One thing I'd like to add to Scott's excellent post --

Airplanes are fragile. This is a consequence of their having to be built as light as possible; the major part of the structure of almost any airplane is sheet metal, aluminum, from .030 - .090 inches ( .75 - 2.25 mm) thick. Those are eggshells we ride around in, folks.

The only exception is the engines, and in the case of a 767 or other modern airliner, the size you see is deceptive -- the engine core, the part that makes the power, isn't much bigger than a typical large V8 auto engine, and all the rest of that cylindrical structure is a hollow shell like the fuselage is.

If you've ever seen a crash site, one of the things that will strike you immediately is that the airplane's in shards, pieces the size of your hand in many cases. In the late '50s we had a bomber being refueled that exploded in the air not far from my house; wreckage hunts were popular for months afterward, aided by the fact that the Air Force would send somebody to tell you what that part was if you turned it in [very exciting to Fifties kids]. The typical piece was less than a foot across!

Planes that hit the ground are typically moving relatively slowly, because the pilots are trying hard not to hit the ground. The rare ones that do go in at high speed just flat fragment.

As here.