Around 10,000ft, Shell 77 exploded into three main sections and impacted the terrain approximately 1.5 miles southwest of where the tail section was found
Tankers, like the Boeing KC-135, permit the refueling of other aircraft in flight, extending their range to permit global missions. Fuel is transferred to the receiver aircraft by way of a boom mounted on the tanker’s lower aft fuselage. Operation of the refueling system is controlled by the boom operator, or “Boomer.” While the receiver maintains formation with the tanker, the boom is flown into contact with the receiver receptacle and transfer occurs (Other versions of the system use a probe and drogue system).
As told by Lon Nordeen in Key Publishing 135 – The World’s Greatest Tanker magazine special, the danger of refuelling operations was brought home on May 3, 2013 when US Air Force KC-135R serial number 63-8877 call sign Shell 77 crashed on a routine mission flying from Manas International Airport near Bishkek, Kyrgyzstan to Afghanistan. The aircraft was from the 22nd Air Refueling Wing based at McConnell and temporarily assigned to the 22nd Expeditionary Air Refueling Squadron, 376th Air Expeditionary Wing. The crew were all members of the 93rd Air Refueling Squadron, 92nd Air Refueling Wing based at Fairchild Air Force Base, Washington. The aircraft’s mission was to refuel coalition aircraft in Afghanistan and then return to Manas, referred to by the US Air Force as the Transit Center. The crew of three, perished in the mishap.
Approximately nine minutes into the flight, the aircraft experienced a series of increasing yaw and roll oscillations known as a dutch roll. The aircraft yawed between three degrees left and right, and banked between five degrees left and right.
Approximately 11 minutes into the flight, the pilot made a right rudder input to roll out of a turn, further exacerbating the dutch roll condition. Ten seconds later, the cumulative effects of the malfunctioning series yaw damper, coupled with autopilot use and rudder movements, generated dutch roll forces that exceeded the aircraft’s structural design limit load factors. This overload caused the tail section to fail and separate in several pieces. The aircraft pitched down sharply and entered a high-speed dive.
The last point captured on the flight data recorder (FOR) was a nose-down attitude of 82° at 21,760ft. Around 10,000ft, Shell 77 exploded into three main sections and impacted the terrain approximately 1.5 miles southwest of where the tail section was found. The three main sections included the cockpit, the centre fuselage and the aft section.
The United States Air Force Accident Investigation Board led by Brig Gen Steve Arquiette found “by a preponderance of evidence, that the Dutch roll was instigated by the aircraft’s Flight Control Augmentation System malfunctioning causing directional instability or rudder hunting, which substantially contributed to this mishap. Other substantial contributing factors included insufficient organisational training programmes, crew composition and cumbersome procedural guidance.”
“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 unrecognised hazardous and difficult situation to overcome,” the general said.
The three airmen who perished aboard Shell 77 were: TSgt Herman Mackey (30) of Bakersfield, California; Capt Victoria Ann Pinckney (27) of Colorado Springs, Colorado, and Capt Mark Tyler Voss (27) of Boerne, Texas.
Photo credit: U.S. Air Force
Artwork courtesy of AircraftProfilePrints.com