Damage to metal aircraft structures is often caused by  corrosion, erosion, normal stress, and accidents and mishaps.  Sometimes aircraft structure modifications require extensive  structural rework. For example, the installation of winglets  on aircraft not only replaces a wing tip with a winglet, but  also requires extensive reinforcing of the wing structure to  carry additional stresses.  Numerous and varied methods of repairing metal structural  portions of an aircraft exist, but no set of specific repair  patterns applies in all cases. The problem of repairing a  damaged section is usually solved by duplicating the original  part in strength, kind of material, and dimensions. To make a  structural repair, the aircraft technician needs a good working  knowledge of sheet metal forming methods and techniques. In  general, forming means changing the shape by bending and  forming solid metal. In the case of aluminum, this is usually  done at room temperature. All repair parts are shaped to fit  in place before they are attached to the aircraft or component.  Forming may be a very simple operation, such as making  a single bend or a single curve, or it may be a complex  operation, requiring a compound curvature. Before forming  a part, the aircraft technician must give some thought to  the complexity of the bends, the material type, the material  thickness, the material temper, and the size of the part being  fabricated. In most cases, these factors determine which  forming method to use. Types of forming discussed in this  chapter include bending, brake forming, stretch forming, roll  forming, and spinning. The aircraft technician also needs  a working knowledge of the proper use of the tools and  equipment used in forming metal. In addition to forming techniques, this chapter introduces  the airframe technician to the tools used in sheet metal  construction and repair, structural fasteners and their  installation, how to inspect, classify, and assess metal  structural damage, common repair practices, and types of  repairs.  The repairs discussed in this chapter are typical of those used  in aircraft maintenance and are included to introduce some of  the operations involved. For exact information about specific  repairs, consult the manufacturer’s maintenance or structural  repair manuals (SRM). General repair instructions are also  discussed in Advisory Circular (AC) 43.13.1, Acceptable  Methods, Techniques, and Practices—Aircraft Inspection  and Repair. An aircraft structure must be designed so that it accepts all of  the stresses imposed upon it by the flight and ground loads  without any permanent deformation. Any repair made must  accept the stresses, carry them across the repair, and then  transfer them back into the original structure. These stresses  are considered as flowing through the structure, so there  must be a continuous path for them, with no abrupt changes  in cross-sectional areas along the way. Abrupt changes in  cross-sectional areas of aircraft structure that are subject to  cycle loading or stresses result in a stress concentration that  may induce fatigue cracking and eventual failure. A scratch or  gouge in the surface of a highly stressed piece of metal causes  a stress concentration at the point of damage and could lead  to failure of the part. Forces acting on an aircraft, whether it  is on the ground or in flight, introduce pulling, pushing, or  twisting forces within the various members of the aircraft  structure. While the aircraft is on the ground, the weight of  the wings, fuselage, engines, and empennage causes forces  to act downward on the wing and stabilizer tips, along the  spars and stringers, and on the bulkheads and formers. These  forces are passed from member to member causing bending,  twisting, pulling, compression, and shearing forces. As the aircraft takes off, most of the forces in the fuselage  continue to act in the same direction; because of the motion  of the aircraft, they increase in intensity. The forces on the  wingtips and the wing surfaces, however, reverse direction;  instead of being downward forces of weight, they become  upward forces of lift. The forces of lift are exerted first  against the skin and stringers, then are passed on to the ribs,  and finally are transmitted through the spars to be distributed  through the fuselage. The wings bend upward at their ends  and may flutter slightly during flight. This wing bending  cannot be ignored by the manufacturer in the original design  and construction and cannot be ignored during maintenance.  It is surprising how an aircraft structure composed of  structural members and skin rigidly riveted or bolted together,  such as a wing, can bend or act so much like a leaf spring. The six types of stress in an aircraft are described as tension,  compression, shear, bearing, bending, and torsion (or  twisting). The first four are commonly called basic stresses;  the last two, combination stresses. Stresses usually act in  combinations rather than singly.  [Figure 4-1] Tension is the stress that resists a force that tends to pull apart.  The engine pulls the aircraft forward, but air resistance tries  to hold it back. The result is tension, which tends to stretch  the aircraft. The tensile strength of a material is measured in  pounds per square inch (psi) and is calculated by dividing  the load (in pounds) required to pull the material apart by its  cross-sectional area (in square inches). The strength of a member in tension is determined on  the basis of its gross area (or total area), but calculations  Tension Stresses in Structural Members 4-2