Because the skin is fastened to the stringers, it is often impossible to drill the rivet holes for the repair splices with the common air drill. These holes can be drilled with an angle drill. When riveting a stringer, it may be necessary to use an offset rivet set and various shaped bucking bars.
Former or Bulkhead Repairs Bulkheads are the oval-shaped members of the fuselage which give form to and maintain the shape of the structure. Bulkheads or formers are often called forming rings, body frames, circumferential rings, belt frames, and other similar names. They are designed to carry concentrated stressed loads.
There are various types of bulkheads. The most common type is a curved channel formed from sheet stock with stifleners added. Others have a web made from sheet stock with extruded angles riveted in place as stiffeners and flanges. Most of these members are made from aluminum alloy. Corrosion-registant steel formers are used in areas which are exposed to high temperatures.
Bulkhead damages are classified in the same manner as other damages. Specifications for each type of damage are established by the manufacturer and specific information is given in the maintenance manual or structural repair manual for the aircraft.
Bulkheads are identifed with station numbers, which are very helpful in locating repair information.
Repairs to these members are generally placed in one of two categories: (1) One-third or less of the cross sectional area damaged, or (2) more than one-third of the cross sectional area damaged. If one-third or less of the cross sectional area has been damaged, a patch plate, reinforcing angle, or both, may be used. First, clean out the damage and then use the rivet formula to determine the number of rivets required in order to establish the size of the patch plate. For the length of the break, use the depth of the cutout area plus the length of the flange.
If more than one-third of the cross sectional area is damaged, remove the entire section and make a splice repair (figure 5-79). When removing the damaged section, be careful not to damage the surrounding equipment, such as electrical lines, plumbing, instruments, and so forth. Use a hand file, rotary file, snips, or a drill to remove larger damages. To remove a complete section, use a hacksaw, keyhole saw, drill, or snips.
Measure the length of break as shown in figure 5-79 and determine the number of rivets required by substituting this value in the rivet formula. Use the double shear value of the rivet in the calculations. The result represents the number of rivets to be used in each end of the splice plate.
Most repairs to bulkheads are made from flat sheet stock if spare parts are not available. When fabricating the repair from flat sheet, remember that the substitute material must provide cross sectional tensile, compressive, shear, and bearing strength equal to the original material. Never substitute material which is thinner or has a cross sectional area less than the original material.
Curved repair parts made from flat sheet must be in the "O" condition before forming, and then must be heat treated before installation.
Longeron Repair Generally, longerons are comparatively heavy members which serve approximately the same function as stringers. Consequently, longeron repair is similar to stringer repair. Because the longeron is a heavy member and more strength is needed than with a stringer, heavy rivets will be used in the L = F+ W+F or = 2F + W
• Cross section of stringer FIGURE 5-79. Deterinining length of break.
repair. Sometimes bolts are used to install a longeron repair; but, because of the greater accuracy required, they are not as suitable as rívets. Also, bolts require more time for installation.
If the longeron consists of a formed section and an extruded angle section, consider each section separately. Make the longeron repair as you would a stringer repair. However, keep the rivet pitch between four- and six-rivet diameters. If bolts are used, drill the bolt holes for a light drive fit.
Spar Repair The spar is the main supporting member of the wing. Other components may also have supporting members called spars which serve the same function as the spar does in the wing. Think of spars as the "hub" or "base" of the section in which they are located, even though they are not in the center. The spar is usually the first member located during the construction of the section, and the other components are fastened directly or indirectly to it.
Because of the load the spar carries, it is very important that particular care be taken when repairing this member to ensure that the original strength of the structure is not impaired. The spar is so constructed that two general classes of repairs, web repairs and cap strip repairs, are usually necessary.
For a spar web butt splice, first clean out the damage; then measure the full width of the web section. Determine the number of rivets to be placed in each side of the splice plate by substituting this value for the length of break in the rivet formula.
Prepare an insert section of the same type material and thickness as that used in the original web.
Make a paper pattern of the rivet layout for the splice plate using the same pitch as that used in the attachment of the web to the cap strip. Cut the splice plates from sheet stock having the same weight as that in the web, or one thickness heavier, and transfer the rivet layout from the paper pattern to the splice plates.
