On August 7, 2001, approximately 1110 central daylight time, a WSK PZL Mielec M-18 agricultural airplane, N4206B, registered to and operated by Bullock’s Flying Service of DeWitt, Arkansas, was substantially damaged during a forced landing following a loss of engine power while maneuvering in the vicinity of Dumas, Arkansas. The non-instrument rated commercial pilot, sole occupant of the airplane, sustained minor injuries. Visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 137 aerial application flight. The flight originated at 1010 from a private grass airstrip near Dumas, Arkansas. In the enclosed Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) the pilot reported that the "backfire plate started opening and closing" while he was in the process of spraying a cultivated field. The pilot added that he reduced the power and pulled out of the field to evaluate the situation. He added that "everything was normal of about 15 seconds" which was followed by a rough running engine and a total loss of engine power. The pilot maneuvered the airplane into the prevailing wind to execute a forced landing. During the landing roll in rough and uneven terrain, the tires sank into the soft ground and the airplane nosed over coming to rest in the inverted position. Examination of the airplane revealed structural damage throughout the airframe as well as spar damage to a wing. The reason for the loss of engine power was not determined.