A Cessna 550, N827DP, was substantially damaged when it impacted an agricultural tractor during a landing roll at Reading Regional Airport/Carl A. Spatz Field (RDG). The captain, first officer, and tractor operator were not injured. Visual meteorological conditions prevailed, and no flight plan had been filed for the flight from Pottstown Limerick Airport (PTW), to Reading. At 15:15, the airplane was cleared to land on runway 31, and at 15:17, the tractor operator contacted the tower on the ground control frequency in order to proceed from the “terminal ramp” to the “north ramp.” The controller then cleared the tractor to cross runway 31 at taxiway D. At 15:19, the crew of the airplane informed the controller that the airplane had struck the front of a tractor at the intersection of runway 31 and taxiway D. Approximately two thirds of the airplane’s left wing had broken off. PROBABLE CAUSE: “The air traffic controller’s failure to properly monitor the runway environment. Contributing to the accident was the tractor operator’s failure to scan the active runway prior to crossing, and the Federal Aviation Administration’s inadequate emphasis on vehicle operator visual vigilance when crossing active runways with air traffic control clearance.”