Also planes are built to be as light weight as possible The skin is pretty thin and reinforced with various structural parts elements to make it stiff enough to not bend with take off and landings and aerodynamic loads, however once its in the air and moving, you have added a lot of kinetic energy. Once this hits the hard ground in a crash, the ground wins. Couple this with the fact that an aircraft is also generally pressurized, and can result in an explosive
decompression that helps fling it apart.
Essentially. The impact and heat breaks down whatever the plane is made of. So technically much of the same matter is still there, but broken down into the composite elements or compounds. The rest is ejected into the atmosphere as smoke. There will likely be chunks that can still be identified as parts of the plane
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