Thank you all for the interest:

@Dirk:
I didn't understand why some of the "normal" enemys in flying mission are sometimes transparent and sometimes not.
It is a cloak mechanism some space enemies have, when they are transparent they are cloaked, they won't shoot at you and cannot be shot.

The collsion will be improved on the surface, I'll also look at the spikedog problem.

Thanks a lot for the feedback.

Firle