The U.S. Air ForceAir Mobility Command has released the results of an accident investigation that examined what caused the May 3, 2013, crash of a KC-135 Stratotanker in the Kyrgyz Republic.
The crew of three, en route from the Transit Center at Manas to Afghanistan on a combat aerial refueling mission, perished in the mishap.
Upon takeoff, a flight control system malfunction, the board found, generated directional instability, causing the aircraft’s nose to slowly drift from side-to-side or “rudder-hunt.” This condition, not fully diagnosed by the crew, progressed into a more dangerous oscillatory instability known as a “Dutch roll.” The board identified that a poor layout of key information in the inflight manual and insufficient crew training contributed to the mishap by detracting from the crew’s ability to act on critical information during their troubleshooting to turn off either of two cockpit switches which may have eliminated the malfunction.
Having not recognized the Dutch roll condition, the crew initiated a left turn to remain on-course along the planned route of flight and used a small amount of left rudder to coordinate the turn. The use of rudder, while in a Dutch roll, increased the aircraft’s oscillatory instability. The ensuing large side-to-side movements of the aircraft varied the crew member’s foot pressure on the rudder pedal which caused inadvertent fluctuations in rudder position. These fluctuating rudder movements, coupled with slight right rudder use while rolling out of the turn, compounded the Dutch roll severity and produced extreme airframe stress that caused the KC-135′s tail section to separate from the aircraft. The subsequent, uncontrollable descent resulted in an in-flight explosion.
“Our hearts go out to the family members and friends of these Airmen,” said Brig. Gen. Steve Arquiette, who led the accident investigation board. “Having attended the memorial service at Manas and later interviewing many co-workers, I know these Airmen were highly regarded and are greatly missed. The investigation team, with the help of our industry and Kyrgyz government partners, pushed through months of intense fact finding for the primary purposes of understanding what happened that day and to honor the crew’s service to our nation.”
The three Airmen who perished are:
- TSgt. Herman Mackey III, 30, of Bakersfield, Calif.†
- Capt. Victoria Ann Pinckney, 27, of Colorado Springs, Colo.
- Capt. Mark Tyler Voss, 27, of Boerne, Texas
A unique combination of six factors – flight control malfunctions, insufficient crew force training, incomplete crew checklist response, use of rudder while in a Dutch roll condition, crew composition, and cumbersome procedural guidance – all came together during the flight’s short 11-minute duration and resulted in this accident.
“The crew encountered a condition that they had not realistically experienced in training, and when coupled with decisions based on their relatively low recent experience levels, were presented with an unrecognized hazardous and difficult situation to overcome,” Arquiette said. “It has been the focus of our investigative team, throughout these months of hard work and travel to the accident scene in the Kyrgyz Republic, to do everything we can to fully understand the facts surrounding this tragic string of events.”
The aircraft was assigned to the 22nd Air Refueling Wing at McConnell Air Force Base, Kan., and was flown by members of the 92nd Air Refueling Wing at Fairchild Air Force Base, Wash. The crew and aircraft were flying out of the 376th Air Expeditionary Wing at the Transit Center Manas.
The report is available on the Air Force Freedom of Information Act Reading Room website, http://www.foia.af.mil/reading/.