On August 1, 1994, at 1745 eastern daylight time, a DeHavilland DHC-8-311, N433AW, operated by Atlantic Coast Airlines and doing business as United Express Flight 6578, was substantially damaged while parked at a gate and struck by a ground support vehicle, at Dulles International Airport, Washington, D.C. The three crewmembers and 50 passengers were not injured. Visual meteorological conditions prevailed and a instrument flight rules flight plan had been filed for the flight operating under 14 CFR 121. In the NTSB Form 6120.1/2, the Regional Flight Manager stated that 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. He further stated: "...Baggage tractor DH4 had broken down. The operator asked the operator of baggage tractor DH65 to push disabled DH4 to the west end of the ramp where it could await servicing. DH65 began pushing DH4...DH4 steered so as to pass 15 to 20 feet to the left of [N433AW's] nose when approaching the service road. As DH4 entered the service road it began to turn right to head west. At this point DH65 had not yet entered the service road and for the next few feet continued in a somewhat southwesterly direction. The left side of the hitch on the front of DH65 apparently contacted the right side of the hitch on the rear of DH4. Since [they]...were actually heading in different directions (DH4 west and DH65 southwest), the effect of the hitch interaction was to jerk the front of DH65 sharply to the right causing the driver to lose his seat and be pitched halfway off the left side of DH65... the driver's right hand pulled the steering wheel down to the right, and his right foot pressed on the accelerator pedal...DH65 smashed into the left side of the nose of 433 doing extensive damage..."