Staff Sgt. Kyle Black, 22nd Security Forces Squadron unit trainer, uses a sign-countersign method to test aircrews as they respond to their assigned KC-135 Stratotankers during an exercise March 1, 2017, at McConnell Air Force Base, Kan. The numbers are used as a security measure to prevent unauthorized usage of aircraft. ( Air Force photo/Airman 1st Class Erin McClellan)

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Size: 2824px × 1740px
Photo credit: © AB Forces News Collection / Alamy / Afripics
License: Licensed
Model Released: No

Keywords: aircrew, aircrews, alert, exercise, forces, security, training, usaf