The commercial pilot was conducting a flight in the Zenith CH750 Cruzer to assess engine performance after installing a replacement engine control unit (ECU) with updated programming.
During the flight, the engine monitor provided an alert regarding a high engine coolant temperature. Shortly afterward, the engine seized.
The pilot attempted two engine restarts, including a complete reboot of the ECU, without success.
He then made a forced landing in an open field in Louviers, Colorado. The airplane hit a barbed wire fence, causing damage to the nose landing gear, engine cowling, right wing strut, and right wing skin.
Also, the pilot noticed that the coolant expansion tank had overfilled due to overheating.
A post-accident examination revealed that the ECU had caused the engine fuel-air mixture to be too lean, resulting in excessive cylinder head temperatures, which caused the engine to seize.
The excessive cylinder head temperatures also resulted in the unseating of the head gasket, which pressurized the coolant jacket and evacuated engine coolant overboard. Coolant was found in three of the four cylinders, and oil was mixed in the coolant under the thermostat, resulting in the rapid rise of coolant temperature.
Probable cause: The engine’s excessive cylinder head temperatures and the subsequent seizing of the engine, which resulted from an engine control unit that caused the engine’s fuel-air mixture to be too lean.
NTSB Identification: CEN18LA042
This November 2017 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.