Thanks one and all. This is approach radar that we are simulating, the idea being to present the student with an a/c descending at a high rate into his crowded bit of airspace (FL120 downwards) with not very much notice from area control. We do not intend to complicate the issue at the moment with structural damage as well.
There are limits to what the computers will do (so the boffins tell me) so those ball-park figures are fine. Mind you, we had a PA28 up to mach 3.0 yesterday. No structural damage either!
Thanks again.