Text-only reference. Published from the recorded official FAA Airframe Chapter 4 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
introduced during the manufacture of rod, bar or tubing by rolling or forging. • Nick—local break or notch on an edge. Usually it involves the displacement of metal rather than loss. • Pitting—sharp, localized breakdown (small, deep cavity) of metal surface, usually with defined edges. • Scratch—slight tear or break in metal surface from light, momentary contact by foreign material. • Score—deeper (than scratch) tear or break in metal surface from contact under pressure. May show discoloration from temperature produced by friction. • Stain—a change in color, locally causing a noticeably different appearance from the surrounding area.
• Upsetting—a displacement of material beyond the normal contour or surface (a local bulge or bump). Usually it indicates no metal loss. Classification of Damage Damages may be grouped into four general classes. In many cases, the availabilities of repair materials and time are the most important factors in determining if a part should be repaired or replaced. Negligible Damage Negligible damage consists of visually apparent, surface damage that do not affect the structural integrity of the component involved. Negligible damage may be left as is or may be corrected by a simple procedure without restricting flight. In both cases, some corrective action must be taken to keep the damage from spreading. Negligible or minor damage areas must be inspected frequently to ensure the damage does not spread. Permissible limits for negligible damage vary for different components of different aircraft and should be carefully researched on an individual basis. Failure to ensure that damages within the specified limit of negligible damage may result in insufficient structural strength of the affected support member for critical flight conditions.
Small dents, scratches, cracks, and holes that can be repaired by smoothing, sanding, stop drilling, or hammering out, or otherwise repaired without the use of additional materials, fall in this classification. [Figure 4-170] Damage Repairable by Patching Damage repairable by patching is any damage exceeding negligible damage limits that can be repaired by installing splice members to bridge the damaged portion of a structural part. The splice members are designed to span the damaged areas and to overlap the existing undamaged surrounding structure. The splice or patch material used in internal riveted and bolted repairs is normally the same type of material as the damaged part, but one gauge heavier. In a patch repair, filler plates of the same gauge and type of material as that in the damaged component may be used for bearing purposes or to return the damaged part to its original contour. Structural fasteners are applied to members and the surrounding structure to restore the original load-carrying characteristics of the damaged area. The use of patching depends on the extent of the damage and the accessibility of the component to be repaired.
Damage Repairable by Insertion Damage must be repaired by insertion when the area is too large to be patched or the structure is arranged such that repair members would interfere with structural alignment (e.g., in a hinge or bulkhead). In this type of repair, the damaged portion is removed from the structure and replaced by a member identical in material and shape. Splice connections at each end of the insertion member provide for load transfer to the original structure. Damage Necessitating Replacement of Parts Components must be replaced when their location or extent of damage makes repair impractical, when replacement is more economical than repair, or when the damaged part is relatively easy to replace. For example, replacing damaged castings, forgings, hinges, and small structural members, when available, is more practical than repairing them. Some highly stressed members must be replaced because repair would not restore an adequate margin of safety.
Repairability of Sheet Metal Structure The following criteria can be used to help an aircraft technician decide upon the repairability of a sheet metal structure: • Type of damage. Crack Stop-drill cracks 4-90 • Type of original material. • Location of the damage. • Type of repair required. • Tools and equipment available to make the repair. The following methods, procedures, and materials are only typical and should not be used as the authority for a repair. Structural Support During Repair During repair, the aircraft must be adequately supported to prevent further distortion or damage. It is also important that the structure adjacent to the repair is supported when it is subject to static loads. The aircraft structure can be supported adequately by the landing gear or by jacks where the work involves a repair, such as removing the control surfaces, wing panels, or stabilizers. Cradles must be prepared to hold these components while they are removed from the aircraft.
When the work involves extensive repair of the fuselage, landing gear, or wing center section, a jig (a device for holding parts in position to maintain their shape) may be constructed to distribute the loads while repairs are being accomplished. Figure 4-171 shows a typical aircraft jig. Always check the applicable aircraft maintenance manual for specific support requirements. Assessment of Damage Before starting any repair, the extent of damage must be fully evaluated to determine if repair is authorized or even practical. This evaluation should identify the original material used and the type of repair required. The assessment of the damage begins with an inspection of riveted joints and an inspection for corrosion.
