Mostly all the airplanes are made up of the light and strong metal because the plane has to fly in the air and it is always tried that the weight of the planeâ€™s body should be minimum. Its frame is mostly made up of aluminum which is a light metal. 747-8 was the last aircraft to be built of aluminum. After that all the aircrafts are made up of carbon reinforced plastic CREP it is also known as composites. So we can say that commercial planes are almost made up of aluminum while the newer planes are made of composite materials. Sometimes military aircrafts are made up of a light alloy named titanium. Also the purpose of aircraft decides about which material is to be used for its body or frame.
Piston in the engine is usually made up of steel but some other major parts of the airplane is made up of aluminum alloys. Jet engines are built up of aluminum, steel and titanium alloys.
An engineer always looks up for the appropriate metal and alloy to be used in the aircraft according to its application.