Every part used in the aircraft, whether it be a stringer, former, bulkhead, longeron, or any attaching hardware must meet safety criteria, not only for the certification period, but for the expected life of the airframe under design conditions of use. Some airframe parts such as wing spar carry through assemblies require high strength. Others, such as cowlings, while not requiring as much strength, still must be capable of withstanding some loading and stress, as well as contributing to an aerodynamically clean design. The stress a particular part must withstand is carefully calculated by engineers. Also, the material a part is made from is extremely important and is selected by designers based on its known properties.
Aluminum alloy is the primary material for the exterior skin on modern aircraft.
This material possesses a good strength to weight ratio, is easy to form, resists corrosion, and is relatively inexpensive. Fittings must be made of carefully selected materials because of their importance of holding the aircraft together under expected stress and loading. The same holds true for important fasteners such as bolts and rivets. It is essential that these parts not fail under stress. It is also essential that these parts not weaken with exposure to stress and weather elements.