Enter the plane, either going up the raft-like object or from the hallway inside the building, and go inside the cockpit. Crouch in the left corner against the wall to avoid being shot 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 or else an enemy may spot you and you will most likely not have enough time to react.