Senior Airman Leslie Whitfield, a KC-135 Stratotanker crew chief, operates an auxiliary power unit during pre-flight operations at Selfridge Air National Guard Base, Mich., Nov. 4, 2018. The KC-135 is used for air-to-air refueling, cargo and passenger lift and can be configured to transport medical patients. (U.S. Air National Guard photo by Tech. Sgt. Dan Heaton