Chapter 4 Aircraft Metal Structural Repair Aircraft Metal Structural Repair The satisfactory performance of an aircraft requires continuous  maintenance of aircraft structural integrity. It is important that  metal structural repairs be made according to the best available  techniques because improper repair techniques can pose an  immediate or potential danger. The reliability of an aircraft  depends on the quality of the design, as well as the workmanship  used in making the repairs. The design of an aircraft metal  structural repair is complicated by the requirement that an aircraft  be as light as possible. If weight were not a critical factor, repairs  could be made with a large margin of safety. In actual practice,  repairs must be strong enough to carry all of the loads with the  required factor of safety, but they must not have too much extra  strength. For example, a joint that is too weak cannot be tolerated,  but a joint that is too strong can create stress risers that may cause  cracks in other locations.  As discussed in Chapter 3, Aircraft Fabric Covering, sheet metal  aircraft construction dominates modern aviation. Generally, sheet  metal made of aluminum alloys is used in airframe sections that  serve as both the structure and outer aircraft covering, with the  metal parts joined with rivets or other types of fasteners. Sheet  metal is used extensively in many types of aircraft from airliners  to single engine airplanes, but it may also appear as part of a  composite airplane, such as in an instrument panel. Sheet metal  is obtained by rolling metal into flat sheets of various thicknesses  ranging from thin (leaf) to plate (pieces thicker than 6 mm or 0.25  inch). The thickness of sheet metal, called gauge, ranges from 8  to 30 with the higher gauge denoting thinner metal. Sheet metal  can be cut and bent into a variety of shapes. 