CREW CHIEFS Air Force crew chiefs conduct pre-flight checks during a mission employment phase exercise on Nellis Air Force Base, Nev., Dec. 7, 2012. The crew chiefs are assigned to the 757th Aircraft Maintenance Squadron. The phase is the capstone, graduation exercise conducted by the U.S. Air Force Weapons School. SHARE: Download: Full Size (0.07 MB) Credit: VIRIN: 560373-E-PKR88-804.jpg Photo Gallery