Well, I have never modelled a sand dune, so I'm not really qualified to give any direct advice on how to do it, but here's my recommendation:
Firstly, use books or the www to study a few photos of sand dunes, especially the ones in the area that you are planning to represent. For example, desert sand dunes have a different shape to beach sand dunes. Beach dunes are usually covered with some form of grass or creeper vine and may be littered with debris; desert dunes are usually barren. Also observe the colour of the dune; desert dunes may be yellow, brown or red whilst beach dunes are usually pale yellow to off-white (or even grey if you are modelling a volcanic pacific island scene).
After you decide on the dune shape, you can use any media available to form the shape. You could use styrofoam, balls of newspaper, wire mesh, rags, or anything that will give you a basic "bump" of the right shape. I have found the balled newspaper and/or wire (or nylon) mesh method to be pretty effective in producing hills, which is then covered with strips of newspaper that have been dipped in a watery plaster (with a little PVA) mix. This will dry to a fairly firm shell that can be built up more if required, covered with plaster for an even surface or just painted.
I reckon that nothing looks more like sand, dirt & rocks than sand, dirt & rocks. I always simulate soil or sand by using finely seived soil and sand. I use a minus 80 mesh sieve (grains passing through the seive are less than 1/80") or even a minus 200 mesh seive to collect the grains. Find a source of soil or sand that is the colour you want, then seive as much as you need.
As for simulating a crashing aircraft... that's a whole different ball game. Obviously, if it's a catastophic crash, there'd be sand and debris thrown into the air, which will be difficult to simulate. If it's a controlled, wheels-up landing, then it might be easier???
Hope my comments help.