U.S. Air Force Airman 1st Class Jason Kerr and Senior Airman Jeffrey Johnson, 757th Aircraft Maintenance Squadron crew chiefs, conduct pre-flight checks during the Mission Employment Phase exercise, Dec. 7, 2012, at Nellis Air Force Base, Nev. The ME Phase is the capstone, graduation exercise conducted by the U.S. Air Force Weapons School. (U.S. Air Force photo/Senior Airman Brett Clashman)