ERA24LA099

During an instructional flight of an intermeshing rotor-equipped unmanned aircraft system (UAS), the remote pilot-in-command (RPIC) received warnings of high fuel pump voltage, followed by warnings of max fuel pump voltage. Recorded data revealed that during these high and max fuel pump voltage spikes, the turbine engine lost power and the collective limiter would engage automatically. While attempting to have the UAS return for landing, the UAS had another partial loss of engine power, and the collective limiter was engaged again and remained engaged for the rest of the flight. The UAS touched down on the ground and then became airborne again; subsequently, the RPIC commanded an engine shutdown. After the engine shutdown, the UAS descended and contacted the ground hard on the skids; subsequently, the rotors contacted two of the vertical stabilizers, which resulted in substantial damage to the vertical stabilizers. According to the airframe manufacturer, it is possible that during a partial loss of engine power, the UAS may become airborne again after contacting the ground, as the autopilot will increase collective when the rotor rpm recovers after contacting the ground. This information was not provided to operators in the Unmanned Flight Manual at the time of the accident, so the RPIC was not aware of the need to engage the auto-land function. A postaccident examination of the airframe and the engine’s fuel system revealed no anomalies that would have precluded normal operation. Fuel samples taken just after the accident revealed no visible signs of fuel contamination or debris. An engine test run was performed using the turboshaft engine, electrically driven fuel pump, and the engine control unit (ECU). The engine operated normally at all tested RPMs and there were no anomalies that would have precluded normal operation. During the test run, a loss of fuel flow was simulated by slowly removing the fuel pickup tube from the fuel tank; the fuel pump voltage would increase to its maximum allowable voltage and the engine would lose power. Given that the main fuel tank was still full at the time of the accident, it is unlikely that the fuel pickup would have become unported during the accident flight. The fuel pump was bench-tested and then inspected. There were no anomalies observed that would have precluded normal operation. Similar cases of high/max fuel pump voltage spikes with accompanying reduction in fuel flow of similar make and model UAS aircraft were reported to the manufacturer and were found to have debris in the fuel system, which degraded the fuel pump’s performance. However, there was no debris found in the accident UAS’s fuel system during the examinations. Based on the available evidence, the partial loss of engine power was likely a result of a reduction of fuel flow to the engine. The reason for the reduction in fuel flow could not be determined.

On January 18, 2024, at 1123 eastern standard time, a SwissDrones SDO 50 V2, N780PA, was substantially damaged when it was involved in an accident near Antioch, Georgia. There were no injuries. The unmanned aircraft system (UAS) was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. According to the operator, the purpose of the flight was for the remote pilot flying to build remote pilot-in-command flight experience. The remote pilot was accompanied by two other employees acting as flight support specialists. The flight crew uploaded the mission into the UAS and performed a preflight inspection without any errors or anomalies noted. The operator reported that prior to the flight, the UAS contained a total of 17 liters of fuel (approximately 4.5 gallons). The pilot commanded the UAS to perform an automated takeoff, followed by a manual control and operations check. The pilot reported the UAS operated normally, and the pilot then commanded the UAS to begin the pre-planned mission in an automated mission mode. While the UAS was approaching the eighth waypoint, the pilot received a fuel pump voltage high warning, followed by a fuel pump voltage max warning from the ground control station. The pilot commanded the UAS to return to the first waypoint so the pilot could land the UAS near the initial takeoff point. While en route to the first waypoint, the pilot received a second fuel pump voltage high and fuel pump voltage max warnings. The pilot switched the UAS to manual control mode and initiated a landing. During the landing, the pilot did not engage the auto-land function (the Unmanned Flight Manual that was current at the time of the accident did not state to engage the auto-land function). During the landing, the UAS continued to descend despite the pilot no longer commanding a descent. The UAS contacted the ground and then began to climb again to an altitude of about 10 ft above ground level. Subsequently, the pilot commanded the engine to shut down and the UAS descended and landed hard. The main rotor blades contacted two of the vertical stabilizers, resulting in substantial damage to the two vertical stabilizers. The operator reported that prior to recovering the UAS, about 14.0 liters (3.7 gallons) of fuel was removed from the UAS’s fuel tanks. The UAS was an intermeshing rotor design and was equipped with a 14.7 hp, Jakadofsky Pro X turboshaft engine. The UAS was equipped with a 13 liter (3.434 gallon) main fuel tank as well as two additional auxiliary fuel tanks with each fuel tank being an additional 13 liters (3.434 gallons). Both auxiliary fuel tanks feed directly to the