N599YY – JAMES E DAVIDSON JR RANS S-7S COURIER – 2024-04-22 NTSB Accident Report

HISTORY OF FLIGHTOn 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. AIRCRAFT INFORMATIONThe 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. AIRPORT INFORMATIONThe 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. ADDITIONAL INFORMATIONWinter-Blend Fuel and Vapor Lock

Investigators researched the automotive fuel used during the accident flight to ascertain its vaporization characteristics on the day and location of the accident flight. State and federal standards governing fuel formulation in the Tucson area define fuel vaporization requirements and limits. Fuel volatility is frequently characterized using a measure called Reid Vapor Pressure (RVP). The RVP value indicates the externally-applied pressure in pounds per square inch (psi) required to prevent vaporization at a fuel temperature of 100°F. The larger the RVP number, the higher the fuel’s volatility, and therefore the lower its vaporization threshold temperature. When comparing two fuels, the one with the higher RVP number will vaporize at a lower temperature and/or altitude than the fuel with the smaller RVP number.

Auto fuel sold at retail pumps during the “winter” period (defined as September 16–May 3...