An Air Force KC-135 refueling tanker that crashed in Kyrgyzstan last year lost its tail, then its right wing, in an uncontrollable high-speed dive before exploding in mid-air, the Air Force said during a report of the findings Thursday.
The 50-year-old tanker had been based at Wichita’s McConnell Air Force Base. Its crew was from Fairchild Air Force Base in Spokane, Wash.
The plane crashed in the foothills of the Himalayas near Chaldovar, Kyrgyz Republic, during an 11-minute flight on May 3, 2013, killing the three-member crew. Its mission was to refuel coalition aircraft in Afghanistan, then return to the Transit Center at Manas, the name of the U.S. base in Kyrgyzstan in Central Asia.
Killed in the crash were Capt. Mark T. Voss, 27, of Colorado Springs; Capt. Victoria A. Pinckney, 27, of Palmdale, Calif.; and Tech Sgt. Herman Mackey III, 30, of Bakersfield, Calif.
Air Force Brig. Gen. Steven Arquiette presented the findings of the accident investigation to members of the crew’s family last month and to reporters during a news conference Thursday.
He said investigators found that a series of factors caused the crash. The main factor was a “Dutch roll,” a pilot’s term in which the plane begins yawing left and right and rolling up and down. The Dutch roll became increasingly violent, finally overstressing the airplane and causing the tail to break off.
Immediately after takeoff, the crew experienced an unexpected and rapid directional change, or a crab.
Early in the flight, the co-pilot said that the plane was “waffling a lot,” Arquiette said. During the climb, the plane slowly turned left, then right.
Nine minutes into the flight, the plane’s oscillations became increasingly severe. The co-pilot tried to decrease them by using the yoke and then twice using the autopilot, but the autopilot made the situation worse, the report said.
Voss then took control of the airplane and used the rudders to turn the plane to stay on course. That made the situation more severe.
Finally, the plane was pushed beyond its design structural limits, which caused the tail section to break from the aircraft, the report said. There had been no maintenance issues with the aircraft, officials said.
Parts of the plane were scattered across a wide area near the village of Chaldovar. Some pieces, including the tail, came down in a grassy valley bordered by steep mountains.
Among the factors leading to the crash, investigators noted, was the crew’s failure to correctly identify the situation and react effectively. Also, a malfunctioning Flight Control Augmentation System caused the instability, which led to the Dutch roll.
The report also pointed to insufficient training programs, crew composition and cumbersome procedural guidance.
The Air Force has since made a number of changes to prevent similar occurrences, Arquiette said. Crews are receiving more and better training on the recognition of Dutch rolls and the proper actions to take, he said.
For example, KC-135 simulators are being modified so pilots can better replicate Dutch roll conditions, and the simulator coursework is being changed to better prepare the aircrew, the report said. Before now, the simulators did not realistically replicate the condition, Arquiette said.
During the investigation, Arquiette said he spent about five hours in a simulator at McConnell, but he was unable to duplicate what the crew experienced.
Boeing and the Air Force are also rewriting the flight manual sections regarding rudder systems operation, malfunction identification and solutions. And Boeing is conducting a study to analyze nine key rudder-systems components, the report said.