You should get this cheat without even knowing it. I'll go through it anyway. Turn to your right and turn right into the little romm with a guy walking out of it. Kill his and pick up the box. Exit the room, and take a left all the way down to the conveyor belt. Go outside and proceed directly to the left towards the house. Go in and kill the two guys, picking up the ignition key. Exit the house and take a left onto the runway. Side step your way along, avoiding all resistance, and get in the plane. Easy, wasn't it?