Structural repairs are essential for maintaining the integrity and safety of aircraft. These repairs are necessary when an aircraft structure is damaged due to factors such as corrosion, erosion, normal stress, accidents, or mishaps. Improper repairs can pose immediate or potential dangers, so it is crucial that metal structural repairs be made according to the best available techniques.
Forming Methods and Techniques
Forming sheet metal requires a variety of tools and equipment, both powered and manual, such as the piccolo former, shrinking and stretching tools, form blocks, and specialized hammers and mallets. Tempered sheet stock is used in forming operations whenever possible in typical repairs. Cold forming, which is performed at room temperature, eliminates the need for heat treatment and the straightening and checking operations required to remove warp and twist.
However, if the material shows signs of cracking during cold forming over small radii, it should be formed in the annealed condition. Annealing removes the temper from the metal, making it softer and easier to form. Parts containing small radii or compound curvatures must be formed in the annealed condition, and after forming, the part is heat treated to a tempered condition before use on the aircraft.
Structural Repair Considerations
The design of an aircraft metal structural repair is complicated by the requirement that an aircraft be as light as possible while still safely carrying all loads. 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. 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.
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.