Accident Overview
A mosquito control aircraft crashed and caught fire after a witness reported hearing engine sputtering. The airplane was conducting mosquito control operations and was carrying Scourge insecticide, a mixture of resmethrin, piperonyl butoxide, and naphtalene.
Witness Account
A witness observed the airplane passing overhead at low altitude. They heard the engines "throttle back, then rev up and sputter." The witness noted that the right engine was sputtering, as indicated by the propeller turning slowly. The airplane then pitched up, rolled to the right, and descended into the ground, immediately catching fire.
On-Scene Investigation
Examination of the propeller assemblies revealed distinct damage patterns. On the left propeller, one blade was curled aft and the other was curled forward. On the right propeller, one blade was curled aft and the other was straight.
Engine Examination
Both engines were disassembled and inspected. Crankshaft rotation produced camshaft, valve, and accessory drive continuity on both engines. No anomalies were found in the lubrication, induction, fuel injector, exhaust, or ignition systems of either engine. Examination of the spark plugs and cylinders showed normal burn patterns and color.
All cylinders produced compression except the right engine's number 6 cylinder. According to the manufacturer, the intake and exhaust valve springs on that cylinder had lost tension as a result of exposure to the fire.
Propeller Governor Controls
Both the left and right propeller governor controls were found in the high rpm position.
Conclusion
The investigation did not identify any pre-impact anomalies in the engines or systems. The valve spring damage on the right engine's number 6 cylinder was attributed to fire exposure after the crash.