Complex sentence: Fitzgerald carelessly lets them operate the controls while he mixes drinks in the back of the plane; soon Benjy's erratic steering knocks him unconscious and the pair have to fly and land the plane on their own.
Split into simpler sentences: Fitzgerald carelessly lets them operate the controls while he makes drinks in the back of the plane and is soon knocked unconscious by the Benjy's erratic steering. The two then have to fly and land the plane on their own.