History of Flight
On April 22, 2024, at 0942 mountain standard time, a Rans S-7S experimental amateur-built airplane, registration N599YY, was substantially damaged during a forced landing near Benson, Arizona. The pilot, who was the sole occupant, was not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
The pilot stated that the flight was intended to monitor the fuel system and engine after recent maintenance. Maintenance included replacement of numerous fuel system components and switching from 100LL aviation fuel to unleaded auto fuel. After taxiing for about 9 minutes, the pilot performed a normal run-up and departed. At approximately 2,200 ft above ground level, the engine experienced a partial then total loss of power.
The airplane's avionics displayed a “check engine” alert and low fuel pressure warning. The pilot switched to the No. 2 engine control unit (ECU) and No. 2 fuel pump, restarting the engine at partial power. He turned back toward the airport. Advancing the throttle caused the engine to run rough and lose power completely.
Insufficient altitude remained to glide to the airport, so the pilot chose an off-airport landing site. The airplane touched down hard on rough terrain, collapsing the main landing gear. The collapsed gear struck both wing lift struts, fracturing the right strut and bending the left strut.
Aircraft Information
The Rans S-7S was a single-engine, experimental airplane, serial number 0510343, built by the pilot from a kit completed in 2019. It was equipped with a ULPower UL350is fuel-injected engine. At the time of the accident, the airplane and engine had about 43 hours of operational time. The pilot had replaced the No. 2 electric fuel pump, both fuel filters, and some fuel hoses subject to a ULPower service bulletin, and completed a condition inspection the day before. The accident flight was the first flight following this maintenance.
Fuel System
The airplane had a combined gravity-fed and suction-pump fuel system. Fuel was stored in two wing tanks (13 gallons each) that gravity-fed a centrally located header tank behind the rear seat. Each wing tank had a vent line; the header tank had two vent lines. Fuel from the header tank passed through a shutoff valve, then through the firewall to an aluminum gascolator. Downstream, the system split into two parallel supply lines, each with an in-line fuel filter and an electric fuel pump mounted on the firewall. The pumps drew fuel from the header tank over about 10 feet forward and 1-2 feet vertically.
Fuel exited each pump under high pressure into a banjo fitting where lines rejoined, then passed through a fuel-flow transducer before entering the engine's fuel-injection system. Unused fuel returned through a second transducer and a return line about 10 feet long to the header tank. The airplane had redundant electric fuel pumps and ECUs, each controlled independently by switches on the pilot's right-hand panel. No engine-driven mechanical fuel pump was installed.
Fuel
The pilot stated that during maintenance, he found low compression in all four cylinders and lead deposits on exhaust valves. The engine manufacturer recommended switching from low-lead aviation fuel to auto fuel with 93 or higher octane. The pilot purchased 91-octane unleaded auto fuel (the highest available in Tucson) within two weeks before April 21, 2024. He mixed it with 100LL avgas at about 85% auto fuel and 15% 100LL to achieve approximately 93 octane. He ran the engine with this mixture several times before the accident flight.
The ULPower installation manual for the 350iS engine stated that Avgas 100LL can be used but lead-free fuel is preferred, and that hot and/or ethanol-containing fuel is more prone to vapor formation, and the pilot must ensure the installation, operation, and fuel choice does not result in vapor lock.
Additional Information: Winter-Blend Fuel and Vapor Lock
Investigators researched the auto fuel's vaporization characteristics. State and federal standards define fuel vaporization limits using Reid Vapor Pressure (RVP). Winter-blend fuel (sold September 16–May 31) in Tucson can have an RVP of 15 psi, while summer-blend has about 9 psi. 100LL avgas typically has an RVP between 5.5 and 7 psi. The fuel purchased was likely winter-blend with an RVP near 15.
The FAA Aviation Maintenance Technician Handbook explains that vapor lock can occur when fuel vaporizes in lines or pumps, forming vapor pockets that restrict flow. Conditions include high fuel temperature, reduced pressure (e.g., altitude), and fuel system characteristics promoting low pressure or turbulence.
Flight Recorders
The airplane was equipped with a Dynon Avionics EFIS/EMS that recorded over 50 parameters at about 16 times per second. Data from the accident flight showed engine running at 08:23:14, ground operation for about 9 minutes, takeoff at 08:32:30, and forced landing at 08:42:30. Fuel pressure ranged from 40 psi pre-takeoff to 47 psi during initial climb, then gradually declined to 42 psi. At 08:39:20, fuel pressure dropped suddenly to near zero, with erratic readings. Fuel flow dropped similarly. Cylinder head temperatures were normal; exhaust gas temperatures rose slightly then dropped after fuel pressure/flow loss. The data pattern was consistent with gas bubbles due to vapor lock.
Tests and Research
Postaccident examination found no external engine damage except to the gascolator (consistent with impact). Tests on electric fuel pumps showed intermittent fuel flow when first activated; a small leak at a pump fitting was corrected. Examination of the fuel system revealed the left-wing tank vent tube was obstructed until a small wire was inserted to clear it.
An engine manufacturer representative noted that aluminum fuel supply lines between the header tank and pumps were 3/8 inch (9.5mm) inside diameter, below the manufacturer's minimum 10mm, and given line length and bends, the required diameter was 1/2 inch (12.7mm). The return line was 1/4 inch, below the 5/16 inch minimum. These undersized lines could reduce fuel pressure and increase vaporization potential.
The ULPower installation manual specified that fuel system lines must have appropriate diameters and be capable of 3 bar pressure and 120 liters/hour flow. It stated the builder is responsible for designing and testing a suitable fuel system.
Full-power ground tests using recovered fuel, with cowling removed, showed normal engine operation. No mechanical malfunctions were found. After these tests, the manufacturer's representative noted that firewall-mounted components (gascolator, filters, pumps, lines) were positioned near the exhaust system and remained hot for over 20 minutes after shutdown. Under heat-soaked conditions, the electric pumps required extended priming time and initially produced cloudy, aerated fuel with cavitation noises, consistent with vapor formation.
The ULPower manual recommended protecting fuel lines and connectors from excessive heat and using heat shields on pumps if mounted forward of the firewall.