WPR24LA130

The pilot’s unintentional use of a winter-blend fuel that was susceptible to vaporization, which resulted in fuel starvation and a loss of engine power due to vapor lock. Contributing to the accident was the airplane’s undersized fuel lines, which limited the available fuel flow and volume.

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.

The Rans S-7S was a single-engine, experimental airplane, serial number 0510343, built by the pilot from a kit that he completed in 2019. The airplane was equipped with a ULPower UL350is fuel-injected engine. The pilot stated that at the time of the accident, the airplane and engine had accumulated about 43 hours of operational time. The pilot had just replaced the No. 2 electric fuel pump, both fuel filters, and some fuel hoses that had been subject to a ULPower service bulletin. Additionally, he completed a condition inspection of the airplane the day before the accident. The accident flight was the first flight following the maintenance and inspection. Fuel System The airplane was equipped with a combined gravity-fed and suction-pump fuel system. Fuel was stored in two wing tanks, each with a capacity of 13 gallons. Fuel from each tank was gravity-fed to a centrally located header tank positioned in the mid-fuselage area behind the rear seat. Each wing tank incorporated a vent line, and the header tank was equipped with two vent lines. Fuel from the header tank was routed forward through a pilot-operated fuel shut-off valve near the pilot seat (see figure). Downstream of the shutoff valve, fuel passed through the firewall to an aluminum gascolator. From the gascolator, the fuel system divided into two parallel supply lines, each delivering fuel to an in-line fuel filter and an electric fuel pump mounted on the firewall. The pumps were designed to draw fuel from the header tank over a distance of about 10 feet forward and about 1-2 feet vertically. Fuel exited each pump at high pressure and entered a banjo-type fitting, where the two supply lines rejoined. The combined fuel flow then passed through an electronic fuel-flow transducer mounted above the engine, before entering the engine’s fuel-injection system. Unused fuel was routed aft through a second transducer and then through a return line approximately 10 feet in length, before reentering the mid-fuselage header tank. Figure: Diagram of fuel system The airplane was equipped with redundant electric fuel pumps and ECUs, each of which could be turned on/off independently via switches located on the pilot’s right-hand switch panel; the airplane was not equipped with an engine-driven/mechanical fuel pump. Fuel The pilot stated that while conducting maintenance on the airplane before the accident flight, he was concerned that the engine had not been producing full power. He discovered that all four engine cylinders had low compression values and were leaking air though the exhaust. He used a borescope to inspect inside the engine and observed lead deposits on the exhaust valves. He said he discussed this problem with the engine manufacturer, who recommended he stop using low-lead aviation fuel in the airplane, and instead change to auto fuel with 93 or higher octane rating. The pilot then purchased the highest-octane auto fuel available in the Tucson area, 91 octane unleaded, within the two-week period prior to April 21, 2024. To attain an approximate 93 octane result, he mixed the auto fuel with 100LL avgas at a ratio of approximately 85% auto fuel and 15% 100LL avgas. He indicated he ran the engine with this new fuel several times before the day of the accident flight. In pertinent part, the ULPower installation manual for the 350iS engine stated that “Avgas 100 LL can be used but ULP engines prefer lead free fuel such as Mogas or Avgas UL91,” and that “Hot AND/OR Ethanol containing fuel is more prone to vapour formation” adding that the pilot must ensure the installation, operation and fuel choice does not result in vapor lock. On April 22, 2024, at 0942 mountain standard time, a Rans S-7S experimental amateur-built airplane, N599YY, was substantially damaged when it was involved in an accident near Benson, Arizona. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot stated that the purpose of the flight was to monitor the fuel system and engine after he performed recent maintenance. As part of the maintenance, he replaced numerous parts in the airplane’s fuel system and changed the fuel from 100LL aviation fuel to unleaded auto fuel. After taxiing for approximately 9 minutes, the pilot completed a normal run-up check and departed from the active runway. As the airplane reached approximately 2,200 ft above ground level, the engine sustained a partial and then total loss of power. The airplane avionics displayed both a “check engine” alert and a low fuel pressure warning on the screen. The pilot switched to the No. 2 engine control unit (ECU) and No. 2 fuel pump and was able to restart the engine, although it only produced partial power. He turned the airplane back toward the airport and continued troubleshooting. As he advanced the throttle control forward to increase power, the engine ran rough and again lost power completely. The airplane had insufficient altitude to glide to the airport, so the pilot chose an off-airport location to land. The airplane touched down hard on rough terrain, collapsing the main landing gear. During the accident sequence, the collapsed landing gear impacted both wing lift struts, fracturing the right strut and bending the left strut. A postaccident examination of the engine and connected fuel system revealed no visible external damage to the engine or accompanying accessories, with the exception of the gascolator, which had damage that appeared consistent with the accident-related damage reported by the pilot. Investigators performed numerous tests on the electric fuel-pumps. During several test cycles, fuel did not flow reliably when the pumps were first activated. After the pumps successfully primed, fuel flow stabilized. A small fuel leak was discovered at an exit fitting in one of the fuel pumps, which was corrected by tightening the related fitting. A comprehensive examination of the airplane’s fuel system revealed that 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. After clearing the vent line, investigators confirmed continuity of the fuel system. An engine manufacturer representative participating in the examination noted that the aluminum fuel supply lines between the fuselage header tank and the electric fuel pumps, as well as the fuel return line routing fuel from the engine back to the header tank, were undersized for the installation. He stated that the installed supply line diameters were 3/8 inch (9.5mm), which did not meet the manufacturer’s minimum 10mm requirement, particularly considering the length of the lines and the presence of tight-radius bends. According to the representative, due to the length of the fuel supply line the inside diameter necessary to meet the manufacturer’s pressure and flow requirements would be 1/2 inch (12.7mm); he said the restrictive system reduced available fuel pressure and increased the potential for fuel vaporization. He further noted that the undersized return line (1/4 inch inside diameter as installed, compared to the 5/16 inch minimum specified) would both increase fuel-pump workload and restrict the volume of fuel returning to the header tank, in turn reducing the opportunity for heated fuel to cool. In pertinent part, the ULPower installation manual for the 350iS engine stated: • “The ULPower fuel system requires a pressurized fuel supply (3bar) that can cope with a fuel flow of minimum 120 litre/hour. Therefore the use of high quality lines with an appropriate inner diameter and adapted fuel connectors [is] paramount.” • “It is the OEM/Builders responsibility to design and test a suitable fuel system before flight testing i.a.w. good practice and local regulations.” • “With 1