Give all contacting surfaces a corrosion-resistant treatment and rivet the component parts of the repair into place. The rivets used in attaching the insert section to the cap strips are in addition to those calculated for attaching the splice plates. Replace all web stiffeners removed during the repair.
An exploded view of a spar web butt splice is shown in figure 5-80.
Top view of butt splice Cap strip Insert Splice plate FIGURE 5-80. Spar web butt splice.
When making a spar web joggle splice, no splice plates are needed. Instead, form the web repair section so that it overlaps the original web suffciently to accommodate the required number of rivets. Make a joggle in each end of the repair section so that the repair piece contacts the cap strips to which it is riveted. Rivet calculation for this repair is similar to that described for butt splicing.
Many forms of cap strips are used in aircraft manufacturing, and each equires a distinct type of repair. In calculating the number of rivets required in an extruded T-spar cap strip repair, take the width of the base of the T, plus the length of the leg as the length of the break, and use double shear values.
Place one-fourth of the required number of rivets in each row of original rivets in the base of the T-section. Locate them midway between each pair of the original rivets. Locate the remainder of the rivets along the leg of the T-section in two rows.
Consider all original rivets within the area of the splice as part of the required rivets.
Make the filler piece of a similar piece of T-section extrusion or of two pieces of flat stock. It is possible to make the splice pieces of extruded angle material or to form them from sheet stock; in either case, they must be the same thickness as the cap strip. Figure 5-81 shows an exploded view of a Insert T-spar cap strip repair. The rivets used in the leg of the cap strip may be either the round-, flat-, or brazier-head type; but the rivets used in the base must be the same type as those used in the skin.
The repair of milled cap strips is limited to damages occurring to flanges. Damages beyond flange areas require replacement of the entire cap strip. To make a typical flange repair, substitute the depth of the trimmed-out area as the length of break in the rivet formula and calculate the number of rivets required. Form a splice plate of the required length and drill it to match the original rivet layout. Cut an insert to fit the trimmed-out area and rivet the repair in place. If the trimmed-out area is more than 4 in. in length, use an angle splice plate to provide added strength.
Rib and Web Repair Web repairs can be generally classified into two types: (1) Those made to web sections considered critical, such as those in the wing ribs, and (2) those considered less critical, such as those in elevators, rudders, flaps, and the like. Web sections must be repaired in such a way that the original strength of the member is restored.
In the construction of a member using a web (figure 5-82), the web member is usually a lightgage aluminum alloy sheet forming the principal depth of the member. The web is bounded by heavy aluminum alloy extrusions known as cap strips.
These extrusions carry the loads caused by bending Cap strip stiffeners Depth Web Splice piece Spar FIGURE 5-81. T-spar cap strip repair. FIGURE 5-82. Construction of a web member.
Stamped beads. Damage to ribs and webs which require a repair larger than a simple plate will probably need a JOIC patch plate, splice plates, or angles and an inser- tion. To repair such a damage by forming the necessary parts may take a great deal of time; therefore, if damaged parts which have the necessary areas intact are available from salvage, use them.
For example, if an identical rib can be located in salvage and it has a cracked web but the area in question is intact, clean out the damaged area; then cut the repair piece from the rib obtained from salvage. Be sure to allow plenty of material for correct rivet installation. Using a part from salvage will eliminate a great deal of hard work plus the heat-treating operation needed by a new repair piece.
FIGURE 5-83. Stamped beads in a web.
and also provide a foundation for attaching the skin. The web may be stiffened by stamped beads, formed angles, or extruded sections riveted at regular intervals along the web.
The stamped beads (figure 5-83) are a part of the web itself and are stamped in when the web is made. Stiffeners help to withstand the compressive loads exerted upon the critically stressed web members.
Often ribs are formed by stamping the entire piece from sheet stock. That is, the rib lacks a cap strip, but does have a flange around the entire piece, plus lightening holes in the web of the rib.
Ribs may be formed with stamped beads for stifieners, or they may have extruded angles riveted on the web for stiffeners.
Most damages involve two or more members;
however, it may be that only one member is damaged and needs repairing. Generally, if the web is damaged, all that is required is cleaning out the damaged area and installing a patch plate.
The patch plate should be of sufficient size to ensure room for at least two rows of rivets around the perimeter of the damage; this will include proper edge distance, pitch, and transverse pitch for the rivets. The patch plate should be of material having the same thickness and composition as the original member. If any forming is necessary when making the patch plate, such as fitting the contour of a lightening hole, use material in the "O" condition and then heat treat it after forming.
