Accident Summary
On November 27, 2017, at about 1520 mountain standard time, a Zenith CH750 Cruzer, registration N956GM, experienced a loss of engine power and was forced to land in an open field near Louviers, Colorado. The pilot, who was the sole occupant, was not injured. The aircraft, however, sustained substantial damage.
Flight Details
The flight was a personal flight conducted under 14 CFR Part 91. It originated from Rocky Mountain Metropolitan Airport (BJC) in Broomfield, Colorado, at approximately 1440. Visual meteorological conditions prevailed, and no flight plan was filed. The purpose of the flight was to evaluate engine performance after the pilot had installed a replacement Engine Control Unit (ECU) with updated programming.
Engine Failure
The pilot flew to his designated phase 1 flight area near Louviers and was conducting autopilot testing when the engine monitor alerted him to high engine coolant temperature. He reduced power and initiated a turn back toward the airport. Subsequently, engine RPM decreased and temperature continued to rise, while fuel and oil pressures remained within normal limits. Shortly thereafter, the engine seized and the propeller stopped rotating. The pilot attempted two engine restarts, including a complete reboot of the ECU, but was unsuccessful.
Forced Landing
With the engine inoperative, the pilot executed a forced landing in an open field. During the landing, the airplane struck an unseen barbed wire fence, which collapsed the nose landing gear and crushed the engine cowling. The right-wing strut was bent, and some wing skin damage occurred. Post-landing inspection revealed that the coolant expansion tank had overfilled due to overheating.
Post-Accident Examination
Upon later examination of the engine, the pilot found that the ECU had run the engine too lean, resulting in excessive cylinder head temperatures. This caused the head gasket to unseat, pressurizing the coolant jacket and evacuating engine coolant overboard. The pilot discovered coolant in three of the four cylinders and oil mixed in the coolant under the thermostat, which led to a rapid rise in coolant temperature and eventual engine stoppage.