. Insu fficient pressure or poorly machined wood su rfaces usually result in thick bond lin es, which indicate a weak joint, and should be carefully guarded against. Some epoxy adhesives require much le ss clamping pressure to produce accep table joint strength. Be sure to read and follow the adhesive manufacturer’s instructions in all cases.
METHOD OF APPLYING PRESSURE to joints in aircraft bonding operations range from the use of brads, nails, small screws, and clamps to the use of hydraulic and electrical power presses. The selection of appropriate clamping means is important to achieving sound bond joints.
Hand nailing is used rather extensively in the bonding of ribs and in the application of pl ywood skins to the wing, control surfaces, and fuselage frames. Small brass screws may also be used advantageously when the particular parts to be bonded are relatively small and do not allow application of pressure by means of clamps.
Both nails and screws produce adverse after effects. There is considerable risk of splitting small parts when installing nails or screws. Metal fasteners also provide vulnerable points for moisture entry in service.
On small joints using thin plywood for gussets or where pl ywood is used for single covering, the pressure is usually applied by using nails or small brass screws. Cloth or thin plywood nailing strips are often used to facilitate removal of the nails after the adhesive has cured. Plywood strips also serve to spread the nailing pressure over a larger area.
The size of the nails must vary with the size of the members. If multiple rows of nails are r equired, the nails should be 1 inch apart in rows spaced 1/2 inch apart. The nails in adjacent rows should be staggered. In no case should the nails
Par 1-18 Page 1-73 (and 1-10) be more than 3/4 inch from the next nearest nail. The length of the nails should be such that they penetrate the wood below the joint at least 3/8 inch. In the case of small members, the end of the nail should not pr otrude through the member below the joint. Hit the nails with several light strokes, just embe dding the head flush with the nailing strip.
Be careful not to crush the wood with a heavy hammer blow.
In some cases the nails are removed after adh esive cure, while in others the nails are left in place. The nails are employed for clamping pressure during adhesive cure a nd must not be counted upon to hold members together in service. In deciding whether or not to remove nails after assembly, the mechanic should exa mine adjacent structure to see whether nails remain from original manufacture.
On larger wood to wood or wood to plywood members, for example in spar repairs, apply pressure by means of screw clamps, such as cabinet maker’s bar, or “C” clamps. Strips or blocks should be used to distribute clamping pressure and protect members from local crushing due to the limited pressure area of the clamps, especially when one member is thin (such as plywood). The strip or block should be at least twice as thick as the thinner member being bonded.
Immediately after clamping or nailing a member, the mechanic must examine the entire joint to assure uniform part contact, glue line, and adhesive squeeze-out. Wipe away excess adhesive.
1-12.1-17. [RESERVED.]
Par 1-18 Page 1-74 (and 1-10) SECTION 2. HEALTH AND SAFETY
GENERAL. The possibility of an injury is an important consideration when working with wooden aircraft structures. The tools and machines used to shape wooden members can be very dangerous. In addition, there are potential health hazards in working with adhesives and finishes. The mechanic should follow manufacturer’s instructions wherever applicable to prevent injury. Federal law mandates that individual chemical manufacturers are to provide Material Safety Data Sheets (MSDS) with health hazard data to all consumers. First aid information and handling precautions must also be identified.
Most of the products used in wooden aircraft construction are flammable. Some, such as dope and paint, may be highly flammable.
SANDING IN AREAS OF EXISTING BOND JOINTS AND FINISHES. Some adhesives used in wooden aircraft construction contain biocides. A commonsense precaution when machining or sanding existing structure is to wear a respirator to avoid inhaling dust products. To lower potential fire hazards avoid using electric sanders around dope, paints, and adhesives.
HANDLING OF ADHESIVES AND FINISHES.
Most adhesives and finish products present at least some toxic potential to users. Injury may occur from skin or eye contact, inhalation, or accidental ingestion. Users should be aware of the manufacturer’s instructions and MSDS.
Appropriate skin, eye, ear, and respiratory protection should be worn whenever indicated.
Shop cleanliness is essential for health and fire safety.
Shop personnel should maintain awareness of others in the work area to assure that bystanders are not injured.
Proper shop ventilation is essential to disperse fumes emitted from adhesives such as resorcinol and epoxy.
1-21.1-26. [RESERVED.]
Par 1-27 Page 1-75
SECTION 3. INSPECTION
GENERAL. Inspection of wooden structure includes some methods, equipment, and awareness of failure modes which are unique to wooden aircraft.
