or something like this, syntax not guaranteed:
call this script where you want it to land, with [heli] exec "land.sqs"
heli=this select 0
#land
heli setpos [(getpos heli select 0), (getpos heli select 1),(getpos heli select 2) -0.5)]
?((getpos heli select 2)=0):goto end
goto land
#end
like i said, no guarantees