Leading Edge Repair The leading edge is the front section of a wing, stabilizer, or other airfoil. The purpose of the leading edge is to streamline the forward section of the wings or control surfaces so that the airflow is effective. The space within the leading edge is sometimes used to store fuel. This space may also house extra equipment such as landing lights, plumbing lines, or thermal anti-icing systems.
The construction of the leading edge section varies with the type of aircraft. Generally, it will consist of cap strips, nose ribs, stringers, and skin. The cap strips are the main lengthwise extrusions, and they stiflen the leading edges and furnish a base for the nose ribs and skin. They also fasten the leading edge to the front spar.
The nose ribs are stamped from aluminum alloy sheet. These ribs are U-shaped and may have their web sections stiffened. Regardless of their design, their purpose is to give contour to the leading edge.
Stiffeners are used to stiffen the leading edge and supply a base for fastening the nose skin. When fastening the nose skin, use only flush rivets.
Leading edges constructed with thermal anti-icing systems consist of two layers of skin separated by a thin air space. The inner skin, sometimes corrugated for strength, is perforated to conduct the hot air to the nose skin for anti-icing purposes.
Damage to leading edges are also classified in the same manner as other damages. Damage can be caused by contact with other objects, namely, pebbles, birds in flight, and hail. However, the major 197
cause of damage is carelessness while the aircraft countersunk rivets and forming countersunk shop is on the ground. heads to get a smooth contour.
A damaged leading edge will usually involve several structural parts. Flying-object damage will probably involve the nose skin, nose ribs, stringers, and possibly the cap strip. Damage involving all of these members will necessitate installing an access Insert door to make the repair possible. First, the damaged area will have to be removed and repair procedures established. The repair will need insertions and splice pieces. If the damage is serious enough, it may require repair of the cap strip and stringer, a new nose rib, and a skin panel. When repairing a leading edge, follow the procedures prescribed in the appropriate repair manual for this type of repair. FiLer Trailing Edge Repair A trailing edge is the rearmost part of an airfoil, found on the wings, ailerons, rudders, elevators, and stabilizers. It is usually a metal strip which forms the shape of the edge by tying the ends of a rib section together and joining the upper and lower skins. Trailing edges are not structural members, but they are considered to be highly stressed in all cases.
Damage to a trailing edge may be limited to one point or extended over the entire length between two or more rib sections. Besides damage resulting from collision and careless handling, corrosion damage is often present. Trailing edges are particularly subject to corrosion because moisture collects or is trapped in them.
Thoroughly inspect the damaged area before starting repairs, and determine the extent of damage, the type of repair required, and the manner in which the repair should be performed. When making trailing edge repairs, remember that the repaired area must have the same contour and be made of material with the same composition and temper as the original section. The repair must also be made to retain the design characteristics of the airfoil.
Damage occurring in the trailing edge section between the ribs can be repaired as shown in figure
5-84. Cut out the damaged area and make a filler of either hardwood, fiber, or cast aluminum alloy to fit snugly inside the trailing edge. Then make an insert piece of the same material as the damaged section and shape it to match the trailing edge, Assemble the pieces as shown and rivet them into place using FIcURE 5-84. Trailing edge repair between ribs.
To repair damage occurring at or near a rib, first remove sufficient trailing edge material to allow a complete splice to fall between the ribs. This usually requires two splices joined by an insert piece of similar trailing edge material or of formed sheet stock. The repair procedure is similar to that for damage between ribs. Figure 5-85 shows this type of repair.
STRUCTURAL SEALING Various areas of airframe structures are sealed compartments where fuels or air must be confined Some of these areas contain fuel tanks; others consist of pressurized compartments such as the cabin.
Because it is impossible to seal these areas completely airtight with a riveted joint alone, a sealing compound or sealant must be used. Sealants are also used to add aerodynamic smoothness to exposed surfaces such as seams and joints in the wings and fuselage.
Three types of seals are ordinarily used. Rubber seals are installed at all points where frequent breaking of the seal is necessary, such as emergency exits and entrance doors. Sealing compounds are used at points where the seal is seldom broken 198
be detected by the change in sound when the instrument passes over it. After completing the test, rensert move the soapless bubble solution from the outside of the aircraft by washing with clear water to prevent corrosion.
