http://www.avweb.com/news/columns/182676-1.html
See "Select The Surface"
"There are some very interesting films made by NASA when it did full-scale crash tests of general aviation airplanes. The gantry arrangement that had been used for training Apollo astronauts for working under reduced G levels was modified to impact airplanes into the ground at specific angles and speeds. The initial tests crashed the aircraft on concrete. The data obtained showed that some impressively high-speed impacts were potentially survivable. At flatter angles the airplane would hit and then slide along the concrete. For a while though, everyone missed the obvious: The concrete simply redirected much of the energy of the crash. While it stopped movement downward, providing a significant deceleration in that direction, it did not absorb all of the energy of the moving airplane. The remaining energy was translated into a long slide. An engineer visiting from one of the manufacturers politely mentioned to the NASA scientists that, in his experience, not too many airplanes crashed on concrete. The light bulb lit and dirt was brought in and layered about three feet deep atop the concrete.
The same crash tests were rerun. The results were dramatically different. None of the impacts was survivable. The dirt compacted about six inches, and then stopped the airplanes cold."