An air tanker that crashed in Kyrgyzstan was rolling to the left and right so violently that the tail section tore free, causing the aircraft to plunge toward the ground and explode in mid-air, an Air Force report said Thursday.
The accident last May 3 killed a three-member crew from Fairchild Air Force Base.
The Air Force report concluded that a series of factors combined to doom the 50-year-old KC-135 aircraft.
The most serious was a so-called "Dutch roll" in which the plane started yawing to the left and the right and pitching up and down early in the flight. The stresses on the plane became so great that the tail section broke off, the report states.
Killed in the crash were Capt. Mark T. Voss, 27, of Colorado Springs, Colo.; Capt. Victoria A. Pinckney, 27, of Palmdale, Calif.; and Tech Sgt. Herman Mackey III, 30, of Bakersfield, Calif. All three were stationed at Fairchild, near Spokane.
The aircraft suffered the effects of what is called sideslip, said Brigadier Gen. Steven J. Arquiette, who briefed journalists on the report from Scott Air Force Base in Illinois.
"Sideslip is basically the airplane skidding sloppily sideways through the air," he said.
The crash occurred about 11 minutes into the flight. The air tanker had departed the Transit Center at Manas, a military base in Kyrgyzstan, on a mission to refuel aircraft flying over Afghanistan.
The crash occurred about 6 miles south of the town of Chaldovar.
The report found that immediately after takeoff, with Pinckney at the controls, the aircraft experienced a "crab," which is movement of the nose left and right about 1 degree in each direction.
The crew did not turn off the yaw damper, which may have been malfunctioning, or the rudder power, which would likely have helped the situation, the report said.
About nine minutes into the flight, the airplane began a series of increasing yaw and roll oscillations. The co-pilot attempted to decrease the oscillations using aileron controls and autopilot. But the autopilot, which was engaged twice, made the oscillations worse, the report said.
Voss took over the controls and used the left rudder to start a left turn. But use of the rudder caused the Dutch roll to get worse, the report said.
The combination of factors caused the roll to exceed the aircraft's structural limits, the report said.
"The tail section failed and separated from the aircraft, causing the (airplane) to pitch down sharply, enter into a high-speed dive, explode midflight and subsequently impact the ground," the report said.
The report concluded that the crew failed to recognize the roll conditions in time to take proper actions.
Contributing factors included the relative inexperience of the crew and insufficient training in how to deal with a Dutch roll, the report said. Arquiette said those issues were being addressed.