On April 4, 1998, at 1120 central standard time, an Aero Commander S2R aerial application airplane, N8984Q, sustained substantial damage during a forced landing following a total loss of engine power during the takeoff/initial climb from the Cushing Municipal Airport, Cushing, Oklahoma. The aircraft was owned and operated by a private individual under Title 14 CFR part 91. The commercial pilot, sole occupant of the aircraft, was not injured. Visual meteorological conditions prevailed for the local flight that was originating at the time of the accident. A flight plan was not filed. During a telephone interview, conducted by the investigator-in-charge, a personal interview conducted by the FAA inspector, and on the Pilot/Operator Aircraft Accident Report(s) (NTSB Form 6120.1/2), the pilot reported that he had completed the aerial application flights for the day and had filled the hopper with approximately 70 gallons of water to rinse out the chemicals. The airplane departed runway 35 and turned outbound at 300 feet agl to the west when the engine lost manifold pressure. The pilot increased the throttle and mixture, the engine backfired through the induction system with black smoke, and the engine "quit." The pilot tried to restart the engine, using the electric primer; however, the engine would not restart. During the forced landing, the pilot maneuvered the airplane to clear power lines and homes. During the landing flare/touchdown, the airplane touchdown hard in an open field, collapsing the main gear and folding the gear under the aircraft. The airplane remained upright as it slid into trees and came to rest in a pond in about a foot of water. Structural damage occurred to the empennage, propeller, and wings. On April 8, 1998, the FAA inspector and the operator examined the airplane. The engine is equipped with a Stromberg PD12-K18 carburetor. The mixture control arm at the carburetor was loose; however, the nut and cotter key were intact. The mixture control in the cockpit was full forward; however, the mixture control at the carburetor was set at less than 50 percent travel toward the rich stop on the carburetor. Black soot consistent with backfiring was found inside the carburetor and the induction intake screen. On April 16 and 17, 1998, at the Guthrie Municipal Airport, Guthrie, Oklahoma, the engine was examined, under the surveillance of the FAA inspector, in the hangar. The engine is a Wright R-1820-76B, serial number W-474086. All the engine cylinders had compression. Engine continuity was confirmed to the accessory case; however, neither magneto rotated when the propeller was rotated. According to the FAA inspector and a mechanic, this lack of magneto rotation indicates that the accessory shaft has sheared inside the rear case of the engine. The mechanic stated, that based on his experience, he has found the backlash off a strong backfire and metal fatigue as possible causes of shaft shearing. The mechanic reported that he did not know if the mixture control arm was loose on its shaft during flight or impact related.