What happened
While performing a low-level pass, the pilot of an agricultural aircraft noticed an odor similar to burning wires and observed a drop in engine fuel pressure. In response, the pilot moved the auxiliary fuel pump to the emergency position. Within five to ten seconds, the engine experienced a loss of partial power. Unable to maintain altitude, the pilot performed an emergency landing into a rolling wheat field. During the landing sequence, the aircraft nosed over and came to rest in an inverted position. The occupants were not injured.
The investigation
An examination of the aircraft revealed that the engine exhaust stack had separated from the muffler prior to the impact. This separation caused hot exhaust gases, which are typically vented overboard, to be directed toward the aircraft firewall. This heat resulted in thermal damage to the electrical wiring and created a vapor condition within the fuel gascolator. Although the pilot attempted to purge the vapor, the effort was unsuccessful. No other anomalies were identified in the engine or fuel system that would have prevented the maintenance of normal operating power.