Introduction
On August 15, 2019, about 1000 Pacific daylight time, an experimental amateur-built Zenith 601XLB, registration N662WB, was involved in an accident near Jamul, California. The pilot, who sustained minor injuries, was operating the airplane as a Title 14 Code of Federal Regulations Part 91 personal flight.
Flight Details
The pilot stated that he had just installed a stall warning system and that the purpose of the flight was to test its 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 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. The engine turned but did not restart. After several more attempts, he declared an emergency and performed a forced landing into an open field. During the landing roll, the airplane struck a large rock, resulting in substantial damage to the left wing and forward fuselage.
Data Recovery
The airplane was equipped with a Dynon SkyView electronic flight information system (EFIS) 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.
Data began at 0927:14. The first 12 minutes showed fluctuating values consistent with engine start, taxi, and runup. After takeoff, EGT and CHT values climbed to 1,400°F and 260°F respectively, engine speed increased to 2,975 rpm, and bus voltage changed from 12.0 to 14.3 volts. During the next 20 minutes, values remained relatively constant, consistent with cruise flight.
At 0957:22, engine speed and fuel flow dropped to zero with a corresponding reduction in EGT. Bus voltage returned to 12 volts while almost 13 amps of current continued to flow. During the next 3 minutes, fuel pressure remained about 45 psi, and GPS altitude decreased from 2,900 to 900 ft. A few seconds later, fuel pressure dropped to zero, and GPS altitude indicated the airplane had landed. The data ended at 1000:28.
The engine was a fuel-injected, four-cylinder UL260iS manufactured by ULPower Aero Engines, incorporating a full-authority digital engine control system with an engine control unit (ECU) managing ignition and fuel flow. The ECU was powered by an external battery and an alternator integrated into the engine. The ECU could operate at supply voltages down to 10 volts. The airplane was not equipped with a secondary electrical source or the optional dual-channel ECU. A 68,000-µF protection capacitor was installed as recommended.
The ECU was sent to the manufacturer for functional testing and data extraction under oversight of Belgium’s Air Accident Investigation Unit. Functional testing found the ECU operated within specifications. The last 3 minutes of data, including engine speed, ECU voltage, and oil temperature, were recovered. The data revealed ECU engine speed matched the EFIS-recorded speed, input voltage remained at 13.2 volts, and oil temperature was about 174°F. The data ended at the same time the EFIS recorded engine speed drop to zero rpm (0957:22), indicating the ECU shut down.
Post-Accident Examination
Examination at the accident site revealed the right-wing fuel tank contained 8 gallons of fuel, and the left-wing tank, though breached, contained about 7 gallons. The engine remained attached to the firewall without significant damage and contained appropriate quantities of oil. No evidence of a catastrophic failure was observed. Engine controls, electrical harnesses, fuel lines, and pumps were intact, and no circuit breakers had tripped. The air filter was free of debris, and fuel pumps activated when energized. The battery sustained impact damage, causing a direct short circuit against the airframe, so functional testing could not be performed.