Here are a few precautionary measures to follow during the testing procedure just discussed. With personnel inside, the area should never be pressurized to a pressure higher than has previously been established during testing with the section empty.
No person who has a cold or who has recently had one, or whose sinuses are impaired in any way, should work in the pressurized section. A qualified operator should be present at the pressurization equipment control panel at all times while the sec- Slug tion is being pressurized.
Pressurization may not always be necessary to FIcURE 5-85. Trailing edge repair near a rib. determine defectively sealed areas. Sealants should be repaired when:
except for structural maintenance or part replacement, as with riveted lap and butt seams. Special seals are required for passing cables, tubing, mechanical linkages, or wires out of the pressurized or sealed areas.
Wires and tubes are passed through pressure bulkheads by using bulkhead fittings such as cannon plugs for wiring and couplings for tubing.
These fittings are sealed to the bulkhead and the wires and tubes are fastened to them from each side. All seals of moving components such as flight controls are subject to wear and utmost care must be used when they are installed. Also, they must be checked regularly.
(1) Sealants have peeled away from the structure.
(2) Seams are exposed through the sealant fillet.
(3) Fillet- or hole-filling sealant is exposed through the smooth overcoating.
(4) Sealant is damaged by the removal and re-installation of fasteners, access doors, or other sealed parts.
(5) Cracks or abrasions exist in the sealant.
Determining Sealant Defects Pressure tightness of an area or section is checked before and after a repair is made. Ground pressurization is accomplished by filling the section with air from an external source through ground pressure test fittings.
With the sections pressurized to a given pressure, locate leaks on the outside of the aircraft by applying a soapless bubble solution to all seams and joints in the suspected area. Air bubbles will locate the general area of leakage. A specific leak is then isolated on the inside of the aircraft by passing the free end of a stethoscope or similar listening device along the seams in the leakage area. The leak can Sealant Repair All surfaces which are to be sealed must be cleaned to ensure maximum adhesion between the sealant and the surface. Loose foreign material can be removed by using a vacuum cleaner on the affected area. Scrape all the old sealant from the repair area with a sharp plastic, phenolic, or hardwood block to prevent scratches, and apply a stripper and a cleaner.
The cleaner should not be allowed to dry on a metal surface, but should be wiped dry with clean rags. Do not remove the cleaner with soiled rags since the metal surface must be free of all dirt, grease, powder, and so forth. The surface can be checked for cleanliness by pouring water over it after being wiped dry of the cleanser. If the surface 199
is not free of oily film, the water will separate into small droplets.
Be extremely careful to protect any undamaged sealant and acrylic plastics from the stripper compound. If artificial lighting is used when the repair is made, be sure the light is of the explosion proof type. Wear clothing which affords adequate protection from the stripper and cleaner so that these chemicals cannot contact the skin. Provide adequate ventilation in the work area. Personnel should wear a respirator when working in an enclosed area.
It may be necessary to replace rubber seals periodically to ensure tight door closure. Seals of this type should be replaced any time there is any degree of damage. Such a seal is usually not reparable because it must be continuous around the opening.
To remove the old seal, remove all the seal retainers from the frame and then pull off the old seal.
Use aliphatic naphtha and clean rags to clean the frame on which the new seal is to be cemented.
Cleaning should be done immediately before sealer installation. Then, using a clean paint brush, apply an even coat of rubber cement upon the metal parts and the seal surfaces which are to be joined.
Allow the rubber cement to dry until it becomes quite sticky. Then join the seal to the metal by pressing it firmly along all contact points. Install the seal retainers and allow the seal to set for at least 24 hrs. before using.
Toluene may be used for cleaning brushes and other equipment used in applying rubber cement. If the rubber cement needs thinning, use aliphatic naphtha.
Seals on pressurized sections must be able to withstand a certain amount of pressure. Therefore, damage to the seals in the compartment or section must be repaired with this question in mind: Can it withstand the pressures required? Pressure sealing must be performed on the pressurized side of the surface being sealed. Make sure that all areas are sealed before completing further assembly operations which would make the area inaccessible.
