well... there are some ways to solve it...
use:
this flyinheight 0 of the choppers init field...
but that won't stop it taking off to pick people up....
try and place a invisible H in the correct place and it stops this (but very much trial and error)
or type this in the choppers init:
this disableai "move"and it shouldnt move at all - but remember to enableai once he needs to take off...
hope one of those suggestions work