U.S. Air Force Staff Sgt. Kristopher Castillo, a 927th Aircraft Maintenance Squadron crew chief, stands under the air refueling boom of a KC-135 Stratotanker during a pre-flight inspection during a unit training assembly on MacDill Air Force Base, Fla. Prior to flight, the crew chief observes aircraft functions for safety prior to departure. (U.S. Air Force photo by Staff Sgt. Brad Tipton)