Sealing compounds should be applied only when the contacting surfaces are perfectly clean. The compound should be spread from the tube by a continuous forward movement to the pressure side of the joint. It is advisable to start the spreading of the compound 3 in. ahead of the repair area and continue 3 in. past it. If the compound is in bulk form, apply it with a pressure gun. Two coats or layers of compound are often required. If this is necessary, let the first application cure before the second is applied. Allow the compound to cure until it becomes tough and rubbery before joining the surfaces.
Curing time varies with temperature. High temperatures shorten the curing time and low temperatures lengthen it. Artificial heat may be used to speed up curing, but care must be used to avoid damaging the sealant with too high a temperature.
Warm circulating air, not over 120°F., or infrared lamps placed 18 in. or more from the sealants are satisfactory heat sources. If infrared lamps are used, adequate ventilation must be provided to carry away the evaporated solvents.
Sealing compounds are most generally used on seams and joints, but they may also be used to fill holes and gaps up to 16 in. wide.
Impregnated zinc chromate tape is sometimes used between seams and joints. Sealing tape is also used as a backing strip over holes and gaps which are 16 to ½ in. in width. The tape is applied over the opening on the pressure side, and a fillet of sealing compound is applied over the tape. Holes and gaps over 16 in, in width are usually plugged with wood, metal caps, or metal plugs on the pressure side of the area; then, impregnated tape and sealing compound are applied over the repair.
Be sure that all forming, fitting, and drilling operations have been completed before applying the tape. With the repair surface area clean, unroll the tape with the white cloth innerleaf away from the metal surface. Leave the innerleaf on the tape until just before the parts are assembled. There must be no wrinkles in the tape, and the parts must be joined together with the least possible amount of sideways motion.
The application of putty sealant is similar to that of sealing compounds. A spatula or sharp-pointed plastic, phenolic, or hardwood block is sometimes used to force and pack the putty into the gaps or seams. Clean the gap or seam with compressed air before applying the putty to the pressure side.
Rivets, bolts, or screws do not always seal properly when used in these critical areas or sections.
When pressure leaks occur around the fasteners, they should be removed and replaced. The holes should be filled with sealing compound and new fasteners installed. Remove excess sealant as soon as possible to avoid the difficulty encountered after it becomes cured.
METAL BONDED HONEYCOMB The introduction of bonded honeycomb (sandwich construction) members in airframe design and 200
manufacture came as a major breakthrough in the search for a more efficient type of structure. Because bonded honeycomb structures are manufactured and perform their jobs in a manner different from the previously used and more familiar conventional structures, new attitudes and methods of repair had to be developed with respect to the advantages, limitations, and physical peculiarities.
CONSTRUCTION FEATURES Sandwich construction design is governed by the intended use of the panel or structure. It can be defined as a laminar construction consisting of a combination of alternating dissimilar materials, assembled and fixed in relation to each other so that the properties of each can be used to attain specific structural advantages for the whole assembly.
Sandwich-constructed assemblies can be found in a, variety of shapes and sizes on modern aircraft.
They may consist of a whole section or a series of panels combined into an assembly. Sandwich-constructed assemblies are used for such areas as bulkheads, control surfaces, fuselage panels, wing panels, empennage skins, radomes, or shear webs.
Figure 5-86 illustrates a section of bonded honeycomb. The honeycomb stands on end and separates facings which are bonded to the core by means of an adhesive or resin. This type of construction has a superior strength/weight ratio over that of conventional structures. Also, it is better able to withstand sonic vibration, has relatively low cost when compared with fastener cost and installation of conventional structures, reduces the number of parts needed, and greatly reduces sealing problems while increasing aerodynamic smoothness.
Facing
- Honeycomb core Bond line • Facing FIGURE 5-86. Bonded honeycomb section.
Special applications of metal-bonded honeycomb may employ stainless steel, titanium, magnesium, plywood, resin-impregnated paper, glass, nylon, or cotton cloth in various combinations.
DAMAGE Causes of Damage The majority of damages to bonded honeycomb assemblies result from flight loads or improper ground handling. Honeycomo structures may also be damaged by sonic vibrations. Such damage is usually a delamination or separation of the core and face along the bond line. (The bond line is the thin line of adhesive between the core and the face that holds the two together.) Occasionally the core may collapse.
Damage Inspection Inspection for damage is more critical for honeycomb assemblies than for conventional structures. A honeycomb structure can suffer extensive damage without any observable indication. Sonic vibration, liquid leakage, internal condensation, or a misstep in manufacture or repair can cause or result in varied amounts of delamination.
