A KC-135R tanker flown by a Fairchild Air Force Base crew shook apart over Kyrgyzstan last May, an investigation shows. The tail section came off and the rest of the plane plummeted through the air until it exploded then crashed, killing the three on board.
Shortly after the tanker took off from Manas Air Force Base, the crew reported the plane was “waffling” and later that it was “bent.”
The tanker was experiencing what pilots call “Dutch roll.” The nose was moving left and right while the wings were alternating up and down. Unless it’s brought under control, the phenomenon can exert too much stress on the structure of a plane.
That’s what happened to the tanker. About 11 minutes after takeoff, the plane exploded in flight and crashed. Killed were Capt. Mark Tyler Voss, 27, the pilot; Capt. Victoria Pinckney, 27, the co-pilot; and Tech. Sgt. Herman “Tre” Mackey III, 30, the boom operator.
The Air Force released its findings on the May 3, 2013, accident Thursday morning in a briefing from Brig. Gen. Steven Arquiette, who led an investigation into the crash of the tanker that went by the call sign Shell 77.
The tanker, which had been in the Air Force inventory since it was delivered by Boeing in 1964, had been in Kyrgyzstan for less than 24 hours. Assigned to McConnell Air Force Base in Kansas, a different crew had flown it to Manas the previous day and reported it had a “right rolling motion,” but the plane had no major maintenance problems in the previous 14 flights.
Voss, Pinckney and Mackey had been deployed from Fairchild to Kyrgyzstan about three weeks earlier and were the first crew to fly the plane after it arrived.
Shell 77 took off with reports of isolated thunderstorms. About two minutes into the flight, co-pilot Pinckney reported over the radio that the plane was “kind of waffling a lot.” Later she said, “the jet’s bent.”
Data from the flight recorder shows the plane was going into a Dutch roll, with the movements getting more pronounced with each wag of the nose and dip of a wing.
“As it grows, it gets to be quite violent,” Arquiette said.
The KC-135R has systems that are designed to help prevent or correct such rolls, but investigators who inspected pieces of the plane determined that at least one of them, a rudder lock lever, was significantly worn. It may not have operated properly and could have increased the problem of the roll.
Corrective action taken by the crew couldn’t stop the roll and appears to have made it worse, based on data from the flight recorder. Eventually, the nose of the plane was swinging from 12 degrees right to 17 degrees left, with one wing lifting up then dropping as the other wing lifted. The stress caused the plane’s tail section to fail and separate from the plane just ahead of the vertical stabilizer.
The tanker dove, nose down at 82 degrees, gaining speed. The stress forced the right wing off. Fuel from the wing tank spilled out and ignited. The plane exploded midair and crashed to the ground in pieces. Debris was scattered over an area more than 2 miles square in the foothills below the Himalaya Mountains.
Investigators know Shell 77’s tail came off first because there was no fire damage on that section.
The investigation pointed to several problems with training and information provided to tanker crews on Dutch roll. Arquiette said when he was a young pilot in the 1980s, he flew the KC-135A, a version with less powerful engines. Crews practiced regularly with how to handle Dutch roll, which was a common occurrence.
“The R is much smoother,” Arquiette said. “When its systems work, they work quite well.”
When the engines were replaced, the revised KC-135Rs also had new systems installed to prevent Dutch roll. Crews train less on the problem, both in the air and in flight simulators. Investigators couldn’t even re-create the conditions of Shell 77 in a simulator at McConnell. Those simulators are being updated to improve training.
As a result of the crash, KC-135R crews will get more training on how to handle Dutch roll. The procedural guidance in the flight manual on how to handle a Dutch roll, he said, is “cumbersome and disjointed.” The manuals are being revised.
Maintenance is being increased and systems for controlling the rudders are being updated. Boeing is evaluating the rudder system, and flight data recorders are being updated.
The changes are important because the Air Force may continue flying the Eisenhower-era tankers through the next decade.
The remoteness of the crash site was among the reasons the investigation took almost twice as long as a standard accident investigation, Arquiette said. Briefing the crew’s family members about the results was “the hardest thing I’ve ever done in my career,” he added.