The simple answer might be: anything. Airplanes have been made of all sorts of material over the past 100+ years, though aluminum and other sheet metals have been the most popular choices.
The basic description of an airplane includes a body made of a strong but light metal that keeps the weight of the plane down to a manageable level. This is where aluminum comes in. According to aircraft history, the last aircraft to be built entirely of aluminum was a version of the 747. Now, the major builders of airliners use what is called a “carbon-reinforce plastic.” This material will lead the way to replacing decades of aluminum airplanes.
Currently, many of the commercial planes still flying are primarily made of aluminum, but as they age they will be replaced by planes with bodies of some composite material. The military has made extensive use of a very strong and light element called titanium. As for some of the other key parts of the planes in use today, aluminum, titanium, steel and various combinations are used.
In the beginning, airplanes had wood frames and fabric skins. These planes were extremely light and thus very fragile. Builders used wire and light pieces of metal to for bracing and support. Eventually the builders went to laminated wood and then to sheet metals that formed a skin around a frame.
One of the keys to improvements in airplane construction was the increase in speed as engines improved. Builders began to use stronger metal alloys, along with ceramics and other combinations. These went along with the changes in aircraft engines, which also became lighter. Aluminum, titanium and other combinations of metals were used to create powerful, yet light engines.
With the latest mixes of carbon-reinforced plastics, the plane is stronger and even lighter than when only aluminum is used. Major changes in wing design and in the materials used have reduced wing failure and metal fatigue considerably. Maintenance is less costly, primarily due to the reduction of breakage and corrosion in crucial parts.
It’s interesting to note that builders once used wood as the primary material for plane construction. At the time this was necessary because wood was widely available. Metal construction was not a refined science in the early years of the 20th century. Along with the extensive use of wood, pilots were separated from the air around them by only a thin skin of some rough fabric, such as canvas. Compare this with the materials used today.
One company has built a replica of a P-51 Mustang using mostly carbon fiber. There are only 12 major pieces to this plane, according to the manufacturer. Even with this very light, plastic-like material, the new plane, which is modeled after a military plane from decades ago, can fly about 400 miles per hour.
The change in use of materials has created some additional problems, however. As the demand for aluminum decreased, the price of that raw material changed. But companies are now finding that the cost of carbon fiber material is going up rapidly.