Enter the plane, either going up the raft-like object or from the hallway inside the building. Go to the front, where the pilot would normally sit. Crouch in the left corner against the wall so no one can kill you through the windshield. If someone else comes in the front without noticing you, knife them. Otherwise, just shoot the people you see in the hallway coming to the plane. Never look through the doorway. If you do and there is someone there, they will shoot you and most likely you will not have enough time to react.