On August 15, 2019, about 1000 Pacific daylight time, an experimental amateur-built Zenith 601XLB, N662WB, was involved in an accident near Jamul, California. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot stated that he had just installed a stall warning system in the airplane and that the purpose of the flight was to test the system’s operation. The initial stages of the flight were uneventful. After performing a series of turns and stall maneuvers, the pilot initiated a return to the airport. A few seconds later, while the airplane was at an altitude of about 2,900 ft mean sea level (about 2,000 ft above ground level), the engine lost all power. The pilot performed troubleshooting steps, including switching the fuel tank selector valve engaging the fuel boost pump, and engaging the starter motor, and the engine turned but did not restart. After several more attempts to restart the engine, he declared an emergency and performed a forced landing into an open field. During the landing roll, the airplane struck a large rock. The airplane sustained substantial damage to the left wing and forward fuselage during the impact.

The airplane was equipped with a Dynon SkyView electronic flight information system (EFIS), which was configured to record GPS data and system and engine parameters, including engine speed, fuel flow and pressure, oil temperature and pressure, exhaust gas and cylinder head temperatures (EGT/CHT), and bus voltage and current.

The data began at 0927:14. Review of the first 12 minutes of data indicated fluctuating values consistent with engine start, taxi, and runup. As the airplane took off, the EGT and CHT values climbed to 1,400°F and 260°F, respectively, the engine speed increased to 2,975 rpm, and the bus voltage changed from 12.0 to 14.3 volts. During the next 20 minutes, those values remained relatively constant, consistent with cruise flight.

At 0957:22, the engine speed and fuel flow dropped to zero with a corresponding reduction in EGT, and the bus voltage returned to 12 volts while almost 13 amps of current continued to flow through the bus. During the next 3 minutes, the fuel pressure remained about 45 psi, and the GPS altitude decreased from 2,900 to 900 ft. A few seconds later, the fuel pressure dropped to zero, and the GPS altitude indicated that the airplane had landed. The data ended at that time (1000:28).

The airplane was equipped with a fuel-injected, four-cylinder UL260iS engine, which was manufactured by ULPower Aero Engines in Belgium. The engine incorporated a full-authority digital engine control system that included an engine control unit (ECU), which managed ignition and fuel flow based on sensor data. Ignition was provided by electrically powered automobile-style ignition coil packs, and the system was powered by an external battery and an alternator integrated into the aft section of the engine. The ECU can operate at supply voltages down to 10 volts.

Even though ULPower manufactured an optional dual-channel ECU, this redundant system was not installed on the airplane. The airplane was not equipped with a secondary electrical source to power the ECU and ignition system in case primary electrical power e was lost. A 68,000-µF “protection” capacitor was installed along with the battery, as recommended by the installation manual. In addition, representatives from ULPower stated that they had no record of an ECU failure.

The ECU had limited recording capabilities and was sent to the manufacturer for functional testing and data extraction under the oversight of Belgium’s Air Accident Investigation Unit. The functional testing found that the ECU appeared to operate within specifications. The last 3 minutes of data for the accident flight were recovered, including engine speed, ECU voltage, and engine oil temperature. The data revealed that the ECU engine speed matched the EFISrecorded speed, the input voltage remained at 13.2 volts, and the oil temperature was about 174°F. The data ended at the same time that the EFIS recorded the engine speed drop to zero rpm (0957:22), indicating that the ECU shut down.

Examination of the airplane at the accident site revealed that the right-wing fuel tank contained 8 gallons of fuel and that the left-wing tank, although breached, contained about 7 gallons of fuel. The airplane examination also revealed that the engine remained attached to the firewall without significant damage. The engine contained appropriate quantities of oil, and no evidence of a catastrophic failure was observed. The engine controls, electrical harnesses, fuel lines, and pumps were all intact, and no circuit breakers had tripped. The air filter was free of debris, and the fuel pumps activated when energized. The battery had sustained impact damage, causing it to become directly short-circuited against the airframe; therefore, functional testing could not be performed.