Inspection of Riveted Joints Inspection consists of examining both the shop and manufactured heads and the surrounding skin and structural parts for deformities. During the repair of an aircraft structural part, examine adjacent parts to determine the condition of neighboring rivets. The presence of chipped or cracked paint around the heads may indicate shifted or loose rivets. If the heads are tipped or if rivets are loose, they show up in groups of several consecutive rivets and are probably tipped in the same direction. If heads that appear to be tipped are not in groups and are not tipped in the same direction, tipping may have occurred during some previous installation.
Inspect rivets that are known to have been critically loaded, but that show no visible distortion, by drilling off the head and carefully punching out the shank. If upon examination, the 30 1/4 felt glued on 1/4 plywood both sides 2 X 3 Canvas or strong cloth tacked on to cover felt 2 X 8 4 8 24 16 30 5 1/45 1/4 2 3/4 CL 444 2 2 2 3/8 35/8 21/8 39/16 35/16 3 75/8 71/4 67/8 55/8 21/8 61/4 55/8 43/4 shank appears joggled and the holes in the sheet misaligned, the rivet has failed in shear. In that case, determine what is causing the stress and take necessary corrective action. Countersunk rivets that show head slippage within the countersink or dimple, indicating either sheet bearing failure or rivet shear failure, must be replaced.
Joggles in removed rivet shanks indicate partial shear failure. Replace these rivets with the next larger size. Also, if the rivet holes show elongation, replace the rivets with the next larger size. Sheet failures, such as tearouts, cracks between rivets, and the like, usually indicate damaged rivets, and the complete repair of the joint may require replacement of the rivets with the next larger size. The presence of a black residue around the rivets is not an indication of looseness, but it is an indication of movement (fretting). The residue, which is aluminum oxide, is formed by a small amount of relative motion between the rivet and the adjacent surface. This is called fretting corrosion, or smoking, because the aluminum dust quickly forms a dark, dirty looking trail, like a smoke trail. Sometimes, the thinning of the moving pieces can propagate a crack. If a rivet is 4-91 suspected of being defective, this residue may be removed with a general purpose abrasive hand pad, such as those manufactured by Scotch Brite™, and the surface inspected for signs of pitting or cracking. Although the condition indicates the component is under significant stress, it does not necessarily precipitate cracking. [Figure 4-172] Airframe cracking is not necessarily caused by defective rivets. It is common practice in the industry to size rivet patterns assuming one or more of the rivets is not effective.
This means that a loose rivet would not necessarily overload adjacent rivets to the point of cracking. Rivet head cracking is acceptable under the following conditions: • The depth of the crack is less than 1⁄8 of the shank diameter. • The width of the crack is less than 1⁄16 of the shank diameter. • The length of the crack is confined to an area on the head within a circle having a maximum diameter of 11⁄4 times the shank diameter. • Cracks should not intersect, which creates the potential for the loss of a portion of a head. Inspection for Corrosion Corrosion is the gradual deterioration of metal due to a chemical or electrochemical reaction with its environment.
The reaction can be triggered by the atmosphere, moisture, or other agents. When inspecting the structure of an aircraft, it is important to watch for evidence of corrosion on both the outside and inside. Corrosion on the inside is most likely to occur in pockets and corners where moisture and salt spray may accumulate; therefore, drain holes must always be kept clean. Also inspect the surrounding members for evidence of corrosion. Damage Removal To prepare a damaged area for repair: 1. Remove all distorted skin and structure in damaged area. 2. Remove damaged material so that the edges of the completed repair match existing structure and aircraft lines.
3. Round all square corners. 4. Smooth out any abrasions and/or dents. 5. Remove and incorporate into the new repair any previous repairs joining the area of the new repair. Repair Material Selection The repair material must duplicate the strength of the original structure. If an alloy weaker than the original material has to be used, a heavier gauge must be used to give equivalent cross-sectional strength. A lighter gauge material should not be used even when using a stronger alloy. Repair Parts Layout All new sections fabricated for repairing or replacing damaged parts in a given aircraft should be carefully laid out to the dimensions listed in the applicable aircraft manual before fitting the parts into the structure.
Rivet Selection Normally, the rivet size and material should be the same as the original rivets in the part being repaired. If a rivet hole has been enlarged or deformed, the next larger size rivet must be used after reworking the hole. When this is done, the proper edge distance for the larger rivet must be maintained. Where access to the inside of the structure is impossible and blind rivets must be used in making the repair, always consult the applicable aircraft maintenance manual for the recommended type, size, spacing, and number of rivets needed to replace either the original installed rivets or those that are required for the type of repair being performed.