TYPES OF DETERIORATION AND DAMAGE.
Wood Decay. Wood is an organic product which is subject to attack by fungi. Fungi are plants that grow on and in wood. The moisture content of the wood nominally will have to be 20 percent or greater to sustain fungus growth. The result of this growth is called decay. Decayed wood exhibits softness, swelling if still wet, excessive shrinkage when dry, cracking, and discoloration. Repair or replace wood if any amount or form of decay is found.
Splitting. Splits or cracks in wooden members occur along grain lines. When the moisture content of wood is lowered, its dimensions decrease. The dimensional change is greatest in a tangential direction (across the fibers and parallel to the growth rings), somewhat less in a radial direction (across the fibers and perpendicular to the growth rings), and is negligible in a longitudinal direction (parallel to the fibers). These dimensional changes can have detrimental effects upon a wood structure, particularly when two parts are bonded together with grains in different directions. This effect can often be seen where a plywood doubler is bonded to a spruce member. As the spruce member dries, it attempts to shrink, but is restrained by the plywood, which shrinks less. The re sulting stress in the spruce member exceeds its cross-grain strength, and a split occurs.
Bond Failure. Bond joint failure is generally due to improper fabrication technique or prolonged exposure to moisture in service. Although none of the older adhesives have been specifically found to fail by simple aging, the mechanic is advised to in spect all accessible joints carefully.
Finish Failure. The finish coat on wood structure (usually varnish) is the last line of defense to prevent water entry into wood and th e resulting decay. Finish failure can be the result of prolonged water exposure, wood splitting, ultraviolet li ght exposure, or surface abrasion.
Damage. Stress, impact, or mechanical damage to a wood structure is caused by excessive aerodynamic loads or impact loads occurri ng while the aircraft is on the ground. Overtightening of fittings can also cause crushing of the underlying wood member and possible bending of the metal fitting.
INSPECTION METHODS. Whenever possible, the aircraft should be kept in a dry, wellventilated hangar, with all inspection covers, access panels, etc., removed for as long as possible before final inspection. The aircraft should be given a preliminary inspection when first removing the inspection covers and access panels and inspected with a moisture meter at this time. If the moisture content is high, the aircraft should be thoroughly dried. If the aircraft is dry, this will facilitate later inspection, especially when determining the condition of bonded joints.
Likely locations for wood structure deterioration should be given special attention. Most damage is caused by external influence such as moisture, temperature extremes, or sunlight. Care should be taken to note all possible entry points for moisture, (i.e., cracks or breaks in the finish, fastener holes, inspection/access openings, c ontrol system openings, drain holes, and the interfaces of metal fittings
Par 1-27 Page 1-76 and the wood structure). The mechanic should also look for evidence of swelling or warpage of the aircraft’s wood structure, which would indicate underlying damage or decay. Particular attention should be paid to the wood structure immediately beneath the upper surfaces, especially under areas that are finished in dark colors, for signs of deteriorating adhesives. Cracks in wood spars are often hidden under metal fittings or metal rib flanges and leading edge skins. Any time a reinforcemen t plate exists that is not feathered out on its ends, a stress riser exists at the ends of the plate. A failure of the primary structure can be expected to occur at this point.
Tapping the wood structure with a light plastic hammer or screwdriver handle should produce a sharp solid report. If the suspect area sounds hollow and soft, further inspection is warranted by the following methods.
Probe the area in question, if accessible, with a sharp metal tool. The wood structure should be solid and firm. If the suspect area feels soft and mushy the mechanic should assume that the area is rotted.
Disassembly of the structure is warranted at this point.
Prying the area of a bond joint will reveal any mechanical separation of th e joint. If the mechanic detects any relative moveme nt between two adjacent wood members, a failure of the bond is evident. Any loose fittings should arouse the mechanic’s suspicion, and the fittings should be removed to check for elongated bolt holes. Disassembly is warranted for further inspection.
Odor is an important indicator of possible deterioration.
During the initial inspection, as the access panels are being removed from the structure, the mechanic should be aware of any areas that smell musty or moldy.
These odors are indicative of the presence of moisture and associated fungal growth and decay.
Visual inspection requires looking at the wood structure both externally and internally for visual signs of decay or physical damage. Any accumulations of dirt, bird nests, or rodent nests are likely places to hold moisture and promote decay.
