WPR18LA120 · Zenith STOL CH750

The pilot reported that he was conducting a personal, cross-country flight in the experimental, amateur-built airplane with three flight legs planned. About 15 minutes after the pilot departed from a remote airstrip for the second flight leg and while the airplane was in cruise flight about 2,000 – 2,500 ft above ground level, the engine experienced a total loss of power. The pilot attempted to restart the engine to no avail. The terrain below and ahead of the airplane was mountainous, so he chose to turn toward terrain that was flatter to conduct an emergency landing, during which the airplane impacted dense brush and then nosed over. According to recorded engine data, when the airplane was about 7,425 mean sea level, the engine rpm began to decrease from about 3,000 to 1,100 over 10 seconds. Eight seconds later, the rpm had decreased to 0, corroborating the pilot’s statement regarding the loss of engine power. Examination and test run of the engine revealed no evidence of any preaccident mechanical malfunctions or failures that would have precluded normal operation. The fuel tank inlet was located on the inboard tank wall in the aft area about 0.25 inch above the tank’s bottom. The forward wall height was about 7.75 inches, and the aft wall height was about 4 inches. The fuel quantity was provided by a float-type sending unit attached to the inboard, forward portion of the tank wall. Because of the tank’s shape, the relationship between the fuel depth and fuel quantity for a given tank attitude and depth measurement location will not be constant. Further, if the airplane is pitched down, the fuel will be pushed forward in the tank, and the float on the fuel sender will rise, indicating a higher fuel quantity than what is actually in the tank. The engine data showed that the airplane departed with about 2 gallons and 9 gallons of fuel in the left and right wing tanks, respectively. The pilot reported that he thought the fuel selector was positioned to the right tank but that could not be verified from the recorded data. Throughout the first 5 minutes of the flight, the fuel quantity continually fluctuated with values between 0.4 and 2.1 gallons and 7.8 and 9.4 gallons in the left and right tanks, respectively. Just before the engine lost power, the fuel quantities in both tanks increased as the airplane assumed a nose-low pitch attitude. Therefore, the fluctuations in the fuel amount and the increase in the fuel amount when the airplane entered a nose-low pitch attitude were consistent with the fuel tank’s shape and the fuel quantity indicator’s position. Based on the tank’s dimensions, when the tank is level, about 0.5 gallon is required to have a 1-inch fuel level at the inlet port. Given that the engine data indicated that there was likely sufficient fuel to reach the ports in both tanks during the 5 minutes of flight before the engine power loss and that the fuel system’s design did not provide a means for air or fuel vapor to vent from the system, it is likely that excess air or fuel vapor built up and subsequently entered the fuel injectors, which prevented the fuel from reaching the engine cylinders and resulted in the total loss of engine power.

Verbatim NTSB analysis and probable cause from the NTSB dataset