On May 12, 2001, about 1030 central daylight time, an Air Tractor AT-401, N1008Q, piloted by a commercial pilot, sustained substantial damage when it nosed over during a forced landing following an in-flight loss of engine power while maneuvering near Warsaw, Illinois. The aerial application flight was operating under 14 CFR Part 137. Visual meteorological conditions prevailed at the time of the accident. No flight plan was on file. The pilot reported no injuries. The local flight departed from an airstrip, near Ferris, Illinois, about 0945. The pilot stated, "At approximately 10:30 AM spraying operation was nearing completion when in a pull out of the field being sprayed the engine made a bang, started running very rough and backfiring. Within a couple seconds the engine had lost all power and died. The pilot landed in a field that happened to be very soft and sandy. The aircraft rolled approximately 100 ft on its wheels before nosing over on its back. After further investigation it was determined a rocker shaft housing had broken on the #4 cylinder. As a result the exhaust valve was not opening. As the intake valve opened, it created backfiring into the induction system causing the engine to fail."