The metallic ring test is the simplest way to inspect for delamination damage. When a coin (25- cent piece) is lightly bounced against a solid structure, a clear metallic ring should be heard. If delamination is present, a dull thud will be heard. A 1-oz. aluminum hammer makes an excellent tool for this type of inspection.
Occasionally, the delaminated skin will "oilcan" away from the core, making visual or thumb pressure detection possible. Punctures, dents, scratches, cracks, or other such damage may be inspected by conventional methods. Scratches should be given special attention since, with such thin material as that used in the metal bonded honeycomb, the scratch may actually be a crack.
A caustic soda solution can be used for testing scratches on aluminum surface panels. If the scratch area turns black after the application of a small amount of the solution, the scratch has penetrated through the clad surface. Caustic soda solutions are highly corrosive and must be handled with extreme care. Thoroughly neutralize the area after application of the solution.
Two additional instruments used in damage inspection of bonded panels are the panel inspection analyzer and the borescope.
Damage Evaluation After inspections on metal bonded honeycomb structures are completed, any damage found must be evaluated to determine the type of repair needed to make the structure serviceable.
Damage to aluminum honeycomb structures can vary from minor dents or scratches to total panel destruction. Damage evaluation charts for honeycomb structures can be found in the applicable section of the structural repair manual for the specific aircraft. The charts specify types of damage, allow.
able damage, damage requiring repair, and figure numbers that illustrate similar repairs for each type of damage.
Once the type of repair is determined, procedures outlined in the structural repair manual should be rigidly followed.
REPAIRS Recommendations for the type of repair to make and the methods or procedures to use vary among the different aircraft manufacturers. Tools, materials, equipment, and typical repairs that might be made on metal bonded honeycomb structures will be discussed in the following paragraphs.
Tools and Equipment Effective repairs to bonded honeycomb assemblies depend largely on the knowledge and skill of the airframe mechanic in the proper use and maintenance of the tools and equipment used in making bonded honeycomb repairs. The design and high quality of workmanship built into these tools and equipment make them unique in the repair of bonded honeycomb assemblies. Therefore, it is essential that the techniques and procedures established for each tool or piece of equipment be known and practiced. Both personal injury and additional damage to the area being repaired can then be avoided.
Router The primary tool used to prepare a damaged honeycomb area for repair is a pneumatically powered, hand-operated router with speeds ranging from 10,000 to 20,000 r.p.m. The router is used in conjunction with the support assembly, bit, and template as shown in figure 5-87.
The router support assembly threads onto the router body. It has provisions to adjust the desired depth of the cut with a locking (clamping nut) mechanism which secures the depth adjustment in place. One complete turn of the support adjustment changes the depth of cut approximately 0.083 in.
Metal-cutting, ¼-in. mill bits are used with the router for removing the damaged areas. The router bits should be kept sharp, clean, and protected against nicks, breakouts, or other damages.
- Router Router Router support template FIcURE 5-87. Router, support assembly, and template.
Router templates are used as guides when removing damaged honeycomb areas with a router. They can be designed and manufactured to the desired sizes, shapes, or contours of the repair. As an example, the multi-template (figure 5-88) can be used as a guide when cutting holes from ½ in. to 6 in. in diameter. For larger holes, a template can be manufactured locally from aluminum alloy 0.125-in.
thick, whereas smaller holes can be cleaned out without the use of a template. The multi-templates should be kept clean and lightly oiled to prevent rusting and to maintain smooth operation during their use.
A routing template may be applied to a flat surface using the following 'procedures:
1,000 Routing template Bridge Upper facing Damaged area FIGURE 5-88. Multi-template. Lower facing
(1) Apply double-backed tape to the edge or FIGURE 5-89. Wedge blocks and application.
rim of the routing template.
