Incident Overview
A commercial pilot had just finished dispensing his fourth load of fertilizer for the day and was returning to the airstrip when he reported a vibration, followed by a loss of engine power. During the subsequent forced landing, another pilot observed the airplane hit a ditch located between two rice fields, after which the aircraft nosed over.
Post-Accident Examination
The integrity of the fuel system was not compromised during the impact sequence. An examination conducted the day after the accident found no evidence of fuel at the accident site. However, fuel stains were present on the bottom of the right wing. The seat belt and shoulder harness were found fastened, and the pilot was wearing his helmet. The curved overturn tube, located in the upper portion of the cockpit, was crushed inward.
Engine and Component Testing
An engine test run revealed that the engine ran rough, backfired, and cut out momentarily throughout the normal power range. When operated on only the left magneto, the engine maintained 28 inches of manifold pressure at 2,000 RPM, but it still cut out intermittently. Removal and examination of the right magneto showed that the internal brass gear splines were worn, and brass particles were scattered within the magneto.
Further inspection of the carburetor, which had been removed, overhauled, reinstalled, and returned to service approximately three weeks prior to the accident, revealed several issues: the float movement was rough, the orifice of the float valve was too large, the float was set too low, and the accelerator pump did not function properly.
Safety Equipment
According to a manufacturer representative, a service bulletin was issued on March 23, 1991, for the installation of a cockpit overturn skid plate as an added safety feature. This skid plate had not been installed on the accident aircraft.