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9/27/01 AC 43.13-1B CHG 1 SECTION 4. METAL REPAIR PROCEDURES 4-50. GENERAL. The airframe of a fixed- wing aircraft is generally considered to consist of five principal units; the fuselage, wings, stabilizers, flight control surfaces, and landing gear. a. Aircraft principal structural elements (PSE) and joints are designed to carry loads by distributing them as stresses. The elements and joints as originally fabricated are strong enough to resist these stresses, and must re main so after any repairs. Long, thin elements are called members. Some examples of mem bers are the metal tubes that form engine mount and fuselage trusses and frames, beams used as wing spars, and longerons and string ers of metal-skinned fuselages and wings. Longerons and stringers are designed to carry principally axial loads, but are sometimes re quired to carry side loads and bending mo ments, as when they frame cutouts in metal-skinned structures. Truss members are designed to carry axial (tension and compres sion) loads applied to their ends only. Frame members are designed to carry side loads and bending moments in addition to axial loads. Beam members are designed to carry side loads and bending moments that are usually large compared to their axial loads. Beams that must resist large axial loads, particularly compression loads, in combination with side loads and bending moments are called beam-columns. Other structural elements such as metal skins, plates, shells, wing ribs, bulk heads, ring frames, intercostal members, gus sets, and other reinforcements, and fittings are designed to resist complex stresses, sometimes in three dimensions. b. Any repair made on an aircraft struc ture must allow all of the stresses to enter, sustain these stresses, and then allow them to return into the structure. The repair must be equal to the original structure, but not stronger or stiffer, which will cause stress concentra tions or alter the resonant frequency of the structure. c. All-metal aircraft are made of very thin sheet metal, and it is possible to restore the strength of the skin without restoring its ri gidity. All repairs should be made using the same type and thickness of material that was used in the original structure. If the original skin had corrugations or flanges for rigidity, these must be preserved and strengthened. If a flange or corrugation is dented or cracked, the material loses much of its rigidity; and it must be repaired in such a way that will restore its rigidity, stiffness, and strength. 4-51. RIVETED (OR BOLTED) STEEL TRUSS-TYPE STRUCTURES. Repairs to riveted structures may be made employing the general principles outlined in the following paragraphs on aluminum alloy structures. Re pair methods may also be found in text books on metal structures. Methods for repair of the major structural members must be specifically approved by the Federal Aviation Administra tion (FAA). 4-52. ALUMINUM ALLOY STRUC TURES. Extensive repairs to damaged stressed skin on monocoque-types of alumi num alloy structures must be made in accor dance with FAA-approved manufacturer’s in structions or other FAA-approved source. a. Rivet Holes. Rivet holes are slightly larger than the diameter of the rivet. When driven, solid rivets expand to fill the hole. The strength of a riveted joint is based upon the expanded diameter of the rivet. Therefore, it is important that the proper drill size be used for each rivet diameter. Par 4-50 Page 4-11PDF Image | AFS-640
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