WestJet flight WJA1536, a Boeing 737-800, was involved in a ground accident at Calgary (CYYC). During pushback from the gate, the flight crew had started both engines. Once the aircraft was on Taxiway BA, the pushback tractor stopped, and the ground crew disconnected the towbar. The aircraft started rolling forward towards the pushback tractor, where the lower nose of the aircraft collided with the roof structure of the tractor. A hole was punctured into the lower fuselage and the tractor became wedged in the fuselage. The windshield of the tractor was shattered. There were no injuries. The aircraft received damage to the lower fuselage and radome. After company maintenance assessed the aircraft damage, the aircraft was towed back to the gate and the passengers deplaned.