Once you've located the ladder to the drug lab, tos your grenades down
there to clear it.  Go down the ladder and clear out all scum waiting
for you.  Look in a dark room and pick up the RPG rocket launcher. 
Order able to place satchel near the barrals.go out the back door of the
coke lab. Head up hill until the satchel charge goes of.  Clear the hut
, then head down hill until you reach the ciff.  You should be able to
see the plane in the hanger.  By this time, the helicopter should be
airborne.  Ignore it.  Blow up the plane with your launcher and they
have no chance of escape.  Now shoot at the heli until black smoke comes
out. It should stop circling you.