Accident Overview
On 3 May 2013, at approximately 1448 hours local time, a KC-135R tanker (tail number 63-8877) assigned to the 22d Expeditionary Air Refueling Squadron, 376th Air Expeditionary Wing, crashed in the foothills of mountains about 6 miles south of Chaldovar, Kyrgyz Republic. The mishap aircraft was destroyed, with total loss valued at $66.3 million. The three crew members—Cpt Victoria Ann Pinckney, Cpt Mark Tyler Voss, and T/Sgt Herman Mackey III—perished in the accident.
Flight Sequence
The aircraft had departed on a mission to refuel coalition aircraft in Afghanistan and return to its base at the Transit Center at Manas. Shortly after takeoff, the aircraft experienced an unexpected rapid heading change, known as a crab. During the subsequent climb, nearly continuous rudder hunting caused the nose to oscillate slowly left and right by about one degree. The mishap pilot commented on lateral control challenges and a possible series yaw damper (SYD) malfunction but continued the mission without deactivating the SYD or rudder power.
Misdiagnosis and Pilot Actions
Approximately nine minutes into the flight, the aircraft began a series of increasing yaw and roll oscillations characteristic of a dutch roll, which was not diagnosed by the crew. The mishap co-pilot attempted to counter the oscillations using manual aileron inputs, as well as two brief engagements of the autopilot. The manual inputs temporarily prevented the oscillations from growing, but the autopilot use aggravated the condition. After the second autopilot attempt, the mishap pilot assumed control and applied left rudder to initiate a left turn. The dutch roll-induced acceleration forces caused small alternating rudder inputs through the pilot's foot pressure, sharply increasing the oscillations. Within 30 seconds, the pilot made a right rudder input to roll out of the turn, further exacerbating the condition.
Structural Failure
The cumulative effects of the malfunctioning SYD, autopilot use, and rudder movements during the unrecognized dutch roll generated forces that exceeded the aircraft's design structural limits. The tail section separated from the aircraft, causing it to pitch down sharply, enter a high-speed dive, and explode in flight. The debris impacted the ground at three distinct locations, creating craters and contaminating approximately 228 cubic meters of soil with jet fuel.
Probable Cause
Investigators determined the cause was structural overstress leading to tail separation, resulting from the crew's failure to turn off the SYD or rudder power and from oscillating dutch roll-induced acceleration forces transmitted through the pilot's feet during rudder use. A contributing factor was a malfunctioning Flight Control Augmentation System that caused directional instability and rudder hunting. Other significantly contributing factors included insufficient organizational training programs, crew composition, and cumbersome procedural guidance.