On May 11, 2007, about 1250 central daylight time, a single-engine Cessna A188A airplane, N4413Q, was substantially damaged during a forced landing following a partial loss of engine power near Clinton, Oklahoma. The airline transport pilot, the sole occupant, sustained serious injuries. The airplane was registered to and operated by the pilot. Visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations part 137 aerial application flight. The local flight originated from Clinton Regional Airport (CLK), Clinton, Oklahoma at 1157. According to the pilot, age 35, as he was pulling up from a low level pass, he smelled something "like wires burning" and observed the engine's fuel pressure drop to approximately six gallons per hour (GPH). The pilot selected the auxiliary fuel pump's "EMERGENCY" position which resulted in an increase in fuel pressure to about 22 GPH. Approximately five to ten seconds later the engine's fuel pressure again dropped to approximately six gallons per hour. Not able to maintain altitude, the pilot elected to land the airplane to a rolling wheat field. Prior to landing, the pilot shut off the engine magnetos and master switch. During landing, the airplane nosed over and came to rest in an inverted position. A post impact fire did not occur. A Federal Aviation Administration (FAA) inspector responded to the accident site. According to the inspector, the airplane sustained substantial damage to the vertical stabilizer and both wings. The inspector further reported that aviation fuel was found in the airplane's main fuel tanks and in the engine's fuel manifold. The NTSB investigator-in-charge (IIC) and a representative from Teledyne Continental Motors examined the airplane following its recovery. The examination revealed that prior to impact, the engine exhaust stack had separated from the muffler. The electrical wiring attached to the firewall in the area behind the muffler was found thermally damaged. The fuel gascolator was also attached to the firewall in this area. No other pre impact anomalies were noted with the engine or the fuel system that would have prevented normal operating power. According to the Cessna Agwagon Owners Manual dated 1970/71, page 2-12, "...if fuel vapor is affecting engine operation, the vapor may be purged by placing the switch in the "EMERGENCY" position while leaning the mixture as required to prevent excessively rich mixture. Successful vapor purging is evidenced by smooth engine operation and steady and normal fuel flow indications with the auxiliary fuel pump switch "OFF.""