main fuel tank. Engine and rotor rpm were maintained by adjusting the fuel flow going into the turboshaft engine, and the fuel flow was regulated through the electrically driven fuel pump by automatically adjusting the fuel pump voltage. The maximum fuel pump voltage was specified to be 5 volts. According to SwissDrones, a “PUMP VOLTAGEHIGH” alert would be displayed on the ground station if the fuel pump voltage went above 4.6 volts. The ground station would display a “PUMP VOLTAGE MAX” if the fuel pump voltage went above 4.9 volts. According to the manufacturer, the collective limiter would activate when there was a drop in rotor rpm of at least 25 rpm and would disengage if rotor rpm returns. The collective limiter was used to force the UAS to descend at a rate that would reduce the damage to the UAS in the event of a loss of engine power. The activation of the collective limiter would not prevent the pilot from controlling the cyclic controls, which would enable the pilot to adjust the UAS’s trajectory to allow the UAS to land in a safe location. The manufacturer reported that since the UAS was not equipped with sensors to detect the height above the ground, the UAS was not capable of adjusting the collective to perform a flare to land softly while the collective limiter is activated. The manufacturer also reported that during a partial loss of power, the UAS may become airborne again after contacting the ground. The UAS contained two avionics units that were capable of recording non-volatile memory (NVM) from the accident flight. The first was referred to in the UAS’ Maintenance Manual as the “avionic box” that recorded multiple flight and engine parameters, as well as alerts. Following the accident, data was downloaded by the operator using the manufacturer's instructions. The data revealed that during the accident flight, the rotor governor rpm had two distinct drops in governor rpm that were greater than 50 rpm for several seconds. During these drops in rpm, the fuel pump voltage dramatically increased to the maximum of 5 volts. When the fuel pump voltage reached 5 volts, the pump voltage max alert was sent to the ground control station. The NVM data also revealed that during these drops in rpm, the collective limiter was activated. The data also revealed that the “Fuel level too low” alert remained off for the entire flight. The second device that recorded NVM data was the turboshaft’s engine control unit (ECU). The ECU could record limited data that could be viewed using a proprietary engine data terminal (EDT). The ECU was connected to the manufacturer-supplied connectors, adapters, and the Jakadofsky EDT to facilitate a data readout. In particular, one of the code readouts stated, “13: FUELPMP-PEAK 5.00V!” The manufacturer reported that a voltage of 5 volts or more was indicative of a fuel supply issue to the fuel pump, a blockage in the fuel system, or a fuel pump issue. An FAA inspector performed a postaccident examination of the UAS at the operator’s facility. Fuel samples that were taken just after the accident were noted to be clear of contamination and debris. All the fuel lines were observed to be installed in the correct orientation and none of the fuel lines were kinked or broken. The airframe fuel hopper tank and fuel filter were inspected and there was novisible contamination or debris observed. The wiring for the ECU, fuel pump, and rpm sensor were all secure and there were no signs of damage to the wires. The engine to drivetrain clutch and the left and right drivetrains were inspected and there were no anomalies or mechanical failures found. The UAS’s turbine engine was undamaged, and all fuel lines and electrical wires were secured to the connection points. There were no signs of foreign object damage to the compressor blades and the compressor rotated smoothly when rotated by hand. The turbine engine, electrically driven fuel pump, ECU, and the two fuel solenoids were removed from the UAS to facilitate functional testing. An engine test run was performed on the accident engine, ECU, and electrically driven fuel pump; the test run used a new fuel tank, fuel pickup tube, fuel lines, and serviceable fuel solenoids. The engine operated normally at all tested rpms and the fuel pump voltage remained within the normal operating range. To simulate a loss of fuel flow to the fuel pump, the fuel pickup tube was manually removed from the fuel tank to observe the fuel pump operation. A few seconds after the pickup was removed from the fuel, the fuel pump voltage went to 5 volts and the engine began to lose power. The two fuel solenoids were tested, and both fuel solenoids operated normally. The fuel pump was sent to SwissDrones for additional testing and a teardown examination. The fuel pump was connected to a serviceable ECU and was operated at several different voltages. The fuel pump fuel flow and pressure readings were correct for the signal voltage that was emitted by the ECU. The fuel pump was disassembled and there were no anomalies found that would have led to a reduction or failure of the pump’s performance. According to SwissDrones, high fuel pump voltage spikes and reduction of fuel flow incidences had occurred to other UASs that used the same fuel pump and engine configuration. In those technical support examinations, it was found that small amounts of debris (10 microns in size) could degrade the fuel pump operation. In response, the manufacturer modified the UAS’ fuel system for increased fuel filtration before the fuel pump.