The mechanic should remove any such accumulations that are found and inspect the area for signs of decay. Decay will appear as a dark discoloration or gray stains running along the grain and often a swelling of the wood member if still wet. Fittings will be imbedded in the wood instead of flush.
Highly suspected structurally damaged areas are shown in figure 1-3. A list of most likely areas to incur structural damage include the following:
Check front and rear spars for compression cracks adjacent to the plywood reinforcing plates, where the lift struts attach, and at the rib attach points on either side of the strut attach points. Triple-check these areas and the spar to fuselage attach points for cracks if the wingtip has contacted the ground, a hangar wall, etc.
Check all metal fittings which attach to wooden structure for looseness, corrosion, cracks, or warps. Areas of particular interest are strut attach fittings, spar butt fittings, aileron and flap hinges, jury strut fittings, compression struts, pulley brackets, and any landing gear fittings.
Check front and rear spars for longitudinal cracks at the ends of the plywood reinforcement plates where the lift struts attach. Triple-check this area if the wing has encountered any kind of ground strike.
Par 1-29 Page 1-77 Check ribs on either side of strut attach points for missing or loose rib-to-spar attach nails.
Check ribs on either side of strut attach points for cracks where the cap strips pass over and under the spars.
Check for cracked leading edge skin and/or failed nose ribs in the area directly in front of the jury strut.
Check the brackets which attach the struts to the spars for cracks.
Check the aileron, flap hinge, and hinge brackets for cracks and loose or missing rivets.
Check all exposed end grain wood, particularly the spar butts, for cracking or checking. Checking, or splitting, of wood spar butts is common on aircraft based in arid areas.
FIGURE 1-3. Likely areas to incur structural damage.
Also check for any cracks that indicate a bond line failure or structural failure of the wood member. Any evidence of movement of fittings, bushings, or fasteners should be cause for concern, and further inspection is warranted. Splits in fabric covering the plywood, especially on upper surfaces exposed to ultraviolet light and water, dictate that the mechanic remove the fabric around the split so the underlying plywood may be inspected for physical damage or decay. When removing metal fasteners from wood, check for evidence of corrosion. Any corrosion present indicates the presence of moisture and the strong probability of decay in the adjoining wood structure.
Any wooden member that has been overstressed is subject to compression failure (e.g., ground loop). Compression cracking and failure of the wood spars in certain aircraft are a continuing problem.
Compression failures are defined as failure of wood fibers on a plane perpendicular to the wood fiber’s longitudinal axis. If undetected, compression failures may result in structural failure of the wing during flight. Compression cracks have been found emanating from the upper surfaces of the wing spars and progressing downward.
The usual locations for cracks have been the front spar at both ends of the reinforcement plate for the lift strut and the front spar rib attach points, both inboard and outboard of the spar reinforcement plate; and the rear spar lift strut and rib attach points. An inspection of both the front and rear spars for compression cracks is recommended.
The two areas where it is possible to identify a compression crack are on the face and top surface of the spar. Using a borescope through existing inspection holes is one method of inspection. An alternate method is to cut inspection holes in the skin.
If inspection holes are cut, they should be made on the aft side of the front spar and the forward side of the rear spar. This will allow the fabric to be peeled away from the spar. Longitudinal cracks may also be detected during this inspection. Loose or missing rib nails may indicate further damage and should be thoroughly investigated. The mechanic may shine a light, at a low angle and parallel with the grain, in the area of the member
Par 1-29 Page 1-78 subjected to the compression load. An area of grain waviness would indicate a potential compression failure. In all cases the manufacturer’s inspection data should be followed.
Moisture Meters are effective tools for detection of excessive moisture content in wood members. An instrument such as this allows the mechanic to insert a probe into the wood member and read its moisture content directly off the meter. A correction chart usually accompanies the inst rument to correct for temperature and species of wood. Any reading over 20 percent indicates the proba bility of fungus growth in the member. Moisture content of the wood should be 8-16 percent, preferably in the 10-12 percent range (this range is during inspection). Where plywood skin covers the spar and the spar would be inaccessible without removing the skin, the moisture meter probe can be inserted through the plywood skin and into the spar to check the moisture content of the spar. The small holes made by the probe are easily sealed.
Destructive testing of sample bonded joints whenever a new bond joint is made, a sample joint should be made with the adhesive from the same batch used on the repair and scraps of wood left over from the repair.
After curing, the sample joint should be destructively tested to ensure proper bonding of the two wood pieces.