(2) Place the template, centering the desired Routing of Damaged Areas cutout guide hole directly over the dam- aged area. When the extent of damage and type of repair surface by using the follwoing procedures: A routing template may be applied to a tapered (1) Manufacture a bridge consisting of two (3) Press the template firmly down over the and with approximately the same degree of angle as that of the panel. (See figure 5-89. wooden wedge blocks at least 6 in. long secured in place; this will avoid any creeping or misalignment of the template during the routing operation. double-backed tape, making sure that it is it from jumping or creeping. When the router is not should be firmly gripped with both hands to prevent or misalignment of the template. approximate depth required to remove the damaged area. During the routing operations, the router the damage will be removed. The double-backed aged area in such a way that it will ensure that all around the damaged area, thus preventing creeping tape is used to secure the template to the surfaces have been determined, the proper size hole of the router template must be located around the dam- The router bit should be adjusted and set for the
(5) Place another strip of double-backed tape (4) Press the wedge blocks firmly in place. (2) Apply a strip of double-backed tape to one (3) Place a wedge block on each side of the during the routing operation. bridge and support the template properly on the top side of each wooden wedge side of each wedge block. damaged area in a position that will cable aircraft manufacturer. follow the repair techniques specified by the appli- and stored properly until it is to be used again. used by the various airframe manufacturers. Always in use, it should be disconnected from the air hose aged bonded honeycomb areas are typical of those The following procedures for the removal of dam- (1) Determine the extent of the damage. (2) Set up and adjust the router, router supblock. port assembly, and end-cutting mill bit for
(6) Place and align the template over the or misalignment of the template during wedge blocks, thus avoiding any creeping operation. (3) Select a routing template and position the the removal of the damaged area. procedures outlined in the discussion on template over the repair area according to templates.
(4) Attach the router's air intake plug in the socket of the air supply.
(5) Accomplish the routing operation.
(8) Use face shield or goggles for eye protection against flying materials moved.
(b) Place the air hose over the shoulder.
(c) Holding the router firmly and at a 45° angle to the surface, place one edge of the router support assembly against the edge of the routing template.
(d) Start the router by depressing the control lever.
(e) Carefully, but firmly, lower the end-cutting mill bit into the material as close as possible to the center of the damaged area to be removed.
(f) Straighten the router to be perpendicular to the surface.
(g) Holding the router firmly, spiral it clockwise to the outer limits of the template's guide hole, removing all the damaged material.
(h) Release the control lever, allow the router to stop, and remove it from the hole.
Disconnect the router from the air supply.
Check the damaged area removed. If additional removal is required, adjust the router's cutting depth and repeat routing operation.
(6) Upon completion of the routing operation, disconnect and clean the routing equipment.
During the routing operations, the aluminum core cells of a damaged honeycomb area tend to bend or close up. Therefore, they must be opened with tweezers and a pen knife before any further attempts to repair are made. At times, the core cells must be trimmed with a pen knife to the shape of the repair hole.
Pressure Jigs Pressure jigs are used to apply pressure to repairs on the under surfaces of honeycomb panels or assemblies to hold the repair materials and resins in place. The pressure is maintained on the repair area until the repair material is cured.
C-clamps, locally manufactured jigs, or vacuum fixtures may be used to apply the necessary pressure to bonded honeycomb repairs.
The surfaces around the repair area must be absolutely clean and free of any foreign materials to ensure a good vacuum when vacuum fixtures or suction types of equipment are used. An application of water or glycerin to the surface areas will aid in obtaining a good vacuum. Normal cleaning, care, and corrosion prevention will maintain the above equipment in good working condition.
Infrared Heat Lamps Infrared heat lamps are used to shorten the curing time of bonded honeycomb repairs from approximately 12 hrs. to 1 hr. A single-bulb lamp will adequately cure a repair up to 6 in. in diameter, but a large repair may require a battery of lamps to ensure uniform curing of the repair area.
The lamps should be centered directly over the repair at a distance of about 30 in. The setup is ideal to attain the recommended 130° F. curing temperature, provided the surrounding areas are at room temperature (70° F.). Warmer or colder sur rounding areas will require that the heat lamps be adjusted to the prevailing condition. Caution must be used when working under extremely cold conditions, since a temperature differential of 150° or more will cause buckling of the surrounding skin surfaces because of thermal expansion.
As with any ordinary light bulbs, the infrared bulbs require little or no maintenance; however, the support stands, wiring, and switches should be handled carefully and maintained properly.
Fire Precautions The potential of a fire hazard generally exists in the area of bonded honeycomb repairs because of the low flash point of the repair materials, such as cleaning solvents, primers, and resins. Therefore, it is necessary that all fire precautions be observed closely. Certain fire safety prevention equipment, such as utility cans, flammable-waste cans, and vapor- and explosion-proof lights, should be used.