Rivet Spacing & Edge Distance The rivet pattern for a repair must conform to instructions in the applicable aircraft manual. The existing rivet pattern is used whenever possible. Corrosion Treatment Prior to assembly of repair or replacement parts, make certain that all existing corrosion has been removed in the area and that the parts are properly insulated one from the other. Approval of Repair Once the need for an aircraft repair has been established, Title 14 of the Code of Federal Regulations (14 CFR) defines the approval process. 14 CFR part 43, section 43.13(a) states that each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in 4-92 the current manufacturer’s maintenance manual or instructions for continued airworthiness prepared by its manufacturer, or other methods, techniques, or practices acceptable to the Administrator. AC 43.13-1 contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions.
This data generally pertains to minor repairs. The repairs identified in this AC may only be used as a basis for FAA approval for major repairs. The repair data may also be used as approved data, and the AC chapter, page, and paragraph listed in block 8 of FAA Form 337 when: a. The user has determined that it is appropriate to the product being repaired; b. It is directly applicable to the repair being made; and c. It is not contrary to manufacturer’s data. Engineering support from the aircraft manufacturer is required for repair techniques and methods that are not described in the aircraft maintenance manual or SRM.
FAA Form 337, Major Repair and Alteration, must be completed for repairs to the following parts of an airframe and repairs of the following types involving the strengthening, reinforcing, splicing, and manufacturing of primary structural members or their replacement, when replacement is by fabrication, such as riveting or welding. [Figure 4-173] • Box beams • Monocoque or semimonocoque wings or control surfaces • Wing stringers or chord members • Spars • Spar flanges • Members of truss-type beams • Thin sheet webs of beams • Keel and chine members of boat hulls or floats • Corrugated sheet compression members that act as flange material of wings or tail surfaces • Wing main ribs and compression members • Wing or tail surface brace struts, fuselage longerons • Members of the side truss, horizontal truss or bulkheads • Main seat support braces and brackets • Landing gear brace struts • Repairs involving the substitution of material • Repair of damaged areas in metal or plywood stressed covering exceeding six inches in any direction • Repair of portions of skin sheets by making additional seams • Splicing of thin sheets • Repair of three or more adjacent wing or control surface ribs or the leading edge of wings and control surfaces between such adjacent ribs For major repairs made in accordance with a manual or specifications acceptable to the Administrator, a certificated repair station may use the customer’s work order upon which the repair is recorded in place of the FAA Form 337.
Repair of Stressed Skin Structure In aircraft construction, stressed skin is a form of construction in which the external covering (skin) of an aircraft carries part or all of the main loads. Stressed skin is made from high strength rolled aluminum sheets. Stressed skin carries a large portion of the load imposed upon an aircraft structure. Various specific skin areas are classified as highly critical, semicritical, or noncritical. To determine specific repair requirements for these areas, refer to the applicable aircraft maintenance manual. Minor damage to the outside skin of the aircraft can be repaired by applying a patch to the inside of the damaged sheet. A filler plug must be installed in the hole made by the removal of the damaged skin area. It plugs the hole and forms a smooth outside surface necessary for aerodynamic smoothness of the aircraft. The size and shape of the patch is determined in general by the number of rivets required in the repair. If not otherwise specified, calculate the required number of rivets by using the rivet formula. Make the patch plate of the same material as the original skin and of the same thickness or of the next greater thickness.
Patches Skin patches may be classified as two types: • Lap or scab patch • Flush patch Lap or Scab Patch The lap or scab type of patch is an external patch where the edges of the patch and the skin overlap each other. The overlapping portion of the patch is riveted to the skin. Lap patches may be used in most areas where aerodynamic smoothness is not important. Figure 4-174 shows a typical patch for a crack and or for a hole. When repairing cracks or small holes with a lap or scab patch, the damage must be cleaned and smoothed. In repairing cracks, a small hole must be drilled in each end and sharp 4-93 Federal Aviation Administration (As shown on registration certificate) 2. Owner 3. For FAA Use Only 5. Unit Identification 4. Type (As described in Item 1 above) 6. Conformity Statement 7. Approval for Return to Service 4-94 8. Description of Work Accomplished 4-95