MOTLEY GARY W ZENITH CH750 CRUZER

Full Narrative

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."