I'm in the level where I've just ran into two of the terrorist leaders. I've already killed the one that was on the ground, but now I'm to kill the one in the helicopter. I've picked up an RPG and an AK-47. While killing the ground troops, I'm supposed to shoot down the helicopter. Problem is, RPG's dont lock on, and I've shot the helicopter twice now in the tail rotor section. What am I missing??? How do I shoot this thing down and move on with the game??