Best way to do this, is to create your helo, with crew somewhere on the island out of harms way in the mission editor and set the helo fuel to 0
(Avoid Createunit where possible, as causes a bit of lag, especially if the type of unit doesnt already exist on the mission already)
MISSION EDITOR
create your helicopter, with crew, and any waypoints it needs
Set the helo's fuel to 0 using the fuel slider
name the helo "Helo1"
Place the helo away from any playing area, eg on an offshore island where players wont be able to get to it
Create a game logic and place it wherever you want the helo to start flying from (We will setpos the helo to this location when it is needed)
Name the gamelogic HeloLogic
and then run the following code when it's required
Helo1 setfuel 1
Helo1 flyinheight 50
Helo1 setpos [getpos Helologic select 0,(getpos helologic select 1), 200]
the value 50 is the crusing height that you want the helo to fly at, once it steadies itself out, edit this flyinheight value to create your desired effect
the value 200 is the height above ground that the helo will be setpos'd to before it starts flying.
This value could be reduced somewhat, experimentation will give you the desired effect
make sure the first waypoint is a sensible path from the gamelogic