With the potential of fire hazards in a honey.
comb repair area, it is necessary to make sure that a suitable fire extinguisher is on hand or is located nearby and ready for use, if necessary. The extinguishing agents for all the materials used in bonded honeycomb repairs are dry chemicals or carbon dioxide; thus, the standard CO, fire extinguishers should be on hand for use in areas where bonded honeycomb structures will be repaired.
Handtools and Equipment In addition to the tools and equipment described 204
in the preceding paragraphs, standard handtools and shop equipment are utilized in the repair of bonded honeycomb structures. Standard handtools and shop equipment used in the shop include an airframe mechanic's tool kit, face shields, scissors, power shear, drill press, horizontal and vertical belt sanders, contour metal-cutting saw, and pneumatic hand drills. The general uses and maintenance of these standard tools and equipment should be famil iar to any airframe mechanic.
REPAIR MATERIALS Cleaning Solvents Before any repair is made to a bonded honeycomb structure, an area extending several inches away from the damage must be cleaned thoroughly of all paint or surface coating. This is best accomplished by the use of paint remover or MEK (methyl-ethyl-ketone) cleaning solvent, In some cases, Alconox, a powerful wetting and detergent agent, may be used for a final cleanup to remove any residue or oils remaining after application of the paint remover or the MEK cleaning solvent.
Paint removers are applied with a suitable size brush. When the paint or surface coating has loosened, it is either wiped off with a clean rag or removed with a nonabrasive scraper. Paint remover must not be allowed to enter the damaged area or be used along a bonded joint or seam because its chemical action will dissolve the bonding adhesive.
These areas should be masked and final cleanup accomplished with the MEK cleaning solvent or emery cloth. The MEK cleaning solvent and the Alconox cleaning agent may be applied with a clean sponge.
After a damaged area has been completely removed, the surrounding surface areas must be thoroughly re-cleaned. This is accomplished by the use of the MEK cleaning solvent and gauze sponges.
The MEK cleaning solvent is applied to the area with one sponge and immediately wiped off with another before it has had time to dry. This cleaning process should be continued until the surface area is lusterous in appearance and clean of any foreign matter.
To determine whether an area is completely and thoroughly clean, a water "break" test can be used.
This test is a simple application of a thin film of distilled water to the cleaned area. Any "break" of the applied thin film of distilled water will indicate that the area has not been cleaned thoroughly enough and the cleaning process must be repeated.
Safety precautions must be closely observed when working with the above solvents, especially when the work is overhead or when working in confined areas. For personal protection, rubber gloves, face shields, adequate ventilation, and respirators should be worn. A CO, fire extinguisher should be on hand or nearby and ready for use if necessary.
Primers Primers are applied to the cleaned surface areas primarily to ensure a good bond of the honeycomb repairs. The primer is applied to the cleaned surface areas with a clean gauze sponge or suitable brush.
It is recommended that the primer be applied as rapidly as possible because it will become tacky in 10 to 15 sec., and it will pull and be ruined by any further brushing. The primer will cure in approximately 1 hr. at room temperature; however, this time may be reduced by the application of controlled heat.
Adhesives and Resins Two types of adhesives presently used in the repairs of bonded honeycomb structures of some aircraft are known as the type 38 adhesive and the potting compound. The type 38 adhesive is applied to glass fabric overlay repairs, and the potting compound, as the name implies, to the potted compound (hole filling) repairs. In addition, the type 38 adhesive may be used as an alternate for the potting compound by adding micro-balloons (microscopic phenolic). The adhesives or potting compounds are prepared according to a batch mix (amount required for the repair) formula. The batch mix Accurate mixing of the adhesive ingredients by batches is considered one of the more important steps in the repair of bonded honeycomb structures.
The correct proportions of the epoxies, resins, and micro-balloons to be mixed into batches, both by weight and/or by volume, are given in the applicable section of the structural repair manual for the Core Material Fiberglass honeycomb core materials (16 in, cell size) are usually used to replace the damaged aluminum cores of the bonded honeycomb structures.
Aluminum core materials are not satisfactory for the repairs because of their flimsy and fragile structure. With this condition, it is impractical to cut the aluminum core materials accurately to the desired repair size. Fibergiass core materials are available 205