Amateur-built ZENITH CH750 CRUZER N956GM accident near Louviers, Colorado

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 analysis

The commercial pilot was conducting a flight in an experimental airplane 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. The pilot made a forced landing in an open field. The airplane struck 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 postaccident 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.

Text from the NTSB. Open the NTSB docket for CEN18LA042 for the full record.