A DHC-8-311, N433AW, operating as United Express Flight 6578, was substantially damaged while parked at a gate and struck by a ground support vehicle, at Washington-Dulles International Airport, D.C. The three crewmembers and 50 passengers were not injured. At the time of the accident the airplane was parked at the passenger gate and was being prepared for departure. The engines were stopped, the chocks were in place, the pilots were seated in the cockpit completing weight and balance paperwork, and the airstair door was open. Meanwhile a baggage tractor had broken down. While the tractor was being pushed out of the way by another tractor. Both tractors contacted each other in the process. This caused the driver to lose his seat and be pitched halfway off the left side. The driver’s right hand pulled the steering wheel down to the right, and his right foot pressed on the accelerator pedal The tractor then smashed into the left side of the nose of N433AW. PROBABLE CAUSE: “The vehicle operator’s improper handling of the baggage tractor resulting in loss of control of a second baggage tractor being pushed and its subsequent collision with the airplane.”