From Airwolf wiki
St. John Hawke
St. John Hawke is a decorated Vietnam War veteran and the long-lost brother of Stringfellow Hawke who eventually takes over as the primary pilot of the Airwolf helicopter.
St. John Hawke is a former United States Army operative who went missing in action for many years during the Vietnam War. His disappearance served as a primary motivation for his brother, Stringfellow Hawke, who spent much of his life searching for him while operating the Airwolf helicopter under the guidance of Dominic Santini.