Any failure in the bond line indicates a cohesive failure of the adhesive. Any failure along the bond line indicates an adhesive failure, which is indicative of poor bonding. The ideal situation is when wood fibers are observed on both sides of the fracture surface. This indicates a failure in the wood, and indicates the bond joint is actually stronger than the wood.
1-30.1-35. [RESERVED.]
Par 1-36 Page 1-79
SECTION 4. REPAIRS
GENERAL. The basic standard for any aircraft repair is that the repaired structure must be as strong as the original structure and be equivalent to the original in rigidity and aerodynamic shape.
Repairs should be made in accordance with manufacturer specifications whenever such data is available.
REPLACEMENT OF DRAIN HOLES AND SKIN STIFFENERS. Whenever repairs are made that require replacing a portion that includes drain holes, skin stiffeners, or any other items, the repaired portion must be provided with similar drain holes, skin stiffeners, or items of the same dimensions in the same location. Additional drain holes may be required if reinforcement under a skin repair interferes with waterflow to existing drain holes.
Make any additional drain holes the same diameter as originals, usually 1/4 inch.
CONTROL SURFACE FLUTTER PRECAUTIONS.
When repairing or refinishing control surfaces, especially on high-performance airplanes, care must be exercised that the repairs do not involve the addition of weight aft of the hinge line. Such a procedure may adversely affect the balance of the surface to a degree that could 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, and that the stiffness, weight distribution, and mass balance are not affected in any way. Consult the aircraft maintenance manual or seek manufacturer’s direction for specific requirements on checking control surface balance after repair and refinishing of any control surface.
SCARF JOINTS. The scarf joint is the most satisfactory method of making an end joint between two solid wood members. Cut both parts accurately. The strength of the joints depends upon good joint design and a thin, uniform bond line. Make the scarf cut in the general direction of the grain slope as shown in figure 1-4.
No grain deviation steeper than 1 in 15 should be present in an outer eighth of the depth of the spar. In adjacent eighths, deviations involving steeper slopes, such as a wave in a few growth layers, are unlikely to be harmful. Local grain slope deviations in excess of those specified may be permitted in spar flanges only in the inner one-fourth of the flange depth.
FIGURE 1-4. Consideration of grain direction when making scarf joints.
SPLICING OF SPARS. Unless otherwise specified by the manufacturer, a spar may be spliced at any point except under the wing attachment fittings, landing gear fittings, engine mount fittings, or lift and interplane strut fittings. These fittings may not overlap any part of the splice. A spar splice repair should not be made adjacent to a previous splice or adjacent to a reinforcing plate. Spacing between two splices or between a splice and a reinforcing plate should be no less than three times the length of the longer splice.
Par 1-36 Page 1-80 Splicing under minor fittings such as drag wire, antidrag wire, or compression strut fittings is acceptable under the following conditions:
The reinforcement plates of the splice should not interfere with the proper attachment or alignment of the fittings. Do not alter the locations of pulley support brackets, bellcrank support brackets, or control surface support brackets. Plates are to be tapered off, as depicted in figure 1-2.
The reinforcement plate may overlap drag wire, antidrag wire, or compression strut fittings, if the reinforcement plates are on the rear face of the rear spar or the front face of the front spar. In such cases, it will be necessary to install slightly longer bolts. The front face reinforcement plate shoul d not overlap drag strut fittings, except when it does not require sufficient shortening of compression struts or changes in dragtruss geometry, to prevent adjustment for proper rigging. Even though take up is sufficient, it may be necessary to change the angles on the fittings.
(Acceptable methods for splic ing the various types of spars are shown in figure 1-4 through figure 1-9.)
Reinforcement plates must be used as indicated on all scarf repairs to spars and the slopes of scarves shown are minimum slopes.
SPAR REPLACEMENT. Replacement of spars is a major repair. Spars may be replaced by new parts made by the manufacturer or the holder of a Parts Manufacturer Approval (PMA) for that part.
Owner-produced spars may be installed providing they are made from a manufacturer-approved drawing. Also, a spar may be made by reference to an existing spar providing sufficient evidence is presented to verify that the existing spar is an original part, and that all materials and dimensions can be determined. The dimensions and type of wood used are critical to the structural strength of the aircraft. Care should be taken that any replacement spars accurately match the manufacturer’s original design.
SPLICING OF BOX SPAR WEBS. Always splice and reinforce plywood webs with the same type of plywood as found on the original part. Do not use solid wood to replace plywood webs. Plywood is stronger in shear than solid wood of the same thickness due to the grain direction of the individual plies. The face-grain of plywood replacement webs and reinforcement plates must be in the same direction as the original member to ensure that the new web will have the required strength. (The method of splicing plywood webs is shown in figure 1-9.)
