Airman 1st class David Liu, 351st Air Refueling Squadron boom operator, tests an oxygen mask before take-off aboard a KC-135 Stratotanker prior to a flight at RAF Mildenhall, England, July 17, 2020. The 100th Air Refueling Wing is the only permanent U.S. air refueling wing in the European theater, providing the critical air refueling "bridge" which allows the Expeditionary Air Force to deploy around the globe at a moment's notice. (U.S. Air Force photo by Tech. Sgt. Emerson Nuñez)