Event Description
According to the flight crew, while starting the number 3 engine, the captain noticed that the tug was out of position for a normal pushback. As the pushback proceeded, the crew heard and felt several rough jolts from the nose wheel area. When the aircraft stopped moving, the wing-walker signaled an emergency stop, and the captain set the parking brake. The tug driver reported not noticing anything unusual during the pushback. While towing the aircraft forward and looking back to align it with the taxiway, the driver heard the wing-walker yell as the aircraft rolled forward and collided with the tug.
The wing-walkers stated that it was raining when the pushback procedure started, and they noted a smell of glycol. The tug appeared to lose traction as the aircraft moved past the main landing gear chocks. The tug driver regained control and continued the pushback. When the tug operator began towing the aircraft forward, the wing-walker noticed that the aircraft continued to roll forward after the tug stopped. The aircraft then collided with the tug, coming to rest with the tug on its right underside.
Damage Inspection
Examination of the aircraft revealed a 5-foot tear on the underside of the belly. Further inspection showed that two ribs and five stringers were damaged.
Tow Bar Examination
The tow bar's hitch head shear pin was found sheared off. Metallurgical examination of the pin shank showed that the pin was separated in two locations. Both fractures displayed a flat surface with a small crescent shape, while the remaining surface showed a grainy texture. These features are typical of a "double shear" fracture. Records indicated that the tow bar was last inspected on September 21, 2005, and the shear pin was replaced on November 30, 2005.