HISTORY OF FLIGHTOn October 21, 2016, at about 0822 eastern daylight time, a runway incursion occurred at the Westfield-Barnes Regional Airport (BAF), Westfield/Springfield, Massachusetts, when a Douglas DC-9-83, registration N425NV, operating as Allegiant Air Flight 4203 (AAY4203) landed on runway 20 while Guard Sweeper 96 was conducting runway sweeping operations on the last 4000 feet of the same runway. After AAY4203 stopped on the runway, the pilot reported they were about 200 feet from the sweeper. There was no reported damage to the aircraft or to the sweeper, and no reported injuries. Instrument meteorological conditions prevailed, and AAY4203 had filed an instrument flight plan for the Title 14 Code of Federal Regulations (CFR) Part 121 repositioning flight from Asheville Regional Airport, Asheville, North Carolina with only crew on board. After the airfield had opened, there were several ground operations being conducted in the movement areas to get the airfield operational for the day. At 0733:21, "crash crew" requested permission to enter the airfield to conduct checks of the arrestor cable located on both ends of runway 20. Additionally, "airport 16" was also on the airfield conducting surface checks of the runway and taxiway lights. At 0738:17, Guard Sweeper 96 contacted the BAF local controller and requested permission to conduct "vac" sweeping on the first 4000 feet of runway 02. At 0746:23, Airport 16 reported clear of the airfield and then at 0749:43 requested the tower activate the approach lighting systems, the tower acknowledged and turned all lights on. At 0751:26, Airport 16 reported the approach lights were all working. The crash crew vehicles that had been on the runway conducting checks of the arrestor cables reported clear of the movement areas at 0812:51. At 0813:36, Massachusetts DOT vehicle "DOT1" requested permission to proceed to alpha to runway 33 plus 1. The local controller instructed DOT1 to turn left on runway 20, cross runway 33 and to proceed east on taxiway A. At 0815:08, DOT1 reported clear of the airport movement areas. At 0817:22, AAY4203 checked on the local control frequency outside of WESIE. The local controller responded with the wind and altimeter followed by a clearance to land on runway 20. The controller added the weather was "just as the ATIS reported, ¾ mile visibility overcast ceiling 200 feet." The pilot acknowledged the landing clearance and weather. At 0822:53, AAY4203 contacted the local controller and transmitted "we were on the runway about 1500 feet short of the end and there was a truck on the runway." The local controller asked the pilot if the vehicle was in the approach lights. The pilot replied the vehicle was on the runway. The local controller acknowledged the pilot and instructed him to make a 180 degree turn and back taxi on the runway. At 0823:10, Guard Sweeper 96 contacted the local controller and said there was an aircraft on the runway. He asked if he had missed a radio call advising of the aircraft. The local controller confirmed the sweeper was still operating in the first 4000 feet of runway 02. At 0825:00, the pilot of AAY4203 transmitted "I don't know what the situation was with the truck, but if he would have been up any further with the visibility the way it was, there's, there would have been no avoiding him." The local controller advised he understood and would follow up. There were no further transmissions pertinent to the incursion incident. Airport and Radar Data Radar data for this report was obtained from the FAA Yankee terminal radar approach control (Y90) located in Windsor Locks, Connecticut. The Y90 plot playback (.PPB) data was of good quality and was part of the standard terminal automation replacement system (STARS). The Westfield-Barnes Regional Airport is serviced by an FAA Federal Contract Tower (FCT) that operates daily from 0700 to 2200. It had two main runways, runway 02/20 that serviced the military air National Guard Boeing F15 aircraft stationed at the airfield and runway 15/33 that primarily serviced the general aviation traffic. The runway incursion incident occurred at the approach end of runway 02. Weather Information The 0818 BAF Automated Surface Observing System (ASOS) observation included winds from 020 degrees at 3 knots, visibility 3/4 miles, runway 20 runway visual range 5000' variable 5500', broken overcast at 200' above ground level, temperature 13 degrees C, dew point 13 degrees C, and an altimeter setting of 29.85 inches of mercury. The air traffic controllers utilized a control tower visibility chart in the control tower to assist in determining tower visibility.