N599YYRans Designs S-7S · 2019

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.

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.

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.

Winter-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 31) is manufactured differently than fuel sold during the “summer” period (defined as June 1–September 15). Specifically, winter-blend auto fuel sold in the Tucson area is formulated to ensure it will combust in cold temperatures, and can be manufactured with an RVP of 15 psi. Summer-blend fuel is manufactured with a lower RVP, typically about 9 psi, and is formulated to prevent vaporization until the fuel reaches a higher temperature. Aviation gasoline fuels, such as 100LL avgas, are manufactured per ASTM standards with an even lower vaporization profile, and typically have an RVP value within the 5.5 – 7 psi range.

Based on the timeframe in which the auto fuel from the accident airplane was purchased by the pilot, it was very likely a winter-blend formulation, with an approximate RVP of 15.

The FAA Aviation Maintenance Technician Handbook – Powerplant, Volume 2, Chapter “Engine Fuel and Fuel Metering Systems,” explains that under certain conditions fuel can vaporize in lines, pumps, or other units, forming vapor pockets that restrict fuel flow in components designed for transporting liquid. The resulting partial or complete interruption of fuel flow is called “vapor lock.” The publication identifies high fuel temperature, reduced pressure (including altitude-related effects), and fuel system characteristics that promote low pressure and turbulence as conditions that can result in vapor lock.

Fuel Temperature: High fuel temperature is one of the main conditions that can lead to vapor lock. Heat from the engine, exhaust system, and surrounding components can warm the fuel in the lines and pumps, and this heating can cause the fuel to form vapor pockets that interfere with normal fuel flow.

Barometric/Atmospheric Pressure: Low fuel pressure is another main condition that can lead to vapor lock. At higher altitudes, atmospheric pressure is lower, so the fuel can begin to vaporize at a lower temperature, making vapor formation in the fuel system more likely.

Fuel System Design: Fuel system features that cause low pressure or turbulence in the fuel can contribute to vapor lock. Sharp bends, steep rises, restrictions in fuel lines, and conditions at the pump inlet that lower fuel pressure or create turbulent flow can allow vapor bubbles to form and disturb fuel delivery to the engine.

Because wintergrade automotive gasoline is formulated to ignite more easily at low temperatures, it is more prone to vaporizing sooner than either summergrade automotive gasoline or 100LL aviation gasoline. Any increase in fuel temperature, operating altitude, or restriction in the fuel system will further increase this tendency to vaporize.

The airplane was equipped with a Dynon Avionics combination electronic flight instrument system (EFIS) and engine monitoring system (EMS). The unit was installed in the instrument panel directly in front of the pilot and recorded more than 50 engine and flight parameters in its non-volatile memory. Each data point was recorded and logged approximately 16 times per second. Stored parameters included altitude, speed, oil pressure and temperature, fuel pressure and flow, various environmental values, GPS location, and generated warnings/alerts.

Data was downloaded from the unit, which encompassed the entire accident flight. The data indicated that the engine was already running when data recording started at 08:23:14, and showed the airplane was operated on the ground for about nine minutes before takeoff occurred at 08:32:30; the off-airport forced landing occurred at 08:42:30.

During the flight, fuel pressure ranged from 40 psi before take-off to as high as 47 psi during initial climb-out. Following takeoff, the fuel pressure gradually declined to about 42 psi as the airplane climbed, and then at 08:39:20 suddenly dropped to near zero. In addition, fuel pressure readings became increasingly erratic as overall pressure values decreased. Recorded fuel flow readings also decreased and became increasingly erratic, then suddenly dropped to near zero simultaneous with the sudden drop in fuel pressure. Cylinder head temperatures (CHT) were within the specified normal operating range; exhaust gas temperatures (EGT) rose slightly and peaked just prior to when the engine stopped, with a sudden drop in EGT after fuel pressure and flow dropped to near zero. The collective pattern of recorded data is consistent with a system containing gas bubbles due to vapor lock.

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 120 litres per hour of fuel flow there will be a large volume of fuel returning to the tank from which fuel is drawn.”

• “Fuel lines and connections must be suitable, with appropriate safety margins, for at least a constant 3bar fuel system with min. 120 litre/hour fuel flow rate.”

• For fuel system feed lines (default/generic installation): “Min 10mm ID fuel suction line/couplings; Preferable 12mm ID”

Investigators performed a series of full-power operational ground tests using fuel recovered from the aircraft. With the engine cowling removed, the engine ran normally throughout all test phases. The examination revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.

Following the operational test, the engine manufacturer’s representative observed that several firewall-mounted fuel-system components (including the gascolator, fuel filters, electric fuel pumps, and associated metal fuel lines) had been positioned near the engine exhaust system. These components remained hot for more than 20 minutes after engine shutdown.

Under these heat-soaked conditions, investigators again examined the electric fuel pumps. The 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.

In pertinent part, the ULPower installation manual for the 350iS engine stated that fuel lines and connectors must be protected from excessive heat (e.g. exhaust) and that they recommend using heat shields on the fuel pumps if they are forward of the firewall.