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A damaged refueling boom can be seen on the back of a KC-10 before it completely fell off during a training flight on Nov. 1, 2016, near Mountain Home AFB, Idaho. Air Force photo Photo from Accident Investigation Board report.

—Brian Everstine

An Air Force investigation found that a sheared rotary crank and the boom operator’s failure to turn off a boom control switch caused a KC-10 Extender’s refueling boom to completely fall off the aircraft and crash into an Idaho field in November.

On Nov. 1, 2016, a KC-10 from the 60th Air Mobility Wing at Travis AFB, Calif., was conducting a training flight to refuel a C-17 and two F-15s near Mountain Home AFB, Idaho. About 46 minutes into the flight, the KC-10’s boom operator lowered the boom and it immediately began to move erratically in a U-shaped pattern well outside of its structural limits, according to an Air Force Accident Investigation Board report.

After two minutes, the boom’s hoist cable broke. Two minutes later, a pivoting support, or gimbal, separated from its mounts. A few minutes later, the boom completely separated from the aircraft and landed in an open field in Idaho. The total loss is valued at $6.53 million.

The investigation found that a sheared Dual Rotary Voltage Transducer rotary crank, resulting from a rotary bearing misalignment, caused the mishap. The misalignment provided the aircraft’s boom control unit with constant and inaccurate roll position indications. This prompted the control unit to compensate with lateral movement commands, driving the boom beyond its structural limits.

The board also found that the boom operator failed to turn off the boom flight control switch in a timely manner. Turning off the switch would have disabled the control unit, and neutralized the boom flight control surfaces, the report states. In addition, maintenance personnel did not comply with technical orders that could have detected the rotary crank issues.