WPR24LA130 · James E Davidson JR RANS S-7S COURIER

The pilot was evaluating the airplane’s engine performance with a different fuel after changes were made to the fuel system. Shortly after a normal takeoff, the pilot noticed a low fuel pressure warning. The engine subsequently sustained a partial and then total loss of power. The pilot restarted the engine, and it momentarily produced power. As he advanced the throttle, the engine again stopped producing power. During an off-airport landing, the main landing gear collapsed.

On previous flights, the engine had produced less power than the pilot expected. In response, he performed an engine examination and observed lead deposits on the exhaust valves. He contacted the engine manufacturer, who recommended switching from 100LL avgas to unleaded auto fuel. The pilot then created a mixture of winter-blend auto fuel and a small, precise amount of 100LL avgas.

An examination of the engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. Examination of the fuel system revealed that the inside diameter of the fuel supply and return lines were smaller than recommended by the engine manufacturer. This undersized condition limited the volume of fuel available to the engine.

A small fuel leak was discovered at an exit fitting in one of the fuel pumps. Investigators could not determine if the leak existed before the accident or if it was a result of impact forces. Additionally, the vent tube in the left-wing fuel tank was obstructed and did not permit airflow until investigators inserted a small wire several inches into the tube, after which the vent permitted air passage.

Accident flight data recorded by the airplane’s engine monitoring device revealed that the fuel pressure and flow both declined slowly as the flight progressed and became increasingly erratic. Shortly thereafter, a sudden and complete loss of both fuel pressure and flow occurred. When the pilot attempted to restart the engine, the available fuel pressure and flow was insufficient to sustain engine operation. Recorded data from the flight was consistent with air bubbles present in the fuel system, followed by fuel starvation due to vapor lock.

After an operational test of the engine, several firewall-mounted fuel-system components (including the gascolator, fuel filters, electric fuel pumps, and associated metal fuel lines) that had been positioned near the engine exhaust system remained hot for more than 20 minutes after engine shutdown. Under these heat-soaked conditions, investigators noted that the fuel pumps required an extended period to achieve priming, and the fuel exiting the pumps initially appeared cloudy and aerated before gradually clearing as normal pressure and flow were restored. The pumps emitted abnormal noises consistent with cavitation during the period when cloudy, vapor-entrained fuel was present.

The locally procured auto fuel was a winter blend, which is formulated in a manner that makes it susceptible to vaporization at warmer fuel temperatures. Because the fuel was routed through metal fuel system components exposed to exhaust heat, the fuel in the system was likely heated to a temperature that resulted in vaporization. Although the investigation also found a small leak at a fuel line exit fitting and a blocked vent tube in the left-wing fuel tank, the engine data and postaccident engine testing was consistent with fuel vaporization. The vaporization of the fuel likely led to a vapor-lock condition and subsequent fuel starvation.

Verbatim NTSB analysis and probable cause from the NTSB dataset