REPLACING SOLID-TYPE SPARS WITH LAMINATED-TYPE SPARS. Solid spars may be replaced with laminated spars or vice versa, provided the material is of the same high quality.
External reinforcements (plywood or solid) must always be replaced as on the original member.
SPAR LONGITUDINAL CRACKS AND LOCAL DAMAGE. Cracked spars (except box spars) may be repaired by bonding plates of spruce or plywood of sufficient thickness to develop the longitudinal shear on both sides of the spar. Extend the plates well beyond the termination of the cracks, as shown in figure 1-10. A method of repairing small local damage to either the top or bottom side of a spar is also shown in figure 1-10.
Longitudinal Cracking of Wood Wing Spars of Aircraft Operating in Arid Regions. Aircraft having wood spars and operating in arid regions may develop longitudinal spar cracks in the vicinity of the plywood reinforcement plates. Th ese cracks result from the tendency of the spar to shrink when drying takes place.
Plywood resists this tendency to
Par 1-44 Page 1-81
Reinforcement plates to be spruce or plywood and should be bonded only, with the ends feathered off at 5:1 slope as depicted in Figure 1-2.
Solid spars may be replaced with laminated spars or vice versa, provided the material is of the same high quality.
FIGURE 1-5. Method of splicing solid or laminated rectangular spars.
shrink and causes the basic spar stock to split (see paragraph 1-2c). Cracks start under the plywood plates, usually (but not necessarily) at a bolt hole or cutout, and usually spread in each direction until they extend a short distance beyond the ends of the plates where the resistance to spar shrinkage disappears. Cracks have also been found in the butt end of spars. Other factors, which have been found conducive to the formation of cracks are poor protective finishes, large cutouts, and metal fittings that utilize two lines of large diameter bolts.
Repairing Cracks Versus Installing a New Spar. The presence of cracks does not n ecessarily 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 perform repair rather than install a new spar or section. However, a generally acceptable procedure suitable for all airplane models is not available. Because of the possibility of strength deficiencies contact the manufacturer. In absence of the manufacturer, the FAA should be contacted for approval before making repairs not in accordance with the manufacturer-approved instructions or the recommendations of this advisory circular. Longitudinal cracking or the recurrence of cracking can be minimized by ensuring that the moisture content of the solid wood portion is within the proper range before bonding . In arid desert areas, during bonding the moisture content should be in the range of 6-8 percent before bonding, but in other areas 10-12 percent is satisfactory. If solid or plywood repair stock is procured from another climatic region, it should be allowed to season,
Par 1-44 Page 1-82
If splice is made where routing is feathered to full width of spar, tapered plates (conforming to the contour of the routing) should be added first;
otherwise, the splice is the same as shown.
Reinforcement plates may be of plywood, the same material as the spar, or material of equal or higher quality; and should be bonded only with ends feathered out with a 5:1 slope.
FIGURE 1-6. Method of splicing solid “I” spars.
in the same storage area as th e part to be repaired, for no less than 2 weeks.
Preventing Cracks. An important step in the prevention of longitudinal cracking, particularly in spar butts, is to ensure that the wood is thoroughly sealed with a penetrating and highly moisture-resistant finish.
Application of a thin, slow-curing epoxy adhesive or sealer can be very effectiv e in slowing or preventing moisture changes in spar butts.
ELONGATED HOLES IN SPARS. In cases of elongated bolt holes in a spar, or cracks in the vicinity of bolt holes, splice in a new section of spar, or replace the spar entirely. If hole elongation or cracking is minimal and the bolt holes are for noncritical fittings, repair (rather than replacement) may be feasible. Obtain approval for any such repair from the manufacturer or a representative of the FAA. In many cases, it has been found advantageous to laminate the new section of the spar, particularly if the spar butts are being replaced.
RIB REPAIRS. Ribs may be replaced by new parts made by the manufacturer or the holder of a PMA for that part. Owner-produced ribs may be installed providing they are made from a manufacturer-approved drawing or by reference to an existing original rib. A rib may be made by reference to an existing rib providing sufficient evidence is presented to verify that the existing rib is an original part and that all materials and dimensions can be determined. The contour of the rib is important to the safe flying qualities of the aircraft, and care should be taken that any replacement ribs accurately match the manufacturer’s original design.