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.

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Photo credit: © Chuck Little / Alamy / Afripics
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Keywords: air, aircrew, aircrews, alert, america, archive, armed, army, defence, exercise, force, forces, military, navy, reportage, security, service, serviceman, servicemen, soldier, states, training, united, usa, usaf