CEN18LA042

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.

On November 27, 2017, about 1520 mountain standard time, the pilot of a Zenith CH750 Cruzer, N956GM, made a forced landing near Louviers, Colorado, after the engine lost power. The pilot was not injured, but the airplane was substantially damaged. The airplane was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The local flight originated from Rocky Mountain Metropolitan Airport (BJC), Broomfield, Colorado, approximately 1440. The purpose of the flight was to assess engine performance after the pilot had installed a replacement Engine Control Unit (ECU) with updated programming. The pilot flew to his phase 1 flight area near Louviers and was performing autopilot testing. The engine monitor alerted him to high engine coolant temperature. He reduced power and turn back towards the airport. Engine RPMs decreased and engine temperature increased. Fuel and oil pressures stayed within normal limits. Shortly thereafter, the engine seized and the propeller stopped rotating. The pilot attempted two engine restarts, including a complete rebooting of the ECU, without success. The pilot made a forced landing in an open field. The airplane struck an unseen barbed wire fence, collapsing the nose landing gear and crushing the engine cowling. The right-wing strut was bent, and there was some wing skin damage. The pilot noticed the coolant expansion tank had overfilled due to the overheating. The pilot later examined the engine and found that the ECU had run "the engine too lean, resulting in excessive cylinder head temperatures. This resulted in the unseating of the head gasket, which pressurized the coolant jacket and evacuated engine coolant overboard." The pilot added that he "found [coolant] in three of the four cylinders, oil was mixed in the coolant under the thermostat…[resulting] in the rapid rise of coolant temp and stoppage of the engine."