ERA24LA099 · Swissdrones Operating AG SDO 50 V2

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.

Verbatim NTSB analysis and probable cause from the NTSB dataset