Start your engines
Senior Airman Michael Daniel, 28th Maintenance Squadron hydraulics journeyman, applies a seal to an auxiliary power unit piston in the 28th MXS Hydraulics Shop at Ellsworth Air Force Base, S.D., Jan. 19, 2012. Used to build up hydraulic pressure, APUs are necessary for starting the engines on Ellsworth’s fleet of B-1Bs. (U.S. Air Force Photo by Airman 1st Class Kate Thornton/Released)