Hello Manish,
Can you describe your setup a little better? In general, the crash is correct, i.e. the aircraft has fallen below ground level. This can happen right after launch if you do not have a "catapult" launch; if the forward velocity isn't high enough the aircraft will descend. You can set an initial velocity to avoid this. See the
simulation page for more info.
If you are crashing otherwise, you may have to review your flightplan and airframe file and adjust things. Could be as simple as poorly tuned control loops or incorrect altitude setting on a waypoint. You can also log your simulation and view results in the log plotter if you aren't sure what is going on, and try to find the problem. Simply get rid of the -n option in the server program.
Hope that helps.
Thanks,
-Stephen Dwyer